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■ F

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I ^ «

PULP AND PAPER

Chemistry and Chemical Technology

VOLUME II:

Properties of Paper
and Converting

I • •

PULP AND PAPER

Chemistiy and Chemical Technology

IN TWO VOLUMES

JAMES P. CASEY, Director of Technical Service

A. E. Staley Manufacturing Company, Decatur, Jlltnois
Formerly Associate Professor of^ulp and Paper Manufacture
State University of New Yor^lColl^ i»\Forestry, Syracuse,
New Yor\ U ^

VOLUME II:

Properties of Paper
and Converting

19 5 2

INTERSCIENCE PUBLISHERS, Inc., New York

INTERSCIENCE PUBLISHERS Ltd., London

Library of Congress Catalog Card Number 51-13781

ALL RIGHTS RESERVED. This book or any part thereof must not be
reproduced in any form without permission of the publisher in writing.
This applies specifically to photostat and microfilm reproductions.

C_ /X. c-

INTERSCIENCE PUBLISHERS, Inc., 250 Fifth Avenue, New York 1, N. Y.

For Great Britain and Northern Ireland:

INTERSCIENCE PUBLISHERS Ltd., 2a Southampton Row, London W. C. 1

Printed in the United States of America

PREFACE

to Volume II

Volume I of this book was concerned with the chemistry of the basic
pulp and papermaking operations, such as pulping, bleaching, stock prepa¬
ration, sheet formation, filling, sizing, and coloring. The properties o
paper and the important paper-converting operations are discussed in this
volume. These converting operations include pigment coating, printing,
coating with resins and waxes, saturation of paper with resins and waxes,
and laminating—operations that are closely related to papermaking. They
are often carried out in the same plant where the paper is made and, in fact,
there is an increasing trend toward carrying out these operations directly
on the paper machine as an integral part of the papermaking process. Con¬
verting operations are so inextricably tied in with the fundamental proper¬
ties of paper that they cannot be discussed without a thorough knowledge
of paper chemistry. By the same token, satisfactory paper for converting
operations cannot be made without a knowledge of the basic requirements |
of these operations. For these reasons, it is highly desirable that the paper
chemist understand the various converting operations. Many paper chem¬
ists are engaged in technical service work and have to follow their com¬
pany’s products into converting plants during times of trouble, or when
experimental work is being carried out. This requires a special knowledge
of these fields which should be a part of every paper chemist’s training. It
is with these ideas in mind that the chapters on paper-converting processes
have been written.

The author received considerable aid in writing this part of the book,
and the names of those who helped in this work are listed in the preface to
\*o1ume I. To them, the author again wishes to express his appreciation.

J.P.C.

Decatur, Illinois
April, 1952

CONTENTS
Volume I

I. Cellulose and Hemicellulose

II. Lignin ..

III. Pulpwood .

3PlV. Pulping ..

V. Bleaching .

VI. Fiber Preparation .

VII. Nature of Fiber Bonding .

VIII. Sheet Formation .

r ' 1 . q ..

IX. Filling and Loading

X. Internal Sizing-

XI. Surface Sizing ....

XII. Wet Strength .

XIII. Coloring .

XIV. Microbiology .

XV. Water .

Author Index .

Subject Index .

248

322

370

417

468

490

548

583

605

654

707

725

741

CONTENTS
Volume II

Preface .

XVI. Properties of Paper ...

Introduction' to Paper Testing ...

Types of Properties Measured ...

Effect of Papermaking Variables on Properties of Paper ..

Sampling and Conditioning Paper .

Wire and Felt Sides...

Machine and Cross Direction.

Physical Properties .

Basis Weight ...

Thickness ...

Density .

Stress-Strain Relationships of Paper .

Stiffness .

Tensile Strength .

Stretch .

Bursting Strength .

Internal Tearing Resistance .

Edge Tearing Resistance .

Folding Endurance ...

Impact Strength .

Softness .

Hardness and Compressibility .

Bulk and Bulking Thickness .

Porosity .

Smoothness .

Formation ...

Uniformity and Precision of Test Methods.

Unsatisfactory Appearance and Dirt .

Two-Sidedness .

Dimensional Stability .

Optical Properties ...

Nature of Light .

Absorption of Light ...

Reflectance of Light ...

Color of Paper .

Methods of Measuring Color .

Measurement of Color by Eye.

Expressing Color in the Munsell System ..

Physical Measurement of Color .

The Recording Spectrophotometer .

Abridged Spectrophotometry .

797

798

799

800
801
804
804

809

810
814
823
826

830

831
835

838

839

843

844
844
846
846
851

856

857
859
861
863

869

870

871

872

873

874

875

876
878
880

VII

VIII

CONTENTS

XVI. Properties of Paper (continued)

Color Specification by the I.C.I. System

Tristimulus System ..

Standard Observer ...

Chromaticity Diagram ..

Dominant Wavelength and Purity ..

Visual Efficiency ..

Three-Filter Colorimetry ...

Brightness of Paper .....* *

Methods of Measuring Brightness .

Importance of Brightness .

EflFect of Dyestuffs on Brightness.

Effect of Pigments on Brightness..

Relationship Between Transparency and Brightness

Comparison of Pulps for Brightness ...

Brightness of Pulp Mixtures ..

Gloss of Paper ...

Methods of Measuring Gloss ..

Importance of Gloss ----

Finish .. • • ....* *.

* Transmittance of Light .

I Metliods of Measuring Opacity ..
N Measurement of TAPPI Opacity
Measurement of Printing Opacity
Measurement of Printed Opacity .

Measurement of Opacity by Transmittance .

Effect of Sheet Weight on Opacity ..

Effect of Refractive Index on Opacity .

Effect of Sheet Density on Opacity ..

Effect of Fillers on Opacity .

Effect of Dyestuffs on Opacity ...

Effect of Waxing on Opacity ..

Effect of Different Pulps on Opacity.

Kubelka and Munk Theory.

Kubelka and Munk Equation .

Determination of K and 5 Values of Pulp Mixtures ....

Factors Affecting K and 5 Values of Pulp..

Calculation of Brightness of Pulp Mixtures from K and

5* Values .

Determination of S Values of Pigments .

Effect of Dyestuffs on K/S Values ..

Changes in Contrast Ratio with Changes in Reflectivity
Changes in Contrast Ratio with Changes in Basis Weight
Other Uses of Kubelka and Munk Theory.

Chemical Properties .. '' *.

Alpha Cellulose, Viscosity, and Copper Number of Paper ..

Amount of Sizing Agents in Paper .

Moisture Content ...

Ash .

Acidity and />H .

880

881

883

886

887

888
888
890

892

893
893

895

896
898
898

901

902

903

904

905
905
907

907

908

909

910

912

913

914

916

917
917
923

926

927
927
929
929

929

930

931

932
938
943

CONTENTS

XVI. Properties of Paper (continued)

Permanence of Paper .

Odor and Taste .

Electrical Properties ...U^ W*

Specific Inductive Capacity (Dielectric Constant)

Dielectric Strength .

Dielectric Loss (Power Factor) .

Properties of Electrical Insulating Papers .

Types of Electrical Papers .

Microscopical Analysis .

Types of Microscopes .

Types of Fiber Stains .

Preparation of Specimen .

Preparation of the Slide .

Examination of the Slide .

Methods of Counting .

Use of Weight Factors.

Iodine Stains .

Herzberg Stain .

The Graff “C” Stain .

Sutermeister’s “A” Stain .

Wilson Stain .

Modified Selleger Reagent .

Laughlin Stain ..

Dye-Base Stains .

Bright Stain .. • •

Kantrowitz-Simmons Stain .

Lofton-Merritt Stain .

Cooking (Bleachability) Stain .

Cyanine-Glycerine Reagent .

Bleach Stain .

Shaffer Stain.

Malachite Green Reagent .

Groundwood Stains .

Stains for Special Fibers.

Speck Analysis .

Coal Specks .

Iron Specks .

Bronze Specks .

Resin and Pitch Specks .

Alum Spots .

Bark Specks .

Color Specks .

Foam Spots .

Slime Spots ..

Decayed Wood .

Calcium Carbonate and Bleach Scale.

Wax and Grease Spots .

Rubber .

Other Specks ...

944

949

950

951

952

953
953
955
957
957
959
959

961

962

963
963
965

965

966
969

969

970

971
■ 972

972

973

974

975
975
975

975

976

977
977

977

978
978
978
978
978
978

978

979
979
979
979
979

roNtrvTS

XVI. Proptrtic* of Piper {i^nhtufd)

StKH 141 Tcthnur*^^ ...

XVII. Uie of Stitittk'i in the Piper Induitry ..

Thfury of SiAti>lics ....

MrtK*¥iii i>( Coniian*«*fi ..

RuU « ot Sanii'ltnf . • * • •. • -.. ^. 4^4 .......

GrotiptHK Aivl Aiul)"fiiif I...44.444-.44.*>*-*

Krct|iiciKy I)i%trtl^utt«xi ..

Char at ten flics vi Irtqurmjr l)tanf>^itkici .

A vfniff r»r M ran ........-

i’;)niie ..... ...•*••••

Stimlinl Drviati'Xt ...

Variance .....

rocfficieiil of VarUtVin .

I tuiiu of Xeliatnlky .

Standird Errew .. - • ..

PiobaWe Km r .

XIaxtmum Exror ..

Significani Fifurr* .

Tests of SiRntfkince .

Determining the Krnnber of Item* to be Measored

Specified Prcci<k}»fi ....

Coenparison of Means ..*... *.- -

Comparison of Variances ....*.

Use of Sutistics in Quality Control.

Definition of Quality ...

\’^altic of Quality Cootrol .....

Normal Quality Lercl .... -. y * .* • •

Cofistructix) of Frequency Distrflwtion

Construction of Control Chart .-......

Use of Forrnulas for Constrwrting Control Quarts .

Data to Indicate Relationship ...- * ...

Correlatjoo Coefficient . -.- -.- -.

for a

^ m m m m m

9^2

985

9ft5

%i>

9d5

989

990

991

992

995
997
997
997
W7

996
999
PJOl
1005
1005

XVIII. Pigment Coating .

The C«-4tmg Process .

J||® V Conventional Coating .

XIaeWne Coating .

* T 3 rpes of Coating XIachines ...

Brash Colters ..

Ron Coalers .

Planographic RoJl Coater...

Roto gt av ur e RoU Coater .

Knife Coalers .

Air Brash Coaters ..

Coating Adhesives .-.* • • *

Properties Xecessary in Coatinf Adhesfves

Casern ...

1007

1008
10(W
1006
1010
1010
1011
1011
1011

1013

1014

10:4

1015

CONTENTS

XVIII. Pigment Coating (continued)

Starch .* ’

Soy Flour and Soybean Protein .

Animal Glue .

Corn Proteins .

Polsrvinyl Alcohol .

Cellulose Derivatives .

Synthetic Resin Latices .

Blending Adhesives .

Coating Pigments .

Clay ..

Nature of Raw Material .

Methods of Processing Clay .

Properties of Clay .

Ion Exchange Properties of Clay ,..

Hydration of Clay .

Shape of Clay Particle .

Particle Size of Clay .

Dispersing Clay in Water .

Flocculation of Clay ...

Calcium Carbonate .

Precipitated Calcium Carbonate ...

Ground Calcium Carbonate .

Titanium Pigments .

Preparation of Titanium Dioxide ..
Properties of Titanium Pigments ..

Satin White .

Zinc Pigments ..

Zinc Sulfide .

Composite Zinc Sulfide Pigments ..

Zinc Oxide .

Barium Sulfate .

Calcium Sulfite .

Calcium Sulfate .

Diatomaceous Silica ..

Luminescent Pigments ..

Colored Pigments .

Adhesive Demand of Coating Pigments

Preparation of Coating Mixture .

Mixing the Pigment and Adhesive ...

Coating Formulas .

Per Cent Solids .

Flow Properties of Coating Mixtures ,

Water Retention ..

Other Colloidal Properties ...

Foam .......

Coating Raw Stock ....

Composition of Base Paper.

Sizing .

Finish and Porosity

1020

1025

1028

1029

1030

1031

1032

1033

1034

1035
1037
1037
1039
1041

1043

1044
1047
1050

1050

1051
1053
1053
1055
1055

1057

1058

1059
1059

1059

1060
1061
1061
1062
1063
1063
1066
1066
1068
1070
1073

1083

1084

1085
1087

1087

1088
1088

XII

CONTENTS

XVIII. Pigment Coating (continued)

Strength of Raw Stock ...

Brightness and Opacity .

Uniformity .

Applying Coating to Paper.

Wetting of Raw Stock by Coating Mixture ... 1090

Measurement of Penetration of Coating Mixture into

Raw Stock .

Effect of Penetration on Properties of Coated Paper.. 1092

Effect of Properties of Coating Mixture on Penetration ... 1093

Effect of Raw Stock Properties on Penetration . 1093

Smoothing the Coating .

Calendering .

Special Calendering Operations . ™

Evaluation of Coated Papers . _ “

Making Laboratory Coatings .

Strength of Coating .. _

Coating Weight ..

Water Resistance .

Ink Receptivity .

Smoothness .

Gloss . ^

Brightness and Opacity .. “ '

Special Coating Processes ... »

Wallpaper Coating .

Varnished Papers .

Opaque Bread Wrap.

Coated Flour Bags .

Sandpaper .

Specialty Coated Papers .

XIX. Printing .

Printing Processes .

Relief (Typographic or Letterpress) Process

Types of Printing Presses .

Forms of Reproduction .

Type Reproduction .

Line Reproduction .

Halftone Reproduction .:.

Multicolor Printing .

Printing Plates .

Electrotypes and Stereotypes ....

Special Printing Plates .

Makeready .

Relief Printing Inks .

Printing Ink Transfer .

Drying Oil-Base Relief Inks .

Solvent Heat-Set Relief Inks.

■' ! Cold-Set Relief Inks .

,. 1131
.. 1132
.. 1133
.. 1133
.. 1134
.. 1134

.. 1134

.. 1135
.. 1136
.. 1137
,. 1137

.. 1138
.. 1138
.. 1139

.. 1141
... 1142

... 1145
... 1146

CONTENTS

XlII

XIX. Printing (continued) II47

Vapor-Set Relief Inks .. 114a

General Requirements of Papers for Relief Printing.

Printability and Print Quality ....

Uniformity . IJ52

Smoothness ... 1155

Ink Receptivity .

Surface Bonding Strength .

Effect of Moisture and /»H in Papers for Relief Printing 1158

Static Electricity .

Coated Papers for Relief Printing. ^

Paperboard for Relief Printing . 1163

Printing with Aniline Inks .

Printing of Newsprint ....

Intaglio Process . ^

Copper and Steel Engraving ...

^7 . 1167

Rotogravure ..

Sheet-Fed Gravure Printing (Photogravure) .

Paper for Intaglio Printing .

Planographic (Lithographic) Process .

Printing from Stone . ii7n

Printing from Albumen Plates (Photolithography) - 1171

Printing by Deep-Etch Process . ^171

Offset Printing .

‘ Planographic Inks . \\7A

• General Requirements of Paper for Offset Printing . 1175

Smoothness ..

Ink Receptivity .

Chemical Requirements of Offset Papers . 1175

Surface Bonding Strength . 1176

Moisture Control with Offset Papers . 1176

Curl in Offset Papers. 1181

Coated Papers for Offset Printing . 1182

Photogelatin (Collotype) Printing . 1184

Silk-Screen Printing (Serigraphy) . 1184

Pressroom Difficulties . 1184

Strike Through . 1185

Show Through . 1186

Offset . 1186

Collecting . 1187

Caking or Fill-up . 1187

Piling . 1188

Crystallization . 1188

Mottling ... 1188

Misregister . 1189

Chalking or Powdering . 1189

Picking . 1190

Tinting and Washing . 1191

XIV

CONTENTS

XIX. Printing (continued)

Scumming .

iJuplicator Processes ...

Stencil Duplicators .

Hectograph (Gelatin) Duplicators ..

Spirit Duplicators .

Lithographic (Offset) Duplicators ..
Letterpress Duplicators (Multigraph)

Automatic Typing ..

Photo-Copying Processes .

Blueprint Process ..

Direct (Diazotype) Process.

Lithoprints .

1191

1192

1193

1194

1195

1196

1197
1197

1197

1198

1199

1200

XX. Laminating and Pasting ...

Laminating Processes .

Types of Adhesive Used .

Aqueous versus Non-Aqueous Adhesives ...

I Water-Resistant Adhesives .

h Heat-Seal Adhesives .

Pressure-Sensitive Adhesives .

Solvent Welding .

Heat Welding ..

Theory of Adhesion and Adhesive Properties ..

Specific Adhesion ...

Mechanical Adhesion .

Effect of Adhesive Properties.

Viscosity of Adhesive.

Tackiness of Adhesive ..

Other Properties of Adhesive .

Effect of Sheet Characteristics .

Application and Setting of Adhesive .

Setting of Adhesive by Solvent Loss ..

Setting by Gelation Due to Cooling .

Setting by Chemical Reaction .

Setting by Gelation Due to Heating ..

Properties of Adhesive Film .•

Curling and Warping .

Aqueous Adhesives .

Starch and Dextrin Adhesives ...

Starch Adhesives for Solid Fiber Combining and Sheet

Lining ... -

Water-Resistant Starch Adhesives for Solid-Fiber Com¬
bining .

Starch Adhesives for Corrugated Board .

Starch Adhesives for Bag Pasting .

Starch Adhesives for Pasting Bristols ...

Starch Adhesives for Tube W^inding and Carton Sealing

Sodium Silicate ...

Use of Silicate in Solid-Fiber Laminating.

1201

1202

1203

1204

1205
1205

1205

1206

1207

1208
1209

1209

1210
1212
1213
1213

1213

1214

1215

1216
1217

.1218

1220

1222

1226

1228

1230

CONTENTS

XX.

Laminating and Pasting (continued)

Use of Silicate in Corrugating.

Protein Adhesives .

Casein Glues .

Soybean Glues .

Blood Glues .

Animal Glues .

Fish Glues ...

Other Aqueous Adhesives .

Gummed Papers and Paper Tapes.

Remoistening Gums .

Sealing Tapes .

Emulsion Type Adhesives .

Rubber Latex .

Asphalt Emulsions .

Resin Emulsion Adhesives .

Lacquer Type Adhesives .

Hot-Melt Adhesives .

Applying Hot Melts .

Papers Used for Hot-Melt Laminating

Wax Adhesives .

Asphalt Adhesives .

Resin Adhesives .

. 1231
. 1232

. 1232
. 1232
,. 1233
.. 1233
.. 1233
.. 1233
.. 1234
.. 1234
.. 1235
.. 1236

.. 1237
.. 1237
.. 1238
,. 1238
.. 1240
.. 1241

.. 1241
.. 1242

.. 1242
.. 1243

XXI. Internal Treatment of Paper with Resinous Materials. 1246

Treatment of Paper with Latices and Emulsions. 1246

Properties of Beater-Treated Papers . 1246

Types of Resins Used in Beater Treatment. 1247

Methods of Applying Latices in Beater Treatment. 1247

External Application of Resins to Paper . 1250

Types of Paper Used for Saturation ... 1251

Types of Resins Used for Saturation. 1251

Saturation of Paper with Asphalt, Oils, and Waxes. 1254

Asphalt-Saturated Papers . 1255

Tracing Papers . 1256

Oiled Papers ... 1256

Stencil Papers . 1257

Paper Plastics . 1258

Resin-Filled Paper Plastics... 1259

Resin-Impregnated Paper Laminates . 1260

Methods of Applying Resin. 1260

Resins Used for Paper-Base Laminates. 1261

Paper Used for Paper-Base Laminates. 1263

Lamination and Molding . 1266

Pulp-Reinforced Plastics .'.. 1268

Properties of Fibrous Fillers. 1268

Effect of Resins . 1269

XXII. Coating with Resinous Materials .‘. 1271

Fundamental Properties of Films . 1272

Adhesion of Films... 1272

XVI

nomuKTf

XXll.'Co«»itMI with RMinotw MatcrtAhi

Film Flwtibitily *.

VP^

Cfi

i i

• V.A'

- —► ^

fS‘:
A ^^

1275
1275
1275
127S

1211

i2ii

12X2

12U

t2U

t2M

12M

12K5

I2R7

1217

12W

12«0

192

Rkdtint

He*t S«»l

Sliptwcc ......

Ga* Permeability of Coated Pa^rr*

W»teT'V*»f»f Rcairtantr
Machinery for Cnatinx whh Rewm

Kidfc Coaiera ......

Roll Coatert 4 .•«‘ 4 ' 4 *>»'»* 4 »»’**»*'******’*'’**‘***

Air Knife Coaler .....

Cootinff with IJM^ipwra ......

Methuia of Apfdyinf Sjleeni CoaliPKa ....444.*4.-4. . 4*4

Solrcnta for Lacnqrrt *.,.*».,4444.

PU«ici»era .......

Mialifirra ....

Cae of Ncm^Film-Ficmmx Rrain* ..

U*e of Film-FormiTur Re«n* .»,.,4..-44.#.4.4...444-4-.4

Uie nf Kitrowlltdoae ,.4.444.**4»4.4»44*4...**44*»»4«

i t>1iC nf FlHrlcelhdO*?

Uk «f potyrmyl Arrtate and Polyvinyl Chloride .*. 44 * 1293

\Jf< of Polyvinyl Chloride*Acetate Coyolywera ........

Uae of Pf4tvin>1idme Chloride Cofwiymera , 4 .. 4 ..*..*.

Uae of Perfyethykne .*...

Uae of Robber and Robber peiivatirea ....

Coating with FmoHiom .,...444.-^««4..4,44..4.*4..4.....4..44

Formolation of Aqoeons Eraulwcau ....,..*...4...44-.4..

Appljing EmoUkiaw « Sort
Xypea of Aijueow Emnisioot *44
Oricanoaob and Plastiacd* ......

Cnatimt with Hot Melts ..j...........

Methods of Apphrin* He* Mdls
Waxtryr vrith Paraffin ......4..

Dry Waxi^ S r..:

Wet Waxme ........44.-.*..-

Waxinx of Paper Cartons ...

Detemniiatxm of Paraffin in Waxed Papers

Waxmir with Ms u vcirs talline Wax.

Use of Rcsin-W*ax Blends as Ho* Mdti

Use od Ethykclhdose . .-^..4444.4.4.... ••

Use of PfdyeAyienc ...

T ^

^ I •

I ^

m m m 0 m m ^ ^ ^

.0 m m

m 0 m m m m

t I

1293

129$

1297

1298

lino

1301

1303

ijoi

1306

1307

1308

1310

1311

1312
U12
1312
1314
1314

1316

Use of Other Resins...*. ”*1

••v2*r Use of Rt*ba- De sUaii v ts ra Ho* Mehs. .

Si^ ■ ... *?•'

XXni. Resins

1319

Xatnr^ Resias ... . ......

‘>4

CONTENTS

XVII

XXIII. Resins {continued)

Rosin Derivatives .

Shellac .

Dammar Resins .

Manila Resins .

East India Resins .

Congo Resins .

Kauri

Accroides and Elemi .

Zein ..

Cellulose Derivatives (Film-Forming Resins) ..

Nitrocellulose ..

Cellulose Acetate .

Other Cellulose Esters .

Ethyl cellulose .

Vinyl Polymers (Film-Forming Resins) .

Polyvinyl Acetate .

Polyvinyl Chloride .

Polyvinyl Chloride-Acetate Copolymers .
Polyvinylidene Chloride and Copolymers .

Polyvinyl Aldehydes .

Polystyrene .

Acrylic and Methacrylic Polymers .

Polyethylene Resins .

Synthetic Non-Film-Forming Resins .

Phenol-Formaldehyde Resins .

Urea-Formaldehyde Resins .

Melamine-Formaldehyde Resins .

Acetone-Formaldehyde Resins .

Alkyd Resins .

Polyester Resins .

Other Resins .

Coumarone-Indene Resins .

Terpene Resins .

Silicone Resins .

Chlorinated Polyphenyls .

Aryl Sulfonamide-Formaldehyde Resins

Rubber and Rubber Derivatives.

Natural Rubber Latex .

Natural Rubber and Rubber Derivatives

Synthetic Rubbers .

Corollary Reading .

Author Index .

Subject Index .

. 1322
. 1324
. 1324
. 1325
. 1325
. 1325
. 1325
. 1326
. 1326
. 1326
. 1326
. 1327
. 1328
, 1329
. 1331
. 1331
,. 1332
.. 1333
.. 1334
.. 1334

.. 1335
.. 1335
.. 1336
.. 1337
.. 1338
.. 1339
.. 1341
.. 1341
... 1341
.. 1343
.. 1343
.. 1344
.. 1345
.. 1345
.. 1345
.. 1346
.. 1346
,.. 1347
,.. 1347
... 1348
... 1351
... 1353
... 1361

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fifi

CHAPTER XVI

PROPERTIES OF PAPER

Although all papers are made from essentially the same raw materjds
there is confiderable variation in the properties of commercial papers,
pur^se of this chapter is to discuss the significance of the «n pape
pro^rties and to discuss the relationship between these properties and

use requirements of the paper.

Introduction to Paper Testing

The characteristics of paper are divided into physical, optical, chermcal,

electrical, and microscopical properties. Physical properties
ness, weight, density, hardness, stiffness, and tensile strength. Optical
properties include light transmittance, light absorption, and light reflec anc .
Chemical properties include pH, moisture content, alpha cellulose, and cop¬
per number. Electrical properties include dielectric strength, specific in¬
ductive capacity, and electrical conductivity. Most of the important prop¬
erties of paper are discussed in this chapter, although certain properties are
discussed in other chapters; for example, wetting properties are discusse
in the chapter on sizing, and water vapor resistance is discussed in the c ap

ter on coating with resinous materials.

Any listing of tests used on paper would be incomplete without mention

of the handling tests in which the paper is tested for such properties as feel,
rattle, snap, and look-through. These tests have been widely used in the
past, and even though physical measurements have replaced them to a large
extent, considerable paper is still purchased on the basis of these tests.
Handle, according to Strachan,' is a sense impression of bulk, regularity of
flatness and finish, and resistance to bending under the paper’s own weight.
An exjierienced papermaker can obtain considerable information about the
paper from handling, but this information cannot be expressed by numerical
values. Hence, the paper industry and paper-consuming industries are
rapidly adopting technical standards. In most cases, however, standard
samples are furnished with each order to which the delivery must conform

in finish, color, and formation.

Certain tests made on paper are primarily use tests which are based on
the element of utility. These tests simulate actual conditions to which the
paper may be subjected in use and are generally very complicated in nature.
They include such tests as the proof press testing of printing papers, coating

* J. Strachan, Papermaker 113, No. 2; 9-11 (Feb., 1947)

797

798

PULP AND PAPER

of rawstock and testing of the coated paper, and laminating of papers on
small laboratory laminating machines. Another example is the measure¬
ment of water vapor resistance on boxboard after fabrication in the form
of the finished package so that it is possible to obtain over-all information
on the creasing, folding, handling, and sealing properties of tlie paper. The
fabrication and testing of corrugated paperboard is another example. This
involves engineering problems which are at least as important as the phjsi*
cal properties of the paperboard; for example, the scoring and creasing of
the paper has a tremendous influence on the over-all streng^th of the box.
Although there is a correlation between the strength of the original paper-
board and the strength of the finished corrugated box, corrugated paper-
board manufacturers place most of their reliance on tests made on the fin¬
ished box, often after it is filled. Compression resistance, deflection at
maximum compression, resistance to wear in the revolving drum tester, and
other similar tests are among those commonly made on the finished box.
The advantage of “use” tests is that they furnish information on the utility
of the paper for a particular purpose. Their disadvantage lies in the in¬
ability to express results in generally understood terms and in their lack of

relationship with other tests.

Types of Properties Measured

Most of the commonly used tests for measuring paper properties are
arbitrary tests. Arbitrary tests are those which are dependent upon ar¬
bitrary instrument design and carefully defined procedure as to size of
sample and other variables. Such tests generally measure a complex prop¬
erty of the paper which is a combination of several fundamental properties.
Because of their complex nature, results based on arbitrary tests are diffi¬
cult to relate to the papermaking operations unless the person making the

test has had considerable experience.

Fundamental tests, on the other hand, give basic information about the

structure of the paper which is capable of better correlation with the paper¬
making operations. Fundamental properties are independent of dimensions
procedures, or instrument design, and hence can be mathematically related
to other fundamental properties. There is an increasing interest in t e use
of fundamental tests for analyzing paper properties, but so far, very ew
such tests are in use. The advantage of these tests is that once the fun a-
mental characteristics are thoroughly understood, it should be possible to
predict the utility of the paper for any purpose.

Effect of Papermaking Variables on Properties of Paper
Most paper properties are dependent upon the fact that the basic con¬
stituent is cellulose. These are called functional properties." Examples of
2 R. H. Mosher and R. J. Bracewell, Paper Trade J. 122, No. 16: 172-174 (Apr.

18, 1946)

XVI. PROPERTIES OF PAPER

799

. << fpafiilv AVltll AVSttClTj

functional properties are the abili^ °'Xwhen°tvet Functional properties
and the tendency of paper to lose strength "h'" of

cannot be changed without chenucal treatnrent of the P

- of ptiti

degree of of Iber treatment, forma-

,ion on the °V ^ the degree of bonding. These

variables must be controlled to obtain a '"“o, Sample,

ties, which often means good writing

surface, good printing "'/"td'teXr L ‘"eV

sired opacity. The amount and type of ^'^'"8 3 hown in pre-

lerial applied to the paper affects the properties, as has been shown p

vioiis chapters.

Sampling and Conditioning Paper

Before anv tests can be carried out, it is necessary to obtain samples

which are representative of the material to be tested. P^per is

tn this rule Ld the old adage that a test is no better than the sample is as

true for paper as it is for other materials. The methods

Td ^ p’Toduc.s have been fully described inTAPPI Standards, and w.l

not L reprated here.’ The theory of sampling is discussed in Chap

^'^^Lce representative samples have been obtained, they should be pro¬
tected from anvthing which will change their properties whii* mrans *at
they should be' protected from excess heat and light and not be allowed to
come into contact with liquids. Paper should be handled as little as possible,
rarticularly when optical tests are to be made, since fingerprints affect many
of the properties. No tests should ever be made on areas containing water-

marks^rrases or other visible imperfections in the paper.

Light has relatively little effect on paper, unless the paper is aged tor a
long period of time. Moisture content, on the other hand, has a very pro¬
nounced effect on the properties of paper, particularly the physical and elec¬
trical properties, and is one of the most important factors affecting the per¬
formance of paper in use, as well as during the period of test. Paper testing
should be carried out in a room maintained at constant temperature (usu¬
ally 73 rt 3.5° F.) and constant relative humidity (usually 50 ±2^).^

• Tht Manufacturt^ of Pulp and Paper. Vol. V, McGraw-Hill Book Co., New
York, N. Y. (1929)

* In England and France, 65% relative humidity and 65 to 70 F, are used.

800

PULP AND PAPER

The time of conditioning should be sufficient to bring the paper to a uniform
moisture content, which usually requires from four to twelve hours, although
certain treated papers and heavy paperboard may require twenty-four to
forty-eight hours. Paper should not be subjected to very high humidity be¬
fore conditioning, since this alters the sheet structure irreversibly, affecting
all the physical properties and certain of the optical properties, such as gloss.*
For very accurate work, where the hysteresis in equilibrium moisture is im¬
portant, paper should be preseasoned for at least two hours at a low hu¬
midity (e.g., 35% relative humidity) before the regular conditioning period.
After the test specimens are properly conditioned, they should be handled

as little as possible and not breathed upon.

Temperature has relatively little effect on the physical, chemical, and
optical properties of paper within the range normally encountered. How¬
ever, temperature does affect some of the physical properties, e.g., differ¬
ences in temperature of 1° C, cause a change in folding endurance of about
3% in the case of bond and ledger papers and temperature influences the
moisture content of paper, even at constant relative humidity, so that it is
best to control the temperature during the testing of paper. Paper moved
from a cold atmosphere into a warm atmosphere (as, for instance, when
paper is moved from a cold warehouse into a warm converting plant) is
subjected to a higher humidity than that which prevails in the rest of the
room because of the cooling effect of the paper on the air in the immediate
surroundings. This condition exists as long as a temperature difference
exists between the paper and the atmosphere. The effective relative hu¬
midity under these conditions can be determined from special charts.® A
rough approximation suitable for cases where only a slight temperature
difference exists can be made on the basis that a 1° F. temperature differ¬
ence changes the effective relative humidity by 2%.

Wive and Felt Sides

Paper has two sides, the wire and the felt sides. The wire side is al¬
most always the rougher because of the diamond-shaped pattern caused by
wire marks. Paper is generally open or porous on the wire side, and closed
or non-porous on the felt side. This difference in smoothness and porosity

on the two sides of paper is referred to as two-sidedness.

The difference in texture between the wire and felt sides is generally
visible to the naked eye if the paper is folded over and the two surfaces com¬
pared directly. If not readily visible, the paper can be submerged in water
or dilute sodium hydroxide solution for a few seconds, blotted, and then ex
amined as above. This loosens the fiber structure and tends to overcome the

B Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J. 104, No.

IS: (Apr. 15, 1937)
ejAPPI Data Sheets

XVI.

801

PROPERTIES OF PAPER

smoothing effect of calendering. Another test is to dry a strip of pap ^ ’
an oven and note the direction of curl, which is usually tow^ard ^ ^ -
\n experienced person can distinguish between the felt and wire sid y
tearing first with the wire side uppermost, and then with the fe
nermost. A more feathered edge is obtained when the wire side is upper¬
most. Examining a strip of paper taken from the fold tester just ^ ^ ^

strip breaks will usually differentiate between the two sides of the paper
since the sheet will generally appear more worn on the felt si e lan o

Cmain physical and optical tests are customarily made on both the
wire and felt sides, or if only one side is tested, the side used is speci e ,
Wire and felt sides should always be taken into account in the use of paper.
In the case of forms to be printed on one side only, best results are obtained
hv printing on the felt side. When laminating high-quality papers,
bristols, it is customary to put the wire sides together m order to keep t e

smoother felt sides on the outside.

Many papers are watermarked by means of a dandy roll which presses
upon the sheet in the wet condition, thereby leaving the sheet thinner m the
outline of the design. In some cases, the dandy roll is covered with fine
wire similar to the machine wire which produces an impression sirnilar to
that on the wire side of the sheet. This paper is referred to as a wove^
jiaper because it has the same surface on both sides, just as it would have if

the sheet had been woven.

Machine and Cross Direction

Paper has a definite grain caused by the greater orientation of fibers in
the direction of travel of the paper machine and the greater tension exerted
on the paper in this direction. This is known as the machine direction. The
cross direction is the direction of the paper at right angles to the machine

direction.

The grain of paper must be taken into account in measuring all physical
properties. In measuring tensile strength, folding endurance, and tearing
resistance, strips of paper are cut in both directions in order to measure the
strength in both the machine and cross directions. The tensile and folding
strengths are greater in the machine direction than in the cross direction,
whereas the tearing resistance is greater across the machine direction. The
grain of the paper must be taken into account in measuring optical proper¬
ties of the paper, such as brightness and gloss.

Papers vary in their ratio of machine to cross direction strengths.
Cylinder-machine papers have a higher ratio of machine-to-cross-direction
strength than Fourdrinier papers. However, even Fourdrinier papers gen¬
erally have from 1.5 to 2.0 times as high tensile strength in the machine di-

802

PULP AND PAPER

rection as in the cross direction. Orientation in the machine direction of
Fourdrinier papers is very marked on the bottom (wire) side of the sheet,
and distinctly less on the top (felt) side. Steenberg and Danielsen^ found
(by adding a small percentage of colored fibers to the stock) that the num¬
ber of fibers lined up in the machine direction on the wire side is about 10
times greater than tfie number lined up in the cross direction. On the felt
side, however, the number of fibers in different directions is roughly the
same, except in the close vicinity of the machine direction, where there are
twice as many fibers as in the other directions. This is shown in Figure
XVI-1, where the results of fiber counting are given in polar coordinates.

Fig. XVI-1. Fiber distribution in polar coordinates
H for machine and cross directions (160 g./m.®).

The length of an arrow from the center of the figure to the curve gives a
measure of the number of fibers lying in that position.

Machine conditions during paper manufacture have a very definite ef¬
fect on the ratio of machine to cross direction strength. The important fac¬
tors have already been discussed in the chapter on sheet formation (Ch.
VIII), and in general, they include the amount of “shake” on the machine,
the velocity of the stock in relation to the wire speed, and to a great extent

the amount of tension applied to the wet sheet.

Ordinarily there is less variation in paper properties in the machine
direction than in the cross direction, because variations occur slowly in the
machine direction, whereas In the cross direction they may occur quite sud¬
denly, on account of uneven flow of stock on the wire, uneven wet pressing,
or uneven tension during drying. Moreover, there is a normal variation

^ R. Danielsen and B. Steenberg, Svensk Papperstidn. 50, No. 13: 301-305 (July
15, 1947)

XVI. PROPERTIES OF PAPER

803

in cross direction strength of the paper, depending upon how far the sample
was taken from the edge of the sheet.

There are several means of determining machine and cross directions
of paper. Some of the methods are as follows r

(1) By visual observation, since with most papers it is possible to see that more

fibers are lined up in the machine direction. The longer diagonal of the wire mark is

always lined up in the machine direction.

■ «

(2) By wetting one surface of a small square of the specimen and noting the axis
of curl, which is always parallel to the machine direction. The curl is due to the fact
that the water causes the under side of the sheet to expand, and since the sheet tends to
expand more in the cross direction than the machine direction, this forces the sheet to
form a cylinder whose axis is parallel to the machine direction.

strips (6 XT/, in.) both directions of the sheet and
no ing e stiffness in the two directions. The machine direction stiffness of thin pa-

f direction stiffness, and this can be detected in a
ness tester or by holding the two strips in one hand and noting which one bends

A X Avy A.

bursting strength is meas-

ured. This test is based upon the fact that paper has less stretch in the machine direc¬
tion than in the cross direction. Thus, when pressure is exerted by t“e r^ber &
Phragm of the bursting strength tester, the sheet expands readily L the c^s di^r

n, but since it cannot expand appreciably in the machine direction it bursts snddenlv
and with the longest rupture in a line at right angles to.the machine i re^ S "

Strinl i 1 " fo'Otas '"durance in both directions

Strips cut parallel to the machine direction will usually show a greater te“lle strensS

and folding endurance than strips cut in the cross direction.

tween machine and era'sTrKti’oVb7toring”tre''“hM^ boTh'd' 't' ®®*”*"‘*

cbine direction, ^ th?r\:;:L:™ rd^ejli"''

Strength as ^errlyequauf^^^^ direction

(eg maximum tensile strength i^n^th^machlnTdirr^^^^^^

out in Chapter XIX it is desirahl^ trh paper. As pointed

direction in the long^ direction and to LTrfheX'tht* ’"'f T

machine direction parallel to the axis of the press roll Dkror '' ^ ^
are also cut with the machine direction in the long dtrect^m'’"'’"''

folds better and the book stays open better if fin 1 Book paper

machine direction runs up and down the page Intaf "7 k7

Jt IS desirable to cut the sheet with fh^ ^i,* j- ^^^mg tabulating cards,

to take advantage of the greater stiffnesTln Iws rertio"n“'’l

lid p~H

.hia is not always practical. Wrap-around “bt; for

804

PULP AND PAPER

erally cut with the machine direction vertical to obtain the greatest flexibil¬
ity about the can.

Physical Properties

Paper is a self-supporting, structural material. It is not an end in it¬
self, since nearly all paper is used as a base for coatings or printing inks, or
used as a wrapping or packaging material, where a strong, well-constructed
material is required. Paper resembles other structural materials, such as
glass, wood, and metals, in that it possesses tensile strength, elasticity, and
strength modulus in bending. However, by convention, tensile strength,
elasticity, and strength modulus are not measured^directly in paper testing,
but instead empirical tests are used. These tests measure the durability of
the paper and its resistance to applied force. The manner of applying the
force varies wdth each paper-testing instrument.

Basis Weight

Weight is the most common specification made on paper, because most
paper is sold on a w^eight basis. Weight of paper is expressed per unit of
area, rather than per unit of volume, as is the case with most other materials,
because paper is used in sheet form and area is more important than volume.
The consumer \vho buys paper by the roll or ream is concerned about the
weight of the paper which he has purchased, because paper which is heavier
than specified will give fewer sheets than paper of standard weight, whereas
paper which is lighter than specified will give a greater number of sheets,
but these will be lacking in thickness, strength, and opacity. The manufac¬
turer may be inclined to make a somewhat heavier paper than specified, but
the user is interested in buying the lightest paper which is suitable for the
purpose. Paper which is only slightly overweight can mean a substantial
economic loss to the paper consumer. It is customary to bill at the ordered
weight unless the paper is excessively underweight, in which case the paper
is billed at the scale weight. Tolerances are generally allowed in basis

weight and quantity ordered.

The common unit for expressing the weight of paper m the Englis
system is the number of pounds of paper per ream, a ream representing
either 480, 500, or, in the case of United States Government Printing Office,
1 000 sheets. In the metric system, the weight is expressed as the number
of grams per square meter of paper (1,550 sq. in.). It is obvious how much
confusion would arise if the weights of different papers were expressed in
all the various trade sizes used. To prevent such confusion, trade custom
has established certain basic sizes which are used as a standard for express¬
ing weights in the different grades of paper. The basic ream sizes for the

various paper grades are given in Table I.

XVI. PROPERTIES OF PAPER

805

TABLE I
Basic Ream Sizes

Grade of paper

Book and offset papers ..

Blotting ......

Bristols .

Box cover ...

Cover .

Afanuscript cover .

Cover .......

Index bristol .

Tag and mill bristol ...

News .

Bond,^ writing, ledger, and manifold .,.

Hanging, waxing, wrapping, and tissue
Tissue (optional) .

^ • m

Ream size, in.

Number of sheets
in ream

25x38

500

19x24

500

22^ X 28j^

500

20x24

500

20x26

500

18x31

500

20x26

1000

25j4x30j4

500

22^x28^

500

24x36

500

17x22

500

24x36

480

20x30

480

be traced to

the sizes es-

nrintincr J . I'^P^rmaking as the most satisfactory for

size of bonds 07
business stationery (8^ ^

sariiy^d^d^^^^^^^^^ ^ -ces-

cor-dtrMer:n"sr

"S" S5?“

17x^2 (SzTl“^^

30K . 41 , 3lxV

paper have their own customary trade sizes Tnnc'ri ui ■ §^^des of
manifested in the adoption of a sincrl h • interest has been

^eets, in order to simplify the computot W Sts'on fdd"

This standard size is widely used in technical work h ^

extent in commercial work. * used to any

Basis weight is the term used in the indu<!trv f
pounds of a ream containing either 480, 500, or

Converting Ream Weight from One Basis Weight to the Other

0 •

es.

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X X

o 40

CO eg

XVI. PROPERTIES OF PAPER

807

size, which is usually the basic ream size. The basis weight of container
boards is generally calculated as the weight per 1,000 sq.ft., whereas the ba¬
sis weight of boxboards is generally figured at the basic size of 25 x 40—
1000. The basis weight of commercial papers varies, depending upon the
grade of paper, use to which the paper is to be put, and the demands of the
user. Standard weights established by the Government Printing Office for
different grades of paper are as follows:

Bond: 13, 16, 20, and 24 Ib.

Ledger: 24, 28, 32, 36, 40, and 44 lb.

Machine-finish book: 30, 35, 40, SO, 60, and 70 lb.
Supercalendered book: 40, 50, 60, and 70 lb.

Coated book: 50, 60, 70, and 80 lb.

Printing Office weights are usually expressed on the basis of
a 1,000-sheet ream, but the above is expressed on a 500-sheet ream to con¬
form to general usage. The weight of writing papers is sometimes ex-
presse as substance number, that is, the weight of a ream 17x22—500
asic size or writing papers). Table II gives conversion factors for con¬
verting ream weight from one basic size to the other.^ To convert from
ream weig t in the left-hand column to basis shown in the other column
mu ip y the ream weight by the factor given in the other column,
p weight IS measured on a specially graduated scale which may be

type (Thwmg), or quadrant of the pendulum type (Scup¬
per, Cady), or the lever type (Toledo). The scale gives a dirertradine fn

rase"of In^the

case of the Thwmg, a small weight is added or withdrawn from the Scale
depending upon whether a 500- or a 480-sheet reading is desired. Because

or SOoTr * ^ ^ is multiplied by either 480

0 500 times, and consequently the scale should be set up where it Ts not

siiaSiP

multiplied by 1 102 (500/454^)°*^
used in tl case ZTt im r:^r "t^s

the two baskets oTthe'^i; so fta l"! s V -

per ream of material applied in treating thTpaper'""^irt^r'’*

type (Toledo) scale, there are two or three^s^les which 7 '''''''

cally the weight of 480, 500, or 1,000 sheets If thp ^ \ automati-

• TAPPI Data She«s »‘tu weight per 1,000 sq.ft.

808

PULP AND PAPER

of Stock is desired, a sample cut exactly 1 ft, square can be used and the
weight read directly from the 1,000-sheet scale.

In weighing paper at the mill, it is customary to use samples which
have been cut accurately to the proper ream size so that the basis weight
may be read directly from the scale. For routine testing, templates are used
for cutting the samples which are usually one-half or one-quarter the ream
size so that the sample sheet taken from the reel must be folded twice or
four times. If a sheet of odd size is used, the following relationship can be

used:

weight in ream
size desired

weight of
sample

area of ream size desired
area of sample weighed

This same formula can be used for converting from one ream size to an¬
other when the additional factor below is used:

number of sheets in ream desired

number of sheets in known ream

When only sm.all samples are available, it is better to weigh on an analytical
balance and convert the reading by means of the following relationship;

number of sheets in

. t., . • u* ream size desired ream size desired

weight in ream _ weight ot sample n ,io2) x—-— x-

size desired in grams length x width of

500

sample in inches

This formula is based upon the conversion of grams per single sheet to
pounds for 500 sheets, using the factor 1,102 (500x0.0022046 Ib./g.).

For best results, paper should be weighed only after it has been con¬
ditioned under standard conditions of relative humidity (50%) and tem¬
perature (73° F.), since paper gains or loses moisture with changes in the
humidity and temperature. Extreme variations in humidity such as occur
between seasons can result in a substantial change in weight. If a constant

humidity room is not available for testing paper, it is possible to make ap¬
proximate conversions for the effect of humidity on weight by using Table
III which shows the percentage of the weight on the scale which should be
subtracted or added to convert to the proper weight at 50% relative hu¬
midity.®

An interesting new device for measuring the basis weight of paper con
tinuously and automatically is the beta-ray basis weight gage.’^®'^^ This de¬
vice consists of a fixed arrangement of a suitable radioisotope and a detector
between which the paper passes as it leaves the last calender stack. The pa¬
per absorbs beta rays in direct proportion to its mass so that the response

9 “Facts about Paper Testing,” No. 4907, Thwing-Albert Instrument Co., Phila¬
delphia, Pennsylvania (1949)

E. A, Crawford and M. Strain, Tappi 33, No. 4: 190—192 (Apr., 1950)

11 A. P. Schreiber, Tappi 33. No. 6: 28A, 30A, 34A (June, 1950)

XVI. PROPERTIES OF PAPER

809

TABLE III

Corrections of Basis Weight for Humidity
Add or subtract the following percentage from the weight shown on the scale

_ A • ^ ^

Relative
humidity. %

Alpha cellulose
and rag papers

+ 1.5
+ 1.0
+ 0.5
0.0
-0.5
- 1.0
-1.7
-2.5
-3.5

Average
papers

+ 2.5
+ 2.0
+ 1.0
0.0
-0.7
-1.5
-2.5
-4.0
-5.0

Groundwood

papers

+ 3.3
+ 2.7
+1.3
0.0
- 1.2
- 2.2
-3.7
-5.2
-7.2

blsis through the paper is a function of the

tiral properties of paper and many of the op-

h ^ properties. It is customary to report physical properties

•r-i' '• —•

The ratio of strength to weight varies with the weip-ht nf tl, . • ^

eral, the highest strength in relation to weight is obtained on

than 35 lb oer ream 12 u 1 oDtained on papers lighter

1 s-5- K;

Thickness

^ hickness of paper is measured in a micrometer as fliri Ai f i.
two circular plane surfaces rahn.iF 1 / • • ^ distance between

8 to 9 p.s.i. VresuSre^ “nder a pressure of

an inch. In the case of paperboLd the t ” tea-thousandths of

in. of thickness; for ex^^e palb^arS

to as 30-point board The thicknesc a- thickness is referred

p..,,

of paper. It is Sy'^mpor^Hn'’?he“i°S^ P''“Porty

«TnSeh' u/vZa^^sS fate

thickness in order to function properlv in taTfr “ ““lately controlled

P.e. -U.1,

r tiu. ^o. zsy -242 (June 29, 1944)

810

PULP AND PAPER

justed for exact feeding and sorting of the cards. Thickness is important
in the case of book papers, since it affects the opacity of the paper and the
amount of impression received on the printing press. Thickness is a factor
in the performance of condenser papers, blotting papers, saturating papers,
wrapping papers, and many other grades. The thickness gage tends to
measure only the thickest part of the paper because of the relatively large
diameter of the micrometer (about K >"•)• Fot s°me uses, variations m
thickness not measurable with ordinary micrometers are of considerable

significance.

Density

The true d^Sy of paper is probably the most important fundamental
paper property. Density is related to the porosity, rigidity, hardness, and
strength of the paper, and, in fact, density influences every optical and physi¬
cal property except sheet weight. Because of its fundamental nature, sheet
density has been suggested as the most satisfactory basis for comparing e

strength and other properties of different papers.'*

The specific gravity of paper cannot be determined by the customary

method of water displacement, and hence it is more appropriate to speak o
the density of paper. Density is commonly calculated by d vidmg the bas
tigte of the paper by the caliper, but since the value obtained m this way

LLity. According to the A.S.T.M, method, density is determined as
follows:

density =

weight in grams per square meter x 0.0001001

thickness in centimeters
Specific volume is often used in place of density; it is calculated as follows.

thickness in micr ons (0.001 mm.)
specific volume = weight in grams per square meter

thickness in mils ^ jg Qg

= basis weight in pounds

(25 X 40—500)

Specific volume is the volume in cubic centimeters occupied hy ^
per under an applied pressure of about 9 lb. per square inch. e y

reciprocal of specific volume. , . , • Kfo;«*afl as a

There are two types of density, density (bulk) which is o _

ratio of the weight to the thickness of the finished paper, an ensi y

is obtained as the weight of the paper to the thickness P P

finished to remove surface irregularities. The difference bettyee

densities is determined by the difference m smoothness be w

calendered and calendered paper. , , , t amount

The density of the final paper is controlled by the type of fiber, am

13 J. d’A. Clark, Paper Trade J. 116, No. 1: 1-8 (Jan. 7, 1943)

XVI. PROPERTIES OF PAPER

.811

of beating, degree of water removal on the wire, amount of wet pressing,
and the amount of calendering. Data by Clark- shows that the dry appar-
ent density of groundwood test sheets is accurately linearly proportion^ to
e oganthm <rf the amount of beating over most of the beating range.

2idX^ ‘*“*’*^ fineness

ff , ® *fin hnminellulose content of the pulp.- The

effect of different pulps on the sheet density is almost a speciL property,
u a e pulps produce sheets of greater density than sulfite pulps. Soda

property*'''* low *nsity and are used tor their bulking

tfill "■ non-fibrous materials affect the density of paper

(fillers being particularly important). The presence of these materbls in

the sheet changes the density from the true density of the fibrous portion

sit thickness there can be but oL den-

ty. Doughty has made three variable plots showing the weight thickness
and solid fraction for several commercial grades of paper, for sor^fom

■" a-k- gives values of

P e tor several different papers, as shown in Table V. The

TABLE IV

Approximate Eakoes op Weight, Thickness, and Solid Fraction

for Several Commercial Papers

Type of paper

Glassine ....

Basis weight,

Ib., 24 X 36—500

Thickness,

0.001 in.

Solid

fraction

Bond, writing, S.C. book .

Cement bag

• io— 4o

30- 70

7c; ion

1.0^ 2.8

2.5- 6.0
9.0-25.0

0.62-0.75

0.45-0.65

0.20-0.45

TABLE V

Specific Volume

OF Several Grades

OF Paper

Type of paper

Bulky groundwood paper
Unbeaten sulfite paper

Glassine .

Cellulose

Specific volume

3.0

1.8

1.0

0.6S

volum^^or different papers varies from about 60 to 70% by

papers L for high-Zisitv

papers. Baird and Irubesky” list the oer cent ^iV h. i ?

grades of paper, as shown in Table VI ^

R 9: 121-124 (Feb 29 1940^

T d'A^o 7'Fffrfp /. 95, No. 10: 111-118 (Lot ’r

J- m No. 9: 121-124 (Feb 940?

, ■ K. Baird and C. E. Irubesky, Tech. Assoc. Pojs if. 2W(May, 1930)

812

PULP AND PAPER

TABLE VI

Per Cent Air by Volume in Several Grades of Paper

Type of paper Air, %

Groundwood ...

News .

Greaseproof .

Bond . ^ 4.2

Glassine .

The increase in tensile strength obtained as a result of increased beating
can be attributed to an increase in the density of the paper brought about by
an increase in the area of fiber contact. Clark^« found that burst and tensile
strength are linearly proportional to density, whereas tear (after a certain
maximum) is inversely proportional to density. Porosity is inversely pro¬
portional to density, as shown in Figure XVI-2, where the effect of solid

csl

-4

- 10 '®

-8

0.2 0.4 0.6

SOLID FRACTION

0.8

Fig. XVI-2. Relationship between porosity (rate of air trans¬
mission) - and solid fraction for various fractions of spruce sulfite
pulp-: (a) long fraction; (b) whole pulp; (c) short fraction.

fraction on the porosity of spruce sulfite pulp is shown for three different
fractions of the pulp;^* Density greatly affects the optical properties, par

ticularly the opacity (see Figure XVI-29).

As strong evidence of the fundamental nature of density. Doughty
was able to show that papers of the same solid fraction have the same tensi e
strength when the same pulp is used, and that papers of constant weig
show a mathematically regular increase in tensile strength with increas
solid fraction.^^ Increaseii beating (after a slight amount) has no e ec o
18 T d’A Clark, Pulp Paper Mag. Canada 44, No. 1: 92-102, Convention Issue
(1943)

1 ® R. H. Doughty, Paper Trade J. 95, No. 10: 111-118 (Sept. 8 , 1932)

20 R. H. Doughty, Paper Trade J. 94, No. ^ 1041 \

21 R, H. Doughty, Paper Trade J. 93, No. 15: 162-167, 172 (Oct. 8 , 19 )

XVI. PROPERTIES OF PAPER

813

tensile strength so long as the solid fraction is held constant. Figure XVI-3
shows the effect of solid fraction on tear, burst, fold, and tensile when un¬
beaten spruce sulfite pulp is made into sheets of different solid fraction by
variation in the amount of wet pressing. The results are similar to those
obtained when the solid fraction is varied by different degrees of beating.
The modulus of rupture of fiberboards prepared from semichemical pulps
as been shown^a to be directly related to the square of the specific gravity
of the fiberboards, as shown in Figure XVI-4.

sheet stren^rth h ” • density is an absolute indication o

f:'r- -

pulp (gum) were stronger a, the same

from either the fines or tbp Inn f.u jr ‘ fraction than papers mad<
wo.^u,s, WH.H.

There are cases when the density of the oaner ran * j •

proportionately increasing the bursting and tLsile stre^hT T c'
same pulp is used. Calendering relatively drv f the

to increase the (apparent) density witbn^f ^ ^ example, tends

burst and tensile strength Cobh^ ^ ^ proportionately increasing the

of starch to the fStii, ^n Lme adlition

» H. D. Tomer, J. P. HoM and S r . T ’

,, ^Aug. 26, 1948) ‘ ^‘^hwartz, Paper Trade J. 127, No 9- 4J_5o

-lu. ses: CToJTt 1^32)

105-108 (Aug. 12, 1937) Paper Trade J, 105, No. 7;

814

PULP AND PAPER

proportionately thUcisr'^

“St r;;JSJ...... ..e p.po. ....... ,

j;ivc.. tl.ick..ess ni.d l>as.s 'y'^''*- valuable inlor...ation about the

A deusity ..ieas..re...e..t on p. 1 » _ s,„ngth properties. For ex¬
paper when taken ... ^ density, the chances are that

kis'L'L'ftrtng-Sl. tough stock, whereas paper which has a high

400.0

200 . 0 -

100.0
i 80.0

- 40.0

Fig. XVI-l.

0.2 0.4 0.6 1.0

SPECIFIC GRAVITY

Effect of specific gravity on modulus of rupture of fiberbo

burst and a very high density is probably made fret,

hydrated stock,- In the latter case, the sheet w.ll have lower s ^

folding strength compared with the paper made from the Ion, fib

Stress-Strain Relationships of Paper

Recently there has been considerable interest in s to't

paper strength based upon stress-stra.n relat.onships. These s
provided an understanding of the nature of paper streng
in the interpretation of other more common tests. ^

Paper strength is a vague term which means nothing un

25 J. d’A. Gark, Paper Trade J. 104, No. 8; 118-120 (Feb. 25, 1937)

XVI. PROPERTIES OF PAPER

815

TABLE VII

Relation- or Tensile Stee.ngth (Pounds per Squ.are Inch) to Solid
FR.^ cTIoN FOR Black Gum and Spruce Pulps

SoImI (ractioa

02U

OJO

0.40

0.50

black g^um sulfite

■ » —■ I 1 ^ ▼=

pounds per square inch

500

1150

2250

3900

spruce sulfite

pounds per square inch

800

2000

3500

5500

whicli the paper is to lie put and iiariicular strenj,nh property desired are
carefully specified. Paper is not necessarily weaker in the cross direction
than in the machine direction merely because the tensile strength is lower
in the cross direction. In some w-ays, paper is stronger in the cross direc¬
tion than it is in the machine direction, as evidenced by the fact that the
japer bre^s first in the machine direction when tested in the Mullen tester.
It is iUogical to speak of paper strength without careful definition, just as it
IS illogical to compare glass and rubber for strength without denoting the
strength characteristics to which reference is made, since glass has high
static strengtli and poor impact strength, w hereas rubber has very little static
strength but go^ impact properties. Certain papers require high static

Ntrength (c.g., index bristols), whereas other papers require high im-
jiact strength (e.g., bag papers).

More discussing stress-strain relationships, it is desirable to define
what IS meant by stress, strain, ideal elastic solids, and ideal liquids. Stress
IS the internal force per unit of area (dynes per square centimeter or pounds

T f'' '*>""31 load. Strain is the ratio of the

i" •“ dimensions. Ideal elastic solids are

those which defom insuntaneously under applied load to a degree linearly

”ll tltuTm > ■ tshich the specimen will bear and

return to Its original shape is called the elastic limit.

in flulr'i^dl^?" ir ''Tk-“"^ idered

..rmwniJlT,! I .a ‘ both elastic

• "r ‘to the stres

in Sle s,r„® " W^h? qf"

stress-strain relationships of I»per by e“stnTunfer‘ho

;iri|.s of iiHi ,r,r The 1 . "eights onto

-Mic. .oadTa^r

816

PULP AND PAPER

This would indicate that the tensile strength is 10 kg., but when a load of
9 kg. was applied, the strip broke, only this time eleven minutes \\ere re¬
quired. Upon continued testing, it was found that the strip would break
under a load of S kg. in a period of fourteen hours, and under a load of 4 kg.
in a period of 220 days. It should be emphasized that both the elastic
properties and flow properties of paper are important in the use of the

breaking load.

TIME, minutes

Fig XVI-6. Strain recovery from a five-minute application
of stress (2.75 kg./cm.). A-B is instantabeons recovery and B-C

is creep recovery.

paper, particular emphasis being placed on one or the other, depending upon

the use to which the paper is to be put. a thp

If successively increasing weights are applied to a strip o paper an ^

stretching (straining) measured, a curve is obtained which starts out as

straight line (following Hooke’s law), but which later curves ‘“^ard th

Strain axis. In other words, the paper starts out as a purely elas c

XVI. PROPERTIES OF PAPER

817

and then, after a 1 to 2^ strain, begins to exhibit flow. If the load is held
constant, the paper will flow indefinitely until it breaks. This delayed flow,
which is known as primary creep, occurs when printing papers are hung un¬
der their own w’eight for long j^eriods in the print shop,^' in wallpapers
which stretch to conform to slowly sagging w’alls, and in other papers sub¬
jected to load over a period of time. Typical creep is showm in Figure

removed, the paper tends to recover its former
shape partially, but part of the strain is non-recoverable, resulting in some
^rmanent set.'* The non-recoverable part of the flow which occurs in the
first straining is sometimes referred to as secondary creep. Strain recovery
is cliaracterized by two separate parts, an instantaneous part and a time-de¬
pendent part, as shown in Figure XVI-6 for a paper made from 75%
groundwood and 25% kraft.^» A high rate of strain recovery is desirable

in some kinds of printing papers to insure reproducible positioning of the
paper under the various printing plates.^®

If the strain on the paper is held constant, the stress will relax with
time. Stress relaxation is the progressive reduction in stress which is re-
quir^ to maintain a constant elongation of the paper; it is somewhat
similar to thi.xotropy m liquids. A paper with a high rate of stress relax-
a ion wi issipate load more readily, and consequently will not break
as rea i y as a paper with a low rate of stress relaxation. Bag papers re-
quire a ig^ rate of relaxation in order to absorb the stresses to which the

a^?ir/ 'r* i."® of

strenirth burin^''^'T^ better than bag papers with a high tensile

1 , rate. Papers to be printed on a web press

tte press*'Vews? T' “h '''''*“•'0" ‘o ''“'st sudden stresses on

the press .\e« sprint made today has lower tensile strength than news-

WuerTo '•'si"* newsprint works better because of its

high rate 'of reSion ' P^P®’’® are widely used because of their

Moer hnf th • tensile strength is lower than that of uncreped

quentiy more ab^rairr^^^^^^ —

l»aper. In general th^ r.u without piling up stresses in the

f^^nning

As pointed out above rate of ;o • slower,

which is rapidlv strained tend? tn i f important, since paper

ing tnie eli.ic t “ ‘‘I (exLit-

■'J Strechan 'L ,7! ‘onds to hold

'•B 2: 9-10 (Aug 1947)

- s: S'f?- 0947 )

- ■■ CuWa 49,

ii r’ Papperstidn. 50. No IS-

'*B. Steenlxrrg. Pulp PaPer \fnr. r j '^^^50 (1947)

Issue (1949) • Mo. 3 : 207-214, 220. Convention

818

PULP AND PAPER

its fitial shape (exhibiting How). Crepecl paper tends to recover its crejic
when stretched rapidly an<l released, but tends to lose its crepe when
stretched slowly. When strait^ed at very high rates, paper breaks into sev¬
eral pieces similar to the shattering obtained when materials like pitch or
glass are strained rapidly. Rapid rate of straining increases the breaking
strength of the paper.

Stress-strain measurements cannot be made with ordinary paper-test¬
ing instruments. The conventional pendulum type tensile tester is not suit¬
able for measuring stress-strain relationship because of its low degree of
accuracy, its complexity (neither stress nor strain is applied at a constant
rate), and finally because the specimen is ruptured during the test. Steen-
berg^^ hasadescribbd suitable apparatuses for measuring stress-strain. Two

ELONGATION. %

Fig. XVT7. Stress-strain curves of paper made at different humidities.

types of apparatus are used, one for slow rate of loading and one for very
rapid rates of loading. In the slow instrument, the paper is strained between
clamps and the stress in the sample recorded in the form of a stress-strain-
time diagram by means of a beam scale which is kept in balance through a
photocell and reversible motor connection. In the rapid instrument, the
paper strip is strained by the action of a free-falling weight. Automatic
stress-strain diagrams can be obtained under constant rate of loading or un¬
der constant rate of extension (straining). Typical stress-strain cun-es for
relatively stiff, brittle paper (tested at 20^0 R-H.) and relatively plastic
paper (tested at 9S% R.H.) are shown in Figure XVI-7 based upon meas¬
urements made with the slow instrument.’^

Stress-strain measurements have the advantage of emphasizing the prc
rupture characteristics of paper, whereas most conventional tests measure
the strength at rupture. From a practical standpoint, prerupture behavior

XVI. PIOPEBTIES OF PAPER

819

paper is a more important considcradon than the strength at rupture,
since paper is ordinarily not used to the point of rupture. The area under
the stress-strain curve gives the energj' per unit of volume. WTien multi-
pli« by the cross-sccdonal area and length of the sample, the total energv'
which tiK ferial can absorb is obtained. At very’ high rates of straining
(impaa), Steenberg** points out that the inertia as well as the rheological
pro^rties influence the response. Under these circumstances, stresses and
«r^ns are prop^ted as shock waves so that stress is unevenly distributed
in the sample. This may result in breaking of tlie paper in more than one

^ weakest point. As the velocity is increased, a point

«|iial to the cohesive rorces and failure occurs immediately at the imoact

pmni. the stress at any adjacent point being zero. The critical velocity^or
newsprint appears to be about 23 m. ner •>

wAin, \

-'.■‘.SIrf. t -

on ascending and descending strest ”

tt»ed for stretching was reversed and ^ *"otor

curve which is slightly concave*^ \vu ^ gives a down-

o«J cycle, the upeurv-e b slightly coL! fn”* in a sec-

niptly when the maximum stress obtained in th" fi ^ ^hen decreases ab-

P»«ntly the first straining cycle results in u h ^ ** reached. Ap-

the paper removing the secondary creep and IcTd^ properties of

PO«nt at the maximuiii stress of the firef cycle TV^ ^ y'eld

-o. Awdmw. aod B. Swenher. c Phenomenon is known

IW) *• •^*'*'^* Po^Pfrstidn. S3. Hn ^ -r ri

820

PULP AND PAPER

as memory effect, since the curve tends to continue m the same path started
in the first cycle. Thus, from an examination of the stress-strain curve, it is
possible to determine the previous strain to which the paper had been sub-

iected since the stress-strain curve levels off at this point.

A stress relaxation curve can be obtained at the same time t at t e
stress-strain diagram is obtained by stopping the moving clamp and plotting
load versus time at constant length. As noted previously, there is a rapid
loss in stress at first, followed by the very much slower loss (see Fig
XVI-6). The loss in stress is generally small compared with the over-all

stress on the sample. a 4 . a

The stress-strain relationships exhibited by paper can be understood

by analogy with a mechanical model consisting of a spring and a dashpot

connected in series (Maxwell element) and a spring in paralle . as shown in

Figure XVI-9. When forces are applied to the model, both springs are

(a)

(b)

Fig XVI-9. Mechanical model to illus-

trate stress-strain relationships of paper. unstrained (a) and strained (b) p P •

stretched in the early stages, but the dashpot is not appreciably moved
This is the period of elastic flow when the stress-strain curve
line When greater force is applied, the dashpot is moved and the stres
strain ^rve fends to bend toward the strain. Steenberg's concept of a
Strained and unstrained paper is shown m Figure XVi-lU.

Moisture content greatly changes the stress-strength „

paper, affecting the elastic, as well as the plastic properties of
hmreased moisture content increases the flow properties or m othei w
elongates the stress-strain curve so that the load is less, but the per cent

•SB. Stecnbcrg, Puli’ Paper Map. Canada SO, No. 3 : 207-214, 220. Convcntio

Issue (1949) Tj Qfpnnlif*ro‘

S'* O. Andersson, B. Ivarsson, A. H. Nisson and B. Steenbe g,

Trade Rev. 133, No. 1: 2-8TS (Jan. 5, 1950)

XVI, PROPERTIES OF PAPER

821

elongation is much greater, as previously shown in Figure XVI-7, where

stress-strain curves are plotted for papers conditioned at three different

relative humidities. Paper bags are sometimes filled under conditions of

igh relative humidity, because the paper has a greater rate of relaxation

w en slightly moist. This increased relaxation permits the bags to be filled

with less breakage, even though the tensile strength of the paper is lower at
the high humidity.

j j * • I ^ measurements are made in only

two directions, the machine and the cross directions. It is desirable in the

STRAIM, %

XVI-11. Stress—strain diagrams
cut in different directions of the paper
tion = 90‘’).

for test strips
(machine direc-

stress and corresn^dt^., ''"^‘ion in

direction are s3n t ‘he machine

Similar information is also illustrated in Fi^me XVIT7 h’’^ .S^^berg.-

load versus extensibility for test strips cut in dinfrlt dtecF

drinier paper. It can be .seen frr.rm +{, ainrerent directions of a Four-

machine direction (90°) but is verv the flow is small in the

Steenberg and cl^s'-£vS t

B. Steenberg, Svensk Pahb. cn ^^tensive stress-strain studies
37 B PaPPerstiZ. 50 N^o ^15 • l 4 ^ 3 So m"’ 1947)

pr l|- (Sep.

Issue No. 3: 20;-214, 220, Convention

822

PULP AND PAPER

to detennine the effects of paper niacliine operation, history of the paper,
and other sirnilar variables on the stress-strain properties. Their work in¬
dicates that a measurable part of the strength variations between the ma¬
chine and cross directions results from the draw on the paper machine, A
marked difference was obtained in flow in the cross direction across the
width of the sheet, paper taken from the edges showing far more capacity
for irreversible flow than paper taken from the middle of the sheet. On the
other hand, there is little variation in the machine direction flow across the
width of the sheet due to the tension on the paper in this direction during
drying. Steenberg^® was able to show that paper dried experimentally un¬
der minimum tension is very tough, bu^ lacking in stiffness. Paper dried

Cross

Direction

Extensibility

Breoking load

Machine

Direction

Fig XVI-12. Breaking load versus extensibility in differ¬
ent directions of a Fourdrinier paper (arbitrary units).

inder increasing tension becomes stiffer and stiffer, until an almost per-
■ectly elastic body is obtained at maximum tension. The ultimate tensi e
strength was not increased to any .appreciable extent in these expenm^s
3 i drying under tension, although other investigators have reported th
Lw increases tensile strength^ (see Ch. VIII). Rance« has shown tha
the expansivity of paper on wetting is reduced by an

to the degree of permanent set obtained on prestrammg. Thus, the tension

exerted on the sheet during drying on the paper mfchine « a

ducing the expansivity of the paper in the machine direction^ |

affects flow properties, as does calendering, which ten s to rea

paper structure and almost entirely eliminate secondary “«P-

cosity of paper calculated from the stress-strain curve tends to mere

-« J. E. Sapp and W. F. Gillespie, PoPer Trad, I. 124, No. 9: 120-123 (Feb. 27,
« Ranee, PaPer-Maker 22-32, 49, SO, 51, Midsummer Issue (1949)

XVI. PROPERTIES OP paper

823

rapidly with beating in the early stages until a maximum is reached, and
then to decrease slightly for prolonged beating,

5' tiff ness

The similarity between paper and other structural materials has already
been mentioned. This similarity should make it possible to determine the
rigidity of paper by methods used with metals, wood, and other structural
materials. Stiffness is related to the flow properties because it depends
upon the ability of the layer on the outside curve of the material to stretch
and on the ability of the inside layer of the curve to undergo compression.

In the case of heavy paperboards which are stiff enough to act as a
earn, static bending has been used for determining stiffness, modulus of
rupture, and modulus of elasticity.^ In one method of testing paperboard
y static bending, the specimen is freely supported horizontally at both
en s on ro ers and a load applied uniformly at the center of the span. The
app ication of load sets up a stress and a proportional resistance to defornia-

constant rate at the center of the beam
ually by adding water at a constant rate to a pail attached to the strip of

by means of a vertical

scale or a deflection needle. Load in pounds or ounces (stress^ can

; / hieh strain increases at a greater rate than stress Brit-
eness can be determined by multiplying the load at the elastic limit by 100

V "KV : brittle body, t is L e

fetion per cent since a perfectly brittle body will break at the first de-

of proportional to Young's modulus, which is the ratio

of the unit stress to the unit strain below the elastic lin.h f 1
of inertia. PancriT.t>l..,-’o the moment

W Te^ 1/ r of (!) divided by the basis w jh

«ula:«.t' ^ be determined by the following for-

stiffness

where £ (Young's modulus) equals the stress/strain in the direction of

„ 5'“?^ and B. Steenberg, IVcrld's Paper

1^185 ToS ^^ 929 ) ““d h- A. Carpenter, Paper Trade /. «>, No 17 •

Idem,

Instrumentation Studies, Paper Trade J. 104 ,

824

PULP AND PAPER

axis of the test strip, T is thickness of sample, L is length of sample, and
IV is width of sample. Young’s modulus, sometimes called the modulus of
elasticity, is a fundamental property which is independent of dimensions.
It is expressed in dynes per square centimeter, or in pounds per square inch,

and is defined as follows:

tensile stress

Young’s modulus - ^gUe strain

_ force X original length
” area x change of length

force/area
change of length
original length

The higher Young’s modulus, the higher the stiffness, and since \oungs
modulus is numerically equal to stress/strain per unit area, increased stiff¬
ness is obtained by reducing the strain for a given load.

There are several instruments for testing the stiffness of paper, but
these fall short of a true stiffness measurement because they displace the
paper beyond its elastic instrument, and because they express the results in
arbitrary, values. Instruments for measuring papermaking stiffness are
usually based upon one of the following principles: (i) a measurement o
the force required to bend a strip of the paper through a given ang e, f
measiirement of the angle through which a strip of paper is bent when
subjected to a definite force; (3) a measurement of the angle through
which a strip of paper bends under its own weight when held m a horizon-

tal fixed position. ^ . n • r a. j o

In the Taber instrument, a specimen (lj4 x2^ in.) is fastened ^

clamp on a pendulum. Load is applied to the lower end of the specimen by

rollers attached to a power-driven disk so that the resulting torque deviates

the pendulum from the vertical. The end point is indirated by the a ign-

ment of the pendulum with the 15° mark on the loading disk. I” ^

out the test, the force should be applied slowly and the average of the deflec-

tion taken for both directions. . - u

The Gurley instrument measures the bending moment which the paper

can withstand in both directions by deflecting a small '’^?v,rt'the

The sample is cut to overlap the top of the pendulum by ^ m. so
sample must be bent sufficiently to shorten it by this distance. Jhe
the sample and the weight on the pendulum can be varied, depe g P
the stiffness. All combinations can be reduced to a
sample 1 in. in width and 3^ in- in length by use of suitable factors,
driven and hand-operated models are used. Stiffness ridings on the (mr^
ley and Smith-Taber instruments measure essentially the same P™P

Stiffness can be measured in the Clark tester as the critical length ot

«Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J- M.
No. 21: 287-290 (May 27, 1937)

XVI. PROPERTIES OF PAPER

825

2-in, strip of paper which just falls over when the framework of the instru¬
ment is rotated through Stiffness is then calculated as LyiOO,

where L is the average effective overhang in inches.

The Tinius Olsen stiffness tester is well designed to measure stiffness
of heavy paper. The specimen is clamped at one end and bent by applying
a load at the free end. There are two scales, a load scale and an angular de¬
flection scale. Actual pounds load is obtained from the load scale reading
times the bending moment used (inches-pounds on weight) divided by the
bending span in inches. Stress-strain curves can be plotted as load scale

rea mg versus angular deflection. Apparent modulus of elasticity can be
calculated from the data using the following formula:

p _ 45* X scale reading

1000

where E = apparent modulus of elasticity
5* = span in inches
w - width in inches
d = thickness in inches

M moment weight on pendulum

‘t> - angular deflection in radians in straight section of the curve.

fensi.y heavy boards can be measured in a regular

the center ou!lX 0 ^^ h plication of load at

ported in load ,t f u The results can be re-

resistance test known 37^ rtag wLlTett'” In ^ “‘"Pi'^sion

per (usually 0.5 x 2.0 in.) is inserted in a ' • of pa-"

ring. The ring of naner i. n, specimen holder in the form of a

maximum ap;ild“^loX mMs^renS T

used for LktagCiafed ?oa^

as thfcfbeTf rhe^Se«'“^!fh-M P^POri theoretically

beards, that the rigidity LcreaaS as L

IS kept constant, and as the square of th^ fK- i thickness when density

stant. He found rigidity to be nmn f ^hen weight is kept con-

at a ^ven thickness. According toTmTtb’ ^ u'’®‘ ‘’^^ity

machine direction to that in the cross dir ^ rigidity in the

s*™a-j, Tzjr"""'-■I-

" s. P. Smith, 'u'ltld’lpai‘J'T‘’^ hto- tl 1 169-172 (Mar. 28 1935 )

-PMi No. 7: 483. 486, 520, 522, \2S%K(.FA^io. Xi,

826

PULP AND PAPER

is from 3.2 to 1 to 4.3 to 1, and this ratio is maintained independently of the
speed of the machine, thickness of the board, or of the type of fiber.

Papermaking stiffness is related to brittleness, rattle, and other less de¬
finable paper qualities. Papers made from highly beaten stocks (bond and
glassine) have higher stiffness than papers made from lightly beaten stocks
(toweling and filter papers). Papers made from pulps high in hemicellulose
content (e.g., straw pulps) are stiffer than papers made from pulps low in
hemicellulose content (e.g., alpha pulps). Papers made from short-fibered
pulps (straw, chestnut, and ground wood) are generally stiffer than papers
made from longer fibered pulps. Papers made from chemical wood pu p
generally have a higher stiffness to tear ratio than do rag papers.^" Van den
Akker^^ conjectures that stiffness in thin papers is more a matter of indi¬
vidual fiber rigidity whereas stiffness in thick papers is more a matter of ex¬
tent of fiber bonding. The addition of starch or sodium silicate to the fur¬
nish increases stiffness. Increasing the moisture content of the paper

appreciably lowers the stiffness.®^ r i j

Stiffness is very important in boxboards, since the utility o a

pends upon its ability to resist bending when filled. The important fun a-
mental propertv in this case is the elastic rigidity, since a satisfactory box-
board is probably not bent beyond its elastic limit. Stiffness is of very great
importance in index bristols, card middles, typing papers, and playing car s,
where the paper must stand upright during use. A certain amount “ a ' '
ness is desirable in bond paper where it is a factor in the handle of the pa¬
per These papers are customarily made by heavy beating an y e
dition of starch or sodium silicate to the furnish. Stiffness is very '‘"P®''™
in corrugating medium for corrugated board, and hence pu ps are c o
which will give high stiffness (e.g., chestnut, straw, or , °

the other hand, stiffness is undesirable in such papers as tissues, ‘“"f ■"«-
printing papers, and labels. Plasticizers are added to glassine to lower the
stiffness. Paper label stock is sometimes pebbled to reduce t e s i n

Tensile Strength

Tensile strength is a measure of the resistance of paper to
sion. It is defined as the force required to break a strip of paper w
a specified length and a width of 15 mm. Most tensile ^ ^

lum lor applying load, which is free to rotate in a vertical P [

is clamped between two jaws (both of which move during e p
plying load), and then the pendulum is swung out from its vertical

60 C. A. Minor and J. E. Minor, Paper Ind. (Apr., 1935). (Obtained from re-

61 j"A. Van den Akker, Tappi 33, No. 8; 398^02 (Aug-, _ 3 1933)

62 F. W. Adams and J. Bellows, Paper 9: 117-120 (Mar. 2.

53 J. Strachan, Paper-Maker 113, No, 2: 9-11 (Feb., 1947)

XVI. PROPERTIES OF PAPER

827

position by a hand crank or by a motor. This applies an increasing load to
the paper until finally the strip breaks. The load at which the strip breaks
is termed the tensile strength and is reported in kilograms for 15 mm. of
w idth, or in pounds per inch of width. Conversion from one to the other can
be made on the basis that 1 kg./15 mm. is as 3.73 Ib./in. of width.

Tensile strength more nearly approaches a fundamental measurement
than other conventional strength measurements made on paper. Tensile
strength is a component of the more complex bursting, folding, and tearing
strengths which are discussed in the following sections. However, tensile

. . paper industry is not a true tensile strength,

since It measures the breaking load per unit of width rather than per unit

of area. It would be more appropriate to use the term “breaking strength”
m reference to this test, but since the term “tensile strength” has become
well established, it is customarily used throughout the industry.

®''^*'oary purposes, tensile strength as measured in the paper in¬
dustry IS indicative enough of the utility of the paper, since paper is ordi-

nari y used m sheet form, but for some purposes, such as when paper is
med as a structural material, “true” tensile strength would be a more useful
1 "sufe expressing a tensile strength-weight ratio can be ob-

mpet Anmh!'"® '7i T ^g^t of the

rr breaking length, a strength-to-weight

si m brl ; 1*’’' ^ =‘"P P^P- *0 caiile te

— can be de-

breaking length
(meters)

tensile strength
(kilograms)

length of strip (m eters)

p , weightof strip (kilograms)

'“Imawlte t‘'’‘ckness, and basis weight,

load is annP H ^ length determinations, the time during which the

break under lid" loTd previously shown, paper will

Conversely, the “ Itl^

tiiai: ir" p--"-HeMri!

driven tensile tesl arrif "rred over /I 7 °'- ™P‘”"

their more uniform rate of loading. ™°dels because of

standard 1^1 lml“b "tH'

depending upon the size of the available Ipll lh‘ ''c used,

3.rips, because of a greater chance than long

“ R. c. Griffin and R W McKinle P . P'l"'”’" There

1936) • • ‘ Paper Trade J. 102, No. 2: 20-21 (Jan. 9.

828

t’lU.P AND PAPEK

may be as much as lOCt? difTcrener in results be*w>eeti a strip lOO fiwv and
a strip 180 mm. in length t.tkcn irom the same paper. In testing handslKets,
it is customary to u.se a strip 100 mm. in length because of the small size d
the sample. Sjx*ciniens smaller tlian 50 mm., the shortest length which can
be used in the ordinary machine, may be tested by extending the strip with
strips of strong gummed kraft jwper.

Tensile strength is .always greater in the machine direction than in the
cross direction because of the greater alignment of fibers in the machine
direction. lncrca.<iing the weight of the pa[jcr, of course, increases the tensile
strength. Ibiwevcr, Doughty** has shown tlial the ultimate tensile strength
in pounds per square inch dccnascs with increasing basis weight (when the
solitl fraction remains the same). This can Ijc attributed principally to vari¬
ation in fiber orientation.

It is intcrc.stinii to calculate what tlic theoretical upf>cr limit of tensile
strength would l)c if the paper were held together by the same force as the
individual fillers. Tlie standard width of the paper strip u!«d in the tensile
test is 15 mm., or 15.000 microns, and assuming that the thickness of a strip

of i)a|>cr (bond) is 0.004 in. or 101.6 microns, the calculated area of the
strip would be 1,524.000 square microns. Assuming that the paper is com¬
posed of 65% fil)er (the rest being air), the area of the fibrous part of the
strip should he equal to 1,524,000x0.65, or 990.600 square microns.
Clark“* gives the area of a spruce fiber as 160 square microns, w'hkh means
that there should be room for 6,191 fibers in tlie cross-sectional area of the
test strip. Then, using Ruhlemann’s** ^*alue of 10 g. for the tensile strength
of individual spruce fibers, the tensile strength of the strip should be
10x6,191 = 61,910 g., approximately’ 62 kg., if the strip were composed of
continuous inter-fiber bonds. However, the actual tensile strength of bond
paper 0.004 in. in thickness is approximately 10 kg. which is only about
16% of the theoretical 62 kg., thereby attesting to the relative weakness of
the bonds between the fibers, compared to the much stronger bonds within
the indiridual fibers. In the same conn^on, Stamm’*’reports that paper
has a tensile strength of only 0.5 to 8.0 kg. per square millimeter compared

to a value of SO kg. for a well-oriented viscose film.

It is obvious from the above calculations that the amount and quality
of fiber bonding is the most important factor affecting tensile length. The
brittleness of the bond is of relatively little importance. Parsons** has
cVinT 4 *n ttiat tVipre i«; a linear relation between the tensile strength of paper

55 R. H. Donghtv. Paper Trade /. 93, Xo. 15; 162-167, 172 fOcL 8. 1931)

5« J. d’A. aark,*Pa/>er Trade /. 118, Xo. 1; 1-6 (Jan. 6. 1944) ,

57 E. Ruhlemann, Thesis, Tech. Hochschule, Dresden il92S) through J- dA.
Oark, Paper Trade J. 118, No. 1: 1-6 (Jan. 6, 1944) , ^ •

Stamm. “Colloid Chemistry of Cellulose Materials.” L'. S. Dept, erf Agr\-,

Misc. Pitbl. 240, (1936) _ ,

59 S. R. Parsons. Paper Trade J. 115, Xo. 25: 314-322 (Dec. 1/, 1942)

XVI. PROPERTIES OF PAPER

829

and the area of fibers in optical contact in the early stages of beating. The
relation is the same, regardless of whether the area of fiber contact is varied
by increased beating or by increased wet pressing, thus indicating the fun¬
damental importance of fiber bonding. Tensile strength tends to show a
slight drop if the pulp is excessively beaten (due to destruction of fiber
structure), but there is no decrease in tensile strength when the sheet is in¬
creased m density by wet pressing (see Figure XVI-3). Fiber length is
e leved to play a role in tensile strength of paper. Kane®® believes that
moderate variations in fiber length have little effect on tensile strength, but

Qark reports that tensile strength is proportional to the square root of
the EverEg'e fiber length by weight.

Tensile strength is important in newsprint and other papers printed

Ls i f bag and wrapping pa¬

pers High tensile strength is needed in asphalt-saturated papers to resist

tUT't “P P^'P- - '“d during drying

calile wraDDin?nrP°'^ Ti ‘ ®“'""’^d paper, spin paper, binder twine, and

^ wrapping paper. These papers are usually made from raw unbleached

machine to obtain as much machine direc

of five Sf M n “"P'" ^““gth in the ratio

hie units of M. D. tearing strength to one unit of C. D. tensile strength

One special type of tensile strength measurement is th. . “““gth.

The principle behind this test is that felnXIduaffiterfirttZ^^^

trL!c‘’stretXf‘fe '“h ‘heTn-

values for the aero-span tensile test on different pulpsf*^'”"^

Pulp

Unbleached kraft ..

Unbleached sulfite . .. 14,000 meters

Unbleached soda . .. 7,700 meters

y, . 4,600 meters

- »d is at best, only an

measuring the loss in individual fiber stren Jh •'* useful, however, for

high acidity, or other degrading effects. ^ '■'^‘‘■‘mg from overbleaching,

“ J. d’A. Dark, Paper Tr^, (Fd>. 29, 1940)

'• P. M. Hoffmkn uL^PaplJr a°' V «• »«) ’

j uDsen, ^aper Trade J. 81. No 22- oit />t

• 216-217 (Nov. 26, 1925)

830

PULP AND PAPER

The ratio between the normal tensile strength and the zero span tensile
IS, to some degree, an indication of the amount of interfiber bond
ing, since the zero-span test does not appreciably increase after a limited
amount of fiber bonding. It is not appreciably affected by the addition of
cementing agents,®^ and beyond a certain minimum amount, it is not greatly
affected by further beating, as shown in Table VIII taken from work done
by Clark®^ on the beating of west coast sulfite in a laboratory kollergang.
Hence, by comparing the zero-span tensile strength with the normal tensile
strength, it is possible to determine what part of the sheet strength is due to
fiber strength and what part is due to fiber bonding. The zero-span tensile
test is also useful for measuring the relative degree of orientation of fibers
in the machine and cross directions of the paper.

TABLE VIII

Efkect^^^ing in Kollergang on Regular Tensile Strength
a^nd Zero-Span Tensile Strength of Spruce Pulp

Unbeaten
Beaten, 4 r.p.g.
Beaten, 8 r.p.g.
Beaten," 32 r.p.g.
Beaten, 64 r.p.g.

Regular tensile

strength

Zero-Span tensile

strength

Tensile, kg.

Breaking

length,

meters

Tensile, kg.

Breaking

length,

meters

3.6

7.8

10.5

12.6

13.9

1750

3800

5120

6300

7050

19.2

24.2

26.4

26.5

27.4

9,450

11.900
12,700
13,200

13.900

6-10 r.p.g. represents treatment given to normal bond paper. Zero-span breaking
length is calculated as:

kg. breaking load x 200,000

basis weight/sq.in. x 3

On certain grades of paper it is desirable to measure the tensile strength
after the paper has been soaked in water for a standard length of tune. 1 h
measurement, which is known as the wet tensde strength, is discussed

Chapter XII.

Stretch

Stretch is the amount of distortion which paper undergoes under tensde
stress. It is usually measured on the tensile tester at the same tune that

iho tensile strength is measured. . ,

In measuring stretch, an initial, stress of about 0.5 Ib. ^lou ®

to the paper in order to remove any cockles or wavmess. T
by applying tension with the fingers. After the stnp is clamped, ^ ^
indicator is engaged with the lower clamp and load is tien appie _
..longation at rupture is read from the stretch indicator to the carest
0.5 mm. For strips exactly 100 mm. in length, the reading can be direc y

XVI. PROPERTIES OF PAPER

831

reported as per cent stretch. However, strips of 180 nim. in length are
preferred for greater accuracy, and in this case, the percentage stretch must
be calculated. Higher readings are obtained from shorter strips, but the re¬
sults are not greatly affected by the rate of loading or by the width of the
strip.

The stretch of creped papers can be measured in the above manner, but

shorter strips are generally required in order to keep the reading on the

scale. No tension should be applied prior to clamping in the case of creped

papers. The rate of loading affects, to some extent, the results obtained on
creped papers.

The stretch mechanism on the tensile strength tester is not very accu¬
rate because of deficiencies in the release mechanism. For precise work or
or papers with stretch less than 2%, a special stretch tester should be used.

devt^f designed for measuring stretch consists of a

flknt specimen horizontally under a definite tension

f ^‘tetch is indicated by a scale

showmg the distance traveled by the plunger before the strip is broken.

he stretch, as measured above, is not a true strain measurement since

I r”pa;e;'T"" f d”' T" f- -

rure ot the paper. This includes the elastic and the inelastic strctrh r.f fv,
paper, as well as the small distance through which th strC eC es tht

s ~hl"'""d" limitaS st7e" h

s a reasonably good measure of the toughness of paper It is a factor in tV.
bursting strength and initial tearing reLtance of nrner Vtrer u - •

papers, rS'’sSiLTgS^

during use. The stretrli I’c n i • , . suDjected to stress

the machine direction o^he oai^? ’if —" direction than in

the stock used in making feX; p"n^rT

and the loss in stretch on aging Ls been d “u

the resistance of the oaner fo .u f ^S®®*^** ^ better indication of

the paper to aging than the more commonly used fold test.

Bursting Strength

hydros“rpre^ure^!^ifuiredt as the

proximate sphere 1.20 in. in diameter Tt^I ■" an ap-

sure is applied to the paper at an increasing rate^h loading. Pres-

the rubber diaphragm at a constant rate of 95 cc glycerine under

IS transmitted to the paper by the riibhcr ^ u "^^nute. The pressure

orifice which causes the paper to expand into ^ circular

-Clark Stretch Tester ^de bvT Tu "" the

Pennsylvania ^ Thwing-Albert Instrument Co.. Philadelphia.

832

PULP AND PAPER

sheet against the orifice. During the period of the test, the paper must be
securely clamped around the periphery of the test area to prevent slippage,
because slippage increases the curvature and area under test which increases

the pressure required to rupture the paper.

The pressure at the time of rupture is recorded on a sensitive, maxi¬
mum-reading pressure gage. The reading includes the pressure exerted by
the rubber diaphragm, as well as that of the specimen, and since this varies
with the amount of expansion of the paper, it constitutes an uncontrolled
variable. According to Clark,®® the bulge of the rubber diaphragm at the
time of rupture varies from 2 mm. for news to over 4 mm. for manila (with
a bulge of 3 mm. corresponding to about 1.5 lb. on the tester). There are
three bursting strength instruments, the Cady, the Mullen, and the Ashcroft.

The results obtained on the bursting strength tester depend upon the
rate at which pressure is applied so that it is best to use motor-driven
models which apply pressure at a controlled rate. Increasing the rate of
loading tends to increase the reading because it increases tensile strength,
which is a component of bursting strength. Other factors which affect the
bursting strength are clamping pressure, presence of air in hydraulic sys¬
tem, calibration of pressure gage, and sharpness of edges of orifice.®® Ex¬
pansivity of the pressure gage and trapped air have the effect of reducing
the pumping rate, which tends to lower the bursting strength reading.
Individual readings should not be less than 25% or more than 75% of the
total capacity of the gage. Routine calibration of the t«ter can be done
by using standardized aluminum foil, which is available in strengt so

and 40.®®

Bursting strength is recorded on the pressure gage in pounds, but the
results are generally expressed as points. The results are sometimes re¬
ported as points per pound (sometimes called the burst ratio) obtaine y
dividing the bursting strength by the basis weight in .pounds using the basic
ream size. Sulfite bond paper usually runs around 1^4 points per
mimeograph around 1 point per pound, and book papers around 0.25 to
0 50 point per pound, depending upon the amount of filler in the paper.
Another way in which bursting strength is reported is as burst factor,
which is obtained by dividing the bursting strength in grams per square
centimeter by the basis weight in grams per square meter. The m e
States Government Printing Office has the following minimum specifications

for the bursting strength of bond paper;

65 J. d’A. Dark, Paper Trade /. 93, No. 19: 207-210 (Nov. 5, 1931)

66 R. C. McKee. C. H. Root and L. R. Ayers. Tech. Assoc. Papers 31. 34;^^

67 F.^T. Carson and F. V. Worthington. Bur. Standards L

68 N. G. M. Tuck and S. G. Mason, Pxdp Paper Mag. Canada 50, No. 3. 13- l .

69 TTesr»n”be^obtaS^ Hurlbut Paper Co., South Lee, Massachusetts

XVI. PROPERTIES OF PAPER

833

Weight (17x22—SOO) 13

Chemical wood bond . jo

25% rag bond . 22

50% rag bond . 26

75% rag bond . 2 Q

100 % rag bond .

The ftre'Is if wT'’'' “

across rXef f h T differential

n all dtrecons, thus building up unequal stresses in the pape^ TheT

^The rr ‘he paper hTfa iTw ftreTch

ubber d ‘h* '^“^ed by the buLLg

rubber diaphragm into a machine direction tension. By usingT special nttf

larT MuZ *'*' ‘ha ot

using the Mullen testZ^‘^angth
by thTfcZTfgTo^r"' ” ‘-"'a strength

PR = 2T

i^zsz:^ • - ™“ O'».««

Since /?, the radius of curvatnrp ri ^ direction per inch of width.

nothing is measured by the bursting tTst” 1 of the paper,

tensile tester Usinn- fh k ^ i cannot be determined by the

.h...o--1.™

..i... .. "-k. *S,- C «.■-

can be obtained from stretch u^ir^a- i / i ^ formula, values for R

Campbell.” I„ substituting in thf folfnTlT Z ’l" a“g«asted by

pressed in pounds per inch of width, and bursttaTs'rZThfe d

Ow'cn, Pu/fi Paper Maa C A <n ponnds. The

” D ToL P / ' ^ I-e

’‘P- T. Carlof “afd f'’V ^Wo (Sept 19421

•,339-353 (Feb., ,"^,0 '‘^““-.ng.on, F„,. y. STrlTZ 2-

. rson and F, V. W„„bi„g.n„, ^ ,

- 1, Carson and F V" WTr^ *u' *

-dS. P, '

0X?Sr'Z°r.-TZ9M,a‘ Canada, P„,p pUb!

834

PULI’ AND PAPER

relationship appears fairly valid for machine-made papers, but not valid
for laboratory handsheets.’®

TABLE IX

Conversion to Values of R from Stretch

Stretch, %

0.52

1.12

2.04

3.22

4.70

6.47

8.58

11.05

R, inches

3.44

2.32

1.75

1.42

120

1.047

0.934

0.848

The bursting test is one of the oldest tests and is widely used for routine
mill control and for speciheation testing because of its simplicity. There has
been considerable criticism of the bursting test as a measure of paper quality
because (1) it is not a fundamental test and (2) it makes no distinction e-
tween the cross direction and machine direction strengths, or between the
wire and felt sides of the paper. Despite its many disadvantages,
the bursting test is still widely used. As a general rule, bursting
follows the tensile strength, but it is also affected by the amount of stre ch
in the paper and, as such, is an indication of the give or toughness of the

^ Sapp and Gillespie" have shown that during the drying of commercia
papers, it is possible for the paper to lose bursting strength while gaining
h, tensile strength if the per cent elongation is decreased
Sion on the wet sheet. Figure VIII-2, taken from work of Sapp G’'''*'
pie»« shows that the bursting strength diminishes approximately as a

straight line function of the amount of stretching of the

whereas the tensile strength goes through a maximum unde

ditions. Cottrall and Gartshore- found that uni-direc.ional tension durmg

drying reduces the strength to a much greater extent an
eqLly by drying on glass plates. By applying weight., of

"4 was Luced to aero and the bursting strength was reduced by 25 to

35%. '

78 W. B. Campbell, Forest Products Laboratories of Canada, Pulp Paper La ..

Qmrterly Rev. No. 16: 1-3 (1934) 120-123 (Feb. 27,

79 J. E. Sapp and W. F. Gillespie, Pa(>er Trade J. 124. No. V.

ml)

""a CCrall and J. L. Gartsliore, World’s Pefer Tra6e Rev. (Oct. 6 thrnngh
Nov. 24, 1944)

XVI. PROPERTIES OF PAPER

835

Two factors are responsible for bursting strength, fiber length and inter-

filier bonding. Increased fiber length makes for a higher bursting strength,

but bursting strength is even more affected by fiber bonding. Increased

gating increases the bursting strength over most of the range, although a

decrease occurs with excessive beating. Part of this loss in bursting

strength on beating can be attributed to disintegration of the fiber, but since

t e urstmg strength is also lowered when the sheet is increased in density

V increased \\et pressing (see Fig. XVI-3), at least part of the loss in burst

nius e uted to a loss in stretch. Bursting strength is predominantly

an internal sheet property, although it is influenced to some extent by sur-

ace sizing. rymg conditions on the paper machine have a definite effect
on bursting strength as already pointed out.

Bursting strength is directly related to the utility of bag and wrappin-

Eta exerted ir'f'

V , bursting test. However, for most papers there is no

irect corre at ion between the bursting strength and the utility of paper.

Internal Tearing Resistance

n.en/mirTV.^''"'® resistance is measured on a pendulum type instru-
n.eans"„f a^Ler ^told ^ .hEtTrulm

.he specinten E!:..'"’, pT 43™ ^ ^rthe:’'
atrument. The pendulum is releaL7and th^s

clamp in the pendulum m^v/a t pecimen is torn in two as the

.om^orh .7donet" The d™ 7, -

ference in potential energy of the p7dulum aMhe"h «.« ^if-

s'cmg. Since the pendulum would swing to the rieht to 7

ance of the pa^r, The final disotaeme ' 7

.he difference in potent.afener^oT n ”7 T ^ of

tearing the paper. The angle is'’measured7^'*

axis as the pendulum which has a consta If ^ mounted on the same

a. .he highest point reachtd^ he“ oiT

held m place by a stop so that the scale swings nfsf it d • ^ ""

reads the ratio of work done (in gram-centimetersTt^The LalteL^ng

836

PULP AND PAPER

length in centimeters of 16 sheets (i.e., 137.6 cm.), thus giving the force
directly in grams. The scale readings on the instrument are intentionally
made one-sixteenth of the actual values in order to have a convenient factor
for converting the results when several sheets are torn at one time. Thus,
tearing resistance as usually reported is the average force in grams required
to sustain a tear on a single sheet of paper. Because the scale indicates one-
sixteenth of the actual force in grams required to tear the sheets clamped
in the tester, the force to tear a single sheet is calculated by multiplying the
reading on the scale by 16 and dividing by the number of sheets tested.

Tearing resistance is sometimes reported as tear factor by dividing the
tearing resistance in grams (per single sheet) by the basis weight in grams
per square meter. It is also reported as tear ratio, .or tearing strength in
points (grams) per pound. The results obtained on strips torn m the ma¬
chine direction are reported as tearing resistance in the machine direction,
and results obtained on strips torn in the cross direction as tearing resistance
In the cross direction. Machine direction tearing resistance is generally less

than cross direction tearing resistance.

The standard procedure should always be carefully followed in making

tear tests in order to keep to a minimum the additional work not directly in¬
volved in making the tear (e.g., work in bending the paper, and work in¬
volved in rubbing together of the edges of the torn paper). In making the
test, a sufficient number of sheets should be chosen to obtain a reading be¬
tween 20 to 60. Two, four, eight, or sixteen sheets can be used. A single
sheet should never be used, since a reading obtained on a single sheet is
never the same as a reading obtained on several sheets (even after the re¬
sults have been corrected by multiplying by 16 and dividing by the number
of sheets tested). If the line of tear deviates more than 10 mm. from the
line of the initial slit, the results should be discarded. Splitting of the paper
often occurs in making the test, particularly with papers of high tearing re¬
sistance. When splitting occurs only occasionally, it is advisable to throw
out results obtained on the split specimen, but if a large number of tes s
show splitting, they should all be included in the average.®^ Some paper-
makers attempt to judge tearing resistance by hand. Hand tearing ^ ers
from tearing on the tear tester in that hand tearing applies the load at a

much lower rate.

In general, the force required in tearing paper is much less than tne
force necessary to break a strip of the paper. The work involved m tearmg
consists of two components, work involved in pulling fibers out o e pape
and work involved in rupturing the fibers.« The tearing resistance of paper
made from unbeaten pulp is almost entirely due to work invoh ed m ov

M S. D. Wells, Paper Trade J. 76. No. 26 : 24&-251 ( June ^ j„,

83 Institute of Paper Chemistry, Instrumentation Studies, P p

No. 5: 13-19 (Feb. 3, 1944)

XVI. PROPERTIES OF PAPER

837

conung the frictional resistance of the fibers being pulled from the paper,
ere is practi^lly no tearing of the fibers, and since the total area of fiber

!:sTnw 'ir ' and the tearing resistance

lo«. After a slight amount of beating, interfiber bonding is increased and

i^ frictional resistance

ear?v stiles ■'''^''ease in tearing resistance in the

not always show this initial increase in tear, and pulps Latent the kt
oratory spetimes do not show an initial rise in tL beLule of «t
tively uniform beating treatment which decreases average fiber length about
as rapidly as cohesiveness is increased.- As beating fs greatly tcreted

rr=: Lttt at--- f

as a result of beatinp anH th’ increases in cohesion and stiffness

smaller area w h -Result thaTL “ “to a

explains wh; rearing res tea ‘ This

tial increase: The fffect ii not .'‘“''"f “ «P°“ beating after an ini-

progressive loss in tearinv resist"* ° u *e"gth, since the same

amounts of cementing agenfs (e g trchf to th't increasing

densihy of the paper ly --sing the

in combinatfortth"the'tt o/bt? “nd when taken

able information on t t ? ^““"''es consider-

rule, tearing resistance varies inverst with . prepared. As a

If the tearing strength is very high and the h V"® strength and density.

It may be taken as evidence that the o i • “®dng strength is very low,

fearing strength is low and the burst vtyLitThf “r*!f

overbeaten. If tearing strength is disor • * T“lp has probably been

with the burst, it can be taken as evid

fiber treatment since, for a given fiber f, 1“ during

highly satisfactory measure o^f the amount f'fiK resistance is a

about by refining. Tear resULcTnd Life' ” r"**"

Ic) ) vary inversely.®® ^ ineasured on the Gur-

Fiber length is a very important factor in f
resistance increases with an increase in fiber lenlra ® Tearing

ength means an increase in the frictional dra '“reased fiber

-1 d-A. Clark. Paper Trade I ^ P"" bber. Oark**

-7'dA^aVp T /ai'ffe' ms:*'

I. A. Clark, Pater Trade J. us. No. feilfreprint)

8.^8

rUU’ AND PAI*RR

States that tearing strength is increasetl according to the square root of the
cube of the average fil^r length (see Oi. VII). Southern kraft pulps made
from pine and pulps made from western woods have a higher tearing re¬
sistance than northern kraft pulps made from spruce Iiecause of their greater
liber length. Tearing resistance is undoubtedly affected by the stress-strain
properties of the individual fibers, a relationship which has been definitely

established in the case of cotton fabrics.*"

Tearing resistance is higher on un.strained paj^cr than on strained
because of the extra work required to straighten out the fibers in unstrained
paper (see Fig. XVI-10). Mason'* points out that high tearing resisUncc
is obtained with papers having a high creep, since creep tends to distribute
tlic load over a wide area of the sheet. This explains why, on wetting of the
paper, loss in tearing strength is not great compared with the great oss in
tensile strength which occurs under the same conditions. The tearing re¬
sistance of creped paper is higher than that of uncreped paper l>ecausc of the

extra work involved in tearing the greater length of paper

The tear test is particularly useful in the evaluation of bag, tag, wrap
ping, building, tissue, certain grades of boxboard, and other grades of pa¬
pers which are subject to tearing strain in use.

Edge Tearing Resistance

From the standpoint of utility, the initial tearing resistance, N\hicli '
the moment of force required to start the tear at the ^ge of «

probably more important than the internal tearing .'i; tUI is

average force required to sustain the tear once it is started,
often considerable difference betrveen the edge (initial^) tearing
and the internal resistance, as shown by cellophane, which has a verj h g
edge tearing resistance and a ver>- low internal tearing resistance.

^ The initial tearing resistance is determined by the tensde
the stretch of the paper. Initial tear can be me^ured in
using a special clamp with a V-notched beam which te^s the
tane^usly^ both edges of the strip. In this method of testing, a^^

paper 0.59 to 1.00 in. in width is threaded under a to '

to both edges of the strip fastened in the upper clamp °f *e tensile to

The load is applied slowly at first, and then at suffiaen ra ^

Tp^men in to fifteen seconds. The results are reported for both the

machine direction and cross direction of the paper. ^

ST O. B. Hager. D. D. Gagliardi and H. B. Walker. Text. Research J. i/. No. /

ss s"g: M^^n, Pulp Paper Ma,. Ca,u.da 49^ No. 3 : 207-214. Convention Fsne
S9 Institute of Paper Qiemislry, Instrumentation Studies, PaP>^r Trade

No. 5: 13-19 (Feb. 3. 1944)

XVI. PROPERTIES OF PAPER

839

The tensile tester can also be used to measure a combination edge tear

an tensile strength by offsetting the test strip in the tensile tester at a defi-

mte angle so tliat most of the tension is applied to one edge of the strip.

ns test has been said to correlate fairly well with performance of the paper

in we printing where breaks are often caused by uneven tension on the
paper.

Folding Endurance

f w • endurance is an empirical test which measures the amount of

ard ™!u^on Th ®

fnlH' ^ ^ instruments commonly used for measuring

mg en “^^nce, the Schopper (a German instrument) and the MIT

(developed by the Massachusetts Institute of Technology). There is no

consistent relation between these two instruments.

6000

fig. XVI-13.

1000 2000 3000 4000 5000

^.'UMBER OF DOUBLE FOLDS

Effect of tension on test strip on folding endurance (rag paper)

width" L'’fit^'"n';,:„ed"bla1 “"‘‘7 f’" “0 '5 mm. in

tension. The paper is then folded" backTn7lrth'’r!‘

from 790 g. to I.OOO g. dnrLgTte period o rTT

instrument is shown" in FigurfXVI llT ‘n ^chopper

folding tests, the strips of paL I s", K

buckling or waviness, and the instrument k *** °f

that the wear is even. Specimens can be r "" ®'fi“**™tit so

determine the evenness of wear. amoved just prior to breaking to

cCafdVnos^,:^^^^ between a spring-loaded

to the right and left th^h a^tf anS

*®L. W. Snyder and F T Carl n p ^ ^

1933) Colson, P.per Traie /. ,d. No. 22 : 27^-280 (June ,,

840

PULP AND PAPER

double folds per minute. The M.I.T. is a more versatile instrument than
the Schopper, since the latter is limited to papers having a thickness no more
than 0.01 in., whereas the M.I.T. will handle papers of widely varying thick¬
nesses by using different jaws. The results obtained on the M.I.T. tester
are affected by the curvature of the folding edges so that each new jaw

should be standardized before installation.

Folding endurance is reported in double folds. Results obtained on
strips cut with the machine direction lengthwise are reported as niachine
direction folding endurance, and results obtained on strips cut with the
cross direction lengthwise are reported as cross direction folding endurance.
As a rule, machine direction folding endurance is higher than cross direc¬
tion folding endurance, although the reverse is sometimes true. The fact

Fig. XVI-14. Stress-strain diagram for indicating folding endurance

that cross direction folding endurance sometimes approaches or even ex
ceeds machine direction folding endurance, in spite of the
strength in the machine direction, indicates that some flexibi i y
property is involved in the folding test. Mason" believes that high folding
endurance is obtained with papers having a high rate of stress re *

relatively high primary creep. Ranee" believes, on the contrary, tha fold

In tWs curve, line ODA represents the elastic spring line which Jh'

stress-strain curve in the absence of plastic flow. Thus, OC is a
the total elastic strainability, and CB the measure of plastic flow. L
is used in place of line OC to compensate for the permanent tension

MdiL^ eLurtnee is, in a sense, a modified tensile strength determina-
« S. G. Mason, Pull, Paper Mag. Canada 49. No. 3 : 207-214, Convention Issue

•e aRanee, Paper-Maker, Midsummer Issue, 23, 32, 49, 50, 51 (1949)

XVI. PROPERTIES OF PAPER

841

tion, but the results are greatly affected by the flexing ability of the paper.
As a rule, folding endurance follows tensile strength in the early stages o
beating, but then falls off more rapidly than tensile stren^h, and eventually
decreases with increased beating or wet pressing (see Fig. XVI-3), due to
the paper becoming more brittle. Lack of adequate folding endurance can
be the result of lack of fiber length, inadequate fiber bonding, or brittleness.
Improper beating or poor formation are causes of poor folding endurance.
Too much surface size (starch or glue) or too much beater starch tends to
reduce folding endurance because of excessive brittleness of the fibers.
Folding endurance is greatly affected by the moisture content of the paper,
more so than the other physical tests (see Fig. XVI-35). Plasticizers in¬
crease the folding endurance of glassine papers by reducing the stiffness.*®

As a rule, rag pulps produce papers of high folding endurance. On the
other hand, groundwood papers, papers containing hardwood fibers, and
coated or heavily filled papers liave poor folding endurance. Mixtures of
pulps do not always produce the expected folding endurance. Mixtures of
rag and sulfite produce papers having folding endurance approaching the
average of the two pulps, but mixtures of sulfite and wood filler pulps pro¬
duce papers having a folding endurance only slightly above that of the

weaker pulp.®*

The general acceptance of the folding test is based upon the fact that it
is the best means of measuring the quality of those papers which are required
to resist considerable handling and folding in use. It is of particular im¬
portance in measuring the quality of high-grade sulfite and rag content
bond, ledger, currency, index, cover, and record papers. The machine
direction folding endurance of bond papers at 65% relative humidity gen¬
erally varies from about 50 double folds for light-weight sulfite bonds to
3,000 or more double folds for heavy-weight rag bonds. The U. S. Gov¬
ernment Printing Office specifies the values given in Table X as the mini¬
mum for the average double folds in each direction of bond and ledger
papers.

Folding endurance is of value for indicating the utility of bag and box
lining papers and for folding boxboards which are to be scored and folded
into boxes. Boxboards are often tested by folding flat between the fingers,
once in each direction, and with, across, and at an angle of 45° to the grain.
The board is considered a bender if there is no break in the surface fibers.
A scoring rule and creaser are sometimes used. Creasing with the grain is
a more severe test than creasing across the grain; creasing diagonally across
the grain is the most severe test of all.

One characteristic of the folding test is that the results vary widely.

N. R. Pike. Paper Trade J. 102, No. 5: 63-^6 (Jan. 30, 1936)

C. G. Weber. ^I. B. Shaw and M. J. O’Learj', Paper Trade J. 117, No. 13: 147-

152 (Sept. 23, 1943)

842

rULP AND PAPER

TABLE X

Minimum Folding Endurance for Bond and Ledger Paper

FOR U. S. Government Printing Office

Bond

Weight (17 X 22—500)

Chemical wood pulp bond .

50% rag bond .

100 % rag bond .

100

150

800

1000

1200 .

Weight (17 X 22—800)

Sulfite ledger . *•*

50% rag ledger . J^^O

100 % rag ledger .

1000

150

1000

Ledger
32 36

100

150 200

1000 1200

12d

300

1500

» » V

300

1500

The question often arises as to how much of the variation s _

orocedure and how much to variability of sample. In a study of the fold g

test Reitz and Sillay“ found that the coefficient of variation

vMonlpressed as^a percentage of the average result) va™s fo^r^

. .1 rpTTinin.*; fairlv constant irom uaicn

j viation expressed as a percentage ' ..

irrades of oaoer but that the coefficient remains fairly constant from bate

-i-e “S ;r

f .1 o AT T T fnlfl tester and the Schopper fold tester. Data obtainea wuu
the M.I.T. fold ^ normal distribution, as shown ni

testers were in close agreement u nn number of

Table XL The expected reproducibility of results

TABLE XI

SHOWIKO DisnuBUTion or Dath OBraiuED ON M.I.T. aNO ScBOPPra Fora Testehs

Limits of
standard
deviations

F±16

X±26

X±36

Normal^ dis¬
tribution

68.3

95.5

99.7

Actual fold data
on M.I.T. tester

67.0

96.0

99.5

Actual fold data
on Schopper tester

65.0

96.0

100.0

. -rp* VA/T 1 5 for three different grades of pape
tests made is shown in Figure ,f,om the curves that

taken from work by Reitz and Sillay. within a few

an unreasonable number of tests are require " has

folds of the true average. For example, ^ le P > produce

„I.M ui.™.(w»

an average reproducible within ' ■ , 948 )

L. K. Reitz and F. J. Sillay, Tech. Asspe. Pppers 31. 98-lOZ (J

XVI.

PROPERTIES OF PAPER

843

the time). Simmonds and Doughty"'’ calculated that 3,648 tests
required to obtain an average value of such accuracy that by

all such averages would not differ from the true average (1.006 “ds) by
more than plus or minus 30 folds when the standard deviation f ^

sinde test is 604 (calculated from «= (a) /(S.D.m) - (<t) /( / )
(604)7(30/3) = 3,648 tests). Because of the variability of this test^a -

Fig. XVI-15. Reproducibility of fold test. Coefficient of variation:
A paper, 26%; B paper, 21%; C paper, 35%.

iiig endurance should not be used as a specification unless a tolerance of at
least 20% is allowed for high-grade papers and 30% for ordinary grades.'’^

Impact Strength

With the increased use of paper and paperboard products for structural
purposes, the paper converter is showing more interest in direct physical
measurement of the structural properties of paper. The use of rigidity
tests has already been mentioned. Impact streng^th is another test which is
being used to an increasing extent, particularly in the evaluation of fiber-
boards for building purposes.

Impact strength is a direct indication of the ruggedness of building
board in use and is related to the ability of the board to resist dog-earing- or
breaking during handling. According to Lathrop and NaflEziger,®®*^® the
impact strength of fiberboards can be greatly improved by the presence of
long resilient fibers in the board. High impact strength is required in pa¬
pers for resin-impregnated paper laminates.

®®F. A. Simmonds and R. H. Doughty, Paper Trade J. 97, No. 25 : 298-303 (Dec.
21, 1933)

S. Spencer, Tappi 32, No. 7: 291-292 (July, 1949)

E. C. Lathrop and T. R. Naffziger, Paper Trade J. 127, No. 27: 540-545 (Dec.
30, 1948)

»9E. C. Lathrop and T. R. Naffziger, Tappi 32, No. 2: 91-96 (Feb., 1949)

844

PULP AND PAPER

Softness

Softness is the lack of harshness when paper is crumpled in the hand.
(Softness is also used in opposition to hardness as evaluated by compressi¬
bility, as indicated in the following section.) The determination of softness
by hand crumpling involves the appraisal of four main physical properties
which contribute to the softness of paper, namely, density, rigidity, com¬
pressibility, and surface smoothness.^“° Of these, the flexibility of the paper
is most important, and usually a fairly reliable indication of softness can be
obtained from a value representing the inverse of rigidity multiplied by the
log of the basic caliper times a constant.^”® An empirical method of meas¬
uring the softness of tissues is based upon a measure of the volume occupied
by a given amount of tissue in a cylinder when crumpled under a definite

load.

Softness is an important attribute of sanitary tissues, facial tissues, pa¬
per drapes, and toweling. In making these grades, a soft pulp is used which
develops a high strength with a minimum amount of fiber bonding. These
papers are often given a slight crepe. Creping increases the softness by de¬
creasing the apparent density, by decreasing the amount of fiber bonding,
and by increasing the elasticity and compressibility of the paper. Dry
creping is generally more effective in reducing fiber bonding than wet crep¬
ing. Softeners (glycerine, diethylene glycol, and diglycol stearate) are

frequently added to the paper.

Hardness and Compressibility

Hardness is the property of paper which causes it to resist indentation

XVI. PROPERTIES OF PAPER

845

the compressibility of the paper by the rate of flow
and a flat metal plate containing raised metal plugs (0.002 m. ) •
is forced downward (by an inverted cylinder floating freely m a container
partly filled with oil) through an open tube to the upper orifice plate against
which the sample is clamped. In carrying out the test, the paper sample is
punched and clamped in the tester on top of the special plate with the raised
plugs. Pressure is applied by means of a lever arm, thereby forcing the
metal plugs into the paper surface. The plugs are designed to allow air
leakage between the two surfaces when no paper is inserted of 100 cc. per
ten seconds. When paper is inserted, air leakage depends upon the softness
of the paper, since the deeper the plugs are forced into the paper, the slower
is the air leakage between the paper and the metal surface. Different
weights can be used depending upon the softness of the paper under test.
The small weight (0.34 lb.) gives a pressure of 300 p.s.i. on the plugs and
the large weight (2.0 lb.) gives a pressure of 1,300 p.s.i. on the plugs. The
test is appreciably affected by basis weight and smoothness of the paper.“^
The results are reported as time in seconds to leak 100 cc. of air. Soft pa¬
pers give a higher reading than hard papers because of the longer time re¬
quired for the leakage of the air.

The surface hardness of paper or paperboard can be measured by the
abrasion test, the results of which correlate fairly well with the erasing prop¬
erties of paper and the scuffing resistance of paperboard. The instrument
used for making this test is called the Taber Abrader. It consists of a
horizontal turntable and center clamp to which the specimen is attached,
and two weighted arms equipped with special abrasive wheels. The abra¬
sive wheels freely rotate on the specimen during the period of test and wear
away the paper surface at a rate depending upon the wearing resistance of
the paper. The speed of rotation of the turntable is 65 to 75 r.p.m., and the
pressure applied by each arm varies from 500 to 1,000 g., depending upon the
size of the weight used. Wheels of varying abrasive qualities can be used.
After each 1,000 revolutions, the abrasive wheels must be resurfaced by
running on a special abrasive disk. During the period of the test, accumu¬
lated dust should be removed with a brush. The results are reported as the
abrasion loss in milligrams per 1,000 revolutions for a test surface of ap¬
proximately 10 sq.cm. The results vary from about 100 mg. for well-beaten
papers to 2,000 mg. or more for soft papers.

In general, hardness follows the density of the paper, and consequently
papers requiring a high degree of hardness should be made from easy beat¬
ing pulps. A high moisture content lowers the hardness of paper.

Hardness is one of the most important properties influencing the print-

loUnstitute of Paper Chemistry, Instrumentation Studies, Paper Trade J 110
No. 23 : 303-309 (June 6, 1940)

846

PULP AND PAPER

ing qualities of paper, since it affects the ease with which the printing plate
establishes contact with the paper surface. In printing (notably letterpress
printing), the softer the paper the better the contact with the printing plate
(see Ch. XIX). Surface hardness is desired in some papers (e.g., box-
boards and writing papers), and these are usually surface treated with starch

or glue.

Bulk and Bulking Thickness

Bulk is a measure of the thickness of a pile of a specified number of
sheets under a definite pressure (usually 7-9 p.s.i.). Bulking thickness is
the average thickness of a single sheet determined from the thickness of a
stack of sheets. Bulking factor is obtained by dividing the bulking thickness
by the ba'sis weight. Bulk factor (bulk index) is the same as the apparent
specific volume, i.e., the reciprocal of the apparent specific gravity.

Bulk is dependent upon the thickness and compressibility of the paper.
The bulk of a stack of sheets will generally be less than the bulk calculated
from the thickness of individual sheets due to the nesting and compacting
«which occurs on stacking. In general, bulking thickness is about 2 to 6%
lower than the figure obtained from the caliper of a single sheet.

Bulk is of practical importance for printing papers to be bound into
book or pamphlet form. There is a special grade of book paper called
bulking paper, which is made very bulky. To make this paper, soda, rag,
or alpha pulps are generally used, the stock is beaten very little, no fillers
are used, and special felts are used on the machine. The finish pven to the
paper is very important, as shown in Table XII gi'-ing the bulk of several
different grades of printing papers m 60-lb. weights (25 x 38—500).
formity of bulk is an important property on all papers, since non-umfor

bulk causes hard or soft spots in the paper roll.

TABLE XII

Bulk of Printing Papers of Different Finish

Finish

Antique .

English finish .

Supercalendered .;.

Sheets to the inch

300

510’

640

Porosity

Paper is a highly porous material, as attested by its low specific gravity
0 5^8) compared with that of cellulose (1.5). Commercial papers con-
ain^s much as 70% air, which is due to (I) true pores, openings

vhich extend entirely through the sheet, (2) recesses, t lose p ..

"ected to one surface only, (,?) voids, those air spaces connected to neither

XVI. PROPERTIES OE PAPER

847

surface. According to Baird and Irubesky,’- the true pore volume of a
typical paper is only 1.6% of the total air fraction, the remaining 98.47o
being the recess and void volume. This explains why greaseproo paper
which contains as much, as 48% air volume^®^ can still function e ^cti^ y
as a greaseproof barrier, since it contains practically no true pores, i he
recess volume is generally higher on the wire side of the paper than on the

felt side. , , • i

The air volume of paper can be calculated as follows, but the air volume

obtained in this way does not distinguish between pores, recesses, or voids.

spe cific gravity of paper
air volume - - gravity of cellulose

Apparent density (obtained from the basis weight and thickness using the
metric system) is generally used in place of specific gravity, since it is im¬
possible to determine the specific gravity of paper by the displacement of

water.

The pore volume of paper is generally expressed as equivalent pore
radius, i.e., the radius of a single pore of length equal to the sheet thickness
which would give the same flow as the average value for all the pores in a
unit area of the paper. The pores in paper form a complicated system of
interlocking, crooked, and criss-crossing channels which range in size from
a fairly large diameter to capillary dimensions. Equivalent pore radius tells
nothing about the size, shape, and distribution of the pores, but it is useful
for comparing the porosity of different papers. Using an air permeability
method and a modification of Poiseuille’s equation,^®* Carson^”'* found the
average effective pore size to vary from 0.2 micron for coated book paper
to 1.2 microns for bond paper. Simmonds^°® obtained values of 0.1 to 4
microns for blotting paper, using a capillary rise method.Foote^°® ob¬
tained a value of about 0.19 to 0.35 micron for unsized bond paper using
capillary rise, contact angle, and swelling measurements.

p. K. Baird and C. E. Irubesky, Tech. Assoc. Papers 13: 274-277 (May, 1930)
R. M. Cobb, Paper Trade J. 100, No. 16: 200-203 (Apr. 18, 1935)

Formula used by Carson:

in which AP is the pressure drop through the paper, P is the applied pressure, 5" is
the mean free path of the gas, n is the number of capillaries, T) is the viscosity, r is
the radius of the capillaries, and I is the length.

^05 F. T. Carson, /. Research Natl. Bur. Standards 24, No. 4: 435-442 (Apr., 1940)
^°®F. A. Simmonds, Paper Trade J. 97, No. 10: 40-42 (Sept. 7, 1933)

The capillary rise method is based on the following equation:

rat

2ir

where h is tlie height of capillary rise, t is the time, cr is the surface tension, t\ is the
viscosity, and r is the radius of a uniform capillary that will give the same rise.

J. E. Foote, Paper Trade J. 109, No. 14: 180-188 (Oct. 5, 1939)

848

PULP AND PAPER

In routine paper testing, porosity is generally measured by the resist¬
ance of a paper sample of given dimensions to the passage of air under
standardized conditions of pressure, temperature, and relative humidity.
The results are expressed in arbitrary units as the time taken for the passage
of a given volume of air or as the amount of air passed in a given period
of time. The flow of air through paper is directly proportional to the pres¬
sure difference (for small pressure differences), to the time of flow, and to
the effective area of the specimen; it is inversely proportional to the thick¬
ness of the sample.^®® Air resistance varies inversely as the fourth power
of the radius of the pores.'^" Two papers may have the same equivalent pore
radius but different air resistance if the papers differ -in thickness, since
equivalent pore radius is independent of sheet thickness, whereas air resist¬
ance is dependent upon sheet thickness.^^^’

The air resistance of paper is commonly measured with the Gurley
Densometer or the S-P-S Tester. These testers measure the time for the
flow of a standard volume of air (usually 100 cc.) through a standard area
(usually 1 sq.in.) of the paper under uniform light pressure. Air is forced
through the paper by pressure resulting from the weight of the inner cylin¬
der floating freely “in an outer cylinder partly filled with light oil. In the
densometer, the paper is clamped in the top of the inner cylinder, whereas
the paper is clamped in the base of the S-P-S tester on top of a per orated
plate. The instruments are equipped with an elastic rubber (Thiokol) gas¬
ket to prevent air leakage laterally across the surface of the paper. This
gasket is part of the densometer, but must be inserted in the S-P-S tester
when porosity tests are being made. The movable inner cylinder is gra u-
ated in units of 50 ml., and the volume of air transmitted is read on this
gage. The time for the passage of 100 cc. of air is taken after t le iimer
cylinder has been released and attained a steady downward movement. 1 he
apparatus should be tested periodically for leakage by testing a sraooti,
non-porous material such as metal foil: leakage should not exceed 10 ml.

of air per hour. ,.

The Williams tester uses a movable inner cylinder and an ou er cy ^

der partially filled with mercury. The specimen ia folded and perforated

and then a small square of rubber (also perforated) is
the sample. The specimen is then placed between the ■'t'M’M-
so that all four surfaces are sealed against escaping air. _
pass out through the paper in a line perpendicular to t le p an

i»9F. T. Carson, Paper Trade 1.

l. «. «,'c. SeLfand P.‘k‘B^ aw, 'Pape} Trade J. 94. No. 24:

nsR^.'Doirhiy! C.'a Seborg and P. K. Baird, Paper Trade J. 95. No. 13:

149-151 (Sept. 19, 1932)

XVI. PROPERTIES OF PAPER

849

Timing is controlled by a mercury column in a small tube in the
apparatus. An electric timer is started when the mercury is raised to the
correct level at the start and is automatically shut off when the mercury

has fallen, due to the air pressure being relieved. ^ r • 112

Air resistance increases greatly with increasing solid fraction.

There is no absolute relationship between solid fraction and air resistance,
however, since papers made from different pulps have different air resist¬
ance, even at the same solid fraction.^^^* Air resistance generally decreases
with increasing amounts of rosin size and filler, with the bulky fillers such
as calcium carbonate causing the greatest decrease in air resistance. Lane
found that low percentages of fillers reduce the air resistance by reducing
fiber bonding, but that high percentages of fillers increase the air resistance
by plugging the sheet. Foote^^® found the average pore diameter to be
greater in the machine direction of the paper than in the cross direction.

Air resistance measurements are commonly used as a control test for
paper manufacture because of the indirect correlation between porosity and
formation and strength of the paper. Porosity is of direct importance in
writing and printing papers, since it is a factor in the absorption of inks.
It is an important property of coating rawstock for pigment-coated papers
where it affects the absorption of adhesive. It is definitely related to the oil
resistance, and hence is an important property of greaseproof and oil-re¬
sistant papers. Albert”® found a linear relation to exist between air resist¬
ance in the range of 10 to 150 seconds on the Gurley instrument and oil
resistance, when the latter was measured by dividing the seconds for oil
penetration by the thickness of the paper square (S/T^). Porosity is an
important property of unimpregnated electrical insulating papers because
of its influence on the dielectric constant.*^''

Porosity is an extremely important property of saturating papers.
However, because of the low air resistance (1-5 seconds) of many satu¬
rating papers, the conventional instruments are not well adapted for testing
these papers. There is, however, a special permeometer for testing highly
porous papers (e.g., filter, tissue, blotting, absorbent, and saturating papers)
which measures air flow as small as 1 cu.ft. of air per minute per square foot
at a pressure drop equal to 0.5 in. of water. This instrument uses a null
method of measurement and is, in principle, a pneumatic bridge consisting'
of a reference chamber and a test chamber through which air is drawn simul¬
taneously into a reservoir. The valve adjusts the suction in the test chamber

113R. H. Doughty, Paper Trade J. 95, No. 10: 111-118 (Sept. 8, 1932)

11 ^ W. H. Lane. Paper Trade J. 117, No. 3: 28-32 (July 15, 1943)

115 J. E. Foote, Paper Trade J. 109, No. 14: 180-188 (Oct. 5, 1939)

11 ® G. A. Albert, Paper Trade J. 101, No. 11: 127-131 (Sept. 12, 1935)

111 L. Emanueli, Technologie und Chemie der Papier- u. Zellstoff-Fabrikation 25
No. 4: 49-51, (Apr. 28, 1928)

850

rULP AND PAPER

SO that it balances the suction in the reference chamber, as indicated by a
zero reading. The valve reading is used as a measure of the air permea¬
bility.

Porosity is very important in bag papers to be filled by valve connec¬
tion where the bags must have a certain porosity to prevent bursting during
filling. In cigarette papers, the porosity must be controlled through the use
of filler (calcium carbonate) to regulate the burning rate. Anti-tarmsh
wrapping papers should be made with as low a porosity as possible in order
to reduce the penetration of hydrogen sulfide, oxygen, carbon dioxide, and
other gases which cause the tarnishing of steel, copper, and silver articles.
Porosity is very important in filter papers used for the filtering of oils,
aqueous fluids, and gases. Gas mask filter paper must be permeable to air,
but impermeable to smoke and fume particles. Perot“® found that the fi -
tering of smoke bv paper is primarily a mechanical effect which is depen ent
upon sheet density, sheet weight, and fiber diameter, but is independent of
fiber length. Straw pulps mixed with cotton or coniferous wood pulps seem
to work better in filter paper than wood or cotton pulps alone because of the

small diameter of the straw fiber.^"® . , .

Porosity, or more specifically, void volume, is of particular importance

in insulating papers made from wood fibers, asbestos, or mineral wool fibers.

The voids in the paper sflould be small so that circulation of

voids is held to a minimum. Some of the papers requiring a high void vol-

Irare sheathing papers used between the walls of homes ^

wind, felt papers used tor heat insulation, and deadening felt used bet

the walls and underneath floors and carpeting to ea en “'1" ' . . j

grade is insulating board used as lath, roof insula ion, outs de shea „,

acoustical tile, and for insulating walls. In making this type o ’

an attempt is made to strike a balance between the amount of g
material, which is used for the structural framework, and die »

.short fiber material, which is used as a filler or "tatenah ^

proportion is 55 to 70% long free fiber, and from 30 to 45%
liiaterial. The fine material is derived from many juices me u i »
stalks bagasse, and grouiidwood. Southern pine is wi ey .aoiv

loug-filiered material. Recently, insulating board has been
■pine groiindwood pulp. The properties of insulation board are app
iiately-™ as follows; weight, 1^ Ih. per square foot; thickness, 1
nial cLductivity, 0.33 E.t.u. per square foot per difference per our
ficiisitv 14 to 18 lb per cubic foot. Another grade of wallboard, know
Sd nude with a relatively high percentage of

118 j, j. Perot, fa^tifrop No. 9: 1300-

ii» S. 1. Aronovsky. G. N. Nelson and E. C. Latnrop, aptr

120 (:.^a Trade J. J25, No. 5: 58-60 (July 31, 1947)

XVI.

properties of paper

851

short-Hbered material in order to produce a board of greater den^Y.
ity, and better finish. This grade of board is frequently coated for use

inside decoration.

Smoothness is concerned with the surface contour or mechanical per¬
fection of the paper surface. Optical smoothness of paper is discussed under

hnish and gloss. _ . .

Probably the most accurate method of measuring smoothness is y

means of a microscope using a micrometer focusing adjustment by which
the microscope is focused on a small area of the paper surface and the mi¬
crometer reading is noted. The paper is then moved to a small predeter¬
mined distance, the microscope again focused, and a now micrometer rea
ing is noted. In this way, a complete and accurate picture of the paper
surface is obtained, but the method is slow and tedious, and suitable only

for research work.

Another direct means of measuring paper smoothness involves the tracing
of surface irregularities, using a surface analyzer, so that a profile of the
paper surface is obtained. Detector needles are used for contacting the
paper surface.^^^’ In one instrument, a steel ball is mounted on a tracer
arm which automatically adds the total movement as the steel ball is moved
over the surface of the paper. The Forest Products Laboratory roughness
tester, which is also based on the tracer principle, uses a detector needle with
a flat end., The needle makes intermittent contact with the paper surface
to avoid side stresses and the plowing effect of continuous tracing, and thus
the paper is essentially stress-free at the time of measurement of its rough¬
ness. The Brush surface analyzer magnifies and records the contour of the
paper and indicates the average variation between the high and the low
areas.

Recently Chapman^^® has described a modified version of the Davis
tester’-^* for measuring smoothness. In this method, the test sample is
pressed against the lower surface of a glass prism and the sample illuminated
by an approximately parallel beam of light passed through the prism at right
angles to the surface of the paper. The amount of reflected light in the
prism having an angle greater than 41° 8' is taken as a measure of the sam¬
ple in contact with the prism. The amount of this light is measured on a
photoelectric cell and compared with the total light measured on a second
photoelectric cell.

121 S. Way, Mech. Eng. 59, No. 11; 826-828 (Nov., 1937)

122 M. Heinig and P. K. Baird, Paper Trade J. 113, No. 15: 195-200 (Oct. 9,
1941)

123 S. M. Chapman, Pulp Paper Mag. Canada 48, No. 3: 140-150, Convention

Issue (1947) *

. 124 M. N. Davis, U. S. Patent 2,050,486 (Aug. 11, 1936)

852

PULP AND PAPER

Routine measurement of smoothness is generally made by air flow
instruments which measure the rate of flow of air across the surface of the
paper. Air flow smoothness testers are based upon the principle that the
volume of air voids between a paper and a plane surface is proportional to
the roughness of the paper, and that the rate of air flow between these two
surfaces is proportional to the volume of the air voids. In carrying out the
test, the paper is clamped under definite pressure and the time taken for a
given volume of air to flow either between the paper and a standard surface,
or between two or more sheets of the paper. There are three principal
air flow instruments used for measuring smoothness, Bekk, Williams, and
Gurley. All are based on the same principle, but there is no constant rela¬

tion between the three instruments.

In the Bekk smoothness tester,”® the time is taken for a given volume

of air (10 ml.) to flow under reduced pressure between the paper surface
and a flat polished glass surface having an area of 10 sq. cm. The air
passed between the two surfaces is conducted through a circular aperture
in the glass surface into a vacuum chamber. A vacuum is maintained in
the chamber by means of a mercury pump, a mercury manometer being use
to show the difference between the pressure in the chamber and the atmos¬
pheric pressure. The paper is placed with the surface to be tested in con¬
tact with the polished glass and a rubber pad placed on the paper, meta
pressure disk is then placed on top of the rubber, and a pressure of 1 kg. per
square centimeter is applied through this plate to the paper by means o a
pressure bar. The air pressure in the chamber is then adjusted to 380 mm.,
and the time is taken for the pressure to drop to 360 mm., representing t ie
passage of 10 ml. of air between the paper and the glass sur ace. e
smoothness values have proved to correlate fairly well with printing
quality This test is widely used by government agencies. . .

In the Gurley tester, the paper is clamped between two optica ly flat
metal surfaces having an effective area of 1 sq. in. Pressure is app le y
an unweighted lever arm to place the paper under a pressure o p.s.i.,
which is lower than the pressure used in the Bekk and is one of the reasons
contributing to the lack of agreement between the two testers. O'-dinari y,
eight sheets of paper are tested at one time, which gives an overall, top-to-
bottom average of si.xteen surfaces in one reading. In carrying out ^
tests, the eight sheets are arranged so that all the felt and all t e w-ire si es
are in contact, and then a hole is punched through the pad of sheets. T
test does not differentiate between top- and bottom-side _

instead gives an average of the two sides. The results are usually reported
as the nLber of seconds required for 50 ml. of air to pass between

125 Sold by A. v. d.

126 B. L. Wehmhoff,

Korput-Baarn (Holland) /imcx

Tech. Assoc. Papers 18, No. 1: 337-339 (1935)

XVI.

853

PROPERTIES OF PAPER

surfaces of eight thicknesses of samples. The
of individual readings from the mean lor hurley S-P-S snm
found to be 6.4 for a number of different papers.'-' In the case ot pape
Lrds, smoothness may be tested in the S-P-S tester by placing two speci-
“the top one having a hole punched in it) between two pieces of
rubber (the top one having a hole in it). The paper su ace to e es
should be placed in contact with the corresponding surface on the othe
specimen so that the air flows out between two similar paper surfaces.

In the Williams tester, the air escape is between two paper surfaces,
the other surfaces of the paper being in contact with rubber-faced clamps to
seal off these sides of the sheet. In making the test, the specimen (at least
2 in in size) is folded over so that the felt sides are in contact, the sample
i*s punched with an in. round hole and then is clamped in the tester at a
pressure of 30 p.s.i. The results give felt-side smoothness, and the test
must be repeated with the wire sides in contact to obtain the wire-side
smoothness, using a new sample each time because of the compression of
the sheet which occurs in the test. Results are usually reported as the
numlier of seconds for the passage of 25 ml. of air between the surfaces of
the test specimens. A correction is sometimes made in the results for the
transverse porosity of the paper by taking readings with a thin rubber disk
(having a hole in the center) inside the folded paper sample to seal off all
four surfaces and thus measure only the escape of air laterally through the
sheet structure. This porosity reading is then used to correct the smooth¬
ness reading, as follows:

smoot hness times porosity
corrected smoot ness - rninus smoothness

This correction is not always used, because the relation between smoothness
and plane porosity remains reasonably constant for a given paper. Further¬
more, some chemists do not feel that the correction is a legitimate one. In
making smoothness tests with the Williams instrument, it is necessary to
maintain a constant temperature, not only for the effect on the paper prop¬
erties, but also to keep the air forced over the sample at a constant value.

As a rule, testers of the air-flow type are fairly well adapted to the test¬
ing of paper smoothness. There are, however, several shortcomings of
this type of instalment. The results are more affected by a few large de¬
pressions than by an equivalent area of small depressions because the air
flow is proportional to the fourth power of the surface channels.^*®* This
place? undue emphasis on accidental defects. The flow of air is affected by

*** Institute of Paper Chemistry, Instrumentation Studies, Paper Trade /. 110,
No. 23: 305-309 (June 6. 1940)

*** S. M. Chapman, Pulp Paper Mag. Canada 48, No. 3: 140-150, Convention
Issue (1947)

»»• InrtniroaiUtion Studies, Paper Trade /. 104. No. 13: 188-191 (Apr. 1, 1937)

854

PULP AND PAPER

the depth of the cavities in the paper surface, which is often not indicative
of the suitability of the paper for a certain purpose, for example, in printing,
where very deep cavities are no more harmful to printing quality than
medium-deep depressions.^®® On the other hand, the flow of air is not
greatly influenced by discontinuous irregularities in the paper surface (e.g.,
wire marks), and these irregularities are often important in the use of the
paper.”® In the case of very smooth papers, such as coated papers, the
readings obtained on air flow smoothness testers are principally a measure
of transverse porosity, rather than smoothness, and are therefore affected
by thickness and density of the sample.”^

® Fig. XVI-16. Effect of pressure on the smoothness of paperboard.

Smoothness is greatly affected by the pressure to which the paper is
subjected at the time of testing, since increased pressure tends to level ott
the surface of the paper. In many cases, smoothness readings at low Pres¬
sure are not indicative of smoothness at higher pressures, as illustrated
Fieure XVI-16, which shows two samples of paperboard which have essen¬
tially the same smoothness at 50 lb. pressure, but markedly different smoo
ness at higher pressures.”® In this case, board B printed more satisfacton y
than board A because of its greater smoothness at high pressures, ncreas

130 s. M. Chapman, Pulp Paper Mag. Canada 48, No. 3: 140-150, Convention

131 Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J. U2,

fp. /. 106. No. 25 : 341-350 (June 28, 1938) , , ,

S55

XVI. PROPERTIES OF PAPER

ine the gage pressure on the sample in U.e Williams tester front 10 to W

p.s.i. incases the smoothness reading front ten to ^ws

Figure XVI-17, which was taken from work done by Qiapman, si
the effect of pressure on the smoothness of newsprint measured y
llunr^surLe conuct between the paper and a piece of smooth glass
\ high moisture content increases the smoothness under pressure, eca
dK st. is nu.de softer and more easily flattened. The effect of relative
humidity on the smoothness o( the paper at various pressures is a s

in Figure XVI-17.

Fig. XVI-17. Effect of relative humidity and pressure on the

smoothness of newsprint.

Increased beating of the stock increases the smoothness of the final
jiapcr. Tlie shake on Fourdrinier machines is another important factor in
improving smoothness. The type of wire and type and weave of the felts
used on tlie machine also affect the smoothness. Increased wet pressing
and increased calendering improve the smoothness, but in general, it is
lietter to make the sheet smooth rather than try to calender it smooth.
Calendering a rough paper so that it is smooth creates hard spots in the
paper which tend to swell when the paper is wet (e.g., with aqueous coating
mixtures). Most printing papers are, however, calendered to increase
tlieir smoothness. Filling improves the smoothness, particularly after
calendering. Surface sizing improves smoothness, and pigment coating
has a verj* definite effect on smoothness, particularly if the coated paper is
supercalendered. The type of pulp has an important effect on smoothness,
Groundwood pulps, as a rule, prcKluce rough papers, because of the fiber

Instrumentation Studies, Paper Trade J. 106, No. 4 : 34-48 (Jan. 27, 1938)
S. M. Chapman, Pulp Paper Mag. Canada 48, No. 3: 140-150, Convention
Issue (1947)

856

PULP AND PAPER

bundles. Southern kraft pulps and some of the West Coast pulps produce
rough papers because of the large size of the fiber. On the other hand, well-
beaten spruce sulfite and rag pulps tend to produce smooth papers because
of the plastic nature of the paper at the time of wet pressing.

Smoothness is related to the appearance of the paper because, as a
general rule, a rough paper is unattractive. Paper with excessive wire and
felt marks, or paper which is lumpy, fuzzy, or badly crushed is considered
unsatisfactory. On the other hand, small even irregularities often give a
pleasing appearance of pattern or texture. The most important factor is
the average size of the surface irregularities, although the size, distribution,
and arrangement of the irregularities are also important. Smoothness is of
importance for writing papers where it affects the ease of travel of the pen
over the surface of the paper. It is very important in printing papers be¬
cause of its correlation with printing qualities, and the subject is discussed
further in Chapter XIX. Smoothness is a factor in the pasting of papers.

Formation

Formation is defined as the uniformity with which the fibers are dis¬
tributed in the paper. It is thus a physical property of the paper, although
it is customarily measured by the degree of uniformity of light transmission

through the paper.

Formation is often judged visually by looking through the sheet at a
uniform light source. Naturally, the results of such an examination cannot
be expressed numerically, and it becomes necessary to compare the specimen
with a standard paper of acceptable formation or to rely on the judgment of
the observer. Paper is said to have a uniform or close formation if the
paper has a uniform texture similar to ground glass when viewed in trans¬
mitted light. The formation is said to be poor or wild if the fibers are
unevenly distributed, giving the sheet a mottled or curdled appearance in

transmitted light.

Formation is affected by the transparency of the paper since, in general,
the more transparent the paper, the more readily poor formation shows up.
For example, waxed paper generally appears more poorly formed than the
same paper before waxing. The color of the paper is another factor, since
blue papers generally appear wilder than white or yellow ones.

Formation can be measured by means of an instrument consisting of a
photoelectric cell which scans the underneath side of the paper and a small
beam of light of controlled intensity which is projected on the upper surface
of the paper.The paper is moved at high velocity through the tester

135 Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J. 103,

No. 27: 383-385 (Dec. 31, 1936) n . -r ^ r rn/ XTn 4-

138 M. N. Davis, W. W. Roehr and H. E. Malmstrom, Paper Trade J. 101, No. 4.

43 ^8'(July 25, 1935)

857

XVI. PROPERTIES OF PAPER

so that the amplitude and frequency of variation in ^

measured on the photoelectric cell. Another suggested method of measu
ing formation is to cut small disks (0.25-0.50 in. diameter) out of the paper
at regular intervals (or from obvious thick and thin spots) ^or weig mg on
a sensitive balance. This method is, of course, too laborious for rou

"^Formation is of great importance because it not only affects the ap¬
pearance of the paper, but also because it influences the physical and optical
properties of the paper. Formation affects both the average value and the
uniformity of values of nearly all physical and optical propertjes. Good
formation is essential in writing and printing papers. Paper to be impreg¬
nated or coated should also have good formation. On the other hand,
tissues and blotting papers do not necessarily require good formation.

Uniiormity and Precision of Test Methods

Uniformity is a highly desirable characteristic in all papers. Uni¬
formity in fiber distribution (formation) is necessary to obtain satisfactory
appearance and good strength properties. Uniformity from top to bottom
of the sheet (lack of two-sidedness) is desirable. Uniformity of properties
from one edge of the sheet to the other, and from one lot of paper to the
other, are essential requirements in high quality paper. Uniformity from
lot to lot is particularly essential in the case of papers to be treated with
substances which must be adapted to the paper, e.g., as in pigment coating,
printing, impregnating, or laminating. If the paper in these operations
does not run uniform from sheet to sheet or from roll to roll, considerable
variation in results will be obtained in the converting operation.

Paper is never completely uniform. Different papers vary in uni¬
formity, depending upon the grade of paper and care used in its manufacture.
As a rule, heavy papers and papers made at slow speeds are more uniform
than light papers and papers made on fast machines. The magnitude of the
deviation of the arithmetical average obtained by a series of tests from the
true value of the universe depends upon (J) the inherent non-uniformities
in the sample as measured by the instrument, (2) the irregularities in the
instrument, and (5) the number of readings taken. Statistics have been of
great help in distinguishing between the uniformity of properties in different
parts of the sample, the difference in properties between lots of the paper,
and differences due to test method variability. The subject of statistics is
discussed in Chapter XVII.

The physical tests commonly made on paper differ appreciably in pre¬
cision, and hence the nature of the test must be taken into account in analyz¬
ing uniformity. Basis weight measurements, for example, usually run
quite uniform on specimens taken from the same lot, since this test tends to

858

AND PAnOI

avrr:i|»r tiiii Mimll l«KalimI vnnatituts in The him •

Kivcn jmitcr nuiy IIiua rtui vrry iiriiiortn rvTti whrti thnrr t% cvwn«lrr^ik
variaiuiii in the foniiaiton or trn^ilf >irenpth The methnri <4 nv^%?inj 5 t;
tearing resistance also temls to average out difTermet* because •es'eral
arc torn at the same time. On lire other haixi, (oUmg enduranoe geoenfly
shows wide variation because of the small area under test at any one

Some years ago, the Institute of l^fier Chemistry^- carried oat a study
of the magnitude of variation in the different profienies oi tyfttcal papers of
.‘•tan<]ar<l grades and Iwists weights. Three different grtoies were used, bond,
mimcograjih, and Inlgrr, so that the results arc iodicaiive of those obtained
on fine pajKTS. The probable errvir for each test (the error plus or minus
from the true average within which a test result has an etjual chance of
falling) was calculated and then expressed as a percentage M the average
reading. Proliahle error was calciibted as:

8.85r(V)

X\X-l

The [)cr cent probable errors for eacli grade were then averaged to give an
average for cacli test based on the three grades of ini'er tested. The resuhs,
which arc rejxirtctl in Table XIIl, indicate tlut maximum deviations omtr

TABLE XIII

V’^AtiswuTV OP pHvsiCAi. Pioponts or PAwa

Pkper pr op erty

Opacity—Bausch A. I.omb (contrast ratio) ...

Basts weight—Thwing basis weight Kale (pounds 17 * 22—500t

Caliper—Schopper micrometer (iiKbes-'1.000) ...

Apparent density (basis wcighv'caliper) ..

Bursting strength—Perkins Mutleii tester (pounds square meb)

(per cent points, pound) .......

Tear—Elmendorf tear tester (grams per 16 sheets) .

Tear-across (graniS'''16 sheets) ....

Tensile strength—Schopper hand-driven medd (poonds) .

Tensile strength—across (pounds) ..

Stiffn«s—Gurley stiffness tester (nulligrams) .

Stiffness—across (milligrams) .-..

Per catt stretch—Schopper hand-driven OMdel .

Per cent stretch—across.

Porosity—Gurley densonietcr (seconds) .....

Folding entrance—Schopper (douWe fcJds) ..

Folding endurance—across (doidde folds) ...

0.?)|
0 566
0iS7
0602

1j612

203

2j65

1.93

2j69

304
4 08
408
6.08
83

in all tests, with the exception of thidotess and basis weight."' It was oo^
eluded that deiTalions, in the main, are the result of lack of unifonnitr m

**• Institute of Paper Cbexnistr}-, Instnaaentatioo Stwfies. Payer Trade I. 106,
Na 3: 28-30 (Jan. 20, 1938)

XVI. PROPERTIES OF PAPER

859

the other ^ t, 3 t method variability. They found

leth^^^Xru'A rphTsJ test under consideration and the paper
“‘""itt wdl kUwL to properties of paper vary with

Lst re!u “by the basis weight and report the result as strength ^

cent points This method of reporting tends to correct for sma _

in resets due ,o differences -thod of

tol^^l't^ursting strength, but seems to ^ entirety

1™ at a proportionately greater rate than the incmase m basis weight.

Unsatisfactory Appearance and Dirt

Unsatisfactory appearance is probably the greatest cause for the rejec
tion of paper. The principal cause of unsatisfactory appearance is dirt?
i.e. the presence of minute particles of foreign matter which seriously detract
from the appearance and brightness of the paper. Dirt is particu ar y
offensive in writing and printing papers and in the higher grades of wrapping

Dirt may be defined as any foreign matter which is embedded in the
sheet and has a marked contrasting color to the rest of the sheet (when
viewed by reflected light), and which has an equivalent black area of 0.04
sq. mm. or more. The equivalent black area is defined as the area of a
black spot on a white background which makes the same visual impression
as the dirt speck has on the paper.^^®- Light-colored spots on white paper
or black spots on colored paper have a lower estimated equivalent black area
than black spots of the same size on white paper. Thus, certain specks may
have a low equivalent black area in unbleached pulp but a high equivalent
black area in bleached pulp. Shives and spots which show up only at certain
angles of observation are not considered as dirt.

Dirt is determined by counting the number of dirt specks in a given
area of the paper and by estimating the equivalent black area of the specks.
Standard comparison charts having a series of black spots (round and
rectangular) of different areas on a white background are used for estimating
the equivalent black area of each spot.^®® The test sheet is laid on a white

K. Reitz and F. J. Sillay, Tappi 33, No. 10: 504-506 (Oct., 1950)

138 tappi Standards call for an equivalent black area of 0.04 sq.mm, or more for
paper and 0.08 mm. or more for pulp.

*2* Charts may be obtained from the secretary of the Tech. Assoc, of the Pulp
& Paper Ind., 122 East 42nd St., New York, N. Y.

860

rULP AND PAPER

background and illuminated with a 50-watt frosted bulb placed 4 ft away.
The standard chart should be held over the edge of the Sf^ecimen and all
specks graded by selecting a spot on the standard chart which is equally
noticeable when both are held at a distance from the eve. Both sides of the
sheet should be examined. Shives or spots which do not appear dark at all
angles of observation should not be counted. 7'he results are generally
reported as the total equivalent area of dirt in .square millimeters of equiv¬
alent black area per square' meter of surface examined, i.e., as parts per
million. In some cases, the results are reported as the number of particles
per square foot of surface, but this is less satisfactory than the above method.

The method described above can also be used for measuring dirt count
of pulp. A surface count is considered satisfactory on pulp since it has
been found that the total area of visible dirt on the surface bears a close
relationship to the total dirt in the whole pulp sheet.^^® A quick estimate of
the amount of dirt in pulp samples can be obtained by comparing with
standard samples of known dirt content.

The total dirt count depends upon (1) the type of light used for
examining the paper (i.e., whether reflected or transmitted light), {2) the
type of dirt specks, and (i) the frequenc\' distribution.^' Some specks
are more readil}’ observed by reflected than by transmitted light, and vice
versa, but too much reflected light reduces the total number of specks ob¬
served because of glare and internal reflection. According to Graflf,'** the
largest number of specks and the largest area of dirt per square meter is
obtained when the sheet is examined with a single 20-watt daylight fluores¬
cent lamp, using reflected light and a chinrest at 14 in. Frequency charts’**
which divide the dirt specks into classes according to size are more useful
in summarizing the data and disclosing the type of dirt which is most objec¬
tionable than just reporting the total of all visible specks without regard to
size. Graflf”’ obtained the results in Table XIV for a mimeo bond. These
results show that while the large specks are few in number, they constitute
an appreciable percentage of the total dirt area. On the other hand, small
specks constitute an appreciable part of the total number of specks, but an
insignificant part of the total dirt area.

TABLE XIV

Dirt Count on Mimeo Bond

Area of speck. Percentage Percentage

Class sq.mm. by number area

Small specks . 0.00-0.05 4.59 0.^

Medium specks . 0.05-0.50 91.00 ■

Large specks . 0.50 and up 4.41 ^

J d’A. Clark, Paper Trade J. 96, No. 26 : 27-28 (June 29, 1933)

J. H. Graff, Paper Trade J. 124, No. 26 : 292-298 (June 26, 1947)

XVI.

PROPERTIES OP PAPER

861

Some causes of spots in paper are slime spots, “lored fibers, color
;rptr\SlrdS^^^^^ m: panldes, pieces

Ss; fez'.?;-::’ fe ~

estimation because they are visible only at certam

The Dulp supply is generally the greatest source o i • /u fu v,,

an analyL of standard mimeo bond that over of the specks (bo y

number and weight) are due to resin specks. „„„t5sfac-

It should be pointed out that there are many other causes of unsatistac

tory appearance, aside from dirt. Some of these are as follows: too pro¬
nounced wire marks, marking caused by seams or bad spots m the wire
wrinkling or cockling of the sheet, crushing due to excessive pressure at
wet end, calender cuts, calender blackening, fish eyes, foam spots, slime
spots, ragged edges, presence of shavings and paper dust, and grainy edges.

Tivo-Sidedness

Paper never has identical appearance and surface characteristics on
both sides of the sheet. This difference in appearance of the two sides of
paper is referred to as two-sidedness. All pape'rs have a certain amount
of two-sidedness in that the top side of the paper is smoother and has a
closer formation than the wire side.

Two-sidedness may be an actual structural two-sidedness due to greater
concentration of size, pigment, or fines on one side of the sheet, or it may be
an optical two-sidedness caused by a difference in the finish or reflectance
on the two sides of the sheet. Most papers, particularly thick papers, show
definite two-sidedness in regard to fibrous composition. This is usually
manifested in a preponderance of larger fibers on the wire side and a pre¬
ponderance of finer, smaller fibers on the felt side. • White papers sometimes
show a higher brightness on the wire side than the top side, due to the greater
removal of the dark-colored fines from the wire side.

Two-sidedness may be caused by one or more of the following factors:
(i) loss of fine fibers through the wire before a mat of the larger fibers is
laid down; (2) slower settling of the fine fibers, due to their reduced specific
gravity and hydrodynamic resistance, with the result that a higher per¬
centage of fines are deposited on the top side of the sheets; {3) removal of
the finer fibers from the underside of the wet sheet after formation, due to
the action of the table rolls and suction boxes. Increased machine speeds
have increased the amount of two-sidedness. Smith^^® has shown that two-
sidedness is not due to increased suction at the suction boxes, but instead is

J. H. Graff, Paper Trade J. 124, No. 26 : 292-298 (June 26, 1947)

J. F. Smith, Paper Trade J. 129, No. 21: 468-470 (Nov. 24, 1949)

862

PULP AND PAPER

caused by a washing or flooding of the underside of the wet sheet each time
the sheet passes over a table roll. Varying the amount of suction (in making
hand sheets in the laboratory) showed that increased suction actually results
in a preponderance of fines on the wire side, the opposite effect from that
obtained on the paper machine.

The composition of the original pulp is a factor in the amount of fiber
two-sidedness. As would be expected, paper made from pulp of uniform

fiber length tends to show less two-sidedness than paper made from a pulp
having a wide variation in fiber length. An examination of a typical bond
paper show’ed about 18% fines on the felt side and about 11% fines on the
wire side,^^® The effect of the greater concentration of fine material on the
felt side is (f ) to increase the bonding strength, (2) to increase the smooth¬
ness, and (i) to improve the writing and printing qualities of this side of
the paper.

Colored two-sidedness exists where there is a greater depth of color on
one side of the sheet. This condition may be due to a number of causes, but
it generally occurs at the same time as structural two-sidedness because of
the greater concentration of more highly dyed fiber fines on the felt side of
the paper.^^® The subject of color tw'O-sidedness is discussed at greater
length in the chapter on coloring.

t Paper is often two-sided in regard to sizing. Lee^^’ was able to show,
in the case of papers two-sided in sizing (as measured by the pen and ink
test), that there is a difference in the amount of rosin on the two sides of the
paper. The usual condition is for paper to show' a good size test on the felt
side and a lower test on the wire side, particularly if the rosin size has been
precipitated in the form of relatively coarse particles and the paper made on
fast machines (see Ch. X).

Nearly all pigment-filled papers have a higher concentration of pigment
on the top or felt side of the paper, due to preferential removal of pi^ent
from the wire side during formation (see Ch. IX). This results in an
appreciable difference in color, brightness, or finish benveen the two sides

of the sheet.

Two-sidedness is ordinarily undesirable, since it results in a difference
in surface properties between the two sides of the sheet which affects the
utility of the paper. In some cases, how'ever, two-sidedness is intentionally
sought after for reasons of economy or improved product. For example,
paperboards made on a cylinder machine are often made intentionally two-
sided by using different grades of stock in the liner and filler plys. In this
way, cheaper stock can be used in the filler and more expensive stock used

1^5 o. Kress and E. J. Loutzenheiser, Paper Trade J. 114, No. 11: 133-136 (Mar.

12 1942)

E. R. Laughlin, Paper Ind. 24, No. 7 : 709-711 (Oct., 1942)

H. N. Lee, Paper Trade J. 103, No. 27 : 386-^90 (Dec. 31, 1936)

XVI.

PROPERTIES OF PAPER

863

in the outside plys. The filling and sizing is often different in the various
plys Calender coloring of paperboard and the lamination of paper to one
s^e of paperboard are other examples of a deliberate attempt to produce a
-two-sided" product. In the case of certain Fourdrimer pape^a two-s.ded
paper is intentionally produced by using a secondary head box for app y
ine stock to the top of the sheet on the paper machine wire at a point where
thf sheet is set on the wire side, but is still sufficiently wet on the top side o
allow comingling of the fibers. In this way, a marked difference in the
properties of the two sides of the sheet can be obtained.

Dhfiensional Stability

The change occurring in the dimensions of paper with changes in the
moisture content is an important consideration in the use of the paper. A
papers expand with increasing moisture content and contract with decreasing
moisture content, but the rate of change and the extent of change vary with

different papers. .

Dimensional changes are due to two factors: (1) the drawing together

(or pulling apart) of the fibers and (2) the contraction (or swelling) of the
individual fibers. Both are involved in ordinary dimensional change, but
the movement of fibers in relation to one another is a more important factor
than the expansion and contraction of the individual fibers. Much of the
actual fiber movement is taken up internally in the paper, but some is trans¬
mitted into dimensional change. The amount involved in dimensional
change is determined in large part by the way in which the sheet is formed,
particularly by the amount of tension on the sheet during drying. Paper
expands much more in the cross direction than in the machine direction, in
part because of the greater expansiveness of the fibers in the cross direction
and more importantly, because of the greater expansiveness built into the
paper in the cross direction on the paper machine. The type of pulp used,
degree of beating of the pulp, and the effect of fillers and starches are other

factors influencing the expansiveness of paper.

Some of the grades of paper requiring a high degree of dimensional
stability are printing papers, map papers, wallboards, machine-sorted classi¬
fication cards, and recording papers. Because of the great importance of
dimensional stability in printing papers (particularly offset), the subject is
discussed at greater length in Chapter XIX. Aluminum-mounted papers
are sometimes used in making map papers where expansion must be held
to a minimum.

The amount of dimensional change can be measured by a suitable
device which measures the length of the paper before and after conditioning
to a definite moisture content. This can be done by clamping strips of

»«G. R. Tennent, Paper Ind. 27, No. 9: 1357-1358 (Dec., 1945)

864

PULP AND PAPER

paper (24 in. in length) between a fixed clamp and a loading clamp which
applies a tension to the paper of 50 g. per inch of width. The change in
length of the specimen during the period of the test can be measured by using
a suitable magnifying device. The device is located in a cabinet equipped
with a fan and a tray to which various salt solutions can be added for con¬
trolling the humidity.^^® In carrying out the test, the specimen is first con¬
ditioned at a relative humidity of 50^ or lower, and then at 80^ or above.
The length is measured at the higher humidity, and the change in length is
noted as the sample is conditioned at successively lower humidities down to
a humidity of 50%. A curve can be plotted showing change in length
versus relative humidity. The final result is reported as the percentage
change of length on decreasing the relative humidity from 65 to 50%.

Expansion and contraction of paper is greater for beaten than for un¬
beaten stock. It is greater for papers made from chemical pulp than for
papers made from mechanical pulp.^®® On an equal strength basis, ex¬
pansiveness is greater for papers made from wood pulp than for papers made
from rag fibers, because more beating is required in the case of wood pulps
to develop the desired strength. Expansiveness is reduced by the presence
of clay or other fillers,”^ and by the presence of plasticizers.^®^ The expan¬
sion which takes place when the relative humidity is changed from 25 to
65% is“shown in Table XV for several different papers.^®® As can be seen

Tablet

TABLE XV

Effect of Moisture on Expansion of Paper
Relative huinidity changed from 25 to 65% at 25 C.

Per cent increase in extension

Type of paper

Bond

Cross

Machine

Ratio of cross to
machine direc¬

direction

tion expansion

0.35

0.13

0.38

0.12

3.16

0.33

0.16

2.06

0.47

0.10

4.7

0.31

0.13

2.3

0.48

0.10

4.57

0.95

0.16

5.93

from these results, the ratio of cross direction to machine direction expan
sion is always greater than 2 to 1. The rate of change of dimensions is e

149 See TAPPI Standards

««S. F. Smith, Paper-Maker 1J9. No. 3: 185-192 (Mar., 1950)

151 C. G. Weber, M. B. Shaw and M. J. O’Leary, Paper Trade J. 11/, Mo. 10

147-152 (Sept. 23, 1943)

162 N. R. Pike, Paper Trade J. 102, No. 5: 63-66 (Jan. 30. 1936)

>63 G. Laroeque, Pulp Paper Mag. Canada 37: 199-209 (Mar., 1936)

XVI.

865

i>roi*erties of paper

pendent upon the porosity of the paper. Porous papers ('-S-

Sange in dimensions more rapidly than non-porous papers (e.g., glass

As mentioned above, the origin of paper expansivemess is m *e steams
built into the paper when the natural shrinkage of the paper is resiste
t ing drying A part of this dried-in strain is permanently recoverable
bv wetting and redrying the paper. If the wetting and drying cycle is
repeated, further contraction takes place, although the per cent ^ntrac ion
decreases with each successive cycle. The results in Table XV were
obtained by Smith^^^ on a sulfite paper. These changes are irreversible.

TABLE XVI

Effect of Wetting and Drying on Contraction of Paper

Wetting and drying cycle

Per cent contraction

1.80

0.55

0.30

Irreversible changes in dimensions also occur when paper is subjected
to high humidity, i.e., 90% or above, for a period of time. The amount of
contraction obtained on four different papers which were measured at 50%
R.H., conditioned at 90% R.H., and then remeasured again at 50% R.H. is
shown in Table XVIThese results show that the dimensions of paper

TABLE XVII

Effect of High Humidity on Contraction of Paper
Per cent contraction which takes place at 50% R.H.
when paper is subjected to 90% R.H.

Paper

Cross direction Machine direction

News . 0.15 0.09

Bond . 0.07 0.20

S.C. Book. 0.17 0.08

Offset . 0.12 0.08

on desorption of moisture are smaller than those obtained on absorption of
moisture, in spite of the fact that the moisture content of the paper is higher
in the former case. This reduced expansion in the face of higher moisture
content is attributed to the gradual release of strains in the paper at high
humidities. Results obtained by Larocque^®® on several different papers

S. F. Smith, Paper-Maker 119, No. 3: 185-192 (Mar., 1950)

G. Larocque, Pulp Paper Mag. Canada 37: 199-209 (Mar., 1936)

866

rULP AND PAPER

are shown in Figure XVI-18 where it can be seen that the various papers
were extended less on desorption than on absorption for a given moisture
content. This effect was noticeable in both the machine and cross directions
of the bond and greaseproof papers, but only in the machine direction of the
newsprint. Once the strains are released by subjecting the paper to a high
humidity, subsequent expansions and contractions take place in a more
reversible and reproductive manner.

BOND GREASEPROOF NEWSPRINT

PER CENT RELATIVE HUMIDITY

Fig. XVI-18. Effect of relative humidity on per cent expansion of paper for

absorption and desorption of moisture.

Curl is a problem closely related to dimensional change. Curl is
affected by the dimensional stability, uniformity of the paper, thickness of
the paper, and amount of two-sidedness. There are several forms of curl.
curl to the wire side of the sheet, curl to the felt side, and curl partly to the
wire side and partly to the felt side. Curl is caused by uneven expansion

or contraction of the paper.

Curl is more pronounced with thin papers than with thick papers, be¬
cause thin papers have less physical resistance to curl. Papers made from
long, highly beaten fibers are more likely to curl than papers made from
short, free fibers, because greater tensions and unrelieved stresses are set up
in the former case. The softer and more flexible the paper, the less its
tendency toward curl. A dense paper will curl under conditions where a
soft, porous paper will remain practically flat. Moist papers tend to curl
proportionately less than papers which are very dry. Loading, as a rule,
decreases curl, since it reduces the overall cohesiveness of the paper.
Rosin-sized papers seem to curl more than unsized papers. Other con¬
ditions causing curl are faulty machine conditions, such as dirty felts or
poorly ground press rolls. Mechanical breaking of the paper as a result
of sharp bending may disrupt the fibers or break the surface layer and

produce curl. i u* i

In general, four types of curl are recognized: {1) inherent curl, which

XVI. PROPERTIES OF PAPER

867

is built into the paper and shows up fg|^g‘;‘’(“)'''st7uctural

rouglily *e same as ’“fjerOTles hi the Lo ’surfaces of the

7aper'"'w moisture curl, 7hichis the result of dimensional change caused

1Xm.taTcraTs—al curl are caused by much the same
factors. These factors involve conditions which result m two-si ^
in some way create a built-in strain in the paper. Inherent cur i
m»"o correct. It is due to uneven drying or finishing, uneven rosin
Sace siring, uneven loading, or by the natural difference m cohesiveness
betwet thTwhe and felt sides of the paper. The most -m-n tj^e o
inherent curl is curl of the grain edge toward the wire side with axis of the
curl parallel to the machine direction.'- This is caused by uneven stress
on the two sides of the sheet which is later released upon sheeting. An¬
other less frequent type of inherent curl is curl of the cross gram edges
toward the wire side. This is caused by tensions built into the sheet in the
machine direction, and since the wire side has the more pronounced gram,
the contraction is greater on that side when the tensions are released.

Water-finished or calender-sized paperboard, upon removal from ^ e
paper machine, tends to curl toward the side which received the most mois¬
ture in the sizing operation. Hence, moisture addition in calender sizing
must be controlled by regulating the number of water boxes used or by con¬
trolling the degree of sizing in the top and bottom surfaces of the board.
Curl can be controlled in the drying operation by reflation of the steam
to the various drier rolls. For example, if the underside of the paperboard
is being dried more than the top side (due to the admission of more steam
to the upper row of driers), built-in strain results on the underside. This
arises from the fact that the underside of the paper is not free to contract,
due to the tension on the paper, whereas the top side, when it dries out later
to the same moisture content, is under no tension and can freely contract,
with the result that the sheet curls toward the upper side.^®® As evidence
that surface contraction is the cause of curl. Smith has shown, in the case of
paperboard, that the measured shrinkage of the top liner (which was de¬
tached from the rest of the board by inserting tissue paper between the first
and second plies on the machine) is the same upon wetting as the shrinkage
calculated from the curl by use of the following formula;

j = lOOf/y

where, ^ is the percentage contraction of the liner, t the thickness of the
board, and y the radius of curl.

1558 ^ E Erspamer and W. D. Rice, Paper Mill News 73, No. 28: 76-78, 88
(July 15, 1950)

156 s. F. Smith, Paper-Maker 119, No. 3: 185-192 (Mar., 1950)

868

PULP AND PAPER

Paper sometimes takes on a permanent curl after it has been subjected
to tension for a long period of time. This is caused by a phenomenon re¬
ferred to as micellar creep which causes an expansion in the outside layer of
the sheet. This phenomenon occurs when paper is stored in roll form, and
is more pronounced in the inner part of the roll. Reel curl is an example of
this type of curl.

Moisture curl is related to the dimensional stability of the paper.
Paper exhibits moisture curl when the two sides of the sheet expand or con¬
tract unequally on account of uneven absorption of moisture. Wrinkling
and wavy edges result from the localized absorption of moisture which sets
up localized strains in the paper. This freqeuntly occurs when paper is
piled in a moist atmosphere or is moved from a cold warehouse into a warm
room. Even if paper is exposed to the same atmospheric conditions on both
sides, uneven expansion may result because of differences in the sizing,
coating, or amount of finish on the two sides of the paper. In this way,
moisture curl is related to structural curl. As a rule, hard-sized papers are
more susceptible to moisture curl than soft-sized papers because they absorb
moisture more slowly and take longer for the internal forces to equalize
themselves. In general, the wire side of paper has greater expansiveness
than the felt side, and consequently nearly all papers curl toward the felt
side if too moist and toward the wire side if too dry. The axis of the curl
is always parallel to the machine direction of the paper, except in the case
of papers showing excessive reel curl or in some forms of inherent curl.

Paper wetted on only one surface, as, for example, paper coated with
pigment on only one side, tends to curl toward that surface on drying.
Curl, in this case, can be attributed to the release of built-in strain on the side
of the paper which is wetted, thereby resulting in a disproportionate con¬
traction of that surface upon drying. In the case of pigment-coated papers,
the water in the coating mixture is responsible for most of the curl, but part
of the curl can be attributed to the shrinkage of the adhesive on dr>'ing of

the coating.^®®

Even if one-side pigment-coated paper is flat after coating, trouble with
curl frequently occurs at high humidities. Curl, in this case, is caused by
the differences in the hygroscopicity and in the coefficients of expansion and
contraction between the fibrous surface and the coated surface, that is, the
layer of coating is physically more uniform and tends to change more or less
equally in all directions, whereas the uncoated surface undergoes a greater
change of dimensions in the cross direction than in the machine direction.
A coated one-side paper may, under some circumstances, curl less than an
uncoated paper if the coating counteracts the greater expansiveness of one
side of a normally two-sided sheet. The hardness of the coating an t^e
amount of coating on the paper affect the amount of curl. Coated two-si e

XVI. PROPERTIES OF PAPER

869

moer eenerally curls less than uncoated or coated one-side paper because o

papers), ping of pap fcarbTn papers,
coating of paper. Curl is a Irequeni nans': f ^

Gummed papers are also mg y P ungummed

toward the gummed side d the ^ help the situa-

oI somewhat. In the case of certain label papers, curl is desirable p is

sometimes intentionally imparted to the paper in ,"‘"^he Lb-

Lee a curl toward the side of the label going against the bottle. The suD

ject of curl is discussed further in the chapters on

by allowing a specimen of the paper to fall The

StTrLorLd as the maximum curvature in degrees per centime er of
arc. The results are obtained by adding the average of the tea 'ngs ° _

fixed angle of 30°, which the other end of the specimen makes with the
horizontal, and dividing by the length of arc of the wetted, curled pt o
the specimen in centimeters. The result is an indication of the tendency

of the paper to curl.

Cockling (localized buckling of paper) is due to local expansion or con¬
traction of the paper. Cockling can be attributed to local variations in basis
weight, moisture content, density or other properties of the paper upon
leaving the paper machine which result in local built-in strains in the paper.

Optical Properties

The optical properties of paper are as important, or more important,
than the physical properties of paper. Some of the grades of paper where
optical properties are of primary importance are printing papers, transparent
wrappings, and writing papers. An understanding of the optical properties
is necessary to understand such important paper properties as opacity,

transparency, brightness, gloss, and color.

The optical properties of paper are determined by the relative amount
of light reaching the paper and the manner in which the incident light is
reflected, transmitted, and absorbed by the paper. The optical properties
are affected by the optical properties of the materials in the paper. Some of
the important factors are: (J) degree of whiteness of the pulp, (2) presence
of white pigments (fillers or surface coatings), (5) presence of dyestuffs
or colored pigments, (^) method of stock preparation and sheet formation,
and (5) presence of minor ingredients (e.g., rosin and starch).

157 See TAPPI Standards

PULI’ AND PAPER

Nature of Light

It was established years ago that white light is cotn|)Osed of a spectrum
of radiations of electromagnetic waves of wavelength between 400 to 70Q
millimicrons, each wavelength or wavelength range being associated with
a different color or hue. When the wavelength range is very small (e.g.,
1—2 millimicrons), the light is known as monochromatic light. Mono¬
chromatic colors can be represented, roughly speaking, by the following
wavelengths (millimicrons) :

Violet . 400-450

Blue . 450-500

Green . 500-570

Yellow . 570-590

Orange . 590-610

Red . 610-700

These are only approximate values, since colors vary from wavelength to
^wavelength. Radiation smaller in wavelength than 400 millimicrons is
called ultraviolet light, whereas radiation greater in wavelength than 700
millimicrons is called infrared; both of these are invisible to the human eye.

Before undertaking a discussion of the optics of paper, it is necessary
to define certain of the terms used in connection with light measurements.
A few definitions are given below:

"The candlepowcr is tlie unit of luminous intensity of the light source.

Luminous flux is the time rate at which radiant energy is transmitted.

Illumination is the density of luminous flux per unit of area, i.e., the quantity of
light that falls per second upon a unit area of illuminated surface.

Brightness refers to the light reflected from the surface; it may be expressed m

terms of flux or in terms of intensity. . . • • i

The reflectivity is the ratio of the light reflected from a surface to the light incident

upon it.

Absorption of Light

Absorption of light refers to the capacity of paper for converting light
energy into other forms of energy, usually thermal. Since all the light
falling on paper is either reflected, transmitted, or absorbed, the amount of
light absorbed can be measured indirectly by subtracting the sum of the
reflected and transmitted light from the amount of incident light. The
absorption coefficient, which measures the degree of conversion of light
energy into other forms of energy, is influenced by the chemical nature o
the sheet constituents (lignin content, dyestuffs, pigments, etc.), an to a
lesser extent, by the degree of bonding and index of refraction of the sheet.
Forni^^® obtained a straight-line relation between the specific absorphon
coefficient and the lignin content of paper up to about 1.85% hgiiin. i e

P. A. Forni, Paper Trade J. 119, No. 11: 108-112 (Sept. 14, 1944)

XVI.

properties of paper

S71

,,r«mce dy.«uffs in ,he sh«t usually results in a selective absorption of

Reflectance of Lujht

When light strikes a smooth, optically flat surface, a high
.He i«rnt'’lt is reHected at an

“ contpute the^raction of the incident 'jg-t

-I^ive index is known.-* Highly finished papers exhibit ^

reflection, although the amount of regular reflection is consid y
than that exhibited by highly polished glass or metal surfaces.

Rpgubr reflection always takes place from the uppermost pa ^

surface^In the case of a perfectly matt surface, the light penetrates beyo
the surface into the body of the material where the light is scattere ap¬
proximately equally in all directions so that it emerges as a semicircle o^

diffused light. The amount of this diffused light

of incidence and the index of refraction of the material. Most of the light
reflected from a paper surface is diffused light. In the case of glossy papers,
the distribution of regularly reflected light about the angle of regular re¬
flection is represented by a hump in the reflection distribution curve.

Reflecting-measuring instruments are of two types: (1) those which
measure specular (regular) reflection (glossmeters), and (2) those w ic
measure diffuse reflection (brightness meters, opacimeters, and spectropho¬
tometers). The sum of the specular reflectance and the diffuse reflectance
nukes up the total reflectance. Color or brightness readings must be evalu¬
ated on the diffuse reflection, care being taken to eliminate the glare of
sftecular reflection, since it is only the diffuse light which penetrates into the

surface of the paper far enough to define the color.

It is customary, when making reflectance measurements, to denote the
reflectance of the paper when backed by the appropriate body. The body
used for lucking is denoted by the use of subscripts. Reflectance (^) may
be reported in any one of the following ways:

(!) which is the reflectance when a single sheet is backed by a black cavity.

(2) Rm, which u the reflectance of a pad of the test paper thick enough so tliat no
change in reflectance occurs when the backing is doubled. (When this reading is made
at a wraeelength of 457 millimicrons, it is known as papermakcr's brightness.) This
is an intrinsic characteristic of paper.

(J) Rm% which is the reflectance of a single sheet when backed with a standard
white body haring an ab«olute reflectance of R'.

R. S. Horrter, *TTie Gloasmeter," Science Section Circular, 456, National
Paint, Vamisb and Lacquer Association, Washington, D. C. (1934)

***R. S. Hunter, Paprr Trad* J. 100, No. 26 : 335-337 (June 27, 1935)

872

PULP AND PAPER

All reflectance values given the designation R should be absolute values
based upon values obtained on a primary standard. A primary standard
can be prepared by depositing a thin film (0.5 mm. or more in thickness) of
magnesium oxide on the surface of a suitable base material by exposing the
base material to fumes of burning magnesium. Magnesium oxide prepared
in this way has a highly diffusing surface and a total light reflectance of about
97 to 98^ ; the reflectance varies less than 1% with wavelength. The oxide
is fairly stable, although it tends to become slightly yellow with time, result¬
ing in a decrease in the reflectance of the light of wavelength less than 550
millimicrons. A block of magnesium carbonate can also be used as the
primary standard if it is freshly prepared by scraping. It has a reflectance
of 97.5% in the red and green, and a reflectance of 96.5^ in the blue.

In reporting results in absolute reflectance, it is necessary to multiply
reflectance readings based upon magnesium oxide as the standard by the
absolute reflectance of magnesium oxide, i.e., 0.97. Thus, assuming that
the reflectance of a paper sample was found to be 85% relative to magnesium
oxide as_^100%, the absolute reflectance of the paper can be obtained by
multiplying 0.85 by 0.97, the absolute reflectance of magnesium oxide. For
regular routine testing, secondary working standards calibrated to an ab¬
solute reflectance based on freshly prepared magnesium oxide are generally
used. These may be enameled iron, opaque glass, or glazed tiles. Reflec¬
tance readings made on an absolute basis can be related mathematically by
using simple calculations, as will be shown later.

Color of Paper

One of the most important optical properties of paper is the color of
of paper. Color has already been discussed from the standpoint of the
coloring process, but the properties of colored paper are discussed in the

following sections.

A perfectly white or a perfectly gray body reflects light of all wave
lengths to the same degree. A colored (opaque) body preferentially reflects
light in one or more of the wavelength ranges, due to greater absorption o
light in the other wavelength ranges. For example, paper colored wit
methyl violet shows preferential absorption of light in the wavelength range
between 550 to 600 millimicrons, which is the yellow part of the spectrum.

It should be emphasized that every dyestuff and pigment acts by ab¬
sorbing light, never by increasing the amount of reflection. Exceptions are
the very bright white pigments (e.g., titanium dioxide) which are brigh er
than the paper, and certain dyestuffs (called white bleaches or w iite yes^
which absorb practically no visible light, but do absorb ultraviolet light
which is re-emitted as visible light. In general, however, the stronger ^
color, the greater is the dulling effect. The amount of dulling resulting fro

XVI. PROPERTIES OF PAPER

873

use of a particular dye depends upon the amount of light whtch the dye

in^^ even thewst

blues, whichare probably the dyesfuffsabsorb

S,r« i'r s- rr“i;»fs

L reflection of red light. a\s a result, they usually produce brighter sheets
L gr^dyc^^ strongly absorb red and blue light, and also an

appreciable amount of the green light. n.vcho-

^ Methods of Measuring Color. Color may be measured by psycho

logical, physical, or by psychophysical means.'** ’? "f “

liurfd optical instruments such as the recording spectrophotometer.
Psychophysical properties are measured physically, but interpreted psyc o
loeicallv, using tristimulus values.

Color belongs to the realm of the psychologist, as well as to the realm
of the physicist and chemist. Godlove and Laughlin'**- illustrate the d|f-
ference in interpretation between the psychologist and the physicist by e
use of a sample of gray paper which, psychologically speaking, appears to
be lialfway between a white (which reflects lOO^o of light) and a black
(which reflects no light), but when examined by the instruments of the

physicist, is found to reflect only 18^ of the light.

Color is not a property of paper; it depends upon the following factors:
the spectral characteristics of the illuminant, the geometrical conditions of
illuminating and viewing, the spectral reflectance of the specimen, and the
characteristics of the observer’s eyes. The only color characteristic which
is the same under all circumstances is the spectral reflectance of the speci¬
men.

The reflectance of light from a sample of paper is a function of the
scattering and absorption characteristics of the paper, the thickness of the
paper, and the reflecting characteristics of the background. There is a
minimum thickness required to obtain the true color of paper. The reason
for this is tliat there must be ample opportunity for light to penetrate the
paper in order for absorption to take place. Since the relative reflectivity
of light of diflferent wavelengths depends upon the thickness of the sheet,
the resultant color depends to some extent upon the thickness of the paper,
lewis'** points out that unfilled papers of both high and low opacity must
be about 0.2 to 0.8 mm. in thickness (about 10-40 fiber diameters) to pro-

R. S. Hunter, “Photoelectric Tristimulus Colorimetry with Three Filters,”
U. S. Dept. Commerce, Natl. Bur. Standards, C429 (July 30, 1942)

*•* I. H. Godlove and E. R. Laughlin, Paper Trade J. Ill, No. 17: 21^223 (Oct.
24, 1940)

I_ C. Lewis, Paper Trade /. 103, No. 22 : 323-330 (Nov. 26, 1936)

874

PULP AND PAPER

duce 99% of the true reflectivity. Coated papers have their final color at
from 0.15 to 0.20 mm., whereas papers loaded with pigment develop their
full color at a still lower thickness.

Increased beating appreciably increases the depth of shade or color in-

tensity, because it decreases the number of air-fiber interfaces at which re¬
flection takes place, thus increasing the opportunity for increased absorption
of light by the dyed fibers. Calendering increases the depth of shade by
about 10 to 15% for the same reason.

Measurement of Color by Eye. Color may be defined as the psycho¬
logical sensation produced in the brain of the observer by the action of
visible light on the retina of the eye. The interpretation is different for

400 500 600 700

WAVELENGTH,

Fig. XVI-19. Comparison of three illuminants for
energy per unit of wavelength.

each obsei^f since some people are more sensitive to color than others
and, in fact, a'few are totally color blind. The results of color matching are
influenced by the mood and health of the observer, although most people are
in fairly good agreement about the matching of colors. A trained observer
can distinguish minute variations in shade which are difficult to measure

with the finest optical instruments.

Color depends as much on the spectral energy distribution of the i -

luminarft as on the reflecting characteristics of the paper. Ordinary incan¬
descent lamps have a high percentage of yellow light, but are deficient m b ue
light The principal illuminants used for viewing paper are north sky light
and special fluorescent (daylight) lamps. The ideal illufninant would be
one having equal brightness at all wavelengths, but such an illuminan is
not available. Comparison of three common illuminants is shown in Figure
XVI-19.^“'’® Sunlight has a nearly uniform distribution of energy throug
out the spectrum. North sky light, on the other hand, has high brightness

103 a Institute of Paper Chemistry, Instrumentation Studies, Trade J. 10 .

No. 18: 285-291; No. 19: 29:1-306 (Oct. 28, Nov. 4, 1937)

XVI, PROPERTIES OF PAPER

875

in the violet end of the spectrum and low brightness in the red end. 1 he im-

L illuminrting light can be illustrated by exanun.ng a green

paper under ordinary white light and under red ig .

near ?reen in ordinary light because it absorbs more red and blue light than
green! but the same sheet will appear red when illuminated by monoc ro-
maticVed light because it reflects a part of the red l^ht. ^or t ^
given above, the color of paper is generally slightly different
in daylight than it is when examined under artificial light, P »

a cerLin paper may appear purple in daylight, appear almost red under

tungsten light, and almost violet under north sky light.

The color sensation measured by the eye depends upon the enei gy is-
tribution of the light entering the eye. The spectral energy distribution
of light reflected from a colored body is determined by im tip ying le
amount of energy present in the light source at each wavelengt y t le
fraction of the incident light reflected at the corresponding wavelength.
The eye is not capable of analyzing for energy distribution; for examp e,
the eye does not distinguish between a monochromatic red and a red com¬
posed of heterogeneous light.

The eye is very sensitive to small differences in hue (wavelength), but
is less sensitive to depth or strength of color. Accuracy m estimating
strength varies with the color, being least in the yellow and greatest in the
red and blue range, but is generally considered to be about The

sensitivity of the eye to wavelength is greatest in the yellow-green range
and decreases to practically nothing in the violet and red range. In other
words, for the same intensity of radiant flux, that is, same amount of energy
radiated per unit of area, a yellow light will produce much more illumina¬
tion (luminous flux) than a red or a blue light. Thus, dyestuffs which
show great differences in reflection in the extreme violet or red range have
very little visual effect on the eye. The relation between luminosity (visi¬
bility) and wavelength is shown in Figure XVI-20, which is the accepted
curve for the spectral sensitivity of the average human eye. The eye is
particularly valuable in judging two samples differing only slightly in shade
and strength, and since this is the case in most commercial color matching,
visual color matching is generally quite effective.

In visual grading, glare should be avoided by viewing the specimen at
an angle of 45° to the incident light; if the light falls on the sheet at 45° to
the normal, the sheet should be viewed on the normal, and vice versa.

Expressing Color in the Mimsell System. There have been many at¬
tempts to express color in terms which can be understood by anyone. One
of the most used systems of specifying color subjectively is the Munsell sys¬
tem, which specifies three properties of color, hue, value, and chroma.

G. G. Taylor, Paper-Maker 118, No. 2: 99-105 (Aug., 1949)

876

PULP AND PAPER

Hue is determined l)y the dominant wavelength of the light reflected
from the paper; it is represented in the Munsell system as an angle on a
vertical plane. Value measures the total amount of light reflected to the
eye; it is represented in the Munsell system by the distance above the hori¬
zontal plane which varies from black on the bottom plane to white on the
top plane. Gtroma measures the per cent departure from a neutral gray
of the same value; it is represented by the distance from the vertical axis
whichi varies from neutral gray at the center to the pure colors at the ex¬
treme distance from the axis.

500

wavelength,

700

Fig XVI-20. Sensitivity of human c>'e and photocell to ^ifferent
wavelengths of light for equal intensities of radiant flux.

The chromaticity of paper is specified by hue and chroma, " 'do'

"''S.: S'—»3

principd^y from the standpoint of vishal or subjective measurment

r ;^ssihie to define color from the and^

of reflectance compared to some standard, w en

unknown are illuminated with a specific luminant and v le .

coitions. In the system developed by ‘"e InternaUo^l
Illumination (I.Cl. System), magnesium oxide is used “ ^ , y

is arbitrarily riven the value of 100. Instruments for ^

differ in three respects, method of illurninatiom, geometry of illuminating

viewing, and photometry. .u r t,f rraoTr he either monochromatic,

In regard to spectral features, the hght njay be e

approximately monochromatic, or non-monochromatic. .

XVI. PROPERTIES OF PAPER

optical studies on Paper and is absolut^y essential

lye carefully specified. The illumination upon a surface inclined to the direc¬
tion of the light rays is less than it would be if the surface were perpen¬
dicular to the light rays. Illumination is directly proportional to the cosine
of the angle which the incident ray makes with the normal, and the bright¬
ness (luminous intensity) of emitted light is proportional to the cosine of
the angle between the direction of the reflected ray and the normal. ossy
surfaces do not, of course, follow these laws. Illumination at 45° and nor¬
mal viewing are specified for reflectance measurements in the International
Commission on Illumination System. The distance between the light source
and the object is important, since the intensity of illumination varies in¬
versely as the square of this distance.

Photocells are used in optical instruments for measuring spectral re¬
sponse. Photocells produce a measurable photoelectric current which de-
jiends on the illumination or radiant flux incident upon the photocell sur¬
face and the spectral response of the cell. The spectral sensitivity of a
photoelectric cell is shown in Figure XVI-20 in comparison with the hu¬
man eye.^®* It can be seen from this figure that the cell is more sensitive
than the eye in the blue and red regions, although filters can be used to make
the spectral sensitivity more closely approximate that of the eye. There are
two principal methods used with photocells for measuring the intensity of

A. W. Thomas, Colloid Chemistry, McGraw-Hill Book Co., New York, N. Y.
fl9^)

’««D. H. Oewel, /. Optical Soc. America 31: 521-527 (1941)

Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J. 105,
No. 18; 285-291; No. 19 : 293-306 (Oct. 28, Nov. 4, 1937)

*•* These curves were taken from bulletin, “Technical Data,” Western Photronic
Cell, Western Electrical Instrument Corp., Newark, N. Y.

rt’LP AND PATFR

lijihl rpllcctpd frcitu tlic satnplc ami Iroui tlic stniuiarcl. ihc dircci Trading
method and the null method. I'lic direct reading methml i.'s l>ase<l on a dirrri
reading of the idiotocurrent produced by the cell on the assumption that
this ])hotoelcctric current is directly projKsrtional toithe reflected light flux,
'riiis relationship is not completely valid, however, which makes it desirable
to use calil»ration curves for the particular instrument set-up when this type
of arrangement is used. 1 he null method is based on the balancing of two
photocurrents by varying the amount of incident or reflected liglit flux to
compensate for differences in reflectance of sample and standard. A num-
l)er of devices can be used to control the amount of light flux incident upon
the photocell, e.g., iris diaphragm, sector diaphragm, polarization, neutral
wedge, or wire screens.'®® The null method is more accurate than the direct
reading method, since it compensates for variations in the photoelectric
cell and light source. It is used in the Hardy recording spectrophotometer.
General Electric photometer, and Hunter multipurpose reflectometer.

The Recording Spectrophotometer. Instruments called spectropho-
otometers are available for measuring reflectance as a function of wavelength.

C urves, called color curves or spectrophotometric curves, can l>e pre]>ared
from data obtained on these instruments showing the percentage of light
reflected for a given wavelength relative to the percentage reflected from a
standard white surface (e.g., magnesium oxide or magnesium carbonate) at
that wavelength under identical conditions of illumination and viewing
Spectral reflectivity curves for several deeply dyed papers are shown in

Figure XVI-21.''® .

Spectrophotometric curves offer a means of measuring the true spectral

properties of paper which can be kept as a permanent record. A mininium
in the spectrophotometric curve represents an absorption band w ic is
characteristic of the sample. The depth of the minimum indicates the pro¬
portion of material causing the absorption. The spectrophotonieter o ers
a means of comparing one colored paper with another, and of obtaining an
idea of what niav be done in making tvv'O colors more alike. e instnime
is particularlv useful in making color measurements on standard orders o
paper tvltich must be kept constant in shade over a long period of time, and

for checking the amount of fading of colored papers. . >

Two samples of colored paper may appear to match, even though ther

spectrophotometric cui^'es are not identical. In this case, however,

colored specimens are different physically, and while they

match under a specific illuminant, they will not match iinder all i unn ^

A color match can be made as long as the color cui^ e of the sample in qu

169 Institute of Paper Cheinistr>% Tnstruraentotion Studies. Paper Trade J. 1 -.

No. 18 : 285-291; No. 19 : 293-306 (Oct. Tsoj 4.

ITOJ. A. Van den Akker. J. E. Todd, P. Nolan and W. A. V ink. /. Op

Av\CT\ca> 37, No. 5: 36^387 (May, 194/)

,n. jmnMtuA

' * l»w

^879

Kiaidi cmrt I* ;:

^ ihr ID bt oialriKd. >

J -

«l die

A-

Mctnl idkctm «id» •!«. ^

cakviMcil <wjm

. ^■ ■ ■

f V

m^rnMht

•e |l»Wy.,'m»«fc

.iprw »

ImhI oC w

b« <! ■* ft ,

if} •’ -~ ^ ' - ■ * J

iciMfiic iIk li(bf

«iaiioiicftm*:^,^V' ■ •'

■■ . ifi' ''

. r IVI4I

v4i'

®'T<:' '■

a twi—I pn—anJ i> Uiwi brubca
bM «l fdaac tnlh cadi odicr)!
and iIk oi^ apoii a atandard
im a adalr wHfratiaf af^m^oii a
tadtct MMr anoiBtf al liflM w that

Thia carrcM^nma a an^St

ao aa to tqmkm fix Mrciifth of
«f ihr dtokal ayalim cwUr oU a‘

1- Ih ^ V. ^

ladpcaKai h||ht a^paMMa vavdci
«d a W<4toto« fat— wl a Hinl'firMii Thr
ifll thr lii^ ialD two kamm/d fMlafiacd hfhl

J I

880

PULP AND PAPER

at right angles to each other, and the Nicol prism is used to change the rela¬
tive intensities of light on the standard surface and the specimen. The light
reflected from the standard is automatically balanced with that reflected
from the specimen by rotation of the Nicol prism. The difference in the
amount of reflection between the standard and the unknown surface is in¬
dicated by the amount of rotation of the Nicol prism. The test is repeated
for each narrow band, and a complete curve is obtained in this way showing
the relative or apparent reflectivity of the paper compared with that of

magnesium oxide.

Abridged Spectrophotometry. In abridged spectrophotometry, a prism
and slit arrangement is not used to isolate the light according to wavelength,
but instead a series of filters (usually 8—9) are used to isolate narrow spec¬
tral bands. Wratten filters widely used for this purpose cover the ranges

given in Table XVIII.

TABLE XVIII

Ranges Covered by Color Filters Used in Abridged Spectrophotometry

W ratten filter number

■ 74

71A

Approximate wavelength
of maximum response
(millimicrons)

445

455

490

530

570

605

630

680

Abridged spectrophotometry differs from true spectrophotometry in
that the spectral bands are not so narrow, and measurement is made at on y
a few points on the color scale. The results are not as accurate as t ose
obtained in regular spectrophotometry, but are suitable for
poses.^^^"^^® Abridged spectrophotometry can be carried out wi
Hunter Multipurpose Reflectometer or the General Electric reflection meter.

Color SpeciBcation by the I.C.I. System. The interpretahon of
color by the spectrophotometer differs from that given by the human eye
because the eye measures the density of color and the spectrophotomete
does not. The eye tends to average the result so that two samples may

171 Institute of Paper Chemistry, Instrumentation

No. 19; 255-267 (May 13, 1937)

172 Institute of Paper Chemistry, Instrumentation

No. 20 : 271-275 (May 20, 1937)

173 Institute of Paper Chemistry, Instrumentation

No. 18: 245-251 (May 6, 1937)

Studies, Paper Trade /. 104,
Studies, Paper Trade J. 104,
Studies, Paper Trade J. 104,

XVI.

PROPERTIES OF PAPER

881

Alike to the eve, even though they have different spectrophotometric curves^
Since there is no clear relationship between physically measure ^ ig

f: Inner in which the human eye sees and '‘f‘i;‘-“Hum

tn obtain a true mental picture of color from a spectral reflectan .
alone. In other words, the spectrophotometer does not measure co or,
rather the properties which are responsible for color.

m ordL to specify color, the spectral reflectance curve must be assesse^d
in relation to the visual efficiency of the eye

chromaticity, can be calculated from spectrophotometric data, as will be de
scrld in ffie following sections. In the I.C.I. color systenn, thr« va ues
Tre used which are somewhat similar to those used in the Munsell system^
These are dominant wavelength (hue), excitation purity (saturation) and
visual efficiency (visual brightness). These are objective measurements, in
contrast to the subjective properties of the Munsell system. _

Tristimtdus System. In specifying color psychophysically, it is -
sumed that the eye reacts as if it contained three sets of color nerves, t is
assumed that color is interpreted through the response of the eye to excita¬
tion of its three-color nerves by a physical stimulus. The problem o co or
matching, then, becomes one of matching the stimulus exciting t e co or.
The stimulus can be matched by combining three primary colors, red, green,

and blue, called the tristimulus values.

The tristimuliis values of the stimulus exciting the color can be deter¬
mined in a colorimeter by manual adjustment of the relative amounts of the
three primaries required to obtain a color match with the sample, using a
divided field.”^ Such a match is valid, however, only for the particular ob¬
server and for the particular conditions of illumination. It is impossible
to establish a positive relationship between the color sensation and the physi¬
cal properties of the color in this way, because color is a subjective property
which depends upon the observer. In order to establish a standard method
for computing the sensation produced by a given physical stimulus, the In¬
ternational Commission on Illumination (I.C.I.) established the standard
observer at a meeting held in Cambridge, England, in 1931.

Standard Observer. The hypothetical standard observer is based upon
the average color matches obtained by a number of observers in England
who were known to have normal color vision. The use of the standard ob¬
server makes it possible to compute universally accepted color sensation

from the physical specification.

Three sets of color data are used to represent the spectral sensitivity of
each set of color nerves. One set has its maximum sensitivity in the blue
region, another in the green, and another in the red, as shown by the color

Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J. 105,
No. 19: 293-306 (Nov. 4, 1937)

882

PULP AND PAPER

curves in Figure XVI-22.”^ These color curves can he used to comimle
the sensation evolved by a given physical stimulus by (7) multiplying the
spectral energy distribution curve of the standard white light by the spectral
reflectance curve of fhe colored paper to obtain a curve showing the energy
distribution of the reflected light and (2) multiplying the resulting curve
by the three sensitivity curves (Fig. XVI-22) to get three new curves rep¬
resenting the degree to which each individual set of nerves is stimulated.

</>

400

700

Fig. XVI-22.

500 600
WAVELENGTH. m/i

Tristimulus distribution functions of three primary stimuli.

As used in the I.C.I. system, the standard observer consists of three
functions of wavelength which show the relative amounts of
stimuli required to match the color of various parts of the "cal equal
energy spectrum. The tristimulns system of color specihcatiou is
tour chosen stimuli of homogeneous radiant energy given m Table, AlA,

TABLE

STANDARn Coordinate System

Coordinates of system

Four chosen stimuli

„„ 0.73467

' . 0.27376

435;8. o.i6658

Standard illuminant fi (approx, mean

norm sunlight ) . 0.34842

0.26533

0.71741

0.00886

0.00000

0.00883

0.82456

0.35161

0.29997

irin .. • /-' r'lAvp

.7.- Ilmrihook ol ChewUtry a,ut /’/ly.tiV.i. Cliemicul Rubber Publisbing Co, Cl

land.

XVI. PROPERTIES OF PAPER

883

.ogether with the coordinates used for specifying the system_ The rtand

Z ohsercer is detennined by three in

responding to the three theoreucal ,, „hich are

F’igure XVI-22, or by three trichromatic coemcients,. ,

fractional values defined as follows:

X = ^

y = y/£» 3’^ £

= c/x, y, c

Table XX gives the trichromatic coefficients and distribution functions
for eoual enerey for each wavelength interval of 5 millimicrons.

The information i.i Table XX is useful because it makes .t poss.ble to
calculate the tristimulus specification from the spcctrophotometnc curve.
This is done bv summing throughout the visible spectrum the product o
tatet! by multiplying the spectral reflectance curve by the spectral energy
of the illuminant by the respective spectral distribution functions, i, y, an

z of the standard observer at the appropriate wavelengt .

Chromaticity Diagram. The information in Table XX makes it pos¬
sible to construct chromaticity diagrams using a' as the abscissa an y as
the ordinate, which gives a complete specification of chromaticity. Only
r and v need be used as coordinates, since by definition, A* + y + 2 - 1, and
hence r is specified by difference when x and y are given. A cl^omaticity
diagram for the spectrum colors is given in Figiire XVI-23.”^ The points
for the l.C.I. ilhimiiiants A, />. and C are given in this figure, as well as the
designation for a i>articular green (point G on diagram). The solid line
connecting the ixiints siiecifying the si>ectrum colors is called the spectrum
locus. A line connecting two points specifying two different colors may be
regarded as the locus of all points specifying colors obtained by mixing the

two colors.

Damiuant iravelcngllt and Purity. It is a difficult matter to visualize
color in terms of numbers such as represented by the tristimulus values,
and hence it is preferred in many cases to use objective measurements of

ilominant wavelength and purity.

Dominant wavelength is defined as the wavelength of light which must
Ik added to the light of the illuminant to match the stimulus of a given color.
It corresponds to the subjective term hue in the Munsell system. Dominant
wavelength is determined on the chromaticity diagram in Figure XVI-23
as the wavelength on the spectrum locus obtained by drawing a line through
the point representing the particular illuminant used and the point specify¬
ing the color. This gives the dominant wavelength, because any color above
the dotted line drawn through the illuminant (e.g., point C in the diagram)

*•* R. S. Hunter. "Photoelectric Tristimulus Colorimetry with Three Filters,”
U. S. Dept. Commerce. Natl. Bur. of Sundards C429 (July 30, 1942)

884

PULP AND PAPER

Wave¬

length,

m|i

380

385

390

395

400

405

410

415'

420

425

430

435

440

445

450

455

460

465

470

475-

480

485

490

495

500

505

510

515

520

525

530

535’

540

545

550

555

560

565

570

575

580

585

590

595

600

TABLE XXi74a

Trichromatic Coefficients and Distribution Functions

OF THE Standard Observer

Distribution coefficients
for equal energy

Trichromatic coefficients

0.1741

0.1740

0.1738

0.1736

0.1733

0.1730

0.1726

p.1721

0.1714

0.1703

0.1689

0.1669

0.1644

0.1611

0.1566

0.1510

0.1440

0.1355

0.1241

0.1096

0.0913

0.0687

0.0454

0.0235

0.0082

0.0039

0.0139

0.0389

0.0743

0.1142

0.1547

0.1929

0.2296

0.2658

0.3016

0.3373

0.3731

0.4087

0.4441

0.4788

0.5125

0.5448

0.5752

0.6029

0.6270

0.0050

0.0049

0.0048

0.0051

0.0058

0.0069

0.0086

0.0109

0.0138

0.0177

0.0227

0.0297

0.0399

0.0578

0.0868

0.1327

0.2007

0.2950

0.4127

0..538’4

0.6548

0.7502

0.8120

0.8338

0.8262

0.8059

0.7816

0.7543

0.7243

0.6923

0.6589

0.6245

0.5896

0.5547

0.5202

0.4866

0.4544

0.4242

0.3965

0.3725

0.8209

0.8210

0.8213

0.8215

0.8219

0.8222

0.8226

0.8231

0.8235

0.8239

0.8242

0.8245

0.8247

0.8251

0.8257

0.8263

0.8246

0.8181

0.8036

0.7760

0.7306

0.6596

0.5638

0.4534

0.3413

0.2359

0.1491

0.0919

.0.0596

0.0394

0.0255

0.0161

0.0099

0.0061

0.0038

0.0024

0.0017

0.0012

0.0010

0.0009

0.0008

0.0006

0.0005

y (rel. vis.)

0.0014

0.0000

0.0065

0.0022

0.0001

0.0105

0.0042

0.0001

0.0201

0.0076

0.0002

0.0362

0.0143

0.0004

0.0679

0.0232

0.0006

0.1102

0.0435

0.0012

0.2074

0.0776

0.0022

0.3713

0.1344

0.0040

0.6456

0.2148

0.0073

1.0391

0.2839

0.0116

1.3856

0.3285

0.01681

1.6230

0.3483

0.0230

1.7471

0.3481

0.0298

1.7826

0.3362

0.0380

1.7721

0.3187

0.0480

1.7441

0.2908

0.0600

1.6692

0.2511

0.0739

1.5281

0.1954

0.0910

1.2876

0.1421

0.1126

1.0419

0.0956

0.1390

0.8130

0.0580

0.1693

0.6162

0.0320

0.2080

0.4652

0.0147

0.2586

0.3533

0.0049

0.3230

0.2720

0.0024

0.4073

0.2123

0.0093

0.5030

0.1582

0.0291

0.6082

0.1117

0.0633

0.7100

0.0782

0.1096

0.7932

0.0573

0.1655

0.8620

0.0422

0.2257

0.9149

0.0298

0.2904

0.9540

0.0203

0.3597

0.9803

0.0134

0.4334

0.9950

0.0087

0.5121

1.0002

0.0057

0.5945

0.9950

0.0039

0.6784

0.9786

0.0027

0.7621

0.9520

0.0021

0.8425

0.9154

0.0018

0.9163

0.8700

0.0017

0.9786

0.8163

0.0014

1.0263

0.7570

0.0011

1.0567

0?6949

0.0010

1.0622

0.6310

0.0008

Table continued

XVI.

PROPERTIES OF PAPER

885

TABLE XX (continued)

Wave-

Trichromatic coefficients

Distribution coefficients
for equal energy

ni|i

605

0.6482

0.3514

0.0004

1.0456

610

0.6658

0.3340

0.0002

1.0026

615

0.6801'

0.3197

0.0002

0.9384

620

0.6915

0.3083

0.0002

0.8544

625

0.7006

0.2993

0.0001

0.7514

630

0.7079

0.2920

0.0001

0.6424

635

0.7140

0.2859

0.0001

0.5419

640

0.7190

0.2809

0.0001

0.4479

645

0.7230

0.2770

0.0000

0.3608

650

0.7260

0.2740

0.0000

0.2835

655

0.7283

0.2717

0.0000

0.2187

660

0.7300

0.2700

0.0000

0.1649

665

0.7311

0.2689

0.0000

0.1212

670

0.7320

0.2680

0.0000

0.0874

675

0.7327

0.2673

0.0000

0.0636

680

0.7334

0.2666

0.0000

0.0468

685

0.7340

0.2660

0.0000

0.0329

690

0.7344

0.2656

0.0000

0.0227

695

0.7346

0.2654

0.0000

0.0158

700

0.7347

0.2653

0.0000

0.0114

705

0.7347

0.2653

0.0000

0.0081

710

0.7347

0.2653

0.0000

0.0058

715

0.7347

0.2653

0.0000

0.0041

720

0.7347

0.2653

0.0000

0.0029

725

0.7347

0.2653

0.0000

0.0020

730

0.7347

0.2653

0.0000

0.0014

735

0.7347

0.2653

0.0000

0.0010

740

0.7347

0.2653

0.0000

0.0007

745

0.7347

0.2653

0.0000

0.0005

750

0.7347

0.2653

0.0000

0.0003

755

0.7347

0.2653

0.0000

0.0002

760

0.7347

0.2653

0.0000

0.0002

765

0.7347

0.2653

0.0000

0.0001

770

0.7347

0.2653

0.0000

0.0001

775

0.7347

0.2653

0.0000

780

0.7347

0.2653

0.0000

j^(rel. vis.)

0.5668
0.5030
0.4412
0.3810
0.3210
0.2650
0.2170
0.1750
0.1382
0.1070
0.0816
0.0610
0.0446
0.0320
0.0232
0.0170
0.0119
0.0082
0.0057
0.0041
0.0029
0.0021
0.0015
0.0010
0.0007
0.0005
0.0004
0.0003
0.0002
0.0001
0.0001
0.0001
0.0000
0.0000
0.0000
0.0000

0.0006

0.0003

0.0002

0.0001

0.0000

Totals. 21.3713 21.3714 21.3715

may be regarded as a mixture of the illuminant and a spectrum color. Thus,
in Figure XVI-23, the dominant wavelength for the color green represented
by the point G is 500 millimicrons for I.C.I. illuminant A and 520 milli¬
microns for illuminant C, based on the standard observer.

Excitation purity is defined as the per cent departure from a neutral
gray for a particular illuminant. Excitation purity can be objectively de¬
termined from the chromaticity diagram as a ratio of the distance between

886

PULP AND PAPER

the points specifying the illuniinant and the sample color to the distance be¬
tween the points specifying the illuniinant and the dominant wavelength of
the sample color. In Figure XVI-23, the purity is (C-G)/(C-520), which
is 25.8%. A purity of 100% means that only light of the dominant wave¬
length is reflected.

Visual Efficiency. For all practical purposes in color matching, it is
usually enough to obtain a close correlation with the dominant wavelength
of the specimen and the sample to be matched, as shown by the spectrophoto-
metric curves; then, if the samples have the same approximate purity and
visual efficiency, the match will be satisfactory. For very exact work, how-

Fiff XVI-23. Chromaticity diagram showing the .r and y values of the spec¬
trum colors, I.C.I. illuminants A, B, and C, and a particular green (6).

ever the chromaticity (dominant wavelength and purity) and visual effi
cienev should be accurately determined. The methods of calculating doini
iiant wavelength and purity have just been described Xisua

efficiency can be calculated in similar manner from spectrophotometnc ■
using values from the standard observer. Visual efficiency defines _
lightness or brightness of the paper and is an important characteristic w iic
•__f .,1 ,Mii*r>ncf»Q other than soecifvitig the color.

XVI,

rROPERTlES OF PAPER

887

ard body is determined by the sum of the product throughout the visible
iLt of the spectral energy of the light falling on the standard tunes ho
luminosity of the light, all at unit wavelength intervals. In prartice, ^
luminosity rather tlian absolute luminosity is used, since only the

spectral energy distribution of the light is known.

In practice, visual efficiency of paper based on the standard observer

can be determined by taking the reflectance reading at regular interva s
of wavelength on the spectrophotometric curve, multiplying each reading y
the corresponding y value of the distribution coefficients for that wave¬
length, and averaging the results throughout the entire spectrum. The y
value can be used in this way because it was chosen so that it is identical to
the standard visibility function (see Fig. XVI-20 and Fig. XVI-22).
Values for y for the various wavelengths can be obtained from Table XX.

Three-Filter Colorimetry. It has just been explained how color can
be completely specified by trichromatic coefficients, visual efficiency, domi¬
nant wavelength, and excitation purity, using spectrophotometric data.
This method of color analysis yields an accurate specification of the color,
but has the disadvantage of requiring considerable time for making the
measurements and integrating the results. In order to speed up the work,
three-filter colorimetry is sometimes used. This is much more rapid than

spectrophotometric colorimetry because the integration with respect to
wavelength is done automatically by using special filters. Three-filter
colorimetry is not, however, as accurate as spectrophotometric colorimetry,
because the photometric conditions (light, filter, and photocell) used in
three-filter colorimetry are not spectrally equivalent to the desired combina¬

tion of standard illuminant and standard observer of the I.C.I., and hence

do not yield a true measure of the color. Threeffilter colorimetry does

serve, however, to fill an important practical purpose.^^^

The three filters generally used in three-filter colorimetry arc green,

amber, and blue. In making determinations, reflectance measurements are

made on the unknown sample relative to magnesium oxide using the three

filters. The x, y, and¥ values for the I.C.I. scales can be obtained from the

following relationships, where A, B, and G represent the readings obtained

with the amber, blue, and green filters, respectively. The reading obtained

with the green filter closely duplicates the ~y (visibility function) in the
tristimulus value.

0.80^+ 0.185

A-* _ _

3 ;= l.OOG

7= 1.185

y = i

2 = T + y+ F

r* ^ _

X -v y + z should equal 1.0

888

PULP AND PAPER

It should be borne in mind that the color match is valid only for the specific
illuminant used.

Calibration of the instrument and filter set-up can be made against
working standards which should be spectrally similar to the samples to be
tested. A wide range of working standards in gray and colored papers are
available which have known tristimulus values.’^^®* The readings which
should be obtained for the amber (A), green (G), and blue (B) filters,
using the values of the working standards, should be as follows:

A = 1.25-r-0.19s
G = y

B = 0.852s

Yellowness is sometimes expressed in three-filter colorimetry as the
difference between the reflectance obtained with the amber (A) filter and
the reflectance obtained with blue (B) filter divided by the reflectance ob¬
tained with green (G) filter—i.e., yellowness equals [(^-B)/G1 x 100.

Brightness of Paper

True brightness refers to the lightness or over-all reflectivity, i.e., visual
efficiency, of the paper. As shown in the previous section, this is a function
of the over-all spectral reflectance of the paper, the ener^ distribution of the
illuminant, the viewing conditions, and the characteristics of the viewer.
While visual efficiency is a function of the reflectance over the visible spec¬
trum, papermaker’s brightness, on the other hand, is based on a measure¬
ment of the reflectance of light from white or near-white papers at a single
wavelength in the blue region of the spectrum, that is, light having a wave

lens^th of 457 inillimicrons. -

Methods of Measuring Brightness. The instruments comnioniy

used for measuring brightness in the paper industry are the General h ec

trie. Hunter, and the Photovolt.

The General Electric instrument employs a null method. Eigh

a single source is directed along two different paths to two photoelec nc
cells which are connected with opposite polarities to the terminals of a gal¬
vanometer ; the amount of light along these two paths is varie y
cal means until the cells produce the same currents as indicated y
ing of the galvanometer. The instrument is operated as ® .

working standard is placed over the opening and the dial ('"s ^

set to read the calibrated brightness of the standard; the ^

removed and the unknown sample is placed over the opening ^e ^
vanometer balanced with the other dial (sector diaphragm) , Ae stands
Is then placed over the opening a second time and the first dial (ins dia

i’«MunseIl Color Company, Baltimore,

177 /. Optical Soc. America, p. 609 (Dec., 1940)

XVI. PROPERTIES OF PAPER

889

Dhram) is again balanced to give the brightness reading. In making the
Uwness determination, a thick opaque pad of the test paper The

tiL direction) and the pads inserted in the brightness tester with the
felt side facing the instrument and the machine direction p
plane of the incident and reflected rays of light. The percentage re ec ion
(based on magnesium oxide as 100.0) is then reported to the nearest 0.1%.
Samples should, of course, be used which are free of all blemishes and aws.

The General Electric instrument in routine use should be m calibra ion
with a master instrument of particular type and design which must be com¬
pletely specified as to its spectral, geometrical, and photometrical charac¬
teristics.”®’ In the master instrument, the effective wavelength (457 ±

0 5 millimicrons) is obtained by a suitable combination of glass optics, lamp,
filters, and photoelectric cell. The spectral energy distribution is defined
to control the rate of absorption of energy in the test specimen and to con¬
trol the intensity of ultraviolet light. The mean angle of incidence of rays
upon the test specimen are maintained at 45° ± 1°, and the mean angle of
reflected rays and the incident rays is maintained between 0 and 1° with the
normal. These and other characteristics of the master instrument are com¬
pletely specified to measure correctly at all times the brightness of paper
relative to that of freshly prepared magnesium oxide. In calibrating the
General Electric instrument used for routine work, both secondary and
working standards (which have been standardized against the master in¬
strument) are used. The secondary standards consist of about five pads of
paper ranging in brightness value from 60 to 85. At least two working
standards made from white opaque glass are also used. During calibration,
the instrument is checked against the assigned values of the secondary
(paper) standards and adjusted so that the readings of these standards
against the working standards agree with the assigned values. The instru¬
ment should give the proper reading on all five or six of the secondary
standards. Calibration should be done every one or two months. If a dif¬
ferent instrument (not the same as the master instrument in design) is
used, the readings may not agree with both the secondary and working
standards. - For these instruments, only the secondary (paper) standards
should be used for calibrating the instrument. Working standards can then
be evaluated by the instrument in terms of the paper standards.

The Hunter instrument, like the General Electric, is based on a null
method. A comparison standard is kept in one beam and the unknown
specimen is compared with the standard in a second beam. Several work¬
ing standards are used covering a wide range of brightness readings. In

See TAPPI Standards

Such an instrument is maintained by the Institute of Paper Chemistry, Apple-

ton, Wisconsin.

890

rULP AND PAPER

operation, the working standard is placed over the test opening, the scale
set at the calibrated reading, and the galvanometer is adjusted to zero by
moving a mirror in the circuit closer or farther away from the photocell.
With the instrument thus adjusted, the unknown sample is then substituted
over the opening and the scale (which moves the photocell in the other cir¬
cuit closer or farther away from the reflectance surface) adjusted to a null
reading on the galvanometer. The brightness reading is then read directly
from the scale. The scale reading is based upon the inverse square

law which states that the intensity of illumination varies inversely as the

square of the distance from the source.

In the Photovolt, a single photocell is used. The instrument must be
adjusted before use by placing the search unit over a black cavity, setting
the galvanometer to zero, and then placing the search unit over a calibrated
standard and adjusting to the correct reading. When properly adjusted in
this way, the search unit is then placed on the unknown specimen and the
brightness is read directly. Calibration charts should be used with this
instrument, since the response of the photocel-galvanometer is nonlinear.
The instrument is sensitive to frequency variation in power supply and is

subject to error resulting from specular reflectance.

Importance of Brightness. The practicality of measuring brightness

as reflectance at a single wavelength is based on the fact that pulps of the
same type have spectral reflectance curves of similar shape, and hence a
reading at a single w^avelength is sufficient indication of the shape of the
curve. Papermaker’s brightness is not a complete designation of the color
of paper, but is a highly useful arbitrary means for measuring the color
quality of white papers. Brightness readings correlate reasonably well with
the lightness of the paper and nearly always correlate well with a visual
rating of samples if the samples have spectral reflectance curves of similar
shape. In order for the brightness reading to be of value in the grading o
different papers, the samples must have a similarity in shape of their re ec
tance curves, and there must be relatively small deviation from neutrality of
the colors involved.^®" If samples of pulp of unknown origin are being
tested, a complete spectrophotometric curve should be taken, but or or¬
dinary mill testing of papers of known history, a brightness rear ng is sa is
factory. A difference in brightness of 1 to 2 points is considered significant.

It is possible to have two papers equal in papermaker’s bnghtnes w iic i
differ widely in “whiteness” and visual efficiency. Subjective grading of
samples for brightness does not always agree with the grading obtaine on
lirightness meters. Brightness, as interpreted visually, is a psycho ogic
combination of purity and true brightness, purity (percentage cevi

170-L. R. Dearth, J. A. Van den Akker and W. A. Wink, TaPh 33, Ko. •

85A-S9A (Oct., 1950) iqa^ioo 1940)

ISOM. N. Davis, Paper Trade J. Ill, No. 14: 184-188 (Oct. , )

XVI. PROPERTIES OF PAPER

891

from a neutral gray) dominating if the physical differences in brightness are
small. Neither papermaker’s brightness nor even true visual efficiency a -
ways correlates with what most people select as the whitest paper. T is
discrepancy is due to the fact that a pleasing white is not always the one
with the highest degree of whiteness. For example, certain slightly tinted
“white” papers may appear more pleasing and “whiter” to the eye than
other papers of equal brightness. A pleasing shade of white depends upon
the personal preferences for hue, most people preferring a blue-white con¬
taining a slight amount of red. An analysis of two white papers was found

to be as follows:

Paper No. 1 Paper No. 2

Papermaker’s brightness (457 mu) . 74.3 77.4

Reflectance of white light . ^2.6 79.3

Paper No. 2, a blue-white, had a higher papermaker^s brightness than Paper
No. 1, a creamy white, but the latter had a greater visual brightness, as in¬
dicated by the greater reflectance of white light. Paper No. 2 would, how¬
ever, be selected by the average observer as the whiter paper because of the
general preference for a blue-white. Color curves on the two papers are
shown in Figure XVI-24.

Fig. XVI-24. Spectral reflectivity curves for a cream and a blue-white paper.

Brightness was designed primarily as a test to measure the effectiveness
of bleaching in removing the yellowness of pulps, and for this purpose,
brightness is admirably suited. The spectral reflectance curve for an un¬
bleached pulp starts off with a relatively low reflectance in the violet end of
the spectrum and rises rapidly to a fairly high value in the extreme red end
of the spectrum; in other words, an unbleached pulp is a yellow of low
purity. Bleaching raises the spectral reflectance curve over the whole ransre.

O '

but the increase is greatest in the blue and violet range where brightness is
measured, i.e., in the neighborhood of 457 millimicrons. This makes the
brightness reading a particularly sensitive measure of bleaching efficiency.

892

PULP AND PAPER

Spectral reflectance curves showing the effect of bleaching on sulfite and
sulfate pulps are shown in Figure XVI-25. Brightness is also well suited
for measuring the permanence of paper, since the change in the color of
paper on aging or thermal degradation is greatest in the blue and violet re¬
gions of the spectrum where brightness is measured. There are isolated
cases when brightness readings do not correlate well with the visual ef¬
ficiency of pulps: for example, in the case of rag pulps made from blue
denims which have a bluish tint and consequent!)' have a high brightness
reading in relation to their true visual efficiency.

Fig. XVI-25. Effect of bleaching on the spectral reflectance curves of sulfite
and sulfate pulps; (a) bleached; (b) unbleached; (c) unbleached groundwood.

Effect of Dyestuffs on Brightness. Brightness readings have no
significance on colored papers, particularly those papers which absorb light
in the range of 400 to 500 millimicrons. Brightness readings on colored
papers rarely correlate with the true brightness (lightness) of the papers.

In practice, a small amount of blue dyestuff or blue pigment is often
added to the stock in making white papers, and the effect is pleasant be¬
cause the average person prefers a blue-white to a yellowish white. How¬
ever, the brightness is not increased by this practice, since the maximi^
brightness of any paper is that of the pure pulp. There is no dyestuff which
will increase the brightness, since all dyestuffs reduce the amount of re¬
flected light. All dyestuffs must either lower the brightness, increase the

opacity, or both.^®^

It is possible to increase the apparent whiteness of paper by adding a
dyestuff complementary to the color in the paper, thus graying the paper and
making it appear whiter. It is possible to increase the whiteness in this

E. R. Laugblin, Paper Trade J. Ill, No. 22; 275—281 (Nov. 28, 1940)

XVI. PROPERTIES OF PAPER

893

way, even though the total reflectance of the paper is reduced, because paper
reflecting only SOfc of the light is considered as white. Lewis^®^ has pre¬
sented cur\’es showing the effect of increasing amounts of a special blue dye
on the brightness, whiteness, and daylight reflectivity; the brightness re¬
mained relatively constant, the daylight reflectivity decreased, and the
whiteness increased as the amount of blue dye was increased.

As pointed out above, it is possible to tint a given stock without ap¬
preciably altering the brightness through the use of dyestuffs which have
very little dulling effect.'" Each dyestuff has a characteristic wavelength
where it absorbs more light than others, and this is known as the wavelength
of maximum absorption. By choosing dyestuffs which do not affect the
reflectance of light of wavelength near that used in determining brightness
(i.e., 457 millimicrons), it is possible to tint the paper without appreciably
affecting the brightness readings. This is commonly done in tinting white

papers.

Reddish blues, violets, and particularly blacks exert the greatest opaci¬
fying and dulling influence. Oranges, reds, and greenish blues are of lesser
influence and, in some cases, almost without effect.'®' Yellow and green
dyestuffs lower the brightness slightly because they absorb an appreciable
amount of light in that part of the spectrum from 400 to 500 millimicrons.
On the other hand, certain blue dyes (e.g., Halopont brilliant blue) may be
added until the reflectance has been decreased as much as 30% at the maxi¬
mum absorption wavelength (.560 millimicrons) before there is any appre¬
ciable re<luction in reflectance at the wavelength (457 millimicrons) where
brightness is measured.'*'

Effect of Pigments on Brightness. The presence of pigments has
a marked effect on the brightness of paper. Colored pigments have much
the same effect as dyestuffs. Ochre has a particularly bad effect on bright¬
ness, since the wavelength of maximum absorption of ochre coincides al¬
most exactly with tliat at which brightness measurements are made.

Some of the white pigments such as titanium dioxide, calcium car-
Itonate, and zinc sulfide, have much higher reflectivity than pulp fibers, and
consequently appreciably increase the brightness of paper. In some cases,
fillers tend to concentrate in the upper layers of the paper, thereby resulting
in brightness differences as high as 2 to 3 points between the two sides of
the sheet. If the pigment is used in a surface coating, marked increases in
brightness are obtained.

Relationship between Transparency and Brightness. Increasing
the transparency of paper reduces the brightness, because increased trans-

**»L. C Lewis. Paper Trade J. 103. No. 22: 323-330 (Nov. 26, 1936)

'«E. R. Uughlin and O. Kress, Paper Trade J. 100, No. 8: 106-118 (Feb. 21,
1935)

'*♦ E. R. Laughlm, Paper Trade J. Ill, No. 22 : 275-281 (Nov. 28, 1940)

894

PULP AND PAPER

l)arcncy means fewer interfaces for the reflection of light, hut greater opjxjr-
timity for the absorption of light. It follows from the above that increasing
the density of paper reduces brightness because it increases the transparency.
Glassine, for example, has a much lower brightness than ordinary opaque
papers, even though the same furnish is used in both cases. Doughty’®-*
reports a drop in daylight reflectance of 5 to 10% for an increase in solid

fraction of 0.2 to 0.5.

LlI

200

400 600 800
FREENESS. SCHOPPER-RIEGLER

1000

Fig. XVI-26. Effect of beating on brightness.

Beating lowers the brightness, as shown in Figure XVI-26, because
of the increase in transparency of the paper resulting from increased beat¬
ing. Wet pressing appreciably affects brightness. It is important to main¬
tain constant wet pressing conditions when making laboratory handsheets
for bleachability determinations in order to eliminate the effect of wet press¬
ing on the brightness of the test sheets. Blackening or loss of brightness on
calendering is caused by an increase in transparency of the paper resulting
from a reduction in the number of cellulose-air interfaces. The are^ w ic
appear dark to reflected light are the areas of highest translucence. The loss
of brightness resulting from overcalendering is always accompanied y a
loss in opacity. The loss in brightness on supercalendering vanes rom
1 to 4 points, depending upon the furnish and calendering conditions.

In the case of lined paperboard, the brightness is dependent upon t
opacity and brightness of the liner.'” The use of pigments m the hner
stock increases the brightness of the board m a dual way, by mcreas g ^
opacity and by increasing the brightness of the liner J^^cr^sing e _ ^
weight of the liner also increases the brightness of the board because
creases the opacity of the liner, thereby resulting in a better covermg-up

185 R. H. Doughty, Paper T rade J. 95 No. 10; < ^mct^3^^1940)

iSG M. N. Davis. Paper Trade J. Tradfj 109 No. 9:

187 w. R. Willets, R. T. Bingham and L. H. Eriksen, I apir Pra • .

91-94 (Aug. 31, 1939)

X\a. PROPERTIES OF PAPER

895

the dull filler stock. The use of low brightness filler stock tends to reduce

the over-all brightness of paperboard.^**

Comparison of Pulps for Brightness. The brightness of pulp be¬
fore any materials are added is a good measure of pulp quality and is al¬
most a fundamental property. It is the most important factor affecting the
brightness of the finished paper. If the brighter of two pulps does not pro¬
duce as white a pajjer as the other, it is true that the brighter pulp can be
nude to produce the whiter paper by changing the dye formula.^**

A definite relationship exists between the lignin content and the bright¬
ness of pul[)S, the effect of lignin being greatest in the region of low lignin
content.’*^ There are other factors which affect pulp brightness, aside from
the intrinsic brightness of the pulp. Dirt specks, finger marks, or other
surface imperfections on the test pads used for making the brightness deter¬
mination will result in low readings. Seasonal variations in the clarity
of the water used in forming the paper affect pulp brightness. Aging causes
the pnlp to become yellow and lose brightness, unbleached pulps and ground-
wfxxl lieing particularly bad offenders. Overdrying tends to lower the
brightness. Bleaching, of course, greatly increases brightness. On a com-
furative lusis, the brightness of different commercial pulps would be ap¬
proximately those given in Table XXI.

TABLE XXI

Brigiitxess of Difff.rf.xt Pulps

Type of pulp Brightness

Unbleached sulfate . 25—45%

Bleached sulfate . 70-85%

Unbleached sulfite ..... 50-65%

Bleached sulfite . 75-85%

Si»ecially purified puli»s. up to 90-94%

Groumlu'ood . 55-60%

Brightness values are inadecpiate for characterizing gronndwood (al¬
though the test is often used) because of the hue of the pulp. A spectro-
pliotometric curve gives more useful information, or the results may be ex¬
pressed in terms of brightness, dominant wavelength, and purity.*®^ The
doftiinant wavelength for gronndwood, both bleached and unbleached, is

***W. R. Willcts, R. T. Bingham and L. H. Eriksen, Paper Trade /. 109, No. 23:
308-310 (Dec. 7, 1939)

‘••J. A. Van den .\kker. P. Nolan and W. A. Wink, Paper Trade /. 114, No 5*
•Vk-52 (Jan. 29. 1942)

’••W. W. Pigman and W. R. Csellak, Tech. Assoc. Papers 31: 393-399 (June,

**• R. M. Kingsbury. F. A. Simmons and E. S. Lewis, Forest Products Lab.,

Paper given at TAPPI Mechanical Pulping Conference, Poland Springs
Maine (Sept. 27-28-29, 1948)

896

PULP AND PAPER

from 562 to 582 millimicrons, 'or in other words, the pulps are essentially
yellow in hue.^^^ The spectral reflectivity curve for groundwood shows
distinctive low values in the wavelength range from 400 to 450 millimi¬
crons (see Fig. XVI-25).^®® Groundwood papers having the same bright¬
ness as bleached sulfite papers, as measured on’ a brightness meter, will
generally appear yellower because of the greater reflection in the yellow
range. Consequently, groundwood papers are generally given a low rating
in subjective evaluation, due to lack of purity (yellowness) of the reflected
light. However, by using a suitable dye, it is possible to tint groundwood
to a color which yields identical subjective and instrumental grading.^®®

Neutral sulfite semichemical pulps which have been bleached with
hypochlorite in the presence of sodium silicate exhibit fluor^cence. Be¬
cause of this fluorescence, these pulps tend to give different brightness read¬
ings on different brightness meters.

Brightness of Pulp Mixtures. The effect of blending two pulps on

brightness can be worked out experimentally by mixing the pulps in various
proportions and determining the resulting brightness, or the results can be
calculated using the Kubelka and Munk equation, as will be shown later.
Calculated and experimental results generally agree closely, as shown in
Figure XVI-27, where the calculated and experimental results of blending
bleached sulfite and groundwood pulps are shown. Similar results are
shown in Table XXII for- mixture_s of bleached and unbleached sulfite pulp.

TABLE XXII

Brightness of Mixtures of Bleached and Unbleached Sulfite Pulps

Pulp in mixture, %,

Bleached sulfite

100

Unbleached sulfite

100

Observed

brightness

59.13
60.20
61.83
62.77
63.69
66.15
67.68
70.40
72.76
76.62

80.13

Expected brightness on
weighted arithmetic
average of two
pulps in mixture

61.23

63.33

65.43

67.53

69.63

71.73

73.83

75.93

78.03

It can be seen from Figure XVI-27 and Table XXII that

values (either calculated or experimental) are lower than would be p

192 j. A. Van den Akker, H. F. Lewis, G. W. Jones and 1^. A. Buchanan. Tappt

22. No. 4: 187-192 ('Apr., 1949) .qa laa mrt 1940)

193 M. N. Davis, Paper Trade J. Ill, No. 14: 184-188 (Oct. 3, 1940)

XVI. PROPERTIES OF PAPER

897

from the weighted arithmetic average of the two pulps in the mixture, the

greatest difference being obtained at about a 50-50 mixture.”^^®®

Lower than expected brightness values are obtained in mixing a pulp
of low brightness with a pulp of high brightness because the duller pulp al¬
ways predominates in the mixture.^®^ The explanation for the accentuation
of the darker-colored fibers in a mixture is due to the greater transparency
of the light-colored fibers, which means that a greater percentage of the
light falling on the bleached fibers tends to pass through the fibers, whereas

100 80 60 40 20 0

% SULPHITE (Rcd=84)

Fig. XVI-27. Effect of mixed furnishes of groundwood and bleached sulfite on
brightness: (—) calculated values from Kubelka and Munk equation; (--) weighted
arithmetic average; ( • ) experimental values. Tappi Data Sheet 147, courtesy Techni¬
cal Association of the Pulp and Paper Industry.

a greater percentage of the light falling on the unbleached fibers tends to be
absorbed. This accentuates the presence of the more opaque and less bright
unbleached fibers.

Because of the above effects, the substitution of bleached groundwood
for unbleached groundwood in a mixed furnish with bleached sulfite results
194 E. R. Laughlin and O. Kress, Paper Trade J. 100, No. 8: 106-127 (Feb. 21,

195 W. J. Foote, Tech. Assoc. Papers 29: 180-182 (Tune 19461

196 See TAPPI Data Sheets ’ ^

191 E. R. Laughlin, Paper Trade J. Ill, No. 22: 275-281 (Nov. 28, 1940)

898

PULP AND PAPER

in a greater increase in brightness than would be expected because of the
relatively high opacity and covering power of the groundwood, compared
with the sulfite. The following example illustrates the point. A 50-50
mixture of bleached sulfite, 80 brightness, and a groundwood, 60 brightness,
was found by calculation to have a brightness of 67.3. If the sulfite bright¬
ness were increased 10 points, the brightness of the mixture would then
become 69.0 (an over-all increase of only 1.7 points). However, if the
trroundwood brightness were increased 10 points instead of the sulfite, the
brightness of the mixture would then become 74.3 (an over-all increase of

7.0 points).

Gloss of Paper

Gloss is a proiierty of paper which refers to the quality of glossiness,
luster, or ability of the surface to show an image. Gloss is a qualitative
propertv which cannot be expressed in fundamental terms. It is related to
luster (the sudden selective reflection of light) and glare (the undesirable
reflection of excessively bright light). In a psychological sense, gloss an
luster connote a pleasing effect, whereas glare connotes an unpleasant

blinding effect? r u- i

Gloss may be described as the characteristic ot the paper surface which

causes the paper to reflect light at a given angle of reflection in excess o

tlic diffuse reflection at that angle. Gloss is thus related to specular re-

'^'^'^^^Gloss is determined by (/) the index of refraction of the paper which

determines the total amount of reflected light and (2) the degree ° °P
smoothness of the surface, which determines the ratio of specularly reflected

“ “Methodr'ot Measuring Gloss, A number of instruments measure
an arbitrary value indicative of the gloss of paper. These are the Ingersdl
Glarimeter, Sheen Glossmeter, Bausch and Lomb G ossn

Hunter Reflectometer and Glossmeter. .

In the lugersoll Glarimeter, it is assumed that the per
of the reflected light is a measure of what the eye sees as S
strument measures contrast or per cent gloss, w iic i is p ^

reglny reflected light to the total light reflected. In other words, the

Ingersoll Glarimeter measures the ratio:

intensity of specular reflection

intensity of specular retlection + intensity of diffuse reflection
.\ notarizing prism is used to separate the light into two beams, one com
laiS diffusely and regularly reflected light

diffusely reflected light. The amount of polarized light is the p ^
Institute of Paper Chemistry. Instrumentation Studies. Poper

108

No. 2; 19-21 (Jan. 14. 1937)

XVI, PROPERTIES OF PAPER

899

the total light hy Ii.<-a,.s ..t a .lividctl liohl, the two halves o( which are tnade
equally bright hv rotating an analyzing Nicol prism. The seal' 5“ "'8
eives a measure of the glossy reflection as a per cent of the total light re¬
flected. The glarimeter works on an angle of 57/2° to the normal on the
assumption that maximum polarization occurs at that angle. Another as¬
sumption is that the index of refraction of the paper surface is approximately
1.57. This assumption is valid enough in regular papers, but leads to ap¬
preciable error in coated papers where the index of refraction may be much
higher than this.*^'*“® The glarimeter does not work well in the case of
colored papers. Another disadvantage is the subjective nature of the
measurements. The instrument works best on papers in the low-gloss

range.

In the Bausch and Lomb Glossmeter, a beam of light is projected on
the test specimen at an angle of /5^ to the normal, and the intensity of the
light scattered is measured on a photometer at an equal and opposite angle.
The Bausch and Lomb Glossmeter measures a value represented by the sum
of the intensities of the specular reflection and the diffuse reflection. A piece
of polished black glass is used as the working standard. At the incident
angle use<l (75®), polished black glass reflects specularly about 26% of
the incident light, but the instrument is adjusted so that the scale reading is
100 with the black glass in place.-®* When a specimen of the paper is placed
over the glass standard, the gloss of the paper can be read directly on the
meter. A pa})er which reflects specularly \3% of the incident light will
liave a gloss of 13/26, or 56% of the gloss of black glass which shows up
directly on the scale as a gloss of 50. One advantage of the Bau.sch and
Lomb Glossmeter over the Ingersoll Glarimeter is the lower operating
angle used. The ratio of specular to diffu.se reflection is over three times
as great at 75® (15® operating angle) as it is at 57/° {52/° operating
angle).*®* The Sheen Glossmeter is similar to the Bausch and Lomb ex¬
cept that it ojierates at an angle of 45® instead of 75®.

(jIoss can be measured with the Hunter Multipurpose Reflectometer,
although the values obtained with one instrument do not duplicate those of
another instrument because receptor field angles are not accurately con-
trolle<l. Reproducible glo.ss measurements can be obtained on the Hunter
filossmetcr. Both s|)ecular gloss (sheen) and contrast gloss can be meas-
ure<l with the Hunter Multipurjxjse Reflectometer. The theoretical pri¬
mary standard for measuring sj)ecular gloss is the perfectly reflecting mir-

»“O. Kress and H. W. Morgan, Paper Trade J. 100, No. 26: 337-338 (Tune 27.

1935 ) vj .

Institute of Paper Qiemistry, Instrumentation Studies, Paper Trade J 104

Na 2; 19-21 (Jan. 14. 1937)

* ' Trurff /. m, .\<i. 26: 337-338 (June 27,

900

PULP AND PAPER

ror, although such a mirror is not physically attainable. Working standards
consist of highly polished pieces of black glass which are accurately cali¬
brated for gloss. In measuring specular gloss, part of the light beam is
taken out by means of a mirror and reflected onto the gloss surface at 45
from which it is diverted to the gloss photocell. (Attachments are available
for measuring gloss at 60 and 75°.) A semi-diffusing mirror actuated by
screw in front of the photocell is used to adjust the fraction of the gloss
beam falling on the photocell when the instrument is being standardized.
The galvanometer is set at zero when the gloss opening is measured; then a
gloss standard is placed over the opening and the galvanometer deflection
noted; finally the paper specimen is placed over the opening and the gal¬
vanometer deflection again noted. Gloss is determined by:

known gloss of standard surf ace x deflection obtained with paper s ample in pla^
-deflection obtained with standard surface m place

Contrast gloss (ratio of specular reflectance to diffuse reflectance) can lie
measured directly on the gloss scale. This is done by lowering the two
mirrors and using only the reflectance opening so that a measuremen is
made directly of the relative amount of light reflected specularly to the
amount refleLd diffusely. The reflectometer then furnishes approximate

relative values of t

_ a pparent reflectance 45°, -45°
contrast gloss— apparent reflectance 45°, 0

This may also be expressed as 1 minus the reciprocal of the above, or^

apparent reflerlan ce 45*. - 45° - appare nt reflectance 45*. 0*
contrast gloss- “ apparent reflectance 45°, -45

There is no single primary standard for contrast piater^n be

standards consisting of matte, white porcelain it

obtained, or a surface painted with white casein paint can be used,

has a value of contrast gloss near unity.

Contrast gloss describes the extent to which paper departs tr

ness. It correlates well with the tendency of paper to ^ _

specularly reflected light. reflect'ce is

low. For this reason, many chemists advocate "th“ to

gloss (percentage of =P^™'“V'‘'Tr"“os^‘ends ifsZ up Terences in
printing quality, and it is best adapted for measuring v 5 g y

faces (e.g., lacquered papers). „„™ring two sheets side by side

Gloss is often measured visually by comparing

under the same conditions of lighting. In most cases, the sheet

XVI. PROPERTIES OF PAPER

901

that the light falls on the paper at an angle of 45° at the front while being
viewed at an angle of 45° from the back.^°* However, easier differentiation
can be made by using lower viewing angles because of the increase in specu¬
lar reflection at low viewing angles. As the angle is lowered down to about
10 to 25°, total reflection increases, specular reflection increases, diffuse
reflection decreases, color effect decreases, opacity effect decreases, an
smoothness effect increases.^®^

Low viewing angles are particularly desirable in the case of colored
paper in order to keep the effect of diffuse reflection to a minimum. For
example, a yellow paper which has the same gloss as a blue paper when
viewed at an angle (field) of 10 to 20° might show a higher gloss than the
blue paper when viewed at an angle of 40°. This is due to the greater
diffuse reflection at the higher viewing angle, and the fact that the eye is
more sensitive to the yellow color.^o^ The viewing angle must not be too
low however, since this makes it difficult to distinguish different samples.
Viewing angles less than 5 to 10° are not desirable.®®^

Harrison^"® found that there is a broad linear relation between gloss
readings obtained on different samples using the Bausch and Lomb, Inger-
soll, and Sheen glossmeters, and the visual gloss determined by a number of
different observers. There are, however, substantial and erratic differences
between instrumental gloss ratings and visual gloss ratings. The Ingersoll
in particular shows poor correlation with visual grading. Kress and Mor-
gan^®^ found that the Bausch and Lomb Glossmeter, which depends upon
light reflection, correlates much better with visual rating than the polariza¬
tion type glarimeter when various grades of pigment-coated papers were
tested.

Importance of Gloss. Many printers demand a glossy paper because
they associate gloss with high surface smoothness and good printing quality.
Gloss is associated with high optical smoothness. In order for regular
reflection to take place, the height of the irregularities on the reflecting sur¬
face must not exceed about one-sixteenth of the wavelength of the incident
light.2®8

Gloss does not always go hand in hand with physical smoothness since
it is possible to have a glossy surface which is quite rough. Adding sand

V. G. W. Harrison, Paper-Maker 118, No. 2; 106-114, 123 (Aug., 1949)
Institute of Paper Chemistry, Instrumentation Studies, Paper Trade J. 104,
No. 1: 6-9 (Jan. 7, 1937)

2®* Idem.

205 Idem.

206 V. G. W. Harrison, Paper-Maker 118, No. 2:106-114, 123 (Aug., 1948)

207 0. Kress and H. W. Morgan, Paper Trade J. 100, No. 26:337-338 (June 27,
1935)

208 R. w. Wood, Physical Optics, p. 43, The Macmillan Co., New York, N. Y.
(1943)

902

PULP AND PAPER

to a glossy paint, for example, does not reduce the gloss, although it does,
of course, make the paint film very rough. However, the average observer
unconsciously tends to downgrade the rougher paper, even though the
gloss is the same.

High gloss is often associated with poor reading quality, due to glare.
The ideal surface should catch and reflect the light when the light source
is behind the observer and at a high angle of incidence, but should absorb
the light when viewed at a low angle with the light source in front. If the
paper surface does diffuse the light the same in all directions, it will exhibit
glare when the light source is in front of the observer, i.e., a higher pro¬
portion of the light will be reflected at the glare angle, thereby leaving less
light for the other angles where it is more useful. The proportion of light
at the glare angle need not be a great deal greater than the diffuse reflection
at that angle in order for the sample to exhibit glare, 4^0 greater light reflec¬
tion at the glare angle being sufficient to cause appreciable glare. Gloss
can be reduced by coating the paper with highly opaque pigments, provided

that the pigment surface is not too highly finished.

Ordinary grades of paper specularly reflect only 0.2% to 6.0% of the
light incident upon the paper at an angle of 45°,^°® although certain coated
liajiers may have much higher specular reflectance than this. Typical gloss
values for several different grades of paper are given in Table XXIII com¬
pared with a standard value of 100 for polished black glass.^^® As pre-
viously mentioned, black glass actually reflects specularly only 2Gyo o

incident light at an angle of 45°.

TABLE XXIII

Gloss ok Different Papers Compared to Polished

Black Glass as 100

Lacquer-coated papers

Magazine cover .

Machine-coated book ..
Supercalcndered hook .
English finish book ...

Typewriter bond.

Mimeograph .

Household waxed paper
Bread wrapper .

Finish. Finish is a broad term which is used in the paper industry to
describe the surface characteristics affecting the appearance an ee ^

S. Hunter, “A Multipurpose ^1940)^^ ^

Commerce, NalL Bur. Slavdards. Rcscarci ^ 1949)’ Measured

ZH. Technical Bulletin ‘-New Portalde 75 Glossmeter h 3 ^^

on Portable glossineler, Henry A. Gardner Laboi atoi le.,

Bethseda, Maryland

XVI. PROPERTIES OF PAPER

903

paper. It is a composite property and includes smoothness, softness, gloss,
and other less definable properties.

Because of its complexity, finish cannot be measured or expressed
by a single value, and is usually expressed as’ high, medium, or low, in the
opinion of the observer. Other indefinite terms used are smooth finish,
glazed finish, English finish, or machine-glazed finish. Pattern is related
to finish in that it is a surface characteristic attributed to variation or pat¬
tern in color, gloss, or surface smoothness.

With paperboard, it is customary to designate the finish by numbers
ranging from 1 to 4; No. 1 finish is a fairly rough surface, and No. 4 finish
is the highest possible machine finish. Other definitions are:

(1) Machine finish: either a high or a low finish, but a finish which is obtained
on the paper machine.

(2) English finish: a special machine finish which is quite high, but which is ob¬
tained without too much gloss.

(3) Glased finish: a finish obtained by moistening the surface of the paper and
then calendering under high pressure.

{4) Machine-glased finish: a finish obtained by drying the sheet against a large,
highly polished metal drying roll, a Yankee drier.

(5) Smooth finish: a finish obtained by the use of pressure rolls or a breaker
stack in the drier section of the paper machine.

(6) Antique finish: a rough finish which may be obtained by eliminating all cal¬
endering.

Drawing paper is an example of a paper requiring a special finish,
since this paper must be fairly rough, but must, at the same time, have a
fine grain. Special felts and special methods of drying and calendering are
often used to obtain the desired results.

Transmittance of Light

Transmittance of light refers to the ability of paper for passing rays of
light through the sheet. There are two ways in which light may be trans¬
mitted, by parallel, and by diffuse transmittance. Parallel transmittance
refers to the capacity for transmitting rays of light without scattering;
diffuse transmittance refers to the capacity for transmitting scattered light.

The sum of the diffuse and parallel transmittance gives the total transmit¬
tance of the paper.

Opacity is a property of paper which is determined by the total amount
of transmitted light (diffuse and non-diffuse). It is defined as the re¬
ciprocal of the amount of light transmission; that is, a perfectly opaque
paper is one which is absolutely impervious to the passage of all visible
light. Opacity is generally measured by the amount of “show through’'
when the material to be tested is held directly against the object to be viewed.

Transparency is somewhat related to opacity, but differs in that it is

904

PULP AND PAPER

determined by the amount of light which is transmitted without scattering,

A perfectly transparent material would be one which reflects, refracts, or
absorbs none of the light falling on the material, but instead transmits all
the light without scattering. If the material has a tendency to scatter light,
transparency will vary with the distance between the material and the object
being viewed.®^^ On the other hand, if the material is non-scattering, vision
will be good regardless of the distance between the material and the object
being viewed. Hence, transparency is measured by the percentage of sma
angle transmission of a beam of parallel light when there is a space separat¬
ing the material and the object being viewed. The transparency ratio, whic
is the true measure of transparency, is the ratio between the parallel trans¬
mittance and the total trahsmittance. The most transparent paper made
(glassine paper) has in the neighborhood of 10 to 40% parallel transmi -
tance for a total transmittance of 60 to 85%. On a comparable basis (65%

total reflectance), a tissue paper would have only 3%
tance.^^^ Transparency ratio is the best method of evaluating highly trans¬
parent materials, whereas opacity measurements are more suitable or re

213

Vetreen'thrtwo extremes of a perfectly opaque and a

rknown as translucency. Nearly all commercial papers are t™"
Lt r they permit the passage of some light. The word opaque .
looselv' used in reference to paper; for example opue bread
have an opacity of only 60%. whereas opaque book paper V
opacity of 90%. The smallest difference m opacity w - P

by visual observation is between 0.3 to 0.6 units. oropertv

Methods of Measuring Opacity. Opacity ,s an

of printing, bond, and writing has minimum

tion of these papers. The T . ■ ... raneing from 86 for

specifications for the opacity of uncoated book

a 45-lh. hook to 90 to 91 for a 60-lh. book (25 x 38-5TO • ^ „i„ed.

Opacity is a fundamental property which ,h‘ amount

In measuring opacity, a photoelectric “ surface Opac-

of reflected tight when the paper is backed by an app p^

ity can best he iiieasiired by i's hacked by a black body

reflectance when a single sheet , ' , ^ , .t ,he paper is hacked by a

to the diffuse reflectance when a single ' P , . ^^^en tacked

white body. The reflectance of the paper IS generally g

ail D. B. Wicker. Pafrr 'P’'"*/' 321 ^ 3 W^(Nov. 26.’ 19361

a.a I.. C. Lewis, Paper Trade J. JW. No 22 32.3JOT t Nov .

aisD. 13. Wick^, Paper Trade J. ' p^p„-Maker IK, No, 6 : 416-420,

213 « V. G. w. Harrison and S. K. U rouiwr, r i

A29 (June, 1950)

XVI, PROPERTIES OF PAPER

905

by the white body than when backed by the black body because of the greater
amount of transmitted light which is reflected by the white body. If there
were no difference in reflection when backed by the white and black bodies,
the opacity would be lOOfc, representing a perfectly opaque paper. If the
reflectance were zero when backed by the black body, the opacity would be
zero, representing a perfectly translucent paper. The ideal contrast ratio
could be obtained by measuring the apparent reflectance of the paper when
backed by a perfectly absorbing body (Ro) divided by the apparent reflec¬
tance when backed by a body having a perfect or unit reflectance (/?i.oo).
A black cavity lined with black velvet having an apparent reflectance less
than 0.5% can be used for the Ro reading, but there is no perfectly reflect¬
ing white body, and consequently a body of reflectance lower than 1.00 must
be used for white backing. There are two methods of measuring opacity
by contrast ratio: (I) TAPPI opacity, using a white backing of reflectance
of 0.89, and (2) printing opacity, using a backing of an opaque pad of the
paper under test. These methods of measuring opacity are discussed in the
following sections.

When instrumental measurements of opacity are being made, the
spectral nature of the light, geometry of illumination and viewing, and the
spectral reflectance of the backing body must be specified. For best results,
monochromatic light should be used such as that obtained from the green
filter used on the photocell in the Bausch and Lonib. Opal-glass standards
can be used for standardizing the opacimeter.

Measurement of TAPPI Opacity. TAPPI opacity is the ratio of
the reflectance of a single sheet backed by a black body to the reflectance of
a single sheet backed by a white body having an absolute effective reflectance
of 89%, that is,

TAPPI opacity = R 0 /R 0.99

( I he ap{)arent reflectance of the backing white body is 91.5%, which is ob¬
tained by dividing the effective reflectance by the absolute reflectivity of
magnesium oxide (97%).) A white background of high reflectivity is
used in measuring T.APPI opacity in order to obtain high contrast with the
reading obtained with the black background. In the Bausch and Lomb
(Opacimeter, the white backing is not placed in actual contact with the
sample, and this, together with other factors, results in white backing having
an absolute reflectance of 89%.*’* For bond papers, TAPPI opacity varies

from about 80 for light-weight sulfite papers up to about 93 for heavier rag
content papers.

^ Measurement of Printing Opacity. Printing opacity is the ratio of
the reflectance of a single sheet of paper when backed by a black body to the

*** D. B. Judd, Paper Trade /. 100, No. 1 : 4-8 (Jan. 3, 1935)

I'ljLr AiM> rAr»^

HM)

Printing njacily incusuro the s.u»ic projicrt)
hh(»w lliruUKh when a iunnl>cr o( sheets printed on
.. ,.;u PrCftrirtcr nt»;iritv is nearlv alwavs a larger v

{>reafcd as AV^*.

\i«ually judgetl ai
stacked

show throuKl' wnen a nninner oi snccis |niiu«.'* win. —i. --

a pile. Printing opacity is nearly always a larger \’alue than TAPI I <noc-

ily, since the amount of light returned from a thick pad erf the |«peT is
. 11 - I_— 1 stafiflard white re*lector ixsed m

^ m. M a a. p • • a * ^ ^ ^ ^ ^ - —

measuring TAP PI opacity.

___ _ A A, V

opacity

sunfrfes

have slightly tlifTcrent TAPPI opacity. *--- \

have very different values of rcUccting |wwer for a single sheet and lor a

pile of sheets, only the ratio of the two reflecting [wmers remaining con-

.stant.’'“ The |K)orcst correlation is observed betw*ecn a white and a ^ay

• _ 1 ___ Kni n til#* .uinr orintincr onacilv, the white

paper. H a wniic anu i«|.i.. ..•**-*•--- i

...prr will l.avc the Rreatcr TAl’Pl opacity. An .llu.tration cf a wW
anti a RCav IHpcr which have cq.al printing opaat.c* tat oncqiuJ TJ^l l
opacities is shown it. Ttthlc XXIV.- There ts ^ ^ m

’ _ ... i'.T _Z.. rxr.nlinrr mtlfl whiell iS O //O IH POWl

cases.

T.XBLE XX1\"

White and Gray Papijt H.amsc EoOAt Printisc Opautiis

BUT l’Nf.>UAL TAPPI OpACITIKS

VMritc

Cr»f giapef

ss%

7<<e

TAr r 1 upaciiy .

Printing opacity .

87.5^

The above illustration is an extreme case and would not occur in
testitl s^.^ generallv only papers of similar brightness are con,p«^or
o" inlrily, tliere is very dose correlation

and contrast ratio for most papers.-’- Printing opaa > i^ ppi opadtv
Printing opacity is less subject to instrument ^

to affect the effective reflectance of the -lute ^

hC be^n the need of a thick pad of the paper for

ever, this objection can ta overcome by from

the Ro and TAPPI opacity* readings, using table, o

rtsU. N. DaAns, Paper Trad^ f

Institute of Paper Cheimstrj*. Instnimentation btwues.

4:30-36 (July 27, 1939) , <,5 x* ia- iRl-184 (Oct- 15. 1931}

SITM. N. Davis. Paper Trade J. 93 Xo^- /. UT^,

*18 Institute of Paper Chemistry, InstruroenUUoo btuoies.

No. 4:31-36 (July 27. 1939)

XVI. PROPERTIES OF PAPER

907

the Kubelka and Munk equation. The means for doing this will be given
later.

Measurement of Printed Opacity. Another type ot opacity meas¬
urement is printed opacity. This test is made by using a sheet printed
on one side witli a solid black color for tlie Ro reading. 1 he opacity is
measured as the ratio of the diffuse reflectance from the uninked side of the
printed sheet to the diffuse reflectance of an opaque pad of the imprinted
paper. This test is designed to measure the show through which occurs
on printing. The results depend upon the amount of penetration of ink
and vehicle into the pajier, as well as the original opacity of the paper.

Measurement of Opacity by Transmittance. Measurement of
oi>acity by reflected light, as done in the case of printing opacity and TAPPI
opacitVt is a more useful method of measuring opacity than a measurement
by transmitted light. The reason for this is that the opacity of most paper
is judged by looking at the paper in reflected light while the paper is backed
by a surface covered with light and dark areas. However, in the case of
jiackaging papers, which are used primarily for their light-protecting
qualities, it is more suitable to measure opacity by transmission than by
reflection of light.

CONTRAST RATIO, Bousch 8 Lomb

Fig. XV^I-28. Relationship between contrast ratio and total
transmittance for transparent papers.

In measuring opacity by transmitted light, a sample of paper is placed
between a standard light source and a photoelectric cell, and the total
amount of light transmitted is conqared with the total amount of light fall¬
ing on the photocell wlien there is no jiaper lietween the light source and
pliotocell. For transjiarent ija[)crs of the same absorptive qualities, there
exists an empirical linear relationship between the total transmittance of

908

PULP AND PAPER

light and the contrast ratio, as sliovvn in Figure XVI-28,“* but for ordinary
papers there is no simple relationship between opacity as determined by

contrast ratio and by transmittance.*®**

Effect of Sheet Weight on Opacity. Increasing sheet weight has

the obvious effect of increasing tlie opacity of the paper. There is, however,

a limiting point beyond which further increases in basis weight have no

further effect on opacity. The results in Table XXV. taken

show the effect of increasing basis weight on TAPPI opacity (Ro/Ko.m)

and reflectance over a black backing body (^o)*

TABLE XXV

Effect of Basis Weight on OPAcrry and Reflectance

Basts

weight

25 X 40—500

Sulfite writing paper
(/?a) = 0.81, ash 0.36%)

Book paper
= 0.83, ash 5.89%)

(Hunter)

Printing

opacity

(Hunter)

TAPPI

opacity

(Hunter)

Printing

^acity

(Hunter)

TAPPI

opacity

111

0.606

0.646

0.677

0.714

0.740

0.778

0.747

0.799

0.831

0.881

0.906

0.959

0.712

0.768

0.807

0.862

0.887

0.944

0.725

0.745

0.769

0.785

0.802

0.821

0.877

0.900

0.929

0.947

0.906

0.994

0.85

0.88

0.91

0.93

0.95

0.99

Effect of Refractive Index on Opacity. The amount of "
fleeted and refracted by paper is determined by the ^

tse constituents, and by the amount of
'reTrh^ttll be « feTndices

C‘t cirS -:::

fleeted at each- boundary is larger the greater the difference betrveen

fractive indices of the two matenals. ^fraction strikes the line

When light rvithin a material of high ^ ^^jium of low

of separation between this materia an \ into the surround-

refractive index, part of the light will p^s ( er r Because the

909

XVI. PROPERTIES OF PAPER

low index of refraction, the light is bent away from the

greater than the angle of incidence. As the angle of mcdet^ce ts

Se angle of refraction increases so tliat eventually the refracted ray e

along the line of separation between the material and "'f'

ium This is known as the critical angle, i.e., the angle of
which the angle of refraction is 90». All the incident hght which strikes
at an angle greater than the critical angle will be totally reflected back into
the material of high refractive index. The critical angle can lie calculated as.

sine angle of incidence _

sine angle of refraction (90")

where m, is index of retraction of surrounding medium relative to that of
the material. Since sine 90“ = 1, » sine critical angle. Thus, it can be

seen that the higher the index of refraction, the greater the critical angle,
and the more light which is reflected. This accounts for the high opacifying

effect of materials of high refractive index.

Effect of Sheet Density on Opacity. Cellulose absorbs very little

light, and conseijuently a homogeneous sheet of cellulose (e.g., cellophane)
is quite transparent. Ordinary paper is not homogeneous, since it is made
up of individual fibers separated by air spaces. Consequently, paper has
a relatively high opacity.

The fiber area in paper is made up of two components, area which is
in optical contact with other fibers and area which is not in optical contact
with other fibers. Optical contact exists when two fibers are bonded to¬
gether in ordinary fiber bonding. The unbonded areas of the fiber are
res|X)nsibIe for the scattering of light, since it is only at the fiber-air inter¬
faces that scattering occurs. No scattering or reflection of light occurs
when two fibers are in optical contact.

The scattering coefficient of paper {S value) is a linear function of the
s|)ecific surface per unit of mass of the paper,*®*' *** and hence opacity must
also be a direct linear function of specific surface, assuming there is no
change in absorption {K value).*** Beating w'ould be expected to increase
opacity, l)ecause beating increases the total surface area of the fibers at
which scattering may occur. However, beating also increases the area of
optical contact between fibers by increasing the area of fiber bonding, and
this tends to reduce the opacity.**’ Pulps of exceptionally high alpha cellu¬
lose content tend to increase in opacity on beating because the amount of
fiber bonding of these pulps increases ver\' little on beating, whereas the
specific surface of the pulp is increased.

S. R. Parsons. Paper Trade J. US. No. 25 : 314-322 (Dec. 17, 1942)

M. N. Davis. Paper Trade J. Ill, No. 14:4CM4 (Oct. 3, 1940)

•** The S' and K values are those in the Kubelka and Munk theory.

»«F. T. Ratliff, Toppi 32. No. 8 : 357-367 (Aug,, 1949)

910

PULP AND PAPER

The fiiticJanicntal sheet property affecting opacity is the density of the
paper. According to Clark,the opacity of paper decreases approximately
linearly with increasing solid fraction over most of the commercial range of
density. Actually, opacity goes through a maximum with increasing solid
fraction, because at very low solid fractions, the paper approaches pure air,
which has zero opacity, and at very high solid fractions (approaching
1.0), the paper approaches pure cellulose, which also has zero opacity.
Doughty--^ found that opacity goes through a maximum at a solid fraction
of 0.3 to 0.5, as shown in Figure XVI-29, when unbeaten spruce sulfite

0.90

SOLID FRACTION

Fig. XVI-29. Effect of solid fraction on opacity. Unbeaten spruce sulfite
wet-pressed into sheets. Basis weight 40 lb., 25 x 36—500.

pulp is wet-pressed into sheets of varying solid fraction. Wicker--® oun
that the transparency of paper increases rapidly as the solid fraction o t e
sheet is increased from 60% on upward until eventually the transparency
reaches a maximum at a solid fraction of about 90%, which is the
upper limit. The last 10% of residual air voids places a limitation on e
traLparency of glassine sheets, since these air voids have an
effect on light transmission. Wicker points out that the b'^ffmgent nature
of cellulose also imposes a limiting transparency less tian ,

It is obvious from the above that one method of produemg *
paper is to process the pulp mechanically through beating so a i
a sheet of high solid fraction which contains a mmiiniim of fib
terfaces. Calendering increases the transparency for the ^

Smoothness of the paper is a factor, since the opaci y en s

the smoothness of the paper is increased. „.u,Vh results

Effect of Fillers on Opacity. The increase in <^aci y ^ _

from filling is due to an increase in the scattering coefficien o

from g 4 . fihpr «;ratter li'^ht independently of each

In filled paper, the pigment and fiber scatter ^ i

J. d’A, Clark, Pulf PoPer Mag.

«« R. H. Doughty, Tech. Assoc ^^iPoa;, 4 1940)

228 D. B. Wicker, Paper Trade J. 110, No. 1.8-14 tJ • .

XVI. PROPERTIES OF PAPER

911

Three interfaces are involved: fiber-air, pigment-fiber, and pig

rncnt'^ir. , • * * r t <acc

Keflecuon and scattering are greatest at the pigment-air mtertace. Less

scattering occurs at the pigment-fiber interface, e.g., m the case ot Utaniiim
dioxide, reflection is IS^o lor perpendicular incidence from air to titanium,
and about 6% for perpendicular incidence from cellulose to titanium.
Tliere is relatively little scattering at a clay-fiber interface, due to the smal
difference in the refractive indices of clay and cellulose. In pigment-hlled
naijers most of the surface area of the pigment is not in optical contact with
ihe fillers, and hence most of the scattering occurs at pigment-air interfaces.
.\nyihing which increases the numlier of pigment-filier interfaces at the ex-
i*^ise of i>igment-air interfaces will lower the opacity. Increased wet press¬
ing of pigment-filled papers tends to lower the opacity, not only becau.se it
increases tlie amount of fiber bonding, but also because it increases the
bonding lictween the pigment jxirticles and the fibers, thereby substituting
pigment-fiber interfaces for pigment-air interfaces.

The scattering coefficients for a numlier of paixrrmaking pigments are
given in the chapter on filling. In choosing a pigment for its opacifying ef¬
fect, it is wise to choose one with a high scattering coefficient (S value).
Pigments with high absorption coefficients (A’ values) also increase the
ofiacity. but they reduce the brightness. The effectiveness of fillers in in¬
creasing opacitv depends upon the type of fibrous furnish, as well as the
amount of beating given the stock.’’* Certain of the white pigments in¬
crease the brightness, as well as increase the opacity of paper. It is pos¬
sible, by U'iing a combination of a white pigment and a dyestuff, to obtain
an exceptionally large increase in ojiacity by maintaining a constant bright¬
ness.’*’

A given amount of pigment used as a filler produces a higher opacity
tiun the same amount of pigment used as a coating.’” Close packing of pig¬
ment particles causes interference which decreases the effectiveness of the
particles as reflecting units, thus accounting for the reduced opacity of coated
|iapers, since the pigments in coating are closely packed compared with the
pigments in paper.***

Mineral constituents left in the paper after the cooking and bleaching
o(>erations increase the opacity, although they have a minor effect when

**»F. A. Steele, Pafer Tradt /. 104, No. 8:129-130 (Feb. 25, 1937)

**•!.. C Lewis, Paptr Trade /. 103. No. 22 : 323-330 (Nov. 26, 1936)

»*' H. Gaegauf and M. Muller, Textil-Rundschau, No. 6 : 222-229; No. 7:258-265
(1947) through BtdU Institute Paper Chemistry, IS. No. 1:31-32 (1947)

*« E. R. Uughlin, Paper Trade J. Ill, No. 22 : 275-281 (Nov. 28, 1940)

*» R. W. Ball and O. P. Lane. Paper Mill 59. No. 21: 20-22, 24-25 (May 23, 1936)
-*• R. H. Sawyur. A Applied Phys. 13, No. 10 : 596-601 (Oct., 1942)

912

PULP AND PAPER

present in less than Excess alumina, starch, or other gelatinous

materials lower the opacity because they increase the area in optical contact.

Effect of Dyestuffs on Opacity. Dyestuffs have a pronounced ef¬
fect on opacity. Laughlin^^s found, at least in the range of tinted whites,
that the fiber, filler, and dyestuff in the paper each absorb and scatter light
independently of the other, and that the opacifying effects are additive.
The effect of dyestuffs on opacity is obvious in the case of a thin, deeply
dyed black paper which may owe nearly all its opacity to light absorption.
The effect is not so obvious in the case of white papers tinted with a small
amount of dyestuff, but even this small amount of added dyestuff tends to
increase the opacity by absorbing part of the light. Blue-white papers are
generally more opaque than yellow-white papers, because the blue-white is
obtained by adding blue dye to the pulp. For example, Steele^” presents
the following data for two different papers made from the same pulp:

Creamy white

Blue-white

Test

0.824

0.936

0.778

0.962

Reflectivity .

Opacity (TAPPI)

As can be seen, adding blue dye to obtain a blue-white color lowered the
brightness, but increased the opacity of the paper. Wicker^^® found t at
certain dyes have the ability to increase the transparency of glassine, but
this is because the dye decreases the diffuse transmittance to a greater e-
gree than the parallel transmittance. The opacifying effect of yestu s is
less on relatively opaque pulps, such as groundwood, than on rag or su e

239

913

XVI. PROPERTIES OF PAPER

roe blue is almost twice as effective in increasing the 0 [»city when *e

ample is tested in anificial light as it is when tested ;

The wavelength of the viewing light also has a considerable effect on trails

parency.**‘

100

WAVELENGTH, m/a

Fig. XVI-30. Effect of dyestuffs on opacity.

Effect of Waxing on Opacity. If paper is impregnated with paraf¬
fin or similar material having an index of refraction close to that of cellulose,
the opacity of the j^aper is greatly reduced. Even if the paper is filled or
coated with talc, clay, or other pigments having a refractive index in the
neighborhood of 1.5, the opacity of the paper will be very low after waxing.
On the other hand, if pigments of high refractive index (e.g., zinc sulfide
or titanium dioxide) are present in the paper, the paper will retain a high
degree of opacity after waxing. Table XXVI shows the amount of light
reflected from different air. fiber, titanium dioxide, and wax interfaces, and
illustrates the effect of titanium dioxide on the amount of reflected light

after waxing.***

TABLE XXVI

Amount or REn.rcnoN or Light at Vabious I.nterfaces

_ . Per cent light

Interface reflected

Celluluse-air . '^•0

Cellulose-wax .. ^-1

Titanium dioxide-wax . 7.5

Titanium dioxide-air . 18.0

The effect of waxing on the opacity of two different unfilled papers and
the corresponding papers after filling with 5% titanium dioxide is:**®

OpttdtT of w&zed Opaettr of waxed paper

paper. oifiDod filled witli titanitun dioxide

20% 57%

50% 80%

L C Lewis. Pa^rr TratU J. 103. No. 22 : 325-330 (Nov. 26. 1936)

Idfm.

***Tkr Hmmdbook, Titanium Pigment Corporation, National I.ead Company, 11
Broadway. New York, N. Y. (1949)

914

PULP AND PAPFR

rahic XXV’ll shows the eH'ccl of a small amount of titanium flioxith*
in clay-coated pajiers rjn the opacity after waxing. It can Ik; seen frotu
this table that increasiuj' the coat weijiht from U to S ll>. increased the
opacity of the w'axed paper Irom 24.5 to only 29.5^r lor the clay-coatefl

* ^ jM. # #

paper, whereas the same increase in coat weight for the sample containing
10% titanium dioxide increased the opacity from 24.5 to 50.5%. This is
the reason \vhy coated papers for waxing (e.g., opaQue bread wrap) must
be coated with titanium dioxide or other pigment of high retractive index.

T.\BLF. XXVII

F.ffect of W.winc on T.APPI Opacity of an Aix Clay-Co.\ted Paper and a
C i-AV-CoATED Paper Containinc Titanu m Dioxide at Different Coat Weights

U, uinvaxed. W, waxed.

One interesting point in connection with coated papers is that waxing prac
tically eliminates the effect of the adhesive. In other words, although the
amount of adhesive used in the coating before waxing has a pronotincec
effect on the opacity, once the sheet is waxed, the adhesive ratio is no
longer a significant factor because the refractive indices of the adhesive an

wax are very close to each other.

Effect of Different Pulps on Opacity. For paper made front a
<Tiven pulp, the opacity is a direct function of the fineness of the fibers, that
Ts. the fiber diameter. The length of the fibers has little or no effert on
oprfeitv.^*^^ since fiber length does not affect the number ot transverse fiber-

air'interfaces. due to the fact that nearly all fibers he m '

sheet and light passes through them only transversely. Short-fibered pulp
such as hardwood soda and groundwood tend to produce papers
tivelv high opacity, but this is due, in the case of soda pulp, to t e re
poor bonding qualities and, in the case of groundn ood, to ^

The same hardwood species cooked by ■the sulfite prw^s
soda process produce papers of low opacitj' because of the hig i »

fiber bonding and the high sheet density. ^rmind-

The effect of fiber size on the specific surface and opacity of grounn^

wood pulps is shown in Table XXVIII. taken from J^y Parsons.^„.

These results show that the s|3ecific surface and the opaci > g
... L. C. Lewis, Pater Trade I. 103. No.

2*5 s. R. Parsons, Paper Trade J. 11:>, Xo. -5. 314-32.. (Dec. ,

XVI. PROrEHIES OF PAPER

915

T.\BLE XXV'III

Errtci or Fibe* Size ok Sptanc Su»f.\ce akd Op.acifyinc

Effect of Gbovkuwood Pulp

Fnctioa

SpeciAc surface.
*Q.cfn./c.

(^vcnxis

tedwkjnO

Printing

opactiy*

12-20

11.400

79.8

12-35

12300

85.0

12-65

13,900

87,4

91.1

12-15U

24,100

13-luie»

47,400

98.8

12-0

34300

92.6

crcasc a> ilic liber Uiiiieiisioiis are decreased. The fines have a dispropor¬
tionately high opacifying effect, dne to their higli light absorj)tion.

Because of tlicir inherent light absorption and light scattering powers,
each major grade of pulp lias a rather consistent or typical effect on the
opacity. However, the ojjacity obtainable with any pulp depends upon the
amount of beating and calendering, and also on other pajiermaking factors
which affect tl»c degree of bonding in tlte sheet. Hence, any listing of pulps
according to their ojiacifying effect is only relative. How’cver, it may be
stated as a general rule that the o|Kicity of pa|)er is higher the lower the
reflectivity of tlic pulp from which the pa|)er is made. Thus, unbleached
pulps produce papers of higher o|>acity than bleached pulps, diic to the fact

OPACITY. X

Fi*. XVI-31. Effect of uifftleachcd (a) and bleached (b) sulfite
P®lp biith with itroutKiwood~— on the o|iacity of newsprint.

916

PULP AND PAPER

that lignin absorbs more light than cellulose. 1 he effect of adding different
amounts of unbleached and bleached sulftte pulps to a groundwood furnish
is shown in Figure XVI-31. Maass^^® points out that the addition of a
little unbleached sulfite to a groundwood furnish has relatively little effect
on the transmittance or reflectance of light, compared with the significant
decrease in transmittance and increase in reflectance obtained when a little
groundwood is added to an unbleached sulfite furnish.

Kubelka and Munk Theory

The theory of Kubelka and Munk has been extremely helpful m under¬
standing the optical properties of paper. This theory is based on the defini¬
tion of two fundamental optical constants; the specific scattering coefficien
(S) and the specific absorption coefficient (K). These coefficients, are
characteristic of any diffusing material such as paper and are define
as follows: (i) K is the limiting value of the absorption of light en g p
unit thickness as the thickness becomes very small; (2) 5 e g

value of light energy scattered backwards per unit of thic cness as
ness becomes veryliall. The /C value is proportional to the average length
of path traversed by the light in passing through the material, ""'c^ely
proportional to the density, and is a function of

Lai to the total surface area per unit mass of the matenal, inverse y pro-
Lffonll to the density, and increases with the refraction index it ^
nosr a iLter of the sheet structure. These fundamental constants K
and S determine the optical properties of paper, and are very usefu i
calculating the effect of basis weight and furnish changes on the c^c an

and opacity of up /o

.x* —.»- »-e

preciable error.^^® ^ e r,.,iltinUed bv X (the thickness of the

If the scattering coefficient, 5, is mult p > ^

paper), the product, SX, ^ ‘‘’c ™,l,iolied by X, the product, XX,

wise, if the absorption coe cien , , nractice X is represented by

is the absorption coefficient of the paper, " 3 p,rifica-

weight instead of thickness, because handled in

tion, and because coefficients based ^ S ; jo^etiuies used as

calculations. . 1 proportionality constant

S weight. The basis weight should be expressed

O. Maass, 1940)

::: ^ry, Im.run,en.a.ion Studies, Faf- Trade /. ,

No. 4: 31-36 (July 27, 1939)

XVI.

PROPERTIES OF PAPER

917

. j oc ^ AH_?nn 9 and are not fundamentally

terms of the standard ream 25 x 40-500. i ana o are

the same quantity. For example, if a sample of paper i P

dry condln, the and S-X values are not S’-f“‘fj

vaL increases because of the greater number of scattering P

unit of thickness. The S' value is a characteristic of ^.

per, and is not influenced by the weight or thickness of the

it is influenced by physical changes in the material rnaking up the p p .

For example, beating affects the S‘ value of pulp, and particle

the S' value of a pigment. Unless otherwise stated, the S values p

in the following discussion are based on weight rather than on thickness

and are in reality 5^ values. , tv , , 4 .- •

Kubelka and Munk Equation. The Kubelka and Munk equation

may be expressed as follows for diffuse light.

R ^1 + (K/S) -\(K/Sy-^2iK/S)Y-^

where K is the absorption coefficient, 5 is the scattering coefficient, and R„
is the reflectance of a pad thick enough to be opaque. If made at the proper
wavelength, the value is papermaker’s brightness. The derivation of

this formula has been shown by Steele and Judd.^^®'

The above equation may be transformed into a more workable form as

shown below:

K/S=[l-Kr/2R

By the use of this simple formula, a single brightness reading (R^) will
serve to give the K/S value for any paper. Table XXIX show's the
relationship between monochromatic reflectance of an opaque pad of paper
(i?„) and the K/S value of the paper. It should be stressed that a single
measurement of R^ gives only the ratio of K to S, not the individual K and
5” values.

Determination of K and S Values of Pulp Mixtures. For most
practical purposes, the K/S of a pulp mixture can be determined from the
K/S values of the components of the mixture. This can be done by means
of the following relation, without having to find the individual K and S
values of each component.

(K/S)

mixture c.{a:a),+Cv(K'/5),

where Ci and C 2 are the fractions of each component of the pulp mixture,
and (K/S)i and (K/S )2 are the corresponding values for each component.

Although the above formula is not fundamentally correct, it works very
well in practice, provided the components are of the same order of magni¬
tude. All that is necessary to use the above relationship is to obtain the

2*9 D. B. Judd, Paper Trade J. 106, No. 1; 5-12 (Jan. 6 , 1938)

259 F. A. Steele, Paper Trade J. 100, No, 12; 151-156 (March 21, 1935)

Table continued

TABLE XXIX (continued)

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REFLECTANCE-,

922

rULl’ ANn PAPER

T —I— 1 1 1— I—I—I 1— j — I — I — I 1— I — I 1—I— I —j I ' i i I ' ' I"

DIAGRAM SHOWING INTERRELATION Of REELCCTANCE, R, ,

REFLECTIVITY, R*. , AND TAPPl OPACITY, Com=

.70

“ "hd

.50

V>'

TMI5 CHART IS CON5TRUCTCD TROM Tnt KueCuHA-

ano-munk formula, it gives:

1. dependence of reflectance , TAPPl opacity ,

AND PRINTING OPACITY ON THICKNESS.

2. INTERCONVERSION OF TAPPl OPACITY AND
PRINTING OPACITY.

3. SCATTERING POWER fSx) FROM

reflectance and tappi opacity.

.'J.

. 1 ,-

.70

.00

SSSfl!

OVtiifiii

I TI OT

li>0

TAPPl OPACITY, Co.09

Fig. XVI-32. Relation between reflecttnce over
TAPPl opacitj% reflecti\-ity. and scattering power
National Bureau of Standards).

xvj_ properties of paper

923

K S value of each component of the pulp mixture from the brightness (/?,)
reading by using Table XXIX, and then to substitute in the formula.

In the case of mixtures where there is a marked difference m A/i
values of the different components, the above relationship cannot be used
because it is necessar>' to use individual K and X values. Once the indi¬
vidual K and S values are known, the following relationship can be used.

CiKi-^C.K:

Si,i C1S1 + C2S2

where Ci is the part by weight of material 1, and C* is the part by weight
of material 2.

To determine K and X values individually, a reflectance measurement
R^t of a known thickness, .V, over a background of reflectance A* is neces¬
sary in addition to the A. reading. The following relation can then be used.

2.303 , lAo.>,-(l/AJl[0.98-AJ

l/A.-A. [0.98-(1 /AJ][Ao.o8-A„1

X i-s found, from the above relationship, by dividing 6 A by A; A is found
by multiplying the A,A value (obtained as described above) by S.

I he calculation of SX from the above formula is obviously quite com-
pbi.-iinl. Ill order to simplify calculation, Kubelka”® has given a .set of
formulas in terms of hyperbolic functions by which exact calculations of
^S*,V can be made conveniently. It is also possible to make rapid, approxi¬
mate graphical solution by use of the curv’es in Figure XV’^I-32, which
show the interrelationships lietween reflectivity (A^), reflection (Ao),
TAPPl opacity (Ao/Ao.„), printing opacity (Ao/A,), and scattering
Imwer SX. Two families of curves are plotted on a single set of co-
»*rdinates. the ordinate Ixring Aq and the abscissa being TAPPl opacity
(- RjRn ‘*). By the use of these curves, a reading of SX can be ob¬
tained from a knowledge of the T.APPI opacity (Co.g») and the reflectivity
fA,). Thus. X v'alues can l)e determined from this chart, if the basis
weight 25 v 40—500 (.V) is known, by dividing X.Y by A'. Knowing the
X v'alue and the reflectivity (A,), the A* value can then be determined from
Table XXIX.

Factors Affecting K and S Values of Pulp. Once the K and .V
values have been determinetl for a given pulp, it can lie assumed that these
values of the pulp remain constant, unless changes are made in the method
of pulp preparation or a new pulp is used. Steele**’ gives comparative X
values for different pulps, as shown in Table XXX, when the pulps are
treated in the same w'av'. These values are based on a basis weight of

22—^00. These are only comparative values, of course, and should
not he taken as absolute values.

“* P. Kobelka. /. Optical Soe. America 38: 44^58 (1948)

*“F. A Steele. Paper Trade /. 100, No. 12: 151-156 (March 31, 1935)

924

PULP AND PAPER

TABLE XXX

Comparative S Values for Several Different Pulps

Type of pulp

Unbleached sulfite .

Bleached sulfite .

Soda pulp .

Bleached kraft .

S value

(17 X 22—500)

0.086-0.111

0.106-0.133

0.174

0.186

Judd^®^ has presented data demonstrating how the scattering coeffi¬
cients of paper can be determined from measurements of conventional opti¬
cal properties. The results obtained on four different commercial papers,
rag writing, sulfite writing, book, and newsprint, are shown in Table XXXI.
The Ro and Co.so values were obtained experimentally, and R„ and SX val¬
ues determined from curves similar to Fig. XVI-32. Coefficient of scat¬
ter per inch and coefficient of scatter per pound were obtained by dividing
the SX values respectively by the thickness of the paper (in inches) and
basis weight of the paper (in pounds 25 x 40—500).

TABLE XXXI

Calculation of Coefficient of Scatter from /?o and Co.w

Data

Paper thickness X, in.

Basis wt. of paper X, lb. (25x40 500) ..

Ash, % .

J^o ...

Co .80 “ J^o/Ro .80 . - • ...

Ra> from chart.

SX from chart .

Coeff. of scatter S', per in.

Coeff. of scatter S^, per lb. (25x40—500)

Rag

writing

Sulfite

writing

Book

Newsprint

0.0028

0.0037

0.0048

0.0056

62.5

0.36

0.34

5.82

0.38

0.683

0.677

0.785

0.605

0.767

0.807

0.937

0.985

0.890

0.805

0.827

. 0.609

2.23

2.40

5.05

4.28

800

650

1050

760

0.059

0.046

0.081

Book and newsprint have a high scattering coefficient, as can ^
rom Table XXXI. The high S' value of book paper can be ascn e

presence of filler (high ash). The high S value „

iscribed to the presence of darkly colored groundwoo . can e

;he results in the table that book paper has about the same

print when S is expressed on a weight basis, but that the v

paper is much larger than that of newsprint when the S value is expre

L'the basis of thickness.''^ The book paper has a much higher g

than the newsprint; the S values (weight basis) of the two paper

284 D. B. Judd, Paper Trade J. 106, No. 1: 5-12 (Jan. 6, 1938)

256 Idem.

XVI. PROPERTIES OF PAPER

925

same, but the K value of the book paper is lower and consequently the K/S

value is lower. • r

The specific scattering coefficient of pulp is a linear function o e

surface area. Because the surface area is reduced by fiber bonding,

would be expected that anything which increases fiber bonding, such as

increased beating or wet pressing, would in turn reduce the S value of the

pulp. The effect of beating on the and K values of a bleached spruce

sulfite pulp is shown in Table XXXII.^=® It can be seen from this table

TABLE XXXII

Effect of Beating on K. and 5" Values of Bleached Spruce Sulfite Pulp

Beating interval (Lampen mill), min.
Freeness (S-R), cc.

5^ (25x40—500)
(25x40—500)

0 IS
835 690

0.886 0.882

0.0537 0.0348

0.0394 0.0275

30 60
520 355

0.851 0.835

0.0298 . 0.0249
0.0389 0.0406

that substantial decreases in 5" occur as a result of beating; the K values
showed an initial decrease, followed by an increase, so that the over-all
change was small. Nordman^®^ has shown that the ratio of the scattering
coefficient to the specific surface varies with the type of beating apparatus;
a special disintegrator and Clark kollergang, for example, produced pulps
having a higher scattering coefficient than a Lampen mill for a particular
specific external surface. The loss in scattering coefficient with beating
also varies with the type of pulp; for example, bleached sulfite loses scat¬
tering power more rapidly than strong sulfite.®®® There are particular
cases where beating actually increases the scattering coefficient, but this
occurs only if the pulps are formed into paper under conditions where fiber
bonding is negligible. For example, increased beating increases the scat¬
tering coefficient of pure alpha pulps, and increased beating increases the
scattering coefficient of ordinary pulps if the sheets are formed in non-polar
liquids.®®®

The effect of wavelength of light on the 5 and K values and reflectivity
(RJ) are shown in Table XXXIII for spruce sulfite pulp.®®® The variation
in K value and with wavelength is to be expected because of the slig htly
yellowish color of the pulp. The decrease in specific scattering coefficient
with increasing wavelength is fairly small and can be attributed to variation
in the internal scattering within the fiber with changes in wavelength.

258 S. R. Parsons, Paper Trade /. 115, No. 25: 314-322 (Dec. 17, 1942)

257 L. Nordman, Svensk-Papperstidn. 52, No. 18 : 441-447 (Sept 30 1949)

258 Idem. ■ '

Idem,

260 S. R. Parsons, Paper Trade J. 115, No. 25: 314-322 (Dec. 17, 1942)

926

I'ULP AND PAPKR

TABU*: XXXIII

Variation in A:«, S. and K Values with \Va\tu>:noth (Spruce Sulpite I’utf)

Wavelength, mji

8, 25 X 40—500

K . 25 X 40—500

420

0.808

0.0659

0.1505

500

0.877

0.0612

0.0528

600

0.899

0.0582

0.0330

700

0.889

0.0548

0 0379

Calculation of Brightness of Pulp Mixtures from K and S Val¬
ues. Since the brightness of paper is a function only of tlie K and S values
of the individual components of the pajier, it is possible to calculate the
brightness of paper made from mixed pulps if the K and S values of the in¬
dividual pulps are known. Tilie 5" value for the pajxir can be calculated by

the following formula;

6'.V = + S2^iy2 + Sa^Wj + ■''

vfhere SX is the scattering power of the paper and the S and W values arc
the scattering coefficients and weights per reams, respectively, of the vari¬
ous constituents. The K value of the paper may be obtained in the same
way, and the ratio K/S obtained by dividing K by S. From this, i?. can
be obtained by means of Table XXIX. The brightness values calculated in
this way agree very closely with experimental brightness values (see Fig.

XVI-27).

Fig. XVI-33. Variation in brightness with changes in K/S %'alue.

If the reHectivitj- (RJ of paper is ®

of the geheral shape, shown in Figure XV -33 tS obtomed. Thu

shows that the reflectance (brightness) falls off ^

mnstrales what every practical ^permaker knows-t^t the a^^»

Sharply decreases the brightness. On tlie other hand, addition of the same
material to a dull pulp will have much less dulling effect

XVI.

PROPERTIES OF PAPER

927

Determination of S Values of Pigments. The 5 value of pign e
ill paper can be determined from the S value of the unfilled paper an
5 value of the paper after filling with the pigment. For example consider
an unfilled paper of basis weight 20 lb. which has a contrast ratio (C. s») of
0 80 and a reflectivity (if.) of 0.84. The SX value of the paper obtained
from the curves in Figure XVI-32 is 2.45, and since the basis weight is
20 lb the S value is 2.45/20 or 0.122. Assuming that a sheet of the same
weight made from the same pulp containing lOfo filler has a contrast ratio
(Co,.) of 0.87 and a reflectivity (R.) of 0.86, the new SX value of the pa-
L obtained from the curves in Figure XVI-32 is now 3 55, and since tte
basis weight is the same (20 lb.), the 5 value of the filled paper is 3.55/20
or 0.178. The S value of the pigment is then determined from the 5 value
of the filled paper, the 5 value of the unfilled paper, and the proportion of

filler (0.10) in the paper as follows:

„ naner (0.1 78) - (I.OQ-Q.IO) ^'unflllea

O pigment ^ 0 10

pope

r ( 0 . 122 )

Opigment *“ v/*U^

When the 5" value of a pigment has been determined, it can be used
in the same way as the value of pulp to calculate the 5 value of paper con¬
taining the pigment. The following relationship can be used where P rep¬
resents the pigment content on a fractional basis:

Sptptt— (1 — P)5’pnIp + .P*S'plgment

This relationship works fairly well up to 5 to 10% filler, but unfortunately
scattering is not directly proportional to the pigment content of the paper.
Consequently, it is desirable to plot a curve of 5* value of the pigment versus
pigment content of the paper for use as a reference.

The S value of a pigment is a measure of its effectiveness in scattering
light. The 5 value is not constant, but varies according to the conditions
of use of the pigment. Thus, 5 value serves as a measure of the effective¬
ness of a pigment under the particular conditions of use. By studying the
effect of manufacturing conditions on the 5" value of the pigment, it is pos¬
sible to determine whether the pigment is giving the results it should. Typi¬
cal S values of different pigments are given in Chapter IX,

Effect of Dyestuffs on K/S Values, Several attempts have been
made to study the effect of dyestuffs on the K and 5' values of paper.
Foote-®^ found, in the case of a single dyestuff, that the scattering coefficient
(S) is practically independent of the amount of dyestuff in the paper for
dyeings up to 1 lb. of dyestuff per 1,000 lb. of stock, but that the S value
decreases appreciably for heavier dyeings. The absorption coefficient {K)
is approximately proportional to the concentration of dye in the range up

2®iW, J. Foote, Paper Trade J. 109, No. 25 : 333-340 (Dec. 21, 1939)

928

PULP AND PAPER

to 3 lb. of dye per 1,000 lb. of stock. In light tints, he found that the ratio
K/S, as determined for the dye alone, is approximately proportional to the
dye concentration; in light to medium shades, he found a straight-line rela¬
tionship between the log K/S and the log concentration of dye, but in me¬
dium-deep to full shades, no direct relationship was found. These results
are applicable to sheets containing a single dyestuff only. The amount of
light reflected by a sheet containing a mixture of dyestuffs cannot be speci¬
fied by addition, since the amount of light reflected is determined by sub¬
traction.

Nolan^®* used the Kubelka and Munk theory to calculate the reflec¬
tivity (brightness) of paper containing two or more dyes by using the K/S
value of the undyed pulp and the K/S values of the pulps containing each
individual dye. He obtained good agreement between experimental and
theoretical spectral reflectivities of handsheets of unbleached sulfite when
colored with mixtures of twm basic dyes. These results can only be ob¬
tained, however, if there is no reaction between the dyes, and if the scatter¬
ing coefficient of the dyed paper is independent of the concentration of dye.

The effect of dyestuffs on opacity can be calculated by assuming that
the dyestuff has no effect on the 5- value ^of the paper. When this assump¬
tion is made, the effect of dyestuffs on the K value can be ca culated i ^ .
is known. The curves in Figure XVI-32 may be used to quickly determine
the effect of dyestuffs on opacity. For example, supposing a *

reflectivity (R ) of 0.80 and a reflectance over black (i?o) of O./U, tn
printing opacit’y is 87.5, and the value of *e paper oMa.ned

from Figure XVI-32 is found to be 2.8. Upon the addition of dyestuff, the

is found to be 0.75, and assuming that the value is no

ofthe dyVd paper is now found by calculation to be 90.5, compared with

87.5 before the addition of the dyestuff.

Van den Akker^^^ has shown how it is possible to calculate t ^

of extractives or colored additives on the color of paper by measuring t
asborption coefficient (a) of the coloring matter in

lengths. The absorption coefficient (a) is obtained by Beers law,
follows:

T=C

-oca

where T is the transmittance of the solution, ar

c is the concentration, and a is the absorption ^ wTelength

to be equal to 2a, so that measuring .fmZ^ t
gives a means of measuring the contribution of the coloring mat

262 p. Nolan, Pafer Trade J. lOS, No. 14: Snov ^1949^ '

«3 1 A. Van den Akker, Tapp! 32, No. 111 498-501 (Nov., 1949)

XVI. PROPERTIES OF PAPER

92y

color of the paper. Calculations are simplified it weight is expressed in

grains per square centimeter. - 4 . ,

Changes in Contrast Ratio with Changes m Reflectivity.

Changes in contrast ratio with changes in reflectivity of the paper can be
read directly from the curAes in Figure XM-32, so long as the weight re¬
mains constant. For example, a paper having a contrast ratio of 0.80 an
a reflectivity of 0.80 would liave a contrast ratio of 0.84 at a reflectivity of
0.70, assuining that SX remains constant. Calculations of this type are
valid only if X remains constant. The reflectivity may be changed by the
addition or subtraction of dyestuff, as shown in tlie previous section.

Changes in Contrast Ratio with Changes in Basis Weight. For
a given reflectivity {RJ. opacity depends upon the product of X times X.
Therefore, it is possible, by the use of SX, to determine from a sheet of
knowTi weight what the opacity will be at some different weight. The SX
value of the pajier sample at the original weight can be calculated from
and /?« readings. The new SX value at the proposed weight can be calcu¬
lated by direct proportion, since SX is directly proportional to weight. For
example, assume that the pajier in question has an SX value of 0.23, con¬
trast ratio of 0.80, /?. of 0.80, and a basis weight of 23 lb. The

problem is to find what the contrast ratio of the paper would be at a basis
weight of 30 lb. when the reflectivity remains constant. The new SX value
will lie (0.23 X 30)/23 = 0.30. Using the curves in Figure XVI-32, the
contrast ratio at /?. = 0.80, and at the new SX value of 0.30, is found to be
0.86. Thus, changing the weight from 23 lb. to 30 lb. would increase the
contrast ratio from 0.80 to 0.86. The soundness of tliis type of calculation
is Iv'ised on the fact that S and do not cliange with increase in weight,
and hence can be used as constants in the calculations.

Other Uses of Kubelka and Munk Theory. By using Figure
.\\T-32, it is possil)!e to obtain considerable information about a given
pa|K*r more or less directly from the charts. For example,^®*"^®® reflectivity
(/?,) may be read directly from values of reflectance (Ro) and Co.so
(T.\PP1 opacity). This permits reflectivity (brightness) to be obtained
from a single small sample. The printing opacity (Ro/R^) may be obtained
from values of Ro and TAPPI opacity by first finding R^, as above, and
then dividing into /?<». This permits printing opacity to be obtained from a
sm.all sample. If R^ and R^ are given, TAPPI opacity may be obtained,
even if a white backing of the proper reflectance (0.89) is not at hand.

Scattering power, SX (a pure number), may be read directly from any
|iair of \*a1ues which serves to locate a point on the graph: for example,
R. and C>;.. Rr. and printing opacity RJR^, or R^ and R^. Scattering co-

D. B. Judd, Paper Trade J. 106, No. 1; 5-12 (Tan. 6, 1938)

*** D. B. Judd. Paper Trade J. 100, No. 1: 4-8 f jan, 3, 1935)

*** D. B. Judd, Paper Trade /. 101, No. 5: 58-59 (Aug. 1, 1935)

930

PULP AND PAPER

efficient, 5, may be found directly in a similar way if the basis weight X is
known. The S value is, as mentioned before, a fundamental constant and
makes it possible to read from the graph the reflectances (-^o) and o])acities

at all weights of the paper.

Chemical Properties

The chemical properties of paper are derived primarily from the type
of wood used, method and extent of pulping and bleaching, and type and
amount of non-fibrous constituents added. The chemical properties of pa¬
per are important because they influence the physical, electrical, and op¬
tical properties of paper.

In a few grades of paper the chemical properties are as important, or
more important, than the physical properties. An example is anti-tarnish
paper, a special grade of paper used for the wrapping of silverware and
polished steel articles. This paper must be free from sulfur and sulfides
and must not contain free acids, chlorine, or strong alkalies, since tiese
cause tarnishing or etching of the metal surface. The best grades of anti-
tarnish paper are made from carefully purified and bleached rag or su e
pulps which have been washed two or more times to remove all bleach
residues. Paper to be printed with metallic printing inks or covered with
“gold” foil must be similarly made, since the metal m the ink or oi wi
tarnish if it comes into contact with paper containing reducible sulfur as low
as 2 p pm Some anti-tarnish papers used for wrapping silveiware are
imprecated with salts (e.g., copper acetate, lead acetate or -- a^
which react with any hydrogen sulfide in the atmosphere to prevent the gas

from coming into contact with the silver. , „

The amount of reducible sulfur in paper can be determine y ea in^

the paper with aluminum foil and dilute phosphoric acid in a flask m
neck of which has been placed filter paper moistened with lead ace a e so
tion The amount of staining of the filter paper is compared with the re

suits obtained in other control flasks containing knonm
thiosulfate. The amount of reducible sulfur should be less han
for non-tarnishing papers. Papers having more than aimunt of r
ducible sulfur will not necessarily be tarnishing, however these pap
io'dd be tested by wetting with distilled water and then heating for three

hours at 150 to 170° F. between two polished silver plates.

ntLr erades of paper in which the chemical properties are as im

portant, if not more important, than the physical properties are P

papers, reproduction papers, safety papers papers 4si„s and

207 Paper-Maker 57-70 (May 1, 1939)

XVI. PROPERTIES OF PAPER

931

ami imist have tlie pro|)er />H, acidity, and filler content for the purpose for
which tlie pajier is intended.

Alf'ha Cellulose. I’tscosi/y, onJ Copper Number of Paper

.\lpha cellulose, viscosity in cuprammonium or cupriethylenediainine,
and copper number determinations are sometimes made on papier to deter¬
mine the probable stability of the paper. The values obtained are aftected
primarily by the quality of the fibrous constituents of the paper, although
certain of the non-fibrous constituents e.xert an appreciable influence. Pa¬
pers having a high alpha cellulose content or a high viscosity generally
contain high-quality fil»cr (rag or highly purified chemical pulps) and are
cliaracterized by having a high degree of stability. Papers having a high
cop|)er number are generally lacking in stability. It has been suggested’®*
that laboratory filter i>aper carry a maximum specification for copper num-
Ijer of 0.5. The copi>er numljer is influenced by the presence of non-fibrous
reducing agents sometimes found in pai>er, for example, starch, calcium
sulfite, zinc sulfide, and melamine resins.

The above tests are carried out on paper in about the same manner as
on pulp, with the exception of a few minor differences in the method of pre-
fjaring the sample. In the case of paj>er, the fibers are close!}' packed and
are often protected by sizings or coatings, which makes it necessary that the
sample be ground.*** The results of these tests must be interj)reted in light
of the amotmt of lignin, moi.sture, mineral matter, and sizing agents in the
paper and corrections made accordingly.

Amount of SiAng Agents in Paper

Connnercial fjajx-rs frequently contain sizing agents such as rosin, jiar-
affin, starch, polyvinyl alcohol, glue, and casein. Analytical tests for de-
temiining the amount of these materials in jmper are important for checking
the retention and for determining the efficiency of these agents. For ex¬
ample. conqxtring the actual amount of rosin size precipitate in jiaper with
the sizing value furni.shes valuable information on the elficiencv of the
sizing o[jeration. There is often coiisideralde discrepancy between the
anmunt of .size present in the j>a|x*r ami the sizing value.

The amount of sizing agent pre.sent in pafK-r is also of interest because

of the effect of sizing agents on the physical properties and permanence of

the paper. Rosin-sized papers sometimes carry a specification stating the

marttNiim permissible rosin content. Waxed papers, on the other hand,

frequently carry a specification stating the minimum permissible paraffin
content.

*** V ‘ Paper 1809. Bur. Standards 39.

No. 1: 21-27 (1947)

^ J. O. Burton and R. H. Rasch. Paper Trade /. 93. No. 4: 41-48 (July 23, 1931)

932

PULP AND PAPRR

The im-thcls of analyzing tor dillferent sizing agents have been imn-
tioned briefly in other sections of this b,K>k. The methods are s,«.fied m
detail ill 'fAl’PI Standards.

Moisture Content

The cellulose-water relationship is the most important relationship in
paper chentistry. The amount of water in the individua
strength flexibility, and sheet-forming charactenstics of the fibers^ The

ntoisture content of paper affects the weight. ^Tm^rm

flaor t ffte' calendering, printing, coating, and

Moisture content is therefore an important paper specification. In t
“ of "t is customary to condition the paper in order to mamU.n

a constant predeter.nined moisture at the r^ing the paper

The test for moisture

in an air-circulat.ng oven at 100 to 105 orted in one of

constant weight is obtained. Mo.sture “"'mote paper or as moisture
two ways, as moisture on the oven-dry weight oj 'he W"

on the “as is” weight of the paper. and in computations which

ing moistures of paper on the

involve reporting the results of Che _ , small sample

making moisture The “as is” method of reporting mois-

(about 2 g.) is amount of moisture in paper as de-

Ses)rv^^hanges in moisture content

during sampling. Uptween the fibers or within the indi-

Paper may b^ing subdivided into coUoidal

vidual fibers, the moisture in the latte
and capillary water. At low vapor pressures m^ of

as colloidal water ('“‘“f (w i" 'h'

moisture contents. co"s.<i^ imbibed

pores) is present. At \er> mgu

water present. ctored under normal conditions (e.g-,

The moisture content " W " i ulr between 5 to lOfc. K the
50% relative humidity at 73 F.) juiong it may have a moisture

paper has not been stored under stan ar c depending upon the

coLnt anywhere from ty^ of paper.

humidity and temperatiire of the Ph has been moved

hfgC^t r^ver humidity, or in other words, whether

XVI. PROPERTIES OF PAPER

933

equilibrium is reached by adsorption or desorption of moisture. This
phenomenon is known as hysteresis. The equilibrium moisture content
of standard newsprint at different relative humidities is given in a e
XXXIV.^” Papers containing less lignin than groundwood would be ex¬
pected to have a somewhat lower moisture content at the corresponding rela

tive humidities (see Table III).

TABLE XXXIV

Equilibrium Moisture Content of Standard Newsprint at Different Humidities

T?ela.tive

Moisture content

humidity

on absorption, %

on desorption, %

• •

2.8

4 *

4.9

6.5

7.2

7.9

8.0

9.2

9.0

10.4

10.5

11.5

12.2

12.7

Kress and coworkers^^’

found that for a given

temperature the

content of paper is fairly well proportional to the relative humidity at wdiich
it is stored. Rutt'='=' found that a high relative humidity is more important
in getting the air to part with its moisture than the actual moisture con¬
tained in the air. On the other hand, McKee and ShotwelP^® believe that
the equilibrium moisture content of paper is more nearly proportional to
the absolute humidity of the air than to the relative humidity. They re¬
plotted the results of Kress for two different papers, as shown in Figure
XVT34, where it can be seen that the relationship between the logarithm
of the moisture content of the paper and the absolute humidity of the air
is a straight line over most of the range.

In general, most investigators^’^"^"® have reported that increasing tem¬
perature decreases the moisture content over most of the range when the
relative humidity of the atmosphere remains constant. Below about 16° C.
and above 40° C., variations in temperature affect the moistvire content even

2^0 Taken from TAPPI Data Sheets

271 O. Kress and P. Silverstein, Paper 19, No. 25: 13-17 (Feb. 28, 1917)

272 A. H. Rutt, Svensk-Papperstidn. 51, No. 18: 413-417 (Sept. 30, 1948)

273 R. H. McKee and J. S. G. Shotwell, Paper Trade J. 94, No. 22 : 286-288 (June
2, 1932)

27* A. R. Urquhart, /. Text. Inst. 14; T183 (1923)

275 R. E. Wilson and T. Fuwa, /. hid. Eng. Chem. 14, No. 10: 913-918 (Oct., 1922)

276 R. H. McKee and J. S. G. Shotwell, Paper Trade J. 94, No. 22 : 284-286 (June
2, 1932)

934

PULP AND PAPER

at the same absolute humidity.-” McKee and Shotwell-*® obtained con¬
trary results in that they found under conditions of constant relative hu¬
midity that the moisture content decreases with increasing temperature on y
up to a temperature of 60° F., after which moisture increases with an in¬
crease in the temperature. Differences in the grades of paper used and
differences in methods of testing and reporting results explain the appar¬
ently ambiguous results obtained for the effect of temperature on moisture

content.

practical considerat.on. 1 lie ^ of firing agents,

pending upon the type of fibrous fnrn s , M Absorption of mois-

iype and amount of filler, and amount of coating r 1

tore is more rapid than desorption, paper en .msnhere The rate

Z as fast as it picks up moisture from .1^ same ^ ,*r

of absorption is very rapid at first, absorption be-

second, but as tl l- 1 -r. r 9 -# No. 22; 284-286 (June

277 R. H. McKee and J. S. G. Shotwell, Paper ia t ■

2,1932)

21^ Idem. tier Shotwell Paper Trade J. 94, No. 22: 286-288 (June

27 U R. H. McKcc and j. S. G. bhotwcii, /

2. 1932)
zsn Idem.

XVI. rtDrEiTiES or papei

935

amic» mudi Vmtr. Tht trend is essrtilially the san»e for all |va|)ers.*** For

{a|«rs. two to throe hours is siiffKieiit to reach cqiiilihriiuii. hut this
dcfinwi* ii|>nii the initial moi>ture wntent of the ia|«*r. the rate of air circu-

and the phrskaU form of the paper. McKee ami Shotwell**® found,
in an examination of new sprint, antique laid paper, and double-coated paper,
that the rate of moisture regain was highest in the case of newsprint and
lowYst in the case of the coated paper. Up to twentv’-four to forty-eight
hours may be required for certain papers to reach equilibrium with the at-
moaphere Penetration of moisture into paper in roll form is. for example,
nittcfa slower than penetration into paper in sheet form. Jones*** found
that if took six months for moisture to penetrate into rolls of new-sprint to
a depth of 5 in when store<l under conditions of high relative humidity.
Prramditioned papers for printing are sonvetimes wrapped in special wrap¬
pers (e.f, calender-waxed papers or asphalt-laminated papers) to prevent
the paper from picking up moisture. As pointed out in Chapter VI, beat¬
ing has no significant effect on the equilibrium moisture content of paper,
although Baird and coworkers*** were able to show that beaten pulps have a
dtghtly higher eqwilihrium moisture content than unbeaten pulps.

The cnmpo<itioo of the paper affects the equilibrium moisture content,
althmigh it has less effect than the relative humidity and temperature of
thr atmosfihrre. Groitndwood papers tend to alisorb more moisture than
unbleached chemical wood hliers. and unbleached wood biters tend to ab-
•nrhmnre moisture than ItleacKe*! filters (see Table 111 ).***•*•• Lignin ap¬
pears to be the agent respon<il4e for the high absorption of moi.sture by un-
Wrached fibers *** Baird and coworkers*** obtained a marked decrease in
the hvgrosrnpirity of sftnice and fir sulfite pulps when the lignin w'as re¬
moved h)' hleaching. as in Table XXXV taken from part of their data.

Jarrell*** (ntind that the a.'di content of paper is a more iiiqtortant factor
than the filwr fumt«h in determining the equilibrium moisture content.
Fillers appear to lower the rf|uililtriitm mni<turr content of the pafier at any

R II. McKer awd J. S. G St^nmxn. Trmdt J W. No. 22 : 284-288 (Tunc

I WJ2»

'I4im

»*»D |awv Ttmi* / tn. No. 5: 57-//0 (Auf. 1.

»*• C O Sekeet. F A S wi od i and P. K. Baird. Ind. En<f. Chem. 28. No. 11:

1245-12S0 (Nuv, IftJo)

**»H. Fay. //Mite# «. 6; 301-203 (1926)

C O Sekneg. F. A. Scm mwI v and P. K Baird, Pe/rr Tradr J. 107 . No. 19 :

22^221 (Sm, 10. 1938)

R. E. WOm and T. Fowa. Imd. Eng. Chrm. 14. So. 10: 913-918 (Oct.. 1922)

sod P. K. Baird. Ind. ling. CAcm 28. No. 11:

li4S-l2S0 (No*. 1936)

•••T. D. JafTrO. Pmptr Trmir /. IS. Na 3: 23-27 (July 21, 1927)

936

PULP AND PAPER

TABLE XXXV

Effect of Bleaching on Hygroscopicttv of Spruce Sulfite

Per cent relative
humidity 25” C.

56.5

Unbleached

Bleached

Desorption Adsorption

32.50
20.90
14.43
9.43
# * • •
6.63
2.77

22.27

14.60

10.50

7.23

w « • «

5.57

2.40

30.70
19.03
13.27
• * « «
8.13
5.93
2.30

20.00

13.47

9.89

• • * *

6.50

5.07

2.07

given hunudity, although the effect abso;p.

e;“ub:Un — surface sirfng with eithec
ri, oTa^:Jglue has little oc no effect on ^H-ffuilib"
tent of paper, although the presence oj^ glue P

such as glycerine or corn syrup, has a at

eontent. The P-nce of t^e ag^s

oi ::iatrhli4. 20 .

L nreviouslv pointed out, moisture affects the

elasticity of paper an offset paper are

effect of relative humtdhy on the physical pr^e

shown in Figure . - changes in relative humidity. Most of

properties particularly affecte g „„flmum at some intermediate

the strength properties pass through P 35

relative humidity. Maximum bursting stegt desorption, corresponding

40% relative humidity on Lside the range of

25 to 65%, bursting /^^^^'aboul^^^ to 35%. At high

Tsttgt^r

290H O. Ehrisman, Papfr Trade /. 103,

=91 P. Klemm, Wochbe. Papier-Fcbr. f ' (1932)

=9= A. Arendt and E. Wathelet, W”"-f'X 13 1935)

'■

171-174 (Oct. 12, 1933)

295 Idem.

XVI. PROPERTIES OF PAPER

937

humidities over 85%. The decrease in tensile and bursting strength at hig i
relative humidities occurs in spite of the fact that the individual fi ers in t e
paper increase in strength under these conditions. The loss m shee
strength must therefore be attributed to a loss in fiber friction resulting rom

films of water absorbed on the surface of the fibers. ^ ^

Folding endurance improves as the relative humidity of the condition¬
ing atmosphere is increased from zero to about 70 to 90%. The increase is

25 35 45 55 65 75

RELATIVE HUMIDITY, %

Fig. XVI-35. Effect of relative humidity on physical properties
of offset paper (test average taken at 45% R. H.).

particularly great between 40 and 65% relative humidity.^®® The increase
in folding endurance with increasing relative humidity is due to an increase
in the flexibilty of the fibers. However, at very high humidities, the tensile
strength of the paper is reduced to a point where the specimen breaks after
relatively few folds. The stretch of paper increases almost continuously as
the relative humidity is increased from 15 to 90%, The effect of humidity
on the stress-strain properties of paper has been shown in Figure XVI-7,

2®6E. O, Reed, Paper Trade J. 79. No. 25 : 227-229 (Dec, 18, 1924)

938

PULP AND PAPER

where it can be seeii that the paper exhibits greater flow at the higher mois¬
ture contents. Increasing moisture content reduces the porosity and the
oil permeability of the paper.^®^

It can be seen from the above that paper should have a certain mini¬
mum moisture content to be at its best. Paper should never be overdried
on the paper machine, since this results in a more or less permanent loss in
strength and flexibility. Cheap paperboards, such as bogus wrapping, are
particularly affected by overdrying. In some cases, overdried paperboard
mav crease properly when first taken from the machine because of the sur¬
face moisture added at the water boxes on the calender stack, but after the
board has aged and the surface moisture is absorbed into the body of the
board, the fibers again become dry and brittle. If paper has been overdried,
conditioning at a high humidity (i.e., 90^ or over) and then bringing back
to normal humidity often results in a permaneiit increase in strength, on ac¬
count of the release of internal stresses set up in the paper during drying."”®
In addition, this procedure tends to increase the bulk and decrease the sur¬
face finish, because of the fibers returning to positions occupied before

calendering. ^ « rn

Paper should be stored at a relative humidity of 40 to 60%. Ihe

moisture content of papers to be folded (e.g., folding boxboard) should
never fall below about 6 to 7% (corresponding to a relative humidity of
about 50%), except for short periods of time, in order to avoid trouble with
poor bending.®”® When paper is to be folded, it is desirable to do the folding
in an atmosphere of about 65% relative humidity. Printing papers must
have a certain minimum moisture, since papers which are too low m mois¬
ture are brittle and tend to develop excessive static electricity on the print¬
ing press.®”" Coated papers, in particular, become brittle and susceptible
to cracking. Papers for printing by the offset process tend to pick up mois¬
ture and curl if they are not at the proper moisture content before printing.
Corrugating papers are steamed just before entering the corrugating ro s

in order to increase the flow properties of the paper.

Ash

The ash in paper may be derived from (1) ash in the fiber, (2) ash
from pigments used in filling, (3) ash from sizing agents (4) ash ro
mineral matter originally present in the fresh water, ( ) as rom p^i
used in coating, (6) ash from alum used in setting the size, an ( )
derived from metallic matter from piping and machinery.

297 G. L. Larocque, Pulp Paper Mag. Canada f 15:

298 c. O. Seborg, R. H. Doughty and P. K. Baird, Paper I race y/,

171-174 (Oct. 12, 1943 ) /'Dec 1 1949)

299 T. G. Hicks, Paper Trade J. 129, No. 22 : 474-477 (Dec. 1,

300 w Sooy Paper Mill 63, No. 33: 12-13 (Aug. 17, 1940)

301 w! H Dahl, Paper Trade J. 108, No. 26 : 329-332 (June 29. 1939)

XVI. PROPERTIES OF PAPER

939

Normally, the ash from unfilled or uncoated papers is not ^

is light and fluffy in appearance, although m rare cases t °

liaper may be as much as If the ash content is over and s de

and compact in appearance, it may be taken as positive evidenc
paper is filled or coated. Printing papers for letteriiress genera y i
ash content of 15 to 257<i because of the pigments used m filling. Book
papers for offset generally Itave an ash of 8 to 157e. Pigment-coated pa-
iKrs generally have an ash content of 20 to or more due to the

pigment used in the coating and also to the pigment used in filling. Pig
iiieiit (clay) content of several typical coated papers is given in Tab e

Bond and writing papers have an ash content which genera y

ranges from 2 to 67o, although opacified bond paper may carry a specifica¬
tion of 690 minimum ash content.

TABLE XXXVI

Clay Content of Several Typical Coated Papers

Paper grade

Litho-coated one side ..

Letterpress-coated two sides ...

Machine-coated magazine-coated
two sides ..

® In this case, the weight is not all due to clay; the coating contains 5.62 lb. of an¬
other pigment.

Basis weight
of base
paper, lb.

Weight of
filler clay, lb.

Weight of
coating
clay, lb.

Total clay
in paper, %

2.15

14.45

27.7

4.05

22.50<*

37.10

4.30

13.6

32.6

Papers made for high strength (e.g., wrapping papers and container
boards) should have a low ash content, since mineral matter tends to reduce
the strength of paper. High ash content is also undesirable in photographic
papers and electrical insulation papers. Filter papers used for analytical
work must contain a negligible amount of ash, and consequently these pa¬
pers are extracted with hydrochloric and hydrofluoric acids.

The amount of ash in paper is determined by igniting a known weight
of the paper in a platinum, alundum, or silica crucible, using a Meker burn¬
er or an electric muffle at about 925® C. Ignition is carried out until the
ash is free from specks of carbon, which generally requires about thirty to
sixty minutes. Ordinarily, the ash will be white in color, but if colored
pigments are present in the paper, the ash will be colored. The results are
reported as the percentage of ash to the nearest 0.1% on the basis of the
oven-dry paper, obtained by correcting for the moisture in the paper. When
special accuracy is desired, the paper may be weighed out in the oven-dry
condition and the results reported to the nearest 0.05%.

An ash determination tells nothing about the nature of the ash. It is

“Kaolin Clays and Their Industrial Uses,” p. 127, J. M. Huber Corp., New
York, N. Y. (1949)

940

PULP AND PAPER

iiiiportsiit, however, to identify the source of the ash, since ash from resid¬
ual chemicals left over from bleaching or dyeing is more harmful to the
strength and permanence of paper than clay or other fillers. Identification
of the ash is also desirable when trying to match competitive papers. Reg¬
ular qualitative procedures can be used for identifying the constituents in
paper ash. A suitable procedure is outlined in 1 APPi Standards for iden¬
tifying the common pigments used in papermaking. Special tests are also
described for measuring the amount of arsenic, titanium, acid-soluble iron,
and similar materials. In carrying out qualitative tests, portions of ap¬
proximately the same size should be used throughout the procedure so that
reliable estimates can be made of the relative quantity of the different sub¬

stances which are present. The report should include a list of all the cations
and anions found in the ash, as well as an approximate indication of the
amounts of each. In addition, the report should list the pigments present
in the paper as indicated by the most likely combination of the ions.

The presence of considerable amounts of aluminum and silica indicates
the presence of clay. Titanium indicates the presence of titanium dioxide,
or one of the titanium composite pigments; for example, titanium and bar¬
ium indicate the presence of titanium dioxide-barium sulfate, whereas ti¬
tanium and calcium indicate the presence of titanium dioxide-calcium
sulfate. Papers containing titanium dioxide appear dark under ultraviolet
li<rht since titanium dioxide absorbs ultraviolet rays. An alkaline ash con¬
taining calcium soluble in hydrochloric acid, but no sulfate, indicates the
presence of calcium carbonate. (If carbonate is present, the paper should
liberate carbon dioxide when acidified.) The presence of calcium and sul¬
fate in the ash soluble in hydrochloric acid indicates the presence of calcium
sulfate (gypsum or crown filler). The presence of calcium and sulfur di¬
oxide indicates the presence of calcium sulfite. (Papers containing ca cium
sulfite liberate sulfur dioxide when acidified.) The presence o su ^e
zinc indicates the presence of zinc sulfide or hthopone

portion of the ash indicates the presence of barium

(with effervescence) indicates the presence

coated paper^ndicates the presence of satin white^ ^f c^^

alone indicates the presence of diatomaceous earth. The presence
:!deraHe a— of magnesium and silica indicates the presence of mag-

nesium silicate (talc, agalite, or asbestine). tViMe are

Traces of various minerals should be reported, even g

generally due to impurities in the pigments, fibers, or m the J -

all papers contain traces of calcium, magnesium, and sulfate ions picked up

XVI. PROPERTIES OF PAPER

941

from the mill water. Some of the impurities commonly associated with the
various commercial pigments are as follows:

Pigment

Impurities

present

Clay

Ca Mg Ti

Titanium dioxide

A1 SO 4

Satin white

C 03

Calcium sulfate

Calcium sulfite

SO 4

Aluminum is almost always present in paper ash because of the alum used
in setting rosin size.

In addition to the regular qualitative tests, serai-micro techniques have
been developed in which the centrifuge is used in place of filtering for con¬
centrating precipitates. Simple qualitative tests for the most important
fillers in paper, based upon examination of the ash under the microscope,
are given below. This method is very rapid, because identification is based
upon the crystal structure of the pigments. The reactions are carried out
in small drops on microscope slides. By using such amounts of materials,
evaporations and fusions can be carried out in a fraction of the time re¬
quired for a regular chemical analysis. Another advantage of microscopic
analysis is that it permits distinguishing between the natural and artificial
pigments which have the same chemical composition, but different physical
form. The refractive index of any pigment can be determined by mounting
some of the pigment in liquids of known refractive index. For double re¬
fractive materials, polarized light can be used.

Qay, talc, and asbestine can be readily recognized by the appearance of
the particles when mounted in water and examined under high power. The
flat plates of kaolin and mica are, characteristic of china clay. The particles
of talc are larger and more or less fibrous, asbestine being very fibrous.

If calcium is suspected, some of the ash should be added to a drop of
water on the slide. A few crystals of iodic acid are then added and the slide
scratched to start crystallization. If calcium is present, calcium iodate forms
as colorless diamond-shaped crystals with high double refraction. If cal¬
cium carbonate is present, the iodic acid will cause effervescence. Often,
however, the carbonate is destroyed when the paper is ashed. In this case,
the ash will give a strongly alkaline reaction with phenolphthalein.

If titanium is suspected, some of the ash should be added to a drop of
dilute sulfuric acid on one corner of the slide. Some ammonium sulfate
should then be added and the mixture evaporated and heated carefully until
strong fumes appear. The mixture is then cooled and distilled water is
added to make a drop about the same size as the original drop. A drop of
hydrogen peroxide is added. Yellow color shows the presence of titanium.

942

PULP AND PAPER

If zinc is suspected, some of the ash should he added to a drop of dilute
nitric acid on one corner of the slide. This should he evaporated to fhvness
and a second drop of dilute nitric acid added and again evaporated to dry¬
ness. This is then repeated for a third time. Finally, a drop of water and
a trace of acetic acid are added. In another drop of water on the same slide,
some potassium mercuric sulfocyanate is dissolved and this drop is then
flowed into the other. Zinc mercuric sulfocyanate separates as character¬
istic feathery crystals which are white by reflected light, and black by trans¬
mitted light.

If barium is suspected, some of the ash should be added to a drop of
dilute sulfuric acid on the corner of slide. This should then be evaporated
until strong fumes appear. When partly cool, breathing on the slide \yll
start crystallization. Barium sulfate crystallizes as feathery crosses with
a marked tendency for two adjacent arms to be longer than the others. Cal¬
cium sulfate crystallizes as rounded grains with hair-hke projections on

c3.cli end

For further information on the methods of examination and for pic
tures of the various types of crystals, the reader is referred to other

sources.^®^' ■

For mineral matter which is not decomposed by heat (e-g<. i anium

dioxide), the weight of the ash may be taken as the weight of the pigment

present. Clay is not changed chemically on ignition, but there is a loss in

water of hydration. It has lieen reported that ashing at STO" C. “““S ': ay

to lose 12.57<, water, talc 1.5%, and magnesium carbonate alniost 50%.

Many pigments are changed chemically by ashing, and hence t e weig

ash camot he reported as the weight of pigment; for example, ylaum std-

fate is decomposed at 1200° C. to calcium oxide, barium sulfate is decom-

pcsed at 1510° C. to barium oxide, ^l^TTuita^^

825° C. to calcium oxide, and zinc sulfide sublimes at 1 .

correction factors must be applied when ashing papers “"y"*'"®
pigments if it is desirable to convert the values obtained to " *

Lrresponding pigment. These factors can be obtained ^ 0 ™ 'Sm e

made on known quantities of the pigment. ?<»■ P’fme Presence
sulfate and barium sulfate, the ignition test should be made m the present

of paper in order to simulate the reducing atmosphere.

Often it is necessary to determine the relative

more pigments in paper. This can be ^T used,

procedure, but this is complicated, and generally simpler methods

E. M. Chamot and C. W. Mason, Handbook of Chemical Uicroscopy, Vol.

Tohn Wiley & Sons, New York (1931)

K P. Geohegan, Paper Trade J. 90 No. 10:

M. Chene, C. Dcissenberg, Canand M^m-B°rre,, and Chiaverina, La

6S: 226-231, 258-261, 290-291, 326-329 (1946)

XVL PROPtJITlLS OF PAPER

943

\ c«nnK» probfcan is to dctcmiiue the rauo oi clay and caiamn caru.
in fiUfd or coated paj^rs. This can be done by detemiming the total ash
by a regular ash determination on the paper. A second sample of the paper
b then extracted u ith dilute hydrochloric acid to remove calcium carbonate,
and then ashed. The difference between the two >^lues gives the ash due to
cakium carbonate. Thb v-alue can be converted to weight of actual calcium
carbonate by multiplying the weiglit of ash by the factor 100 56 to convert
from calcium oxide to calcium carltonate. The weight of actual clay m the
l^ier can be determined by multiplying the weight of ash obtained ou the
acid-extracted paiier by about 1.J6 to correct for the loss of water of hydra¬

tion on ignition.

It b frec|ueiitly desirable, in the case of pigment-coated iiajiers, to e
trrmine whether tlie ash is derived from the filler or from the surface coat¬
ing. Thb can be done by stripping the coating from the paper by soaking
the paper in an enzvme solution for one hour at aO® C. and then brushing
off the coating with a camel’s-hair brush (see Ch. Will). The ash in the
coating can be determined indirectly by substracting the ash of the decoatecl
jiafier from the total ash of tlie original paper.

Acidity and pH

Tlie acidity is important because of its effect on the jiermanence of
l«i| 4 *r. The acidity may be determined as water-soluble acidity (or al¬
kalinity), or as the hydrogen ion concentration (^H) of the paper extract.
The hvdrogen ion eoncentration (/*H) is more indicative of the stability of
paper than the total acidity.

Water-soluble acidity ts determined by grinding the paper and digest¬
ing in boiled distilled w ater for one hour under reflux at 98 to 400® C The
digested fillers are then filtered on a Budiner funnel (w'ithout filter paper),
and the resulting extract is titrated w'ith either 0.01 N XaOH or 0.01 N
HQ. as the case warrants, using phenolphthalein as the indicator. The
results are reported as f»er cent of SOi or jier cent of NajO on the basis
of the moist lire-free paper.

The p\\ can lie determined by either hot or cold extraction. In the
coki extraction, 1 g. of pafier (either cut or ground) is macerated with a
'tirring rod in 20 ml. of distilled water at 20 to 30® C. Then, 50 ml. more
of water b added and the sample is left for one hour, after which the pH is
determined in a meter. In the hot extraction, the same procedure is
kiflowcd. except tlut the samfdc is digeste<l at 95 to 100® C. under reflux
for one hour, iiistcafl of merely standing at roi>iu lemiM*rature. The li<»t ex-
tractiim iiirthod is more widrly use<i than llic cold extraction, although the
ould extractkin probably gives Uie more nearly tiuniial /’ll of the pajier.
Hoc extractkin ordiiarily gives a f H aliout 0.5 lower tlian cold extraction

944

PULP AND PAPER

due, according to Launer,®°® to the increased hydrolysis of aluminum salts
at high temperature. In the case of wet-strength papers made with mela¬
mine-formaldehyde resin, hot extraction may give a higher than cold
extraction due to hydrolysis of the resin and liberation of free melamine.®®^
Because of the difference in results obtained with the two methods, the
analytical report should always state which method was used. An indica¬
tion of the surface pYi of paper can be obtained by streaking the paper with

indicator solution, e.g., brom cresol green.

Acidity in paper is derived principally from the alum used in sizing.

Acidity may also be derived from bleach residues left in the pulp, absorp¬
tion of acid gases from the atmosphere, or from the presence of organic acids
found in the pulp, sizing agents, or coating materials. Paper may also pick
up acid by contact with the skin during handling. Filled or coated papers
are likely to have a high alkalinity, due to the presence of alkaline pigments.

The pK is very important for bonds, ledgers, and index papers which
are intended for permanent records. Some of the generally accepted speci¬
fications are as follows:

Type of paper Minimum pH

Papers for permanent records .

White bonds .

Colored bonds .

The />H is important in converting papers (e.g., papers for saturating, im-
pregnating, or coating), since the pH is likely to affect the materials used
in these converting operations. The pH apparently has some effect on *e
initial strength of the paper. Lihby and Dohne- report that bursUng
strength is reduced approximately 10r<- in going from a />H of 4 to 6 when
alum is used for setting rosin size. On the other h^d, the bursting *
is increased approximately over the same pH range when alum-alu-

minate mixtures are used for setting the size.

Permanence of Paper

A lack of permanence in paper can be manifested either by
coloration of the paper, (2) a loss in strength of the ^ ® jT

in chemical properties, e.g., increase in copper
cellulose or decrease in cuprammomum viscosity. The exte
changes in paper on aging depends upon the type of fiber, t ^
type of secondary ingredients present, and the atmospheric -on^^s to
Jhich the paper is subjected. Changes are highly undesirable in p p

306 H F. Launer, Paper Trade J. 110, No. 10: 1^5-140

307 c. S. Maxwell and W. F. Reynolds, 280^186 (Nov. 27,

308 w. P. Dohne and C. E. Libby, Paper Trade J. 113, No. 22. k

1941)

XVI. PROPERTIES OF PAPER

945

quiring a high degree of permanence, e.g., record papers and base paper

^The best method of measuring the permanence of paper is by natural
aging. However, this method is obviously impractical for relatively per¬
manent papers because of the time involved. In order to shorten the time
involved, accelerated test methods have been devised which utilize light an
heat. The instruments used for measuring the extent of color fading by
light (usually from a carbon arc) are called Fade-Ometers. These lamps
vary considerably in the rate at which they fade paper, due mostly to vari¬
ations in line voltage, and some lamps may require from 2 to 3 times as long
to produce the same fading as others.^"® One means of overcoming this
difficulty is to compare the results with a paper of known fading quality.
Fade-Ometers do not have the same relative spectral energy distribution as

sunlight.

Heat tests may be used to measure the relative stability of paper, al¬
though heating does not always give results comparable to natural aging
because of the dehydration which occurs at high temperatures. In carrying
out accelerated aging tests using heat, paper strips are heated in an oven
at 105° C. for sevent 5 ^-two hours and then tested for folding endurance.
The results are reported as per cent retention of folding endurance after
the heated samples have been reconditioned under standard conditions.
This test is most widely used for testing the quality of high-grade rag papers
which normally retain about 90 to lOO^o of their original folding endurance
under these conditions. The test is not suited for the testing of groundwood
papers.

Klemm®*° reported as early as 1929 that pure cellulose lasts indefinitely
without discoloration. Commercial papers, however, vary greatly in their
resistance to aging. The grades of paper having the greatest permanence
are those made from pulps possessing high cuprammonium viscosity, high
alpha cellulose content, low copper number, low lignin content, and low
pentosan and gamma cellulose content. The U. S. Government Printing
Office carries a specification of not less than 95% alpha cellulose for rag
papers (e.g., book, bond, ledger, and index papers) required for permanent
records.®^^ The copper number must not exceed 1.0. Groundwood pa¬
pers have the lowest stability. Papers made from chemical wood pulps are
much more stable than groundwood; sulfite pulps are more stable than
sulfate; and lightly bleached sulfites are more stable than highly bleached
sulfites.®^^ Rag papers have the highest degree of permanence of the com-

Paper-Maker 64, No. 6: 52 (Dec., 1947)

310 P. Klemm, Paper Trade J. 89. No. 6; 53-58 (1929)

3” M. S. Kantrowitz, Paper Trade J. 128, No. 25: 231-234 (June 23, 1949)

3>2 H. W. Giertz, Svensk Papperstidn. 48, No. 13: 317-323 (July 15, 1945) through
L. E. Wise, Paper Ind. 29, No. 6 : 825^29 (Sept., 1947)

946

rULP AND PAPER

moil papers, and are specilied when niaxiniuin permanence is reijuircd.
Mixed rag and chemical wood fibers are specified for many grades of per¬
manent papers, and for these grades it is customary to make routine micro¬
scopic examination of the paper to see that it meets the specified fiber

content.

Lignin is particularly sensitive to light and undergoes a photochemical
reaction in sunlight leading to a, darkening of the lignin. It is this photo-
cliemical reactivity of lignin which is responsible for the rapid deterioration
of groundwood papers. The rate of fading of groundwood papers in sun¬
light is very rapid in the early period of exposure; it appears to be nearly
proportional to the sejuare root of the exposure time. Even a small
amount of groundwood in a mixed furnish has a pronounced deleterious ef¬
fect on the photochemical stability of the paper. This effect on the color
change is disproportionately large for small groundwood content.

The effect of light on aging depends upon wavelength. Cellulosic
fabrics have, for centuries, been whitened by exposure to air and sunlight,
and yet it is known that papers tend to yellow in sunlight. This apparent
anomaly can be explained by the difference in effect obtained with radia¬
tions of different wavelengths. Groundwood papers have a characteristic
heavy absorption of radiation in the ultraviolet region, and it is this range of
wavelength which is primarily responsible for the color change on exposure
to li^^ht. It has been shown=‘^"'that electromagnetic radiations of wave¬
length longer than 480 millimicrons have a slight bleaching effect on ground-
wood, in contrast to the dulling and yellowing effect which occurs at shorter
wavelengths. The amount of oxygen present is a factor, since it has been
shovvn^^® that groundwood papers fade at a slightly greater rate m e

presence of oxygen.

Heat by itself, at least in certain temperature ranges, has relatively
little effect on discoloration. This is demonstrated by the fact ^ “

no appreciable difference in brightness when papers are aged at 20 _

at elevated temperatures, so long as aging is carried out in t le *
ever, increasing the temperature in the presence of light has J

effect on fading because of the effect of temperature on the photocheimcal
reactions. The discoloration of thick insulation boards, which are ^
nionly dried in ovens at 300 to 400» F„ is a serious problem in the in^
lating board industry. Much of this board is made from groundwo P p.
and the discoloration comes from the lignin in the pulp. On t le o ■

3 i«J. A. Van den Akker, H. F. Lewis, G, W. Jones and M. A. Buchanan, of '

22, No. 4: 187-192 (Apr.. 1949) , , ^27, No. H:

nn ?;Nolan, J. A. Van den Akker and W. A. Wink. r 7 n^ac J. ,

101-105 (Sept. 13. 1945)

:ii5 Idem,

XVL r*OPE*IlES Of PAPER

947

Mclaim*** in tbc o£ sulnte pulps, that certain reactions occur

m the drjinj oi the paper (in tbc presence of ^t and moisture) which re¬
dact the reflectance somewhat, but tend to improve the light stability of the
paper. He found that machine-dried papers ha\'e a greater subility than

air'dried pu^-

In at^ t i on to the degrading effect on lignin, light also tends to degrade
C]fl)idoie, hemicelluloses, and other carbohydrates in the fiber, as well
as fs**. resms, and waxes. Lewis and Fronmuller*” found tlut ultraviolet
light from a Fade-Onwter tends to bring about a shortening of the cellulose
chain molecules, although sunlight has relatively little effect on the cellulose
fraction in groundwood papers It is the light rays shorter than about
JOO imlltfnicrons which ha\*e the most detrimental effect on cellulose.***
The offending substances in bleached wood pulps appear to be heniicellu-
kwes,*** and there afipcars to be a direct relalionsltip between hemicellulose
content and the yellowing tendency of bleached pulps. Of all the henii-
ceflulosie materials, the degraded celluloses have the greatest effect.**®
Gieru*** showed that yellowing in bleached pulps was caused principally by
the decemposed carbohydrates produced by the bleaching operation. On
the other hand. Schwalbe*** pointed out that the addition of glucose im-
proses the color subility of groundwood. and proposed that the mechanical
pulp tt more hat- and l^ht'sensitive than the original wood because of the
removal of soluble carbohydrates during grinding.

Even M the pulps used in making paf>er are of high purity, the pafx^r
win not he permanent if ocher conditions are not favorable.*** High hu-
tnidiiies are more detrimenUl to permanence than low humidities. Ton-
gren*** found in tests conducted at 100® C. (and rebtivc humidities of 25%
and above) that the rate of discoloration upon aging increases linearly with
rrlalive humidity. McIntyre*** obtained a minimum aging rate with
hkachnl sulfite pulp at a relative humidity of about 20%, as shown in
Figure XVI-36. Different pulps react differently to the effect of humidity,
and hence it it desirable in accelerated aging tests to maintain a normal

••® j. W. Mclmrrc. P»prr Trarfe /. 109, Ka 24: 317-325 (Dec. 14. 1939)

H F. Lewis tad D. Fronoadler. Pafrr Trtdt /. 121, Na 14: 133-136 (Oct. 4.
IMS)

«• W HirschkiBd. D J. Pyc and E. G. Thototwon, Paper Trad* J. 105, No. 18:

(Oct. 2iL 1937)

**®H. W Cierti. PapprrtfSdm. 4i, No. 13: 317-323 (July 15. 1945)

sod E F. Korth. Paper Trade /. 109, No. 24; 326-329 (Dec. 14,

"* 15. 1945) thr

I- E. UTst. Paper imd. 29. Na 6: 82S-829 (SeiiL 1947)

Attslandhrft. I
(Dec. 1933)
<Auf. 25. 19.38)

••J. w. Uclotyrc Paper Trade /. 109. No. 24? 317-t?c rrw te

948

PULP AND PAPER

moisture content by controlling the relative humidity to about 55%.®^®
The presence of excessive moisture at elevated temperatures promotes a
hydrolytic attack on the fibers. Papers for permanent records should be
kept at constant relative humidity, since frequent changes in humidity re¬
sult in repeated expansions and contractions of the paper, causing excessive
wear on the fibers. For this reason, valuable historical documents are gen¬
erally kept in rooms maintained at constant relative humidity.

Fie. XVI-36. Effect of relative humidity on the discolora¬
tion rates of air-dried (a) and machine-dried (b) pulps.

The effect of pn on the stability of paper is well known, toda^
hemical wood and rag papers for papers

brightness at a

if lone exposure of paper to sunlight is a lowering of the pH of *e P P
7 ven sulfur dioxide in the atmosphere will cause ®

r„d sulfur dioxide and iron in the same paper

n the pulp from the bleaching operation cause discoloration and

’* RoSn size lowers the permanence of W"; stre'n^h

results in a yellowing of obtained a straight-line rela-

(particularly folding strength). Tongren obtaine

sie J. W. McIntyre K. Buchanan, Tapfi

327 J A. Van den Akker, H. u. vv. j

No. 4 : 187-192 (Apr 1949 j pM, p,o. 140, 7 PP-

328 A, E. Kimberly and A. L. Emiey, Dur. o

(1933) rr j T ms KTo 26- 333-337 (June 29, 1939)

31. L K. Johnson, Paper Me J 108 No 26 • « 35 ^ ,, 38 ,

330 J, C. Tongren, Paper J raae J.

XVI. PROPERTIES OF PAPER

949

tionship between the rosin content of paper and the aging rate (change in
K/S number) when the two were plotted on log paper. The degradative
effects of rosin are due to oxidation, and consequently, papers sized with
modified rosins art considerably more stable than papers sized with natural
rosin. Starch and animal glue have no degradative effect in themselves,
but in cases where the fold and bursting strengths are due in part to surface
sizing with these materials, there is a tendency for the strength to decrease
on aging. Writing inks, especially those of low pH, increase the rate of
deterioration of paper on aging. On the other hand, coatings, lacquers, or
printing on the surface of the paper often act as protective layers and reduce

the amount of deterioration on aging.

Fillers have little degradative effect on paper. In fact, the alkaline
fillers tend to prolong the life of the paper.®®^ Zinc sulfide is supposed to
increase the permanence of paper by absorbing some of the harmful light
rays. The effect of dyestuffs on permanence of cellulose is not well under¬
stood. It is possible that certain dyestuffs protect cellulose from the de¬
structive effects of light. Other dyestuffs are apparently capable of trans¬
forming absorbed radiant energy in such a way as to cause deterioration
of the fiber. It has been reported®®® that direct dyes protect cellulose,
whereas acid dyes are harmful.

Odor and Taste

Odors and tastes are very difficult to describe, either for quality or in¬
tensity. There are no basic units in which these properties may be ex¬
pressed, and hence there is no method by which they may be classified or
compared, except in the personal opinion of the tester. An expert who can
classify odors and thus help’to track down their origin is invaluable at times.

Paper and paperboard readily absorb tastes and odors. As a rule,
paper does not hold tastes or odors permanently, but rather tends to transfer
them to other absorbent materials. Hence, foodstuffs packaged in paper or
paperboard containers tend to pick up tastes and odors from the package.
For this reason, paperboard used for packaging foodstuffs should be lined
with high-grade virgin pulp or the inner surface protected in some way.
Often, paperboard for packaging is coated with a thin film of sodium silicate
or starch on the side which comes in contact with the food product.

Several odor transfer tests are used for the detection of off-odors in
paper. Butter or plain milk chocolate placed in contact with (or stored in
a closed container with) the suspect paperboard is a rather sensitive test,
since these substances are easily upset in flavor balance by a small amount
of off-odor substance. Attempts have been made to isolate and classify the

8*32 T 20 pp. (i936)

Textile Weekly 30: 164-167 (1942) ^ ^

950

PULP AND PAPER

odor constituents of paper by absorbing or dissolving these constituents in
different solvents or media, but the results are rather inconclusive.

Every effort should be made to eliminate the source of odor or taste in
paper. Paperboard itself generally has an odor described as “paper odor,
but this is ordinarily not objectionable, since this type of odor is low in in¬
tensity and not readily picked up by foodstuffs. This odor varies according
to the type of pulp used; for example, paper made from alpha pulp is prac¬
tically odorless, whereas paper made from kraft pulp has a typical sweet
coumarin mercaptan-like odor.«=*^ One source of taste and odor in paper
is the process water, and for this reason, the process water should be care-
fully treated, particularly if the paper is to be used for the pac ^agmg o oo
products. In addition to odor picked up from water, disagrerable musty
sour, or putrid odors may be imparted to paper as a result of slime grow
in the mill system, decomposition of oils in groundwood pulps, spoiled
starch or protein sizing agents, and other similar sources^ Papers sur ace¬
sized with glue or papers coated with casein frequently have an
able musty odor, particularly during the summer months.- toxicants

added to control slime growth may impart a chemical or ° °

to the paper The printing on the paper may be responsible for odo .
Other sources of odor are anti-foaming agents, dispersing agents, foreign
master added with waste paper stock, and contaminants such as oil and

kerosene.

Electrical Properties

Paper is among the cheapest and best electrical insulating

known. Although the amount t'porTa^^Hnsulating

not large compared with other fields, paper y V

and,® additiormust meet speciheatmns on ^Xn^heS^l

tricil properties are, in general, dependent f ' ^ ,,ed.

properties of the paper. Lulation paper

are: (f) high dielectric constant J jtm (diefectric

electric strength (electrical resistance), (-) The electrical properties

loss), and {4) 'Ta e given in Table XXXVIl.”'

of cellulose and a typical insulating paper » . . , the life of the

The ultimate value of electnea pa^rs is -.uyoical life of electrical
paper under the particular conditions of use. The physical

. K T R Siostrom TapH 33. No. 6: 58A, 60A (June.
333 Report on talk given by L. B. Sjostrom, yi

1950) er e/ r 7 /)^ No 26* 333-337 (June 29, 1939)

.«L. K. Johnson, No. 7 ': 807-817 (July. 1939)

33 S G. T, Kohman. hid. hng. Lfiem. jj.,

XVI. PROPERTIES OF PAPER

951

TABLE XXXVII

Sample

100% cellulose
Paper .

Dielectric Properties of Cellulose and Paper

Dielectric

constant

Power

factor

Resis- Dielectric

tivity, strength,

ohms/cm. volts, d.c.

81 . 10“ 25x10"

1.2-4 0.001-0.002 > 10“ 2 x 10"

Density

1.56
0 . 2 - 1.2

equipment depends upon the life of the paper used as the insulation. In
a.-c. units, life is affected principally by the magnitude of the dielectric loss,
whereas in d.-c. units, life is a function of degradation and points of low
resistivity.®*® The temperature of insulation in use is generally about
100® C.,*®^ and consequently electrical insulation paper should be as free as
possible of chemical deterioration resulting from high temperature. Im¬
pregnated papers generally have a shorter life than unimpregnated papers.

Specific Inductive Capacity {Dielectric Constant)

Specific inductive capacity is related to capacitance. Capacitance is
defined as the ratio of the charge on a condenser to the potential difference
between the terminals of the condenser. The specific inductive capacity
(dielectric constant) of paper is the ratio of the capacitance of a two-plate
electrical condenser when the space is filled with the paper to the capacitance
when the space is under vacuum.

Pure cellulose has an exceptionally high dielectric constant which is
due, in all probability, to the oscillation of polar hydroxyl groups on the
cellulose molecules, or to strongly adsorbed moisture.®®®'®*® The most re¬
liable value for the dielectric constant for solid native cellulose appears to
be 6.0 to 6.1.®*^ The dielectric constant of paper is much lower, generally
varying from slightly over 1 to about 2.5®*® The specific inductive capaci¬
tances of a few common materials are given in Table XXXVIII. The di¬
electric constant of cellulose increases with increasing temperature.

Paper has a lower dielectric constant than pure cellulose because of
the high percentage of air voids in the paper. To overcome the low dielec¬
tric strength, paper used for insulation in power transmission cables, or as
the dielectric in wound capacitors, is impregnated with oil, wax, or certain

336 C. Delevanti, Jr., and P. B. Hansen, Paper Trade J. 121, No. 26- 241-249
(Dec. 27, 1945)

337 C. H. Pike, Paper-Maker 111, No. 1: 11-12 (Jan., 1946)

338 G. T. Kohman, Ind. Eng. Chem. 31, No. 7: 808-817 (July, 1939)

338.^. Campbell, Proc. Roy. Soc. (London) A78: 196-211 (1907)

H. A. DeLuca, W. B. Campbell and O. Maass, Can. J. Research 16, Section B
No. 8 : 273-288 (Aug., 1938) ’

3^> C. R. Calkin, Tappi 33, No. 6: 278-285 (Jan., 1950)

3^-C. Delevanti. Jr., and P. B. Hansen, Paper Trade J. 121 No 26 - 241-249
(Dec. 27, 1945) ' ^

952

ruLT AKD rAna

TABLE XXXVlll

SnKinc I»*i»ucTiv» CAfACttAwc** <0 StxtMAi. C*M0om IIatokma

.. Z1-2J

Wns'Ml .

. 2-6

. S-V

1 til 1

I lUC ♦•♦••••

. ... . About S 5

resins, liecausc these nuterUU have greater speahe mdortive ofadtf tlaii
air. Impregnation raises the dieleclnc constant d resak* in a

nKJfC etjuiiable distribution ol the electric field. Hoarever, any cam in
caixaciLince by increasing the 5|iecific inductive capacity o( the bnp rq^
is limited by that of the |>aper, since combiiutioo |iaper-otl dickcincs Ubiw
as a scries.*** Residual gas (air) in impregnated papers u a cause oi poor

perfonnance.

DUlectrk Strgngtk

The dielectric strength of papier is measured by the ability
to offer resistance to the passage of electrical spark disch^ The
trie strength can be determined by pbdng a spedmen of paper «
thickness between electrodes and subjecting it to voltage stroa

down occurs.**^ , ... _. .v>«^

In pnetiCT. . high dielectric strength « required m

and insulating papers to be used under conditions ol htgh

roltage. charges are built up on the fiber surface,

side of the fiber, and positive on the other. BiMuse of

ance of the fibers! these charges do not neutralize one ««tto. brt

they act to oppose the electric field presented bjr the

trie strength of Uk fibir is «ce^ »«> ‘"e

dielectric strength has been exceeded, a is

the paper and it no longer has the same chelect^^

referred to as die break*down strength of the paper a^ **

breakdown strength of the hber it^. k- the Akknes*

pressed in terms of the total voltage at breakdown Avided by

ofthepaper sh«t factor cootritatmg to the di*^

The most important mdii^^ snenj^ indhidtal fiben.***

trie strength of paper is the dielectiK strength ^

,« D. O. Adams. Pafer Tr-lr I. J2. Na 7: 6W2 (FA H »«)

f TTO Ko. 26: 2S5-2S5 (Dec. 28,19W)

XVI. PROPERTIES OF PAPER

953

In the ease of impregnated papers, the dielectric strength is a complex
function of the dielectric strengths of both the paper and the inipregnant.**®
The dielectric strength of impregnated papers decreases with an increase
in sheet density,”’-”* due apparently to the increased percentage of fiber to
impregnant. in general, the dielectric strength decreases with an increase
in the thickness of the paper, because of the greater proportion of fiber to
air as the thickness is increased. The time of voltage stress and frequency
affect the dielectric strength.

Dielectric Loss {Power Factor)

Dielectric loss is an important consideration in capacitor paper, because
losses represent energy* loss which must be dissipated as heat, a problem
in capacitors. The heat generated per unit of volume is proportional to the
square of the voltage gradient, the dielectric constant, and the power fac¬
tor. ••• Since both high voltage gradient and high dielectric constant are a
necessity in capacitors, power factor is the only value which can be reduced.
It has been shown that the dielectric losses in oil-impregnated papers de¬
pend upon the dielectric losses in the paper.*®^

Power factor and dielectric constant can be measured in an apparatus
consisting of a conjugate Sobering bridge, a source of alternating current,
a three-stage amplifier for accurate detection of the null-balance point, and
a test condenser and chamber.*”"”* The presence of metallic ions and the
presence of lignin is associated with a high loss factor, the effect being
greatest, at high frequencies.*”

Properties oj Electrical Insulating Papers

All electrical insulating papers must meet high physical, chemical and
electrical standards. Electrical papers meet their most severe tests in ca¬
pacitor papers and in high voltage cable wrappings. In order to meet the
strict requirements, most electrical insulating papers are made from kraft
pulp, linen rags, cotton rags, or manila hemp.

The important physical properties of electrical insulation papers are
thickness, density, finish, porosity, tensile strength, and tearing resistance.

•♦•D. O. Adams. Pa^tr Trade /. 122, No. 7: 65-72 (Feb. 14, 1946)

J. A. Van den Akker, Paper Trade J. 119, No. 26: 255-255 (Dec. 28, 1944)

*♦* J. B. WHiiidiead. Elee. Eng. 59: 660-663T (Dec., 1940)

•‘•F. M. Clark and V. M. Montsinger, Gen. Elec. Rev. 28 : 286-290 (1925)

H. H. Race, R. J. Hemphill and H. S. Endicott, Paper Ind. 22, No. 8 : 792-798
(Nov.. 1940)

idem.

•*• J. A. Van den Akker. Paper Trade /. 119, No. 26 : 253-255 (Dec. 28 1944)

T* if* ^ Supplement to

A. S. T. M. Standards. Pan III 375-405

•” C R. Calldns. Tappi 33, No. 6: 278-285 (June, 1950)

J. A. Van den Akker. Paper Trade J. 119, No. 26 : 255-255 (Dec. 28, 1944)

954

PULP AND PAPER

Density, in particular, has an important effect on dielectric strength, dielec¬
tric loss, and dielectric constant. The true dielectric constant (K) of paper
is related to density by the relationship which states that (/v ~ 1 )/{K + 2)
is proportional to density.-’®®

The important chemical properties of electrical insulating papers arc
moisture content, ash, acidity, electrolyte content, and conducting particles.

Cellulose is a good insulator only so long as it is dry. An increase in
moisture content brings an increase in the specific inductive capacity and
power factor and a decrease in the dielectric strength.®®® The effect of
moisture is particularly bad on dielectric loss (power factor).®®^ It is de¬
sirable to keep the moisture content of insulating paper down to about
0.1%, but there is no advantage in going to a moisture content lower than
this.®®® The low moisture content required in insulating papers makes it
necessary to subject the paper to severe drying treatment. Ordinarily, the
paper is heated under vacuum until the moisture reaches the required value.
This results in some degradation of the paper. Ash generally varies from
about 0.5 to 0.8% for condenser tissue, to about 1.0 to 3.0% for telephone
wrapping paper and laminated insulation.®®®

An important property of insulating paper is the number of conducting
particles in the paper. The presence of electrolytic materials, such as chlo¬
ride ions left over from the bleaching operations, are especially deleterious,
particularly in direct-current work. Because of this factor, papers bleached
with hypochlorite are rarely used for electrical insulation. Conducting
particles are measured by the number of electrical breakdowns when the
paper is subjected to voltages somewhat lower than those obtained as the
dielectric strength values.®®® In making the test on thin paper (papers
under 0.0015 in. in thickness), the paper is placed between two metal elec¬
trodes, a flat metal plate and a smooth metal roller. The plate is connected
to the grounded pole of a 110-volt current (a.c. or d.c.). The metal roller,
a set of telephone receivers, and an 80,000-ohm resistor are connected in
series, and the other terminal of the resistor is attached to the live pole of
the 110-volt circuit. The number of conducting paths is taken as the num¬
ber of clicks heard in the telephone headset when the roller is passed over
the paper at a rate of 5 to 20 f.p.m. The results are reported as the number
of clicks per square foot of paper. For thick paper (0.0015-0.030 in.), a
.somewhat different set-up is used, and the number of conducting paths
taken as the total number of holes burned in the paper. The moisture con-

3D0C. Delevanti, Jr., and P. B. Hansen, Paper Ind. 28: No. 2 : 288-289 (May,
1946)

E. W. Greenfield, /. Franklin Inst. 222: 345-358 (1936)

.158 G. T. Koliman, Ind. Eng. Chem. 31, No. 7: 808-817 (July, 1939)

Tests for Insulating Papers D202-48T, A. S. T. M. Standards on ap
Paper Products. American Society for Testing Materials, 1916 Race =.t.,

Philadelphia 3, Pa.

XVI.

PROPERTIES OF PAPER

955

tent ol the paper at the time of test must be no higher titan TJo. 'I'he ntaxt-
^m numL' oi tolerable eonducting particles per square 'oo‘ vanes from
less than 10 for dielectric capacitor paper to less than 0.01 for p

bo&rds * * I

Impregnated insulating papers are made by impregnating paper witi

paraffin, linseed oil, shellac, phenolic resins, chlorinated naphthalene, an

chlorinated diphenyl.*" Anthraquinone has been used as a chemical s

lizer and elemental sulfur has also been suggested for t e same pur

nose.®®* It has been shown in many instances that the electrical properties

of impregnated papers cannot be accurately estimated from the electrica

properties of the paper and impregnant.

Types of Electrical Papers

Unimpregnated, impregnated, and vulcanized papers are among the
types used in electrical insulation. Some of the most important electrical

insulating papers are discussed below.

Condenser tissue®®* is a light-weight paper which is used in electrical

capacitors for radio transmitters and receivers, and in telephonic equip¬
ment. This paper is made as thin as possible compatible with good elec¬
trical insulating properties. From the standpoint of high dielectric strength
and life of the condenser, it is desirable to use several sheets of thin paper,
rather than a single sheet of equivalent thickness. By using several thin
sheets in place of a single thick sheet, the probability of failure due to im¬
perfections is greatly reduced. The thickness of condenser paper varies
from 0.0003 to 0.0015 in., and the weight from 4^2 to 12 lb. (24 x 36 500).
The finish, which is quite high, is obtained by supercalendering. High den¬
sity is important in condenser tissue, because density Is directly related to
the dielectric strength of the paper and the capacity of the condenser.
Highly porous papers are not suitable at all. The paper should be free of
pin holes and conducting particles. Good condenser paper should have a
low ash content, although the type of ash, rather than the amount, is of
most significance. Soluble salts (measured by the specific conductivity of
an aqueous extract of the paper) are particularly objectionable, chlorides
being the most harmful. The paper should be as close to neutral as possible.

Most condenser tissue is made from linen, cotton, or sulfate pulp, or

360 H. H. Race, R. J. Hemphill and H. S. Endicott, Paper hid. 22, No. 8: 792-798
(Nov., 1940)

361 D. A. McLean, L. Egerton and C. C. Houtz, hid. Eng. Client. 3S, No. 11;
1111-1116 (Nov., 1946)

362 D. A. McLean, U. S. 2,339,091 (Jan. 11, 1944)

363 L. Egerton, U. S. 2,287,421 (June 23, 1942)

36* Not to be confused with electrolytic condenser paper, which is a much heavier
paper used in electrolytic condensers. It is made open and porous to pick up
the maximum amount of electrolyte.

956

PULP AND PAPER

mixtures of these pulps. At one time, linen papers were used exclusively
for capacitors, but linen papers have the disadvantage of gaining in power
factor at temperatures greater than 70° C. Papers made from kraft pulp
stand up better under heat and voltage,®®® The greater resistance of wood
fibers may be due to the presence of natural anti-oxidants in kraft pulps.®®®
Sulfite pulps are not so acceptable as kraft pulp because of the instability of
the electrical and physical properties. It has been suggested®®^ that the
superior qualities of kraft pulp, as compared to bleached wood and linen
pulps, for d.-c. units is due to the greater ion exchange ability of kraft pulps
which causes the paper to adsorb hydrogen ions produced as a result of
breakdown of the impregnant in the paper.

McLean®®® reports that controlled oxidation of kraft papers results in
an improvement in insulation resistance, perhaps because of the immobiliza¬
tion of the ionic material through adsorption or chemical reaction. Cal¬
kins®®® found that the presence of carboxyl groups causes increased dielec¬
tric loss at higher temperatures. The source of excessive loss is evidently
in the non-cellulosic constituents of the fiber, i.e., the lignin and the hemi-
celluloses.®®®'®^® Miller and Hopkins®^® found, in the case of spruce kraft
pulps, that the most critical factor is the hemicellulosic content. They ob¬
tained the lowest power factor at a hemicellulosic content in the neighbor¬
hood of 20%, but found that the ash content and tannin were also a factor.
Delavanti and Hansen®^® found that acid washing and, to a lesser extent.

water extraction are effective in reducing the power factor of kraft pulp.

Insulating tissue or coil-winding paper is a thin paper, ranging from
0,0005 to 0.003 in. in thickness, which is used for wire wrapping in trans¬
formers, radio wires, and magnet wires. The basis weight generally ranges
from 5 to 30 lb. (24x36—500). The requirements are largely the same
as those for capacitor paper. The paper must not contain corrosive ma¬
terials which will attack fine copper wire.

Cable paper is strong paper which is used in the winding of high- and

low-voltage cables, and in transformers, coils, and magnets. The paper is
generally applied spirally in the form of strips. Winding is done by machine
at high speed and high tension so that paper having high machine direction

305 D A. McLean, L. Egerton, G. T. Kohman and M. Brotherton, Ind. Eng. C
34. No. 1: 101-108 (Jan., 1942)

306 D. A. McLean, U. S. 2,339.091 (Jan. 11, 1944) , P

307 D. A. McLean, L. Egerton, G. T. Kohman and M. Brotherton. Ind. Eng.

Chem.34, No. 1: 101-109 (Jan., 1942)

368 D. A. McLean, Ind. Eng. Chem. 39. No. 11: 1457-1461 (Nov.. 1947)

369 c. R. Calkins, Tappi 33. No. 6: 278-285 Gune, 1950) 241-249

370 c. C. Delevanti, J., and P. B. Hansen, Paper Trade J. 121. No. 26 . 241 24^

371 R R Miller and R. J. Hopkins, Gen. Elec. Rev. 50.^o. 12 = 20^4

372 c. Delevanti, Jr., and P. B. Hansen, Paper Trade J. 121. No. 26. 241 ^

(Dec. 27, 1945)

XVI. PROPEXTIES OF PAPER

957

tensik strength and stretch, and high cross direcuon tear is required.
Winding is often done under controlled humidity conditions. The
relauvely heavy and generally has a basis weight between 50 to Ub lb.
(24 X 36—500). Paper made from manila hemp has long been considered
the standard because of the high strength requirement, but kraft and niLX^
kraft and hemp papers have been used. ITie paper must 1^ free of pm
holes, and must not contain conducting particles (eg., iron). High
diekctric strength is. of course, \ery important in cable papers. Dielectric

are also an important consideration.

Pressboards made on a wet machine in thicknesses ranging from 0.W5
to 0.125 in. are used as a spacing and insulating medium in electrical
equipment. The stock used is generally rag or mi.xtures of rag and kraft.
Vulcanized fiber is used for electrical insulation in winding motor arma¬
tures. The desirable features of vailcanized fiber are good meclianical
strength and high dielectric strength. Certain grades of low caliper are

known as fish paper.

A special tyqie of electrical paper is that used in certain recording in¬
struments. Thb paper must have a high conductivity and consequently is
filled with 505 b or nwrc of carbon black.

.Microscopical Analysis

In addition to the chemical, physical, and optical tests commonly made
on paper, it is possible to obtain considerable information about the proper¬
ties of paper by examination w ith a microscope. Among the important uses
of the microscope in paper mill w'ork arc those for fiber length analysis,
determination of fiber species, dirt and speck analysis, determination of
amount of fiber treatment, study of rosin and starch sizing, and examination
of paper for fillers.

The v'alue of a microscope examination depends to a large extent upon
the skill of the microscopist and his ability to relate his observations to prac¬
tical application. The value of the microscope has been mentioned at fre¬
quent interv'als throughout this book. Its use for identifying fit)ers and
measuring the length of fibers has already lieen mentioned. The following
sections are concerned with the identification of fibers according to the proc¬
ess by whkh they were pulped and purified. Dirt and speck analysis and
other special uses of the microscope arc also discussed in this section.

Types of Microscopes

There arc several types: the optical microscope, the ultramicroscope,
and the electron microscope. For the examination of the crystalline struc¬
ture of nuteriab, x-ray analysis is very valuable.

The resolving power of the optical microscope is limited to objects no

958

MJLJf ANt) rAfER

closer together than the wavelength of light. Even with the best models it
is impossible to increase resolution beyond 1000 x to 1400 x, which means
that it is impossible to resolve objects less than 0.2 micron in size. When
the object is less than about 0.5 micron (0.0005 mm.) in diameter, the image
is not a true geometrical outline of the particle, and this lack of outline be¬
comes worse the smaller the particle, down to a size of about 0.1 micron,
where the object completely vanishes from view.

In spite of the above limitations, the optical microscope has consider¬
able value in the paper industry where it is widely used for dirt speck
analysis, for fiber measurement, for determining the degree of beating, and
for making fiber analysis.

The ultramicroscope can be used to investigate particles smaller than
those which are visible in the optical microscope. The “slit” ultramicro¬

scope consists of an ordinary optical microscope in which a slit of light is
used at right angles to the microscope to illuminate the particles by scat¬
tering the light and making them visible as spots of light against a dark
background. The ultraniicroscope shows up particles as small as 5 milli¬
microns, and with special refinements, as low as 3 millimicrons. Particles
which are not visible in the ultramicroscope are called ultramicroscopic par¬
ticles and are considered to be in true solution. The ultramicroscope is
sometimes used to count the number of particles present in a colloidal dis¬
persion. The size of the particles can be determined from the mass of the
dispersed phase (obtained by evaporation or filtration) and the number of
particles in the dispersed phase (determined in the ultramicroscope).

The electron microscope is a newer instrument which uses a stream of
electrons in place of ordinary light. In the electron microscope, pictures
can be taken which later can be enlarged to show useful information' at
ca. 50,000 X to 100,000 x. The advantage of the electron microscope over
the ordinary optical microscope lies in its improved resolving power, and
the fact that its ultimate resolution does not decrease appreciably with an
increase in magnification. It has a resolving power down to 0.01 to 0.004
micron, and promises to be effective down to 7 A. It has a depth of focus
of 7.0 microns at its greatest resolution. The electron microscope is o
great value in studying the problems of pulp and paper manufacture be¬
cause it permits study of materials which are not visible in either the op¬
tical or ultramicroscope. .

Like ordinary visible light, x-rays are electromagnetic radiations.

They differ from light in having a very much shorter wavelength, being ap-
])roximalely 10 ® cm. or 1 angstrom unit (A.) in length, compared wit i
about 10 ' cm. for light. These rays are much more penetrahng than eiec-
Iron beams and are diffracted by atomic planes deep within the ’

whereas the electrons in the electron microscope are diffracted mostly jy

XVI. PROPERTIES OF PAPER

959

planes lying near the surface. X-ray photographs can be takim l)y means
of special photographic film. Considerable experience is necessary in order
to interpret the results, but in the hands of a competent physical chernist,
they shed considerable light on the configuration of crystalline materials.

Types of Fiber Stains

The use of stains for the identification of different fibers became of
importance after the introduction of wood fibers in papermaking. When
only rag fibers were used, there was no necessity of determining fiber com¬
position, but after the introduction of straw and wood pulps, it became es¬
sential to find some means of distinguishing between wood, straw, and rag
fibers. Later, as the number of different wood pulps increased, it became
necessary to find a means of distinguishing between groundwood, soda,
bleached and unbleached sulfite, and bleached and unbleached sulfate.
Satisfactory stains have been developed for distinguishing between these
wood pulps, but the problems of identification are becoming greater all the
time because of a broadening in the types of papermaking fibers and because
of special methods of pulping and bleaching. Some of the relatively new
pulps having different characteristics are hardwood sulfite, bleached ground-
wood, and semichemical pulp.

Fiber analysis is important for a number of reasons. It furnishes a
means by which the mill chemist can check on current pulp production. It
is useful for analyzing old papers purchased as a source of pulp. It is a
means for analyzing competitive papers. Fiber analysis is useful in setting
up specifications on a given grade of paper and checking on whether the
specification has been met. Thus, in the manufacture of rag content papers,
a fiber analysis showing the percentages of rag and sulfite fibers serves as
a check for both the mill and the purchaser to see that the paper has met
specifications. A fiber analysis gives information on what may be expected
in permanence and performance of the paper. Bond and ledger papers are
sometimes specially watermarked to show the rag content. In the case of
papers for the U. S. Government Printing Office, watermark consi.sts of
the seal of the United States with a halo of stars, the number of which de¬
pends upon the rag content. Thus, one star signifies 25% rag, two stars
signifies 50% rag, three stars 75% rag, and four stars 100% rag. Chemi¬
cal wood papers for the U. S. Government Printing Office are not water¬
marked.

Preparation of Specimen

In preparing the specimen for staining, it must first be reduced to the
fibrous state. The sample may be either a pulp, a highly sized paper, an
impregnated paper, or a specially treated paper. Pulps are readily broken

960

PULP AND PAPER

Up in water, with the exception of groundwood, which must be given special
treatment.

Rosin-sized papers should be torn into small pieces, placed in a small
beaker, and boiled in a 1 ^ sodium hydroxide solution to remove the size.
After boiling, the liquor should be decanted and the sample washed with
distilled water, covered with 0.05 N hydrochloric acid, and then washed
again in distilled water. If the specimen contains wool fibers, the caustic
treatment must be eliminated.

Impregnated or coated papers must be given special treatment. As¬
phalt- or tar-treated papers can be digested in heavy or medium coal tar oil
on a steam bath for one hour and later extracted with benzene to remove
the asphalt. The digestion in oil may have to be repeated one or more times
before extracting with benzene. Waxed papers can be Soxhlet-extracted
with ether, alcohol, ethyl acetate, or acetone to remove the wax. Rubber-
treated papers can be Soxhlet-extracted with chloroform, dichlorobenzene,
nitroanisole, isopropylbenzene (cumen), or similar solvents to remove the
rubber.®^®’ The sample should then be boiled in a very dilute solution of
wetting agent. Parchment papers can be broken up by soaking in acid
(either concentrated HCl or a mixture of 50 parts of water and 50 parts
concentrated H 2 SO 4 ) until the paper starts to break up. The action of the
acid should then be stopped immediately by diluting in water or by neutral¬
izing with NaOH.®^^ Viscose-treated papers can be disintegrated in hot
calcium nitrate solution,®’® Coated papers can be prepared by soaking in
a suitable solvent and then rubbing the coating loose with a special abrasion
device. Dyes can be removed by treatment of the paper with bleach liquor,
nitric acid, alcohol, ammonia, hydrochloric acid, hydrosulfite, or similar re¬
agents, depending upon the type of dyestuff present. Melamine-treated
wet strength papers can be defibered with the aid of hydroxylamine hy¬

drogen chloride.

After the sizing or impregnating agent has been removed, a small
amount of the wetted sample is rolled in a small ball between the fingers
until the fibers are thoroughly loosened (chewdng in the mouth is some¬
times used as a method with samples containing groundwood). Next, the
small pellet of pulp should be placed in a large test tube and shaken with a
small amount of water. In some cases, about 10 to 12 glass beads may be
added to help in breaking up the fiber bundles, but shaking should not be
so severe that the fibers are disintegrated. Water should be added gradu¬
ally with repeated shakings to obtain a fiber concentration of about 0.05 to

373 J. H. Graff, Pulp and Paper Microscopy, The Institute of Paper Chemistry,
Appleton, Wisconsin (1942)

374 TAPPI Standards , , _ _

376 j. H. Graff, Pulp and Paper Microscopy, The Institute of Paper Che try,

Appleton, Wisconsin (1942)

XVI. PROPERTIES OF PAPER

961

0.1%. The fibers are now ready to be transferred to a microscope slide
for staining.

Preparation of the Slide

There are many techniques for preparing microscope slides for staining
and counting. In the TAPPI method, the fibers are transferred to the slide
by means of a dropper consisting of a glass tube (15 cm. long and 6 mm.
internal diameter) with one end fitted with a rubber bulb and the other end
carefully smoothed, but not constricted. After the fibers are thoroughly
mixed in water, the dropper should be inserted into the fiber suspension
5 cm. below the surface, two bubbles of air expelled, and the tube filled to
a distance of about 13 mm. The entire contents of tlie tube should then be
transferred to the slide, making 4 drops in all. This is repeated until the
slide is uniformly covered with drops. A metal dissecting needle can be
used for spreading the fiber suspension evenly over the slide, after which the
water is evaporated by heating the slide in an oven or on a hot plate where
surface temperature is about 50 to 60° C. Tapping the slide with the point
of the dissecting needle during drying helps to maintain uniform distribu¬
tion of the fibers. The slide should be left until it is completely dry, and
should not be touched with the fingers. In a properly prepared slide, the
fibers should be evenly distributed at a low density on the slide so that
individual fibers can be examined. If the fibers are too dense, the stain
will not take properly and the fibers will be difficult to count. Some chem¬
ists prefer to draw a line on the slide 1 in. from each edge, using aluminum
stearate dissolved in benzene to keep the fibers on the slide.

One of the most important considerations in the preparation of micro¬
scope slides for fiber analysis is the matter of cleanliness. All slides and
cover glasses should be thoroughly washed with soap, rinsed in distilled
water, and kept in 50% alcohol. The slides should be polished with clean
cheesecloth before using. Unless the slides are perfectly clean, the fiber
suspension will no spread evenly and there will be a tendency for iodine
(in iodine-base stains) to precipitate around the dirt specks.

When the slide and fibers are thoroughly dried and cooled, the fibers
are ready for staining. The methods of applying stains vary according to
the type of stain used. In applying iodine base stains, the stain is added to
the fibers, a cover glass is applied, and the stain is allowed to stand on the
fibers for one to two minutes. The excess stain can then be removed by

pressing the slide gently between two blotters or by tilting the slide against
a piece of filter paper.

There are a large number of stains available to the paper chemist.
Graff^'* lists the following stains as being useful:

962

PULP AND PAPLR

(7) Stains for clcterinining groundwood or woody fibers in paper, c.g., pbloro-
glucinol and aniline sulfate.

(2) Iodine-metallic salt stains for differentiating between groundwood, chemical
wood, and rag pulps, e.g., Herzberg stain and “C” stain.

(i) Dye-base stains for determining the degree of cooking, bleaching, and purity
of the fibers, and for the purpose of differentiating between unbleached sulfite and
unbleached sulfate pulps, e.g., Kantrowitz-Simmons, and Lofton-Merritt

(4) Special stains for positive identification of jute, manila, flax, hemp, and other
special fibers,

(3) Stains for determining the amount and type of sizing and coating on paper.

In preparing all stains, chemically pure reagents should be used and
the dry ingredients should be carefully weighed on an anal>'tical balance.
The liquid fraction should be carefully measured volumetrically. In mak¬
ing up concentrated salt solutions used in the stain, a Westphal balance
generally gives best results. Distilled water should be used both for pre¬
paring the stains and for preparing the fiber suspensions, since tap water
contains enough dissolved inorganic matter and is often so alkaline that it
throws off the color reaction. Stains must sometimes be adjusted after they
are made in order to obtain the correct color reactions, and for this pur¬
pose standard pulps should be kept on hand. Color charts are available for
certain stains to indicate the color reactions to be expected on different
fibers, but these charts are often confusing unless the observer has had

considerable experience.

Examination of the Slide

The light used for examining the slides is very important and should
be kept constant during the period of the examination. Once the con¬
denser on the microscope has been set, it should not be changed. North
daylight is best, but when this is not avaliable, special daylight lamps can
be used with good results. A daylight fluorescent lamp placed about 10 to
12 in. from the mirror of the microscope has the advantage of furnishing a

constant source of acceptable light.

One fact which is well known to anyone who has used fiber stains is

that it is impossible to ascribe any positive color to the stained fibers. This
is due to the fact that different batches of stains vary slightly in their stain¬
ing properties and, more importantly, because different pulps vary in their
de<^ree of cooking and bleaching. Hard, medium, and soft pulps have quite
different staining reactions, as would be expected from the differences in
the amount of lignin and other impurities which they contain. Even t e
various fibers from the same cook will sometimes show quite different stain¬
ing characteristics, particularly unbleached fibers cooked by the sul e

process.

XVI. PROPERTIES OF PAPER

963

Methods of Counthuj

The percentage of different fibers in an unknown sample can be deter¬
mined by the estimation method or by the dot count method. In the estima¬
tion method, an estimate is made of the relative proportion of different fi¬
bers in the microscope field. This method is fairly accurate in the hands of
an expert, but requires considerable experience. It is losing favor among

chemists.

In the dot count method, each individual fiber is counted as it passes
under the intersection of two crosshairs on the eyepiece. A mechanical
stage is used for moving the slide in a straight line across the field. Kach
fiber is counted once every time it passes under the crosshair, regardless of
whether it has been counted before. However, if a single fiber remains
under the crosshair intersection for a considerable part of its length, it
should be counted only once. All material w'hich is longer than it is wide
should be considered as a fiber or as a fiber fragment. Fiber fragments
should be given a value according to their relationship to the full fiber length.
In samples containing groundwood, there will be a considerable number of
fiber fragments in the groundwood portion. These fragments should be ig¬
nored if they are no longer than they are wide, but if they are slightly longer
than they are w'ide, they should be counted as one-quarter to one-si.\th of a
fiber. All other parts of groundwood should be counted as single fibers.
In the case of fiber bundles, each fiber in the bundle should be counted
separately.

In making a fiber count, it is customary to count three lines horizon¬
tally and tw'o lines vertically on each slide so that the area of the slide is
well covered. Some analysts prefer to count five lines horizontally, omit-
ting the vertical counts. As a rule, only one type of fiber is counted each
trip across the field, and the second fiber is then counted on the return trij)
along the same line. Some analysts, however, prefer to count all fibers in
a single trip across the field, \vhich is probably the most accurate method
when there is a sharp difference between the fibers. If the fibers are hard to
differentiate, the best method is to count the fiber \Yith the most obvious
color on the first trip and then count the total number of fibers on the re¬
turn trip, subtracting the two readings to obtain the number of the second
fiber type. A total of at least 400 to 600 fibers should be counted for each
unknown sample. When a fiber count is properly carried out, the per cent

error from the true average of the fiber composition .should be less than
2 to

Use of IVeight Factors

Fiber counting in the above fashion gives the percentage by number of
each type of fiber in the unknown sample. However, the papermaker han-
H- Graff. Paper Trade J. 101, No. 2: 14-28 (July 11, 193,S)

964

PULP AND PAPER

dies pulp by weight, and it is therefore necessary to convert the results into
percentage by weight. Weight factors are used for this purpose. In re¬
porting results, it should be clearly stated whether the report is in per¬
centage by number or percentage by weight.

Weight factors are necessary for converting from percentage by num¬
ber to percentage by weight, because all fibers do not weigh the same, as
shown in Table XXXIX.^” For example, in an equal weight of soda

TABLE XXXIX

Average Numbers of Fibers per Gram of Pulp

Probable error ± 5,000

Pulp

Number of fibers

. 8,550,000

Xvag .. , . . ... -.

Softwood U. S. western sulfite .

Softwood B. western sulfite.

. 9,250,000

. 9,880,000

. 6,800,000

. 7,050,000

. 9,525,000

. 9,225,000

j^oixwooQ xy * iviciiL

Softwood U. B. southern kraft .

. 5,500,000

and rag pulp, there will be twice as many soda fibers as rag fibers. Since
rag fibers are taken as the basis of comparison, the weight factors for these
two pulps are therefore 1.0 for rag and 0.5 for soda fibers. The ^
factors generally used for the different commercial pulps are listed in table

TABLE XL

Weight Factors for Converting from Percentage by Number to Percentage

BY Weight

Pulp

Rag ..

Sulfite (spruce) .

Sulfite (western hemlock)
Sulfite (hardwoods) .. -.
Kraft (eastern U. S.) ..
Kraft (southern pine) ..

Kraft (hardwoods) .

Soda (hardwoods) .

Groundwood (softwoods)
Straw and bast fibers ....

Factor

1.00
, 0.90
1.20-1.23
. 0.60
. 0.90
. 1.5S

. 0.70
. 0.50
. 1.30

. 0.55

XL. Weight factors are ordinarily not affected by bleaching or ^

^-ee of beating. Isenberg and Peckham- found that ^

3 T7 J. H. GraflF, M. A. Schlosser and E. K. Nihlen, Tech. Assoc, apers

531 (June, 1941) „ 'r 37 Mra in- .527 528 (Oct., 1950)

37 iai. H. Isenberg and C. L. Peckham, Tappt 33, No. 10. 5//, wo t

XVI. PROPERTIES OF PAPER

965

different weight factor from regular rag pulp, and that the weight factor
varied in the case of second cut linters from 1.50 to about 1.05 when the
freeness due to beating was changed from 850 to 450 (S.R.).

Iodine Stains

Staining is a colloidal phenomenon. ^lost of the staining reactions are
based upon the fact that lignin has an affinity for certain salts and basic dyes,
that hemicelluloses have an affinity for iodine, and that cellulose has an
affinity for direct dyes. The blue color obtained upon the addition of an
iodine stain to a bleached pulp is due to the adsorption of iodine by the
hemicellulose in the pulp (in the same way that starch is stained blue with
iodine). Zinc chloride in the stain increases the hydration of the hemicellu¬
lose, resulting in greater iodine adsorption, and hence intensifies the blue
color. Highly lignified fibers, on the other hand, are stained yellow rather
than blue because lignin has no readily available hydroxyl groups to adsorb
the iodine in a definite pattern, and because the lignin prevents the zinc
chloride from swelling the hemicellulosic material in the fibers.

The amount of iodine adsorption by the fibers is in direct relationship
to the moisture content of the fibers, and consequently the fibers should be
thoroughly dried before staining.*^* In all iodine stains, the color reaction,
in going from an uncooked pulp, to a cooked pulp, to a completely purified
pulp, is from yellow, to blue, to red. Thus, with iodine base stains, ground-
wood fibers stain yellow, chemical wood fibers stain blue, and rag fibers
stain a wine red. The iodine stains include the Herzberg stain, Suter-
meister’s stain, Selleger's reagent, the Graff “C” stain, Wilson’s stain, and
Laughlin’s stain, lliesc stains are discussed in the following sections.

Herzberg Stain. One of the most important of the iodine stains is the
Herzberg stain. This stain is useful for differentiating between rag,
groundwood, and chemical wood pulps as a class, but cannot be used by the
average observer for differentiating l>etween the various chemical wood
pulps. The wide usage of this stain is due to its ease of preparation and
its traditional use in the paper industry.

The color reactions obtained with the Herzberg stain depend upon the
amount of lignin and hemicellulose in the fibers. Heavily lignified fibers
such as groundwood, uncooked manila hemp, raw straw, and jute stain
yeUow. Highly purified fibers such as cotton and linen rag pulp and
bleached manila hemp stain a purplish pink to a wine red color. Most
ax)ked and bleached sulfite and soda pulps stain a blue color, although there
IS a tendency toward a reddish purple in the case of the highly purified
grades, and a tendency toward an olive gray in the case of the raw grades.

wi) Withrow, Paper Trade /. 91, l^o. 24; 51-55 (Dec. 11,

966

I’ULr AND PAPER

Hardwood soda libers stain a deeper blue than softwood sulfite filjcrs be¬
cause of their thicker fiber walls. Of the synthetic fibers, vinyon is left
uncolored by Herzberg stain, whereas cellulose acetate fillers are stained
yellow and viscose fibers are stained a deep blue or violet.

If sufficient care is used in making the Herzberg stain, it should lie
satisfactory as originally prepared. However, it is sometimes necessary to
adjust the stain. If the stain produces too weak a blue on chemical pulps,
additional zinc chloride should be added. If the red and yellow colors are
weak, additional iodine can be added. The addition of water tends to
weaken the predominating color. Some analysts prefer to keep several lots
of stain on hand which have been specially adjusted so that they work best
on certain fiber combinations. A stain which works well on groundwood
and unbleached sulfite does not work well on rag and bleached sulfite or

soda.

There have been many variations in the preparation of the stain which
is now generally known as the Herzberg stain. The standard method of
preparing Herzberg stain, as accepted by TAPPI, is as follows.

Solution A: 40 cc. of zinc chloride solution of 1.80 specific gravity at 28* C. This
solution can be made by adding about 25 cc. of distilled water to 50 g. of cp. anhydrous

Solution B: 5.25 g. of dry potassium iodide and 0.25 g. of dry iodine in 12.5 cc. of
distilled water. (Mix the dry ingredients together and add water, drop by drop.)

Add solution B to solution A and, after mixing thoroughly, pour the solution into
a tall narrow cylinder and allow to stand until the precipitate has settled. The clear
Jotation is .ha. pipetted into a dark bottle and a leaf of iodine added. The stam should

be renewed every month or so.

In using the Herzberg stain, a few drops of the stain are “

the dry libers on the slide, a cover glass placed over the fibers, and e
stain is left for at least one minute. At the end of this time, t *
stain is removed by pressing the slide and cover glass between two blotter ,

or by tilting the tong edge of the slide into contact with a blotter^

The Graff “C” Stain. Graff has developed a series of lodme stains

which are improvements on older iodine stains. The most "

these is the “C” stain. In using the “C" stam, between ‘"j®
of the stain should be added to the dry fibers on the ^bde and t
covered with a watch glass. After standing exactly trvo minutes, the ex«ss
stain should be removed by pressing the slide between blotters or by t g

one edge against a blotter. . ,.j • j* .. e.r,iiitinn

The “C” stain is made by combining potassium lodide-i^me
with definite amounts of metallic salts. It is prepared as follows:

To 20 cc. of standard aluminum chloride solution (Uo 10 "cc. zinc

10 cc. of calcium chloride solution (1.36 specific gravity at 28 C.) ana

XVI. PROPERTIES OF PAPER

967

chloride solution (1.80 specific gravity at 28* C.), add 12.55 cc. of iodine solution made
by adding 50 ‘cc. of di.'itilled water to 0.9 g. of dry potassium iodide and 0.65 g. dry
iodine. After mixing well, the solution should be poured into a tall narrow vessel and
placed in the dark for 12 to 24 hours. At the end of this period, the clear portion is
pipetted into a dark, glass-stoppered dropping bottle and a leaf of iodine added. The
stain should be kept in a dark cool place when hot in use.

The “C” stain differentiates between a wide variety of fibers, including
raw, medium, fully cooked, and bleached softwood sulfite pulps and raw,
medium, well-cooked, and bleached softwood sulfate pulps. The stain also
can be used to distinguish between softwood and hardwood pulps, hard¬
wood kraft, sulfite, alpha, and soda pulps, as well as jute, manila, hemp,
and straw pulps. In order to be able to differentiate between all these
various fibers, considerable experience is necessary, but in the hands of
an experienced observer, the “C” stain is extremely useful.

One of the most difficult jobs in fiber analysis is differentiating between
bleached sulfite and bleached sulfate, but this can be done fairly well by
the “C" stain; under the “C” stain, bleached sulfite fibers stain a light
reddish to pinkish blue (vinaceous fawn), whereas bleached sulfate fibers
stain a darker blue which may have some red (violet plumbeous).

Differentiation with the “C” stain should not be made on the basis

of color alone, since the depth of color and clarity of the color are equally

important, particularly in the case of unbleached pulps. For example,

with a given specimen of unbleached sulfite pulp, the “C” stain is likely

to produce colors ranging from yellow, to a light tan, to a gray, almost

colorless, fiber, depending upon the degree of purification of the individual

fibers. It is possible to obtain an approximate indication of the bleach-

ability of a sulfite pulp by counting the number of undercooked (yellow)

fibers in the specimen. Unbleached sulfate pulps also show considerable

variation in color in the same sample. The range of colors is very much

the same as that shown by unbleached sulfite, but whereas the colors with

unbleached sulfite are light and clear, the colors obtained with unbleached

sulfate are muddier and much darker. Bleached fibers do not show as wide

variation in color as unbleached fibers because of the greater uniformity of
the bleaching action.

The colors developed by the “C” stain on different pulps are given
m Table XLI. However, it should be pointed out that these are only
approximate colors, and each observer must make his own interpretations
based upon examinations of known pulp samples. Color charts are avail¬
able for the “C" stain, and these are helpful in becoming familiar with this
stain.

968

PULP AND PAPER

TABLE XLI

Colors Developed by Graff “C” Stain

Groundwood

Vivid yellowish orange
Rag

Red to reddish orange
Manila

Light yellowish green (raw)

Jute

Yellowish orange (unbleached)

Light yellow-green (bleached)

Straw, Bamboo, Cane, etc.

Light yellow to weak greenish yellow (raw)

Greenish gray to bluish gray (unbleached and bleached)

Sulfite Coniferous Pulps
Generally yellow to gray

(A) Softwood unbleached sulfite
Yellow, cartridge buff

(B) Softwood lightly bleached sulfite
Pale vinaceous fawn

Pallid purple drab
Very light vinaceous gray

(D) Softwood West Coast bleached sulfite
Pale purplish vinaceous

Softwood alpha pulps

Pale brown to pink and red

(A) Softwood unbleached alpha
Cinnamon buff

(B) Softwood lightly bleached alpha
Light purplish lilac

Softwood sulfate pulps

Greenish yellow to dark bluish gray

(A) Softwood unbleached sulfate
Mineral gray

Ecru drab

(B) Softwood bleached sulfate
Pale violet plumbeous
Glaucous gray

Hardwood sulfite pulps

Pale yellow-green to purplish blue

XVI. PROPERTIES OF PAPER

969

TABLE XLI (continued)

(A) Hardwood bleached sulfite
Very light grajdsh blue violet
Very pale Windsor blue
Pale grayish vinaceous
Pale purplish vinaceous
Hardwood alpha pulps
Reddish orange to dusky red
(A) Hardwood alphas
Pinkish vinaceous
Etruscan red

Light vinaceous Congo pink
Light vinaceous
Hardwood soda pulps
Blue-green to blue, Russian blue
Hardwood sulfate pulps
Blue-green to blue

(A) Hardwood unbleached sulfate
Light vinaceous purple
Orient blue

(B) Hardwood bleached sulfate
Alice blue

Sutermeister “A” Stain. There are two Sutermeister stains, both
of which belong to the class of iodine stains. The original Sutermeister
stain was applied in two stages, but Graflf®^® modified the stain to a one-
solution stain which he called the “A” stain. The “A” stain is prepared as
follows;

Solution A: Calcium chloride solution of 1.36 specific gravity at 28“ C. is prepared
by adding about 100 g. of cp. calcium chloride to 150 cc. of distilled water.

Solution B: Prepared by adding 50 cc. of distilled water to a mixture of 0.9 g. of
dry potassium iodide and 0.65 g. of dry iodine.

Add 5 cc. of solution B to 45 cc. of solution A and, after mixing well, pour the

mixture into a dark glass-stoppered bottle. Add a leaf of iodine and keep in the dark
when not in use.

The colors developed by this stain are as follows;

Red or brownish red—cotton, linen, hemp, ramie

Dark blue—bleached soda (from hardwoods)

Bluish or reddish violet-bleached sulfite and thoroughly cooked unbleached sulfite.

Greenish—jute, manila hemp, and the more lignified fibers in unbleached sulfite.

Wilson Stain. One of the newer iodine stains is the Wilson stain
This stain has several advantages over the older iodine stains, among which .

H. Graff, Paper Trade J. 100, No. 16: 45-50 (Apr. 18, 1935)

970

PULP AND PAPER

are greater ease of preparation, less overlapping of colors, and more uniform
coloring on any one type of pulp. The Wilson stain is prepared as

follows

Iodine crystals cp. 0.8 S-

Cadmium iodide cp. 35.0 g.

Distilled water . 50.00 cc.

Heat the above to 110° F. and break the iodine crystals with the end of a stirring
rod. Decant the solution from one beaker to another to observe the point where no
dissolved solids remain. Then add the following in order, stirring:

Distilled water . 90 cc.

Formaldehyde .

Calcium nitrate cp., Ca(N0s)2 4H20 . 70 g.

Cadmium chloride cp., CdCl 2 2.5H20 . 20 g.

Formaldehyde has a stabilizing action and tends to improve the brilliancy of the
colors.

In using Wilson stain, a few drops of stain are placed on the dry fibers
on the slide, a cover glass applied, and after about ten seconds, the excess
stain removed from the edges of the slide with a blotter. The slide may be
examined at once. Some of the colors developed are as follows:

Soda—purple
Linen and rag—^pink
Kraft—brown (some gray)
Unbleached sulfite—colorless
Bleached sulfite—lavender
Bleached sulfate—^blue
Cotton—red

Groundwood—yellow
Straw—predominately green

This stain is particularly useful for distinguishing between unbleached

sulfite and unbleached sulfate fibers. Qtain

Modified Selleger Reagent. Selleger's reagent .s -

which is particularly useful for distinguishing ® Whereas

straw, and esparto fibers, since these fibers stain a distinct blue, where

most other fibers have a pale rose or yd'o™ are

There are two methods for making Sellege g >

described below;

Method I: Dissolve 100 g.
solution, made by dissolving 8 g. of KI in JO cc.

Method II: Dissolve 0.267 g. of potassium iodine ^0 ’ ^ ^

iodine and let stand 2 weeks, shaking each day. Then dissolve
calcium nitrate. The stain is then ready for use.

380 N. F. Wilson, Paper Ind. 27, No. 2: 215-216 (May, 1945)

XVI. PROPERTIES OF PAPER

971

The colors produced by Selleger s reagent are as follows:

Bleached coniferous woods—flight rose
Unbleached coniferous woods—clear yellow
Poplar and birch—violet blue

Esparto and cooked straw—blue "

Manila and hemp—wine red

Linen and cotton—rose-brownish

Groundwood, jute, uncooked straw—deep yellow

The stain is good for about two to three months.

Laughlin Stain. A special stain for differentiating between bleached
sulfite and bleached sulfate fibers has been introduced by Laughlin*®^ and
elaborated upon by Stocker and Durant.^®- This stain is prepared as

follows:

Solution A (basic orange stain) : 0.7 g. Dupont Basic Orange 3RN, SO ml. dis¬
tilled water, 50 ml. 95% ethyl alcohol. The dye is dissolved in the water and then the
alcohol added.

Solution B (dilute hydrochloric acid, approximately 0.02 N).

Solution C (regular Herzberg iodine stain).

Solution D (chloride stain) : 50 g. zinc chloride (fused sticks), 15 g. calcium
chloride (anhydrous).

Make up to a volume of 100 ml. with distilled water and add 0.5 ml. 0.02 N hy¬
drochloric acid.

It is not advisable to keep the solution for a period exceeding two months.

The method of using this stain is as follows. A representative sample

of the unknown is boiled in acidified water (4 drops of concentrated hydro¬
chloric acid to 50 ml. of distilled water) for about one hour. The acidified
water is decanted and replaced with distilled water, after which the sample
is again brought to a boil. The sample is swirled around in a flask con¬
taining a few glass beads until it is well dispersed. The sample is diluted
to the proper consistency and slides prepared. Enough of solution A is then
added to cover the fiber (approximately 0.5 ml.). This is allowed to stand
on the fibers for one minute, after which the slide is blotted with a clean
filter paper and rinsed once with solution B, and then blotted dry. Enough
of solution C is added to cover fibers (approximately 0.5 ml.), allowed
to stand thirty seconds, and then the slide is blotted. Finally, one drop of
solution D is added and the cover glass immediately placed on the slide.
After one minute, the colors develop and the slide is ready for examination.
Differentiation is difficult after a period of thirty minutes, as there is a
definite fading of color.

E. R. Laughlin, Canadian Industries, Ltd. Report on Fiber Analysis M!ethods
(June, 1938)

382 p w, Stocker and L. G. Durant, Pulp Paper Mag, Canada 46: 67-69 (Apr
1946) ■’

972

PULP AND PAPER

The results obtained in using this stain are summarized below

Pulp

Acid cooks

Unbleached sulfite.

Bleached sulfite

(approx. 80 G.E. brightness) ..

High bleached sulfite ^

(approx. 90 G.E. brightness) ..

High alpha bleached sulfite .

Colors observed

Medium to dark amber
Pale tan

Will stain a very pale blue which fades out in
one to two minutes to a pale tan.

Will stain a very pale blue which fades out in
one to two minutes to pale tan. Practically un¬
stained.

Alkaline cooks

Unbleached kraft.

Regular bleached kraft ...
High alpha bleached kraft
Bleached soda .

Dirty gray to khaki, some blue
Medium blue
Pale to medium blue
Deep to medium blue

Dye-Base Stains

Several stains use dyestuffs as their main base. In these stains, dyes
are taken up by the fiber,? according to complicated relationships between
the purity (lignin content) of the fibers and the dyeing properties of the
dyestuff. These sta'ins are used principally for determining the extent o
cooking and extents bleaching of the fibers. Some of the important dye-

base stains are described below. , .

Bright Stain. The Bright stain is used for differentiating e e

bleached and unbleached pulps, and for indicating the approximate ^
of cooking. The stain consists of three solutions, which are made as

^ ^ • o 7n <y m EcCLi • bffnO in 100 ml. distilled wstcr.

Soluhon A. Z./U g. cp. i ec.i 3 ujta.w ^ inn ml of dis-

Solution B: 3.29 g. cp. potassium ferncyanide (K 3 Fe(.CN) 6 )

^vMition C: 0.50 g. benzopurpurine (Dupont Purpurme 4B c .)

50% ethyl alcohol. The solution must be warm to keep the dye m so
Solutions A and B should be filtered every day just before use.

In using this stain, a small amount of pulp S-) for

50-ml. beaker containing equal parts of solutions .4 an an
one minute. The pulp is then screened, squeezed dry, an ^

shaking in about 500 ml. of distilled water. The sample ts a^ " s
and squeezed dry, after which it is placed in 50 ml. of —

warmed gently for about two minutes. T e pup gjjdes

squeezed dry, and washed in 500 ml. of distilled water, after w

are prepared. , the fer*'**''

The lignin in partially cooked, unbleached fibers re ^ fj^ers.

ferricyanide to Prussian blue, which is taken up by t e u

XVI. PROPERTIES OF PAPER

973

Bleached fibers, on the other hand, contain no lignin and hence do not take
up a blue color, but absorb benzopurpurine in the subsequent stage, thus
taking on a red color. Unbleached fibers vary in color from blue to red,
depending upon the degree of cooking. The colors developed^®® are as
follows: 20% cooked, light to dark blue; 40% cooked, light bluish gray;
60% cooked, blue-gray with purplish pink markings; 80% cooked, strong
purplish pink; 100% cooked, red. Graff®®® has shown that the number
of red fibers in unbleached sulfite pulp increases almost directly with a de¬
crease in the permanganate number.

Kantrowitz-Simmons Stain. One of the most useful modifications
of the Bright stain is the Kantrowitz-Sinimons stain. This stain can be
used to differentiate between groundwood and unbleached fibers on one
hand, and bleached fibers on the other. The solutions used are the same as
those in the Bright stain.

In using the Kantrowitz-Simmons stain, three drops of solutions A
and B are added to the fibers on the slide and allowed to remain one minute.
The excess solution is then removed by blotting with filter paper, a few
drops of solution C are added, the specimen is covered with a cover glass
and left for. two minutes. The excess stain can then be removed by care¬
fully washing with distilled water, playing the water gently at one edge
of the cover glass. Too much washing removes the red color.

The color reactions obtained with the Kantrowitz-Simmons stain are
as follow's; red, bleached sulfite, soda, sulfate, and rag; blue, unbleached
sulfite, sulfate, groundwood, and jute. Ordinarily, this stain is quite re¬
liable, but care must be exercised in the staining technique to obtain the
true color reactions. Sloppy technique leads to some bleached fibers
taking on a blue color, or some unbleached fibers taking on a red color.

Lofton-Merritt Stain. The Lofton-Merritt stain was developed to
aid in distinguishing between unbleached sulfite and unbleached sulfate
fibers. This stain is highly useful, once it has been checked against known
fibers, but it is not reliable unless it is standardized every day. Lofton-
Merritt stain is prepared as follows:

Solution At 2 g. of Mslachite green in 100 cc. of distilled water.

Solution B; 1 g. of basic fuchsine in 100 cc. of distilled water.

The two solutions are kept separately in tightly stoppered bottles and mixed to¬
gether just before use. Ordinarily, the two solutions are mixed in the proportion of
1 volume of solution ^ to 2 volumes of solution B. but the ratio varies, depending upon
t e dyes used, and consequently the correct ratio must be determined on known sam¬
ples of pulp In making a batch of stain, it is best to mix the two solutions drop by
drop until the correct color reactions are obtained with known fibers.

Ap”«onrWif^mtaOwT Chemistry,

974

PULP AND PAPER

In using Lofton-Merritt stain, the compound stain is added to the
fibers on the slide and allowed to remain two minutes. At the end of this
period, the excess stain is removed by means of hard filter paper or, better,
by playing a fine stream of distilled water at one end of the slide so that
the water washes gently over the fibers. Then, three to four drops of 0.1^
hydrochloric acid solution (1 cc. cone. HCl diluted to 1 liter of water) are
added and left for about thirty seconds. The acid is then removed with
a blotter and a few drops of distilled water added. A cover glass is then
placed on top of the fibers and the slide pressed gently between two pieces

of blotting paper to remove any excess water.

When properly made and used, Lofton-Merritt stain produces a blue

to blue-green color with unbleached sulfate fibers and a purple or lavender
color with unbleached sulfite fibers. Since a blue color is also produced
with groundwood fibers, this stain can be used to determine the amount
of unbleached sulfite in mixtures containing groundwood, unbleached
sulfite, and unbleached sulfate fibers. Highly purified (i.e., bleached)
fibers do not absorb any color. Some of the newer unbleached sulfate pulps
do not stain well with this stain. In testing the stain, if any sulfite fibers
are green, too much Malachite green is present. If any sulfate fibers are

Durple, too much fuchsine has been used. _

Cooking (BleachabiUty) Stain. The cooking stain is useful as a

means of measuring the approximate degree of cooking. In t is staining
iXque, the fibers are first treated with a solution of Ma achite ^e^
and then with a solution of Congo red. The colors produced
areen for relatively uncooked fibers to red for the completely cooke •

If green fibers predominate, the pulp is undercooked, whereas a predom -
nance of red fibers indicates an overcooked pulp. Graff"®* gives t e vano
colors the following arbitrary values:

Extent of

Color cooking

. 10%

Variations of green . .. 2o%

Blue with green markings ^ ..

Red with strong green markings .

Red with faint tint of green .

Lightly broken red . 100%

Pure red .

Graff and coworkers“> found that the cooking stain correlates well with the
lignin content and permanganate number for fie used

Cyanine-Glycerine Reagent. Cyanme-glycerine reagent can

J. H. Graff. P«IP e-d fafer MkroscoPy, The Institute of Paper Chemistry.

3 ss^:t Hedlrand TR Graff, Paper Traie J. lU. No. 5 : 52-56 (Aue. 1,
1940)

XVI.

PROPERTIES OF PAPER

975

to determine the bleachability of pulp. This reagent consists of a satura
alcoholic solution of cyanine diluted with water and one-third of its volume
of glycerine. The following color reactions are obtained, according to

Graff

Unbleached sulfites .

Bleached sulfites .

Unbleached sulfates .

Bleached sulfates .

Tracheids

Cell walls Pits

Blue

Colorless

Blue

Colorless

Center deep
blue, out¬
side dark
blue

Colorless

Ray cells

Cell walls Pits

Blue

Dark blue

Colorless Dark blue

Blue None present

Colorless Colorless

Bleach Stain. The bleach stain is useful for measuring the approxi¬
mate degree of bleaching. This stain is based upon the amount of absorp
tion of a basic dye, gentian violet, by the fibers. Lignin has a strong affinity
for basic dyes and cellulose has little or no affinity, so that the amount of
gentian violet absorbed by the fibers is related to the amount of lignin in the
fibers. Thus, the color reaction is related to the bleach requirement of
the pulp. Lightly bleached sulfite fibers stain a dark blue with this stain,
whereas highly bleached sulfite fibers are stained only a faint color. Soda
and sulfate fibers also stain in proportion to the degree of bleaching.

Shaffer Stain. The Shaffer stain,which is made by dissolving 1 g.
of C. P. brazilian in 175 cc. of water, has been suggested for distinguishing
between bleached sulfite and bleached sulfate fibers. This stain is not
used to any extent in fiber analysis.

Malachite Green Reagent. A 2% solution of Malachite green in
Jicctic acid is useful in measuring the extent of bleaching, since unbleached
fibers stain blue-green to green, partially bleached fibers stain blue, and
fully bleached fibers show up colorless.

Groundwood Stains

It is often desirable to detect the presence or absence of groundwood
fibers in papers without making a quantitative determination. This can
be done by the use of special stains which are applied directly to the paper.

Phloroglucinol stain is made by dissolving 1 g. of phloroglucinol in
50 cc. of alcohol and 25 cc. of concentrated hydrochloric acid. It produces
a carmine red color on papers containing groundwood. The depth of color
is an indication of the amount of groundwood present. However, a very
light color does not necessarily indicate the presence of groundwood pulp,
.since unbleached chemical wood fibers sometimes show a slight color with

H. Graff, Pulp and Paper Microscopy, The Institute of Paper Chemistry.

Appleton, Wisconsin (1942)

R. W. Shaffer, Ind. Eng. Chem., Anal. Ed. 5, No. 1: 35-36 (Jan., 1933)

976

rULP AND PAPER

this slain. Jute fjcnerally produces a reddish color with this stain. TIk
stain is relatively unstable and should not be made up long Wforc being used.

Aniline sulfate stain produces a yellow color when applied to [jajjer
containing a high percentage of groundvvood fibers. In addition, this
stain also produces a yellow color with raw unbleached sulfite and sulfate
pulps. To prepare aniline sulfate stain. 1 g. of aniline sulfate is dissolved
in 50 cc. of distilled water and the solution acidfied with one drop of erm*

centrated sulfuric acid.

Other groundwood stains include /’-nitroaniline and dimcthyl-/>-
phenylenediamine. The latter can be used to differentiate between bleached
and unbleached pulps, since it produces an orange color \\ ith mechanical
and unbleached sulfite pulps, and a yellow color with Reached sulfite pulp.

One interesting method for distinguishing between softwood and hard¬
wood groundwood is to stain a slide first with aniline sulfate and then
with aqueous methylene blue. When treated in this way, hardwood ground-
wood fibers generally stain a bluish green color, whereas softwood ground-

wood fibers stain yellow.

Stains for Sl’cdal Fibers

Manv special stains can be used for the detection and identification
of special non-woody fibers. The stains which are used for this purpose
are so varied that it is impossible to cover all of them, and the r^der is re¬
ferred to special reference books on the subject.^®® The ph) sica prope les
of non-woody fibers are discussed in the chapter on pulp fibers.

A few of the stains which are useful for examining non-\\oo > >ers

are listed below:

Neocarnnne can be applied to the fiber, (after d«otorizing
following are some of the color reactions obtained ' „„„

tion of Neocarmine W: linen, deep blue; cotton, redd sh blue. jute,
wool, yellow: viscose, red; acetate rayon, light green-} e o\i. ^

With Neocarmine B, the following reactions are obtamed: ^

Flax and hemp can be distinguished mth cj-Mine ^illrion diluted with a small
rated solution of cyanine in water is made and then

amount of water and about one-third of its .cabling glvceiine.

on the slide should be heated and then washed “"/XTSatoed are:

water, and alcohol. The slide can be mounted in glj-cerme. The colors

flax, colorless; hemp, greenish blue. reauired for roofing

A determination of the amoimt of woo in paper is ^ determined bv any of the
paper and linoleum base stock. The amoimt of w«l ( 7 ) Kjeliahl method.

LSowing methods; (f) solution n ami «b^rs is TlOtO^-

( 4 ) microscopic counting of fibers, ine we g ^

388 j H Graff, Pulp and Paper Microscopy, The Institute of Paper

XVI. PROPERTIES OF PAPER

977

Speck Analysis

It is frequently the job of the pap6r analyst to analyze and identify
dirt specks in paper in order to determine the source of the dirt. In this
work, the analyst must use his ingenuity, particularly in mills using waste
paper or rags, since almost anything may be present in paper made from
these materials. Many helpful hints are given in the literature, but it is
sometimes necessary to devise new methods. In most cases, the dirt can

best be examined under a dissecting microscope.

In analyzing dirt specks, the sample of pulp or paper is first wetted
in water and then the marked specks are removed, using iridium needles
and a dissecting microscope, or any microscope magnifying about 50 to 100
times. The specks can be arranged on a microscope slide and then identified
either by chemical or solubility tests, or from the physical appearance of the
specks. Browning and Graff have developed a chart for speck analysis
which is used in certain paper mills. Most of the following methods have
been taken from their chart. Some of the solubility tests used are as

follows:

( 2 ) Water—for dyestuffs and alum specks.

( 2 ) Alcohol—for some dyestuffs.

(3) Alcohol plus 1% cone. HG—for resins and pitch.

(4) Benzene—^for resins, oil, grease.

(5) 5% Sodium hydroxide (at 100° C.)—for casein and wool.

(d) 10% Nitric acid—for copper and lead specks.

(7) Hydrofluoric acid—for silicate.

Some of the staining or treating agents used are as follows;

(1) Fiber stains—for identifying fibers.

( 2 ) Phloroglucinol—^for shives, bark, etc.

(3) Iodine-iodide solution—for starch spots.

(4) 2% Ferrocyanide solution—for copper (brown) and ferric ifon (blue).

(5) India ink—for slime-forming bacteria.

( 6 ) Millon reagent—for casein (brick red color).

(7) Raspail test (sugar and cone. H 2 SO 4 )—for rosin spots (raspberry red).

(8) Bleach solution—^for shives, bark, etc. (which decolorize when treated with
bleach solution).

Coal Specks. These are black, insoluble particles which are often in
a pulverized condition, due to the action of the calender rolls. They can
be distinguished from iron spots by testing with a magnetized needle, or
by the ferrocyanide test. Large coal specks can be readily smeared when
rubbed across the sheet. If the speck tends to break and show sharp
edges, it is probably a cinder.

Iron Specks. Iron specks can be determined by treating the speck
with 10^ hydrochloric acid-solution, removing the excess acid by draining
onto a blotter, and moistening each speck with a small drop of potassium

980

PULP AND PAPER

albumin fixative) and the paraffin removed by soaking with xylene. The
slide can then be examined with transmitted light.

Special Staining Techniques

It is possible for an experienced observer to obtain considerable in¬
formation on the type and location of sizing agents in paper by microscopic
examination of the paper. This is particularly true with starch-sized
papers where it is possible by staining with iodine-iodide solution to deter¬
mine approximately how much starch is present, whether the starch was
applied as a surface or internal size, and if sized internally, whether the
starch is uniformly distributed throughout the sheet.

Special staining techniques can be used for detecting the presence of
resins (pitch) in pulp fibers. Either Sudan orange 2R or Sudan black
can be used for pitch detection. These dyes are dissolved in 72^ alcohol-
glycerine mixture (3 volumes of alcohol and 1 volume of gl 3 ^cerine) to form
a saturated solution. Sudan orange stains pitch red, and Sudan black
stains pitch blue. In using this method of staining, the dye solution is added
to the fibers on the slide and left until the pitch spots are stained (approx¬
imately one hour for unbleached pulps and two to three minutes for bleached

pulps), and then the excess dye is removed.

CHAPTER XVII

USE OF STATISTICS IN THE
PAPER INDUSTRY

Theory of Statistics

In statistics, a universe is considered as being made up of a large group
of related individual measurements which are known as the popuMion.
A normal universe is one in which the individual measurements are distrib¬
uted in accordance with the Gaussian law of chance, known as the normal

law.

The theory of statistics is based upon two fundamental concepts;^ (i)
that there must exist an equality of opportunity for the chance occurrence
of each item and (2) that nature has a very precise and orderly plan for
variation which is revealed whenever some variable factor is measured, and
the items are grouped numerically in the order of increasing or decreasing
magnitude. Thus, if a sufficient number of items are selected at random,
these will appear in proportion to their frequencies, and the data thus se¬
cured may be used to construct a replica of the universe. This replica can
then be studied by statistical methods. To be useful for this purpose, all
data must be homogeneous, that is, all data must be obtained under the
same test conditions.

In industrial applications, the primary concern is with estimating char¬
acteristics of the universe of data from the evidence in small samples. Sta¬
tistics are of great value in handling the data obtained in sampling, instru¬
mentation studies, paper testing, and quality control. Data may be of two
types: ( 1 ) data representing single test measurements on a series of similar
objects and (2) data representing a series of like tests on a single object.
Most data are of the first type. The second type is used, however, to fur¬
nish information regarding errors of measurement.

Precision is the reproducibility or spread of results about the average.
Accuracy is error or bias, and refers to the displacement of the mean result
from the true average. Precision can be low, while accuracy of average is
quite high. It is important in statistics to differentiate between accuracy
and precision. Some of the purposes for which statistics may be used are:
(I) to determine the accuracy of a given set of data; (2) to determine how

1 C. C. Forsaith, “Statistics for Foresters,” Tech. Bull. No. 62, New York State

College of Forestry, Syracuse, N. Y. (Mar., 1946)

981

982

PULP AND PAPER

many tests must be made to secure any specified precision; (3) to determine
the variability of some property or group of properties; (4) to determine
the factors which influence the variability of properties; (5) to deter¬
mine when results are significant.

The following is a general, illustrative approach to the application of
statistics in the paper industry. It does not present an integrated attack on
the problem of control in a paper mill, but instead illustrates some of the
methods useful in analyzing paper testing data.

Methods of Comparison

There are two principal methods of testing or comparison. The first
method, called the method of attributes, is typified by go-no-go gages, by
which each item is tested to see whether it is within limits or outside limits. ■
The other method, called the method of variables, is based upon an exact
measurement for each dimension examined. The latter method requires
greater skill in measuring, but it requires a smaller sample and makes pos¬
sible more exact calculations. It is the method most widely used in the pa¬
per industry.

All things are variable if the method of comparison measures small
enough differences. Variability is due to lack of precision of the measuring
device or to inherent, non-uniformity of the quality being measured. Use
of more precise measuring devices reduces the error, but there are limits to
which this correction may be applied. A rough working rule is that the
measurement scale should be several times as close and precise as the vari¬
ation which it is desired to measure.^ Repetition of the test reduces the er¬
ror, since the error of average is inversely proportional to the square root of
the' number of observations. This law, which is known as the law of least
squares, states that (1) the precision of the results obtained is proportional
to the square root of the number of readings made; (2) the degree to which
a sample is representative of the whole is proportional to the square root ot
the number of specimens taken; (5) the degree to which the effect of a given
condition (when all other conditions are constant) is reflected in t e resu
is proportional to the square root of the number of runs ma e.

Rules of Sampling

Any universe can be specified in detail by a number of mathematic^
constants, or statistics. If the statistics of the universe are knovra, ,t is v ^
easy to calculate the statistics of samples taken from the I"J

tice however, the process is reversed, in that samples are taken and a
alyaed, and from these data the statistics of the universe ^

with a degree of precision which depends upon the number of values ma »
up the sample. Sampling is used when the whole data are too large to be

2 J, H. Toulouse, Tappi 33, No. 1: 44-48 (Jan., 1950)

XVII. USE OF STATISTICS

983

handled by themselves. In this way, generalizations can be made about the
entire population from an analysis of the sample. The accuracy o samp mg
should be governed by the precision of the analytical methods, since it is
illogical to apply analytical methods of high precision to a sample which may
have a large error. Conversely, it is wasteful to spend the time ta mg
highly accurate samples for testing by analytical procedures of low precision.

There are general rules for the sampling of fluids from a batch, solids
from a batch, fluids in flow, and solids in movement These rules are de¬
scribed in books on engineering principles. The taking of samples must e
completely random, so that the selection is purely by chance and each item
has an equal opportunity of being selected. The sample must have the same
degree of variation and the same distribution of the different degrees of
variation as the original material from which it was taken; in other words, it
must be a representative sample. A perfect sample should represent a pre¬
cise model of the population.

All samples must, of course, be protected from change after sampling
and before testing. In commercial work, sampling methods must be care¬
fully agreed upon by both the consumer and producer, since many different
sampling plans may be used. Testing methods and calibration of testing
instruments must also be carefully specified in advance. For most purposes,
TAPPI or A.S.T.M. standard methods of sampling, testing, and calibration

can be used in writing specifications.

In order to set up an adequate control system, data for design of its
final form must be taken at a time when there is no variation in the system,
other than chance sampling. In other words, control must come first so
that the samples will be taken under the same conditions at all times, with
no disturbing environmental influences. There must be no unknown or un¬
recognized causes of variability in the quality of the product or, in the words
of Shewhart,^ there must be no assignable causes of variability. Assignable
causes of variation are due to differences in machines and in sources of raw
materials. Chance causes are due to peculiarities of the machine, skill of
the operators, etc., which may be considered as normal.

Samples are rarely a true replica of the universe, so that the question
arises as to how large a sample should be taken to obtain the desired preci¬
sion. This can be answered from a study of the frequency distribution
curve, which shows the variations to be expected in the material to be tested.
The method of constructing a frequency distribution curve will be taken up
shortly. If the frequency distribution is normal, certain exact mathematical
calculations can be applied to the measurement to be made on the material.
For example, the range within which 99.7% of the fraction defective of

3 W. A. Shewhart, Economic Control of Quality of Manufactured Product, D. Van
Nostrand Co., New York, N. Y. (1931)

984

PULP AND PAPER

values obtained by a series of samplings would be expected to fall can be
expressed by

P±3MP(l-F)/n

where P equals the average fraction defective of values from a number of
groups and n equals the number of samples used in each group. This equa¬
tion shows that it is necessary to quadruple the sample' size in order to
double the precision.

The practical or specific probability curv’e for the material in question
may not be exactly the same as the normal probability curve, but it is as¬
sumed in statistical analysis that the set of figures is drawn from normal
distribution. It is true, moreover, that the distribution of the means of
small subgroups of values gives a curve very close to normal, even when the
original frequency curve is considerably different from normal.

Grouping and Analysing Data

Statistical methods for handling data consist of (1) grouping the data
into easily understood form and (2) analyzing the data by accepted meth¬
ods. Most original data are obtained in chronological order and conse¬
quently may need to be regrouped to be easily understood. One method is
to arrange the data according to ascending order of magnitude in a series
called an ungrouped frequency distribution. This simplifies the analysis,
iDut is unsuitable for the handling of large amounts of data. Data may be
presented in a grouped frequency distribution by dividing the data into a
number of conveniently sized groups, called class intervals or. cells, which
are arranged in a column of decreasing size. A frequency distribution can
then be constructed from these grouped data by checking each value accord¬
ing to the class interval into which it falls. This gives the number of times
a measurement falls in that class interval.

Frequency Distribution Curve

A grouped frequency distribution curve can be obtained by plotting
the data from the grouped frequency distribution, using the interval group¬
ings as the X-axis and the frequency as the F-axis. In other words, the
frequency distribution curve is obtained from the data as follows:

(1) The range of the largest and smallest value is determined. These values are

then plotted on the X- 3 xis. • 4 . arp definite

(2) The above range is then divided into equal ® /"Nervals There ^e detm^^

rules for the choice of number of intervals. In general, there should be

dais intervals, about 7 or 8 for each 100 measurements. The boundaries of the cells

should be halfway between two possible observations. ^

• (3) The class frequencies are then determined as the nu

class^mtervahi^^ the class frequencies (ordinates) are plotted against class intervals
(abscissae).

985

XVII. USE OF STATISTICS
#

If desired, relative frequency can be plotted m place of frequencjc The
relative frequency is a percentage which is obtained by d.v.dmg the fre¬
quency of each class interval by the total number of observations. 1 hus,
the relative frequency is the fraction of the total number of observations

lying within each class interval.

If a symmetrical bell-shaped curve, which meets the other qualifications
of a normal probability curve, is obtained by the above procedure, it is taken
as an indication that the values vary according to the laws of normal proba¬
bility, and that the sample is a good replica of the population from which it
was taken. The goodness of fit of the sample curve with a normal bell¬
shaped curve will show whether there is anything abnormal about the
sample. A normal probability curve is represented in Figure XVII-1.

Fig. XVII-1. Normal probability curve.

This curve is symmetrical to A, which is the average or mean value of the
quantity being measured. Other characteristics denoted by this curve are
that small deviations from the average are more frequent than large devia¬
tions and that positive deviations occur as often as negative deviations.

Characteristics of Frequency Distribution

Grouped frequency distribution curves greatly condense the original
data, but it is frequently desirable to use characteristics or statistics of the
frequency distribution to condense the data further. Two properties of
the data are most important, the average (mean) and the variability. The
variability can be expressed in a number of ways, the most important of
which are range, standard deviation, and variance. Other characteristics
of frequency distribution are sometimes used, but most of these are falling
into disuse because of certain shortcomings or because of their lack of suit¬
ability for industrial work. The number of observations should always be
given.

Average or Mean. _The average of the test values may be calculated
as the arithmetic mean (Z), that is, the sum of the variables divided by the
number of units. In statistics, a bar over the symbol indicates an average
of that value. The mean is the most widely used statistic, but it is badly

986

PULP AND PAPER .

distorted by extreme values and fails to give full information about a sample.
Because of the large influence of extreme values, it is customary in calcu¬
lating the mean to discard extreme values, but particular care should be
exercised in carrying out this procedure if the number of observations is

small.

The mean of a small group of individual items may be obtained by add¬
ing all items and dividing by the number of items. However, when a large
number of items is to be analyzed, this method is too cumbersome, and it is
then necessary to obtain the mean from grouped data. This is done on the
assumption that all values in a class interval are distributed evenly.

Range. Range is the diflference between the largest and the smallest
values. Range furnishes limited information, since it gives nothing about
the intermediate results. The greatest use of range is in quality control
charts, as will be explained later.

Standard Deviation. The most useful measure of variability of
the test values is the standard deviation represented by the Greek letter
sigma (a). Standard deviation is the root mean square deviation of the
test values from the arithmetic mean. The standard deviation is of value
for comparing the scatter or dispersion of sample data with that obtained

in other studies, or with that of normal probability.

A small standard deviation means that the values are closely clustered
about the arithmetic mean, whereas a large standard deviation means that
the values are widely scattered. In Figure XVII-1, a is the point of inflec¬
tion of the curve and represents one standard deviation. The area under
the curve between the two ordinates -c and +o- represents the relative
likelihood or the probability of occurrence of a value in the range between
these ordinates. In the normal distribution curve, the percentage of the
total numbers included in the limits for 1, 2, and 3 standard deviations are

as follows:

Y± Ict—68.3%

]r±2o—95.5%

X± 3g— 99.7%

50% of the area of the curve falls within the limits of ± 0.67 a, which means
that any value chosen at random has a fifty-Bfty chance of falling wit m
this range. Table I gives the total percentages of items included for i
ent ranges of u above and below the mean. Thus, if a given d>stnb“ti“ h
an average of 100 and a standard deviation of 5, the chances are 1 to .1 that
any given figures will fall outside the range of 90 to 110 (because ‘bis
"plus or nZs 2 deviations). On the other hand, if the distribution has an
average of 100 and a standard deviation of 1, then «« 'hances are
15 772 that any given figure will fall outside the range of 96 to

XVII. USE OF STATISTICS

987

TABLE I

Table of Probability Integrals

Number of standard devia¬
tions (<r) above and
below the mean

0.2

0.4

0.6

0.8

1.0

1.2

1.4
1.6
1.8
2.0

2.5
3.0

3.5
4.0

4.5

Percentage of total items
included in speci¬
fied range

15.9

31.1

45.1

57.6
68.3
77.0

83.8
89.0

92.8

95.5

98.8

99.7

99.8

Chance of any item, taken
at random, falling witnin
given range

0.2 : 1
0.5 : 1
0.8 : 1
1.4 : 1
2.2 ; 1
3.3 : 1

5.1 : 1

8.1 : 1

12.9 : 1
21.0 : 1

79.6 : 1

369.4 : 1

2126.6 : 1
15771.8 : 1
147057.8 ; 1

99.9

In ungrouped data, the standard deviation is obtained hy (1) obtaining
the difference between each value and the arithmetic mean, (2) squaring
each difference and then averaging the square, and (2) taking the square
root of the total. The definition of standard deviation is:

<7" =

2(X-X)

but a more convenient formula is;

CT® =

5(X2) -

(2X)

A problem taken from Forsaith* serves to illustrate the calculation of
standard deviation from ungrouped data.

Five fibers from the wood of Pinus lanibertiana Dougl. were measured, and the
following lengths were obtained: 4.35, 7.35, 4.65, 6.4^and 4.35 mm. The sum of these
lengths (27.15 mm.) divided by 5 gives the average X = 5.43 mm. The deviations from
the mean will be;

X X Xx Xi*

4.35 - 5.43=-1.08 1.166

7.35 - 5.43= 1.92 3.686
4.65-5.43 =-0.78 0.608
6.45 - 5.43= 1.02 1.040

4.35 - 5.43=-1.08 1.166

7.666

The standard deviation is a= V7.666/S = 1.24 mm.

4 C. C. Forsmth, “Statistics for Foresters,” Tech. Bull. No, 62, New York State
College of Forestry, Syracuse, N. Y. (Mar., 1946}

988

PULP AND PAPER

The standard deviation (1.24 mm.) thus obtained proved to be less
than that (1.40 mm.) obtained from the analysis of a far larger number of
fibers. This may be due to a failure to pick items towards the limits of the
range, a probability which may be counteracted by using n — 1 in place of n
in the above formula. When this is done, the standard deviation is found
to be 1.39 mm., which is very near to the value obtained by using a far
greater number of items. The number (n — 1) is known as the degrees of
freedom of the deviation, which is defined as the number of observations

minus the number of constraints on the system.

It may be safe to employ the following general rules with respect to

the use of a small number of items: (I) if the range of the small sample in
comparison with that of the population is cramped (less than 50%), use
n- 1.; (2) if the range of the small sample is 50% or more of that for the
entire population, use n; (5) if the ranges are nearly equal to that of the
parent, use n + 1; (4) if the total number of observations is less than 25,

use w -1.

Differences from arbitrary

Values

mean of 50

-1

+ 1

+ 3

+ 2

-2

+ 2

-1

Sum of differences .... +4

Square of
differences

Adjustment for mean = 50+[+4/24] = 50.17
Correction factor = (4)V10 = 0.8
Corrected sum of squares = 24 - 0.8 = 23.2
Mean square of deviations = 23.2/n -1 - 23.2/9 - 2.
Estimated standard deviations = V2.58=1.61

In grouped data, the standard deviation is obtained by (^)
averageSion of each group from the arithmedc mean (2) ^uanng to
average deviation for each group, (5) multiplymg by the freq«n.^y for each
grLp {4) adding the totals and dividing by the number of .terns, and (3)

calculations, it is often desimb. to

shift ^1 values to a^new scale. For example, in the ab-J-ple, .t uo^d

be convenient to subtract 4.35 from all values of X, g
dimensions 0, 3, 0.3, 2.1, and 0 which are easier numbers to handl .

XVll. USE OE STATISTICS

989

shifting does not alter the variance, since the variability is not affected by
changes in the mean. This procedure is most beneficial when using the al-

ternative formula for calculating or.

Another way of avoiding work with decimals is to use an. arbitrary
mean of a whole number. Deviations are then taken from this whole num¬
ber and a correction factor applied at the end, as shown on p. 988, where
50 is taken as an arbitrary mean in place of the true mean of 50.4.

Variance. The magnitude of the variability can be determined
from a property of the data known as the variance. The variance of a set
of numbers is taken as the sum of the squares of the deviations from the
mean divided by the number. In other words, the variance is the mean
square. It is equal to the square of the standard deviation. When small
samples are used, the divisor is taken as the number of independent values
from which the sum of the squares is composed, i.e., usually one less than
the number of quantities.

Coefficient of Variation. The coefficient of variation (F) is a
measure of the relative dispersion. It serves to compare two instances in
which the same absolute dispersion occurs about two unequal arithmetic
means. It is a percentage obtained by dividing the standard deviation by
the arithmetic mean, as follows: Coefficient of variation (F) = (cr/X) x 100.
It is the standard deviation calculated on a mean of 100.

Limits of Reliability

In many cases of technical analysis, a single sample is taken as repre¬
sentative of the aggregate. It is often desirable to know the adequacy of
the sample in representing the whole. The limits of reliability can be ex¬
pressed in terms of the probable error, maximum error, or the standard
error. For example, the results may be expressed as X ± probable error.
In many cases, it is useful to use the formula X db oo-, in which a is obtained
from tables based upon the number of observations made and the statistical
probability.®

Standard Error. A very useful statistic in the handling of sample
data is the standard error (SEm) of the mean, or in other words, the differ¬
ence between the mean obtained from the analysis of chance samples and the
true mean of the universe. The value of the standard error depends upon
(i) the degree of variability of the items in the universe from which the
sample was taken, as measured by the standard deviation, and (2) the
number of items taken for the determination of the standard deviation of
the universe. The formula for SEm, the standard error of the mean is
SEm = cr/Vw.

6 AS.T.M Manual on Presentation of Data, p. 41. Published by the American
oociety for Testing Materials, Philadelphia, Pennsylvania (1940)

990

PULP AND PAPER

As Stated above, SE^ theoretically represents the standard deviation of
the individual means of different samples al)out the perfect mean of the
universe, a the theoretical standard deviation of the items in the universe,
and n the number of samples tested.® In practice, however, it is customary
to let SEm equal the standard error of a single computation, a the standard
deviation of the items included in the sample about the mean of the sample,
and n the number of items in the test. This distortion of the true values
can be justified on an empirical basis. It has been found from experience
that when 30 or more readings are taken, the probability is two to one that
a mean obtained by a single analysis will not deviate from the perfect mean
by more than one standard error, 21 to 1 by more than two standard errors,
and over 300 to 1 bv more than three standard errors* (see Table I).

Stated differently, it means that the true value would fall within =t 1 stand¬
ard error from the arithmetical mean in 68 times out of 100, within ±: 2
standard errors 95.5 times out of 100, and within ± 3 standard errors 99.7
out of 100. For example, in a problem where SEm = 0.076 and X = 3.441,
the chances are 2 to 1 that the true mean lies somewhere between_the limits
of X ± SEm or 3.441 0.076, and 19 to 1 between the limits o( X ± 2SEm

or 3.441 ± 0.152. As another illustration, suppose that an analysis of 81
samples of paper shows that the average basis weight is 60 lb. andj^e
standard deviation is 1.89. The standard error is then SEm= 1.89/V81 =
0.21, which means that the average of a second set of samples would have
a two to one chance of falling within the range of ± 0.21 lb. from the av¬

erage of the first set, or within the range of 59.79 to 60.21.

Probable Error. The probable error is 0.6745 of the standard

deviation. It is the error which will not be exceeded by 50^ of the cases,
or in other words, it is the error on either side of which any deviation from
the true average is equally likely to fall. P.E. = 0.6745 a/y^«. The use o
the probable error is decreasing because of its lack of meaning.

Maximum Error. The maximum error is considered to be dcr,

which normally includes 99.73% of all values measured. In other words,
the maximum error is not the greatest possible error, but rather the error

which will not be exceeded in over 0.27% of all cases.

Significant Figures. In obtaining data, the apparent degree o

precision should not be carried in excess of that justified by the data- o
carry a value beyond the significant figures involves unnecessary wor
implies a relative precision which is not justified. As a ^ ^

Lie measurement included in the data, that .s, U the ^

read to the nearest 0.1 lb., tl,e figures should be earned to the nearest

C. C. Forsaith, “Statistics for Foresters " Tej^Bull.
College of Forestry, Syracuse, >{. Y. (Mar., 1946)

No. 62, New York State

XVII. USE OF STATISTICS

991

0.001 lb.' Rejection of figures should be done after computations, accord¬
ing to the general rules for rounding off figures.

The problem often arises as to what data in a compilation of data

should be discarded. Extreme values obtained in large sets of data can be
discarded so that all extreme values which are recorded have significance in
terms of their probability of occurrence.® However, any values outside 3
standard deviations indicate assignable causes of variation and may be dis¬
regarded in computation only if the causes are identified and steps taken to
prevent their recurrence.

Tests of Significance

It is often of importance to know (i) whether or not a given individual
or a given sample of n individuals belongs to a certain population or (2)
whether or not there is any significance between the means of two different
sets of numbers, i.e., whether the two sets can be considered as being drawn
from the same or from different populations. These are known as tests of
significance.

Tests of significance are based upon the probability of occurrence.
Probability is generally expressed on a scale of 0-1, or on a percentage basis.
Zero probability means that a certain event will not occur, whereas a proba¬
bility of 1 means that the event is certain to occur. A probability of 0.05
means that there is a 5% probability of occurrence. A probability level of
95^ is generally used in industrial work as a test of significance, which
means that the event will occur 19 out of 20 times. It means that 95% of
the individuals lie between — 1.96 a and + 1.96 o-of the mean. Larger val¬
ues of probability are an even stronger indication that the results are sig¬
nificant.

The data can be used in one of two ways. They can be used (7) to
determine the number of items to be measured for a specified precision or
(2) to determine the significance of differences between means. Both tj^pes
of problems are illustrated below.

Determining the Number of Items to Be Measured for a Specified

Precision. The problem often comes up of how many tests must be

made to obtain an average which will not differ by more than some quantity

from the true average in a certain percentage of cases. Variance must be

taken into account, since a small variance means that a fewer number of

tests will be required to determine a value with a given precision than is

the case when the variance is large. Problems of this sort are handled on

the basis that sample means distribute themselves about the population mean

A.S.TjM. Monuol on PTssentotion of Data, pp. 44—45. Published by the American
Society for Testing Materials, Philadelphia, Pennsylvania (1940)

8 C. C. Forsaith, “Statistics for Foresters,” Tech. Bull. No. 62. New York State
College of Forestry, Syracuse, N. Y. (Mar., 1946)

992

PULP AND PAPER

with a Standard deviation equal to a/\/n, or 1 standard error. Thus, if the
mean of n samples taken from the population is more than 1.96 cr/y/n (95%
probability) different from the true mean, it can be concluded that the
sample was not drawn from the population under consideration.

In tackling a problem of the above type, the range allowed must first
be stated as an absolute or relative value. A series of tests is made to de¬
termine the mean and the standard deviation. Then, using the formula for
the standard error of the mean, SEm = a/^/n, the necessary number of tests
can be calculated. A problem from Forsaith® serves to illustrate. In this
problem, the question is asked: How many fibers should be measured in
order to obtain a mean which shall not deviate from the true mean of the
population by more than 5% of the average length, and with a 95% proba¬
bility which, as stated above, is the accepted probability level for problems
of this sort. First, the mean and standard deviation for a single analysis
are obtained, and then these are used to determine the number of items
which should be chosen for a subsequent sample in order to insure the
above accuracy. Assume that the data have shown a mean of 3.44 and a
standard deviation of 0.630. Then, 5% of the calculated mean is
3.44x0.05 = 0.172 mm. A 95% probability is 19 to 1, or approximately
2 standard errors. The number of samples to be tested (n) can be calc^
lated as follows, where E equals the expected value of the mean and X
equals the mean of the sample.

( 1 )

( 2 )

( 3 )

t =

f =

X-E

SEm

(T/\jn

X-E = tSE

\/n

n =

2 X 0.63
0.172

= 54 fibers

The same technique can be used to evaluate the number required for any
deviation or any probability, provided the constants are made available by

a previous analysis consisting of at least 30 items.

Comparison of Means. In comparing results obtained on two or

more materials from different sources, the problem often arises of determin¬
ing the significance of means. A typical example is the comparison of
bursting strength readings obtained on two different lots of paper. If tlie
means of the two samples are fairly close together, ‘h"'''"''''
overlapping of individual readings. The problem may be (J) to
whether there is a significant difference in means or (2) to ^ei^m-ne tow
many readings must be taken to reduce the chance of selection to g

0 C. C. Forsaifh, "Statistics for Foresters," Te<*. Bull. No. 62, New York Stat
College of Forestry, Syracuse, N. Y. (Mar., 1946)

XV’^II. USE OF STATISTICS

993

probability. A single reading on each paper will tell practically nothing
about the comparative status of the two papers, but by increasing the num¬
ber of readings in each set, the means will var>' within narrower limits, i.e.,

the probability curve will show less spread.

In practice, the number of readings in each set is increased until the

chances of mistake reach the point of acceptable risk. The standard devia¬
tion of the individual readings should first be obtained either from previous
knowledge of the usual spread, or by direct calculation from 30 or more
readings. A working rule used by some^° is to increase the number of
readings until the standard deviation of tlie two sets is reduced to 0.4 times
the difference it is desired to distinguish with reasonable certainty. The
general equation used for more exact calculations^’^ is as follows.

X-X' /«i X W2

f =-\-

a \ Ml + «2

where X = 2(A')/«i, X* - 2 (X')/W 2 , and

712

Ml + M2 — 2

[MX)V

The value of t obtained by substitution of values in the above equation is
compared with the required value of t for the probability level and degrees
of freedom (Ni + Nt-2) obtained from suitable tables (see Table IP’).

Toulouse’® has reported on an extensive study made on two different
kraft liner papers having the following characteristics;

Board A Board B

Number of tests . 100 100

Maximuin burst . 117 p.s.i. 124 p.s.i.

Minimum burst -. 74 p.s.i. 89 p.s.i.

When grouped in subgroups of three, board A had an average of 95.68 and
l)oard B an average of 105.69. Calculations show that grouping in this
way made it possible to distinguish the stronger from the weaker only 90%
of the time, and that grouping in subgroups of 12 was required to distinguish
betA\een the two papers every time. From this, it can be concluded that
when two normal kraft boards are only 10 p.s.i. apart in bursting strength,
samples of twelve are required to conclude with certainty that the series of
tests are different. In order to differentiate with the same certainty in the
case of two boards differing by 5 p.s.i., a grouping of twenty-four values

^®P. H. Prior, World’s Paper Trade Rev. Ill: TS 50-58, 66-70, 91-96 (Apr. 7,
May 5, June 2, 1939)

K. A. Brownlee, Industrial Experimentation, Chemical Publishing Company,
Brooklyn, N. Y. (1947)

>*J. H. Toulouse, Tappi 33, No. 1: 44-48 (Jan., 1950)

PULP AND PAPER

TABLE II
Values of t

8
9

0.10

2.92

2.35

2.13

2.02

1.94

1.90

1.86

1.83

o.os

12.71

4.30
3.18
2.78
2.57
2.45
2.37

2.31
2.26

0.02

31.82
6.97
4.54
3.75
3.37
3.14
3.00
2.90
2.82

0.01

63.66
9.93
5.84
4.60
4.03
3.71
3.50
3.36
3.25

1.81

2.23

2.76

3.17

1.80

2.20

2.72

3.11

1.78

2.18

2.68

3.06

1.77

2.16

2.65

3.01

1.76

2.15

2.62

2.98

1.75

2.13

2.60

2.95

1.75

2.12

2.58

2.92

1.74

2.11

2.57

2.90

1.73

2.10

2.55

2.88

1.73

2.09

2.54

2.86

1.73

2.09

2.53

2.85

1.72

2.08

2.52

2.83

1.72

2.07

2.51

2.82

1.71

2.07

2.50

2.81

1.71

2.06

2.49

2.80

1.71

2.06

2.48

2.79

1.71

2.06

2.48

2.78

1.70

2.05

2.47

2.77

1.70

2.05

2.47

2.76

1.70

2.04

2.46

2.76

1.70

2.04

2.46

2.75

1.68

2.02

2.42

2.70

1.67

2.00

2.39

2.66

120

1.66

1.98

2.36

2.62

1.65

1.96

2.33

2.58

636.62
31.60
12.94
8.61
6.86

5.96
5.41
5.04

4.78
4.59
4.44
4.32
4.22
4.14
4.07
4.02

3.97
3.92
3.88
3.85
3.82

3.79
3.77
3.75
3.73
3.71
3.69
3.67
3.66
3.65
3.55
3.46
3.37
3.29

ivoukl have to be used. To obtain the same certainty with boards differing

by 3 p.s.i., a grouping of 48 would be required. outlined con-

As a further example of the usefulness o the
sider the problem” of a mill which is purchasing wood

suppliers. It is known that these suppliers fvary
ent areas, and consequently it appears probable that the woo^^

13 Problem taken from pamphlet, "Sampling Theory and Practi
State College of Forestry, Syracuse, N. i.

XVII. USE OF STATISTICS

995

i„ respond to the different eonditions under which it is the

purpose of the mill, the wood which has the J P ,]

LcVit will produce more pulp per cord, and the price per cord from ^

suppliers is probably the same. In a case like this, it
w,^ from all suppliers be sampled and tested under conditions
give a high degree of precision in the resulting averages, sm
of dollars’ worth of wood may be purchased on the basis o e
the other hand, the wood may be known to be of low density *
suit, be purchased at a reduced price in comparison with wood of „ ^

density. The question in this case is whether or not the ^ice
is sufficient to justify purchase of the low density wood Here a^m the
precision of the sampling and testing procedure must be of a hig
\n example based on the actual e.xperience of one mill- in sanyhng pu -
diased pulp wood is given in Table HI. the figures tabulated were ob¬
tained as a result of sampling large shipments of spruce wood from two
sources. Because the size of samples is large in thts case, n can be used in
place of »i - 1. The formula then becomes

/ =

X-X'

where

ay* = + ao'/tJa-

It will be seen that the clitYerence between the densities of the woods from
these two sources is 1.93 lb. per cubic foot, and this difference is several
times as large as the standard error of the averages themselves. Calculation
of t using the above formula shows that the difference noted is truly signifi¬
cant. Therefore, purchase of wood from source 11 should result in a
marked decrease in wood cost per ton of pulp, assuming that the cubic
content of solid wockI per cord from both sources is the same.

Cowpartson of Variances

In the above sections, it has been shown how to compare the mean of
two groups of numl)ers. Another problem frequently encountered in in¬
dustrial work is the comparison of the variabilities of two groups of num-
l»ers.

In laboratory experiments, it is possible to hold all variables constant
while the variable under observation is changed, but this classical labora¬
tory method is almost impossible to apply in industrial work. Statistical
methods of analysis can be applied in mill experimentation, however,
whereby the particular ^'ariables to be studied are selected, and each of
these variables changed separately. The assumption is made that the effects
of other variables are at random and that they will show in the measure-

996

PULP AND PAPER

TABLE III

Variation of Density of Pulpwood from Two Sources

Oven-dry density
green volume
Ib./cu, ft.

Wood from
source I,

Wood from
source II,

Class

intervals

Class midpoint

frequencies

16.0

to 16.99

16.5

17.0

17.99

17.5

18.0

. .18.99

18.5

19.0

19.99

19.5

20.0

20.99

20.5

21.0

21.99

21.5

22.0

22.99

22.5

23.0

23.99

23.5

24.0

24.99

24.5

25.0

25.99

25.5

26.0

26.99

26.5

107

27.0

27.99

27.5

115

28.0

28.99

28.5

29.0

29.99

29.5

30.0

30.99

30.5

31.0

31.99

31.5

32.0

32.99

32.5 .

33.0

33.99

33.5

34.0

34.99

34.5

35.0

35.99

35.5

36.0

36.99

36.5

37.0

37.99

37.5

760

n a

it «

((

192

Analysis of the data given above produces the following results
Spruce wood from source.I » , n.

Mean density . 27.04 lb-/cu- ft.

Standard deviation . 2.85

Standard error of mean .... 0.103

Spruce wood from source II ooo7

Mean density .. 28.97

Standard deviation . 2.58

Standard error of mean- 0.186

ments as variance due to experimental (residual) error.In a ^

this sort, all test results will tend to group themselves about the true averag

in an orderly way, except where there have been assigna e measure

tion due to changes in one of the variables. Thus, it is possible^
the size of the variance for each property measured, compare with
ual variance, and then compute the probability of its ^

ducting an experiment in this way, information is o aine
effects of one variable are affected by the other variables, i.e., th

14 H. G. Burbidge, Pulp Paper Mag. Canada 51, No. 3 : 16&-173. Convention
Issue (1950)

XVII. USE OF STATISTICS

997

interaction. The mathematical calculations involved have been well de-
scribed in other sources.'®

Use oj Statistics in Quality Control

Definition of Quality. The word “quality” is used in many di^
ferent ways, and consequently it is necessarj' to define the term before it is
used. In general, quality refers to the goodness of the product, so that a
high quality product is considered as a good or excellent product. Quality

has both an objective and a subjective side.

The most useful index of quality is a sum of those characteristics which

are of importance to the user of the product. No properties which are not
of significance to the customer should be included in the quality index.
Each significant characteristic should be weighted according to its demerits

or seriousness.'*

Before pajier quality can be measured, some means must be found for
expressing quality in terms of the physical characteristics of the paper.
The significant characteristics of paper quality may include variables such as
weight, tensile strength, and moisture, or attributes such as wrinkling and
dirt counts.'^ Such rigid specifications as complete freedom from wrinkles,
holes, and dirt sjiecks should not be included because they are obviously not
capable of being met in large shipments of paper. A good specification can
l)c obtained only after a thorough study of the consumer’s needs and the
manufacturer’s product and processes, which means that a minimum of six
months to one year is usually recjuired to obtain the necessary information.'®

Value of Quality Control. The value of quality control is undis¬
puted. All industrial products are variable in quality, and it is therefore
desirable to state the probability with which a given sample will fall within
s|K*cific limits. Quality control provides a statistical means of detecting lack
of control of (piality. Once quality control is established, there will be a
saving in cost, due to fewer rejects and less need for inspection.

Normal Quality Level. Once a means of measuring quality has
l)ecn found, a nonnal quality level must be established to have a yardstick
by which assignable causes of variation can be detected.'® This means set¬
ting up limits of variability outside of which it is worth looking for assign¬
able causes of variation. After the limits within which the data should
fall have been established, any values falling outside these limits are an

'* K. A. Bro^^nlee, Industrial Experimentation, Chemical Publishing Company,
Brooldyn, N. Y. (1947)

‘•J. B. Catlin and J. G. Strieby, Paper Trade J. 125, No. 15; 161-164 (Apr. 10,
1947)

" J. G. Strieby, Paper Trade J. 124, No. 24: 515-521 (Dec. 9, 1948)

»» Idem.

»• Idem.

998

PULP AND PAPER

indication of the existence of causes of variability which can be found and
eliminated.^®

Statistical methods have been of great value in quality control and are
now being used in the paper industry.In practice, quality control
resolves into the problem of determining from a given set of data represent¬
ing fluctuations in quality of the product whether or not the product is under
control. Prior to setting up control charts, it is necessary to obtain basic
data from which chance variability may be calculated for the particular
process. These data must be based on results obtained over a long enough
period to determine whether the process is truly uniform. Then, statistics
can be used for setting up limits within which future results must fall.

The specification of quality should include some statistic of the product.
This may be simply the number of defective individuals or the percentage
of defective individualsor the statistic may be more complex, such as the
aA'erage (arithmetic mean) of the test values or a measure of the variability,
such as a multiple of the standard deviation or the range. The number of
demerits per unit may be plotted against the time at which samples were
taken. Another more satisfactory method, which eliminates the varying
width of control area due to size of samples, consists of plotting the standard
deviation of demerits per unit against the time at which samples were taken.

Construction of Frequency Distribution. All statistical methods
are based upon the fact that the data, when obtained under constant
controlled conditions, can be plotted so that they result in a symmetrical
curve known as the normal distribution, or probability, curve (see Fig.
XVII-1). Changes in the probability curve can occur because of changes
(either scheduled or unscheduled) in the processing conditions. It is of
prime importance to know whether changes in the curve are due to a change
in average value (i.e., a shift of the curve to the right or left), or due to a
lack of uniformity (i.e., a change in the shape of the curve), since t e
remedy is different in each case. When the process is operating satisfac¬
torily, the average will be centered between the specification, and tl^ curve
will be narrow and symmetrical with a predetermined relationship between

ts heighth and its width. , ,1 .1 r,A

Since the practical use of statistics prevents the testing of all the pr

ict, it is customary to take samples at random. This still gives a arge

Troup of data which is hard to analyze, but which can be han e y ®

"istical methods. Even though the original test measurements are no no -

20 W. A. Shewhart. Economic Control of Quality of Manufactured Product, V.

Van Nostrand Co., New York, N. Y p Man Canada 48,

21 N. S. Grant, O. A. Mason and H. F. Donnelly, Pulp Pap 9-

No. 3: 151-158, Convention Issue (1947) Convention Issue

22 C. A. Bicking, Pulp Paper Mag. Canada 49, No. 3 . 181 188, Co

23 j, G.^Strieby, Paper Trade J. 127, No. 24: 515-521 (Dec. 9, 1948)

XVII. USE OF STATISTICS

999

mal, it has been found that a very nearly normal distribution can be obtaine
by averaging small successive subgroups of the data and making a frequency
distribution of the averages."** The subgroups should be selected so that
variations within subgroups may be considered as due to chance causes,
even though variations between subgroups are due to assignable causes.
The data in each subgroup can be summarized into the average, standard
deviation, and sample size. The procedure is described in the following

section.

Construction of Control Chart. There are several different ways
of expressing the results, but the type of control diagrams originated by
Shewhart are the most generally used. The control chart is a simple tool
which can be understood by any supervisor. It is helpful because it

furnishes a basis for taking corrective action.

Bicking^* has described in simple and lucid fashion the construction of
a Shewhart control chart, using basis weight measurements taken on sam¬
ples of paper from consecutive reels. The following data and explanation
are taken from his work showing how the data on basis weight are handled
in both table and chart form. See Tables IV—VIII and Figures XVII-2 to
XVII-5 (pages 1001-1002).

The curve in Figure XVII-4 expresses the normal condition of the
process or, in other words, the inherent variability of the process. Limits
of dr 3(r are used as the control limits although some use 2a and even 1.5<r
for signal points. From this it may be assumed that any average of sub¬
groups of four successive tests which deviate further from the average than
the limits (dr 3a) set by the curve is not a chance cause. If a greater varia¬
tion than this occurs, it is worth while to search for an assignable cause of

TABLE IV
Original Data^^

These values are listed in order of production
Basis weight, grams per square foot

37.5

37.6

34.8

37.2

34.9

36.2

32.2

36.0

32.8

37.6

37.7

31.4

36.4

37.7

36.2

34.5

37.4

33.3

36.8

35.4

33.8

36.6

38.6

38.8

37.2

38.2

32.8

39.6

36.8

39.0

35.4

33.8

36.6

38.6

38.8

37.2

38.2

32.8

39.6

C. A. Bicking, Tech. Assoc. Papers 30: 135-139 (June, 1947)

1000

PULP AND PAPER

TABLE V

Ungrouped Frequency Distribution's
These values are arranged in order of magnitude
Basis weight, grams per square foot

31.4

34.9

36.8

37.6

32.2

35.4

37.7

32.8

37.7

32.8

33.3

36.2

38.2

33.5

36.2

37.2

38.6

33.8

36.2

37.2

38.8

34.5'

36.4

37.4

34.8

36.6

37.6

39.6

TABLE VI

Grouped Frequency Distribution's
Values are grouped into different measurement intervals
showing number of values in each interval
Basis weight, grams per square foot

Cell No.

Class intervals

30.55- 31.55

31.55- 32.55

32.55- 33.55

33.55- 34.55

34.55- 35.55

35.55- 36.55

36.55- 37.55

37.55- 38.55

38.55- 39.55
39.55^0.55

Frequency

TABLE VII

Averages of Successive Groups (4) of Basis Weights^®
Averages are taken for each successive group of four basis weights from ongina

37.2

36.7
35.4
36.9

37.7

37.3
36.0
33.2

35.7

2 s C, A. Bicking, Tech. Assoc. Papers 30: 135-139 (June, 1947)

XVII. USE OF STATISTICS

1001

TABLE VIII

Distribution of Antr-^ges of Subgroups-®

Cen No.

8
9
10

Boundaries

Frequency

30.55- 31.55 0

31.55- 32.55 0

32.55- 33.55 1

. 33.55-^.55 0

34.55- 35.55 1

35.55^.55 2

36.55- 37.55 4

37.55- 38.55 1

38.55- 39.55 0

39.55- 40.55 0

variation. On the other hand, it is uneconomic to look for a cause of
variation if the average of all the subgroups falls within these limits.

If Figure XVII-4 is rotated clockwise, the ends of the curve can be
extended as horizontal dotted lines to provide upper and lower control
limits, as shown in Figure XVI1-5. The final step consists of plotting the
subgroup against time, using the above limits as horizontal limits on the
cur\e, as shown in Figure XVII-5. Then, if any value falls outside of
these prescribed limits, it can be assigned to some new variation which is
causing the system to operate at a new value.

»-

o£

^ O’ 40
iG »

4
OQ

• 10 20 30 40

REEL NO IN PRODUCTION ORDER

Fig. XVII-2. Data obtained in chronological order.*®

Use of Formulas for Constructing Control Charts. It is unneces¬
sary to follow all the above steps, since the same results can be obtained by
the use of relatively simple formulas which are given in texts on statistics.
Formulas are available for two distinct methods of control:

(f ) Control with respect to a given standard (to determine uniformity

of quality). This method requires two sets of formulas and is too con¬
fusing for practical control work.

Control with no giver standard (to determine the constancy of

samples).

PULP AND PAPER

1002

Fig. XVII-3. Frequency distribution curve lor indi¬
vidual results.25 The vertical scale shows the frequency
of test measurement at each class interval.

Fig XVII-4 Frequency distribution for subgroup averages.^^
This is similar to Figure XVII-3 with the exception that the ex¬
tremes of variability have been irtihed out by the groupmg procedure.

XVII. USE OF STATISTICS

1003

For systems where no standard is given, the central line and con to
limits can be obtained as follows, where n is the number of samples tested.
The use of range (R) is so efficient that use of control charts for a is

scarcely ever warranted.

Control limits

Central line

Using factors

Using formulas

==. 0

Averages using 0 .

^±A7d

^-^CeVn

Averages using R .

^-^d^Vn

Standard deviations .

.... 0

Bia and 8*0

Rancrpc ...

DstR and DtR

R ± 3<sr

The factors A2, and D, are obtained from suitable Tables,

e.g., Table X which lists the proper values based on the number of items in

each of the subgroups.^®'

An example of the use of formulas for calculating control data for
averages and ranges is shown in Table IX, based on the data previousl}^
analyzed in curve form. The successive sets of four test results are ar-
ranged horizontally in subgroups and the mean {X) and the range (a)
are calculated as shown. In this case, no standards are given. X is the

TABLE IX

Control Chart Computations from Original Data on Basis Weight

Subgroup

Measurement

, g. /SQ. It.

36.0

37.2

38.0

37.6

37.2

2.0

37.0

36.2

36.8

36.6

36.7

37.2

33.5

34.9

36.0

35.4

3.7

37.7

36.4

34.5

39.0

36.9

4.5

38.6

38.2

37.6

36.2

37.7

2.4

37.0

37.7

37.4

37.3

0.7

35.4

38.8

32.8

34.8

36.0

6.0

32.2

32.8

31.4

36.2

33.2

4.8

33.3

33.8

37.0

39.6

35.7

6.3

Grand average . 36.23

Average range .;. 3.47

average of the four measurements in each subgroup. R (range) is the dif¬
ference between the highest and lowest weight in each subgroup. X is the

E. S. Pearson, The Application of Statistical Methods to Industrial Stand¬
ardization and Quality Control,” British Standards Inst., London (1935)

A.S.T.M. Manual on Presentation of Data, p. 20. Published by the American
Society for Testing Materials, Philadelphia, Pennsylvania (1940)

1004

PULP AND PAPER

grand average obtained from the averages of each subgroup. R is the
average range obtained from the averages of each subgroup.

The limits for the averages (mean) were obtained as follows:

X ± A 2 R = 36.23 ±. (0.729 X 3.47) = 38.76 and 33.70 g./sq. ft.

The limits for ranges were obtained as follows:

£>3^= 0 X 3.47 = 0
D^R = 2.282 X 3.47 = 7.9 g./sq.ft.

As just mentioned, the values for A 2 , D 3 , and were obtained from
Table X.

TABLE X

Factors for Computing Control Chart Lines—Small Samples

Chart for averages

Chart for ranges

Number of
observations
in sample, n

4
s
6

10
11
12

Factors for
control limits
A2

1.880

1.023

0.729

0.577

0.483

0.419

0.373

0.337

0.308

0.285

0.266

0.249

0.235

0.223

Factor for
central line
di

1.128

1.693

2.059

2.326

2.534

2.704

2.847

2.970

3.078

3.173

3.258

3.336

3.407

3.472

Factors for
control limits
Di D*

0.076

0.136

0.184

0.223

0.256

0.284

0.308

0.329

0.348

3.268

2.574

2282

2.114

2.004

1.924

1.864

1.816

1.777

1.744

1.717

1.692

1.671

1.652

Averages using R
Ranges .

CENTRAL LINES AND CONTROL LIMITS

Central line

. ^

. Rr

Control limits

T^2R

D 3 R and DiR

The various steps in setting up a control chart may then be summarized
as follows:

_ (i) Obtain the average of from 15 to 25 sets of test averages for the base period

^^^'(2) Calculate the upper and lower limits as outlied above.

(3) On a suitable chart, draw a line to represent X and dotted lines to repres

the upper and lower control limits.

(4) Plot average's on the chart. Also plot ranges between range limits.

All points falling outside the control limits should be investigated by ch«h
tests and a search made for assignable causes. All future tests s low g

xvu. USE OF STATISTICS

trend toward the ogotrol limits should be taken as a sign of approac^ng
trouble. After all ass^;nable causes have been eliniinated, a new base
period be established and new control! limits set up.

/)dtA to Indkatf Kflationship

Correlation CoeflBcient. A problem that frequently arises is me
^ <d the relation between tw o variables. One n^hod^ of measuring
relationship between two \’anables which v-ary in linear fashion is the
relation coefficient (r)’, which s'aries from + 1.0 for perfect positive cor-
lion to -1.0 for perfect negative correlation. When no correlation

TABLE XI

Tams or tub CtntBLATiox Coernaurr

41#

40S

•at

o.ai

0.001

0.9M

0.999

1.000

.*4)1)

.9S0i

.900

0.990

0.999

JOS

.934

r* -959

.992

729

jii

J82

.917

.974

U/t

754

.833

.874

.951

/>2I

707

.7J9

.834

.925

5a2

j666

’■^750

798

.898

Jr-

349

332

716

.765

321

£02

J8S

735

.847

497

376

658

708

.823

.476

.553

.634

.^44

JOl

457

532

312

.661

.780

44)

314

392

.641

.760

.426

.497

374

.623

742

.412

4S2

.558

306

.725

.400

4^4

543

390

'.708

JS9

456

.528

375

444

316

361

379

369

433

503

349

.665

360

423

337

352

323

381

.445

.487

.597

296

349

409

.449

354

275

325

381

.418

'319

257

304

.358

393

.490

24J

2B7

338

372

.465

211

273

322

354

.443

211

250

295

32S

.408

.195

232

274

302

380

ISJ

217

256

283

,357

173

205

242

267

337

164

195

230

254

321

S ^ ^

10(V)

rui.p AND PAPF.R

exists, r « 0. The c«»efticient of correlation is calculated as follows:

(y-J)

where:

:i(.r-.F)* = i(a:*)- —

- (iv)*

To test for significance, the value of r obtained by use of the above
formula should be tested to see if it is larger than would have been obtained
in tthe absence of correlation. This is done by comparing the calculated
value of r with the value obtained from suitable tables, e.g.. Table XI for
tilt proper significance level and degrees of freedom, using n-2 for the
degree of freedom. If the value of r is p-eater than that for the proper level
of significance; a positive correlation exists; for example, if greater than the
value of r at the 0.5% level, there is better than a 19 to 1 chance that there

is a real correlation.

28 K. A. BrowTilee, Industrial Experimtniation, Chemical Publishing Ccmipany,
Brooklyn, N. Y. (1947)

CHAPTER XVIII

PIGMENT COATING

I’ai)er coating is an inijMjrtant part of the paper indiistr 3 \ It is a wide
tield which includes the application of man}' different types of coatings to
the base paper. This chapter is concerned with pigment coating in which
an atjueous mixture of pigment and adhesive is applied to the surface of
the pai'ier. d his type of coating is used primarily for the production of

printing jiajxTS for magazines and catalogues.

Some of tlie earliest uses of pigment-coated papers were for wallpapers,
Ik).k coverings, and other fancy papers. Box coverings were often highly
glazed by the flinting process to produce a glossy coating which was usually
colored to increase its attractiveness. In contrast, wallpapers were gen¬
erally dull-finished. At one time, these various types of coatings were
probably regarded as separate processes, but today it is customary to group
into the class of pigment-coated papers all papers which are coated with a
pigment-adhesive composition.

The first use of coated book papers probably dates back to the intro¬
duction of the halftone process of printing. It is debatable which came
first, the halftone printing process or coated paper, but it is certain that
neither one could have been fully developed without the other. Fine half¬
tone printing plates do not reproduce well by the relief process on uncoated
jMiper, even on the smoothest of machine-finished sheets. Coated paper,
however, has made it possible to reproduce the finest halftone printings and
to control such properties of the paper surface as ink absorbency, ink re¬
ceptivity, smoothness, gloss, and brightness.

Most of the coated papers are used in printing. If it were not for the
demand of coated papers by the printing industry, coating would he of minor
importance. .\s it is, however, the field of coated papers has become one
of the mo.st important parts of the paper industry, due to the demand for
letter printing surfaces on the part of the printer.

\\’allpai)ers and decorative papers have always been coated on one side
only. The earliest coated book papers were also coated on one side only,
and it was not until 187o that paper coated on two sides was made.^

There are three basic raw materials used in pigment coating: (1) the
I>igment, (2) the adhesive used for bonding the pigment, and (3) the ba.se
[lafier, or rawstock, to which the pigment is bonded. Each will be taken up

* W. B. Wheelwright, Modern Lithography 10, No. 7: 47, 49 (July 1942)

1007

1008

PULP AND PAPER

separately. The coating mixture is usually prepared by dispersing the pig¬
ment and adhesive separately in water and then mixing the two together.
The pigments in common use are clay, calcium carbonate, satin white, and
titanium dioxide. The principal adhesives used are starch, casein, and cer¬
tain synthetic resins. The following is an example of a typical coating
formula, although widely different formulas can be used: clay, 100 parts by
weight; modified starch, 15 to 20 parts by weight. The clay is first dis¬
persed in 60 parts of water containing a dispersing agent. The starch is
cooked with 34 parts of water and then mixed with the clay.

The Coating Process

After the coating mixture has been prepared, it must be applied to the
paper in the coating process, which consists of applying the fluid coaUng
mixture to the base paper, smoothing, drying, and calendering the coating.
All these are important steps in the coating process and must be carefully
carried out in order to obtain a satisfactory coated paper. These steps will
be discussed in the following parts of this chapter, but first, the different
types of coating processes will be described.

Conventional Coating

Coating is of two types, conventional and machine. Conventional coat¬
ing is carried out as a separate operation in a coating plant The
plant may be part of the paper mill, or it may be operated independently,

in which case the raw stock must be purchased from an outside sou .
Conventional coating was the earliest method of coating, and until recent^,
was the only method used. Conventional coating has always been a lo
speed process designed to make high-quality papers, but because o
competition of machine-coated papers, conventional coating p a
become interested in replacing their old slow-speed coalers "

high-speed machines. The highest quality coated papers are P

duced by conventional coating.

Machine Coating

Machine coating is carried out on the paper machine as
of the paperniaking operation. The paper is coated an coating

paper machine speeds. Because of its mass prM^g

made possible for the first time the development o* c»‘e P

papers at low cost. Machine coating was P"™"‘^cl iHustra-

need for cheap mail-order catalogue paper which would .. j

Ls better tL standard uncoated P^P-^/V’^WoVa
gather with the development of new rapid settin^, in )

tion in the publication field.

XVIII. PIGMENT COATING

1009

There are two reasons for the low cost of machine-coated papers.
First, as just mentioned, machine coating is a high-speed, mass-production
process which turns out a large volume at low unit cost. Second, the coat¬
ing mixture, which is usually made of starch and clay, costs less than an
equal weight of fiber (depending, of course, upon the relative price of pulp,
coating adhesive, and clay), and since coated papers are sold on a weight
basis, this replacement of fiber by an equal weight of cheaper coating ma¬
terial results in a saving in manufacturing cost. The situation with ma¬
chine-coated paperboards is somewhat different, since board is not sold on
a weight basis, and hence the cost of coating must be carried by the board
mill or passed on to the consumer in the form of a higher price for the

coated board.

Attempts to coat paper on the paper machine date back to 1880 or
earlier. Prior to the advent of true machine coating, several processes were
developed for surface filling or “washing” the paper surface with pigmented
coating mixtures. The application of coating in this way served to fill in
the pores of the paper with pigment, but still left the surface in a pre¬
dominantly fibrous state. The Wheelwright patent® granted in 1916 de¬
scribed the addition of pigmented coating mixture at the size press. The
Fair process (see Ch. IX) provided for the addition of pigment to the wet
sheet on the paper machine wire, but did not include the use of a binder.
Coating at the size press was tried in the early days of machine coating,
but coating weights were low, on account of the low solids of the coating
mixtures used, and the coating had a tendency to soak into the paper, leaving
the surface fibers exposed. All these early semi-coating processes used
coating mixtures of low solids contents, and hence were limited to the
application of very light-weight coatings. In this way they differed from
modern machine-coating processes, which can produce heavily coated paper
on one or both sides having a smooth surface and excellent printing quality.
Usually, the weight of coating applied is between 5 to 8 lb. per side per
ream, although heavier coatings up to 18 and 20 lb. are possible.

In machine coating, the coaters are located between drier sections.
The wet coated sheet is passed over at least one drier with the coated side
away from the drier surface, so that the coating is dried sufficientlv to
prevent sticking when it comes into contact with the hot drier surface.
Passing the sheet over 5 to 6 additional driers completes the drying. Coat¬
ing may be applied to both sides at once, or each side may be coated sepa¬
rately. If each side is coated separately, the wire or bottom side is usually
coated first, the paper passed over 6 to 8 driers to dry that side of the
paper, and then the felt side is coated, followed by another 6 to 8 driers.

Wheelwright. U. S. 1.195.888 (Aug. 22. 1916) and U. S. 1,258.840 (Feb.,

1010

PULP AND PAPER

Oil certain grades, two a])|)lications of coatiiij]^ may l>c made to one side of
the ]iaper.

Coaters for machine coating must have a rugged construction and yet
he capable of fine adjustments. They should he located on the same floor as
the paper machine and must not be so complicated that they cannot be
operated liy the regular paper mill crew at regular machine speeds.

Types of Coating Machines

Egan'* has described ten basic methods of coating: dip, knife, cast, roll,
brush, air brush, spray, print, extrude, and strip. All these methods are
not in general use for pigment coating. The most important processes are
described below. It should be emphasized that many of these coating

processes are protected by patents.

Brush Coaters

Brush coating is used exclusively for conventional coaling.
ventional brush coating, the coating is applied by means of a round brud,
spatter brush, felt-covered applicator roll, felt blanket, or )y
sheet The round brush is the most satisfactory meth .
rolls and felt blankets are often used, but the felt tends to till cit¬

ing and become hardened. Small driven doctor rolls may be
xcess coating, after which the coated paper is passed
reciprocating hair brushes of graduated fineness, vyhich spread the coatir^g

eveSy over'the surface of the sheet. In some -““/J ’

located so that it acts on the sheet as it passes around ^e lieadjoll^

sheet may be supported by a cylinder °,L “ "V ^^3 coming into con-
where the brushes act on the coating J t^X'Tes3 and
tact with the paper have the the various brushes

the lowest frequency to their Vibrations. )

are so adjusted that they are out of phase w supports

coating is smoothed, the sheet is passed over an air

the paper and prevents the wet coated on one side

::'Kf “ S'.'t rJW1.- - “■

'"""b™; ■it™ r "i”V

since its original development, “ teause of the reciprocating

brushes used. In a standard brush '"^ye of the paper,

fper ream 25x38-.';00) are usually applied to each side ot

3 F. w. Egan, Paper Trade !. 121. No. 17: 163-169 (Oct. 25, 1945)

XVllI. PIGMENT COATING

1011

The coated paper is mostly ot high quality; it is generally graded into firsts,
seconds, and thirds.

Roll Coalers

For many years, brush coaters were the only type of coating machine,
but with the advent of machine coating, many new types of coating machines
were developed. Among the most important of these are the roll coaters
in which coating mixture is applied and spread on the surface of the paper
l>y means of rubber-covered metal rolls. Roll coaters are used mostly for
machine coating, although they can be used for conventional coating.

There are two principal types of roll coaters, the planographic and the
gravure. These coaters can handle coating mi.xtures of higher viscosity
than can lie satisfactorily handled by brush coaters, which permits the use of
coating mixtures of high solids content. Modified starches are particularly
well suited as the adhesive for the roll coating process. Low-viscosity
modified starches have made possible coating mixtures up to 55 to 70%
solids, which are about ideal for coating on the paper machine. On the
other hand, casein has not been used to any extent for roll coating opera¬
tions because it must l)e used at low solids, which leads to an excessive dry¬
ing load on the machine and a tendency for the coating to stick to the driers.
Most machine coating with roll coaters is done with coating mixtures of 58
to G2% solids. Much has been learned about the fundamentals of coating
from the operation of high-speed roll coaters. The knowledge gained from
roll coating has been of great value in understanding other coating processes.

Planographic Roll Coater. One of the most important of the roll
coaters is the planographic coater, of which there are several designs. One
design consists of a fountain roll, a series of small, smooth distributing and
oscillating rolls, and a smooth jirint roll. The rolls are alternating chrome
and niblier covered. The coating is picked up by the fountain roll and
worked between the distributing rolls which meter and distribute the coat¬
ing and transfer it to the print roll from which it is applied to the paper.
Another design consists of two metering rolls and a rubber-covered print
roll. One of the metering rolls is chrome plated and the other is a rubber
covered roll.

Another coater is somewhat similar except there are two systems of
rolls which split the coating and then recombine it ju.st before printing on
the paper. The amount of coating mixture applied to the paper can be
regidated by the setting between the rolls or by the relative speed of the
rolls. The print roll turns in the same direction as the paper travels.

Rotogravure Roll Coater. 'I'lie rotogravure-offset coater consists
of a soft rubljer feed roll, a knurled or an engraved roll, a doctor blade, a
rubber-covered offset roll, and a backing roll of cast iron or hard rubber.
1 lie feed roll revolves in the coating pan where it picks up the coating mix-

1012

PULP AND PAPER

ture and applies it to the knurled roll. The excess coating is removed from
the knurled roll by means of the doctor blade. The coating may be printed
directly on the paper from the knurled roll, or it may be transferred first
to an offset roll and then to the paper. An offset roll is beneficial in re¬
moving the design or pattern which the knurled roll tends to leave on the
paper, and hence results in a smoother coating. The feed roll is driven
separately at slow speed, and the knurled roll and offset roll are driven at
the same surface speed as the paper. A drawing of an offset rotogravure
print coater is shown in Figure XVIII-1.

Fig. XVIII-1. Diagram of an offset rotogravure print coater

(courtesy John Waldron Corp.).

When the coating is first picked up by the offset roll, it is in
of discontinuous areas corresponding to the recessed areas ^ knurl
or engraved roll, but these areas tend to flow together into a continuous film
on the surface of the offset roll. When the coating is transferred to *e
paper the film splits, with most of the film sticking to the paper, but a
Lying with the offset roll. This film-splitting accounts for the ribbed tex-

turc in the costing* ‘naner with

It is difficult to vary the weight of coating applied f f^^-oatiiw

gravure coaters. The only method of changing the weight ^ry “
applied to the paper with this type of coater is y ^ applied

solids of the coating mixture, since the volume o c g
is predetermined by the engraving on ‘he print to" . " , j

Jhe solids content of the coating mixture must w^L -

correct amount of coating under '‘7; . JLb “d (or the roll

to be changed, the solids content will have to be adjusted f

changed).

* G. Haywood, P<.Per Trade J. 127. No. 3 : 314-316 (July 15. 1948)

XVIII. PIGMENT COATING

1013

The viscosity of the coating mixture used in gravure coating must be
controlled in order that coating will distribute well .without producing a
pattern on the paper. Too low a viscosity causes flooding of the nip be¬
tween the knurled roll and the offset roll, whereas too high a viscosity pre¬
vents proper flow or spreading of the coating on the offset roll. The vis¬
cosity also affects the amount of coating transferred to the paper. However,
the proper viscosity is determined by the working properties of the coating
mixture on the rolls and cannot be changed from the optimum point to con¬
trol the amount of coating applied.

Knife Coaters

Knife coaters are used for machine coating, primarily for the coating
of paperboard. In the knife coater, an excess of coating is applied to the
paper by means of a roll revolving in a pan of the coating mixture, and the
excess coating is then removed and the remaining coating is smoothed by
passing the paper underneath a stationary knife blade. An important factor
in knife coating is the draw or tension on the sheet, since the amount of this
tension determines the pattern of the coating and the amount applied. A
sheet which is run loose will receive more coating, but the pattern will not
be so good as a sheet which is run with a tight draw. The draw can be
readily adj usted, since the coater is generally located between drier sections.
The coated paper has a moisture content of about 40 to 50% when it goes
to the driers.®'®

In addition to the straight-edge knife coater, there is also the Mayer
type coater in which a small revolving rod is used in the knife edge. This
rod is wound with wire, the size of which determines the amount of coating
spread on the paper.

Another special type of knife coater, which is in the early stages of
commercial use, consists of a pneumatic diaphragm which supports the pa¬
per as it passes under a straight-edge knife. Low air pressure is main¬
tained in the diaphragm to level out the surface of the paper and furnish a
resilient support.

Casein is the principal adhesive used in knife coating, and some syn¬
thetic latex is often used with casein to improve the flexibility of the coating.
The amount of coating applied varies from about 1 to 4 lb. of coating per
1,000 sq.ft. The base paper for knife coaters must be exceptionally smooth
and possess a high degree of internal strength. Beater starches are fre¬
quently used in the base paper for strengthening the surface. It is desirable
to use finely woven felts on the paper machine to keep the surface of the
paperboard as smooth as possible.

® J. R. Simpson, Paper Trade J. 128, No. 13; 109-111 (Mar. 31, 1949)

^ Paper Trade J. 128, No. 12; 12-13 (Mar. 24, 1949)

1014

PULI’ AND PAPER

j'lir Brush Coatcrs

Another type of coaler is the air hrusli or air knife coaler. In this
coating process, an excess of coating is applied to the j)a])er hy roll apjdica-
tor, and the excess coating is then blown oiT the sheet by means of a thin
air jet which strikes the wet coating at an angle where the sheet is wrapped
around a breast roll. The air jet can be regulated for velocity and width of
stream in order to regulate the amount of coating left on the paper. The
velocity of air leaving the’ air doctor varies from 20,000 to 40,000 f.p.m.,
depending upon the pressure used. The excess coating is removed in the
form of small drops which are separated from the air by baffling and by loss
of velocity of the air. This coating is collected in pans, returned to the
supply tank- and recirculated. The excess coating removed from the paper
amounts to about 30 to 509^> of the original coating applied to the paper.
There are two commercial air knife coalers, the \\^arren air knife and the

Micro jet.

I Air brush coalers produce a uniform coating even oh papers having a
Very rough surface wallpaper stock). They cannot be used for coat¬

ing heavy boards which are too stiff to be wrapped around the breast roll.
The coaler is used for both conventional and machine coating.

The properties required in a coating mixture for air knife coating are
approximately the same as those required for brush coating, with the ex¬
ception that coating mixtures of a wider range of viscosities can be handled.
There is some tendency for air brush coalers to classify the pigment in the
coating mixture, resulting in a build-up of the larger particles. However,
this difficulty can be eliminated by better dispersion of the pigment and the
use of pigments of finer particle size.

Coating Adhesives

The adhesive exerts a profound influence on the properties of the
mating mixture and the properties of the final coated paper. The functions
f the adhesive in pigment coating are as follows: (7) to seiw’e as a carrier
)r the pigment so that the coating mixture will have the proper flow pro^r-
es • (2) to bond the pigment particles together in the dried coating and to
ond the pigment to the body stock; (5) to control the absorption o
rinting ink during printing of the coated paper.

Properties Necessary in Coating Adhesives

An adhesive, to be satisfactory, should have a
trength and good color. Moreover, the adhesive should not
iroperties of the pigment, but should produce with the pi^e ^
hX higWv receptive to printing ink. The adhesive must ha« he
icosity for the solids content at which the coating mixture is to be

XVIII. PIGMENT COATING

1015

and should have strong filming properties to prevent excessive penetration
of the coating mixture into the raw stock at the time of coating. The ad¬
hesive should be compatible with the pigment and should produce a coating
mixture with a high degree of colloidal stability, so that the properties of
the coating mixture do not change during the period of use. Finally, the
adhesive should have enough plasticity so that the coating will not powder
or dust during calendering.

The most important single property of the adhesive is the pigment
bonding strength, because this property determines the amount of adhesive
required to hold the pigment on the paper. Each grade of coated paper has
a fairly definite strength requirement, and the amount of adhesive necessary
in the coating mixture to meet this requirement is determined by the pig¬
ment bonding strength of the adhesive. The pigment bonding strength of
the adhesive indirectly affects all the properties of the coating, because the
amount of adhesive in the coating affects the opacity, brightness, color, and
.smoothness, as well as the strength. If the ratio of adhesive to pigment is
excessive, the quality of the coating will be poor, from a printing standpoint.

The pigment bonding strength of the adhesive is determined by (i) the
cohesive strength between the adhesive molecules and (2) the adhesive bond
formed between the adhesive, the pigment, and the raw stock. In this re¬
spect, pigment coating is related to the fundamental laws governing all
adhesiv'e action, although pigment bonding is more complicated than most
adhesive problems.

The earliest adhesives used in pigment coating were animal glue and
gum arabic; later, casein was widely used. Today, the principal adhesives
are starch, casein, and soybean protein. Most of the casein is used in con¬
ventional coating, and most of the starch in machine coating. Other ad¬
hesives used in lesser amounts are polyvinyl alcohol, water-soluble cellulose
derivatives, synthetic latices, and soy flour. The properties of the various

adhesives and the methods of preparation of various adhesive solutions are
discussed in the following sections.

Casein

Casein was the most important adhesive used in pigment coating prior
to the advent of machine coating, after which starch became increasingly
important. Casein is still an important adhesive for the coating of high-
grade papers or papers in which a high degree of water resistance is de¬
sired. Most of the casein is used in conventional coating with brush coaters
or air knife coaters, although some casein is used for machine coating par¬
ticularly by the knife coating process.

^sem is a by-product of the dairy industry and is prepared from skim
milk by coagulation in the form of a curd. About lb. of casein are ob-

1016

PULP AND PAPER

tained from 100 lb. of milk. Casein in milk is combined with calcium and
is associated with some calcium phosphate in the form of colloidal com¬
plexes which are suspended in an aqueous solution of lactose and mineral
salts. Hydrochloric acid, lactic acid (from natural souring), and rennet
are the common coagulating agents used for the separation of casein from
milk. The properties of casein depend somewhat upon the coagulating
agent used. In acid coagulation, the />H of the milk is lowered to the iso¬
electric point of the casein (/’H 4.6-4.7) with hydrochloric acid. In the
natural souring method, the milk is held in vats at a temperature favoring
fermentation until sufficient lactic acid is formed to coagulate the casein.
Rennet (obtained from cows’ stomachs) is used as the coagulant in the
other important process. Since rennet coagulates casein best at a />H
around 6.0 to 6.4, the casein is less hydrolyzed than acid casein. After co¬
agulation, the curd must be washed to remove free acid, sugar, and mineral
salts. A high ash content is an indication that the casein has been poorly
washed. The final step is drying of the casein, and care must be taken to
avoid excessively high temperatures, since this forms a case-hardened

product which is difficult to dissolve.

Casein is a relatively non-uniform product, unless it is carefully

Idended. Blending is customary with the large distributors. Both domestic
and imported (mostly Argentinian) caseins are used. In Europe, French
casein is considered the best. All the different types of caseins (acid, ren¬
net, and self-sour casein) have been used in pigment coating, although eac

type seems to work somewhat differently.’

Casein arrives at the mill either as a crude dried curd or as a coarsely
ground powder. Excessively fine particles are undesirable because they in¬
crease the difficulty of wetting, and because they appear to be associa e
with excessive foaming. Milham* gives the desirable fineness ru e ha
the casein should pass a 10-mesh screen, and not more than '0% ^ould
pass a 40-mesh screen. An approximate analysis of casein is giv

Carbon .

Hydrogen .

Nitrogen .

Sulfur .

Phosphorus ...

Moisture .

Ash .

Fat .

Specific gravity

53.5%

7.13%

15.8%

0.72%

0.71%

5.5- 10.5%

1.5- 4.5%
0.2-2.5%
1.25 to 1.31

1 E. O. Whittier, S. P. Gould R. W. Bell B. Shaw and G. W. Bicking,
Eng. Chem. 25, No. 8: 90^908 ^ - 23-24 (July 2, 1936)

: 11 S-nd I w! Bio,:14: 203 (.913)

XVIll. PIGMENT COATING

1017

Casein always contains some fat. Excess fat causes trouble with grease
spots in the coated paper, but up to 225^0 fat can be present without caus¬
ing trouble.** . .

Casein should always be stored in a dry' place, but not where it is ex¬
cessively dry, since tliis lowers the moisture content and reduces the solu¬
bility.** Casein is likely to become infested with weevils or worms on
storage. Casein infested in this way dissolves more readily (often re¬
quiring only 40 to 50^ of the normal amount of alkali), but the pigment
bonding strength of infested casein is greatly reduced (sometimes being

only 50^ that of uninfested casein).

Wlicn casein is stirred into water, the individual granules imbibe water

and swell, but do not dissolve unless the medium is highly alkaline or highly
ackl. .Mkali and heat are generally used for dispersing casein in pigment
coating, .\bout 5 to 6 parts of w'ater are used for each part of casein. The
casein should be soaked in cold water for at least fifteen to thirty minutes
and stirred during this period in order to swell the casein curd and permit
subsequent penetration of the alkali. The alkali is dissolved in a small
amount of remaining water and is added to the thoroughly wetted casein
while agitating, after which the suspension can be heated to 130 to 140° h.
aixl held there for fifteen to thirty minutes, or until the casein is completely
dis{)ersed.

The type of alkali used is very important. Sodium, potassium, and
ammonium hydroxides are more rapid solvents than the weaker alkalies
such as lx>rax and trisodium phosphate. The hydroxides of alkali metal
earths, e.g., calcium and magnesium hydroxides, are slower solvents than
the »odium and potassium hydroxides and are not generally used because of
their tendency to flocculate pigments used in the coating mixture. Caustic
Mjda reputedly prriduces denser and clearer films than other alkalies, al¬
though the films arc somewhat more brittle. Milham'* reports that sodium
carbonate produces a fairly clear and tough film, although the film has
relatively poor varnishing properties; it tends to produce a foamy disper¬
sion, unless a large excess of alkali is used, or unless it is used in combina¬
tion with ammonia or some other alkali. The phosphates and silicates give
somewhat porous films having relatively poor varnishing properties. Am¬
monia produces a fairly dense and tough film which has a good bonding
strength and readily lends itself to waterproofing; if ammonia is used, it
should be added at the end of the heating period.

The amount erf alkali required for dissolving casein is greater than the
stoichiometric amount, because considerable alkali is adsorbed on the surface

** ^ O. Whittkr, S. P. Gould. R. W. Bell, M. B. Shaw and G. W. Bid
£■#. Ckfm. ^5, Na 8: 90i-908 (Aug.. 1933)

II» Na 11: 160-162 (Mar. 14. 1940)

*• E. G. Mahani, Tappi BuU. 57 (Aim. 6 1945^

1018

PULP AND PAPER

of the casein micelles.’“ The amount of alkali depends somewhat upon the
type of pigment with which the casein is to be used. For example, less al¬
kali is required when satin white is used as the pigment than when clay is
used. Mixed alkalies are frequently used for dissolving casein. A few

typical formulas are given in Table I.

TABLE I

Typical Formulas for Dissolving C.^seix for Pigment Coating

Formula 4

Fomula 1

100 parts casein
8 parts sodium
carbonate
8 parts borax

600 parts water

Formula 2

100 parts casein
10 parts sodium
carbonate
3 parts ammonia
(26* Be.)

550 parts water

Formula 3

100 parts casein
5 parts sodium
hydroxide

100 parts casein
8 parts ammonia
(26“ Be.)

550 parts water 600 parts water

Too much alkali should not be used in dissolving casein, particular!)
the stronger alkalies (e.g., sodium hydroxide) are used, since this irings
about rapid decomposition of the casein. Casein solutions
cold usually have greater adhesive strength than those prepared by heatup
because of the greater hydrolysis that takes place under
heat. Casein solutions having a pH of 10.5 or above are S'* 1®' P
decomposition. Most casein coating mixtures have a pH between 9.0 a
10.0, and even at room temperature casein will hydrolyze s °wy _
pH, with a resulting breakdown of the casein into ammonia an
imposition products. Thus, casein coating mixtures cannot be held
definitely, even though protected from spoilage with Ptese*^

Casein produces relatively viscous solutions compar eoncen-

coating adhesives; the 'a" concentration of

9% solids, but are pseudoplastic at a concentration o j,,

creasing the temperature lowers the viscosity, and » particularlv

casein solutions results in a permanent reduction in m P

if the pH is very high. The high viscosity and '"y
flow makes casein undesirable for certain paper coa i g p

The conventional manner of using casein has mixture of

alkali and then add the casein solution to » separate ^ j j

pigment and water. However, newer methods have been d P

prSce coating mixtures of lower viscosity .""d pj-ment-ivater

properties.’® In one procedure, dry casein is added P

13/, Phys. Chem. 29: 769 (1925) <- • -ii? no-in *

14 R. W. Bell and S. P. Gould, /. Dairy Set ?? No 5' 212-218 (May,

15 j/w. Smith, R. T. Trelfa and H. O. Ware. Tappx 33. No. 5.

1950)

XVIII. PIGMENT COATING

1019

mixture and solubilized in the presence of the pigment. In another proce¬
dure, casein is dissolved in water at 25 to 30^o concentration, using. 3 to
5% sodium hydroxide on the weight of the casein and a temperature of
110 to 125° F. for twenty to thirty minutes, after which the dry pigment is
added. Sodium sesquisilicate is added later to reduce the viscosity.

Casein has a high pigment bonding strength when properly dispersed.
A curve showing the relative bonding strength of a number of different ad¬
hesives is shown in Figure XVIII-2. T. his curve shows the amount of
dry casein (in comparison with other adhesives) required for each 100 lb.
of dry clay in order to obtain a coating which will not pick at various Denni¬
son waxes. About 12 parts of casein will bond 100 parts of clay for most
grades of coated printing papers.

PER CENT ADHESIVE ON WEIGHT OF CLAY
Fig. XV1II-2. Comparison of adhesives for pigment bonding strength.

The particular advantage which casein has over most other adhesives
is the ease with which its film can be made water-resistant. Treatment of
a dry or semi-dry casein coating with formaldehyde, alum, acids, or lime
will produce a highly water-insoluble coating. It is also possible to add
formaldehyde directly to the casein coating mixture if it is added in very
dilute solution under vigorous agitation. Mixing ammonia with the formal¬
dehyde before adding to the coating mixture will reduce the coagulating
effect of the formaldehyde on the casein. Small amounts of formaldehvde
added to the coating mixture do not produce an immediate waterproofing
of the dried film, but water resistance develops slowly with time.

1020

PULP AND PAPER

A qualitative test for casein in coated papers can be made by boiling the
paper in 1% caustic soda solution, neutralizing the filtered extract, and
treating with Millon’s reagent (mercury dissolved in nitric acid). The
appearance of a red color on heating indicates the presence of casein. A
quantitative test for casein can be made by the Kjeldahl determination.

Starch

Starch has become the most widely used coating adhesive because of
its low cost and good working qualities. The introduction of machine coat¬
ing was responsible for the wide acceptance of starch. Starch can be used
to prepare high solids coatings which are particularly well adapted to the
demands of machine coating. Most of the starch used is domestic corn
starch. Other starches which have been used to a lesser extent are tapioca,
white potato, Avaxy corn, waxy sorghum, sweet potato, rice, and wheat
starch.

Native or unmodified starches are unsuited for most pigment coating
because of their high viscosity, which precludes the preparation of high
solids coating mixtures. Furthermore, in the case of corn, native starch
has a tendency to gel or retrograde, thereby producing a coating mixture of
high yield value. Most native starches contain two fractions, amylose and
amylopectin. The amylose fraction (fraction made up of linear molecules)
has greater pigment binding strength than the amylopectin (branched
chain) fraction,"® but has less dispersing action on the clay and greater
tendency to increase in viscosity and yield value on standing. When the
amylose fraction is modified to lower viscosity, it tends to lose strength
much more rapidly than the amylopectin fraction. For the above reasons,
it is desirable to use starches having a minimum of amylose. Waxy varie¬
ties of starch are entirely free of amylose and hence make good coating ad¬
hesives, but these are not available in sufficient quantities or at low enough
cost to meet the demand. Fortunately, however, certain of the modified
starches are relatively low in amylose, e.g., oxidized starches and dextrins.

Most coating is done with high solids coating mixtures, and hence
modified starches of relatively low viscosity are required. However, it
should be pointed out that all modified starches tend to lose strength as the
modification is carried to products of lower and lower viscosity. This loss
in strength depends somewhat upon the type of modifying agent use , eac
type of modifying agent has definite limitations beyond which it is not prac¬
tical to reduce the viscosity without causing a serious loss in strengt . ^

principal types of modified starches used for coating are dextrins (trom
corn), oxidized starches (from corn), and enzyme-conyerted stare es
(from corn or tapioca). Modified starches made by acid hydrolysis ha^e

10 R. W. Kerr and N. F. Schink, Paper Trade /. 120. No. 8: 77-80 (Feb. 22, 1945)

XVIII. PIGMENT COATING

1021

a relatively high amylose content and tend to produce coating' mixtures of
high plasticity and low strength; hence, they are not widely used for paper
coating, although they find application in certain special coating processes.
(These starches are prepared by hydrolyzing starch in water slurry using
mineral acids. They are commonly referred to as thin-boiling starches.)
In the early part of 1900, the use of starch acetate (Feculose) was tried
out in England, but although it was quite successful at that time, it is not
used today because it is up better than the cheaper oxidized starches.

There are three principal types of dextrins, white dextrins, canary
dextrins, and British gums. All are made by roasting dry starch, but white
dextrins are made with relatively high percentage of acid and low tem¬
perature, whereas British gums are made without acid, using high tempera¬
tures and long converting periods. Canary dextrins are intermediate to
white dextrin and British gums in the amount of acid and heat used. Dex¬
trins are used in coating when coating mixtures of exceptionally high solids
are required, e.g., 60 to 70% solids. Highly converted dextrins have the
lowest viscosity of any modified starch product. Canary dextrins and
British gums are best suited for coating, since they have higher pigment
binding strength than the white dextrins.

Oxidized starches used for coating are made by treating starch in
slurry form, using alkaline sodium hypochlorite. Oxidized starches are
available in a wide range of viscosity but, in general, only the highly modi¬
fied products of low viscosity are used in coating. Oxidized starches are
not available in as low a viscosity range as dextrins, and hence cannot pro¬
duce coating mixtures quite so high in solids content. One particular ad¬
vantage of oxidized starches is their natural dispersing effect on pigments
(similar to that of casein); this, however, has become of lesser importance
since the introduction and widespread use of inorganic dispersing agents
(e.g., polyphosphates) which has made the dispersing action of the adhesive
of minor importance. At one time, oxidized starches were about the only
type of modified starch used for coating. They are still used to a large
extent for both machine coating and conventional coating.

Coating starches are generally cooked before being mixed with the
pigment. This is done by stirring the starch in the required amount of wa¬
ter and heating to a temperature of about 200 to 212° F. and holding it
there for ten to fifteen minutes. Heating and agitation tend to reduce the
viscosity of the cooked solution, but the solution should be well agitated
during preparation to help stabilize the viscosity. After cooking, the starch
solution can be cooled before being added to the pigment slurry if a jacketed
tank is used. If the tank is not jacketed and steam is injected directly into
the starch during cooking, it is possible to withhold part of the water at
first and add it at the end to aid in cooling. Best results are obtained with

1022

PULP AND PAPKR

some modified starches if the starch solution is mixed with the pijjnient at
a temperature of about 140° F. One process’* calls for cooking the starch
in the presence of the pigment. The strength of the coaling is increased by
1-2 Dennison waxes by this method, but there is the obvious disadvantage

of having to cool the entire batch of starch and pigment.

The general procedure for converting starch with enzymes has been
discussed in Chapter XL The same procedure is used for converting starch
for coating, except that the conversion is carried out at higher concentra¬
tion, and consequently higher percentages of enzyme are required. If the
starch is to be used for high solids coating mixtures, approximately 1 to
enzyme will be required. (The exact amount depends upon the type of
enzyme: the newer enzyme concentrates can be used in much lower con¬
centration.) Enzyme activity is a function both of time and temperature.
Enzymes, however, vary in their response to time and temperature varia¬
tion. Certain enzymes depend mostly on time of conversion (doubling the
conversion time results in twice as much conversion), whereas other en-
zvmes lose activity after a short converting period. Certain enzymes result
in a progressive loss in strength of the starch as the viscosity is decreased,
whereas other enzvmes convert starch over quite a range of viscosity with¬
out appreciable loss in strength. The effect of degree of enzyme con^^rslon
on the viscosity and adhesive strength of corn starch is shown in Figure
XVIII-3 In these experiments, starch solutions converted with various
percentages of enzyme were made into coating mixtures with clay, the
coating applied to paper, and the strength of the resulting coating
by means of the wax pick test. It is to be noted that both the ^‘scosity of
starch and the adhesive binding strength are lowered as the degree
Version by enzyme is increased. The loss in binding strength is essential!)

linear whereas the decrease in viscosity is essentia y ‘

When converting corn starch'with enzymes, it is ge

use low percentages of enz>mie and relatively long reason

than hi<^h percentages of enzymt and short concerting pen .

an ftarch graLles do not convert at th, same mU,

desirable to convert relatively slowly ° converted slowlv will

conversion. As a general rule, starch which has been -n er ed^o^W

have greater adhesive strength than stare \\ iic ordinary

idly Certain enzymes are more heat-resistant a ' the end

lemperatures are not enough to inactivate completely to ^

of the converting period. In such cases, l^^^ich can 1«

in combination with heat. There are a number

nsed to inactivate enzymes, some of which are phosphates, silicates,

I; ™ NO. 5: 253-256 (May, 1950,

XVIII. PIGMENT COATING

1023

acid salts, copper sulfate, sodium chlorite, mercury compounds, and formal¬
dehyde. Iron salts are not particularly detrimental to enzyme activity.

One process^” calls for converting starch with enzyme in the presence
of the pigment in a definite and predetermined time cycle and at a constant
rate of agitation. With this process it is possible to convert at high solids
content without running into excessive thickening of the starch, the whole
process being controlled automatically. The resulting product has a higher
pigment bonding strength compared to starch converted in the regular
manner. When using this process, the />H of the starch-pigment-enzyme

PER CENT ENZYME ON STARCH

(/>

Fig. XVIII-3. Effect of degree of enzyme conversion on the viscosity and

pigment bonding strength of starch.

mixture must be adjusted to between 6.5 to 7.0 during conversion. The
conversion can then be carried out in the normal manner by heating to ap¬
proximately 160® F. and holding there for fifteen to thirty minutes. At the
endj)l the converting period, the entire mixture must be heated to at least
2(X) h. to cook the starch and destroy the enzyme (this imposes a problem
m cooling the batch). Some clays have a tendency to adsorb the enzyme
and prevent its action on the starch. Why some clays work well and others
do not has not been determined, but the difficulty can be overcome in most
cases by adding a small amount of protein or sodium silicate to the clay be-

«W. L. Craig, U.
1945)

S. 2,360,828 (Oct. 24, 1944)

and U. S. 2,388,526 (Nov. 6,

1024

PULP AND PAPER

fore the enzyme is added to satisfy the adsorptive demand of the clay par¬
ticles,*" or by increasing the amount of agitation during the converting cycle.

In another process for converting starch for high solids coating, the
starch is converted with enzyme in the presence of the total amount of water
to be used in the coating mixture and dry clay added to the starch after
conversion. The clay must be the pulverized, predispersed type, and the
coating mixture must be kneaded to bring about a good mixture of starch

arid clay.

Starch has fair pigment bonding strength (see Fig. XVI11-2), although
it is not quite so good as casein. Most starches have pod color and produce
coatings of high brightness and good printing qualities. Ordinarily, starch
tends to produce coated papers of lower finish than casein. Another differ¬
ence is that starch-coated papers have lower oil absorption than casein-
coated papers, due to the higher percentage of adhesive used in starch-coated
papers. Starch has the advantage over casein in that it has less odor, less

tendency to foam, and less tendency to spoil.

The only major shortcoming of starch is that starch-coated pa^rs

cannot be made water-resistant. Water-resistant coatings are desired in
the case of washable wallpapers, coated litho, coated tag papers, coated
fancy wrappings, coated paperboard for soap boxes and medicines, and
other cases where the .coated paper is likely to come into contact with water
during use. Casein is generally used for these grades of paper, ®
coated papers do not meet the water resistance requirements. Howeye ,
consideraL time and effort have been expended trying to produce wate -
resistant starch coatings, but the efforts have only been partially successful.

Fairly good results have been obtained through the use '

De resins and dimethylolurea, but they have not been good enough for ^

bXhe major disadvantages of this process. Three methods which hare
given fairly good results are described below.

Method 1: Cook 75 parts of oxidized starch. 25 parts 7 0 ^ CqoI the

parts of water a. 200 to 2.2- F for 10 to U rnin^es a^^.H o^a^^
mixture and add to the pigment slurry in the regular ^

coating mixture to 5.0 to 5.5 with hydroc is"beneficial in maintaining a high

dispersing agent such as sodium order to obtain water resist^e

fluidity of the coating mixture at \ temperature, and even then,

by this method, the coated paper most be subjected to mgn pe

the water resistance is only fair. cooked starch and pigment in the

Method 2: Prepare the coating mt^ure ''7* “ fcrmaldehvde resin on the
usual manner. To this, add about ^5^“ ^sV of a catalyst (e.g., am-

weight of the starch, and after miHng, add 7““* “ ,1,,, u,hich might be usrf

. monium chloride) based on the weight of urea-formaldehyde resins

are ammonium oxalate and diarnmoninm phosphate, home u

SOW. L. Craig. U. S. 2,360,828 (Oct. 24, 1944)

XVIIL PIGMENT COATING

1025

2 to thirkm thf cnfttinc •**•! incre»4* its thixotfojwc ch*r4Ctcr, whereas

others to ^ecrcasie the vHoooity of the coating mixturr The addition of the
catalyst to the fating muitarc aJv-ajrs results in an appredabk increase in viscosity,
la totoe «*»«*«. best residts are uhcained by cuoldng the starch and resin together, in*
stead of the reshs directly to the coatif^ mixture.

Metk6d 3: Thu method, which is used mostly in tsoard mills, invol^'es a precoating
(d the paper with aavDOciiiaB chloride or altan prior to the application of the starch-
rr»ia atxtwe. The ahan or ammoniom chloride solution at a concentration of

ab:iul 1 fb per galloa is applied to the board on the calender stacks, and then the reg¬
ular stareh-day coating cuotasning about IS to 2S% resin is applied os^r this precoat.
TIm mrdkjd gives the highest aratrr resistance of any of the above methods and elimi*
aaies the thtekenaig effect obtained when Oie ammonium chloride is added directly to
the coatiag mixture.

In aU the above proersses. the coated paper should be subjected to as
much heat a» poasiUe in order to speed up the reaction lietween the starch
and ream. II possible, the sheet should be heated to approximately 212° F.
and held there fur twenty to sixty seconds. Ttie f H of the liase paper is im¬
portant, since f li values above 7 tend to retard curing of the resin. Time
u another important factor, since tite water resistance tends to increase on
agit^ t^ to about one week’s time. Even under ideal conditions, the coat¬
ings oUatned are not waterproof, but merely water-resistant; the bond is a
wrt*»trength bond similar to that otiCaincd in wet-strengh palters.

Another method of producing insoluble starch coatings is with an¬
timony salts s|i*cihcally puUssium pyroantinionate (K,H,Sb.Ot).” Sat-
idactf^ water rrsioance is obuined with 10% of pyroantinionate on the
weight of the starch when use<l at a ^11 7.0 to 7.8, but the coated |>aper con-
uining antimony salt must be heated to a tmi|>erature of 220° F. for 30 to
120 tretmds. The results are fairly satisfactory if high tem[)eratures can

hr used, but like all schemes for making starch water-resistant, the process
IS rather ex)ienstvr.

The presence of starch in coated papers can lie delected qualitatively
by the wdiiie lest. The amount of starch can be determined quantitatively
by the standard method of starch analvsii.**

Soy Flour and Soybron Protein

^ ^ isolaied soytiean protein are two products derived from

«oy|tfa m wrhtth are used in paper coating. Of the two products, isfdated
mrybm proidn is much more suitable for coating; it can generally be sub-
ttituted for casein on an equal weight basis. Soy Hour is used onlv in small
q«amii«^ and then generally as a kw-cort extender for casein. '

^^***^’ 24 : 263 -

"fe?) ***^ Mo. 7: 328-332 (July.

• S«r TAPPI StomMs

1026

PULP AND PAPER

There are two processes for making soy flour, the cxpeller and the
extraction processes. In the expeller process, dehulled soybeans are forced
at high temperature (350° F.) and great pressure through expellers which
mechanically force out the oil. In the extraction process, dehulled soybeans
are first flaked to destroy the cell structure and then extracted with petro¬
leum solvent to remove the oil. After the oil is removed, the oil-free residue
is ground into soy flour. The two types of flour (expeller and extracted)
differ appreciably in their properties. One difference is in their oil con¬
tents, which is about 7^o in expeller flour and about 1% in extracted flour.
The protein, which is the important constituent of the flour, is more de¬
natured in the case of the expeller flour than in the case of the extracted
flour because of the high temperatures and pressures used in the expellers.
A comparison of solvent-extracted and expeller soy flours is given in

Table II.

TABLE II

Comparison of Extracted and Expeller Soy Flours
All percentages given on dry substance basis

Analysis

Moisture, %

Oil, %

Protein, %

Water-soluble protein, %
Fiber, %

Carbohydrates, %

Ash, %

Extracted

Expeller

soy flour

6.0-6.5

4.2^.5

07-1.0

6.0-8.0

55.0-56.0

48.0-50.0

43.0-45.0

2.5-2.8

2.5-2.6

32.0

6.5

5.0-5.5

6.5

Sov flour is sometimes used as the adhesive in the manufacture of
washable wallpaper, and it is sometimes used as an 50

the manufacture of regular coated papers. Soy oti elf

to 55^0 usable protein, and 45 to 50% inert matenal, h^n by « _

produces a weak coating. Most of the C to

Usible by alkali and hence results in small specks in the anting, h

^ TXr. . .

To obtain the maximum adhesive stren^h, soy

• fln.ir Tncreasine the amount of sodium hydroxiae it»-

persion of the flour, increasing . .not «ndinm hvdroxide on the

XVllI. PIGMENT COATING

1027

l«sis of the dry soy flour is used. The following represents a typical for¬
mula: 100 parts soy fl<mr. 400 to 500 i>arts water, 8 to 14 parts of 50%
sodium hydroxide solution. Heating the dispersion to about 130° F.
slightly increases the bonding strength and apprecialdy reduces the vis¬
cosity, but prolonged heating should be avoided, since this discolors the
flour and tends to reduce the bonding strength. The addition of a small
amount of carbon disulfide increases the adhesive strength,-^ but produces
a slightly discolored coating. Soy flour dispersions are difficult to keep
without spoiling, and for this reason, a preservative should always be in¬
cluded in the formula.

A more desirable adhesive for paper coating than soy flour is isolated
soybean protein (trade name, Alpha protein). Soybean protein is made
from extracted sovbean flakes. The oil-free soybean flakes are treated with

a solution of alkali to dissolve the protein, which is then removed by filtra¬
tion and subsequently precipitated in the form of a curd by adjusting the
/>H of the solution to the isoelectric point of the protein, i.e., />H 4.0—4.5.
The curd is dried to produce a granulated protein of greater than 95%
j)urity. Almost any alkali can be used for extraction, but some alkalies are
better than others. Sodium sulfite is different from other alkalies in that
it produces a protein very similar to that in the original soybean. The native
soybean protein has a very large molecule and tends to form solutions of
high viscosity. It has low adhesive strength when dispersed with weak al-
kalie.s, although its adhesive strength is good when dispersed in strong
alkalies. If too much alkali is added, the protein gels, but if still greater
amounts of alkali are added, the dispersion passes through the gel stage and
liecomes fluid again. The native protein is also sensitive to heat, tending to
gel at tem[)eratures in the neighborhood of 160° F. Because of these dis¬
advantages of native soybean protein, it is customary to modify the protein
during manufacture so that it has high adhesive strength when dispersed
in weak alkalies and is no longer sensitive to heat.

The manufacture of soybean protein is carried out on a large scale, and
consequently the process is readily adapted to large-scale manufacturing
methods and controls. One patent^^ describes a method of heat-treating an
aqueous suspension of oil-free flakes at a />H of 4.5, which coagulates the
heat-sensitive proteins and prevents their appearance in the final product.
.\nother patent'® calls for treating the protein with hydrogen peroxide in a
mildly alkaline medium to produce a modified protein which has a lower
viscosity than the native protein. Modification is believed to result in a
reduction in molecular weight and an unfolding of the globular protein into
a linear shape. This gives the soybean protein properties which are verv

I. F. Laucks, U. S. 1,805,773 (May 19, 1931)

-»P. L, Julian and G. C. Engstrom, U, S. 2,238,329 ( Apr, 15, 1941)

=*R. H. Hieronymous, U. S. 2,274,983 (Mar. 3, 1942)

1028

PXJLP AND PAPER

similar to those of casein. Special low-viscosity soybean protein is available
conimercially.

Commercial soybean protein works very well as a pigment-coating
adhesive. It has good pigment bonding strength (see Fig. XVIII-2), being
approximately equal to casein in this respect. The color of the bleached
protein is good, although the first commercial product was dark in color
and produced coatings of low brightness. It is not as readily waterproofed
as casein, and has slightly more tendency to foam. In general, however, the
coating industry has accepted soybean protein as a satisfactory substitute
for casein.

Soybean protein is dispersed with alkali in the same way as casein, but
soybean protein is more susceptible to heat than casein, and consequently
should not be heated over a temperature of about 130® F. In general, soy¬
bean protein produces more fluid coating mixtures than casein, as shown
particularly in the. range of high adhesive to pigment ratios.

Animal Glue

In addition to casein and soybean protein, there are several other pro¬
teins which are used as pigment coating adhesives. One of the most im¬
portant of these is animal glue. Animal glue was among the first coating
adhesives used, but today it is used to a limited extent only, for coated spe¬
cialty papers, e.g., playing card stock, wallpapers, metal-coated papers, an
other grades where a high gloss and a water-resistant coating is desire .

Technical gelatin is used in photographic papers.

Either hide or bone glue may be used for coating, but the former gi'Ca
the best results. In general, the higher the grade, that is, the higher the
jelly test, the greater the pigment bonding strength. A rnedium grade g ue
has about the same pigment bonding strength as casein. Accor ing to
Sweatt,^' animal glues should fcontaih a small amount of grease to eep
down foam and help reduce dusting on the calenders, but too much gr^
is undesirable because of the danger of forming grease spots m the coating.

Glue coatings can be “hardened” or made water-resistant by after-

treatment of the coating with chrome alum or formaldehyde, d--
glues, by mixing about 1 to 35b of alum with the glue solution. The ^ J
grades of glue require less tanning agent than the lower gra es o
the same degree of water resistance.*’ Glue coatings cannot e ma

•water-resistant as casein coatings.

Corn Proteins

Another class of proteins which are of interest to
chemist are the com proteins, which are repmsented by crude com >
or by the purified corn protein known as zein.

ITH. B. Swealt, Paper M. 27, No. 6; 882.B84 (Sept,: 1945)

XVIII.

PIGMENT COATING

1029

Corn gluten, as obtained front the regular corn rehniug
about 60'"c protein and ■fOtt'o starch. There is, however, a process for p
dLng a'pure destarched gluten. In isolating the gluteti, the ci«de ™rn
Iten i! treated with acid at the isoelectric point of the protan tn order o
Lubilize and remove the starch, leaving only the protetn.
product is sold as destarched gluten. Corn gluten has never a w.de

usage in the paper industry, principally because of its very dark coloi e
the difficulty of dispersing corn gluten in aqueous media, ut
gluten has been used as the adhesive in wallpaper coating. The prote ^
diswrsed with ammonium hydroxide and rosin soap, but the co or o e
solution is quite dark, and the protein must be bleached with peroxide or
other bleaching agents for best results.- Sulfonated tall oi and sulfonated
castor oil have also been used for dispersing corn gluten, but a high per¬
centage of dispersing agent is required. A formula which has been use
is: 100 parts corn gluten, 500 parts water, 50 parts sulfonated castor oil or

sulfonated tall oil, and 50 parts of lO^o sodium hydroxide.

Zein is available commercially in the purified form, and is much su¬
perior to the crude gluten for most purposes. Zein is soluble in alcohol
and may be dispersed in this medium when used as a protective coating, but
alcoholic solutions are not practical for pigment coating. The best dispers¬
ing agents for zein in aqueous media are sodium resinate, sulfonated castor
oil, and sulfonated tall oil. Chemically modified zein of low viscosity is
avaliable commercially. It is dissolved in water to which are added oleic
acid, dry rosin size, and an alkali such as sodium hydroxide or ammonia.
When ammonia is used, the film is more water-resistant than when sodium
hydroxide is used.- James- found that pigment coatings made from zein
are not sufficiently water-resistant for lithographic papers, and recom¬
mends’’ the use of a hea\w metal salt (e.g., lead acetate) to add water re¬
sistance to the coating. Coatings made with zein cannot be tested by the
usual wax test, because zein is sufficiently thermoplastic to bond to the hot

wax.

Polyznnyl Alcohol

One of the most promising of the synthetic resins for pigment coating
is poKwinyl alcohol. Except for its high cost, this material is almost an
ideal pigment coating adhesive. It has an exceptionally high pigment bond¬
ing strength (see Fig. XVIII-2), and this fact helps to offset the cost dis¬
advantage, since less adhesive is required in the coating formula. For most
types of coated papers, 3 to 8 parts of pol}winyl alcohol per 100 parts of
pigment are sufficient for good-bonding strength.

« J. W. Gose, U. S. 2.284,800 (June 2, 1942)

»» C. R. Keim, Paper Trade J. 129, No. 21; 464-465 (Nov. 24, 1949)

A. L. James, Paper-Maker 104, No. 4: 148 (Oct., 1942)

A. L. James, National Lithographer 50, No. 1: 58 (Jan., 1943)

1030

PULP AND PAPER

Polyvinyl alcohol is made in two different grades (see Ch. XI). One
grade dissolves fairly readily in cold water, whereas the other must be
heated to 70 to 85° C. to effect complete solution. Both types can be ob¬
tained in high, low, or medium viscosity. The tj-pe requiring heating is the
one generally used for pigment coating work. In the process of dissolution,
dry poljwinyl alcohol should be added to cold water (20 C. or lower) and
stirred until all lumps are broken up and a smooth mixture is obtained. In
some cases, this may require considerable stirring. Once the dispersion is
smooth, it may be heated with constant stirring at a temperature of 70 to
85° C. until a clear solution is obtained. The solution can then be cooled

and added to the pigment in the regular manner.

Polyvinyl alcohol does not produce a water-resistant coating. Water-

resistant coatings may be obtained by adding dimethylolurea or urea-formal¬
dehyde resins in a manner similar to that described for starch.^® In gen¬
eral, polyvinyl alcohol is more easily waterproofed than starch. About
10 to 207 o dimethylolurea based on the weight of the alcohol is sufficient to
produce a fairly high degree of water resistance where a catalyst (hydro¬
chloric acid or ammonium chloride) is used, and the coated sheet is heated
at approximately 200° F. for twenty to sixty seconds. Ammonium chloride
should be used cautiously, inasmuch as it has a tendency to thic en e
coating. Some people prefer to add the resin and catalyst to t e pigmen

before the adhesive is added.

Cellulose Derivatives

The water-soluble cellulose derivatives, such as methylcellulose the

sodium salt of glycolic acid ether of “ ^^ting’

and hydroxyethylcellulose, have been used as adhesives in PS ^

Methylcellulose has a high bonding strength (see Fip XVIII )
produLs iW flexible coatings. It has a high degree o ~ to “il

and consequently its use is indicated when a high

ink varnish or lacquer is desired.^* Methylcellulose is soluble in cola

ter,’ but is best dissolved by soaking for twenty to thirty ^

amount of boiling water and then adding the remaining amount

"dtei (see Ch. ^ai ,„ethylcellulose as a coahng adhesive is

its high vLosity. The product is available commercially m a number^^^

different viscosities, but even the lowest viscosity gra e ...nared bv

high solids coating mixtures. Water-resistant coatings can be p P

Bulletin, -Polyvinyl Alcohol," E. I. du Pont de Nemours i Co., Wilmington.

Delaware (1946) , p r Peterson Paper Trade J. ID, No. 2: 16-20

33 R. M. Upright, M. Km and F. C. Peterson, raper

(Jan. 8, 1942)

XVIII. PIGMENT COATING

1031

mixing with urea or melamine-formaldehyde resins m a manner similar to

that described for starch. .

Carboxymethylcellulose has been used as a coating adhesive. It has a

higher pigment bonding strength than methylcellulose (see Fig. XVIII-2).
It is best dissolved by adding to water at 60 to 70° C. with vigorous agita¬
tion (see Ch. XI).*

Hydroxyethylcelhilose is a water-soluble cellulose ether which can be
prepared by treating alkali cellulose with ethylene chlorohydrin or with
ethylene oxide. The product is fairly soluble in dilute aqueous solutions
of alkali up to a relatively high degree of substitution because of the presence
of hydroxyls in the substituted group. Alkali-soluble grades of hydroxy-
ethylcellulose have been suggested as pigment coating adhesives in a novel
process.®^'In this process, hydroxyethylcellulose is dissolved in dilute
sodium hydroxide and mixed with the pigment. Immediately after coating,
the wet coating is brought into contact with a dampening roll which applies
a solution of a precipitating and neutralizing salt, such as ammonium sulfate.
This precipitates the hydroxyethylcellulose from solution by exchanging
ammonium ions for sodium ions, thus forming sodium sulfate and am¬
monium hydroxide. Since the cellulose derivative is not soluble in am¬
monium hydroxide, it is immediately precipitated from solution. The paper
then passes through a set of squeeze rolls which remove the excess salt
solution, together with the excess water. The ammonium hydroxide is
evaporated during drying of the paper, leaving a substantially neutral coat¬
ing. When applied in this way, the pigment bonding strength of hydroxy¬
ethylcellulose is very good, and the coating has a high degree of water
resistance and excellent flexibility. The finished sheet is level and needs
no calendering. A suitable formula is

93 parts clay; 40 parts of water (^H 10.25 with NaOH).

2.5 parts NaOH, 14.5 parts water.

7.0 parts hydroxyethylcellulose, 88.0 parts water, 5.0 parts NaOH.

The final coating mixture contains 5% sodium hydroxide on the total
weight of water. There is 7% binder on the basis of the pigment solids,
and the total solids content of the coating mixture is 40%.

Synthetic Resin Latices

Synthetic resin latices have been used to an appreciable extent in paper
coating. Latices are generally not practical as the only adhesive, but they
are useful in blends with other adhesives.

A latex of styrene-butadiene copolymer (sold by the Dow Chemical
Company under the trade name of Dow Latex 512-K) has been widely used

Paper Mill News 72, No. 19: 12-14 (May 17, 1949)

35 D. R. Erickson, Paper Trade J. 128. No. 25 : 243-244 (June 23 1949)

35 D. Robert Erickson, Tappi 22, No. 7 : 289-291 (July, 1949)

1032

PULP AND PAPER

in paper coating since 1946. This material is sold in the form of a milky
aqueous dispersion containing 45^ solids. It is an anionic latex with a
/>H of 9.0 to 10.5. It is compatible with casein, glue, soy flour, soy protein,
and other proteins. Although it is not completely compatible with starch,
it works well with starch if it is first stabilized with a small amount of ca¬
sein, about 3% on the latex solids. Originally, the latex was used only in
oft-machine coating with casein coating mixtures, but some latex has been
used in machine coating. To work satisfactorily in high-speed roll coating,
the latex must have a high degree of stability to withstand the high shearing
stresses. The latex cannot be used with satin white and certain grades of

English clays, but it works very well with domestic clays.

Synthetic resin latex improves the smoothness, gloss, flexibility, an
wet rub resistance of the coating. It acts as a permanent plasticizer for
other adhesives and improves their flexibility, water resistance, and calen¬
dering properties. The wet rub resistance and strength of the coating im¬
prove with age, and there is very little loss in flexibility, whic ^is a_ e me
advantage over the results obtained with natural rubber latex. The p g
ment bonding strength of the latex is high, although it cannot be m^sured
by the wax pick test, due to the thermoplastic nature of the resin. g
eL, a replacement of 25 to S0?o of the regular adhesive with latex will give

the desired results.

Blending Adhesives

A number of different adhesives are suitable for paper coating. Be

cause each adhesive has its good points and each has its ^

have been made th develop suitable blends of two or more adhesives

will combine the good properties of each j r»sem and starch

Many attempts have been made to develop a blend of

which would combine the low cost of starch and t e 2°° protein and

of casein. Nearly all these uniform

starch adhesives are incompatible. It is quite aimc

a”»■

r “"‘Sri .1 'b.

or nothing toward increasing the water the adhesive

of the starch, unless it is present in ig en » , „£ e^sein to a starch

is essentially all casein. The addto of a stnaH amount oj ca
coating mixture reduces the bonding strength Mixtures o
Sn and starch behave similarly to mixtures of a"d aprfng

The mutual incompatibility of starch and casern ''_as
World War II in the preparation of a special coa mg for sh.pp »

S> E. K. Stilbert, R. D. Visger and R. H. Lalk, Tappi .

1950)

XVIII. PIGMENT COATING

1033

used for the packing of synthetic rubber. The rubber, as shipped in those
days, was in a very tacky condition and tended to pull fibers from the side of
the container. This fiber acted as a serious contaminant, since it interfered
with the subsequent processing of the rubber. A small amount of pigment
or adhesive in the rubber was not considered harmful, and consequently
coated papers were tried, but these, too, were unsatisfactory because the
tacky rubber would pull loose not only the coating, but also some of the
underlying fiber. It was found, however, that by precoating the paper-
board with a starch solution and then applying a clay-casein coating over
the dried starch film, a coating was obtained which would rupture between
the starch layer and the clay-casein layer and not pull any fiber from the
board. The same effect could have been produced by using any two adhe¬
sives which had a low degree of compatibility.

There is a modern trend in pigment coating toward double coating, i.e.,
the use of two or more coatings on the same side of the paper. Different
adhesives are sometimes used in the separate coatings, but this poses the
problem of finding compatible combinations.

Many combinations of the common adhesives are not truly compatible,
but there are a few combinations which have enough merit to warrant their
use. Soy flour, for example, is compatible with both casein and soybean
protein, and was widely used during World War II as an extender with
both these products. It was found that up to 25 to 30% of the total binder
could be replaced with finely ground soy flour without reducing the strength
of the coating or seriously detracting from its other qualities. Animal glue
and casein have also been used together in the manufacture of special grades
of coated papers.

Methylcellulose-starch and polyvinyl alcohol-starch have been sug-
•gested as possible blends in order to combine the desirable properties of the
synthetic resin with the low cost of the starch. The synthetic resins con¬
tribute film-forming power, toughness, superior adhesive strength, and
flexibility to the coating, and in some cases improve the flowing properties
of the coating mixture. Starch is incompatible with methylcellulose, how¬
ever, and consequently mixtures are not satisfactory, although newer prod¬
ucts of improved compatibility with starch have been produced. Blends of
carboxymethylcellulose and starch offer some promise because of their
better compatibility.

Coating Pigments

The pigment is a highly important component of coated paper. The
principal functions of the pigment are to fill in the irregularities of the paper
surface, to produce an even and uniformly absorbent surface for printing,
and to improve the appearance of the coated sheet. In one sense, the ad-

1034

PULP AND PAPER

hesive is only of secondary importance, since its principal function is to bind
the particles of pigment to each other and to the paper surface.

A pignient must have certain physical and chemical properties in order
to be suitable for paper coating. A good pigment should possess all or
most of the following properties; good dispersibility in water, correct par¬
ticle size distribution, high opacifying power, high brightness, low water
absorption, non-abrasive qualities, chemical inertness, compatibility with
other ingredients of coating mixture, low adhesive demand, and, if colored,
a high tinctorial power and color permanence. Because the qualities of the
coated paper depend to a large extent upon the pigment, it is highly impor¬
tant to choose the proper pigment or pigment blend for the coating formula.

Clay accounts for approximately 90% of the pigment used in paper
coating, but a large number of other pigments are used m smaller quan¬
tities Many grades of coated paper w'ould be impossible to produce with¬
out the use of these other pigments. The principal pigments used in paper
coating, aside from clay, are titanium dioxide, precipitated calcium ^r-
bonate, water-ground calcium carbonate, calcium sulfate, calcium sulfite,
barium sulfate, satin white, and zinc pigments. The properties o t ese
various pigments and the methods of preparing the pigment slurry are dis¬
cussed in the following sections. An attempt has been made in Table
to list the common coating pigments, together with the outstanding proper¬
ties of each. ^

TABLE III

Outstanding Properties of Common Coating Pigments

for printing ink

1. Medium-grade clay—general body
^ High-grade clay—finish

3. Precipitated calcium carbonate—opacity, brightness, a ni >

4. Water-ground calcium carbonate—^brightness, dull ms

5 Titanium dioxide—opacity, brightness ^

6. Blanc fixe— weight, chemical purity (photographic papers)

7. Zinc oxide—opacity

8. Satin white—finish, brightness

C/ay

Clay is the most important single pigment used
rhvs have long been used as filling or loading agents, but uith

years, their use as coating pigments has from

’ In the past, clays varied considerably m their
the same producer. There is still some variation, but day
are seeking new ways of improving the uniformity of thei^ pro

development of machine coating has been im¬
provements made in the physical properties of clays, ^ *.s ope

poses more exacting requirements than were former,

XVIII. PIGMENT CO.ATING

1035

Nature of Raw Material. The term “clay” is used in different indus¬
tries to refer to a variety bf minerals which differ widely m chemical an
physical properties. In the paper industry, the term “clay” generally refers
to a naturally occurring hydrous aluminum silicate, known also by the
names of kaolin or china clay. Kaolinite, which has the formula
AUO. • 2SiO. • 2H,0. or AU(Si.O»o) (OH)^ is considered as the parent
substance of kaolin, but impurities such as iron oxides, mica, bauxite, cal-
cite, and unchanged feldspar are also present. Other important clays are
those belonging to the illite group, in which some of the alummiim has
Ijeen replaced bv iron and magnesium, and the montmorillonite group
((()H)*AUSi,0» XHjO), in vyhich part of the aluminum has been re-

place<l by magnesium and ferric oxide.

Small amounts of montmorillonite are used in the paper industr>% par¬
ticularly for deinking, improving the retention of fillers, and eliminating
rosin s{x>ts in pai)er. Bentonite (vol clay) is composed of several members
of the montmorillonite group; it is formed from the devitrification and
panial decomposition of glassy volcanic ash and is found principally in
Wyoming. It is a highly colloidal clay. Kaolinite sometimes contains
small amounts of montmorillonite. but this is considered as an objection¬
able contaminant liecause of its adverse effect on the flow properties of the
clav. Much of the fundamental work done on clay has been done on mont¬
morillonite ty|>e clays, rather than with kaolinite, although kaolinite clays
are much more imjiortant from a pa|>ermaking standpoint. The reader
should \ic cenain of the distinctions l)etween these two types of clay. Ka¬

olinite clays come mostly from England and from the southern part of the
United States, particularly Georgia and South Carolina. Domestic clays
are now used to a far greater e.xtent than imported clays.

Clavs are fonnetl by the reaction of a weak acid (silicic acid) and a
• ^

weak lose (aluminum hydroxide), which are derived from the parent
nxrk as a result of weathering. The silica and alumina groups combine with
each other in various projjortions to form a clay particle. Almost any ratio
of liasic to acidic constituents mav result. In kaolinite, the ratio of silica
to alumina is always close to 2 of SiOj to 1 of AI 2 O 3 , whereas beidellite
f member of montmorillonite group) contains approximately three silica
to one alumina, and Ijentonite (also a member of montmorillonite group)
contains about si.\ silica to one alumina. Hence, clays have variable chemi¬
cal and physical composition, de|)ending upon their source. Chemical anal¬
ysis can be made on clay by dissolving the clay in sulfuric and hydrofluoric
acid.s, or by fusion with alkali. An approximate analysis of Georgia (ka¬
olinite) clay is given in Table IV.**

*• Kanlin Qays and Their Industrial Uses,” J. M. Huber 0)rp,, New York
N. Y. (19491

1036

PULP AND PAPER;.

TABLE IV

Chemical Analysis of Typical Georgia Clay

AUO 3 . 39.42

SiO* . 44.32

Fe203 . 0-67

TiO* . 1-29

CaO . 0-68

MgO . 0-21

H 2 O (combined) . 13.95

Some clays have a lower pH than others, that is, they are isoelectric
at a lower pH than other clays, because of a higher ratio of acidic to basic
groups in the clay particle. Thus, beidellite is more acid than kaolinite
because it contains a higher proportion of silica. The pH of domestic
(Georgia) clays is usually between 4.3 to 7.0. The pH is generally higher
the lower the concentration of the clay suspension, but remains reasonably
constant at concentrations over 25The acidity of clays, which is de¬
termined by titrating a suspension of the clay with 0.1 iV sodium hydroxide
using phenolphthalein as the indicator, is important, since clays having a
high acidity cause trouble by thickening protein adhesives, such as casein

or soybean protein. 1 .u* v.

Clays are classified into two types, residual or primary clays, whic^

remain where they were formed, and transported or secondary clays, w ic
are eroded and redeposited in another locality. The Georgia kaolinite clays
are sedimentary clays, whereas the English clays are of primary origin. As
a rule, the primary clays contain a relatively high ° ^ ’

and other foreign materials which must be removed before the ^^y s ^
able for papermaking. On the other hand, secondary c ays ar

free from errit Eticl micE. . «

The quality of the final clay depends to a considerable exten upo

purity, composition, and uniformity of the clay deposit, ^ ^ Weoosits

L Ts posJble from grit, mica, and other impurifes. Many chy depos.ts

are unsatisfactory because of the presence of impunties. F

qnarta is the commonest impurity, and .t .s .rouhlesome

large particles, as well as small grams of silt. A q
impurity is iron, which tends to lower the reflectance of clay.
tent of clay generally varies from 0.15 to 2.0% expresse as^^^

the weight of the clay, and this is mostly dioxide

American kaolinite clays usually contain sma amoui . ‘ manganese,

(about 2-4%). Other impurities are calcium, magi

.sodium, and potassium.

80 “Kaolin Clays and Their Industrial Uses,” p. 104, J. M. Huber orp.,
York, N. Y. (1949)

XVIII. PIGMENT COATING

1037

Methods of Processing Clay. Clays are mined by both
and underground methods. In the open-face method, the overburden
:rtpped oland the clays are mined by means of power shovels or scrapers.
Clays are processed by (I) dry process, or air separation, « (2)
ess or classification in water suspension, the latter giving the mo

pletCirefining^y it is first precrushed and then finely

ground. Air flotation using a whizzer separator removes impurities w^ ic
are highest in specific gravity, but there is no essential change in t ^

size or brightness of the clay. Air-floated clays contain about 1 27o mois¬

ture and are light and fluffy in appearance, although some air-floated clays
are treated with water to form small pellets; they are generally shippe
in 50-lb. paper bags or in bulk. Air-floated clays are used for mg o

paper, but are not suited for paper coating. . j •

In the wet process, the clay is suspended in water, treated with a dis¬
persing agent, and then is passed through separators or continuous centri¬
fuges where it is classified into fractions of controlled particle size range.
The brightness is often improved as much as three points by fractionation.
The clay is then flocculated and separated from the water by settling and
filtering, and finally by drying. The clay is sometimes bleached with zinc
or sodium hydrosulfite to reduce the ferric iron to ferrous iron and produce
a whiter product. Bleaching increases the brightness of the clay by 2 to 7
points, making a total possible increase of about 10 points in brightness on
refining. Dryii^ of the clay is an important step, since overdrying tends
to produce a clay which is hard to redisperse in water. Most of the English
clays and many of the high-grade American clays are water-processed. The
products are used in paper coating and, to some extent, for filling. Ordi¬
narily, water-processed clays contain from 3 to S% moisture. The lump
type washed clays have a higher density than air-floated clays and are gen¬
erally sold in 100-lb. paper bags or in bulk. Special pulverized washed clays
are produced; these have a lower density and are generally sold in 50-lb.
paper bags or in bulk. Water-treated clays have higher brightness, lower
screen residue, reduced abrasiveness, and more controlled particle size
compared to air-treated clays.

Clay is sometimes sold in slurry form at a solids content of 70%, using
insulated tank cars for transporting. Clay in this form is desirable for paper
coating, because the physical properties of the clay particle have not been
affected by the high temperatures normally used in drying the clay. This
results in coatings of higher finish and less tendency toward dusting, and
increases the life of the calender rolls used in calendering the coated paper.

Properties of Clay. Clay has a specific gravity of about 2.57 to 2.63.
The index of refraction is about 1.56, although clay has been reported to

1038

PULP AND PAPER

contain another component or components of much higher index of re¬
fraction.

Most clays do not exceed a brightness of 85%, and certain clays have a
brightness as low as 65%. Recently, however, improvements have been
made so that clays are now available up to a brightness of 95%. Most clays
are specified as to brightness. In making a brightness test on dry clay, a

sample passing a 200-mesh screen is compacted in a metal cylinder under
uniform pressure of 30 to 60 p.s.i.'*®''*^ A thoroughly cleaned glass plate is
placed upon ordinary bathroom scales, and the metal cylinder is placed on
top of the plate. About 15 g. of dry clay are added to the cylinder and, with
the cylinder held firmly against the plate, a plunger is inserted and forced
downward to give the desired pressure on the scales. The plunger is then
removed, the cylinder is lifted from the plate, and a brightness reading is
made on the smooth, exposed surface of the compacted clay at a wavelength
of 457 millimicrons. The moisture content of the clay affects the reading.

since there is a progressive loss in brightness as the moisture is increased
from 2 to 10%.^^’^^ For this reason, the clay should be carefully dried
before testing at temperatures not over 105° C. so that the moisture content
is below 1.5%. A study (made by Edgar Brothers Co., McIntyre, Georgia,
in 1948) of the testing of clays for brightness in a number of different lab¬
oratories has shown that the probability of missing the true brightness at
the 10% probability level is 1.12 points, and at the 5% level, is 2.25
if the clay is not pulverized before testing. If a pulverizer is used, the
values are reduced to 0.80 point at the 10% probability levei and 1.61 points
at the 5% probaliilit}^ level. It is recommended that brightness tests

made in triplicate to obtain worth while precision.

If the moisture content is too high, clay does not flow properly an
comes difficult to handle. If it is too dry, clay dusts badly. Ihe
content should be taken into account in calculating the solids content ^
high solids coatings. The moisture content of clay may be determine
(7) toluene distillation, (2) by electronic measurements based
lationship between moisture content and electrical capacity,
a definite weight of clay in a definite weight of distilled water, t g ^
a small amount of pyrophosphate and determining he specific grauty
with a .sensitive hydrometer, or (4) by oven drying a 100 to 05 U

cause clay tends to lose moisture slowly, it is desiia ie o content

at least ten liours. C lay loses water above its normal mois ^
when heated at temperatures over 105° C. Ihe amount o wa

-•0 1. L. Lester and S. C. Lyons, Pal>cr Trade J. 125. No. 17: 189-195 (Oct.

41 N. Millman, Paper Trade J. 127,^ No. l^M^ Hube^ Corp.. New York,

42 “Kaolin Clays and Their Industrial Uses, p. 97. J. M. Hube P

N. Y. (1949)

XVIII. PIGMENT COATING

1039

pends upon the type of clay and the temperature as shown- m Figure

XVIII-4.«

■m

Fig. XVIII-4. Effect of heat on water loss of kaolinite and halloysite.

Ion Exchange Properties of Clay. The free acidic and basic residues
on the surface of the clay particle are free to dissociate into ions or to react
with ions in the surrounding medium. In the natural state, varying amounts
of ions, such as Ca"^, Mg^, K^, and Na'^, are adsorbed on the surface of the
clay particle. Surrounding the clay particle is a micellar atmosphere con¬
taining adsorbed and unadsorbed ions which bring the clay particle into
equilibrium with the surrounding medium. This forms a diffuse double
layer about the clay particle, as shown in Figure XVIII-5. This layer has

Free water

►

Counter charges

+ -f

Fig. XVIII-5. Showing diffuse double layer surrounding
clay particle (courtesy Edgar Bros. Clay Co.).

an important effect upon the hydrating properties and flow characteristics
of the clay.

Government Printing Office, Paper 185-6,
1934, p. 140 f ,

1040

PULP AND PAPER

The colloidal properties of clay are due to the colloidally active surface
layer surrounding the more or less stable core. Thus, the colloidal prop”
erties of the clay depend as much upon the activity of the clay surface as
upon the particle size. It is conceivable that a clay with a large particle
size might exhibit greater colloidal properties than a smaller particle size

clay, due to the greater activity of its surface.**

The firmness with which adsorbed ions are held by clay follow s the

following decreasing order: Al^, Fe-", H*, Ca-, K% and Xa* for cations,
and OH-, PO4—, SO4-, and Ch for anions.*® By nature, clay adsorbs
hydroxyl groups more strongly than hydrogen atoms; hydroxyl ions are

Fig. XVlII-6. Effect of pH on the exchange capacity of claj.
not displaced except at extremely low pH. “

to a clay suspension both the however, the

with equal firmness (at the correct pH) , it AabU4 a ,

sulfate ion is adsorbed more strongly ^ particle ad-

The pH of exchange neutrality is the p neutral salt Below

sorbs an equal number of cations and amons ^ress of

this ?H, an excess of aruons is “^sorbed a „eLality for ordinary china
cations is adsorbed. The of e>.\^

clav occurs at a pH less than pH , ^ ^ exchange

properties of clay are of little practical H

«W. W. Meyer. /. Research Na,l. Bur. S.andaris 13. No. 2. 24

45 Albert, Tra,<s. Electrochem. Sac. 73: 173-182 (1938)

XVIII. PIGMENT COATING

1041

neutrality and the exchange capacity of an electrodialyzed clay in equihb
nun. with sodium chloride is shown in Figure XVIII-6, which rvas taken

from work by Meyer.^* . ,

The composition of clay, i.e., the ratio of acidic to basic residues, de¬
termines the ion exchange characteristics. A clay with a high percentap
of acidic residue tends to have a high exchange capacity for cations In
general, the exchange capacity varies from 15 to 100 milliequivalents of ex-
changeable cations per 100 g. of clay.^* This depends upon t ie t)pe o
clav, as shown in Table which gives the base exchange capacity of a
number of clavs as reported by Grim.‘* Hauser- points out that bentonite
undergoes base exchange in the interior of the particle and hence is not
affected by fineness of the particle. In comparison, the base exchange of
kaolin clays, which have a dense structure, depends mainly on the amount
of external surface. Thus, the base exchange properties of kaolinite would

be expected to increase with fine grinding.

TABLE V

Base Exchange CAPAarv of Different Clays

Milliequivalents

Clay per 100 g.

JktontmoriUonite . 60-100

illitc . 20-40

KaoHnhc . ^15

The higher the ratio of acidic to basic residues in the clay, the lower
the isoelectric pH of the clay, w'hich means that more hydrogen ions will
be required in solution to suppress the ionization of the hydrogen atoms on
the surface of the particle.

Hydration of Clay. Water is held by clay in three ways: (2) water
of combination held in the fonn of hydrous oxides; (2) water of hydration
held on the surfaces of the clay particle by molecular attractions; (3) capil¬
lary, or free, water held in the fine capillaries of the clay particle by surface
tension. Kaolinite contains two molecules of chemically combined water,
or about 13.96%, whereas halloysite seemingly has four molecules, even
though they both have the same ratio of alumina to silica.

The amount of water of hydration increases as the particle size de¬
creases, since this increases the specific surface. The thickness of the water
layer about the clay particle is dependent upon the base exchange capacity

**W. W, Meyer, /, Research Natl. Bur. Statidards 13, No. 2: 245-258 (Aug.,
1934)

«E. A- Hauser, Paper Trade J. 105, No. 7: 111-113 (Aug. 12, 1937)

••R. E. Grim, “Modem G)ncepls of Qay Materials.” p. 245, Report No. 80.
Dept of Registration and Education, Urbana, Illinois (1942)

1042

PULP AND PAPER

of the clay and the number and type of adsorbed ions/” The clays which
are most active in base exchange exhibit the greatest hydrating character¬
istics. Thus, kaolinite has low hydrating properties because of its low base
exchange, whereas illite and montmorillonite are more hydrous than ka¬
olinite because of their greater base exchange capacity.®” Rowland®' con¬
siders the hydration capacity of the clay as one of its most important
properties, because it affects the water-holding capacity and also the flow

characteristics of the coating mixture.

Meyer®^ has shown that substitution of an ion Iwwer in the Hofmeister,
series (e.g., calcium) for one higher (e.g., sodium) reduces the amount of
imbibed water. In other words, calcium ion has a lower hydration capacity
than sodium ion, and consequently the substitution of calcium for sodium
reduces the hydrous nature of the clay. According to Meyer, certain clays
may imbibe a layer of water up to 23 to 45 millimicrons in thickness when

treated with sodium.

The crystal sheets in a highly hydrated clay are held together less
firmly than those in a slightly hydrated clay. Consequently, even weak
shearing forces may cause the crystal planes in a highly hydrated clay to
slide over one another and result in a subdivision of the clay particle. This
may occur very readily wdth montmorillonite clays, but is probably not
realized in the case of the less cleaval)le kaolinite clays. According to
Grim,®® the ease of subdivision of a hydrated clay is due to the thin films of
oriented water molecules which adhere to the surface of the crystals and act

as a lubricating film.

Clays lose their hygroscopic moisture at 100° C., and lose their chenn-
cally combined water at about 500° C. The eiifect of heating on halloys.te
and kaolin is shown in Figure XVni-4. Heating clay at very high tem¬
peratures is believed to bring about an irreversible dehydration and ag-
Ugation of the clay particles,“ resulting in less freedom of movement for
the crystal layers. Such calcined clay has much lower hydrating properties

than the original clay. This anhydrous type of clay
sibilities for paper coating because of its high brightness (90-92) and lo
gloss characteristics, but i.nfortimately it is extremely abrasive, and coat mgs
containing it are marked easily on contact with metal, and they wear pr
iug plates at an excessively high rate.

4(»w. W. Meyer, J. Research Natl. Bur. Standards 13, No. 2. 245 258 (A g.

so cf H. Sheets, Paper Trade J. No. 3: 1941)

B. W. Rowland, Paper Trade J. 112, No. 26. 311 313 (J G45-258 (Aug.

62 W. W. Meyer, /. Research Natl, Bur. Standards 13, o.

63 R E. Grim, J. Am. Ceram. Sac. 22, No. 5: Nn^3- 25-28 (Jub’

64 h! W. Rowland and H. J. Allison, Jr., Paper Trade J. 111. No. 3. 25

18, 1940)

XVIII. PIGMENT COATING

1043

Shape of Clay Particles. Alumina and silica are combined in clay to
form a crystal which is monoclinic. The individual layers are arranged in
the form of hexagonal plates. Rowland and Allison®^ visualize a clay par¬
ticle as similar to a pack of cards where the individual cards are, under cer¬
tain conditions, free to slide over one another. ?^early all elementary pa-

Fig. X\'III-7. Kaolinite clay (10,000x) (courtesy Institute of Paper Chemistry).

hig. X\ III-8. English clay (ll.SOOx) (courtesy Institute of Paper Chemistry).

permaking clays^ are thin-plated,although MarshalF® believes that clay
particles below 500 millimicrons are not so markedly plate-shaped as the
larger particles. An electronmicrograph of the finer particles in kaolinite is
shown in Figure X\ 111-7, where the typical hexagonal shape is quite ap¬
parent. Figure XVni-8 is an electronmicrograph of an English clay. The

**194^) ^ ^ Kregel. Paper Trade J. 114, No. 12: 139-145 (Mar. 19,

®*C. E. Marshall, J. Soc. Chem. Ind. 50: 444-450 ; 457-462 (Dec. 4, 1931)

1044

PULP AND PAPER

gloss-producing qualities of clays result primarily from their plate-like
structure.

Particle Size of Clay. The particle size is an important property of
clays, since it determines the opacity, the brightness, the ink receptivity, the
adhesive demand, and the flow properties. Particle size has become of in¬
creasing importance in recent years. Originally, screen or sieve tests were
used for measuring particle size, but this method no longer gives enough
information because of the large size of the openings in the screens
(325-mesh screen having openings equal to 44 microns, 100-mesh screen
having openings equal to 149 microns, etc.). Screening through a 325-mesh
screen has been widely used in the past to determine the amount of grit,
mica, or trash in clay, and this test is still used somewhat with interior clays
which contain a considerable amount of mica. An acceptable clay should
contain less than 0.02% foreign matter on a 200-mesh screen. However,
screen analysis is no longer an adequate criterion of the quality of paper-
laking clays, and newer methods of particle size analysis have been

‘^^’’^sStaientation methods constitute the most practical means of measur-
hm oartide size. Sedimentation curves can be made by plotting the weig
ot°v;gment settled out against the time of settling. High-speed centrifuges
mav be used to speed up the settling. The radius of the particles can then
calculated accoaing to the well-hnown Stokes etjuation, excesses

Cuirlccurate down to a particle size of about 1 micron, n maknng s^
mentation studies on clay, the clay is first deflocculated and then centrifuge
.-It low solids The sediment is collected m calibrated glass tubes an

volume read at selected time intervals. Another "

iviri icle size is bv taking readings on a clay suspension at re^,

with the' Botiyoucas hydrometer, a specially designed hydrometer winch

calibrated to read the grams of material m suspension.

I, should be pointed out that particle size ™ basis,

means very little unless all nieasurcments are made on P‘

Thus, the reader should use discretion m ‘ comparable

reported lor clays or other pigments unless he is ce . . 1

oil., of analysis have been used. The value o particle s.z^^^^^

from scdimenlalion methods are not absolute v.a , .

;,rd:=

r.r “Kaolin Clays and Their Industrial Uses, pp. 99 100, J.

York, N. y. (1949)

XVIII. PIGMENT COATING

1045

distribution curves (e.g., curves based on sedimentation methods) can be
used. The fonner method stresses the fine particles in the mixture, whereas
the latter method stresses the coarse particles. In general, a pigment which
has most of the particles in the optimum range will have greater hiding
power, reduced light absorption, and greater over-all reflection than a pig¬
ment containing some large and some small particles.

EQUIVALENT SPHERICAL DIAMETER IN MICRONS

Fig. XV'III-9. Particle size range of several commercial
papermaking clays (courtesy Edgar Bros. Clay Co.).

COAT WEIGHT, lbs. per reom (one side)

Fig. X\ III-IO. Effect of particle size on the covering power of clay (brightness vs.

coat weight.)** Brightness of raw stock 60%, of clays 85%.

Commercial kaolinite clays do not contain much material in the range

commonly accepted as colloidal.*® Marshall®^ states that English china

clays contain practically no particles less than 0.1 micron in diameter.

»*D. F. Wilcock, Ind. Eng. Chem. 33, No. 7 : 938-940 (July, 1941)

‘•L. T. Work and I. H. Odell, Ind. Eng. Chem. 25, No. 5: 543-549 (May 1933)
••C G. Albert Trans. Electrochem. Soc. 73: 173-182 (1938)

** Marshall, Ceram. Soc. Trans. 30: 81 (1931)

1046

PULP AND PAPER

Sometimes clays are graded for convenience into coarse, intermediate, and
fine, depending upon the range in which the majority of the particles fall.®*

The range is usually as follows;

Grade clay
Coarse
Intermediate
Fine

Majority of particles

Over 10 microns
Between 2 and 10 microns
Less than 2 microns

•

Filler clays contain some relatively coarse particles (about 40 to 70
microns in diameter). Coating clays generally range from about 30 mi¬
crons as an upper limit to about 0.1 micron in sue. /O to 90/c of P”'
ticks being smaller than 2 microns. Machine coating clays are graded for
particle size and generally contain a greater proportion ot smaller particks,
L range being from 8 to about Od micron. Particle ^ze <l'»on
curves for several different commercial clays are shown in Figure X .
Regular coating clays generally contain 709b finer than 2 microns an ma-
chL coating clays usually contain 80% finer than 2 microns.

The proper size for coating clays depends on the faults to be •

The thickness of the dried coating on the average coated book paper ( 5-lb.
Lat£ is neighborhood of 12 to 15 microns, so that obviously no

ImeS particle should, be greater than this in diameter. On the othe
tod pipnents which are too small would increase the adhesive demand
1 ’ A reason It would appear that a pigment composition containing

X. " “■ “is" “

1 u,. ™ost desirable composition from a coating standpoint, do some

toe? ro^timum pdrtick size- depends upon the weight of coating

p'^'rticle size has a very definite effect on the covering power bright-

r 1 naners The effect of particle size of clay on

ness, and gloss of clay^coa p p . j p^per is shown

covering power as measured by the brightness of » p P
in Figure XVIII-10 for two grades of coating clay nating

particle size distribution.®*

Diameter,
microns

Above 10
5-10
4-5
3-4
2-3
1-2
0.5-1
0-0.5
Finer than 2

Clay 1. %

0.0
3.3
2.8
4.8
8.6
15-4
17.2
47.9
80 5

Clay 2, %

1.0

6.6

4.2

6.8

9.4

15.3

16.0

40.7

72.0

^Kaoim^Sayfand Mr' Ind" York

63 “Kaolin Clays
XT Y. 0949)

XVIII. PIGMENT COATING

1047

It can be seen from Figure XVIII-10 that the finer clay has the higher
brightness. This is due partly to the natural higher brightness of the finer
clay and partly to the greater number of air interfaces arising from the
greater surface area. Fine clays produce coatings of higher gloss than
coarse clays, clays of particle size less than 2 microns producing the maxi¬
mum gloss. For the two clays used in the above example, clay No. 1 (the
finer clay) produced a gloss of 39 compared to a gloss of 37.5 for clay

2.®® However, fine clays do not stand calendering as well as coarse
clays and are more likely to produce blackening on calendering.®* Where
gloss is of primary importance, it is undesirable to have more than 5^ of
the particles over 2 microns.

Dispersing Clay in Water. In order to prepare a satisfactory coating
mixture, it is necessary to reduce the clay to the proper state of subdivision.
Most clays are received at the coating mill in the dry form, and they must
first be mixed with water and then agitated. Good working or flow proper¬
ties of the coating mixture are dependent upon the proper dispersion of the
clay. The properties of the final coated sheet are also enhanced by dis¬
persing the pigment as completely as possible. Many coating mills use
“dough” mixers, mechanical dispersing equipment such as ink mills, pebble
mills, or colloid mills, but these are generally used on the coating mixture
after the adhesive is added and not on the pigment dispersion.

Clays as received commercially are not dispersed into their ultimate
particle size, since the unit particles are held together firmly in the form of
aggregates. Even violent agitation will not break the aggregates down per¬
manently, unless a dispersing agent is used. However, a clay (kaolin)
suspension containing particles all of which were larger than 3 microns was
found to have all the particles reduced to a size less than 3 microns after
milling with sodium silicate.®®

Dispersing agents are essential in preparing free-flowing clay suspen¬
sions at high solids content. The best dispersing agents are sodium silicate,
sodium pyrophosphate, sodium hexametaphosphate, specially purified so¬
dium lignosulfonate, and alkaline casein. Some of the ordinary alkalies,
such as trisodium phosphate, sodium hydroxide, and sodium carbonate, are
effectiv'e, but not so effective as the above. Silicates of high silica content
make better dispersing agents than the more alkaline grades, silicate of the
ratio Na20 • 4Si02 being best. The most effective dispersing agents (e.g.,
sodium silicate, sodium hexametaphosphate, and sodium pyrophosphate)
produce highly fluid suspensions at 75 to S0% solids using only 0.2 to O.S%

65 D ^ 26: 384-385 (June 30, 1938)

1 Trade /. 96. No. 22; 36-39 (June

1048

PULP AND PAPER

dispersing agent on the weight of the clay. Figure XVIII-11 shows the
relative efficiencies of several different dispersing agents. Tetrasodium
pyrophosphate is used quite extensively for clay deflocculation. It is not
so efficient as sodium hexametaphosphate, but it is cheaper. Another ma¬
terial that has found some favor for clay deflocculation is sodium tetraphos-

phate (NasP-iOis). xi

Some clays are difficult to mix into water without balling up. 1 he

best procedure is to add the water and a dispersing agent to the mixer first
followed by the clay. Another method is to add part of the water, followed
bv part of the clay, and then gradually to add more clay and w^ater at inter¬
vals It is desirable that the clay suspension have sufficient viscosity w re
it is being dispersed in order to offer resistance to the mixing blades. It

03 0.4 0.5 0.6

PER CENT DISPERSING AGENT

Fie. XVIII-11. Effect of different dispersing agents on the viscosity
of clay-water suspensions (courtesy Edgar Bros. Clay o.).

1 nresent there will be insufficient rubbing and shearing

)0 much water is present, mere w ^ hr^aWdown in par-

ction between the clay particles to produce e ^ probablv

icle size. Of all the pigments used m paper coatmg, 1 )

hanged more than any other by the work done “P““

hat the greatest changes m milling take^p ace increase the

iig the lowest percentage of dispersing agen . i present to satisfy

. “cosity of the slurry unless enough

ill the newly exposed pigment sur ace w ic is added to the

It makes a difference whether the dispersing g

.ale. ..fare or aller Ih. clay 0. JI ,|j. dirpcrsm.

* ,i;; -

n T r T77 Vo 9- 9&-104 (Aug. 26, 1943)

66 K. A. Arnold. Paper Trade J. Ih, Ao. v.

XVIII. PIGMENT COATING

1049

should be based on the weight of the clay rather than on the amount of wa¬
ter present. The amount of water in the mixture has some effect, however,
since Arnold®® found that the amount of dispersing agent required for
minimum viscosity increases as the amount of water in the slurry is de¬
creased. The correct amount of dispersing agent should be carefully de¬
termined and then rigidly adhered to thereafter in order to maintain a uni¬
form coating mixture. If too much dispersing agent is used, a rethickening
or a flocculation of the clay may result, particularly when using sodium
silicate, which is sometimes quite critical in its effect on the viscosity. If
less than the required amount of dispersing agent for minimum viscosity
is added, the clay suspension may show an intense thixotropic build-up.®^
The mechanism of dispersing agents is not well understood, although
it is generally believed that they increase the acid character of the clay
particle surface, thereby creating more surface activity. The negatively
charged ions of the dispersing agent are probably adsorbed on the clay
particle, thus increasing the negative charge®® and the over-all net charge
of repulsion. In addition, dispersing agents also increase the exchange ac¬
tivity, thereby increasing the hydration by building up the double layer
about the clay particle. Presumably, then, dispersing agents work in two
ways, by increasing the thickness of the liquid film about the clay particles
and also by increasing the charge on the clay particles, both of which reduce
the mutual attraction between the particles. Because of the weaker attrac¬
tion, the particles become more readily mobile, and the clay dispersion has
greater fluidity.

When alkaline dispersion agents are first added to clay, there is no
change in /»H while sodium ions are replacing hydrogen ions on the floccu¬
lated clay particle. After base exchange is complete, the concentration of
free hydroxyl ions increases, thereby resulting in a sudden rise in pH and
a preferential adsorption of hydroxyl ions on the clay particle.®® As a rule,
neutral or acid salts do not make good dispersing agents, because acids are
formed as a result of base exchange. Disodium hydrogen pyrophosphate is
an exception, since it produces deflocculated clay suspensions under fairly
acid conditions. Most deflocculation agents result in a pH of 6 to 7, but
additional alkali (sodium carbonate or sodium hydroxide) up to a pH of
7 to 9 generally results in a further reduction in the viscosity

Clays vary considerably in the ease with which they can be dispersed.
As a general rule, clays of low surface activity yield suspensions of low vis-

G. H. Sheets, Paper Trade J. 116, No. 3; 22-30 (Jan. 21, 1943)

®®/. Phys. Chem. 44, 1-12 (1940)

«»B. K. Asdell, Paper Ind. 29, No. 7: 1035-1042 (Oct., 1947)

^0 “Kaolin Clays and Their Industrial Uses,” Chapter 13, J. M. Huber Corp., New
York, N. Y. (1949)

R. Willets, Tappi 32, No. 4 : 201-208 (Apr., 1950)

1050

PULP AND PAPER

cosity and require only a small amount of deflocculating agent for minimum
viscosity. On the other hand, clays of high surface reactivity tend to pro¬
duce viscous, thixotropic suspensions and require high percentages of de-
flocculating agent for minimum viscosity. In this connection, Sheets'^
points out that less dispersing agent is required for minimum viscosity with
a hydrogen clay than for a clay containing exchangeable cations, because
exchangeable cations tend to react with the anion of the dispersing agent.

Flocculation of Clay. Some agents act oppositely to dispersing
agents, that is, they thicken clay suspensions. Cations, in general, floccu¬
late clay, and the flocculating tendency in decreasing order is as follows for
anion-dispersed clay: H% Ah"-, Ca"", Ba"", Mg"", K", and Xa". The order
follows the usual lyotropic series, with the exception ot the hydrogen ion
rH"), whjch has a disproportionately greater flocculating effect because of
its small size, which permits it to come into very intimate contact wit t e
clav particle. Alum causes rapid flocculation of clay suspensions, ue m
Al^ replacing Na", whereby the greater charge and adsorption of .
reduces the negative charge on the particle. Further addmon of alum m-
creases the concentration of Al" in the intenmcellar solution and. at
same time, increases the H* concentration, thereby bringing the suspension

Cla^s wWMain polyvalent cations (e.g., calcium,
aluminum) are difficult to disperse. Conimercial clays
small amount of soluble salt, but this should be kept to a
interferes with the proper dispersion of the clay.

carbonate sometimes contains enough free lime to cause "f ^

sfisnensions and satin white has been known to cause trouble in the s me

sives sometimes contain sufficient cations to cause trouble

Dkyandlamide is effective in reducing the flocculat on wWi -u
presence of salts and is used in this way for controlling the viscosity

well drilling muds.^*

Calciinn Carbonate

Calcium carbonate is a pigment which is
portance to the paper industry. Two types are used, precipitated ca

and water-ground carbonate. . r for paper coating.

Calcium carbonate is used tor the filling o p p _ with

Increasing amounts are being opacity, and

clay, where the car ona e ei^^^ calcium carbonate pigments were

ink receptivity of the cla>. made in recent

coarse and poor in color, but improved products have been

G H Sheets Pafer Trade J. 116, No. 3 : 22-30 (Jan. 21, 194«

MWin Cl8, 4or Instance.” .American Cyanamid Co. (W46)

XVIII. PIGMENT COATING

1051

Calcium carbonate has at least two, and possibly three, allotropes. The
two common ones are calcite, which is trigonal, and aragonite, wh.ch is or¬
thorhombic. The water-ground (natural) carbonates are entirely caci e,
whereas the precipitated grades contain some aragonite. The products are
non-hygroscopic and contain no water of crystallization. The partic e size
usually averages 2 to 3 microns, but often is as high as 7 to 8 microns, de¬
pending upon the grade. Certain of the high finish carbonates for paper
coating are much finer than this, containing many particles less than 0.1
micron in size. The brightness of calcium carbonates is generally in the

range of 93 to 98.

Calcium carbonate disperses readily in water, and dispersing agents
such as are necessary with clay are not always required. Precipitated cal¬
cium carbonate produces somewhat thicker slurries than the water-ground
product, and dispersing agents are required at concentrations over 30 to
35% solids. Casein is one of the best dispersing agents, and slurries up to
70% solids can be prepared using a small amount of casein. Oxidized
starches and sodium hexametaphosphate make good dispersing agents, al¬
though the effect obtained with polyphosphates is not as permanent as that
obtained with casein. Special dispersing methods, which will be described
later, are necessary in order to lower the naturally high adhesive demand
of calcium carbonate.

Calcium carbonate is relatively insoluble in water. The />H of car¬
bonate slurries is generally in the neighborhood of 7.0 to 7.5, although it
may be as high as 9.0 to 10.0 for some of the precipitated grades. Calcium
carbonate has the disadvantage of breaking down in the presence of dilute
acids, thereby liberating carbon dioxide.

Precipitated Calcium Carbonate. Precipitated calcium carbonate
can be made by several different methods: {!) by burning limestone to
lime, which is then dissolved in water and precipitated as calcium carbonate
by adding either carbon dioxide or sodium carbonate; (2) by the metathesis
of calcium chloride and sodium carbonate; or (3) as a by-product of soda
pulp mill operation. The last method generally produces a product of
lower purity than the other methods because of contamination with silicates,
iron, manganese, and other impurities which are picked up in the pulping
operations. Lime mud from the sulfate process may be used, but consid¬
erable processing is necessary to remove the blue-green color. Some of

the finer precipitated carbonates are sold in pulp form, but most are shipped
in the dry state.

In all the above processes, it is necessary to maintain careful control
of the temperature, concentration, and amount of agitation during precipi¬
tation in order to obtain a product of suitable and uniform particle size.

1052

PULP AND PAPER

Sears and KregeF* found that the properties of precipitated calcium car¬
bonates often vary greatly, depending upon the conditions of precipitation.
The particle size can be controlled in the carbon dioxide process by con¬
trolling the following variables;’® (1) type of lime; (2) the degree of agi¬
tation in reaction tanks (the more violent the agitation, the finer the par¬
ticles) • (3) the temperature (the lower the temperature, the finer the
particles) ; and (4) the rate of carbonation. Lime quality has an important

influence on the purity of the final pigment.

Fic XVIII-12. Precipitated calcium carbonate (3000x)

Courtesy Powdered Material, Research Laborator.es).

Precipitated calcium carbonate is composed of *'"7 P"'”'
antl held together by crystalline forces, giv.ng the P^^-de a P
and a rough exterior. The particles are rm^tb 03 m. on ^

2 nticrons in length, but due to the.r tmustul ,0 ^

siirtace area. The surface area o precipi a . | , 5(^,5 range from

.TO sn.,n. per gran, where the eflfect.ve d.amet r ° ,al-

to 2.0 microns.’” Because of the.r porous ’ > _^P^. ^

cii.... carbonates have h.gh o.l a..d '“'L electron micrograph

sive re(|uiren.ent when used ... toa y ,^5 19 ,

.. G R. Scars and K. A. Krcgcl. rr I. 114. Ro. 12. 1

J,fR. Davidson, Paf. />«.- Ma. Canada dd. No, 3 : 205,211. Sixth Wart.»c

..Pdotfcoinunication. A. R. '

“‘'p'^1*\*''’'"cT’luKics*aiTT! G, piichow, Pate ' Traie !■

77 A. R. Liikciis, C. u. LriiKies

18.3-100 (Apr. 10, 1941)

XVIII. PIGMENT COATING

1053

of precipitated calcium carbonate *own g

chemicals from the " is u'sed as one of the re-

ture. For example, if bme or so h>^ materials, giving the prod-

actants, the final product is i ^5^ neutralized with acids or acid salts,

net a high fH, unless the j ^^jmm carbonate is used, an

tation to prevent any i, in dull, medium, and high

Calcium carbon^U^enU^ are’only relative since

finishing grades. Ho\\e\er, tnes ,• i, finishine pigment in the sense

probably no calcium -bonate is trul^ of calcium

that refined clays ar . ^ finishing grades include the

:"oun^ gTaTes" M finishing types must be dispersed at rela-
dvely low solids because of their high water absorption.

TABLE VI

Particle Size Distribution (in per Cent) of Different Grades

OF CALauM Carbonate

Type

Dull finishing ..
Medium finishing
High finishing

• • • *

Approximate

Ingersoll

gloss

Average
particle size,
microns

3-5
0.5-2
Under 1

1 and under 1-S

Particle size r ange, microns

S-10

10 %
0
1

2 %

88 %

Ground Calcium Carbonate. Ground calcium carbonates are made

by water-grinding a pure natural limestone rock (at least 9 % .J’

and then mechanically separating the ground product into suitable par i
size. French chalk is considered to be the highest gra e.
bonates, in contrast to precipitated carbonates, are composed o so i rag
ments with a relatively smooth surface. An electron micrograph of water-
ground calcium carbonate is shown in Figure XVIII-13. The sur ace area
of water-ground carbonates is about 4 to 5 sq.m, per gram, and the median
diameter is about 0.8 micron.’® The brightness is about 90, compared with
about 95 to 97 for the precipitated grade, and the specific gravity about Z.Z

to 2.7.

Titanium Pigments

Titanium pigments are used in paper coating in the form of titanium
dioxide and in the form of the composite pigments, i.e., mixtures of titanium
dioxide and other white pigments. Titanium dioxide is available in two
modifications, the rutile and anatase. The rutile has a more compact crystal

TAPPI Monograph No. 7, “Pigments for Paper Coating,” Tech. Assoc, of the
Pulp & Paper Industry, 122 East 42nd St., New York, N. Y. (1948)

1054

PULP AND PAPER

Structure and a higher refractive index (2.70) than the anatase (2.55).
The anatase is most commonly used for papermaking, but the rutile form
is becoming of increasing importance. It has approximately one-third
greater hiding power than the anatase form. The brightness is 97 to 98,

and the specific gravity about 3.88.

Extended or composite titanium pigments consist of mixtures of ti¬
tanium dioxide with either barium, calcium, or magnesium piginents. Ti¬
tanium-barium pigment contains 70% barium sulfate; titanium-calcium
pigments contain 70% calcium sulfate; and titanium-magnesium pigment
contains 70% magnesium silicate. Composite pigments can be prepared by

Fig. XVIII-13. Water ground calcium carbonate (3000 x)
(courtesy Powdered Materials Research Laboratories).

recipitation of the extender pigment on the titanium dioxide, or
le titanium dioxide and extender pigment together either before or

Titaninm pigments are used in coating to increase the ^

ess, and whitL.css of clay coatings. They also increase the sn—^
„d rmish of the coating. The gloss is orilinarily not att^ted As a
lanitiiii piginents form a minor part of the pignient lirnish, b it tl

n the properties of the coated paper is ••'l’l>'™;''’'‘’,|“;^2im^,ig^^^
ionally high hrightiiess and o,,acifying pcnvei. ^ ^ ; pe-

spccially useful in light-weight papers which are dehciCT 1 • ■

a'lise of their high index of refraction '’'S'™; ^ ‘coat-

,1 coatings which are impregnated with wax. I'or the same

XVin. PIGMENT COATING

1055

as suTh bTrlther is obsained (ro.« ibneni.. ore wbich has th. app—
tonimU FtOTiO,. Thr iron is present m belli the '

The titanium dioxide content of ilmenite ore varies Irom 3. to .. /c.

Prior to World War II. most of the ilmenite was obtained ^

.Hit since the war. domestic de,..sits hare been oviened "P . ^ ^ ^

time. N-ew York. New Jersey. Florida. North Carolina. \ irginia. >

cmiine are the i>rincii>al sources of domestic ilmenite.

In the prcdiiction of titanium dioxide from ihiiemte ore «re t>^h-st
dried then pulverized and reacted with strong snitur.c acid to for it. a dr;
ake ' This cake is then dissolved in warm water to produce a solution of
titanium sulfate and iron sulfates. This solution is clarihed to reiiioi e ex¬
traneous materials, and the solution is then cooled so th.at some ot die fer-
rcHis sulfate (cop,<ras, crystallizes out. The solution is killed to hydrdyze
the soluble titanium sulfate to the insoluble hydrated oxide, which is thti
filtereil and washed to lower the iron content below 0.01 The next step
i. calciiation at a tem,K^rature lietween 800 to 1100= C. to change the amor-
1 ,Imus titanium hvdrate to the ervptocrystalline dioxide, having a
fractive index and a purity of 98% TiO». Finally, the product is milled
in the wet or dry .state to break down aggregates. 1 he properties of the
final pigment dqieml on the purity of the ore, concentration and acidity of
the solution, the presence of a suitable miniber of nuclei for seeding during
precipitation, aiul the tenifierature of calcination. Pigments from unseeded
solutions tend to be cc«rsc in size,’* and to lie agglomerate, rather than
crystalline, so tliai they are not an article of commerce. The calcination
operation is prol»ably the most important ojieration in determining the
lihysical properties of the pigment. As the calcining temperature is in-
cr^seti above a tem|«rature of 850= C., the size of the pigment particles

liecomes larger, as shown by reduced oil absorption.

Properties of Titanium Pigments. Titanium dioxide is available in
both water-dispersible and oil-dis|>ersible grades. The w'ater-dispersible
(anatase) grades are readily susjiended in water at concentrations of 75%
solids without the use of dispersing agents. The oil-dispersible (rutile)
grades require the use of dispersing agents, such as sofiium hexametaphos-
phate or gum arabic, the latter being the most effective. Sodium silicate is
not suitable as a dispersing agent, because it reacts with titanium.

The average particle size of titanium dioxide is about 0.3 to 0.6 micron,

^L. T. Work. S. B. Tuwiner and A. J. Glostcr, Ind. Eng. Chan. 26, No. 12;

(Dec. 19J4)

1056

PULP AND PAPER

and the particles are uniform and roughly spherical in shape. A photo¬
graph of a flocculated and deflocculated titanium dioxide suspension is
shown in Figure XVI11-14. The results in Table VII, taken from work of

* * /.**:’^* /• 7 . * >

• -j- •

(A)

Ficr X^TII-14. Titanium dioxide flocculated (A)

photographs by Mr. Harry Green, courtesy Interchemtcal Corporahon).

Martin,®®'®^ show the range of particle size of several different titanium
pigments.

TABLE VH

Particle Size Distribution of Titanium Dioxide

(Water-Dispersing Types)

By Beaker type centrifuge method

Size interval, microns

Percentage of total

Over 1.0.

1.0-0.5 .

0.5~0.4 .

0.4-0.3 .

0.3-0.2 .

Under 0.2..

Mean surface diameter, microns.

Incremental surface area,

sq.m./cc. of ..

Solids in final solution, %..

Average distance between particles,

microns ...

1.3

9.7

19.5
18.8
34.2

16.5
0.28

21.4

1.21

6.3 ’

25.5
16.0

35.5

15.5
0.28

21.4

1.0

6.5

17.5
28.2

36.4

11.4
0.29

21.0

3.0

2.0

I. 3
36.3
49.0

II. 4
0.26

23.2

5.0

2.5

5.3

27.5
52.0
12.7

0.26

23.2

10.0

1.9

1.5 0.92 0.72 0.50

0.8

172

66.0

16.0

0.23

25.9

15.0

0.39

. ,. 7 “Pigments for Paper Coating,” Tech. Assoc. Pulp &

80 TAPPI Monograph No. 7, P

i^bsie.

XVIII.

PIGMENT COATING

1057

The principal titanium pigments used for coating
tanium dioxide and the rutile titanium ” f prLfpitated

made by precipitating dO^e rutile uUe due to

calcium sulfate. The titanium-calcium pigment is slightly solu ,

*e slbility of the calcium sulfate; this is not a

paper coating, although it is a disadvantage for use ^

the highest hiding power of the composite pigments, although it is less tha
hat S p-e titanium dioxide. It has a particle size about I-ce that of
pure titanium dioxide, or about 0.5 to 0.8 micron. Titanium (™tde) cah
cium pigment can replace titanium dioxide in coating m the ra
2.5 parts of the mixed pigment to one part of the pure titanium dioxide
without any loss in optical properties.'* As a general rule, Ae mi.xe pi^
ment is cheaper than a combination of the anatase titanium dioxi e an y

on an equivalent basis.

~ • T T T f * i

Satin white is a pigment which has enjoyed rather widespread use in
the past. It is not so widely used as formerly because of the relatively high
cost and the difficulties in handling. Satin white is used chiefly for its high
gloss characteristics and its bright color. It produces a coating which is

many times bulkier than a clay coating.®®

Satin white is used with casein, but is rarely used with starch. It
forms a rather viscous coating mixture with casein which tends to increase
in viscosity (and may even gel) on standing. Satin white is sometimes dif
ficult to disperse, and consequently it is a wise precaution, when using satin
white, to grind the coating mixture to eliminate lumps. The amount and
type of alkali used for dissolving casein for use with satin white is an impor¬
tant factor, and generally ammonia, trisodium phosphate, or borax are pre¬
ferred. When properly made, casein-satin white coatings have a high de¬
gree of water resistance.

Satin white is made by reacting hydrated lime and aluminum sulfate
(iron-free alum) to form calcium sulfoaluminate, which Sutermeister®^
considers to be best represented by the formula 3CaO ■ AI 2 O 3 • 3 CaS 04 •
3 IH 2 O. The solutions are mixed cold (anhydrous sodium sulfate may be
added with the alum), and then heated to start the reaction, which is exo¬
thermic. The mass becomes quite thick. It may be dewatered on a filter
press and sold as a paste containing approximately 70% moisture, or it may
be made to contain 30 to 33% “paper dry” solids without dew^atering. The

82 W. R. Willets, Tappi 32, No. 8: 349-356 (Aug., 1949)

88 J. Phillips, Thesis, “Study of Properties and Structure of Pigment Coatings.”

New York State College of Forestry, Syracuse, N. Y. (Dec., 1949)

8*E. Sutermeister, Paper Ind. 15, No. 12 : 696-698 (Mar., 1934)

1058

PULP AND PAPER

conditions of temperature and agitation and the method of addition of in¬
gredients affect the final properties of satin white. Small amounts of gum
arabic are added as a stabilizing and dispersing agent. Commercial prod¬
ucts are usually quite variable in composition and properties. Satin white

should be protected from freezing.

The lime used for making satin white should be free of iron, because
too much iron in the lime results in a pigment having a slight yellow color.
Lime is frequently used in excess, so that the final product is strongly al¬
kaline usually containing about 17 to 25% lime on an air-dry basis. Be¬
cause of the alkalinity of the product itself, it is impossible to determine
the amount of excess lime by titration. Satin white contains a considerable
amount of water which it tends to lose at high temperatures. Heating to

Fig. XVIlI-15. Satin white (lO.OOOx) (courtesy Institute of Paper Chemistry).

I00» C results in a loss of about 25 to of the air-dry “‘I ^

loss occurs if heating is continued at higher ttnnp^tures. An

electron micrograph of satin white is shown in it,tire .

Zinc Piyuicnls

The tnost important sine pigments are ilseVto

composite zinc sullitic pigment, hthopone, Ihe. . 1 j,

SO.UC extent in |«,.er coating when a pigment of high refractive

’'‘"’"kne Sulfide. Zinc sulfide is made by

sohilioii of soluble zinc salt. started and the

dried, and calcined. During cakinat o , y ‘^ b rpfnctive index to a
siillhle is changed from a hydrated niaterial of low f

cry.stalliiio siihstance of high refractive (-' J' ,„f,perattires

crystals starts at a calcination temperatiire of 450 t. .xt P

»VHI. mmiSI COATING

1059

• ?

ilM ifciAii 750' C, ihrfT U A irodrocv lo |ir.«lu« cxcwsnfly rj*

llA> A MHaU |«tkk .IK. il«

■ iiili««tii iH of OJ wicroo** Thr |onicle» ak unifonii aikI contAin jitac-
licAB. Ml •ndn idiiAl |iAn!tfe» ««f 10 micToo. Zmc «il6dc u unrtAble m

iIk a,nun of Add., icwfioe *«• hyOT"*™ »>"«>* “

% ITlXr. TV bri*tanoA U 94 to 96. And iV ^cific *TA.it> »>««• •* ®-
CKtAin pAdr. of lint «UMr ak hic«y luniin«c«nt T1>«» t.tado
ak aiDKd Ai Kta«»ly Inch l«n|>rfAHiK.. And cMocqumtly tV lortKlo
i. OAornVi COATK, TAncinc from About I nucnm lor the
fra4r« to 5"lo 10 taknm^ «r mofr lor ihr pho«iihL»rr5iccni jsra
Comowte Zinc Sulfcde Pigment*: GwipoMle nne sulti<

Im«t htm kmmn (or it-nr limr. i)ne |in«luct include* a coif
fine mMt with fwedfitttwl Innum *ul(ale in ap|»roximately a
\oollirf prudun iiiclndr* a o«*ifatk« erf line *ulhde. Iiariunf

in .hr «.« a( 25^ rinc sulhde, 60^ barium

15^ tiOfihnn liihi^nnr i* ihr <4de*t rinc »ultidr coi

; it b hr thr eo|irfcif»itaiioo erf line tulfiele and l»ai
•ohilnm ol Inrievn folMr and rinc *ulfale. The final ii
tain. J0^» wmc •ntede and 70% barium *uHate. usually
mrne ducrefr unk^ The rrirartive inde* i* l•ctwfm 1 84 to

Zmc Oaide Zinc uaide it r^raHy Imming rinc in air and

(«itctii« Ihr ttaidf in Hweial (aliric lia«* In one |frcjcrt». pure line i*
hnrwd In anatbrr prema. rinc ore U burned in mixture with a*al.

Zmr oaidr ha* a hich re(rartive index (2.01) and a good white c*dor.
Zxm oxnie hat an exceptionally high tfiedhc graeity. which it in the neigh-
horhood of 5j6. It b aulbblr in acid toluiion* and U there(e»re impractical
lo Hie M a hlirr in the p r et e n ce of alum. It can W uted in pajiericoating, al-
dKMfh it lend* lo react with catein lo cau»e geling of ihe coaling mixmre.
Bmnrr of ihrte hnncation*, rinc ejxide b not widely used in papennaking.

Bsnmm S«//a/r

b awd m the paper industry in two dmerent forms:
aa the miaral minrral product, haryle*. or as the artificial product, blanc

hxr Both are umS (or (tOmg and aartmg.

Baryte* b (ound m many parts erf the United States, in particular, in
MbiiMipfrf. Gaarfia, and Tcnnrasce. In |ireparmg the pigme^ (ew market,
the minrrai b hit! g ^ *^rrd hkI separated into the desired particle siie. The

then trea^ with acid to remore impitrities. dried, and reground.

**|_ T W<e% Md I If OdrfL /wf Bm$. Obw. 25, So. S: Si} S44 (Ifsjr. 19}})
••O Kiev sad H M Crr. Psfor Tradr /, 9^, Kw lOr 127-1 JO (Mar. 9. 1933)
** tf. A HcAadx Jtaewi Treaiir Jfaaddy 25. Ko. 6: 29S-296 (lone. 1944)

1060

PULP AND PAPER

Barytes occurs in both the amorphous state and
state and generally has an average particle size of

Blanc Hxe can be made from either barytes or witherite, a barium car¬
bonate ore found in England and Euroj*. When

barytes, the finely ground — Ltd tm the ash

sulfate to barium sulfide. 1 he soluble suinoe

and precipitated as the sulfate y a ® leeching the

fate or “^e is treated with hot hydrochloric acid to form barium

^Sttris then filtered .d -L^tttcte ■

fr«=t r r.:r. “S4..

B?anc fixe has a finer particle size, -

grit than barytes. lUs ” erage
ing about 35% water, ihe average p

neighborhood of 0.5 ^ “LLing of high-grade photographic paper,

Blanc fixe is used for ‘ ^ high brightness.

where it produces a coating of ig . ■ papers where a metal stylus

It is also used in special grades of coa _ P hng. Coatings

is employed in place of pen and ink or marking o^

Lm .%ial' ^et used in making lithofmne, a zinc-barium

n:. forms of barium sulfam have

er^tsSn water
pH of terium sulfate suspensions is generally

Calcium Sulfite .

Calcium sulfite is a white JLenTSHTs^?) and is

filling and coating of paper. I is an^ P ^ and the refrac^

“txths'^The'Stn-varies from 92 to 96. The pigment

minTabout 7<?0 water of sulfite, and many of

There are several processes P produce a gra

these are patented.'-' One process, which

s. U. S. 1.984,188 (Dec. 11,19M)

8» u. s, 2,191,465 Feb. 27, IW
90 u. S. 2,210,405 (Aug. 6, 1940)

01 U. S. 2.375,786 (May 15, 1945)

09 u S. 2,413,321 (Dec. 31, 1946)

XVIII.

PIGMENT COATING

1061

suitable for the filling of paper, consists of

sulnte as prepareu duuvc wn nroress consists

size and improves the product for paper coating. Another process

nf treating calcium carbonate with sulfurous acid.

SlSrsulfite has not been used to a great extent for W- ™

because of its high adhesive demand and low ^eat-

mem of L Sgment so that only about 20^0 starch is necessary to obtain
"ng whfch passes a No. 6 wax. Calcium sulfite must be made up in
Wr solids than clay because of its high water absorbing properties. It
lartetspertd with the aid of about 0.75 to 1.0% of sodium hexameta-
phosphate on the weight of the pigment. Coatings made with calcium sul-
fite have high brightness and good ink absorbency.

Calcium Sulfate

Calcium sulfate is used in the paper industry in two forms; the natural
product, gypsum (CaSOd * 2H,0), and the artificial product, known as
pearl hardening, pearl white, pearl finish, crown filler, or alabastme. The
natural product, gypsum, is mined, ground, and sold as a fine POwder. I
has plate-like crystals. Artificial calcium sulfate is generally made by re¬
acting solutions of calcium chloride and sodium sulfate. The final product
contains a considerable amount of free water. It may occur in the orm
of flat plates or needle-shaped crystals, depending upon the amount of com¬
bined water. r ir

A calcined gypsum is made by heating to remove water of crystalliza¬
tion. This product is sometimes known as pearl filler, calopone, or alabas¬
ter. It generally has a finer particle size than the natural gypsum. This is
one of the most highly soluble pigments (0.2 part soluble in 100 parts of

water ).

Diatomaceous Silica

Diatomaceous silica (known also as kieselguhr or infusorial earth) is
a type of silica which is obtained from the fossil remains of microscopic
plant life known as diatoms. It is mined as siliceous skeletons of diatoms,
which are found in diatomaceous earth. The material is chemically treated
and air-separated, and is sold as a very fine, white, grit-free product. A
calcined product with special properties is also available.

The particles of diatomaceous silica are amorphous and quite fragile
and are characteristically diverse in size and shape. The average size is
about 10 microns. Diatoms, if acicular, are about 1 to 5 microns in diameter

1062

PULP AND PAPER

and 10 to 50 microns in length, and if circular, they have a mean diameter
of approximately 5 microns. Because of their highly porous nature, the
particles have an enormous specific surface which approaches 10 to 50

sq.m, per gram.”'*

Diatomaceous silica has a brightness (G.E.) from 65 to 95. The
calcined product has an appreciably higher brightness than the regular
product. The moisture content varies from 5 to 7% for the regular product

to about 1 % for the calcined product.

Diatomaceous silica is sometimes used in small quantities in paper
coating to reduce gloss and to increase the resistance of the coating to
blackening on calendering. In high concentrations, it makes the coating

very abrasive.

Luminescent Pigments

Some inorganic pigments possess photoluminescent properties, that is,
they are capable of absorbing radiant energy and converting it into visible
light. There are two types of luminescent pigments: {1) fluorescent pig¬
ments, which emit light only when the exciting light source (i.e., black
light) is shining on the pigmented surface, and (2) phosphorescent pig¬
ments which continue to emit light after the light source (either black light
or visible light) is removed. The latter have an afterglow of two to three
hours for some pigments, and up to twenty to twenty-five hours for others.

Commercial luminescent pigments are usually sulfides of zinc, cad¬
mium, strontium, or calcium. These are manufactured by mixing the pure
sulfides with small amounts of activators and then heating at hig emp
ture in special furnaces. The phosphorescent pigments of short afte g
are specially prepared zinc sulfides or combinations of zinc an
sulfides. The phosphorescent pigments of long afterglow ^ ^
binations of calcium and strontium sulfides. The average size of fluomscen
pigments is about 1 micron, whereas the size of phosphorescen pig

is o^eiicrally 3 ebout 5 to 10 microns. ^ mao

Luminescent pigments are being used m the pro °

papers, instrument dials, display papers, gift and J j ’ or

.rc ThP niameiits can be used for filling, pigment coating, o
safety papers. p fluorescent pigments and the phosphorescent

pigments of short afterglow (zinc c^ phosphorescent

regular filling and , J„g because of their

pigments cannot be used for filli „ P ^ , ,

sensitivity to water, and consequently tbe use ot tnese

• +• AT? Lukens Powdered Material Research a ra

93 Private communication. A. • 1948T

tories, Cambridge. Massachu^ ^ 1^. 297 (June, 1944)

94 M. A. Heikkila. Rayon TexUle Monthly 2 o, iNo. 0.

XVIII. PIGMENT COATING

1063

type coatings. The coarse size and relatively high cost limit the use of
luminescent pigments.

Colored Pigments

Most of the pigments used in paper coating are white, but in some
cases, it is desirable to use colored pigments. These may be added to the
regular clay coating mixture, or they may be used as the on y pigmen

in the coating mixture. • j t-u ■

Colored pigments are used where permanency is required, ihe pig¬
ments commonly used are:

Yellows—chrome yellows (barium and zinc) and iron oxides.

Blues—ultramarines, iron blues, cobalt blues.

Browns—^umbers and iron oxides.

Greens—cobalt green, Malachite green, emerald green.

Reds—sienna, red lead, Indian red, orange lead.

Blacks— lampblack, carbon, and bone black.

Colored pigments should be finely dispersed in order to obtain the highest
possible color strength. In some cases, trouble is experienced with small,
hard lumps of color producing streaks in the coating. These colored par¬
ticles are so small that they are passed through the screens used for remov¬
ing coarse material from the coating mixture. In order to avoid this
trouble, it is best to disperse the pigment in water or in a mixture of alcohol
and water before adding to the coating mixture. With some colored pig¬
ments, it is necessary to use large amounts of water, and this should be taken
•into consideration in calculating the solids content of the coating mixture.
Grinding of the coating mixture in an ink or colloid mill also helps to break
up any undispersed particles.

Colored lakes, both in dry and paste form, are widely used in coating.
These are available in a wide range of colors and produce deep-colored
coatings of excellent brilliance.

Adhesive Demand of Coating Pigments

One of the most important properties of a pigment is its adhesive de¬
mand, i.e., the amount of adhesive required to bind the pigment to the paper
so that it will not dust on calendering or pick during printing. A low ad¬
hesive demand is a desirable pigment property because it means that a low
percentage of adhesive will be required in the coating formula to meet the
strength demand of the coated paper. The strength which is obtained with
a definite amount of any given adhesive is determined by a complex set of
factors which include the characteristics of the raw stock and the total solids
of the coating mixture, as well as the properties of the pigment.

In studying the adhesive demand of different pigments, coated sheets
are made by hand, using varying percentages of adhesive, and then these

1064

PULP AXD PAPER

coatings are tested for strength with a series of special waxes (see section
on “Evaluation of Coated Papers”). In general, the strength of the coating
increases as the amount of adhesive is increased, but the relation between
the wax pick test and the amount of binder used in the coating is not a
linear function. Usually the wax number increases rapidly with an increase
in the amount of binder in the range of low wax number (i.e., low coating
strength), but the slope decreases in the range of higher strength, and the
curve eventually levels off. Hughes and Roderick®" found that precipitated
calcium carbonate differs from other pigments in this respect in that the
wax number increases more rapidly in the ranges of higher number
than some of the other pigments. The comparanve adhesive demand of
different pigments depends somewhat upon the type of adhesive used as the
binder. Comparative adhesive demands for leveral different pigments
based upon the percentage of casein required for the coated paper to pass
a No. 5 Dennison wax are shown in Table VI1I.“

TABLE VIII

CoMranax.™ Aohes.ve Dem.xos or Ssvcxc Piom.xts ^

Required for Coated Paper to Pass a No. S Denn s .

Casein required, %

T3rpe of pigment

12-13

Regular clay .. .. 12-14

High-finish coating clay (domestic) ...

High-finish coating clay (English) .....^.

Water-ground natural calcium carbonate . 20_30

Precipitated calcium carbonate . 45,55

Satin white .

Curves showing the casein demand of different pigments over a wide range
are shown in Figure XVni-16-

It can be seen from Table M ?• u, inw adhesive demand,

ground calcium carbonate have a compamttvely bw ndte

whereas precipitated calcium carbonate and satm white

lively high adhesive demand. rlavs have a much

In a certain particle size range. Engli* ,, 25 to

higher adhesive demand than American c ay ° jhesive demand than

3oVo more adhesive. Titanium has a « h.gter adtesw

cla;. There are no available figures on demand

although in general, it appears somewhat different

than clay. Comparative adheswe deman y ^

,,he„ comparisons are made in the range of very bgh ^

.. A E. Hughes and H. F. Roderick, ^ ’■

Jl: irT.'d'Sow, P.er We f. H. -

183-190 (Apr. 10, 1941)

XVIII. PIGMENT COATING

1065

cUy and precipitated calcium carbonate may have the
mand when compared under conditions of high
hesive demand depends somewhat upon the method o ma 'e
nioment particularly in the case of calcium carbonate.

On; of the most important factors affecting the adhesive demand i

the average particle size of the pigment. In general, fine particle size c ay
require more adhesive than the coarser grades of c'ays-”

S:k“ has pointed out, however, that

true with domestic clays. On a comparative basis, different grades of

0 5 10 15 20 25 30 35 40

PARTS CASEIN PER 100 PARTS PIGMENT

Fig. XVII1-16. Comparative adhesive demands of different pigments.®® (1)
Titanium dioxide, (2) lithopone, (3) blanc fixe, (4) domestic clay, (5) English clay,
(6) calcium carbonate, and (7) satin white.

calcium carbonate have the following adhesive demands; dull finishing
(large particle size), 10% ; medium finishing (medium particle size), 25% ,
and higli finishing (small particle size), 35%.®® Pigments containing large
particles tend to dust more on calendering because of the tendency of the
heavy calender rolls to crush or shatter large aggregates.

Very probably the particle size distribution, as well as the average size
of the particles, is also important in determining the adhesive demand of the
pigment. From a theoretical standpoint, a pigment containing particles of
different sizes should require less adhesive than a pigment composed of
particles of uniform size, even though the average size is approximately
the same in both cases. The pigment containing particles of varying sizes
should pack better and thus leave smaller pores between the individual par-

R. D. McCarron and B. W. Rowland, Paper Trade J. 96, No. 22: 272-276
(June 1, 1933)

•• W. A. Kirkpatrick, private communication to the author (June, 1950)

®*T.\PPI Monograph No. 7, “Pigments for Paper Coating,” Tech, Assoc. Pulp &
Paper Ind., New York, N. Y. (1948)

1066

PULP AND PAPER

tides to be filled with adhesive. * Such a concept is analogous to that used in
concrete mixing, where the engineer chooses an aggregate of rock of vary¬
ing sizes in order to strengthen the concrete. However, it has been reported
that such compositions tend to produce a mottled finish m coated papers.

The amount of adhesive required in blends of two or more pigments
is not always the arithmetic average of the adhesive demands of the pig¬
ments involved. In some cases, the amount required for the mixture is far
less than would be expected. For example, Roderick and Hughe.s»"“ point
out that a blend of clay and precipitated calcium carbonate containing 10 to
25% of carbonate has almost the same adhesive demand as straight clay,
although calcium carbonate by itself has a considerably ig er a esne

demand th§in clay.

Preparation of Coating Mixture

After the pigment dispersion has been prepared and the adhesive dis
solved in water, the next important step is the preparation o t e coa in
mixture (or coating color, as it is sometimes called).

Mixing the Pigmeyit and Adhesive

The method of mixing the pigment and adhesive nro^rtLtf

termining the working qualities of the coating mixture and the _

added to the pigment slurry while stdl hot, or m some cases, b>

Starch and pigment together.^®^ colloid mill, homog-

Mechanical grinding of the coating mixt • ^^^e of the

enizer, or ink mill is one way of insuring equi-

adhesive and pign«nt, thereby brinp g

librium more rapidly. Grinding m jo dis-

strength obtained with some starches, p rending generally lowers

perse “very well and have low bonding stre^ » S
the viscosity of the coating mixUire. Gni^ding

(undisintegrated) pigment partic es w ic , produce surface

coating mixture, would cause ^ je special

irregularities in the coating. pnatinff mills; in many cases, their

. dispersing machines are not use in a ^ mixing which

use is not warranted, since a leav) ii gg |,ave very serious op-

is necessary. Some of these djspers. g machine „„

crating disadvantages, such as hig ^16 (OA

1.0 H. F. Roderick and A. E. Hughes, Pofrr Trade /. M ho.

Ruff. u. S. 2,140,394 (Dec. 13, 1938)

XVIII. PIGMENT COATING

1067

narts, and incorporation of toani in the coating mixture. A special
ImowTi as the Rafton mill, consists of a very high speed circular saw which
strikes a jet of the coating slurry projected into the path of the saw.

Fig. XVIII-17. Effect of kneading precipitated calcium carbonate slurries for 45
minutes at different solids content in the presence of water and casein'®^ (courtesy
Wyandotte Oicmicals Corp.).

10 I

9 -

1-8

5 10 15 20 25 30 35

Per Cent Starch in Final Coating Color

Fig. X\’1II-18. Effect of lateading precipitated calcium carbonate slurries for 45
minutes at different solids content in the presence of water and starch^®^ (courtesy
VV'3randotte Giemicals Corp.).

One interesting development has been the use of special dispersion
eijuipment such as colloid mills, rod mills, and kneaders for reducing the ad¬
hesive demand of precipitated calcium carbonate. For many years, exces¬
sive adhesive demand was a deterrent to the use of precipitated calcium
carbonate in {>aijer coating, but recent developments in dispersion methods
ha\*e reduced the adhesive demand by about 10 to 25^, depending upon the

ruuf AND rArti

10(iK

fSk ti fura
•r fti

^radr o( carU>i«itc Koderick*** *** 4wcnly*^

dlirnj, (J) ttw pupiKitl and l«'tAl anwwtti oi idhrurr
millcfl in a ctmtlilnjii axvi thm diluted «Tth 'nut, (?) pif*

inrnt and a Mtwll amount of adhrsivr may I* kiUAJbd mbit fia^tc Mif
thru the rmuiimni: adhesive ami water addni Uiihet 1»nr«t a’. ^

a trnnrndcMi- rf<lth l»on in tlir adheM\T demand »d ^
rxtrnl uf the rediKtiiMi in adhe^u-e drutaml dq^l» o|*"
mTsiiiK o)uijmirnt and lli^ sulwU cwitm the mrx}

knradinK e<|UiiHnrut mkIi as d^ni^h nuarr. |«ijjnixef». diitw ^ to

70 to 75S' Md..U can U um.!. it ball imllm,; i. ^ dun^JJJ -
70'': solids can use.J Hgure X\ 111*1/ *lll

r^luci.., in a.1hc»Kr .h.h

,l.f r, «..lar nn-th-l..{ »nh M*k t.n*

m.cnl.’.l a .mthod u( Knmtine a.juc^«> >u»p»««» «< ^

luh » dctlnccnUling *g.n. to reduce the .dh^cT

Trenuuen. .d oleum. or!.m..c TJ^

Tl.e effect i> due to a breaking down .d Uie

hrv imrec >n tlut it is free to act to lU fullcM extern as a •**”'"* ^

“ ^ 1.1^ xHtwonee is made aeailable to funclioo at a Umdiag

As a result, more of itie adnesir^e s u j final maitnc are

agent, with the effect tliat the strength and density of the 6
"^'TL tl« exattug mixture is used.« should be

dirt particles. sh^ he Ic

c,«rs« n«tcri.tl. The gnt r«amed on a IC^

than 0.1 to 0-e in a P^‘rJ^P^„X'^,™*!„.xtme to pick
nviting. there is a tend^ m . renweed

• linf or paper fiWrs ^ ci^.;i^ng screens are drdraHc ta

the coating mixture IS rearcuUled. belt-ckami^-

this purpose.

Coating Formulas

■n« two maior ingredkn.s of t,»

and the pigmem. ^

upon the results desired, ^_ P adhesive comprises «■

to hi. T^e water contenf generaUy tr^ - - ^

i«! H- F. Roderick Pat^ MSI Pr«^Lerf Cakiaffl Cariwaaae.

2.343.243

XVIII. PIGMENT COATING

1069

versely, the solids content ranges from 30 to 70%. Table IX shows several
typical coating mixtures.

Grade of
coated paper

Brush-coated
book paper

Brush-coated
magazine paper

Brush-coated
folding enamel

Machine-coated
book paper

TABLE IX

Typical Coating Formulas

Pigment

composition

100 parts clay

80 parts clay
20 parts calcium
carbonate

75 parts clay
25 parts satin
white

80 parts clay
20 parts calcium
carbonate

100 parts clay

Dispersing
and other
agent

0.3 part sodium

hexametaphosphate
0.2 part soap
0.3 part sodium

h exametaphosphate

0.2 part soap

0.3 part sodium

hexametaphosphate

0.1 part sodium
pyrophosphate

0.2 part sodium

hexametaphosphate
0.2 part soap

Adhesive

Per cent
solids

20 parts oxidized
starch

25 parts enzyme
converted
starch

18 parts alpha
protein

15 parts casein

15 parts dex-
trinized starch

Many coating formulas are made with pigment blends in order to com¬
bine the desirable properties of two or more pigments. For example, clay
and calcium carbonate may be used together in order to derive a high finish
from the clay and a high brightness from the carbonate. Clay-satin white
and clay-titanium dioxide are other typical combinations. On the other
hand, adhesives are seldom blended, with the exception of latex which is
widely used with casein and in some cases with starch.

Commercial coating formulas usually contain a number of minor in¬
gredients, in addition to the adhesive and pigment. These materials are not
an essential part of the coating mixture, but are added for special effects.
Among the materials which may be added are the following; eveners, anti¬
foaming agents, wetting agents, dyestuffs, and pigment-dispersing agents.
These include such materials as pine oil, sulfonated oils, soaps, wax emul¬
sions, and resin emuslions.

Pine oil and sulfonated oils (e.g., sulfonated castor oil and sulfonated
tall oil) are sometimes added to the coating mixture, usually in proportion
of 0.1 to 0.5% on the total volume of coating mixture. The principal func¬
tion of these agents is to aid in smoothing of the coating and to prevent the
formation of pinholes. Pine oil also has a definite preserving action, and is
widely used in wallpaper coating for this purpose. Another function of
these ingredients is to improve the flexibility of the coating, especially dur¬
ing the winter months. Too high a percentage of these materials must be
avoided, however, since they lead to the formation of oil spots in the coat-

1070

PULP AND PAPER

ing. Getty’”® found that coating mixtures can stand much higher per¬
centages of sulfonated tall oil compared with other sulfonated oils without

showing oil spots or other bad effects.

Anti-foaming agents are frequently added to the coating mixture, and

the use of these agents will be discussed later in this chapter. Softening
agents, such as invert sugar, glycerine, or corn syrup, may be used on occa¬
sion to increase the pliability of the coaling, but the use of these materials

is not general. . r

Wax emulsions and soluble soaps (ammonium stearate and sodium

oleate) are frequently added to starch coatings to reduce dusting during

calendering (by increasing the pliability of the coating) and to improve the

leveling properties of the coating. Wax emulsions are frequently used in

casein coatings for the production of friction-glazed papers. Soluble soaps

should be added as the last ingredient after the coating mixture has een

lowered to room temperature. Insoluble soaps (e.g., calcium stearate) may

be added to improve the brightness and gloss of the coating and to preven

rapid absorption of ink when the paper is printed. ^ tr~

is another material sometimes used in paper coatmgs. However all

materials must be used with caution, because “'f“f

SmiJ iTs'^ In roll coating, the rubher.overed offset rol —

undergoes more rapid deterioration when these ingredients a

“‘‘*'sheUri's''sometimes used in paper coating formulas where water re-
sistaiwe i d ired. The shellac is usually dispersed with ammonia in th
Lllowing proportions before being added to the coating mixture. lb-

shellac, 2-3 qt. ammonia (26° Be ), 50 gal. j

Dyestuffs are used for tinting wh-off shades o'

blue dyestuffs being the most w J „„ nr increase the brilliance

the coating, dyestuffs may be added to tone up

of the coating. The precautions to be jhe dye¬

stuffs in coating are the same as t ^ ose m o jr^^ >

stulT should always be dissolved m w brilliance and fast-

ture. Acid dyestuffs are generally used " " of them

ness to light and heat. Basic dyes are extreme y b«

are too fugitive to light, alkali, and moist hea to ^ m " ^
of colored coatings. Direct dyes tend to produce dull coatings.

Per Cent Solids

The per cent of solid matter coating.

rer of ^---slng inn>or.aiice sinc^ the

los E. Getty, Paper Trade J. 11^, wo. o. r'

PIGMENT COATING

1071

High-solids coating mixtures are desirable because

chiL speeds. Dickerman and Riley>«« report that a solids cont nt of 53fe

iwrmits a machine speed of 930 f.p.m., compared with only 750 ‘ P.™- "
Lting mixture of 45fe solids. The higher operating speeds obtainable

with cLing mixtures of high solids is readily understood when “
that a coating mixture of 407o solids requires the evaporation of ,

lb of water per ream of paper for each pound of dry coating applied,
whereas less than 0.5 lb. of water need be evaporated for a coating mix ure

of 70% solids. . • a. 1 • 1 -.

It is customary in coating terminology to divide coatings into hig -

solids coating mixtures and low-solids coating mixture^ lese erms

are quite flexible, but in general, cover the range of 30 to 50% solids for low-

solids coating mixtures and 50 to 70% solids for high-solids coating mix-

A high-solids coating mixture may be obtained by one of two methods.
In the first method, the coating formula can be the same as that used in
low-solids coating, except that part of the water is withheld. This produces
a coating mixture of very high viscosity, and if earned far enough, produces
a mixture of putty-like consistency. Such a coating mixture has on y a
limited use, and while it can be applied by certain high-speed roll coaters,
it cannot be handled by many conventional coating machines. Increasing
the solids content of a coating mixture in this way not only increases the
viscosity, but also increases the plasticity of the coating mixture. The sec¬
ond method of obtaining a high-solids coating is based upon special formulas
containing low-viscosity adhesives so that even though the solids are high,
the coating mixture is fluid and workable. The flow properties of this type
of coating mixture are governed by the viscosity of the adhesive and, to a
lesser extent, by the flow properties of the pigment.

The most important factor determining the practical range of solids
content is the viscosity of the adhesive. Casein has a relatively high vis¬
cosity in the native state but by suitable techniques it can be used in coating
mixtures at 60 to 63% solids. Starch also has an extremely high viscosity
in the native state and, if used as such, would necessitate a coating mixture
of very low solids. However, starches are generally used in modified form
(i.e., after conversion by enzymes, oxidizing agents, or dextrinizing agents)
so that their viscosity is greatly reduced. If a low viscosity starch is used,
coating mixtures up to 70% solids and even higher can be produced.

Another important factor determining the optimum solids content of
the coating mixture is the type of pigment used. Pigments requiring only
a small amount of water to produce a fluid dispersion are particularly use¬
ful for preparing high-solids coating mi.xtures for machine coating. Water-

G, K. Dickerman and R. W. Riley, U. S, 2,425,231 (Aug. 5, 1947)

1072

PULP AND PAPER

ground calcium carbonate produces a dispersion of higher fluidity than
precipitated calcium carbonate, but it cannot be used in machine (roll) coa^
ing because it fractionates out at the nip. Satin white requires a high ratio
of water to pigment to produce a fluid dispersion. Certain types of clay
produce more fluid dispersions than other clays. Clays of small particle size
and low hydration capacity produce more fluid dispersions than clays of
large particle size and high hydration capacity.*® An important factor in
calculating solids content is the moisture content of the pigment, since a
change in moisture content from 6 to 10% can cause a variation of nearly

3% in solids content in high-solids coating mixtures.*®®

The optimum solids content of a coating mixture is determined in part
by the type of coating process used, because certain coaters can handle more
viscous coating mixtures than others. Each coating process has certain
requirements in the matter of viscosity, yield point, and other rheological
properties of the coating mixture which cannot be exceeded without running
into operating difficulties. For example, coating mixtures for brushtype
coaters must have a low viscosity and a low plasticity in order to sprea
easily on the surface of the paper by brushing. In order to attain ese
properties, the solids content of the coating mixture must be relatively low
usually in the neighborhood of 30 to ; « the solids content ts much
greatfr than this, the coating is likely to show brush marks. An brush
coaters and centrifugal spray coaters also require coating mix ure
relatively low solids, usually 38 to 45%. although coating J

.60% solids can be handled when low-viscosity starches ^^e “e
adhesive. Roll coaters can handle coating mixtures of muc ig ^

because they can handle coating mixtures of much h-gher vrscosUy.
ability of roll coaters to handle coating mixtures up o
accounts for the wide use of roll coaters in machme coating
solids are desirable to reduce the drying load on the mac infc

solids for a coating mixture for knife coating is o scratched

coated papers must dry rapidly so that the coa mg w

by the drier surface. advantages to

In addition to increased machme speed, ^

operating at high solids. For example, the ‘ ^ ^^„ce so with

is generally improved as the per cent so i s j

starch than with casein, and more so -'‘h rhe coating

others. The effect of increasing solids content on t » ^

as measured by the Dennison wax number sho

ratio of starch to pigment was used, and the same amount o ,

1.1 K. A. Arnold, ^ (Oct 23,

108 I. L. Lester and S. C. Lyons, rap

1947)

PIGMENT COATING

1073

TABLE X

Effect of Solids Content of

OF Coating Mixture on Strength of Coating

Solids content of
coating mixture, %

Dennison
wax number

3.5
4.0

4.5

5.5

was applied to the paper m

all cases, but the per cent solids of the coating

mixture was varied.

Flow PfopcTtics of Cootiiiff

Rheology (flow) is a subject of great interest to the coating chemist,
particularly in the machine coating field. The rheological properties o
coating mixtures for machine coating determine the amount of coating
which will be applied to the paper under a given set of conditions, and also
determine the leveling out properties of the coating after application. Un¬
less the coating mixture has the proper flow properties, improper trans er

will result and weird patterns will be produced.

Before discussing the flow properties of coating mixtures, it is de¬
sirable to discuss briefly the subject of flow in general. Flow occurs in a
liquid as the result of applied force. Viscosity is the resistance of a fluid
to flow when subjected to a deforming force. If the velocity gradient is
considered as taking place between parallel planes within the fluid, then
velocitv becomes the rate of shear at a given shearing stress. The unit
measure of viscosity is the poise. A liquid has a viscosity of 1 poise when a
force of 1 dyne exerted longitudinally will move a 1 cm. square plane 1 cm.
per second in reference to a second plane 1 cm. away. There are four basic
types of flow: Newtonian, plastic, pseudoplastic, and dilatant, as shown

in Figjure XVIII-19.

Newtonian (viscous) flow is the simplest type of flow. A material
exhibiting viscous flow starts to flow as soon as force is applied, and the
rate of flow increases in direct proportion to the applied force. If a truly
viscous fluid is forced through a capillary tube (e.g., Ostwald viscometer)
at different pressures, the volume of flow will be directly proportional to the
applied pressure gradient. The curve obtained by plotting rate of flow
versus pressure is a straight line passing through the origin, and the slope
of the curve is inversely proportional to the viscosity (see Fig. XVIII-19).
Thus, viscous flow is represented by the relationship discovered empirically
bv Poiseuille:

V = kP

where V is the volume of flow, P is the shearing stress, and ^ is a constant.

1074

PULP AND PAPER

Bcc 3 .us 6 tlic consistency curve (plot of ra.te of slie<ir versus stress foi ecjual
time intervals) for a Newtonian liquid is a straight line passing through
the origin, the liquid can be defined rheologically by a single value, the co¬
efficient of viscosity. The coefficient of viscosity is proportional to the co¬
tangent of the angle made by the consistency curve and the stress axis and
is numerically equal to the number of dynes required to induce a unit rate
of shear. In viscous flow, viscosity is independent of rate of shear up to the

point of turbulence.

\^W A .1 i-;

</)

U-

SHEARING STRESS

Fig. XVIII-19. Four basic types of flow.

■ Plastic flow is exhibited by liquids which do not flow untd “
definite minimum force, called the (if

XVni-19). Plastic flow is represented by the followun^ relatio P I.

the flow is viscous after the yield point has been exceeded):

V = y -I- kP

where y is the yield value. exceeded is called

The resistance to flow after the yield value has 1 een exce

plastic viscosity and is represented by the slope o w cu

cosity is thus the shearing force required to me uc ^ , 1 ,^

excess of the yield value. At least two pom s

consistency curve of a plastic materia , t e yie p ^ nroperly measured
determine the plastic viscosity, ^ le < va tie j ^ ^ flow by

in viscometers at low rates of shear because o 1 K of

which the material moves as a .solid plug " ^ ^ 5 ^,^ yieffl value

fluid at the surface where the shearing ;„oiflifty of the lua-

results in a property Iviiowu as storm , ^-ssible for a given material

terial to flow under its own weight It is quite pos ^e for^a^^^

to have a high yield value while the 1 ’'.“‘'^' ^ ^ whereas

example, mineral oil may have a high ,„o. High yieW

a clay slip may have a low viscosity, but a high ynid

XVIII. PIGMENT COATING

1075

value in the case of a coating mixture results in a mixUire which is difficult
to handle in the return lines and which has poor leveling properties on the
liaper. Most coating mixtures exhibit a yield point, although if the yield
value is low, it ma}^ not be apparent except by measurement with a suitable
viscometer. All plastic suspensions are flocculated and have an internal
structure which must be overcome before flow will take place. It is this
initial force necessary to break down the flocculated structure which is

responsible for the yield value.

In both viscous and plastic flow, the consistency curve is a straight
line. However, in two types of flow the consistency curve is not a straight
line, and these are known as pseudoplastic flow and dilatant flow. In
pseudoplastic flow, the consistency curve bends away from the stress axis
at high rates of shear, but the curve extrapolates to the origin and thus
resembles Newtonian flow at low rates rates of shear in that there is no
real yield value (see Fig. X\'1II-19). Pseudoplastic flow is attributed to
internal structure which is broken down by increasing rates of shear, thus
reducing the frictional resistance. As a result, rate of shear increases faster
than linearly with an increase in shearing stress. As a general rule, time
has no influence on the rate of lireakdown, that is, the rate of shear at a
given shearing stress is the same for increasing as it is for decreasing stress,
although there may be a slight hysteresis effect at very high rates of shear.”**

Dilatancy is the opposite of pseudoplasticity in that the consistency
curve bends toward the stress axis (see Fig. XVI11-19). In other words,
increasing the shearing force produces a smaller increase in rate of shear
tlian ex^iected. Dilatancy is a condition of minimum voids. In a dilatant
system, the dispersed particles and the liquid are in such close balance that
there is barely enough liquid to fill the voids. When flow is initiated, the
void volume is slightly increased, thereby creating a partial dryness which
increases the resistance to flow. Thus, dilatancy is found only in highly
concentrated, deflocculated dispersions, since flocculation prevents dilatancy
by making it impossil>le for the particles to assume a position of minimum
voids.

.\n extremely important flow characteristic in coating mixtures is
thixotropy. Thixotropy is somewhat the .same as pseudoplasticity in that an
increase in shearing stress produces a larger increase in rate of shear than
expected, but there is the important difference that thixotropic changes are
atTected by time, whereas pseudoplastic changes are produced instanta¬
neously. Thixotropy is believed to be due to an internal structure which
imparts rigidity to the system. This structure is broken down and a vis-
cr)siiy etiuilibrium is reached if the system is agitated for a period of time
at constant rate of shear. However, once the shearing stress is removed.

Green. /. Applied Phys. 13. No. 10 : 611-622 (Oct., 1942)

”®R. N. Wcltmann, Ind. Eng. Chem., Anal. Ed. 15 : 424-429 (July IS, 1943)

1076

PULP AND PAPER

the structure responsible for thixotropy reforms and the system returns
to its original state, but a significant time interval is required for this to
take place Thixotropy is difficult to measure, and the usual criterion of
measurement is the hysteresis loop which is produced by the up- and down-
curves of the consistency diagram. A loop is produced because the upcurve
bends away from the horizontal or stress axis, whereas the dowiicmve is
essentially straight, except at the lower end (see Fig. Xyni-20) . Thixo¬
tropic behavior can be superimposed on any of the other basic types of ow.

SiiKzle-point flow measurements are satisfactory for defining e ow
properties of Newtonian liquids, but multipoint consistency curves ob-
taiifed under known conditions are required in the case of non-Newtoniany
This is necessary since a single reading taken at one particular rate of shear
is not sufficient to define the shape of the curve for non-Newtonians. Even
three or four points are often inadequate for non-Newtonians, since thes
often fail to disclose the complex nature of the curve as, for examp ™

coating. Hence, special viscometers are fare

.oeasure the llow at different rates of shear. The niost p ^

rotational type viscometers which, in genya , ^ The most

cometers consist of a c}! which rotates and a

whereas the MacMichael viscometer consists of a cup w

cylinder which is stationary. susnended by a fine

111 the MacMichael viscometer a meta disk is suspe^^^

torsion wire and a cup m which the ™ ^ ^ ^ I the

r.p.m. When the cup is filled with the or torque

through which the torsion wire is ff .ff„,h„ted from 0 to 360«.
which the wire is twisted is measuied • s

The degrees twist represents the app le . degrees torque is

represents the rate of f'Attained if the coating mixture is

Iriily viscous. A viscosity figure can be f the degrees

taking the slope of the line, or the y^scosi y . The fluidtty,

which is the reverse of f e vscosi is obffimrf ,,
twist liy the r.p.m., usually at 56 r.p.m.

XVIII. PIGMENT COATING

1077

viscous, plasticit}- wiU be evident, and this can be measured by extrapolating
the cur%e to the point where it intercepts the vertical axis. The value at
zero r.p.m., is taken as the yield point. The mobility (vvhich equivalent
to the fluidity in a truly viscous dispersion) is measured by subtracting t e
yield point from tlie degrees twist and dividing by the r.p.m. in or er to
obtain the slope of the cui^e. The shape of the curve indicates the type of
flow. For example, a straight line represents either plastic or viscous flow;
a line curving downward toward the horizontal (r.p.m.) axis represents
pseudoplastic flow; a line culling away from the horizontal (r.p.m.) axis

represents dilatant flow. .

A method of measuring thixotropy on the MacMichael viscometer is

illustrated in Figure XVni-20.“‘'‘ The degrees twist are plotted against

Upcurve obtained at
increasing speed settings

Downcurve obtained at
decreasing speed settings

^ Areo of loop meosures
thixotropic breakdown

REVOLUTIONS PER MINUTE

Fig. XVIII-20. Thixotropy as indicated by MacMichael viscometer.

r.p.m. for both increasing and decreasing speed settings. At increasing
speeds, a typical curve bending toward the r.p.m. axis is obtained, whereas
at decreasing speeds, a straight line is obtained. The area between the two
curves is a measure of the thixotropic breakdown. The slope of the down-
cur\’e in Figure X\TII-20 is a function of plastic viscosity, and the inter¬
cept of this line with the vertical axis is a function of yield value.

Still another method of measuring thixotropy is by subjecting the
coating mixture to a high rate of shear for an appreciable perio'd of time,
redircing the rate of shear to a lower value, and then recording the build-up
in viscosity with time.

The Brookfield synchrolectric viscometer is another widely used in¬
strument. This instrument consists of a spindle which is rotated by means
of a small synchronous rotator through a special, calibrated spiral spring
which measures the viscous drag on a calibrated dial. Viscosity readings
can l»e made at different speeds. Viscosity (in centipoises) can be plotted

S. Hoaglond, “The Rheology of Surface Coatings,” R-B-H Dispersions. Inc.

(1946)

1078

PULP AND PAPER

against rates o£ shear (rate of spindle rotation), and a flow curve thus ob-

tained. . . , „

The rates of shear obtained in tlie nip of high-speed coating lo s are

much greater than those obtained in the rotational viscometers described
above Thus, it is possible that different flow behavior occurs during coa -
ing than is indicated by MacMichael, Brookfield, or Stormer vi^ometer
reLings. Special high-shear viscometers have been developed which are
capable of developing shearing forces more nearly within the range eve
Jed in high-speed coating processes.—- One of the most interesting o
these is the Hercules Hi-Shear viscometer which produces a neogram
plot of rate of shear versus shearing stress for equal time interva s. is
i^!:;rlent consists of two concentric cylinder a cup - ^ob ^hich fit
verv close tolerances. In operation, the cup is hlled with coating mix ,
and the bob is lowered into the liquid. The bob can be rotate ^

,0 1,050 r.p.m., which develops rates of ^^ear a" “
than the conventional viscometers. The cup rotates y i

niltted through the liquid, and

deflection of a calibrated set p ^ rnm.I is reached in the

the control handle so that jg reversed back to zero speed.

Two curves, the upcurve and downcurve, are .hear

matically plotted on a graph the ° shearing stress (torque)

‘T t tml/viscous. the two

in dyne-centimeters X 10 • “ ^ material is thixotropic,

curves coincide and pass downcurve is linear.

the upcurve bends toward the vertica axr , . , ^ horizontal axis.

„ J material is dilatant, the in a straight

line to the horizontal axis to determine y>rfd = 500 and

By taking readings on the upcurve at ^ plastic vis-

1,000 r.p.m.), it is possible to “ «>’f thixotropy (M)

cosity («), yieM ^ Vhe'leveling index, which is a dimensiontes

ratio of the coefficient of for high-shear roll

portant measure of Ae suitability ^ ,vill level after

coating, since it indicates the rase w htained if the coating mixture

leaving the nip. A high leveling m ex viscositv. Normally, the

has a high degree of thixotropy „f the coating niix-

. ;:;f ll^sThm ftTp^^nts the amount ^^

1948)

XVIII. PIGMENT COATING

1079

plastic viscosity inherent in the coating formulation."^ Values for leveling
Index less than 0.20 to 0.25 are considered unsatisfactory, whereas values
over 0.35 indicate good performance. A ty^pical cur\e obtame on le
Hercules Hi-Shear viscometer using a bob 2.5 cm. m height is sho^^n m

Ficure XVI11-21. Calculations are showm below.

Instrument constants are required for calculating y^^ld value and
plastic viscosity. A constant for all rotational viscometers is 9^55 but .b
and C are instrument constants for the particular bob ^ PfJ"

ticular example below, the bob constant S = 3.758 x 10'* and C - 0.0163 .

Torque at 1,000 r.p.m. (from curve) 7ioo* = 448,000 dyne-cm.

Torque at 500 r.p.m. (from curve) rsoo = 309,000 dyne-cm. ^

Torque axis intercept of extrapolated straight line portion of downcurve (from curve)

71 = 91,000 dyne-centimeters.

^ _9.SST«S_

Apparent viscosity at 500 r.p.m. A«o-

r.p.ra.

9.55 X 309,000 x 3.758 x IQ-" _ 2 22

500

poises.

9.55(7,o»-7,)5_

Plastic viscosity at 1,000 r.p.m. Uum- ^ p ^

9.55(448,000 - 91,000) x3.758xl0:^ ^

1000

._9.55(7«o-7,)5_

Plastic viscosity at 500 r.p.m. t/wo- rpln

9.55 (309,000 - 91,000) x 3.758 X 10" _, ^

500

Yield stress 7 = 7,C = 91,000 x 0.01034= 1487 g./sq.cm.

Rate of shear coefficient of thixotropy 3/ = 2((/» - (/O/lnfwgViOi*)

2(1.56-1.28) _ 0^ _Q dyne/cm.-sec.

2 X 2.3 X logio2 0.69

Leveling index L.I. = - - = = 0.32.

1 ?«

Arnold"* suggested a method of measuring flow behavior which is in
close agreement with the effects obtained on commercial roll coaters. This
method is based upon the use of a heavy metal roll which travels down an
inclined plane at high speed and passes over a small quantity of coating mix¬
ture placed on a sample of paper on the inclined plane. The area over which
the coating is spread by the action of the roll is an indication of the flow.
A fluid coating mixture will be spread over a greater area than a viscous
coating mixture, and a thixotropic mixture will spread over a greater area
than a dilatant mixture.

The correct rheological properties depend primarily upon the type of
coating process in which the coating mixture is to be used. Brush coating

"»J. W. Smith, R. T. Trcifa and H. O. Ware, Tappi 33, No. 5: 212-218 (May,
1950)

K. A. Arnold, Paper Trade J. 117, No. 9: 9&-104 (Aug. 26, 1943)

1080

PULP AND PAPER

is best carried out with coating mixtures exhibiting Newtonian or slightly
dilatant flow. According to the results in one mill, coating mixtures for
brush coating must have a MacMichael viscosity between 25 to 35, an a
yield point below 10, and preferably below 5. These results are ase on
values obtained with the MacMichael viscometer at 25 C, ^^mg tie ^

type plunger and a N o. 26 torsion wire. If the yield point is over , rusi

marks will be visible in the coated paper.

1000

900

800

LiJ

700

600

500

400

300

200

2 4 6

TORQUE, dyne-cm. x 10

Fig. XVIII- 21 . Consistency diapam of coating mix¬
ture obtained on Hercules Hi-Shear viscome

Coating mixtures for machine or roll coating. A

nnt and greater ^Isticity ^n heavy-bodied coating mix-

1 are used for high-speed -H coafng s.nce —

Dating mixture to become more ui u ^ produce a level

oating roll, thus permitting the —Jo flow J

urtace. Excessive penetration of S ^ shearing stress is removed,

lated by the rethickenmg which occurs a desirable in order to

„ certain cases, a dilatant coating mixtime may Je

nhibit excessive penetration of the coa g j mixtures having a

,f its application to the Jiaper, but ordinarily, coating

XVIII. PIGMENT COATING

1081

marked dilatant character tend to produce coatings which are rough and
mottled, due to a piled effect on the surface. .\s mentione a
leveling properties of coating mixtures for high-sv^ed roll coahng are be t
measured by a high-speed rotational instrument such as the Hercules

shear viscometer. The importance of the correct flow ° J

taining proper leveling of the coating on the paper is discussed later in the

section on smoothing the coating.

\ large number of variables affect the flow properties of the coatin^
mixture at the time of application to the paper. Rowland”* lists the fol¬
lowing : clav mineral species, particle size of clay, base exchange of clay
absorritive qualities of raw stock, solids content of coating mixture, amount
and type of adhesive, amount and type of mineral dispersing agent, pressure

of coating rolls, and sj^eed of machine. j i «

The relationship which exists between the per cent solids and the How

liehavior liave been discussed in the preceding section. This is an impor¬
tant relationship, inasmuch as increasing the concentration of either the ing-
ment or the adhesive in the coating mixture increases not only the viscosity,
but also the yield point and the tendency toward anomalous flow. The effect
of solids content on the flow properties is different for different coating

mixtures.

The optimum flow properties of the coating mixtures are related to
the absorliency of the coating raw stock. Dickerman and Riley”® list the
following requirements for coating mixtures to be used on absorbent and
non-absorlient papers. The Stormer viscometer, with a 500-g. load on the
driving pulley and a temperature of 40® C., was used for measuring flow
properties. The viscosity is reported in seconds for 100 revolutions of the

rotor.

Sheet type

Viscosity.

sec.

Yield value
dynes/sq.cm.

Mobility,

rhe

Abvorbcnl ....

Xo^-absorb^nt . ^

The particle size and particle size distriliution of the pigment has an
imjiortant influence on the flow pro^ierties of the coating mixture. As a
general rule, the finer clays tend to produce the more viscous coating mix¬
tures. Clays containing a high percentage of large particles (i.e., 8-10 mi¬
crons) tend to produce dilatant coating mixtures, whereas clays having a
high percentage of small particles (0.8-1.0 micron or lower) tend to pro¬
duce thixotropic coating mixtures. Clays of low hydration capacity tend to
produce low-viscosity dilatant coating mixtures.^^’ Another extremely
imponant factor is the degree of dispersion of the pigment. If the pigment

”» B. W. Rowland. Paper Trade J. 112, No. 26; 311-313 (June 26, 1941)

"•G. K. Dickerman and R. W. Riley, U. S. 2,425,231 (Aug. 5, 1947)

*** K. A. Arnold. Paper Trade J. 117, No. 9 : 98-104 (Aug. 26, 1943)

1082

rULl’ AND PAPER

is flocculated, the coating mixture will have a higher viscosity and a higher
Yield iioint than if the pigment is well dispersed or deflocctilated.

■ Although part of the viscosity of a coating mixture must Ik attrihutesl
to the pigment, the adhesive is responsible tor most of the body or vis-
cositv In general, the higher the ratio of adhesive to pigment the higher
the viscosity of the coating mixture. The coating chemist should reprd the
I,i<mient and adhesive as two very active colloidal substances which are in
comiietition for the water, dispersing agent, cations, and other minor iit-

Zlients in the coating mixture. This concept is ^

clav which is an extremely active colloidal pigment. t is i e y
Si’striWion of water and ions occurs when clay and adhe- «
into contact and for this reason, sufficient time should be allowed alter

niWing to permit changes to take place and to allow the "ix^

come to a Lte of equilibrium. The final effect may be quite different
that which is obtained upon the initial mixing of the ingredients.

The effect which is obtained upon the “dheshrwill

is difficult to predict. It is X mixed with

■cause thickening vyhen nii.xed with o y . redistribution of

another clay. p'dration of the clay unfavorably in one

case, and favorably in the otherj-knd^^^ f^'^Ter dlection. Starch
affect the flow J „( dispersing agent

may produce a thickemn^ y ^ _r Jicnprsin? aeent below

j i„„, *. •»““ * »■

that necessary for maximum ui y. remove enough dispersing

agent to reduce the amount to th Y always co-

die point of maximum d” Version. By the

incide with tlie maximun ^ alwavs produce the most

same token, the most easily dispersW^^^^^^ , re-

fluid coating mixtures. As a mixture than is required in the

quired for minimum viscosity in a co g because of the

Ly-water slip, particularly if The use of alkali'

tendency of the starch to rob the clay of d'sper g » ^^^^ination with the
(about 0 . 1^0 sodium hydroxide ’ -jl particularly if starch is

dispersing agent is helptul in owenng mixtures only the

used as the adhesive. In the case ^ dispersion of the

minimum amount of dispersing agent "“J j adhesive, since exces-

pignient should be added prior to the ^ddit h „( day.

sWe dispersing agent in presence of casein causes mart

i« B. W. Rowland, Paper Trodc /. N|-

1.9 W. R. Willets, Tappi S3. No. 4 : 201-208 (Apr.,

XVIII. PIGMENT COATING

1083

Casein and starch adhesives produce coating
flow properties. Casein coating mixtures are normally ^^nngy and exhib

Newtonian flow at low solids and are generally plastic and s ig ^ T
at high solids. Casein coating mixtures are widely use in ^

knife coating where low solids are permissible and viscous flow is esi ^ ,
but they are not well adapted to high-sohds roll coating ecause o
tendency toward dilatant flow. However, suitable coating mixtures can le
prepared by modifying with sodium sesquisilicate using special techniques
already described. Starch coating mixtures are generally used a g
solids where they produce mixtures which are short and which exhibit
thixotropic flow. Because of their high-sohds and thixotropic nature,
starch coating mixtures are widely used for roll coating. Soaps are fre¬
quently added to starch coating mixtures to increase the thixotropic prop¬
erties. About ammonium stearate, or 2 to Ivory Flakes, based on
the starch, is generally used. This should be added while the starch is hot;
if the starch has been cooled, the soap solution should be heated, l oo muc i
soap should be avoided, since this causes a reduction in the thixotropic

effect.

TJ/ ^ t T? ^4- M / I *mi

Many coating chemists use a test which measures the water retention
qualities of the coating mixture. This test is usually carried out by floating
a small piece of unsized paper of medium weight on the surface of *e coat¬
ing mixture and taking the time in seconds for the liquid in the coating mix¬
ture to pass through the paper and change the color of indicator powder
which has been dusted on the surface of the paper. The indicator powder
may be finely ground potassium permanganate, or it may be a mixture of
dyestuff and inert ingredients in the proportions of 1 part blue dyestuff,
5 parts soluble starch, and 45 parts powdered sugar. The end point is
taken as the time for the first marked change to occur in the color of the
indicator. The use of a reading lens helps to judge this point.

The value of the water retention test is based upon its usefulness in
predicting the rate at which the coating mixture will release liquid to the
base paper. If the coating mixture has an inordinately low water retention,
water will leave the film of coating too rapidly, with the result that the
coating layer increases in viscosity and does not have time to level off and
produce a smooth surface. On the other hand, if the water retention is too
high, two things may happen: (i) either the coating will take too long to
dry, with the result that it is marred by the drier surface, or (2) the coating
will have too much after-flow, with the result that, it forms puddles. The¬
oretically, there is an optimum water retention for each grade of raw stock;
a coating mixture with a low water retention will work better on highly
sized paper than on a weakly sized sheet. High water retention is desirable

1084

PULP AND PAPER

in thixotropic coating mixtures for roll coating, since a high water-holding
capacity tends to resist the release of water to the base paper when the

coating layer is thinned by the action of the coating roll.”®

In most cases, a high water retention goes hand in hand with a high

adhesive retention and high coating strength. The test is o ten use as an
indication of the adhesive retentive powers of the coating ^‘^^ure Coa -
ing mixtures of high adhesive retention are required when the coated paper
■Jto be printed with gloss inks in order to retain a larger proportion of ad¬
hesive in the coating. However, strong coatings can be
in^ mixtures of low water retention, e.g., when polyvin>l alco
the adhesive. There appears to be a better relationship etween wa e
tention and dusting tendencies than between w^ater retention an streng i.
This relationship is based on the assumption that the adhesive m a coa mg
^Ir retention tends to strike into the base paper to such

Tdtgree that the layer of coating is robbed of adhesive, thereby producmg

' '™“iLVr “nhllllul “rnfluencerpredominantly by the viscosi^
of the coating a*e“ pi^tirt^hd

:: trrr o; an .u.

casein and polyvinyl alcohol produce “a ’"8 mixtures

tention than starch. When compare o^ casein in water retention

however, starch coating mixtures are the qu the same

because of the higher percentage o s arc retention because of

basis polyvinyl alcohol coatings have a very low water retent,o

the low percentage of adhesive which on water reten-

Although the adhesive has „„ of the pigment

tion, the degree of <impers,»n and ^ g example, tend to

also have an important nflu«nce. y t,,e less hydrous

produce coating mixtures stock more slowly,"

clays, because they release the ..rnduces coating mixtures of

Bentonite, which is a is much less hydrous than

elay, and consequent y rt ® ^icle size of the pigment ,n-

readily to the raw stMk. Decreasing ine 1

creases its water-holding properties.

Other Colloidal Properties

The surface tension of the coating

ing is applied by roll coater, because this „,la Surface

of the ribs which are formed by the ° . f (Pe coating mixture,

tension is, of course, related to the wetting qualities

XVIII. PIGMENT COATING

1085

and hence affects the penetration of the coating into the base ^ ^

of the ingredients used in the coating formula, such as eveners a
foaming agents, have a pronounced effect on the surface “ .

The colloidal stability of the coating mixture is important, pa y

when the coating mixture is held at the mill for severa ^

used Colloidal instability is manifested by a change in visco y

fixture upon standing. If .he coating mixtute contatns starch^as

the adhesive, there is likely to be an increase m viscosity caused y
gradation of the starch. On the other hand, if the coating
casein as the adhesive, thinning generally results from the hydrolytic effect
of the alkali on the casein. Willets^^^ found that casein coating mixtures
made with silicate as the dispersant markedly decreased in viscosity on
standing, whereas mixtures made with sodium hexametaphosphate as t e
dispersant increased in viscosity on standing. In addition to these effects,
spoilage may occur if the coating mixture is allowed to stand around or
several days in hot weather. Spoilage reduces the viscosity of the adhesive,
hut the coating mixture itself may thicken, on account of flocculation o t e
clay by the acid produced during spoilage. Even a slight amount of spoilage
will result in an appreciable decrease in the strength of the coating before
there is any apparent odor. For these reasons, coating mixtures should be
protected by preservatives if they are to be held for any length of time.

The presence of oil or grease in the coating mixture may result in oil
spots due to poor wetting of the paper. Oil or grease may be present as a
result of contamination from leaky bearings, too high a fat content in casein
or animal glue, or too much soluble oil (e.g., pine oil) added as an anti-foam.

Foam

Foaming of the coating mixture is a frequent source of trouble to the
coating mill chemist. Pinholes, visible with a hand lens or even with the
naked eve, are produced as a result of foam breaking and forming a hole in
the semi-dried coating. In some cases, foam may become so excessive as
a result of air being whipped into the coating mixture that the coating mix¬
ture loses its fluidity and becomes difficult to handle.

Foam is of two types, coarse and fine. Coarse foam, commonly called
froth, collects on top of the coating mixture and ordinarily is not trouble¬
some. Fine foam remains dispersed throughout the body of the coating
mixture. It is usually very troublesome and is difficult to remove, once it
has been allowed to form.

Agitation, whether it takes place in mixers, pumps, or pipelines, greatly
increases foam by whipping air into the coating mixture. Consequently,
excessive agitation should be avoided as much as possible. Common causes

1086

rULP AND PAPKR

of foam are opn pipelines vvl.ieh disclrarge the coating mixture into open
tanks, and leaky pumps which suck air into the coating mixture. In r^l
coating, air may whipped into the coating mixture by the action o tlic
rolls. Siioilage often causes a foamy condition, due to the liberation o ps
bubbles from the decomposing adhesive. This tyiie of foam is amwt in
possible to eliminate. If the coating mixture contains calcium carbonate,
a drop in />H (due to spoilage or other causes) will cause excessive oam.

All lots of casein do not foam to the same extent: some caseins toaiii
very little whereas others foam very badly.”" Mixing a oamnig c^in
vvit'h a nou-foaiuiiig casein usually results in a non-foaimng mixture, indi-
^tin^hat lack of foaming is due to the presence of a natural anti-foaming
r' fmmine casein The use of sodium carbonate as a dispersing

agent for casein causes foam, because ot the carbon dioxide which is rel^

rs: k—i. - -

It is seldom that starch coating ,8’cause any

Even oxidized Le been ’neutralized during

trouble in coating lively to foam badly; hence, other

manufacture with sodium which do not decompose to

alkalies snch as sodium phosphate or not po

produce carbon dioxide, are used for “ ,han-casein, but

commercial soybean protein contains an a f g

Most coating mills use some type of j^^mula as in-

This anti-foam is added as a regu « pa ^

surance against the occurrence of foam, sme ^ ^

and disappear for no apparent reaso . ° ^jgh molecular weight

foams are the sulfonated oils, pine oi, phosphates, other petroleoni

alcohols, skim milk, ether, kerosene, n pjer trade names. These

fractions, and certain proprietary “the fine foam
last are generally surface tension _ep „hctine.

luE. Sutermeister, Pilfer W Nwr 1,45)

res E P. McGinn, Pefer M- H

XVIII. PIGMENT COATING

1087

been that the addition of an anti-foam to a coating mixture which has al-
rZXg^to foam does very little good. Best results are obtained by
adding the agent either to the pigment dispersion or to the adhesive solution
before the two are mixed together. Most chemists prefer a mg e an
foam to the pigment while it is being dispersed in water.

Coating Raw Stock

Pigment coatings hide or cover up some imperfections in the base pa¬
per. This fact has caused some paper chemists to regard the
of little importance, except as a mere earner for the coating, an i las ,
in some cases, to the use of inferior base stock. Aetna ly. however he
properties of the tody stock have a great influence on the quality ot he
finished coated paper. Milham‘“ regards the base paper as presen mg
most important technical problems for the paper coater. mong t le prop
erties of the base paper which are of importance are the formation, color,
finish, sizing, strength, moisture content, porosity, brightness, and opaci y.

Composition of Base Paper

Most coating raw stock is made from lightly beaten, weakly sized stock.
Coating raw stock is classed as a special grade of book paper and is usua y
made and finished in the same general manner as book paper. A mixed
furnish containing bleached sulfite and either soda or deinked stock has
long been considered the ideal furnish. Several mills are now using sulfate
pulp in place of sulfite particularly when high coat weights are to be ap¬
plied. Some grades contain a proportion of rag pulp to improve the folding
properties. Groundwood offers excellent possibilities, and it is likely that
increasing amounts of groundwood pulp will be used, particularly bleached
groundwood. Several typical examples of coating raw stock furnishes are

given in Table XI.

TABLE XI

Typical Examples of Coating Raw Stock Formulas

Furnish

Fiber

Example
No. 1, %

. 50

Example
No. 2, %

c:„ifote .

Non-fibrous, % of fiber

. 0.50

AlllTTl .* -

. 1.0

1.5

Filler .

.. 5 (clay)

123 E. G. Milham, Paper Mill 60, No. 47: 15, 24, 26 (Nov. 20, 1937)

1088

PULP AND PAPER

Bearse and Barclay^^^ list the structure of a typical coating raw stock
as follows: 50 to 25% supporting fibers, 40 to 70% filler fibers, 5 to 15%
filler pigment, 1 to 10% sizing and other ingredients, 40 to 60% air. The
supporting fibers (e.g., sulfite) supply the strength and flexibility, and con¬
sequently should be as continuous and as well bonded together as possible.
The filler fibers (e.g., soda) furnish resiliency-and bulk and act as the base
for the coating. The filler (e.g., clay) contributes to the brightness and
increases the receptivity of the paper for the coating. The air space makes
the sheet receptive to the coating mixture and, in addition, supplies the
printing cushion. The sizing in the sheet regulates the receptivity of the
paper to the coating and the amount of penetration of the coating into the

paper.

Sizing

In most grades of coating raw stock, light to medium sizing is used, but
the optimum amount of sizing depends upon the properties of the coating
mixture and the type of coating process. The subject of raw stock sizing

is discussed in more detail in a later section.

Considerable interest has been shown, in the use of wet-strengt paper

for coating rawstock.'“ A degree of wet strength is desirable '

helps to prevent breaks during the coating operation, thereby permitting the
use of weaker pulps in the raw stock. Because wet-strength papers have
better dimensional stability, less trouble with curl would be expected. In¬
creased wet bonding of the surface fibers would prevent their being ru

Up and worked into the coating.

Finish and Porosity

Two highly important properties of the base paper are finish and p^^

00 porous, and tL surface should be smooth, even, and free from imperf -
tions In machine coating, it is common practice to pass the paper ju t

coating between two heavy rolls known as —
stack These are usually steel or chrome-plated rolls operated V
up to 100 lb. per lineal inch of nip often are wat^ P-

sticking of the paper. Coating raw stock should not have t^ S

since this results in a weak bond between “‘>^8
High bulk is desirable in coating raw s

M n.rrlav Pai>er Trade J. 120. No. 12 : 24, 26 (Mar. 22,

124 N. I. Bearse and E. H. Barclay, t'aper tuu ,

6 ,r D !, Am Piilo Paper Mill Supts. Assoc. 1945: 202-203

125 T? TCtitnlcr Booh^ Arn. iru p P /t i'? io 47 ^

m N; 12: 26 (Mar. 22,

1945)

XVIII. PIGMENT COATING

1089

uniform calendering of the coated paper without producing hard spot .

A soft, bulky sheet will coat better than a hard sheet because it has less
tendency to curl. Furthermore, a soft bulkT sheet is more resilient and
forms a'better cushion when the coated sheet is printed. The bulk must not
lie too high, however, since the strength of the paper will then be dehcien .

Strength of Raiv Stock

The strength of the raw stock is important because it is related to the
strength of the final coated sheet. High tearing strength and high folding
endurance of the body stock are good indications of the amount of handling
the coated pajjer can stand. A high degree of fiber bonding is desirable, be¬
cause this in part detennines the resistance of the coated sheet to picking
during the printing operation. In particular, the surface fibers of the base
paper should be well bonded to the body of the sheet m order to prevent
their l>eing niblied loose and worked into the wet coating. In general, it
may be stated that increased beating of the base stock tends to increase the
over-all strength of the coated sheet (see Table XII). By the same token,
the presence in the raw stock of substances such as rosin and clay causes
a reduction in the strength of the coated sheet. The base paper must not be
too hard and dense, since this causes trouliles such as curling and streaky

coatings.

Brightness and Opacity

To the casual observer, it would appear that the color, cleanliness, and
brightness of coating raw stock are of little importance, inasmuch as the
jiajier is covered by a layer of coating. This would be true if thick coatings
were applied, but in most commercial papers, the layer of coating is thin
enough that the raw stock determines in part the brightness of the coated
|»aper. For this reason, it is desirable to use bright, clean paper for coating
raw' stock. If light coatings are applied, the body stock should have a
brightness as close as possible to that of the coating. Roll marks, brush
marks, or other imperfections in the coating will be less conspicuous if the
coating and raw stock have the same brightness than if the raw stock is ap¬
preciably darker than the coating. Large dirt specks should be avoided in
the raw stock, since these tend to show through even the heaviest coatings.

Opacity of the raw stock is an important factor when light-weight
coatings are applied. As a rule, groundw'ood papers make the most opaque
raw' stocks, and papers made from w'ell-beaten rag or chemical wood pulps
make the least opaque raw' stocks. Fillers in the raw stock help to increase
the over-all opacity, as does increasing the basis w'eight of the raw stock.
The various factors affecting the brightness and opacity of coated papers
are discussed later in more detail.

1090

PULP AND PAPER

Unijormity

Uniformity in the coating raw stock is of the greatest importance. The
weight, thickness, finish, smoothness, and the porosity should be as uniform
as possible across the width of the sheet.'"® Paper which is wild tends to
absorb coating unevenly, resulting in a mottled coating. Two-sidedness of
the raw stock causes trouble if the paper is coated on both sides, inasmuch
as the printing qualities of the coating will not be the same on both sides.

Applying Coating to Paper

One of the least understood phases of the coating process is the one
involving the application of coating mixture to the base paper. This is,
however, a very important phase, because it is during this period that the
components of the coating mixture adjust themselves to their final state on

the paper. • r

The film thickness of a wet coating is about 0.0014 to 0.0017 in. for a

book paper coated with 25 lb. of coating per ream, 25 x 38-500 basis coated
two sides, using a coating mixture between 35 to 40% dry solids. After
drying, the film of coating is approximately 0.0005 in. or 15 microns in
thickness. On machine-coated papers, the wet film thickness is even less,
being about 0.0005 in. (for an 8-lb. coating applied to one side, using a
coating mixture of 60% solids). These thin layers

chine Adjustments in order to obtain an absolutely smooth and level coating.

Wetting of Raw Stock by Coating Mixture

The initial wetting of the raw stock by the coating mixture
Most coating mixtures readily wet the raw stock, on account of tte
affinitv of the coating ingredients for the paper. In casein coatings,
Av^Uingt irnte/by Aiigh ; Cobb and Lowe«- point out that «sem

coating mixtures wet paper more rapidly than water. There ^ ct J
Z weAting to be increased by the action of the brushes m brush coating and

by tbe action of the rolls in roll coating. Once the coating has P™P >
applied, it must be maintained in place until it is tho.ug^ d™i T ^
flow properties of the coating mixture must be such
not run and produce an uneven or wavy coating.

Measurement of Penetration oj Coating Mixture into Rate Slock

The period during which penetration can take place is “^“Uonds
tion, since most coatings are substantially set within tour to seven

IM I. J. Friel, Paper Milt (Jan 21 'l932)

XVIII. PIGMENT COATING

1091

after application. This short period of penetration makes penetration a
difficult subject to study. Furthermore, the coating mixture may penetrate
into the raw stock in one or more of three different ways: (1) the coating
mixture may penetrate as a whole, (2) the adhesive may preferentially
penetrate into the sheet, leaving the pigment behind, or (3) the water in
the coating may penetrate to a greater extent than either the adhesive or
the pigment. It has been stated that the casein in casein-clay coating mix¬
tures penetrates the raw stock at least 3 microns beyond the farthest pene¬
tration of the pigmeiit.^^^ Segregation of the pigment and the adhesive may
occur as a result of the gravitational pull during drying.^^® If this occurs,
the pigment would tend to settle to the paper surface, leaving a thin layer
rich in adhesive at the surface of the coating. On the bottom side of the
sheet, the reverse would occur, namely, a layer rich in pigment would tend
to form on the exposed surface of the coating. Obviously, these effects, if
important at all, would be most pronounced in coatings containing pig¬
ments of high specific gravity. If mixed pigments were used, there might
be a tendency for the heavier pigments to settle to a greater extent than
pigments of lower specific gravity. This difficulty is common in the drying
of paints, and sometimes leads to a change in the color of the paint, which is
referred to by paint technologists as “flooding” or “floating.”^®®

There is no very good method of measuring penetration. Single-
terry^®® used a microscope to measure the depth of penetration by embed¬
ding a sample of the coated paper in a liquid mixture of urea-formaldehyde
resin and zinc chloride, which was then allowed to harden, and finally was
ground and polished into a section about 10 to 20 microns in thickness.
Casey and Libby^®^ used a microscope for measuring the depth of penetra¬
tion of starch in starch-clay coatings by trimming one edge of the coated
sheet, staining with iodine solution to bring out the starch, and then ex¬
amining under the microscope, using reflected light. Phillips^^® measured
adhesive and water retention by removing wet coating from the base paper
by scraping at various time intervals and measuring the solids content and
ash content of the wet coating layer. Lafontaine^*® and others suggest that
an angle of contact test be used on coated papers to indicate the amount of
adhesive which has left the coating.

«iR. M. Cobb, Paper Ind. 12, No. 4 : 753 (July, 1930) ; /. Rheology: 158-166
(1930)

^32 S. Hoagland, “The Rheology of Surface Coatings,” R-B-H Dispersions, Inc.
(1946)

333 C. R. Singleterry, Paper Trade J. 113, No. 18: 233-236 (Oct. 30, 1941)

334 J. p. Casey and C. E. Libby, Paper Trade J. 127, No. 25 : 522-529; No. 26*
530-536 (Dec. 16, 23, 1948)

335 J, Phillips, Thesis, Study of Properties and Structure of Pigment Coatings,”
New York State College of Forestry, Syracuse, N. Y, (Dec., 1949)

336 G. H. Lafontaine, Paper Trade J. 113, No. 6; 63-65 (Aug. 7, 1941)

1092

PULP AND PAPER

Efect of Penetration on Properties of Coated Paper

Penetration of water and adhesive into the base paper affects the
spreading properties, the strength of the coating, and the ink receptivity
of the coating. There must be some penetration of coating into the paper
in order to anchor or bind the coating to the paper, but the coating must
not strike into the paper to an excessive degree. There is an optimum water
and adhesive transfer from the wet coating to the paper for proper leve mg
of the coating, for best anchorage to the base stock, and for the preven ion
of dusting.^®^ The penetrative qualities of the coating mixture must e

adapted to the absorptiveness of the base paper.^^®

When the coating mixture first comes into contact with the raw stock

the water penetrates and brings about a swelling and disarrangement of the
surface fibers. Too much penetration of water at this point results in
Imrtiorand wrinkling of the paper. After the initial penetration of
liouid the network of fibers on the surface of the paper becomes swol en,
thus tending to block off the pores of the paper and inhibit furt er
fiou rrcoating mixture. There must be sufficient penetration of adhe-

Ibout the sutce fibers so that these fibers are we 1

However, excessive penetration of adhesive ea s calen-

* a. 4 . f Unas mnpr for reducing penetration, e.g., reducing pe
treatment of the paper lor reui g i ,,natinp- whereas re-

by increased sizing tends to reduce the strong i o increase

(hieing penetration by increased beating o le^ as^^^ however, strength of

the strength of the coated sheet. " ’ . ,,„nirth of the base paper

the coating is more affected by changes in the syength of the

than by changes in the depth of penetrahon »*e

Some of ,J raw stock, rheological proper-

raw stock are as follows. ^ fibers and the coating m-

"•r; ::sfr:St- ss—.s

other in complex fashion.

..r B. W. Rowland, Paper Trade

1S8G. K. Dickerman, Paper MtU Hq. (May 7, 1949)

130 G. Davidson, Paper Mill News ’ ' jg. 233-236 (Oct. 30, 1941)

140 c. R. Singleterry. Paper Trade (Jan. 7, 1947)

141 N. I. Bearse, Private communication to the autnor u

XVIII. PIGMENT COATING

1093

Effect of Properties of Coating Mixture on Penetration

The properties of the coating mixture which influence penetration are
viscosity, yield point, degree of anomalous flow, water retention, per cent
solids, and pH. It is well known that high-solids coating mixtures produce
stronger coatings than low-solids coating mixtures for the same per cent
composition. This effect has been ascribed to the reduced migration of
adhesive from the coating layer, in the case of the coating of high solids
content. Phillips^^^ found, however, that increasing the solids content of
the coating mixture decreased adhesive retention in the coating layer in
spite of the fact that the strength of the coating was increased. The greater
strength of the high solids coating, despite the lower adhesive retention,
can be attributed to better distribution of adhesive in the base paper. The
over-all viscosity of the coating mixture is made up of two components, the
viscosity of the mineral component and the viscosity of the adhesive com¬
ponent. It is the viscosity of the adhesive component which is primarily
responsible for the extent of penetration of adhesive into the paper. Thus,
if the viscosity of the coating mixture is held constant while the solids con¬
tent is increased, the penetration of adhesive into the paper will actually in¬
crease because of the lower viscosity of the adhesive which is present. Some
coating mills make it a practice, when testing coating adhesives in the lab¬
oratory, to make tests at several different solids content in order to deter¬
mine the effect of solids content on strength. As a general rule, coatings
containing starch as the adhesive lose more in strength than coatings con¬
taining casein as the adhesive when the solids content of the coating mixture
is reduced.

Increasing the pYL of the coating mixture tends to increase the pene¬
tration of the coating mixture into the paper. The optimum pH depends
upon the degree of sizing of the raw stock; hard-sized raw stocks work best
with coating mixtures of higher pH value. The optimum pH value of the
coating mixture is generally between 7.0 and 9.0, depending upon the de¬
gree of sizing in the base paper. Coating mixtures containing precipitated
calcium carbonate may have a pH up to 10.0, due to the presence of free
alkali in the carbonate.

Effect of Ratv Stock Properties on Penetration

The important properties of the raw stock which determine the amount
of penetration are sizing, porosity, and moisture content. The adhesive in
the coating mixture will tend to penetrate into the paper beyond the pig¬
ment if the base paper is weakly sized. If a given coating mixture is ap¬
plied to two raw stocks differing widely in sizing, the ink receptivity of the

J* Phillips, Thesis, Study of Properties &nd Structure of Pigment Coatings "
New York State College of Forestry, Syracuse, N. Y. (Dec., 1949)

1094

PULP AND PAPER

‘g

coating will be higher in the case of the weaker sized raw'stock, indicatini

that more of the adhesive has penetrated into the paper.

The optimum sizing for the raw stock depends upon the characteristics
of the coating mixture. Casein coating mixtures require less sizing in the
raw stock than starch coating mixtures.^^* High-viscosity coating mixtures
require less sizing in the raw stock than low-viscosity coating mixtures.
Dickermaii^*® reports that unsized papers work best for machine coating,
the absorbency of the paper being controlled by the type of furnish and de-
^rree of beating Milham'"« points out that there appears to be an advantage
hi maintaining the body stock at a />H of 5.5 to 6.5 for starch-coated sheets
The effect of rosin size on penetration is due primarily to a lowering of
the contact angle between the liquid in the coating mixture and the paper.
However, the electrostatic charge on the pores of the paper is also of im¬
portance. Rowland^^' points out that the starch adhesive in a starch-clay
coating mixture will penetrate readily into the sheet if the paper is nega¬
tively charged, whereas if the paper is positively charged, peneUation vm
be hindered. These effects are independent of water penetration effects,

since they occur on unsized papers. .

Another property influencing penetration of adhesive is the moisture

content of the base paper. It was found m the eariy ^

Te li!ng in strength. This was caused by ^ces^^P-™ tot

a bsolulely dry towel. Hence, in the production of ™ach.„.coa«
naners it was found advisable to control the moisture content of the has
paper within limits, usually between 10 to 15%. If ‘ho paper has a much

Wlmr moisture cmttent tkn this, the adhesive in the coat.ng will oak
f rner excessivelv. On the other hand, if the base paper is too dry,

the coating will not bond’ properly. The depth of '"he

smrch-clay coating mixture increases linearly m unsued base paper as

moisture content of the sheet is increase rom ^ o

Pigment coatings do not dry evenly, but instead form d«l' a

spots during drying. This uneven drying is caused par > Ij

plication of coating, with the result that the ‘f”"''." ^|, 3 ^„H,encv of the

Ihe thicker spots, and partly to a lack of uniformity m absorbs.

i« J p Casey and C. E. Libby, Pofrr Trade J. 127, No. 25 : 522- ,

i.r^NM^“ei?raid'K’HTarc.ay, Pa^r Trade /. ;2d. No. 12: 26 (Mar. 22

Dickerman, Paper Mill 61,

146 E. G. Milham, Tappi BuU. 5/ (Aug. ^ 1945)

147 R. W. Rowland, Paper Trade 522-529; No. 26.

148 J. P. Casey and C. E. Libby, Paper Trade J. 127, No.

530-536 (Dec. 16, 23, 1948)

XVIII. PIGMENT COATING

1095

base iiaper caused by uneven calendering or sizing. Davidson"^'^ and
Cagle*’’" showed that the s])ots which are the last to become dull during the
drying of the coating are the spots which show the least receptivity to ink,
and are the most susceptible to picking. It is believed that the adhesive
shows less penetration into the base paper in these areas, thus accounting
for the low ink receptivity and poor strength.

Smoothing the Coating

After the coating has been applied to the paper, it must be smoothed
or made level. W^hen first applied to the raw stock, the wet coating is usu¬
ally in a roughened condition, due to the brush or roll marks left on the
surface. Immediately after application, the coating tends to spread out and
jiroduce a level surface on account of the force of surface tension. The im-
[)ortant factor is the flow behavior of the coating mixture during and im¬
mediately after the coating has been apjilied to the paper and subjected to
the agitation of the coating rolls or coating brushes. The leveling proper¬
ties depend upon the water-holding capacity of the coating layer, because
too rapid penetration of water into the base paper tends to set the coating
before it is level. The requirements for proper leveling of the coating de¬
pend upon the type of coating process.

In brush coating, the coating mixture should be fluid under small
shearing stress and have little or no yield point so that the brushes can
spread the coating evenly over the surface of the paper. Part of the work of
brushing is consumed in overcoming the viscous resistance to flow, and part
is used in breaking down the structural resistance (thixotropy). The
coating mixture must not have too high a viscosity, since this will prevent
even spreading, but on the other hand, the coating mixture must not be too
fluid, since this causes running. It should have a high water retention in
order to retain its fluidity long enough after application to the paper to be
smoothed out by the action of the smoothing brushes. The rate of body
build-up after brushing should not be too rapid, inasmuch as this prevents
after-flow and tends to leave brush marks in'the coating. In order to meet
the above requirements, it is customary to use coating mixtures of low
solids content. Either casein or starch can be used as the adhesive in brush
coating formulas.

In air knife coating, the coating is smoothed by the action of a fine air
jet which blows across the surface of the coated paper. Ordinarily, this
type of coater produces a very smooth coating, even on rough-surfaced
papers. In general, the requirements of a coating mixture for air brush
coating are similar to those for brush coating, although coating mixtures of
somewhat higher solids content and somewhat higher viscosity can be used

*«»G. Davidson, Paper Trade J. 110, No. 24 : 318-320 (June 13 1940')

J. H. Cagle, U. S. 2,286,259 (June 16, 1942)

1096

PULP AND PAPER

in air brush coating. Anything which disturbs the air stream, such as
dried coating on the lip of the air slot, will cause streaks in the coating.

The requirements for roll coating are quite different from the require¬
ments for brush or air knife coating. Coatings appiled by roll coaters must
set very rapidly, and hence high-solids coating mixtures are used. The
viscosity is high, and in some cases, the coating mixture is semiplas ic a e

time of its application to the paper. When the print roll ' '

paper, the coating is “necked out” which leaves ribs or ridges m the wet
mating aim. The size of these ridges is determined by the forces o su ace
teitsiom flow properties, rate of shear at the nip of the rolls and the rate
of separation between the coated paper and the deposi ,

The coating is subjected to very high rates of sh^r in the nip
coaters due to variation in peripheral velocity at different points m the
1 cILd by a difference in the softness of the rolls and by the fact to
TsoL point in the nip the coating color is substantially stationary whde

the rolls are moving at hi^h speed. hphavior. In the

ierted in the roll nip causes radical changes in _

r“t b s:;:.- Sit ;:S

was that thixotropy resulted in after-flow ^

freshly applied paint, but this effect is^now ^^e^i^ the coating

isset. A more acceptable theory than ^

the roll nip (of the or^ er of 10 thixotropic mixtures

mum at the face of the roll, an coatin^^ tends to rupture

—01 ::iit-“

*:efo7rotropy required to obtam P“«-;j;“/Sre;
roll coating varies with the «*er fow characten^

for example, pseudoplastic mix “re available measure of the leve -

tonian or plastic coating mixtures. coefficient of

ing properties is the so-called leveling this index has

shear thixotropy to the plastic viscosi y. ^ies of Coating Mix-

already been discussed (see sec ion prodace a more pronounce

tnres”). Non-thixotropic coating mixtures pr^««

surface pattern than thixotropic (ing mixtures produce

„t,m viscosity next to the roll surface. Dilatant coa

isi J. F. Halladay, Jl/oifcrii PnrMiiW 17, No. 10; 111-

XVIII. PIGMENT COATING

1097

the worst results of all, since they exhibit a point of maximum viscosity next
to the roll surface. Hence, dilatant coating mixtures produce large ridges
or piles of coating on the paper, because of the increase in viscosity at the
roll surface which increases the resistance to separation of the roll and
coated paper surface. Such surface irregularities indicate a lack of suf¬
ficient fluidity in the coating mixture during the period when the coating

is sulijected to the action of the coating rolls.

The weight of coating applied is a factor in determining the leveling

properties, since even with rheologically acceptable coating mixtures, theie
is a limited range of coating weights within which pattern-free coatings can
be obtained.^” If the coat weight is too low, a striped pattern is usually
olitained, while if the coat weight is too high, a stippled pattern is usually
obtained. The better the over-all leveling properties of the coating mixture,
the wider the range of coat weight which can be applied without pattern.

Drying

Conventional oft-inachine coated papers are dried by two different
methods, festoon drying and the straight-pass system. In festoon drying,
the coated paper is looped ov’er a series of wooden sticks and carried
through a drying tunnel. In the straight-pass system, the coated paper
is carried through a drier tunnel without being looped over sticks; if the
jiaper has been coated on two sides, the paper must be supported by air
blown against the sheet in the direction of sheet travel until it is dry enough
to come into contact with a festoon or some other type of carrier. The
temperature of drying is quite important, and there has been considerable
discussion among paper coaters about high-temperature versus low-tem-
jierature drying. Festoon drying represents low-temperature drying, i.e.,
about 120 to 150® F., whereas the straight-line system represents high-
tenqierature drying, i.e., 250 to 300° F. Festoon drying generally takes
from twelve to fourteen minutes, and this long drying cycle permits a uni¬
form drying and allows the paper to assume its final shape without being
subjected to tension or uneven temperature. High-temperature drying in
the straight-line system is more likely to produce overdrying, curling, and
warping than low-temperature drying, and may result in poor calendering
conditions and a weaker coating. The sheet itself rarely reaches these high
temperatures, however, because it is in a wet condition and therefore cannot
exceed the wet bulb temperature .luunaity prevailing in the drier,

in the coating of heavy insulating boards, it is customary to dry at tem¬
peratures in the neighborhood of 600 to 700° F.

Drying machine-coated papers is quite different from drying conven¬
tionally coated papers. Machine-coated papers are dried on the regular

>”J. W. Smith, R. T. Trelfa and H. O. Ware, Tappi 33, No. 5: 212-218 (Mav
1950) ^

1098

PULP AND PAPER

paper machine driers, using conduction for transferring heat. Very short
air draws are used compared with the long air draws used in the drying of
conventional coated papers. The drying of machine-coated papers must be
carried out rapidly because of the short air draw. Care must be taken to
have the driers immediately after the coater in good condition in order to

prevent marking of the wet coating.

The speed of drying depends upon the amount of water in the coating

and the ease with which the water is released to the atmosphere. 1 he. ease
of water removal depends on the degree of hydration of the various com¬
ponents in the coating. Coatings which contain hydrous clays are more
difficult to dry than coatings containing non-hydrous clays. Coatings con¬
taining calcium carbonate are, as a rule, very easy to dry. Satin w ite
tends to lose varying amounts of water, depending upon ffie temperature o
drying.'®" It is customary to dry satin white coating at higher temperatures
than those usually used on clay or clay-calcium carbonate coatinp.

After the coating is dried, the paper is reeled and sent to the finis.nng
department. The presence of coating on the nnderside o one-s.ded coated
paper causes trouble in reeling and consequently should be avoided.

Calendering

Most coated paper is supercalendered. Supercalendering
per is similar to the supercalendenng of uncoated papers exc P
Ion-filled rolls are used which are softer than the cotton- or paper hUed

rolls used on uncoated paper. Machine-coated papers can ^ ^

ficial finishing on the paper machine calenders, but in or
finish, they must also be supercalendered. ^

erally supercalendered on a stack consisting > 1600 to 1800

of 1000 to 2000 pounds per lineal inch, anc a ^ P ^ lo.ver

f p m.'®"” Regular coated papers are generall) superca

e 400 to 800 pounds per fineal inch,
pressures, e.g., ^uu lo i Bering is to increase the gloss and

The primary purpc..v. oi uilen g the coatin<^ must be

smoothness of the coating. In order to accomphsh tins

plastic enough to smooth out under the ptessure sufficiently

out crushing the interior of the paper. that the gloss will

plastic, it will not produce a level surface, -I* „

l,e low. On the other hand, if the coat.ng ts too lasttc, he 1

hlackeued. Blackening is a phenomenon pigment

hrightness, due to close compression a »

surfaces. Albert''- feels that blackening is caused by a

nn. J. D. Davies, Pnfrr Tn, Tech- Asme.

163 a “i^achinery for Coating, Tapp „

Pulp & Paper Ind., . •» • • ^ (Aug. 5, 1937)

354 c. G. Albert, Paper Trade J. 105, Wo. o. ov

XVIII. PIGMENT COATING

1099

ing during calendering, resulting in the formation of ridges and valleys m

the final coating, . .

The results obtained on calendering depend upon the type ot pigment,

t>*pe of adhesive, adhesive-to-pigment ratio, moisture content of coating,
and presence of waxes or other ingredients. The most important single
property in calendering is the moisture content of the coating. The higher
the moisture content, the greater the smoothing, but the moisture m clay-
coated papers at the time of calendering must not exceed 4 to 4.5 fo in order
to avoid the danger of blackening. The critical moisture content is even

lower for coatings containing satin white.

The pigment composition of the coating greatly affects the results ob¬
tained on calendering. Some pigments naturally take a much higher finish
than others. Satin white, certain types of clay, and the fine photographic
grades of blanc fixe produce very high finishes. Talc is another high finish-
ing pigment and is used in friction or flint-glazed papers to the extent of
6 to 107® of the total solids. Calcium carbonate is usually classed as a dull
finishing pigment, but certain grades can be made to produce a fairly high
finish. The hydrous clays produce a higher finish on calendering than the
less hydrous clavs Ijecause of a greater shifting of crystal planes under the
stress of calendering.^’* Calcined clay, which has no hydrating properties
and presumably no cry'stal cleavage planes, produces coatings of relatively
low gloss. Some pigments blacken more readily on calendering than others.
Most clays will blacken if the moisture content of the coating is over 4%
or if too much pressure is used, the hydrous clays being particularly bad.
Calcium carbonate has less tendency toward blackening, and Roderick and
Hughes’“ recommend the use of calcium carbonate with clay to reduce
the blackening effect. With the dull finishing grades of calcium carbonate,
the moisture content at the time of calendering can be as high as 7 to
without danger of blackening. The particle size of the pigment is an im¬
portant factor affecting the calendering properties. In general, the finer
the panicle size, the higher the gloss after calendering. Fine clays show
a greater loss of brightness on calendering than coarse clays,’*^ hut the
initial brightness of fine clays is higher than that of coarse clays, so that the
net effect is a coating of higher final brightness.

The amount of adhesive in the coating is another factor affecting the
calendering operation. The higher the ratio of adhesive to pigment, the
lower the finish of the coating after calendering. The type of adhesive is
also important, since starch-coated papers do not, as a rule, calender to as
high a gloss as casein-coated papers, but part of this difference is due to

VV. Rowland, Paper Trade /. Ill, No. 16 : 207-212 fOct. 17, 1940)

E. liuglws and H. F. Roderick. Paper Trade J. 109, No. 11: 128-133 fSent
14. 1939)

S. C Lyoos, Paper Trade J. 106, No. 26 : 384-385 (June 30, 1938)

1100

PULP AND PAPER

the higher percentage of adhesive generally used m starch-coated papers.
The amount of adhesive in the coating is determined by the strength re¬
quirements, and ordinarily cannot be varied to regulate the finish.

Calendering tends to lower the strength of the coating slightly, but the
strength should not be lowered more than about a single wax (in the Denni¬
son series) if the calendering operation has been properly carried out A
serious loss of strength as a result of calendering is indicative of lack of
plasticity in the coating. If this occurs, the addition of plasticizers or an

increase in the moisture content of the coating may be helpfu .

Some coatings tend to dust on calendering, i.e., very fine particles of
coating accumulate on the calender rolls. Coatings containing coarse pig¬
ments generally dust more than coatings containing fine pigments, since
hoarse figments tend to spatter or break off the coating under he pressure

dusl^g; dusting usually occurs on coated papers 'o™

pver dustine sometimes occurs on papers of high strength (

the wax pick test), in which case the dusting is probably due to excessive

inetratiL of adhesive away from the top layer of the coating, '“"'‘K
Sin surface layer which is deficient in adhesive and consequently tends to

coated papers, but me laue arlbpsive f resin emul-

containing a high percentage of thermoplastic res

ing of starch-coated papers “tax emulsions, syn-

suitable plasticizing agents, such as fatty ^ ’ luble metallic soaps.

thetic resin emulsions, and aqueous ^ to 10% of ammonium

Dickerman and Riley- recommend the addition °

stearate on the basis of the starch adhesive. J^e addiUon o

of casein to a starch coating =°"^“^\fTnt™ls L no apparent
those troubles which appear and disappear at intervals

Special Calendering Operations ^

There are several types of

on coated papers to produce speaal effects On ,„tton-filled roll

these is the friction calender, rills which can be heated with

158 G. K. Dickerman and R. W. Riley, U. S. 2.425,231 ( ug. ,

XVIII. PIGMENT COATING

1101

four times, and this produces a polishing action on the coating. An excep¬
tionally high finish can be obtained on the friction glazing calender, but the
process is limited because of high cost to a few special grades of coated pa¬
pers, e.g., papers for box lining and other decorative purposes. Starch-
coated papers are considered unsatisfactory for friction glazing because
they tend to discolor from the heat generated in the process. Casein coat¬
ings containing a high finishing white pigment, some wax, and some talc
are generally used. The amount of friction-glazed paper is limited because
of the slow production speed, which is about 200 to 500 f.p.m. The nip
pressure in friction calendering is generally about 300 pounds per lineal

inch.

Another special finishing process is known as flint glazing. This is an
ancient and almost obsolete process in which a flint stone is used for im¬
parting an exceptionally high finish to the paper. In the early days, the
paper was rubbed by hand, but in present-day machines, the paper is rubbed
mechanically back and forth with a flint stone while the paper is held against
a smooth wood surface or against wide leather straps. This process is used
only where a very high finish and high bulk are desired, since it is ex¬
tremely slow and expensive (speed about 7 f.p.m.). The greatest use is for
fancy box coverings. A small amount of wax and talc are added to the
coating formula to lubricate the coating and to impart an exceptionally high
finish.

Another special treatment sometimes given to coated papers is that
given in a brush machine. In this process, the coating is subjected to the
action of rapidly revolving bristle or nylon brushes of graduated fineness
while the paper passes over a large drum, after which the paper is super-
calendered. The action of the brushes imparts a velvety or suede finish to
the coating. The process is used mostly on high-grade coated papers con¬
taining satin white as part of the pigment.

Another method of obtaining an exceptionally high finish in coated pa¬
pers is by cast coating. This type of coating involves the use of a large,
highly polished nickel. Monel or chromium-plated drier against which the
wet, freshly coated paper is pressed and allowed to adhere until thoroughly
dry.^5® When dried in this way, the coating takes on the finish of the drier,
making it possible to obtain a uniformly high finish as long as the surface of
the drier remains unmarred. Care must be taken to see that there is no
slippage of the coating against the drier and that no entrapped air is drawn
into the nip. The moisture content of the paper at the time of stripping
from the coating is rather critical. Wax is sometimes incorporated in the
coating mixture to aid in stripping. The gloss obtained in the final paper in¬
creases with an increase in the amount of adhesive in the coating formula,

D. Bradner, U. S. 1,719,166 (July 2, 1929)

1102

PULP AND PAPER

which is different from the result obtained in regular calendered papers.
The surface of the finished paper is soft and porous compared with ordinary
supercalendered papers, and the coating tends to mar easily. Cast coating
may be used as a machine coating process, or it may be used to apply a sec¬
ond coating on an already coated paper.

Coated papers receive special care in sorting because they must be

relatively free of imperfections. Some of the common defects which occur
in coated papers are brush marks from the coating brushes, roll marks, pin¬
holes, oil spots, dirt specks, and cracked coating. In order to remove the
imperfect sheets from the good ones, highly trained inspectors are used

to sort the paper.

Evaluation of Coated Papers

Coated papers are of a large number of different pdes and new
grades are constantly being developed. A few of the more important class

cal grades are listed below.

* J J, t, TViU ?<; a general grade which includes all coated papers (e.g.,
(i) Coated book. This general g orinting or publication where

magazine, pamphlet, book, and brochure) i ^

fine halftones are used 25 >0^500) 60 to 80 lb. being most common. The

rr^ffralu matt fb"'Coated art papers are used where ea-

ceptional finish is desired for special which are coated on one

(2) Coated label. This is taTrade Ts coated box cover or'box

iLteC;::; ta'.rt:::^ou^o„e side Cbu. not .aimed, for

This is a coa.d P- ^ ^ ^Ldt"

process. It is ““‘f "^situr grade is coated offset paper which is usoallr

on the lithographic blanket. A similar grau

coated on two sides.

1 Lifh hnve alreadv been discussed, ordinan y

Machine-coated papers, \v They are used for magazines

contain less coating than regu ar wallpaper will be discussed later,

and for opaque bread wrappers. board for folding boxes,

Many grades of board are also ““ isTne, at waliboards.
coated blanks (for calenders and adv ^tecial type of paper in which

Coated pa,«rs shot,Id monertks ifimpotte. Coated

™pers olten have low over-all strength and/or foWmg p
these properties are of secondary ™P° continuous surface which

because the coating provides a prided by luicoated papet-

makes a far better printing for these desirabk

The pigment in the ^ bind the pigment particles to

properties; the purpose of the adhesive

XVIII. PIGMENT COATING

1103

one another and to the surface of the sheet so that the coating will

pulled loose by the printing ink. Some of the important ^

papers are strength of the coating, smoothness, ink receptivity, bright ,

P^TOient-coated papers are a special grade of printing paper, and con¬
sequently they are evaluated by many of the same procedures use ""
coated printing papers. These procedures are described m Chapter Xl\
to which the reader is referred. The most satisfactory method of evaluating
coated papers is by printing press runs, preferably on a commercial press,
but if that is impossible, on a laboratory proof press. In testing on a press
selected samples of coated paper are run on the press under standardized
conditions, and the resultant printing is graded for quality as excellent,

good, fair, or poor.

Making Laboratory Coatings

Coating formulas can be readily tested in the laboratory by {1) making
a small batch of coating mixture, using the proper adhesive and pigment
combination, and (2) applying this coating mixture to a suitable sample of
raw stock, using suitable small-scale equipment. One method of coating
handsheets is by the brush-out method in which a special badger hair brush
is used for spreading the coating over the surface of the paper. More satis-
. factory results can be obtained with film applicators, such as the ^lartinson
coater or Bird applicator. The Bird applicator**"* is available in three sizes
(0.0005, 0.0015, and 0.003 in.) for applying coatings of different weights.
The Martinson coater*®* can be adjusted to apply coatings in any weight
by raising or low'ering the knife blade. With knife coaters, there is a tend¬
ency to apply heavier coatings as the viscosity of the coating mixture is in¬
creased and as the speed of the paper relative to the coating blade is in¬
creased. Another laboratory coating device in wide use consists of a
metal rod spirally wound with wire. The amount of coating applied is de¬
termined by the size of the wire wrajiped around the rod. This coating
device is available in a large number of different sizes, so that a wide range
of coating weights can be applied.*®’

A laboratory size air knife coater is available. This piece of equipment
will coat sheets 8x23)4 in. in size, and with special equipment, will handle
a continuous web of paper. This coater (Microjet) is furnished complete
with motor-driven coating roll drive and automatic air supply for the
nozzle.*®® There is a special knife-type laboratory coater (Diatron coater)
which is suitable for-coating a continuous wel) of paper. A pneumatic di-

Bird Machine Company, East Walpole, Massachusetts

161 Martinson Machine Co., Kalamazoo, Michigan

162 R. D. Specialities, Rochester, New York

163 John Waldron Corporation, New Brunswick, New Jersey

1104

PULP AND PAPER

aphragm is used for holding the sheet against the knife blade.’** This
coaler is equipped with radiant tyi>e heaters for drying the coated payier.

In addition to the above laboratory coating devices, there are many
custom-built roll coalers which are used for coating pai^cr in the lalK>ratory.
These roll coalers are constructed at the coating plant as small-scale rq4icas
of the commercial coating machine used in the plant. These coalers make
it possible to reproduce conditions in the lalioratory similar to those which
are encountered in commercial operation.

Strength of Coating

The term “strength” has a different meaning when applied to coated
papers than when applied to uncoated i^apers. In the case of coated papers,
the strength refers to the resistance of the coating to the shearing stress o a
viscous film such as occurs during the printing of the paper with a tacky
ink. Ordinary tensile strength tests have very little significance for coated

^^^^One of the earliest tests used to determine the strength of coated le¬
pers was the wet thumb test. In this test, the thumb was ^r^oistened
pressed firmly against the coating, and held there tor a few ^

thumb was then removed and examined for bits of coating. This
materially affected by the water sensitivity of the coating and proved un¬
reliable in comparing starch- and casein-coated papers. ^ •

hands of an experienced man, it ser\ed as a rough gui e P

various casein-coated papers. «. * in tViic test

The next test to be employed was the sealing J"

a piece of sealing wax was melted over a low-temperature ^s flame
thL was pressed against the coating and allowed to cool .
was cooled, it was puUed from the coating and
strength of the wax was high, some of the coated paper .

moved; the results were judged by whether the wax

coating layer or m the base paper, or, mo ^ of .his test

caused a coating pick or a body stock pick. Tte shortco ^

was that the strength of the coating was f appear

of the base paper. Hence, with a weak base pa^r, ® ^ „o„ld

strong by caparison, whereas with a strong base paper, the coating

appear weak by comparison. methods of testing,

In order to overcome the shortcomings o strengths. The

a new test was developed using a series of waxes of 6”^ „„re

conuiiercial waxes>« used for this "“^'o^^rtiofs to obtain a

wax:e> (e.g., barberry and camauba) >” out by firs.

series of waxes of graded strengt or pu . F^anunghani. Mas»-

■ K. Dennison Wax Series, Dennison Manufactnnng Co, F

rhii<;etts

1105

XVIII. PIGMENT COATING

nulling then plying it against the coatmg, allowmg tt to c<»l fo

fifteen minutes, and finally renmving with a gentle

are numbered from 2 to 12 or higher according to their s‘renph The
suits are reported as the highest wax which shows no piclc It is m onna
uve to report whether the pick is a coating or a body stock pick. This te,

is commonly called the wax pick te^t. -o r\

.\nother method of using the wax test is to press a No. 1- Dennison

wax on the paper in a special clamp having an opening exactly 0.5 m. m

diameter, attaching the lower end of the clamp to the bottom jaw o

tensile tester and the end of the wax to the upper jaw, and then measuring

the pull required to rupture the paper surface.

Inasmuch as waxes are widely used for the testing of coated papers,

it is worth while to consider some of the factors which influence the wax
test. Kirkpatrick'" has listed the following factors as being most impor-
unt probably because they influence the crjstal structure which is set up
in the cooling wax at the surface of the coating : (1) rate of cooling of wa.x
(2) ultimate temperature of wax, and (3) humidity. High humidities and
high temperatures cause the coating to pick at a lower wax number. Con¬
sequently, the wax test should be carried out under controlled conditions of
temperature and humidity in order to obtain consistent results. Another
important factor affecting the result is the moisture content of the coating,
since some coatings are affected by moisture more than others. For ex¬
ample, the wax tes’t on casein-coated sheets is only slightly reduced if the
coating is wet, whereas that on starch-coated sheets is greatly reduced.
Annis'" has obser\ed that a thin coating of casein applied over a casein-
clay coating increases the wa.x test. This film of casein reduces the adhesion
between the wax and the coating and does not actually strengthen the coat¬
ing. Such a condition might occur during normal coating if the pigment
should settle against the base paper, thereby leaving a layer rich in adhesive
on the surface. The wax test is a measure of the adhesion between the wax
and the surface of the coating, as well as a measure of the strength of the

coating and Itase paper. This fact accounts for many of the apparent anom¬
alies in results obtained with the wax pick test. A high wax test does not
insure freedom from picking during printing, since a hard-sized paper may
p 9 « 2 high wax test because of lack of adhesion between the wax and the
coating. However, when such a sheet is |»rinted, tacky inks will be required
to bond to the hard-sized surface, and these may l>e sufficiently strong to pull
the body stock apart.

The proper interpretation of the wax test requires considerable ex¬
perience and a knowledge of the use to which the paper is to be put. For

** W. A- Kirkpatrick 11, Paper Trade J. 109, No. 12: 156-158 (Sept. 21, 1939)
>**H. M. Annis, Paper Trade /. 96, No. 2: 21-22 (Jan. 12, 1933)

1106

PULP AND PAPER

example, Upright and coworkers^®^ report that a higher wax number (about
1 wax higher) is required for casein-coated papers than for methylcellulose-
coated papers in order to obtain a sheet which will not pick on the press
under the same conditions of printing. Likewise, starch-coated papers
sometimes require a higher wax number than casein-coated papers. Coat¬
ings containing satin white are usually stronger than the wax test would
indicate. The type of printing process used for printing the paper must e
taken into account. In general, the following wax tests are the minimum
values required for the different grades of printing papers based upon the
wax which shows no pick.^®® Higher pick resistance is required for certain

types of printing (see Table III, Ch. XIX).

Dennison K & N wax

Paper wax

11 90

Uncoated onset . .

Coated letterpress book . gQ

Coated offset or litho book .

According to Griswold,coated paper for
No. 8 wax. but normally a No. 6 wax is considered , J

paper contains thermoplastic materials such as rein “

tei is of limited value for judging the strength. One case has bee

norty» wLre a sheet with a wax test under 2 (which would normally be

L”::^dered" unprintable) was used in normal con-

Because of its shortcomings, the wax test has J ^

siderable criticism. Much of ‘f st’i mL^ evaluat-

cult to devise a satisfactory and, at the same ti^^^.^^ P

ing the strength of coated papers. T ^nmniercial press, but as

is by means of an actual printing press run on a _ t^e condi-

this is often not feasible, attempts have been

Most of these tests involve comparison °* ‘“(L are some-

printing qualities. Proof press runs "^”’8 ^ tin plate

times used to measure strength. ^ of’standard paper of

known printing properties and “ ed by "acli sample. A

one-half the area of the opening in 1 j ,t,e

brass roll covered with tacky ink is then roUed back a

1.1 P. M. Uprigbl, M. Kin and F. C. Peterson, Pn^r Trade /. 1

.«s w“ A.\irkpmrick 11, PWer Trade /■ (May^tw ' ,.

’.r.S'-RB—; LTingelhart a'd M. Zucher, Pa^r Trade d. IdT »• '

7. S4. No. 131 39-40 (Mar. 31. 1927)

XVIII. PIGMENT COATING

1107

opening until one of the papers is picked. This gives a comparison of

strength between the unknown and the standard sheet.

Several new types of pick testers operating on the principle of small
variable speed printing presses have been introduced. These differ in con¬
struction, but are all designed to measure the critical speed at which picking
occurs when a film of ink of predetermined thickness is applied to the paper
under controlled conditions. The film of ink exerts a pull on the coating
which increases as the speed is increased. The speed is varied during test¬
ing until the coating is ruptured, and the lowest speed at which rupture oc¬
curs is taken as the critical speed. In early instruments of this tN'pe, the ink
was applied to a pair of finely polished rolls which were then rolled over
the surface of the paper,*'* but in more modern instruments, the ink is ap¬
plied to flat ink plates and the paper is clamped to the cylinder. In one in¬
strument of this tyjie, known as the Warren M. P. tester,*'* a test strip
(IJ'j in. wide) of standard coated paper of known strength and a strip of
the unknown pajjer under test are clamped side by side on the surface of a
rotatable cylinder. A plate liearing a printing ink film of controlled thick¬
ness is then driven at a known controlled speed into simultaneous printing
contact with both strips. The siieed of the printing block is increased until
picking occurs on one paper, and then is further increased until it occurs on
the other jiaiicr. Thus, a ratio of the relative strengths of the two papers is
obtained. In another instrument, known as the Davidson-Pomper tester,***
four samples of paper x6 in.) are placed on a rubber-covered gripjier
cylinder while a tacky testing ink (I.P.I. Tack Graded Black #6- #8) is

applied to two stainless steel ink plates. The instrument is then set at a
speed near the value at which picking is expected to occur, and the inked
plates are fed to the tester. The samples are then inspected for picking,
and new tests are made on fresh samples at lower or higher sjieeds until
picking occurs. The range of sf)eed is 0 to*740 f.p.m. The ink temperature
is controllcfl by means of a thermostatically controlled heater. Using an
apparatus similar to that described above, Davidson*** found that the rela¬
tive critical si^eeds varied from 200 to 250 r.p.m. for coated letterpress
papers to 250 to 500 r.p.m. for coated offset papers. The tem])erature of
the test ink must fie held within narrow limits, since temperature variations
of 5® F. cause lack of reproducibility.

Some of the factors which affect coating strength are pigment compo¬
sition. amount of adhesive used, tN^pe of adhesive used, method of prepara¬
tion of coating mixture, solid.s content of coating mixture, properties of body

*** Staff of the Institute of Paper Chemistry, Paper Trade J. 123, No. 18: 24-29
(Oct 31. 1946)

Made by S. D. Warren Co.. Cumberland Mills. Maine
Made by John Waldron Corp., New Brunswick, New Jersey
***G. Davidson. Paper Mill News 72. No. 19: 16-18 (May 7, 1949)

1108

PULP AND PAPER

stock, type of coating equipment, conditions of drying the coated paper, and
type and degree of calendering. Strength of coated paper is a complex
function of the strength of the coating layer, the strength of the base paper,
and the type of bond formed between the coating layer and the base paper.
From a strength standpoint, the coated sheet must be regarded as a whole_

It is impractical to draw a sharp line of distinction between the coating and
the base stock, inasmuch as the coating penetrates into the Sbro^ sys em^
producing a gradual transition from coating to fiber. mg ^ ,

Le*" have published photomicrographs showing the '”'8“’“'; Tfiherl
between the coating and the base paper. In some regions,
may become completely embedded in the coating

coatine may penetrate and fill voids deep m the body of the paper Uu g
roatinv the action of the brushes or coating rolls tends to disturb the sur¬
face fibers of the paper which results in better contact between the coating

Xn the coated paper is ruptured, the zone of ^

interface. If the coating layer is mechan.c^ly J j,

rupture will occur either at the interface or in he pap _

known as a body stock pick, or if the pic mg o g

is known as splitting. Failure often occurs Ick.- The

number than that which causes pic tmg tenacity to the coating

reason for this is that the wax bonds peate t nac

than to the body stock so that there is actually a greater pu PP

body stock when the coated paper is teste^^ strength of coated paper
V most important factor deter«^^^^ stre^^ ,,,

is the amount of adhesive use in -^^reases the bond between the

amount of adhesive m the coating or ^ ^ ^ase paper. If the

pigment particles and also strengthens the coating

coating has good strength, the prac ica « ^ffsized ” A comparison of

as ■•hard-sized” or if w^k, he re™ .^itsTves has been shown in

the pigment bonding strengt adhesive in the coating im

Figure XVI 11-2. Increasing the amount . nrimnally occurred

creases the wax pick test, .^^n though ° ; penetrates sufficiently

in the base paper. Since it is '^^XUgth of the paper, the

far into the base paper to „duced bonding between the

increased wax pick test \l. 233-236 (Oct. 30, 1941)

176 c. R. Singleterry, ^ ^ 53-59 (Aug. H. 1^^^^

177 H. N. Lee, Paper (Jan. 12, 1933)

178 H. M. Annis, Paper Trade J. 96, No. - .

XVIII. PIGMENT COATING

1109

wax and the surface of the coating caused by the higher percentage of ad-
hesive in the coating layer.

The properties of the pigment which affect coating strengl i lave
discussed earlier in this chapter under adhesive demand of coating pig¬
ments (see Table VHI and Fig. XVIII-16). The important properties
arc type of pigment, particle size, and particle size distribution.

The strength of coated paper depends upon such properties of the raw
stock as the amount of sizing agent, amount of filler, and the density of the
paper. The effect of raw stock hardness on the strength of coated paper

is shown in Table

TABLE XII

Erru.^ or R.vw Stock Hardness on Stre.sgth of Co.ated Paper

AS Measured by Wax Pick Test

Raw stock

Soft ..

Moderately hard
Wry hard ....

W ax pick
test on
raw stock

Wax pick
test on
coated sheet

6.5 4.2

11.0 5.1

16.5 6.6

Ordinarily there is a difference in the strength of the coating on the
two sides of coated paper. As a rule, the coatings applied to the felt side are
weaker than the coatings applied to the wire side,*®® although Singleterry
obtained better bonding on the felt side. These effects are related to the
two-sidedness of the paper, i.e., to differences in sizing, filler content, and
percentage of fines on the wire and felt sides of the raw stock. Other factors
which influence the strength of coated paper have been discussed through¬
out this chapter.

Coating Weight

The weight of coating applied in commercially coated papers varies
from 1 lb. per ream per side for the very lightest semicoated papers made
on the paper machine to about 15 to 17 lb. per ream per side for the regular
grades of brush-coated papers. According to present practice, a 60-lb.
litho coated one side generally contains alx)ut 15 to 16 lb. of coating per
ream; a 70-lb. letterpress sheet coated botli sides generally contains about
12 to 13 lb. of coating per side; and a 55-lb. machine-coated magazine paper
coated both sides generally contains about 8 lb. of coating per side.
Highly filled book papers, e.g., regular supercalendered book paper, may

be given a very light coating on only the wire side in order to replace the

filler lost through the wire.

G. Landrv Paper Trade /. 110, No. 11: 152-160 (Mar. 14, 1940)
‘••H. M. Annis, Paper Trade /. 96. No. 2 : 32-34 (Jan. 12, 1933)

*•* C R. Stnglrterrj', Paper Trade J. 113, No. 18: 221-236 (Oct. 30, 1941)

1110

rULP AND PAPER

Uaryns-coaucl (.hotographic pai^rs may have up m 33 Ih j^-r side,

coating licing applied in three successive coats. 1 here is a trend m modem

coating to appi nvo or more coats on the highest grades of letterpress and

mrcoated'lLpers. Both of these coats may Ik- appl ed on the j«^r

chine or one coat mav be applied on the machine and another off he im^

chine’or both off the inachine. The clay contents ol severM typical coated

naners a r'iven in Table XXX\T in Chapter XVI.- Tlie thickness of

Z deirf coating film ordinarily varies from about 5 microns tin machine-

1 ♦ iiartiit 12 to IS microns on brush-coatcd paper^.

coated we" 'I i„g coated papers to determine the

It IS someumcb casein-

amount of coating ^ in a solution of suiuble

coated papers, by ( ) ' | adhesive and release the pigment and

proteolytic enzyme to dissolve the ao camel s-hair brush until

(2) gently brushing the ™ — of

all visible coating is removed. Tbe amo

mined by the difference in \\eig i o ^ ^ ordinarily be desized

removal of the coating. Starch-coate ^ ^ ^rcentage of

without using an '“yme. tote^ pa^ S consequently they

synthetic resin emulsion canno determine the amount of coating rm

pose a problem when it is neces ^ determining the amount of ad-

L paper. The desizing test «n he u.d ^-"^ng; this is done by

hesive in the coating, as w ■ \,t the coating as determined above

taking the difference between the ^he amount of mineral

and the amount of mineral matter ™ ^ , ,he decoated paper

r -HaP^^ after converting from ash

affects the strength, ink receptn it>. ng ’ ^ properties ot

paper. The effects of ^oat wmgh‘^n the st^rengt^ a^^_^ P^ m

coated paper are shown in Fi^^ - casein-clav coating containing 13 ^
These results were obtained usi g ^ moderately hard raw stock,

casein on the weight of the ^PP 20 lb. (25 x 38-500) was

Under these conditions, a coa t, ^ ^ , further changes

Lnd to be tbe optimum point abo« which here we

in the gloss and strength properties. Howe'er- ' F ^ ^

continued to increase with further ^ ^ ,-„rther in a later section

coat weight on brightness and opa^'^ ^ ,ied coating lowers the

According to Landes,-increasing the weigMo pP ^

wn-KaoUn Clays and Their Industrial Lses. P- .

York, N. Y. (1949)

183 See TAPPI S^darfk ^52-160 (Mar. 14. 194U1

184 c G. Landes, Paper Trade J. UU, NO.

XVIII. PIGMENT COATING

ink recejAivity and the w'ax pick test. In nmny cases,
ceptmty is increased by increasing coat weight.

how’cver, the ink re-

ti'ater Resistance

Water resistance is an important property for those grades of coated
paper which are likelv to come into contact with moisture during use.
Among these papers are washable v^-allpapers. coated tag, coated labels,

COAT WEIGHT, lbs. (25 * 38* 500)

Fife X\'III-22. Effect of coat weight on physical and
optical properties of coated paper.

coated decorative papers (e.g., Christmas wrappings), and coated paper-
board for pharmaceuticals and soap boxes. Coated offset paper should have
some water resistance, since the paper is subjected to conditions of high
moisture during printing. It is desirable, although not necessary, to have
a small degree of wet-rub resistance on all grades of coated book and maga¬
zine papers in order to pre>'ent smudging during handling.

WTien coatings of high water resistance are required, casein or soybean
protein is generally used as the coating adhesive. Either of these adhesives

1112

rtOJT

rAfu

»ill ,.t.Kl.KC « luKhly w,>«

l>cn» h.»r»lcn«l with (oriitaUlrh>^te. On the oUkt

,Kr, «c vrry .o «-»■-. .l.hco**. « » P*'** “ ^

r«is.»n« .o .t.rch.o«.r,l .hrr,. W .hrj« .

„ .le,criW.l «r..rr,

»imf»lc nirthorl «Vvisr«n»v Hewcit ami COW •’rti

.V ,.1««1 on a Okt. ct «u«k » «»». - « ^ ^

r„rn.l W> oml <hr olge o( the cn»t«l ^ Th^

while the two Sheris are heUl firmly in i4ace, the ^

the coated iMper onto the black l»l«. “MnC ka*«^

nibbing action tran.fee^ sotne o( llw ^ ThTmteMiiy «(

a »hi.e ,ra., on .Ik Mack .-r-r ^ ^ A

...eclunical method of ^Tbeeo deacriM by

which lintshes the i-iwr while submerged in wa.

Black,’** ^a.»;rV»^f

Ink Reetf‘tn\iy

Ink receptivity and ink

of coatr<l pa^icrs, Impro>cd mW q< '^,ed naprr*- There ia an op-

,.ges «™h'^^ of croted ,«,«.

tinnim ink receiHiMt) lor c» h ,,s-,.hi- the mk r«*p-

;i.v should V« nuinuined near tnis^.™^-^ ^ ^

, the ^etratun, of priming Hd. on the paper. Too higb

,(the halftone «reen and ^ ^ , p,,. prhtt.

nk absorption k undesirable becao« U to make an actnal

,riming test on,the paper s^ 's^^A^Kthod te,*'"*

,rd sample, but this is nm al^«n^._^^^^K 4

which is widely used .nv;olves tlw dissolved in a

ink. a special mk containtng ,»» ink b applied to the snrfacr «< the

varnish * In this meth^ ,^,uh un^carefully

coated paper by means of a apply a heavv 61® ^

dittons. The essential of time (us^ 2

(2) to »“ow the him » excess inkanddrywithademid^^

minrnes). and (a) to remo^ the ” n»surtng »

nunierical ralue for the tnk "«P^ m Ss

,«o s Hewett R R Carter R J Omni. W**

(June li *S7-a68 (Ocl- IW

XVIII. PIGMENT COATING

1113

reflectance of the inked sheet on a photometer. This gives an indication o
the amount of ink which has been absorbed by the coating. The darker the
inked spot, i.e., the more ink that is absorbed, the lower the reflectance.
The brightness of the stained spot varies linearly with the logarithm ot the
elapsed time.*®' Usually a reciprocal of the reflectance reading is taken so
that an increase in absorption of ink results in an increase in the numerica
value for ink receptivity. This method of testing gives fairly good compari¬
sons between coated papers containing the same pigment formulation, but
different pigments in the coating produce different hues and prevent an
exact quantitative indication of comparative ink receptivity.*®® If the coat¬
ing is highly alkaline, the dye in the ink will be turned a purple color.

Other methods of measuring ink receptivity and oil absorption are
described in Chapter XIX. Kirkpatrick*®® mentions the use of the wax
test as a guide to the ink absorption of coated papers and suggests as a
safe rule that the paper should show a Dennison wax test no more than 2

higher than that established as a minimum for strength.

A number of factors affect the ink receptivity and ink absorption of
coated papers. The amount of adhesive in the coating is the most impor¬
tant factor. The lower the adhesive-to-pigment ratio in the coating, the
higher the ink receptivity and ink absorption^ of the coating. Excessive
penetration of adhesive into the base paper during coating increases the ink
receptivity, since this reduces the amount of adhesive in the coating layer.
Starch-coated papers have a higher ink receptivity than casein-coated papers
when equal amounts of adhesive are used because of the weaker film-form¬
ing property of the starch. However, in coatings of equal strength, where
more starch than casein is normally used, the ink receptivities are more
nearly equal, and sometimes starch-coated papers have lower ink recep¬
tivity than casein-coated papers.

The type of pigment used in the coating is another important factor
determining ink receptivity. The particle size of the pigment is important,
inasmuch as pigments of large particle size produce coatings of higher ink
receptivity than pigments of small particle size. The type of pigment is
also important. Clays from different sources vary considerably in ink re¬
ceptivity. Calcium carbonate pigments produce highly ink-receptive coat¬
ings, particularly the precipitated grades. As evidence of the strong ink
receptivity of precipitated calcium carbonate, Roderick and Hughes*®®
showed that coatings containing precipitated calcium carbonate have a

*** A. Vort and J. S. Brand, paper presented at 35th Annual Meeting of TAPPT,
New York City (Feb. 20-23, 19^)

>**C G. Landes, Paper Trade J. 110, No. 11: 152-160 (Mar. 14, 1940)

*“W. A. Kirkpatrick. Paper Trade J. 109, No. 12: 156-158 (Sept. 21, 1939)

’••A. E. Hughes and H. F. Roderick, Paper Trade J. 109, No. 11: 128-133
(Sept. 14. 1939)

1114

PULP AND PAPER

hiEher ink receptivity than coatings made with clay, even when both are
sized to the same wax pick test (where the calcium carbonMe coating con-
tains considerablv more adhesive than the clay coating). T^e u«e of saiin
white in the coaling formulation generally increases the ink receptivity of
the coating, probably because of the porous nature of satin white ctatings.

The relationship between the strength and ink

taining different pigments is shown in Figure X\ 111-23- Calendering

23456789

K AND N INK RECEPTIVITY

cium carbonate, (d) satin white.

• 11 (T + r.n tVif> ink receotivitv of coated papers, just as
also has an appreciable effect on the ink recepu .

it has on uncoated papers.

Svtoothness

One of the principal objects of the printing plate

which will insure better contact ^ smoothness is determined by the

than is possible with uncoated pap . ^ curface Smoothness is usu-

contour or degree of continuity of the „ sheeU of the

ally measured by the rate ot ow o air paper and a smooth stand-

coated paper, or between one sheet of the “ated

ard surface, using the some proce ure s printing by breaking

A rough-coated surface causes trouble P

the contour of the halftone dot, pto is about 0.074 nnu

.1 ■ X” r,L -S.. I” "■ “i'S

cation of the magnitude o r ii J 110 No 26347 (June 27,

1.1 A. E. Hughes and H. F. Roderick, Pefer Traic /. 110. ■

1.1 rsiternieister. Paper MiU .Vrwr «, No. 25 : 76. 78 (June 21, 194 )

XVIll. PIGMENT COATING

1115

printing can be obtained by examining the coating «ith a hand en> or .
croscope magnitviiig bet» een 20 to 50 diameters. Any tiling « huh show s
up as WiigTf appreciable size at this magiiitication will affect the printing
wLn fine lialitones are used. Another simple test is to examine the coating
visually with a light source behind the sheet. This will show u|i (as a
result of the difference in the amount of light transtinttcd) any imperlec-
lions which are important enough to cause trouble in limiting.

A nunilier of detects commonly occur in coated papers. Papers coated
with casein or sovbean protein often contain pinholes caused by foam m the
coating mixture.’ Pinholes are aiused by foam which does not break until
the coating is iiartiallv dried and is no longer fluid enough to fill the hole
left bv the ruptured bubble. This leaves small pits in the dried coating
which vary from almost invisible spots up to pits which are 0.064 mm. in

Another tyi>e of surface irregularity is that caused by improper applica¬
tion and smoothing of the coating. In brush-coated papers, improper flow
results in the presence of brush marks. In roll-coated papers, imperfections
in the form of ribs or ridges often occur. The fre.iuency and .size of these
ribs are determined by the surface tension of the coating mixture and the
size of the coating roll: in some cases, a smaller smoothing roll is placed
after the larger coaling roll in order to reduce the ribbed effect. At about
75 to St) ridges jier inch, the eye can no longer perceive these irregulari¬
ties,*''* but even so. they still interfere w'itli the reproduction of halftones
made from screens containing 100 or more lines per inch. Machine-coated
jiajicrs are likely to have surface imi>erfections, due to scratching of the

coating on the drier rolls.

()ther conditions w’hich may result in imperlections in the surface of
coaled j»a|)ers are the presence of loose fibers, dirt, or other extraneous
material in the coating mixture. Starches which contain line filaments of
nylon originating from the nylon reels used in screening the starch have
even lK*en rejioried as causing trouble. Cotton fibers cause less trouble
liecaiisc they lake printing ink more readily than nylon. Particles of undis-
perseil adhesive or pigment in the coating have also been known to cause
trouble. Undisper.sed jiigmeiit particles produce lumyis in the dried coating,
whereas adhesive particles tend to produce depressions caused by a drying-
out and contraction of the adhesive. The latter condition may produce pits
which are somewhat similar to foam spots, but which can be distiguished
from foam sjxits by their irregular sha|)e. liither starch or casein coatings
may contain these pits if the adhesive is iu*t ]iro])erIy di.spersed, in addi¬
tion. certain starches may contain tiiidispersed fragments of starch granules
which tend to produce very small i»its in the coaling,''** as shown in h'igure

Hatladav. yf<*dern Packaging 17, No. 10; 111-115, 140, 142 (June, 1944)
J. P. Case>'. Paper Ind. 26, No. 2: 157-162 (May, 1944)

rULP AND PAPER

1116

XVIIl-24. Pits produced in this way are barely visil)le with a hand lens
magnifying 20 times, and consequently do not cause trouble except possi ) >

“ou emuS i« tCcoating mixture will cause trouble by forming oil
spots in the coating. Oil which has leaked into the coating froin the k,
in-s on the mixing tanks forms an unstable emulsion uduch is broken during
he drvln. process, thereby producing oil spots. Lxcessive amounts of
added oil '(e g pine oil or sulfonated oils) will sometimes produce a similar
‘condition.^ (3ii spots in paper show up by fluorescing under ultraviolet light.

Fig. XVIIl-24. Pits ill coating caused by undispersed fragments of sta

Coated papers (except cast "XiTrirlVo^ttetrse^

so that the smoothness of the coating ep considerable difference m

which the coating can be calenderec adhesive in the coat-

the ease of calendering, depen mg g, points out that the smoothness

ing and the type of pigment used. adhesive to pigment in

obtained passes through a after the point of maxiiiiiini

the coating formula is mcieasec. j J^ses with increasing adhesive,

smoothness is reached, the smoothness decreas

and consequently coated papers adhesive. The

generally less smooth than those w • percentages of adhesive

decrease in smoothness vvdnch occurs which causes

are used is caused by a l e notion in ^ pigment and the pigment
it to calender less readily. 1 .1^^ ihaiiiiini dioxide, clav.

size are other iniportant factoi s. . . 1 ^;^,, ywootlines-^

cium carbonate, and i„ „„oofliness by calendering

Clav-coated sheets arc generally mcicaseti

XVni. PIGMENT coating

1117

more than calcium carbonate dlrS

ticity. The other factors affecting the caienae ^ i

been discussed.

Gloss

rioss is a measure of optical smoothness. In the early production of
coated papers, an attempt was made to obtain the

today it is realized that high gloss is not required m all p ^1 P
Excessive gloss is disturbing to the reader. A matte or dull-fin P p
makes the best reading surface, even though the paper does not prm qiii

a highly finlhed paper. Coated papers for « "
high brightness and smoothness, but not show g are

light at high angles. It is quite possible to prepare a siee wi ^

Sbt pfinting%ualities and still avoid gloss, although ^ |

not agree with this statement. For this reason, some specular glo

sirsiblc from the S3.1cs st 3 rn(ipoint. . . „ . <■

The amount of calendering is the most imporjant factor aflecUng gloss.

Some pigments calender to a higher gloss than others, an m t is respec ,
satin white and certain clays excel. The finer clays produce ^

ines than the coarse clays. Titanium dioxide is considered a high finishi „
oiement. On the other hand, calcium carbonate and blanc fixe are consic -
ered dull finishing, since they calender to a lower gloss Water-groun ca -
cium carbonate produces a lower gloss than the precipitate gra e. is
possible to control the gloss independently of the brightness throng i proper

choice of pigment. „ • ^ •

In general, the gloss of the coating decreases as the adhesive-to-pig-

ment ratio is increased. In some cases, however, the adhesive increases

the light-deflecting surface and increases the glare. Ordinarily, starch-

coated papers do not calender with as much gloss as casein-coated papers.

On the other hand, glue-coated paper gives a comparatively high gloss, and

glue is sometimes used as the adhesive when maximum gloss is desirable.

Latices, which are thermoplastic, are conducive to the production of highly

finished coatings.

• Brightness and Opacity

Most coated papers are printed or used for their decorative effect, and
consequently the optical properties of the coating (brightness, color, and
opacity) are highly important. A paper with a white, bright surface is de¬
sired, and these properties are derived from the coating layer. In the ideal
coated paper, the optical properties would be primarily those of the coating
and not those of the raw stock.

In order for the coating layer to fulfill its functions, it must (1) cover
up the base paper through the effect of the opacity or hiding power of the

1118

PULP AND PAPER

pigment and (2) present a white, bright surface obtained through the nat¬
ural brightness of the pigment. A coating which is deficient in either hiding
power or brightness will not be satisfactory. For example, a white, trans¬
lucent coating would be unsatisfactory because it would not cover up the
base paper. Likewise, an opaque, colored coating would be undesirable
because of its low brightness.

If the base paper is low in brightness and the amount of applied coat¬
ing is small, the hiding or opacif 3 ung power of the pigment is a more im¬
portant factor than the brightness of the pigment. Under these circum¬
stances, pigments of high hiding power produce brighter coatings than
pigments of low hiding power if the brightness of the pigments is the same
(or even if the brightness of the pigment of high hiding power is slightly
the lesser of the two). On the other hand, when the amount of applied
coating is large, the hiding power of the pigment becomes secondary to the
brightness of the pigment. Stated differently, it is better to have a pigment
which will cover up the body stock rather than a pigment which has a high
natural brightness of its ^wn when low coat weights are applied. When
heavy coat weights are applied, it is better to have a pigment of high bright¬
ness, rather than high covering power, because the pigment so completely
obscures the body stock that the brightness of the coated paper is essentially
the brightness of the coating. Pigments of high covering power are always
desirable because they produce the desired opacity at lower coat weights

than pigments of low covering power.

In commercial coating, the coat weight is generally less than that re¬
quired to obscure completely the body stock, and consequently the hiding
power of the pigment is highly important. The coat weight necessary with
clay coatings to obtain complete coverage of the raw stock, i.e., enough
coverage so that additional coat weight has no further effect on the bright¬
ness, varies from 14 to 50 lb., depending upon the particle size distribution
of the clay"«= (see Fig. XVIII-22). The effects of coat weight and pigment
composition on the brightness of coated paper are shown in Figure
XVIII-25 for two raw stocks of different brightnesses.^®® The pigmen
composition was 100% calcium carbonate in one case and 70% calcium car¬
bonate and 30% titanium dioxide in the other. The titanmm dioxide and
the calcium carbonate had approximately the same brightness, but o course
the hiding power of the titanium dioxide is higher than that
bonate. It can be seen from Figure XVIII-25 that the brightness o jhe

raw stock is an important factor at low coat weights, and ^ ,

when pigments of low covering power (e.g., calcium carbona e) ^
than w^hen a pigment mixture of high covering power (mixture of calc

I96 0 W. Callighan and C. G. Albert. Year Paper Mill Supt

Assrc. 1945 r 184, 186. 188, 190. 192. 194. 196, 198. 200

196 TAPPI Monograph No. 7, "Pigments for Paper Coating, p. 3. Teel.

of Pulp & Paper Ind., New York, N. Y. (1948)

XVilL PIGMENT COATING

1119

orbonale and titanium dioxide) is used. Somewhat similar data are pre¬
sented in Table XIH. showing the effect of raw stock brightness and

100

COATINC VrCIGHT. Ib«^ (25 i 36*500)

Fif. XVIII-2S. Effect of coat weight and pigment composition on
the bnghtnest of coated paper. (—) 70% calcium carbonate and 30%
thanium dtuxide; (***) 100% calcium carbonate.

oott weight on the brightness of coated paper when a XOO^o clay coating
if compared with a coating containing 90% clay plus 10% titanium di¬
oxide.*** Because of the great hiding power of titanium dioxide, a '5-lb.

TABLE XIII

Emcr or Raw Stock Buchtmess and Coat Weight

l|L ON‘BaiCHTNCS8 OF COAIEO PaPER

RdfoctirHy, %

mmtk bndrtM*: 74.e 77.5 »0.a

_ » |a .

2SK*«fS^

Al der

iO%rtOt

Aflclsr

10% TiO.

Alt cUjr

10% TiOt

77i)

79J)

80j0

82.0

83i

83i)

81i

85i)

ais

86.0

810

, 8SjO

82J

86.0

^.0

87.0

8ZjO

860

83j0

86.0

84.5

S7 5'

coating containing 90% clay and 10% titanium dioxide has approximately
the same brightness as a 10-lb. coating containing 100% clay.*** If ti-

Tkf Hmdhcck, Titanium Pigment Corporation
111 Broadwap, New York, N. Y.

W. R. Wilk^ PtPtTjlmd. 2S. No. 10; U^U

National Lead Co.,

1120

PULP AND PAPER

tatlium pigments are substituted for part or all of the clay, the coating will
have a higher brightness, but heavier coat weights may be recjuired to obtain
the ultimate brightness and covering power.’-”' Satin white has less hiding
power than clay, whereas calcium carbonate has slightly more hiding power
than clay.

The over-all opacity of coated paper is determined by the opacity of the
coating layer plus the opacity of the raw stock. The coating layer is the
more important factor if heavy coat weight or coatings of high hiding power
are used, whereas the opacity of the raw stock is the more important factor
if light coat weights or coatings of low hiding power are used. Table
Xiv^oo shows the effect of raw stock opacity and coating weight on the
over-all opacity of coated paper; a 100% clay coating is compared with a
coating containing 90% clay plus 10% titanium dioxide.

TABLE XIV

Effect of Raw Stock Opacity and Coat Weight
ON Over-all Opacity of Coated Paper

Contrast ratio, Bausch and Lomb, %

Raw stock opacity : 58.

coat weight,

All

lb., 25 X 40—500.

clay

69.0

78.0

84.0

87.0

66.0

10%

All

10%

TiOj

clay

TiOi

74.0

78.0

84.0

81.0

86.0

89.5

86.0

90.5

92.0

88.5

93.0

76.5 85.0

All

10%

All

10%

clay

Ti02

clay

TiOi

82.0

84.5

88.0

90.0

87.0

90.5

93.5

90.0

93.5

92.5

95.0

92.0

95.0

93.5

96.0

As mentioned above, an ideal coating pigment should have both high
brightness and good covering power. Brightness is determined principally
by the natural brightness of the pigment and the particle size distribution.
Some of the pigments which have a naturally high brightness are calcium
carbonate, titanium dioxide, and satin white. Clay, lithopone, and blanc
fixe have a lower brightness, clay generally having the lowest brightness o
all. The brightness of coated papers made from different pigments is s io\mi
in Figure XVIII-26.^"* However, in interpreting these data, it should e
remembered that the various pigments have unequal adhesive demands (see
Fig XVIII-16), and hence, the lirightness obtained on coated papers o
equal strength would be quite different from the values indicated in Figure

XVI11-26. . ;

Hiding power of the pigment is determined by the index o re
a,«l the particle size distribution. The optintum average particle sire an

199 R Willets, Tappi 32, No. 8: 349 356 (Aug., „_i t pad Co,

200 Taken from The Handbook, Titanium Pigment Subsidiary, National L

New York, N. Y.

XVin. PIGMENT COATING

particle size distribution for coating pigments have not been accurately de¬
termined, but it is known that the presence of pigment particles apprecia > y
less than 0.25 micron in size tends to decrease the brightness of the coat-

Calcium carbonate pigments containing a high percentage o
particles less than 0.2 micron in diameter produce a dull gray coating
A particle size between 0.5 to 1.0 micron is considered optimum for
clay from the standpoint of hiding power. Since coating clays do not
differ appreciably in their refractive index, the relative hiding power of
clays is primarily a function of their particle size. An important considera¬
tion, as mentioned above, is the percentage of clay particle within the range
of 0^5 to 1.0 micron, since these are the jiarticles having the greatest opaci-

PARTS CASEIN PER 100 PARTS PIGMENT

Fig. XVIII-26. Effect of different pigments on brightness of coated paper.'*-*'
(a) Satin white, (b) titanium dioxide, (c) calcium carbonate, (d) lithopoiie, (e)
English clay, (f) blanc fixe, and (g) domestic clay.

fying effect. Pigment flocculation in the coating reduces opacity by reduc¬
ing the probability of light striking a pigment particle. The effect which
is obtained is shown schematically in Figure X\ ni-27 where it can be
seen that the coating containing flocculated pigment permits the passage of
more light than the coating containing deflocculated pigment. Some of the
pigments which are associated with good hiding power are calcium car¬
bonate, titanium dioxide, satin white, and zinc sulfide. The effect of differ¬
ent pigments on opacity of coated paper is shown in Figure XVIII-28.'®^
When comparing pigments for opacifying effect, all comparisons should be
made on coatings of equal brightness, since the opacity increases with de¬
creasing brightness. With high brightness pigments such as titanium, it
is possible to obtain additional opacifying effects by reducing the brightness
with dulling agents.

The opacity of the coating layer depends upon the difference in the
refractive indices and the relative proportion of the various components in

A. R. Lukcns, C. G. Landes and T. G. Rochow, Pa(>cr Trade J. 112, No. 15:

183-190 (Apr. 10, 1941)

202 G. F. A. Stutz, /. Franklin Inst. 210, No. 1: 67-85 (July, 1930)

1122

PULP AND PAPER

the coating, including air, pigment, and adhesive. The greatest difference
in refractive index is between the air and pigment. Thus, increasing the
amount of adhesive in the coating reduces the opacity because air is being
replaced by adhesive (see Fig. XVIII-28). Increasing the degree of calen¬
dering decreases the opacity because it reduces the proportion of air.

SURFACE

PISMENT

.■^VEHICLE

■BASE

X /^ »5UR FACE

PIG-MENT

■/^VEHfCLE

base

Fio-. XVIII-27. Showing how flocculation affects the transmission of light
through pigment coatings. Top figure, light entering deflocculated ^st
Lower figure, light entering a flocculated system (courtesy Interchemical Corp.j.

The brightness is reduced with increasing adhesive (see Fig.
XVIII-26) because the brightness of the adhesive is less than tlmt o
pigment, and also because the adhesive is fairly transparent and hence -
duces the total amount o'l light reflected and refracted. Starch gener >
reduces brightness and opacity somewhat less than an equal amoun 1
sein or soybean protein because of the slightly higher brightness of starch.

of tiv man hifMy

SeyhKtm. pfwtein* proAm

1 'i^- ■fev._ I** .
HHIF91K94M PvJ^* ^ iS^pN

kf^bbM and «|iacity ol coaled

C)cfarr miautt of chaficf«« ^
odvr i o qwf e w wm< of the

«iknnr« (Mod it dttrrmmrd

s.?

Iry llv laiffalaFM and ofiacily
oat yt if l ht ia dr u n mn td by co

tkM by opndiy and bnghtira

Canling Prartnaaa

of itir dncaaMMi ba» Wen Ww i t cd W mrlWaU
gradra of coaled aa coated

fnaara ' TWte tfe. boaiever. a manber of C*^dca

ipccial Iccfmiqttra

«rn W anenl rrifect* Irani the coating of oiach

For 0^ tbing* ^wall|i^ne ia oiatod

m wittai

4 ^ V* n

glne, fiardit toybean

1124

PULP AND PAPER

l»r()tein, soy Hour, or gum arable. In the secoiul .stage, a to]) coating in
colored desigirs or patterns is printed upon the ground coating.

(juni arabic (Egyptian gum) was one of the earliest adliesives used in
wallpaper coating. This gum is obtained from the Anglo-Eg)'ptian Sudan,
w’here it is produced as an exudation of a tree (acacia verek) which grows
in a semi-wild state. The gum is formed beneath the bark during the rainy
season and collected in November by tapping the tree. The first gatherings
are light in color, and the later gatherings are darker. The gum must be
aged, and for the high-quality grades, must be cleaned and processed. In
modern wallpaper coating, gum arabic has been replaced almost entirely
by other adhesives. Animal glue, casein, soy flour, soybean protein, and
synthetic resin latices are the principal adhesives used in the production of
washable or water-fast wallpaper, whereas starch is used in the non-wash-
able grades. The washable grades can be washed with soap and water
without disturbing the coating. Very little w^allpaper is actually washed
by the housewife, but washable paper is useful in cases where the wall be¬
comes spotted, and it also helps the paperhanger who may have to remove
paste spilled on the paper. Casein makes the most highly washable paper,
with soybean protein next, and animal glue last. Finely ground soy flour
mav be used in making washable wallpaper in the proportion of 25 to 50%
of the total adhesive demand if the remaining adhesive is some other protein,
such as casein or soybean protein. Soy flour has also been used as the
entire adhesive with fairly good results, but a higher percentage of adhesive
on the weight of the clay is required. If soy flour is used, the flour should
be ground so that 98 to 99% passes a 270-mesh wire, since particles which
are much larger than the openings in a 200-mesh sieve will not pass through
the nip of the roll running against the paper, and consequently these par¬
ticles tend to accumulate in the coating mixture.^"®^

The first, or ground, coating is used in wallpaper manufacture to fur
nish a proper background for the top coating and to make the finished paper
fast to light by covering up the yellowing of the groundwood. This coating
is similar, to regular paper coating except that less adhesive is ^ed in t e
coating formula and, as a result, the coatings are much weaker. Ordinarily,
between 5 to 10% of adhesive on the weight of clay is sufficient for e
ground coating. Coarse grades of clay are used compared with the c a>
used in the coating of magazine papers, because a rough, matte
desirable. Only rarely, if at all, are the more expensive pigments «sed. si _
as calcium carbonate or titanium idoxide. Clays of 80 to 81% ,

are adequate for most ground coatings. Ground coating is not w ’

but is used as a base for the colored coating, which is subsequent V _

the second operation. A typical ground coating for washable w p P

G. Davidson and J. H. Cagle, Paper Ind. 29, No. 3 : 364-366 (June, 1947)

203

XvIII. pigment coating

1125

is: 100 parts medium-grade clay, 7 parts casein, 135 parts water, 0.3 parts
pine oil. The casein is dispersed with alkalies in the usual manner. Then,
enough casein solution is added to the clay slurry to give about 7% casein
on the weight of the clay (dry basis). The ground coating can be applied
by brush, air knife, or roll coaters. The coated paper is then dried and is
ready for the top colors to be applied in the proper design. About 14 to
22 lb. of ground coating are applied per ream (24x36—480).

The top colors (sometimes called "stainers” or “toppers”) are applied
by a special coating machine consisting of a series of printing rolls arranged
around the periphery of a large drum about which the paper is wrapped.
Each printing roll applies a different color to the paper in the proper se¬
quence and proper register to produce the required design on the paper.
The colors are similar to the ground coating, except that higher adhesive-to-
pigment ratios are used, generally a total of 8 to 14% adhesive on the weight
of the pigment—and finer clays are often used. Colored pigments or lakes
are added to the coating mixture to obtain the desired colors. As many
as fifteen to twenty diflFerent top colors may be required for a single print¬
ing although generally eight to thirteen are used. Printing is done either
from aluminum alloy rolls or from wooden rolls covered with pieces of felt
which are held in the proper place by brass strips. The printing rolls re¬
ceive the color by means of a felt sleeve, known as a sieve cloth, which re¬
ceives color from an individual color pan. Special coating mixtures are
often used in the top colors, e.g., coatings containing 100% titanium dioxide
are sometimes used for the highlights. The top colors must have a higher
viscosity than the ground coating mixture in order to pick up properly on
the printing rolls.

A third operation is carried out in making the washable grades of wall¬
paper. This consists of treating the surface of the coated paper with a
solution of hardening agent, i.e., formaldehyde or alum solution, to harden
the coating and make it water-resistant. Formaldehyde is not widely used
as a hardening agent because of its disagreeable odor; alum is used almost
exclusively, even though it has the disadvantage of reducing the flexibility
and permanence of the paper. A typical hardening solution used for treat¬
ing washable papers is: 20 lb. papermaker’s alum, 10 lb. sodium acetate,
50 gal. water. The function of the alum is to coagulate or reduce the solu¬
bility of the protein adhesive in the coating so that the coating can be washed
with a wet rag without removing the coating. The aluminum ion is the
agent mainly responsible for the hardening effect, and other aluminum salts
work about as well as alum. Starch-coated papers are not treated because
they cannot be made water-resistant by this method. After the hardening
solution is applied, the paper is dried in either flat-bed or festoon type driers.

1126

PULP AND PAPER

hot air ducts being used to aid in drying. The finished paper is then wound

into rolls.

The raw stock used for wallpaper is a grade of paper known as hanging
stock. It is made principally of groundwood, together with enough sulfite
(usually about 15 to 20%) or other long-fibered stock to give the paper
enough strength to pass trough the coating machine and the drying opera¬
tion. The only significant difference between hanging stock and newsprint
is the greater weight, higher percentage of sulfite stock, and the greater
amount of rosin sizing (1 to 2y^% rosin size) in hanging stock. Some wet
strength is desirable, and the paper must have considerable stretch when
wet so that the wallpaper hanger can match patterns even though there are

large irregularities in the wall.

Another grade of raw stock called No. 1 Hanging is used in sma
amounts for the more expensive wallpapers. It is usually made from
bleached soda and sulfite pulps. All hanging stock is lightty calendered
since a bulky sheet with a rough, toothy surface is desired. s «

is generally sold in weights of 38, 42, 50, 58, 66, 74 82, and 98 lb. on the
basis of a ream size of 24 x 36-480. The finished wallpaper is sold by
rolls (width 20 in. and length 9 yd.). The weights of the ^"8‘ng^ oi
listed above are frequently designated in the trade as 9, 1^, 14 16, 18^
90 and 24 oz., respectively, on the assumption that raw stock of the g
weight in pounds will result in a roll of wallpaper of standard length

width weighting the corresponding ounces.

There are very few tests made on the finished wallpaper. T

nick test is rarely Led, principally because the coating .is so weak. One
L*de test wHch^^ commU Lde consists of rubbing the ground coating

0 to 5?‘i-s with a small piece of paper, and then --Hy
surface of the paper tor the amount of coating removed. A ®

shLTd oroduce vLy little dust under these conditions; otherwise, the per¬
centage of adhesive in the ground coating 100

A <rnod irrade of washable paper should withstand at leasr

rubs when wet with water and rubbed with a sp^ial dewce
rubber sponge covered with cloth gauze and weig ^ ^

A diluteLap solution is used to wet paper, and

withstand more rubs in this medium p naoer is taken from

Ub test is "XmytndiTo imVove slightly on aging for a

Si:dtXid Lt nl^ a Jwhich time it remains fairly constant.

Varnished Papers

Some grades of coated paper, such as Spirit varnish

label paper, and playing cards, are given a coating with clear sp

204 Test devised by United Wall Paper, Chicago, Illinois

XVIII. PIGMENT COATING

1127

in order to increase the luster and durability of the surface. This is known
as varnishing, and is discussed further in the chapter on protective coating.
Varnished papers are generally coated with pigment before varnis mg in
order to provide a better surface for printing. The various steps include
coating the paper with pigment to improve the printing qualities, pinting
the paper, and finally varnishing to improve the appearance and increase
the durability. The pigmented coating not only provides a better printing
surface, but also reduces the consumption of varnish by reducing penetration

of varnish into the paper.

Coated papers for varnishing are made in the same manner as regular
coated papers. There are, however, several important differences. Or¬
dinary coated papers are not satisfactory for varnishing because they are too
absorbent and cause the varnish to strike into the coating. In preparing
coated papers for varnishing, the type of pigment and adhesive, and the
ratio of adhesive to pigment are factors to be considered. The finer the
pigment and the greater the filming properties of the adhesive, the greater
the varnish resistance. Table XV shows the amount of different adhesives
which are required in order to obtain a coated sheet of good varnish re¬
sistance, using two different clays, with average particle size of 1 and 5
microns.

TABLE XV

.\mou.nt of Adhf.si\^ Required in Coated Papers for Varnishing

FOR Clays of Different Particle Size^os

Average particle
size of clay,
microns

Adhesive required in coating based on clay
Casein, % Animal glue, % Starch, %

5 30 40 60

1 16-18 22 2^30

It is apparent from Table XV that higher percentages of adhesive are
required for coated varnishing papers than for regular coated papers. The
percentage of adhesive required is greater the larger the average particle
size of the pigment. In the matter of different adhesives, it is evident that
starch is the poorest and casein the best. Alkaline coatings seem to resist
varnish better than acid coatings; the presence of about of soap in the
coating is sometimes helpful.^®® After coating, the paper should be calen¬
dered at high moisture content (e.g., 4%) in order to obtain the highest
possible gloss. The raw stock is an important part of coated papers for
varnishing. The raw stock for coated varnish papers should have approxi¬
mately the same properties as uncoated varnish papers. In particular, high
density is important.

Private communication to the author from N. I. Bearse, Champion Interna¬
tional Paper Company

1128

PULP AND PAPER

Tests made on coated papers for varnishing are the same as those
made on regular coated papers. The wax pick test is much higher, occa¬
sionally being over a Dennison wax of 10. If the paper is to be covered
with solid printing before varnishing, there is no need for special varnishing
properties, since the printing ink will provide a satsfactory base for the
varnish. If, however, the paper is to be printed in only a small area, the
varnishing properties are important. The best method of testing coated
varnishing paper is by applying a film of spirit varnish to the paper by
means of a laboratory coater and then comparing the results with a stand¬
ard paper for gloss, discoloration, and loss of opacity.

Opaque Bread Wrap

Paper for opaque bread wrap is coated with pigment to offset the loss
in opacity which would otherwise result on waxing. In this application,
a pigment with a very high index of refraction is required. Clay is prac-
ticaliv valueless so far as increasing the opacity after waxing is concerne
because of its low index of refraction. Either 100% mixed titanium pig¬
ment (30% rutile titanium dioxide and 70% calcium sulfate) or o %
pure titanium dioxide and 60 to 70% clay are commonly used as the pig-
r^ent in coated opaque bread wrap. Paper coated with —„ p.grnen
loses opacity on waxing, but the loss is not as great as that which occurs
with clay-coated paper. For example, Willets'“ has shown that ‘he opaci y
decreases from 70 to 2ifo on waxing lOOre clay-coated W;-
opacity decreases from 72 to 42% upon waxing a coated J

25% titanium calcium pigment and 75% clay, based upon a - 3.
tcontaining 25% starch adhesive) applied to 25-lb. raw stock.

^ alter amounts of adhesive are generally used in coatings for oW-
bread wrap than in ordinary grades of coated paper m order tha the c
i^Twill nit absorb too much wax. One interesting point here is haUhe
high amount of adhesive used in these papers is not harm u o g

the same refractive index, and thus it m _ and opacity of

hesive or wax is present {or coated bread wrap is:

',00 rxe“rnium pigment. 30 to 40 parts st-h^ f
Tter. The coating is frequently applied on a large Yankee drier.

Coated Flour Bags

Another special grade of coated paper is that used jccjijihing

„ags for the reuil market. The familiar blue and ' a

eSly coated on the outside with a coating similar to a regular

20G w. R. Willets, Tappi 32, No. 8: 349-^356 (Aug., 1949)

XVlII. riCMEKT COATISC

1129

oi^liie. except th»i more adhesive is used in the coatinf fonmila. A high
ftiewth is mpured in this grade o^ i«pef in order to resist the picking ac¬
tion cf the eueedinglv tacky inks med in |»riiiUng. Starch-coated
base not been iatts{actor> because the high |jercentagf of starch required

tends to dull the coating.

Sand(n|ier is a spccul l>"pe of coated paper whidi is quite different from
tV ff^drs of coafed papers already discussed. In the proce^ of making
sandpaper, grains of abrasise such as flint, garnet, silicon carbide, or emery
are bound to the base paper with an adhesi\‘e made from aniiiial glue or a
synllvtic resin Synthetic resins such as the alkyds and phenolic resins
are mr*i as list adhesise arhen the paper is to be used (or a*et sanding in wa¬
ter, ahereas ammal glue u used for regular |^per for dry !aiiiling. Hide
ghsr is «*«*»*< because ^ its higtirr strength compared to bone glue.

Bleached and unbleached kraft iHilj^s and rag stock are the common
pulps used in the manufacture of abrasive |iapers. In some of the best
grades, manila hemp may be used. Wet-strength paper is used for the wet
grades and ordinary paper for the dry sanding grades. In some
cases, the paper nwy be impregnated with paraffin or linseed oil and then
with a resin dissolved indinseed oil. which acts as the binder. The
paper is usmlly quite heavy, ranging from <40 to 90 lb. for hand sanding
grades to >130 lb for mechanical sanding grades (24x36—480). Four-
dm^ P*P^ used for hand sanding grades and cylinder ])a|iers some¬
times used for mechanical sanding grades.

In the coaling process, a la)**^ of adhesive is first aiqdicd to the jiaper.
and then grains of abrasive are dro^qied as evTnly as possible on* the wet
adhesive film. Then, an anchoring coat of adhesive is applied over the
abrasive to hold the individual grains more firmly. .It is necessary that tlie
abrawve particles be well wetted by the tiinder to obtain the maximum
himding effect In recent years, a new method of distributing the abrasive
has been deve l oped which uses an electric field of alxnit 40,(XO to 50.000
volu b et we e n the drive belt carrying the grains of abrasive and the glue-
coalrd paper held about ^ in. away. As the grains enter the electric field,
they are picked up and transferred to the pa{>er. The electric field lines up
the panicirs of abrasive in the direction of their long axis, and since they all
have the mat charge, they are kept in uniform distrilnition over the suHace
of the paper.

5frrtafly Coated Papers

A iHMubcr of ifwcialiy ouled pa(^s are useil fur making faiKv greet-
mg cards, tags. laheH. menus, natch lirMes, wra|i|iings. ami similar sfiecial
acma This held of paper ootiiq^ is highly sfiecializrd, and many types

1130

PULP AND PAPER

of products are made for small special uses. Among the most important
grades are mica coated papers, metallic coated papers, flock coated (velour)

papers, and luminescent papers.

Mica coated papers are made by coating the paper with an adhesive,
and then while the adhesive is still wet, adding finely ground mica. Metal¬
lic coated papers are made by dusting fine metal powders on the surface of
freshly coated paper or by mixing finely ground metal powders (e.g., gold,
copper, or aluminum) or metal oxides with a suitable adhesive and then
coating the mixture on the paper. If metal oxides are used, it is customary
to burnish the coating on a friction calender or on a brushing machine.

Flock coated papers are made by dusting the freshly coated adhesive
surface while the adhesive is still wet with fine flock made from cotton, wool,
rayon, silk, or animal fiber. An electrostatic device is used to cause

orientation of the fiber. • 4 . ....

Luminescent papers are made by coating the paper with a mix u

adhesive and_ appropriate luminescent pigments (see section on luminescen
pigments in this chapter).

CHAPTER XIX

PRINTING

Approximately 90% of all paper is printed by some means or other.
Many grades of paper are made primarily as a base for the printing. n
these grades, the printability of the paper is the most important property,
although strength and other physical properties of the paper must e a e

quate to meet the functional requirements.

Printing is a complex colloidal phenomenon. It involves the applica¬
tion of a liquid or plastic material (the printing ink) to a colloidal fibrous
membrane (the paper) under conditions of high speeds, with the object of
producing certain optical effects on the printed paper. Slight variation in
printing ink, paper, or press conditions can throw the system out of balance.

The paper chemist is primarily interested in the quality of the paper,
but in order to form an able opinion of its printing qualities, it is necessary
that he understand the basic steps in the printing operation, complex and
variable as they are. Unless the papermaker possesses this knowledge, he
can never be certain that the paper is properly used. Paper cannot be
changed after it has reached the printer, and consequently any major ad¬
justments made at the time of printing must be made in the ink or in the
press conditions. The only recourse which the printer has when he con¬
siders the paper at fault is rejection of the order. To be certain that the
paper is not rejected unfairly when the ink or pressroom conditions are at
fault (or at least unsuited to the paper), the paper chemist must be familiar
with the properties of printing inks, the conditions of pressroom operation,
and some of the common remedies which the pressman is likely to use,
many of which may be unsuited to the paper. The proper paper for a given
printing depends upon the type of printing press on which the paper is to
be printed, the type of printing process, and the type and method of drying
the ink. The printer should likewise make an effort to learn more about
the properties of paper. Cooperation is the keynote, and although there is
much more of this than formerly existed, the printer and the papermaker
can profitably learn more about each other’s problems.

Printing is an old process, having been used by the Chinese as early as
the eighth century. The industry was given a great impetus by the intro¬
duction, about 1450, of movable type, the invention of which is generally
attributed to Gutenberg. Other significant developments came later, such

1131

1132

PULP AND PAPER

as the devolopinent of photoengraving in England in 1852, and later on, the
invention by Ives of the halftone process, whicli for the first time made it
possible to obtain tone gradations in printed illustrations. The develop¬
ment of pigment-coated papers was another important step, because it made
possible papers of very high surface smoothness which were particularly
well adapted to halftone printing.

Printing Processes

There are three principal printing processes: relief, planographic, and
intaglio. In the relief process, sometimes called the letterpress or typo¬
graphic process, the ink is held on a raised printing surface. In the plano¬
graphic process, usually called the lithographic or offset process, the ink is
held on a plane surface. In the intaglio process, called the photogravure or
rotogravure process, the ink is held by recesses in the plate surface.

Relief or typographic is the oldest form of printing, and more printing
is done by this process than any other. Planographic or lithographic print¬
ing was not developed until about 1798, but did not have a very wide use
until the offset press was developed about 1906. Since that time, offset
lithography has increased rapidly and is still expanding in use. Offset has
not replaced relief printing, but has increased the total amount of printing
paper used. Intaglio printing is important for certain types of printing;
its greatest use is in rotogravure. Another printing process is si ^-screen
printing, which is a fast growing process for short runs on special mu i-
color printing. Collotype and photogelatine printing are special printing
processes classed with lithography, since they are a water-m< process.

They may be handled as direct or offset lithography.

With the introduction of offset printing, it became possr e o p
presses at much higher speed than formerly obtainable with the re le proc¬
ess. This stimulated interest in methods of increasing t e spec
presses. Substantial gains in speed have been made as a resu _ P

press design, improved papers, and in particu ^veb

today sheet-fed relief presses run as fast as sheet-fed offs p
relief presses run at even higher lineal speeds.

Printing involves the local application of ink to the

of dots, lines, characters, and solid areas. f']'^°i.base varnish

may be spot-varnished or overprinted, y vv iic a sp ’ . j j which is
is applied to the paper. This is different from ^

Operation. Spot-varnishing and oveqirinting P-f"'^ p,ess

because of their rapid drying properties which cause gumini g
transfer rolls.

XIX. PRINTING

1133

Relief (Typographic or Letterpress) Process

The relief process is the most widely used of the printing processes. It
is best suited for long runs to be printed in type and for jobs requiring fine

detail in illustrations.

In the relief process, the ink is applied to a raised printing plate surface,
by a system of rollers made of rubber, glue-glycerine, vulcanized vegetable
oils, or plastics. From the printing plate, the ink is then transferred to the
paper under pressure. During printing, the ink film splits, part of the
film transferring to the paper and part remaining on the plate. The ideal
situation, from the standpoint of good printing, is to transfer as much ink
as possible to the paper, because in this way the plate is cleaned for the next
Inking. If ink is allowed to build up on the plate, there will be a tendency

to fill in the printing form.

Types of Printing Presses

Three types of printing presses are used for relief printing: the platen
press, cylinder press, and the rotary press. The three types of presses differ
mechanically, as shown in Figure XIX-1, but all are designed to accomplish

Ploten

Cylinder Flat-bed

Rotary

Fig. XIX-1. Types of presses for relief printing.

the same purpose, namely, to bring the paper and the printing form together
under pressure.

The platen press, sometimes called a job press, consists of two parts,
the platen and the printing form. The printing form is inked by rollers
and then a sheet of paper is forced against the form by means of a flat
platen. The form and platen may advance toward each other, or the form
may be stationary and the platen movable. All the form is printed at one.
time in a platen press. Platen presses are very useful for small forms but
they do not provide as good impression as cylinder presses.

The cylinder flat-bed press consists of two parts, the impression cylin¬
der and the printing form. The paper is fastened to the impression cylinder

1134

PULP AND PAPER

and moves with the cylinder. As the cylinder turns, the paper is brought
into contact with the printing form which advances at the same rate as the
cylinder is turning. When the impression is complete, the cylinder lifts and

the printing form returns to its original position.

In the rotary press, the printing plates are fitted around a plate cylinder.
The paper passes between the plate cylinder and the impression or backing
cylinder, which rotate at the same speed. Rotary presses can be used for
continuous web printing or for sheet fed printing. They are the most com¬
monly used printing press. Web-fed rotary presses are much faster t an

sheet-fed presses.

FoT'i'iis oj RepToductiou

Reproduction has three forms: type, line, and halftone. These are

discuMed^bdow-^^uc^ion consists of letters, numbers, and charac¬

ters Type can be set as individual movable pieces by hand, or set mechani¬
cally in tL form of slugs, or reproduced from typewritten copy. Ty^ s
set by hand only when small amounts of copy or special forms are involved.
Snd String involves the arranging of individual pieces of t^e in a com-
Dosine stick placing the set type in a form and locking in p ace. n
Sn St^g. intertypes and linotypes, which are
chines operated by a keyboard, are used to cast s u^ yp
the letters for each line in one piece, matrices being used for the ^st

the slugs from molten metal. Another mechanical

Typewritten copy can matter for lith-

Photo-type setting is a direct method ot ^ g yp linotype

ography in which the photo-typesetter functions si y

machine except that it composes type P'’°‘“STde«n different classifica-
Many different type faces are grouped mm eleven d «erei

tions, based on origin and general characteris ic^ ^

determined by the type of printing process, finish on the paper,

artistic effects desired. mnrerned only with full

Line Reproduction. Line ,eprodLtion lays

color values, e.g.. black, white, or colored, tecause I T

down the ink in die form of lines or solid areas wit „ake

Until the invention of the j ,;„ork has been greatly

line drawings for illustrations. t oug i reproduction of pencil

reduced in recent years, line cuts are sdll used toi P

and ink drawings and for hand lettering. ^.^tive must first be made.

In preparing a line reproduction or a ne a

There are two methods by which this may ’ j. j by (f)

and the dry plate method. In the wet plate method,

XIX. PRINTING

1135

coating a piece of clean plate glass with albumen, (2) coating with collodion,
(3) treating with silver nitrate, and {4) finally exposing in a camera o
the drawing or photograph to be reproduced. The negative thus produce
is stripped from the glass and reversed on another glass plate. T is p a
used as a negative and placed with film side next to a sensitized zinc plate
coated with albumen and bichromate of ammonia. The assemb y is en
exposed to strong arc light which hardens and insoltibilizes the P^ti^iis o
the coated plate corresponding to the light areas on the negative. e p a e
is covered with ink and washed in water to remove the unhardened portion
of the coating. Next, the plate is treated with dragon’s blood (resin) to
protect further the hardened areas, and the plate is placed in dilute nitric
acid to etch the unexposed areas of the plate. Treatment with dragon s
blood and etching is repeated until the correct depth of cut is obtained,
after which the plate is touched up and cleaned. The printing areas of the
plate consist of a series of solid lines or solid areas which are broken up by
non-printing recessed areas which do not receive ink in the printing op¬
eration.

Halftone Reproduction. Shadings can be obtained with the halftone
process ranging all the way from solid colors to whites. The halftone proc¬
ess is used in relief and planographic printing for pictorial reproduction.

A relief halftone plate consists of a mass of raised areas or dots which
prints ink to form a corresponding mass of dots on the printed paper. The
size of the dots on the printed paper determines the gradation in tone of the
picture. The larger the dots and the closer they are on the paper, the
deeper the color of the impression. The smaller the dots, the farther apart
they are, the lighter the color. When a black ink is used, it is possible to
obtain color ranging from black through gray to white, depending upon
the size of the dots.

The copy for halftone reproduction is in continuous tone which may
be in the form of a photograph, artist’s painting, and copy which requires
the middle tones or halftones. If colored copy is reproduced in black and
white, the colors reproduce in a range from almost white to black. The
original blue reproduces almost white, white reproduces a light gray, gray
reproduces a light gray, green reproduces a dark grey, and brown and red
reproduce almost black.

A halftone plate is made from the copy by first photographing the copy
through a special screen to produce a halftone screen negative. The screen
consists of two sheets of glass ruled with parallel lines which are cemented
together with the lines on one plate at right angles to the lines on the other
plate to form a screen. The distance between the lines on the screen deter¬
mines the number of dots on the plate. The size of the dots depends upon
the detail in the copy. The highlights of the copy reproduce as small dots,

1136

PULP AND PAPER

widely spaced, whereas the dark areas reproduce as larger dots, closely
spaced. The number of dots is the same in both the highlights and the
dark areas since the same screen size is used.

The. sensitized plate used for preparing relief halftones is either a cop¬
per plate sensitized with a mixture of photoengraver’s glue (fish glue) and
ammonium bichromate, or a zinc plate sensitized with albumen and am¬
monium bichromate or shellac and ammonium bichromate (cold top en¬
amel) . The plate is prepared by making a contact print of the screen nega¬
tive onto the sensitized surface, after which the plate is washed in water to
remove the soluble portion of the coating. The light-hardened portion on
the plate is left intact, and this is further hardened by heating. The un¬
protected areas of the plate are then removed by etching (in feriic chloride)
or by electrolysis. The plate may then be touched up, i.e., etched deeper to
brighten the printing, or burnished, if a deeper color is desired. Depth of
etching varies from about 0.002 to 0.003 in. for the highlights to about 0.001

to 0.002 in. for the shadows.

Most couiniercial work is done with screens ranging from 50 to 13
lines per inch. Screens from 50 to 100 lines are considered as coarse, while
screens from 110 to 200 lines are considered as fine. A 133-line screen pro¬
duces in the neighborhood of 17,000 dots per square inch. Each dot must
stand apart on the printing plate to receive printing ink. and each dot must
be faithfully reproduced on the paper. If more than 25% of the dots are
missing or distorted, the printing will be worthless.^ Fine workmanship
in the preparation of the plate and smooth-finished paper are required o

obtain satisfactory results from fine screens (see Table I).

Multicolor Printing. The wide range of colors usually foun in co -

ored copy can be faithfully reproduced by multicolor If , 3

are usually obtained by printing only three or four colop. e a
used are red, yellow, and blue. These colors are enough to produce any de

sired shade, but for best results a fourth color, black, is use .

The copy for color printing can be a water color drawing, an P

ing, a transparency, or other work. If the copy ff niiah color
negatives are prepared for each color by photograp mg color*’ wanted,
filters to eliminate all colors from the negative ^ ji-

If color correcting is necessary, this can be accomp i y , ,

rectly on the halftone dots or by changing the stained masks use

back . patterns, screen angle rotation is used in making

Tn printing four colors, the three .strong colors (e.g., red,

I'l'alk Ity Mr. R- A. Dielim, rei)rintc(I in 7 33, No. 2. 42A, 44A.

(Feb., 1950)

XIX. PRINTING

1137

may be separated equally at 30® and the fourth color (e.g., yellow)^ posi
tioned between the red and blue at 15°, i.e., yellow at 90°, red at 75 , blue

at 105°, and black at 45°.^

Separate plates must be prepared for each color and each color printed
separately. Transparent inks are used so that the desired colors are repro¬
duced by the overlapping dots of ink on the paper. The classical method
is to print the yellow plate first, then the red plate, then the blue, and lastly,
the black plate. Sometimes, however, the black is placed down first as a
key plate for registration. Some pressmen put the red down first and others
the blue, depending upon the color effect that they w'ant.

Printing Plates

As mentioned earlier, relief printing is done from raised surfaces which
may be metal, wood, rubber, linoleum, or other suitable material. The ear¬
liest typographic printing, practiced by the Chinese as early as 1000 b.c.,
was done from wood blocks on which the characters were carved in reverse
by hand. Wood-block printing was used extensively in Europe and con¬
tinued in use there until movable type was invented about 1450. The in¬
vention of movable type was a notable advance in printing, since it per¬
mitted the reuse of individual pieces of type for many jobs. Hand setting
of movable type is still used for small jobs, display type, and special forms,
but for production jobs, movable type has been discarded in favor of indi¬
vidual solid plates because of the very high machine speeds used on modern
presses and the necessity of running the same job on several presses at the
same time.

Electrotypes and Stereotypes. The original type or halftone plate
may be used for printing short runs of less than 50,000 impressions, but for
longer runs, duplicate plates must be made from the original forms.
Duplicate plates are made as spares in case of abnormal wear of a form, or
in the case of batter or damage to the first plate. They are made when
more than one up is to be printed, that is, when the same order is printed
on more than one press at a time, and when two or more copies of the same
form are printed on a single large sheet and separated later. There are two
principal methods of making duplicate plates, electrotyping and stereo¬
typing.

Electrotype plates are used when it is desirable to reproduce a number
of plates from the original type or halftone plate as, for example, when
long-run, high-grade printing jobs are required for rotary printing of flat
sheets. Electrotypes can be made by pressing the original plate into wax
and coating the depressed wax surface with graphite to make it conductive,
but a more modern technique is to mold in tertaplate and vinylite, and spray

D. J. Andella, Paper Trade J. 128, No. 4 : 35-39 (Jan. 27, 1949)

1138

PULP AND PAPER

with silver salts to give the electric contact for plating. The matrix is then
electroplated to deposit a layer of copper about 0.006 to 0.008 in. thick.
The resulting copper shell is then removed from the wax and filled with
electrotype backing material, a special alloy similar to type metal. After
the plate has been filled, the back is planed to a definite thickness, the non¬
printing areas are routed out, and the plate is mounted on wood, metal, or
patent steel base and fitted to the press. Electrotype plates give an average
run of 1,000,000 impressions. If there are prospects of abnormal wear due
to the type of paper or ink used, they can be run in nickel or chromium
plating.

Stereotype plates are widely used for the printing of newspapers and
advertisements, and to some extent for the printing of magazines when
screens of 85 lines or less are used. In making a stereotype from the orig¬
inal plate, a mold of the plate is made in matrix paper (flong) by pressing
the slightly moistened matrix paper upon the plate under high pressure.
The matrix paper is a heavy, absorbent paper that has a high stretch and
compressibility and a smooth surface so that it will accurately reproduce the
detail in the plate surface. After molding, the paper is dried and placed
in a molding box which is filled with molten stereotype metal consisting of a
mixture of lead, tin, and antimony. This forms the stereotype plate. This
is a rapid and relatively cheap method of reproducing plates.

Special Printing Plates. Because of the non-flexible nature of the
metal plates used in relief printing, only those papers having a high resil¬
iency and a reasonably smooth surface can be printed when fine details are
desired. This limitation on papers suitable for relief printing has stimu¬
lated interest in the use of flexible plates which would permit the printing
of rough papers. Rubber plates made from plastic molds have been used
for the printing of rough-surfaced papers for paper bags and sales books.
These plates are good for about 200,000 to 500,000 impressions when oil-
base inks are used. A special combination machine known as a printer-
slotter equipped with rubber printing plates is used for printing corrugate
boxes. Rubber plates are also used on aniline printing presses and these are
good up to 1,000,000 impressions. Plastic plates molded from the ongina
metal plates have been used to a limited extent for specialized printing.

Makeready

The basis of good printing by the relief process is a firm
After the plate is prepared, it must be properly planed down an a jus ^
the press to secure the desired impression on the paper. Plowever,
steps by themselves are not enough to insure a uniform impression e ^
of lack of uniformity in the printing form, and consequently t le pres

must further adjust the impression by the process of makeready.

XIX. PRINTING

1139

Makeready involves the building up of a layer of packing of the proper
thickness upon the impression surface on which the paper is to be printed.
This involves the building up of a number of plies of paper under the tym-
pan, the top manila sheet (usually oiled) to which the guides are fastened
and on which paper is printed. On a rotary press, this involves the building
up of a packing of a number of sheets around the impression cylinder under
the top sheet. After this initial preparation, a sheet of paper is fed through
the press and printed. This sheet, which is called the “makeready sheet,”
reveals the inequalities of impression and indicates the amount of correction
necessary in the packing. Corrections are made by applying makeready
tissue to the makeready sheet to increase pressure where the impression is
weak and then substituting this spotted up sheet for one of the sheets used
in the packing. A second sheet of paper is then printed, and if the impres¬
sion is still not satisfactory, a second makeready sheet is spotted up with
makeready tissue and placed on the cylinder under the top sheet. This is
continued until the desired impression is obtained. The final packing
should be firm and solid, since a soft, spongy packing will result in em¬
bossing of the printed paper toward the end of the press run.

An absolutely flat makeready is not always desirable, since extra im¬
pression may be required in certain areas of the printing. Solid printing
areas and heavy, large type require extra impression, and consequently an
extra thickness of tissue is applied on the makeready sheet where these
appear. In the printing of halftone plates, an overlay is used to vary the
thickness of the makeready so that the greatest impression is obtained in the
solid areas and the least impression is obtained in the highlight areas. Gen¬
erally, from 0.001 to 0.003 in. more of printing pressure is required in the
solid areas of a halftone compared to the highlights of the halftone.

Relief Printing Inks

Printing inks consist of a pigment suspended in a liquid medium called
the vehicle. The early relief (letterpress) inks were made by the printer
who boiled linseed oil until it had the desired consistency and then mixed
this oil with carbon black. Today, the printer can buy printing inks which

vary from thin fluids to viscous or plastic materials. There are many types
of printing inks.

Pigments are used in printing inks to impart color, opacity, body, and
proper flow to the ink. In most cases, black inks are used against a white
paper background, since this furnishes the maximum contrast, but black
inks are also widely printed on colored papers. The hiding power of the
pigment is due to the interception of light rays, by absorption, reflection,
and refraction. The results obtained depend upon the type of the pigment,
the nature of the pigment surface, and the amount of pigment surface ex-

1140

PULP AND PAPER

posed. Hiding power increases as the pigment is subdivided until the di¬
mensions approach the wavelength of light. With further subdivision,
hiding power decreases.

Carbon black and lampblack are the two principal pigments used in
black inks. Carbon black enjoys a far greater usage than lampblack because
it dries to a higher finish and has a greater tinctorial value. Carbon black
particles are nearly spherical in shape, are quite uniform in size, and have
a large surface area, because of their internal porosity.^

Many inks contain colored pigments. Natural pigments which have
been treated so that they are free of grit are sometimes used in printing
inks, e.g., umber, ochre, sienna, Indian red, and iron yellow. However,
most of the colored pigments are synthetic organic and inorganic pigments,
some of which include the iron blues, chrome yellows, chrome greens, Ver¬
million (mercur}' sulfide), cadmium red, cadmium yellow, zinc yellow,
calcium, barium, or aluminum lakes of acid dyes, insoluble azo dyes, phos-
molybdic or phosphotungstic lakes of basic dyes, and white pigments (tita¬
nium dioxide, lithopone, zinc white and white lead).

Metallic pigments (e.g., aluminum, copper, or brass powders) are used
in special inks for the printing of booklets, advertisements, and labels. The
important consideration in inks of this type is a high leafing value, since
this increases the brilliance of the film. The size, shape, and surface con¬
ditions of the pigment flakes are factors in determining the leafing qualities.
Aluininuiii pigment is generally coated during manufacture with stearic
acid which forms a thin, oriented film on the surface of the aluminum and
causes the flakes to stay on the surface of the vehicle. Aluminum powders
are fairly stable, but bronze inks should be used as soon after preparation
as possible and agitation of the ink held to a minimum, since bronze pig"
ments tend to react with the fatty acids in the vehicle and become dull. In
printing with metallic inks, heavy ink films should be applied under a mini¬
mum of pressure in order to obtain maximum leafing.

Extenders or fillers are sometimes added to printing inks to reduce
the price, extend the coverage, and increase the body of the ink. Sonic of
the common extenders are barytes, clay, blanc fixe, whiting, and silica.
Other fillers and loaders may be added, but they should be used with cau
lion, since too much extender results in an ink which lacks snap.

Increasing the pigmentation of the ink increases the color lalue ant
improves the print quality. However, in practice, pigmentation is limits
by the working properties of the ink on the press and the absorbency o t le
paper being printed. Too little pigmentation results in a long ink, i^he

too much pigment results in a short ink.

The compounding of printing ink requires careful control by an exper

3 O. J. Brown and W. R. Smith, htd. Eng. Chem. 34, No. 3: 352-355 (Mar., 1942)

XIX. PRINTING

1141

ink maker. The vehicle is first added to a large mixer and then the pig¬
ments are added graflually with stirring, after which the mixture is ground
in a roller or hall mill until the pigment agglomerates are dispersed to the
correct particle size. Milling affects the viscosity, interfacial tension, and
degree of dispersion of the ink. Inks which have been poorly milled will
give trouble by filling in of the plate. The vehicle in poorly milled ink tends
to separate from the pigment and penetrate excessively into the paper, and
for this reason, an ink which tends to penetrate the paper excessively can
sometimes be improved by a second milling. Ink which is well dispersed
immediately after milling, but tends to flocculate later, may cause a filling-in
of fine halftones, hut such an ink can usually be redispersed by relatively
weak mechanical forces.

Printing Ink Transfer. It is desirable to say something about the
mode of transfer of ink from the ink rollers to the printing plate and from
the printing plate to the paper. The amount of ink applied to the plate by
the rollers is controlled by setting the fountain keys which open or close
the fountain and then adjusting the stroke of the ink roll to increase or de¬
crease the ink flow. The proper adjustment of the fountain is very im¬
portant, best results being obtained with the minimum amount of ink neces¬
sary for the desired intensity of color. Proper fountain adjustment, to¬
gether with proper preparation of the plate and makeready, govern the
relative amount of ink deposited in the various printing areas on the paper.

1 hese variables are controlled so that the areas of solid printing and areas of
heavy type receive the greatest amount of ink. When halftone plates have
solid areas which contain halftone dots, the pressman may force the plate
to fill up the spaces between the dots so that this area prints solid.

When printing on a non-porous surface (e.g., metal or cellophane),
the ink film tends to split fifty-fifty between the printing plate and the sur¬
face being printed. In the printing of a porous material such as paper,
more than 50% of the ink is transferred, the exact amount depending upon
the porosity and smoothness of the paper and the printing pressure. On
^ery porous papers, the ink readily penetrates the paper, resulting in easy
removal of ink from the plate. Inks are transferred to paper by a com¬
bination of absorption and adhesive strength. The greater the effect of the

adhesive strength and the less the effect of absorption, the brighter the
printing.

The thickness of the ink film on the printed paper has an important
effect on the quality of the printing. Thick films improve the contrast,
whereas thin films give a gray appearance to the print. However exces-
sively thick films may result in messy printing. In the case of newsprint
Prior found that a film of 0.45 micron was too light, a film of 1.1 microns

« P. H. Prior, Paper Trade J. Ill, No. 15 : 223-228 (Oct. 11, 1935)

1142

PULP AND PAPER

about normal, and a film of 1.8 microns too thick. For art printing, he
found 1.9 microns about right. Since ordinary paper is about 100 microns
in thickness, it can be seen that the film of ink is relatively thin in compari¬
son with the paper. Prior has shown that most ink films have a reticulated
appearance under the microscope, caused by alternate thick and thin areas
of ink. He attributes this unevenness to a pulling out of the ink into fine
threads, which cause it to print on the paper in the form of little heaps.
The ink receptivity of the paper, as well as the flow properties of the ink

affect the evenness of ink coverage.

Drying Oil-Base Relief Inks. Inks containing a drying oil base (e.g.,

linseed oil) have been the standard inks for ordinary relief printing for
many years. The functions of the drying oil in an oil-base ink are to im¬
part the necessary fluidity to the ink and to act as the binding and hardening
agent when the ink film dries on the paper. (Drying oils are considered to
be those oils with an iodine number greater than 120.) Vegetable oils
(linseed, china wood or tung, and perilla) are most commonly used in this
type of ink. Linseed oil produces a fairly soft, slow-drying film whereas
china wood oil is very rapid drying. All oils must be heated or o le
develop the correct viscosity before being mixed with the pigment prior o
ink manufacture. This bodying of the oil changes the molecular arrange¬
ment and increases the drying rate. Bodied oils are .

stand oils ranging from No. 00000 (the lightest) to No. 9 the hea^v.«t)^
During drying of the ink on the paper, the ink is trans orin
fluid or semi-fluid state to a solid state. Drying takes place in " - ;

by absorption of ink vehicle into the pores of the paper «
action and hy a hardening of the ink film on the paper The S

erned by the viscosity of the ink and the porosity of the paper
place very rapidly. The second, or hardening, %P;;;Xwly.

During this stage, the drying oil absorbs oxygen *!’7' 1 f^ par¬
ing from a viscous liquid to a hard, solid film which binds th p g

tides tightly to the paper. rnllers resulting

If the ink dries too fast, the ink becomes tacky on the ro lers, r

in picking, whereas if the ink dries too drying of

paper and the ink feels wet. In order to control the Tfo,

drying oil-base inks some non-drying oi-ba^

the printer to add driers to the ink. Driers are or^

resinates, linoleates, octoates, and °. ^4, the paper by

manganese) which increase the rate of drying of th 1 fil P

increasing the rate of absorption of oxygen. Cobalt md^s .^
drier, being four to five times as effective a ^a„d man-

eral, cobalt is classed as a surface drier, lead as an

XIX. PRINTING

1143

ganese as a combination surface and internal drier. Driers sometimes lose
their effect after the ink has been standing around for some time. The loss
is greater for inks pigmented with lake colors containing aluminum hydrate
and for carbon black, and for this reason, a relatively high percentage of
drier is used in inks containing these pigments. The loss in effectiveness of
driers in the presence of these pigments can be explained by the fact that
the active metal group of the drier becomes adsorbed next to the pigment,
leaving the inactive organic group projecting into the vehicle.® Certain
pigments can adsorb large quantities of drier, although recently improved
driers have been developed to overcome this difficulty.® Too much drier is
detrimental to the working properties of the ink, and in some cases may ac¬
tually retard the drying. Generally speaking, more drier is required dur¬
ing the summer months when the humidity is high and drying is likely to be
slower. Inks to be overprinted are sometimes made slow-drying on pur¬
pose in order to prevent them from hardening before the succeeding colors
are printed over them. Anti-skinning agents, which are in reality anti¬
oxidants, may be added to prevent the formation of a skin on the ink, and
while these agents offset the effect of the drier to some extent, they are useful
in preventing drying or hardening of the ink on the press.

Natural and synthetic resins are frequently used in printing inks, either
in combination with drying oils or alone as the only vehicle in special solvent
type inks. Resin-oil combinations, known as oleoresinous varnishes, are
made by mixing the resin with the drying oil (linseed or tung) and heating
until the desired viscosity is reached. A very fast-drying varnish can be
made by heating rosin-modified phenolic resins with china wood oil at 450
to 580° F. for thirty to thirty-five minutes. Oleoresinous varnishes con¬
taining drying-oil alkyds of the short-oil type, or rosin-modified maleic type
resins can be made in approximately the same way. Oleoresinous varnishes
have, in general, higher gloss and better holdout properties than regular
linseed oil varnishes, and for this reason, are frequently used in making
h^gli'gloss inks. The presence of the resin improves the sharpness of detail
by keeping the ink on the surface of the paper, gives the ink non-scratch
properties by increasing the hardness, and improves the adhesion, which is
a particularly important factor in the printing of non-porous materials such
as cellophane and wax paper. Oleoresinous varnishes ordinarily have im¬
proved resistance to alkali, acid, and greases; for example, a resin-modified
drying oil containing a coumarone-indene resin is said to make an alkali-
resistant ink suitable for printing soap wrappers.^ Because of the greater
grease and chemical resistance, the higher gloss, and the improved holdout

5W. F. Harrison, hid. Eng. Chem. 25, No. 4: 378-381 (Apr., 1933)

®W. C. Walker, Tech. Assoc. Papers 31: 440 ; 442 (June 1948)

^ I. Bernstein, U. S. 2,401,898 (June 11, 1946)

1144

PULP AND PAPER

properties, compared with ordinary varnishes, clear oleoresinous varnishes
are widelv used for overprinting. Where special raised efTects are desired,
e.g., in the printing of letterheads, business cards, and announcements, the
ink may be dusted with a resin which adheres to the wet ink film and is
fused there upon baking. The effects resemble those obtained in engraving.

Waxy or greasy compounds, e.g., petrolatum, paraffin, and wool giease,
are sometimes added to printing ink by the printer to reduce the tack and
prevent offset. Waxy or oily substances should not be used if overprints
are to be made, since they tend to migrate to the surface of the ink film
during drying, making it difficult for other colors to trap over them because

of a lowering of the tack of the ink.

The rheological (flow) properties of the ink determine its working
properties on the press and its penetrating qualities on the paper. Voet
and Brand® found that the rate of penetration of printing ink into paper is
directly proportional to the fluidity of the ink vehicle, but not to the fluidity
of the compounded ink. The temperature of the ink is important to the
extent that it affects the fluidity of the ink vehicle. Printing inks are non-
Newtonian fluids generally exhibiting plastic pseudoplastic, dilatant, or
thixotropic properties. Inks are, however, subject to high rates of shear on
the printing press. This tends to overcome any yield value and thixotropy,

thus permitting even distribution of ink on the rollers. •• a

For best results, printing ink should have the highest viscosity an

yield value consistent with good operation on the press and goo pnn g
properties on the paper being used.® High yield value alone resu ts m an
ink which is too short; the ink tends to back away from the fountain. Bac
ing away from the fountain caused by too short an ink can be correc e
adding a bodied varnish, e.g., No. 2, No. 3, or No. 4. Too short an ,
suits in mottle, a condition of ridges in the ink film. ° ”

absorbency can be printed with inks of high viscosity an ng P

but the flow properties of the ink must be such that the mk *s.r.butes
on the press. For relatively non-absorbent papers the '"k
a penetrant, although usually a fairly heavy bodied varnish rv
aWe tack will produce good results. When printing ^

a small amount of light mineral oil may be helpful in ;'X.

of the ink, thereby producing more rapid penetration into the i p
during offset. Thinner inks are required the faster the spee
A subject closely related to flow is the tack o t le in •, w
monly visualized as the pull resistance exerted by the ink on le p
face. The pull exerted between the printing |)late an ^ area

upon the tvIK- of ink, ride of separation of the plate and ,»per

. A. Vuel and J. S, hraiid r^r Tn-dcL 122 '

'•> H. J. Wolff, Paper Trade J. 116, No. 12. 13- 134 ( « • .

XIX. PRINTING

1145

of contact, and film thickness. At the high rates of shear existing on a
press, plastic viscosity is the dominant rheological factor aflfecting tack,
yield point being of little influence. According to Voet and Geffken,^” the
dominant factor in ink tack is not the force, but the energy of film separa¬
tion which they found to be proportional to the plastic viscosity of the ink
raised to the 1.5 power. They visualize film separation as taking place
primarily by rupture in a solid pattern resulting from transversal vibrations
induced at the paper-ink interface. A certain amount of tack is necessary
to cause adherence of the ink to the paper; if the tack of the ink is not suffi¬
cient for the type of paper and press conditions used, the reproduction will
be poor. On the other hand, too tacky an ink is likely to rupture or pick
the paper surface. In multicolor printing, the various inks must be graded
in tack to secure proper trapping, i.e., blending of the different colors on
the paper. The first-down ink (usually yellow) must be relatively tacky so
that it will lift the second-down ink (usually red). The second-down ink
should be more tacky than the third-down ink (usually blue), which in turn
should be more tacky than the last-down ink (usually black). Inks differ in
their rate of increase of tack with increased press speed,” and hence it is
possible for two inks to change their relative order of trapping as the speed
of the press is increased, with the result that false color values are produced.
The tack of ink can be measured by special tachometers.^^ The tack can be
measured qualitatively by an experienced pressman by rubbing a small
amount of the ink into a thin film on a piece of paper or a glass, and then
repeatedly pressing a finger on the film and pulling it away to measure the
pull resistance developed by the ink.

Solvent Heat-Set Relief Inks. The trend in printing is toward higher
press speeds, and since the limiting factor in operating at higher speeds is
the drying of the ink, many new rapid-drying inks have been developed.
One of these new inks is a solvent-base, heat-set ink. Solvent type inks
have been used for many years in the gravure process. Relief inks having
improved drying properties were first made by adding a volatile solvent to
regular relief inks, but within recent years, improved solvent heat-set inks
have been developed for relief printing. These inks have made possible
greatly increased printing speeds on rotary web presses.'^

Heat-set inks (Flash-Dri, Heatset, or Vaporin) for relief printing con¬
tain a high percentage of volatile solvent having a definite evaporation tem¬
perature. The solvent must be relatively non-volatile at room temperature
and very volatile at elevated temperatures, or it may be a mixture of sol-

Voet and C F. Geffken, Paper given at 35th Annual Meeting of TAPPI
Commodore Hotel, New York City (Feb. 23, 1950) ^ At-Fl,

Rheological Structures, p. 133, John Wilev
& Sons, New York, N. Y. (1949) ^ ^ vviiey

^2 R. L. Drake, Paper Mill 62, No. 29 : 24-26 (July 20, 1940)

1146

PULP AND PAPER

vents of different volatilities. Resins are used as the binder. A typica
composition of a heat-set ink is as follows: pigment, 20% ; resins, plasticizers
waxes, 45%; solvent, 35%. Recently four-color process inks in heat-set

formulation have been developed.

After printing with a heat-set ink, the paper is passed t roug an ov
where a gas flame plays upon the printed surface, igniting an urning ^ e
volatile constituents of the ink and baking the resinous binder. Other
methods of drying employ hot air chambers, infrared
drums which vaporize the solvent, leaving the resin to bind
particles. The heated paper is then passed over coo ing
paper back to room temperature and to complete the setting y ma mg
tack-free. Separate drying units are generally used for each side.
Heat-set inks are used mostly for the relief printing of magazines an
cataSs whet instant drying il desired. (Special types have also been
developed for planographic printing.^) The chief disadvantage of heah

i £??. ,k. .1 -i»., .1. i.p«» » 'rr."S

of the paper, me s hv the use of more expensive

20 % during drying, which must be offset y

to speed up the drying process. So far, ^"xhiTtyp'e oTink is

ful and have not been „hich is’solid at temperatures

tLt l^to 180 ;f! but whichUmes fluid when heated to temperatures

above 200 to 220° F. , . . j hv means of heated

Cold-set inks are picked up from heated ““ trrptinted on the

rolls and applied to hot printing plates ^ p heated to a

paper. The fountain, ink 220° F.).

temperature above the flow pom o ^ surface, the layer m

When the hot ink contacts the relative y P^P^

contact with the paper increases m the viscosity of the

heated printing surface remains highly flmd^ S n e t

ink is greatest in and the film tends to break

ships in printing ink transfer P plate."* Because of

near the printing plate, leaving very ^i _ reason, cold-set

this, there is less tendency to remain on the surface of

inks do not penetrate to any ext , obtained by pass*

the paper in sharp relief. the film and permit

ing the printed paper over a heated cylm

IS Am. Ink Maker, PP-21-24 (Dec 1944) 13 ^ I 94 O)

14 F. G. Breyer, Paper Trade J. 110. No. .

XIX. PRINTING

1147

further penetration. The final film is not very hard, and the major prob¬
lem has been in finding an ink which is tough enough to withstand scuffing.

Vapor-Set Relief Inks. Vapor-set inks (Vaposet, Moisture-Set,
Hydri, etc.) are inks based on a different principle from other inks in that
the ink does not harden by oxidation or by evaporation, but rather is changed
from the liquid to the plastic state by coaguladon of the ink through the
absorption of moisture by the ink film. The ink contains a vehicle com¬
posed of a synthetic resin and an organic solvent (alcohol) having a hig
boiling point and a low vapor pressure.^® The resin is soluble in the or¬
ganic solvent and a limited quantity of water, but is insoluble in the pres¬
ence of a higher percentage of water. One^ patented type of vapor- or
steam-set ink is based upon the use of a special, hard, high-melting rosin-
modified maleic resin which is dissolved in diethylene glycol. A suitable
formula^® for the vehicle is: rosin-modified maleic resin, 100 parts; diethyl¬
ene glycol, 100 parts. Add resin to diethylene glycol and heat to 300° F.
and hold for ten minutes until a clear solution is obtained. The water tol¬
erance of the above vehicle is 3.0 to 3.5 cc. per cubic centimeter of vehicle.
When the water content of the ink exceeds this critical value, the resin is
coagulated and the pigment is occluded and precipitated with the vehicle in

the form of a hard film.

Under ordinary pressroom humidities, the ink maintains its fluidity,
but when the printed paper is subjected to a condition of high moisture
(e.g., when the printed paper is sprayed with steam or water vapor), the
ink film picks up moisture and forms a thin, hard layer on the surface. The
exterior of the ink film is dry and hard, which prevents the printed sheet
from offsetting, but underneath the surface the ink remains in a perma¬
nently plastic condition. Even when not sprayed, this ink will usually set
and harden on most papers in ten to thirty minutes.

The advantages of this type of ink are greater speed in printing, elim¬
ination of offset, lack of any odor from oxidation of vehicle, and freedom
from bleeding upon waxing. These inks are particularly suitable for print¬
ing waxing papers (bread wrapper), chewing gum wrapper, multi wall kraft
bags, and paper carton stock to be used for the packaging of foodstuffs. On
kraft bags, the high moisture content of the paper facilitates rapid setting
even without the external application of moisture.^^ Some of the earlier
steam-set inks were unstable under high moisture conditions, but recent

15 D. R. Erickson and F. D. Elliot, Paper Trade J. 112, No. 22 : 273-274 (May 29
1941)

16 Tech. Bulletin, “Amberol,” pp. 14-15. Rohm and Haas Co., The Resinous Prod¬
ucts Division, Philadelphia, Pennsylvania (Apr., 1948)

11 Report on talk given by J. Watson, Tappi 33, No. 6: 60A, 62A, 64A (June,
1950)

1148

PULP AND PAPER

changes have improved the stability.^®* These inks cannot be used in
planographic printing because of the presence of water.

General Requirements 0 / Papers for Relief Printing

Each printing process has definite requirements which a satisfactory
paper for that process must meet. In the following sections, the important
properties of papers for relief printing are discussed. Many of these prop¬
erties are important for other printing processes as well, and in many cases
may be considered as general requirements for all printing papers.

Among the grades of paper used for relief printing are book papers
(coated and uncoated), catalogue, news, mimeograph, folding boxboard,
bond, ledger, cover, blotter and bristol. The majority of book paper is made

from a furnish of bleached soda or deinked stock, plus some sulfite or sul-

fate stock. A t)^pical furnish would include 20 to 30% sulfite, and 70 to
80% soda or deinked stock. Groundwood pulp is used in some grades
mainly to lower the cost and also to increase the opacity, to reduce strike
through, and to improve the uniformity of printing. Rag or high-quality
chemical pulp may be added if the printed paper must stand considerable
handling or folding in use or if permanence is required. The stock for book
paper is generally beaten very lightly. The sizing is light or eliminated
entirely, since the printed paper is generally not written upon with aqueous

inks. Up to 30% filler may be added.

Printability and Print Quality. Printing may, for convenience, be

divided into two major steps, (J) transfer of ink to the paper and (2) dp
ing of the ink film on the surface of the paper. The transfer of ink is in¬
fluenced by the ink receptivity and smoothness of the paper, whereas the
drying of the ink is influenced by the oil absorbency of the paper (when
oil-base inks are used) and the drying characteristics of the ink. It should
be emphasized that both the transfer of the ink and the drying of the ink

are functions of the paper, as well as of the ink.

The printer commonly speaks of the printability of paper. Prmtabi -
ity is not an accurately defined property, but instead is a broad, general term
referring to the property of a paper which yields printed nmtter of goo
quality. The best method of testing paper for printability is by printing
runs on a commercial printing press. The next best method is by labora¬
tory tests on a proofpress. An engraver’s proofpress such as a No.
Vandercook or Hacker press^® is suitable, but several alterations m tie

18 F. J. Jeuck and C. A. Rietz, U. S. 2,390,102 (Dec. 4, 1945)

Am. Ink Maker 25, No. 4; 42-44 (Apr., 1947) _ \fiehle

20 Proofpresses are made by Vandercook and Sons, Chicago, Illinois, and

Printing Press and Mfg. Co., 2021 Hastings, Chicago, Illinois

XIX. PRINTING

1149

jiress are desirable for best results.*^ Glue-coated rollers are reconiniended
for use with heat-set inks, whereas synthetic rubber rollers are necessary
for use with moisture-setting inks. Means of applying an ink film of uni¬
form thickness to the plates must be used; the ink should be applied by

volume, rather than by weight, since inks diflfer in specific gravity. Accu¬
rate plate height is essential and a micrometer should be used to check the
])Iate height and packing. The impression cylinder of the press is undercut
a definite amount and then built up with a patent ba,se to provide the de¬
sired printing j)ressure.

Considerable skill and judgment is required in the testing and grading
of papers on a proofpress. However, when the test is properly carried out
under strict control lyv a skilled operator, it is possible to obtain consider¬
able information relative to the printing qualities of the paper. An indica¬
tion'- of the printability of the jiaper can be obtained by comparing the
print obtained on the unknown sample with the print obtained on a standard
paper for evenness of coverage, mottling of solid colors, and sharpness of
detail in halftones. A value indicative of the ink receptivity can lie oli-
lained by measuring the amount of ink left on the printing plate. The ink
alisorption of the paper is indicated by the evenness of alisorption of ink by
the paper. Any tendency toward offset can be detected by laying a sheet
«»i pa|)er over a freshly printed siiecimen and feeding the two papers through
the press. The picking tendency of the paper can be measured by suc¬
cessively printing the specimen with inks of progressively increasing tack.

If a commercial press or a proofjiress is not available for testing the

l)rmtability of paper, simpler methods can be used. In one method of test¬
ing, originated by Rekk and used by others,*®'-® a weighed amount of print¬
ing ink is transferred to a piece of plate glass of known area. The ink is
spread over the glass in a film of uniform thickness, using a rubber roll.
T wo samples of ])apei, the specimen to be evaluated and a standard sample,
are placed side by side on the inked glass plate, covered with several sheets
nf paper, and then a 25-lb. lirass roller is rolled back and forth four or five
times over the entire area. The samples are then removed from the plate
and compared visually or photometrically for degree and uniformity of
blackness. Printing smoothness and formation can be evaluated from the
uniformity of the inked surface; ink absorbency can be evaluated from the

--1 drill.

j- Iml. 2S>. N„. 3: 438440 fjiiiie. 1947)

4« ole

”cole„tfi"„e"‘l947,''°""' 3^ 159-162,

1150

PULP AND PAPER

blackness of the print.This test is much simpler than testing on a proof-
press and can be carried out quite readily by non-technical help. One sig¬
nificant difiference between this test and the results obtained in commercial
printing is that the paper is left in contact with the ink for a longer period
of time, and only solid areas are printed.

The print quality of the final printed paper is determined by the print-
ability of the paper and the method of operating the press. Among the ele¬
ments determining the quality of the printing are the amount of contrast
between the printed and unprinted areas, finish of the printed and unprinted
areas, the uniformity of solid and halftone areas, legibility of printing, and
show through. Print quality is usually measured by a visual examination
of the print, but methods for measuring objectively the tone reproduction of
halftone prints and the uniformity of solid prints have been suggested.^"

Tone reproduction, or the faithfulness of reproduction of the printing
plate, is highly important in determining the quality of halftone printing.
Halftone dots produced by relief printing show a halo or doughnut effect
caused by a ring of ink of greater thickness around the circumference of
the dot. This may be due to too much ink or to the inking rollers pressing
against the top of the dots, leaving a thin film in the center and an over¬
hanging rim of ink around the edges. Because of the pressure used in
relief printing, there is generally some embossing on the back of the printed
paper. This is different from the effect obtained in offset printing where
the halftone dots produced are solid black without squeeze and show no
embossing on the back.

In solid prints, uniformity or evenness of color density and glossiness
of the ink film are the important factors in determining print quality. Solid
prints sometimes show mottle or bright spots which lower the print quality.
The thickness of the ink film and degree of blackness or density of the ink

are important factors. ,

Ease of reading is determined in large part by the contrast between

the ink and the paper surface. Black ink on white paper gives the greatest

contrast liecause white paper reflects the greatest amount of light and black

ink reflects the least amount. Brightness is the most important factor, but

the contrast is not only one of brightness, but also one of hue and satura

tion. Printing on off-color or tinted papers gives less contrast than print

ing on ))ure white papers, liut liecause the long and short lij^ht lajs '

nary light do not focus well on the retina of the eye, a slightly yellow bac '

■“ioj. H. flardsley and L. Morin, Pul/^ Pat'cr Mat]. Camida -/.S’. No. 3. 159 1

Convcnliiin Issue (1947) Ig:

•-•1 K. nudidahl. M. I'. PolRla.se and 11. C. Schwalbe. / I nidc J. —.

14.1 ( M ay 2. 194())

XIX. rRIXTIXG

1151

ground is sometimes preferred.” Better contrast is obtained on yellow-
white (cream) paper than on blue-white iiaj>er, when both papers have the
same papermaker's brightness. However, the most desirable tint of white
depends upon the color of the ink used for printing the paper. Duplicator
ink, for example, appears purple on w’hite papjer, and much darker, almost
black on yellow piapter.

Uniformity. The most important property of paper for any printing
press is uniformity. The printer can regulate the ink or press to print
practically any paper, and so long as the paper is uniform, no trouble will
occur. On the other hand, if the paj>er is non-uniform from lot to lot or
sheet to sheet, the quality of the printing will suffer, and in extreme cases.

niay have to be stopped at intervals to adjust the ink or press con¬
ditions. Press makeready must be prefiared for a paper of definite thick¬
ness, and consequently any variation in thickness aw'ay from the standard
will cause a variation in impression, resulting in poor printing. Ink re¬
ceptivity, oil absorbency, smoothness, porosity, and other similar proi>erties
should also be as uniform as possible, not only from sheet to sheet, but also
over the entire area and on both sides of the paper.

If only one side of the paper is to be printed, the felt side is generally
u>ed. since this is the smoothest and best printing side. Of minor impor¬
tance IS the fact that printing on the felt side permits the watermark to be
read on the same side as the printing is read. If both sides of the paper are
to be printed, the wire and felt sides should l>e as nearly alike as possible to
diminate the difficulties resulting from differences in smoothness. Gen¬
era y sixaking. trouble with two-sidedness is more frequent on machinc-
hnished and suj^rcalendered papers than on coated paiier.

MmI pnnting papers for sheet-fed presses are cut machine direction
long. After the paper is printed on one side, it is turned over the long way
and printed on the reverse side. Thus, the same edges are used for grippers
^ guKfc, when printing both sides of the paper, and hence no specL rtn-

^h«^ dW on both sides), or sheettvise

Lt^r^v It T " T ™ «--ons, the

T, ' ‘I’' -^overse side.

naper since d'ff special guillotine triinniing of the

paper sin« a different gnpper edge is used in each case.

wire TAs.''sWsTo’rTl^ ** .P™"»“"oed felt and

tions impair the quilitv of since these imperfec-

I-. quality 01 the pnnting. Lumps and grit should be absent,

"1. No. 17: 2I(L22.1 fO«

1152

PULP AND PAPER

since they are likely to damage the printing plate. According to Strachan,^®
particles about 0.1 to 0.05 mm. in diameter do the most damage to printing
plates. Paper for sheet-fed presses must be stiff enough to prevent wrin¬
kling when passing down to the guides and when adjusted to position by the
joggers.

Some of the troubles which occur in printing can be attributed to paper
which has been damaged in manufacture or in shipping. In web printing,
pressroom troubles may be due to poorly w'ound rolls, which develop
wrinkles or cause breaks, wrinkled paper, tight or loose edges, or improp¬
erly made splices. When printing individual sheets, trouble may be experi¬
enced with paper cut out of square, torn sheets, or paper containing me¬
chanical defects. Rough edges may prevent the sheet from coming down to
the guides evenly and may cause more than one sheet to be picked up by the

feeders at one time.

Smoothness. Printability of paper can be measured by press runs or
simulated press runs, as described earlier, but printability can also be evalu¬
ated from the individual characteristics of the paper, among the most im¬
portant of which are smoothness, gloss, formation, finish, ink receptivity,
porosity, bulk, hardness, opacity, brightness, and uniformity. One of the
most important single properties of paper for relief printing is the smooth¬
ness or finish of the paper, because this determines the ease with which the
printing .plate can be brought into contact with the paper. The printing
plate carries an ink film about 3 to 10 microns in thickness, only about
one-half of which is transferred to the paper. No contact will be ma e
between the plate and the low spots in the paper if the depressions m the
paper are deeper than the thickness of the ink film at the time of impression

Actually, papers having a wide range of smoothness or finish are use
for relief printing, some of the most common finishes on uncoated
being: (1) antique finish, a very soft uncalendered finish used where bu
is important; (2) eggshell finish, a dull rough finish vvh.ch ^

surface finish on an egg shell; (2) machine finish, a fairly smoot
produced on the paper machine; {4) English finish, a dull but smooth fim h
which is produced on the paper machine; (5) superca en ere ms ’
high finish obtained on the supercalenders. The low-finish gf® “ ^
and eggshell) are suitable for type or line illustrations only, i ® ,
sible to obtain faithful reproduction of halftones on such roug i
Medium-finish grades (machine and English) are suitable P

of 85- to 100-line screen halftones, whereas the ® .

calendered and coated paper) will print fine screen (150 )

illustrations. There is considerable difference of finer

screen size best adapted to various papers. Paperniakers ten gi

19 J Slrachan, Paper-Maker 117, No. 5 : 331-332 (May, 1949)

XIX. PRINTING

1153

screen values for a given paper than printers do. Table I gives data based
upon long-run good printing jobs (press runs of 500,000 to 4,000,000),

TABLE I

Suitable Screen Sizes for Halftone Reproduction on Various Papers
FOR Long-Run Printing Jobs by the Relief Process

Screen size, lines

Type of paper

50-85

85-100

110

120

133-150

News

Bonds and ledgers, English finish, and machine finish

Supercalendered book, although quite a bit of supercalendered book
is also printed with 100 screen

Machine-coated and brush-coated papers, and sometimes super-
calendered paper, when plates from same original are used for
both coated and supercalendered papers

Very special work where fine detail is required on highly finished
coated papers

which serve as a guide for production work. English finish papers can taki
133-screen printing, but only for high-grade, very short-run work, and no
for production operation. Even when printing type, smoothness is a facto:
in determining the quality of the results. For a given type face, the print
ing will stand out better and appear heavier when applied to smooth, hard
surfaced papers (e.g., coated or supercalendered papers) than it will oi
rough, soft-surfaced papers (e.g., antique or machine-finished papers).

If the paper is rougher than desired, the printer may have recours<
to one of two alternatives: (J) he may use greater printing pressure, oi
{2) he may apply a thicker film of ink. There is a limit to the amount o:
pressure which can be applied, since high pressure causes mottling and, ir
extreme cases, causes the printing form to break through the high spots or
t e paper, resulting m a condition known as' “punching.” If the paper h
too rough, punching occurs before the pressure is sufficient to insure gooc
contact with the low spots of the paper. Increased thickness of ink filir
improves the intensity or color contrast of ink on the paper, but because

^ T mileage is reduced, the likelihood

of fill-up IS increased, and the tendency toward offset is aggravated. Dif-

ffieTT attributed to differences in

the surface characteristics of paper.®"

The smoothness in which the printer is interested is not the smooth-

ne!I of theT' “"'^7 "T"'’ conditions, but rather the smooth-

s of the paper under the pressure of the printing form This is called
printing smoothness, which might be defined ^ 1- I . ^

which approaches a smooth surface when the I k- Tf ^ ^

compamble to that which it receives during prinLg,« sfncelhU p::::""

Cramer, Paper Trade J. 114 No 12- 14f=L-i4Q /'itT ^

S. M. Chapman, Pulp Paper Man Cn a /to xt 19, 1942)

sue (1947) 3^ 140-150, O)„vention Is-

1154

PULP AND PAPER

may be as high as several hundred pounds per square inch, printing smooth¬
ness is determined hy the softness and resiliency of the paper at high pres¬
sure, as well as the initial smoothness. Softness and resilienc 3 L in combina¬
tion with smoothness, are better indications of printability than smoothness
alone.There are several laboratory instruments^’’ for measuring the
smoothness of paper under low pressure, but the results do not always agree
with results on a commercial press. Chapman’^ believes that a desirable
instrument for measuring printing smoothness would he one which meas¬
ures the fraction of the paper surface which approaches within 0.0001 in. of
a smooth surface under a definite pressure approaching the pressure used
in printing. For a further discussion of smoothness, the reader is referred
to Chapter XVI.

All j^apers reach perfect printing smoothness if the printing pressure
is great enough.’^ The amount of pressure required depends upon the
hardness of the paper and the cushion provided.

A soft, resilient paper, even though fairly rough, will print better than
a hard paper which is fairly smooth, because the soft paper will act as a
cushion under the printing form, whereas the hard paper will remain rela¬
tively non-compressible. With soft papers, the wire marks disappear at
moderate pressure, whereas very high pressure is required to eliminate the
wire marks in hard papers.^® An indication of the softness of paper can he
obtained by measuring the caliper and/or the smoothness of the paper at no
load, and under a pressure of about 200 to 250 p.s.i. An indication of the
resiliency can be obtained by measuring the thickness after release of the
pressure and comparing with the original thickness of the paper.

Calendering of the pai^ef is necessary to obtain the higher finishes
required for halftone reproduction. Calendering closes the surface pores
of the paper, but does not appreciably affect the rate of internal oil pene¬
tration,®^ although it does result in a slight decrease in initial oil absorp¬
tion. The best printing papers are those which are formed evenly on the
paper machine and not calendered excessively, since excessive calendering
causes blackening of the paper and produces hard spots which have poor
ink receptivity. If hard spots are present in the paper, the ink can be ad
justed to increase its penetrating qualities, but this produces a general dul

32 G. L. Larocque, Paper Trade J. 106, No. 26; 36^372 (June 30, ^^^cinothness

33 Bekk Smoothness Tester: Gurley Smoothness Tester; Williams Sm

Tester (see TAPPI Standards)

3“* S. AI, Chapman, Pulp Paper Mag, Canada 48, No, 3.

140-150, Convention Is¬

sue (1947)

35 j H. Bardsley and L. J. Morin,

Paper hid. 29, No. 3; 438-440

(June, 1947)

36 Idem.

37 B. L. Wehmhoff, R. H. Simmons and D. H. Boyce

, Paper Trade J. 96, No. 4:

48-52 (Jan. 26, 1933)

XIX. PRINTING

1155

ing of the printing.^® If it is necessary to reproduce fine halftones on rough
papers, a small area of the paper may be crushed flat by pressing with a
heated brass plate (called hot smashing) and the paper printed in the
smoothed area. In general, however, calendered papers are used where
halftone reproductions are required, hot smashing being reserved for special
effects. The presence of fillers in the paper increases the smoothness, the
finer particle size pigments being most effective.^®

Ink Receptivity. Ink receptivity is the property which causes paper
to accept printing ink at the instant of contact between the paper and the
ink. It is a surface phenomenon which is determined by the ease with
which the ink wets the paper and by the uniformity and density of the im¬
pression obtained M'hen a normal amount of ink is used. The ink recep¬
tivity is considered good when the paper accepts the ink readily and uni¬

formly over the whole surface. Ink receptivity is a somewhat different
concept from oil absorption and oil penetration. Ink receptivity is primarily
concerned with the transfer of ink to the paper, whereas oil absorption and
oil penetration are primarily concerned with drying of the printed ink film.
Ink receptivity is a function of both the paper and the ink; it is not a prop¬
erty of a particular paper, but rather is a property of paper when tested in
combination with a particular ink.

It is difficult to measure ink receptivity independently of oil absorption,
and as a result, most tests measure a combined value of the two. A so-
called ink receptivity test is sometimes made on paper by smearing a stand¬
ard ink (K and N. ink) on the paper, leaving the ink for a definite period of
time, removing the excess ink, and measuring the depth of color. This test
is used particularly on coated papers and is discussed further in the chapter

on coating. It is best to compare the results with a paper of known ink
receptivity.

Another method of measuring a so-called ink receptivity value involves
printing a weighed strip of paper with an excess of ink, using a solid plate
with an area of 100 sq.cm. The surplus ink on the paper is removed by
o mg, and the printed paper is then reweighed to determine the quan¬
tity of ink absorbed by the paper. An indication of the ink receptivity can

^ measuring the amount of ink

left on the printing plate. This method is, however, subject to the personal

m erpreta^n of the person making the test, and at best is only
cation of the results to be expected in commercial printing.

ecause most printing inks have an oil or resin base, oils such as castor

fioir U L^'lielrVl t^'^^T'^ to measure the ink receptivity and oil absorp-

helfiful to distinguish between (1) oil absorption, which refers

” G. Cramer. Paper Trade J. in, No. 7 : 78-82 (Am?. 14 1941)

G. L. Larocque. Paper Trade J. 106, No. 26: 368-372 (June 30, 1938)

1156

PULP AND PAPER

to the surface penetration of oil and is roughly the same as ink receptivity,
and (2) oil penetration, which refers to the extent of penetration of oil into
the interior of the paper. It is the former, i.e., oil absorption, which is most
important in ordinary relief printing, since regular relief inks do not pene¬
trate more than 1 to 7 microns below the surface of the paper. In fact, in
halftone printing, the spreading of the halftone dots may l)e nearly as im¬
portant as the absorption of ink into the paper.^®

Oil absorption is best evaluated by the initial resistance of the paper to
the break through of oil.^^*'*" In one test, a drop of castor oil or a standard
mineral oil (SAE 20) is placed on the paper and quickly spread into a thin
film by pulling the paper under a weighted rubber roller which exerts a
definite pressure (i.e., 2 lb.) on the paper. The time is taken for the
initial break of the oil film, as evidenced by a 75% loss in sheen. In a
variation of this test, a drop of heavy white mineral oil is placed on the sur¬
face of the paper or paperboard and spread into a thin film by allowing a
heavy 8-lb. roller to run down a 1 to 50 inclined plane over the drop of oil.
The time is taken for the partial absorption of the oil into the paper or pa¬
perboard. A special photometer is used to measure the end point which is
taken as either the time required for a 50% reduction in the gloss of the
oil film or as the gloss after a definite time has elapsed, i.e., twenty seconds."

Another test sometimes used for measuring the resistance of paper-

board to ink is the butyl carbitol test in which the time is taken for the
complete absorption of one drop of butyl carbitol (diethyleneglycol mono¬
butyl ether) applied to the paper from a tube located 3 in. above the sample
and calibrated to deliver at the rate of one drop per second. The butyl car¬
bitol should contain a small amount of water so that it is in equilibrium

with the atmosphere.

The results obtained with the above tests serve as a rough guide to the
printing qualities of the paper. Too low an oil absorbency indicates that
the ink will be slow in drying and will tend to set-off on the back of the
printed sheet (offset). Too high an oil absorbency means that the vehicle
of the ink will be carried too far into the paper. This causes a loss of opac¬
ity and, in extreme cases, results in staining of the back side o^t e s ee .
The proper absorbency for the paper is governed by the type o^ m use ^
Papers such as bonds, ledgers, and coated grades are genera y ^
with inks which dry mostly by oxidation, and hence a relatively low degree

of absorbency is desired. Papers such as

dered book are generally printed with inks which dry partly by a s p

40 R. M. Cobb and D. V. Lowe, Paper Trade J. 98, No. 12: 151-153 (Mar.

41 G. L. Larocque, Paper Trade J. 106, No. 26: 22 ^ 1945 )

42 V. V. Vallandighani, Paper Mill News 68, No. 52: 1^16 ( ^

43 V. V. Vallandigham, Tech. Assoc. Papers 29: 523 5 5 (J •

XIX, PRINTING

1157

and partly by oxidation, and hence a higher degree of absorbency is de¬
sired. Papers and paperboards to be printed with high gloss inks require
a very low absorbency, and these papers are generally surface-sized with
starch, carboxymethylcellulose, or similar sizing agents to produce an oil-
resistant surface.

The oil absorbency of paper is decreased by increased calendering, re¬
duced filler content, and increased density. In filled papers, the oil ab¬
sorbency is generally higher on the felt side because of the greater concen¬
tration of filler on that side of the paper.^^ Because of this higher oil ab¬
sorbency, a slightly greater amount of ink must be carried on the printing
plate when printing the felt side of the paper.

Surface Bonding Strength. The surface bonding strength is an im-
piprtant property of printing papers. Poor bonding results in picking (a
lifting of the surface coating or fibers away from the body of the paper) or,
in more extreme cases, body stock splitting. Picking of inked fibers off the
surface of the paper leaves a white spot on the surface of the paper where
ink should be. The loosened fibers may clog the type, contaminate the ink,
and eventually show up on a subsequent sheet as an inked spot surrounded
by a white ring.

Picking frequently occurs with poorly sized machine-finished book
papers, the felt side being more likely to give trouble than the wire side.
Like most printers’ problems, picking can be caused by a number of differ¬
ent conditions, for example, weak paper, faulty press, poor makeready, or
too tacky an ink (see section “Pressroom Difficulties”).- Another trouble
resulting from too soft a paper is linting or dusting, which is particularly
prevalent when the cutter knives are dull.

Beating increases the bonding strength of the paper and reduces pick¬
ing troubles. However, paper for relief printing should be made with the
minimum amount of beating and jordaning necessary to obtain proper for¬
mation and minimum picking, since excessive beating builds up internal
strain in the paper, reduces the softness and resiliency, and reduces oil
penetration. Increasing the basis weight is of no value in reducing picking
troubles, since this does not increase the amount of fiber bonding. Starch
and glue sizing improve bonding. Fillers improve the over-all printability,

but excessive amounts of filler are likely to cause trouble with picking and
dusting during printing.

Opacity. Paper for printing should have high opacity in order to pre¬
vent show through of printing on the back of the printed sheet. Show

through IS most apparent when halftones or solids are printed on one side
ot the paper and type on the other.

A high original opacity is helpful in preventing show through, but it

M. S. Kantrowitz, Paper Trade J. 128, No. 25 : 231-234 (June 23, 1949)

1158

PULP AND PAPKR

no ^^tiaraiiU’P aj^ain.st its occiirroiue. since the ori};in:il miaeity <n' ilu r
may be reduced by excessive penetrati».*n of oily veliicle into the jiajier. 'I his
occurs when the paper has a hijxh oil alisorUmcy and is most prevalent in
the case of low-linish book papers, newsprint, and mimeograph pajK-rs.
Fillers such as clay increase the original opacity of the paper and are or¬
dinarily considered to reduce show through, but clay has relatively little
opacifying effect in the presence of the ink vehicle and may actually increase
show through by increasing the oil absorbency of the paper/'^ High-opacity
fillers, e.g., titanium dioxide, are much more effective than clay.

The tendency of printing papers to exhibit show through can be meas¬
ured by the printed opacity test. In this test, the sample of paper is printed
in a solid black and then the opacity is measured, using the hack of the
sheet. This test takes into account the degree of penetration of ink into
the paper, as well as the original opacity of the paper. The subject of

opacit}' is discussed further in Chapter XVT.

Effect of Moisture and pH in Papers for Relief Printing.
Moisture content affects the strength, dimensional stability, ink receptivity,
finish, softness, and all-round printing qualities of paper. Control of the
moisture in printing papers is essential for the control of register, curling,
creasing, static electricity, and cockling. The optimum moisture content
for papers for ordinary relief printing is about 4.5 to 5.5%

Too low a moisture content results in a hard, brittle paper with poor
printing qualities. Damp paper prints better than dry paper, due to its
greater resilienev which causes the paper to flatten out better under the
printing pressure. IMany years ago, it was the practice, for high-quality
relief printing, to dampen the paper before printing. (In copper and steel
engraving, the work is still wet before printing.) Excess static and trou e
with curl result when the moisture content is too low. If the moisture con¬
tent is less than about 4%, the paper will lose considerable strength w en

printed with heat-set inks. .

Too high a moisture content leads to loss of finish, misregister, coc '-

ling, and curl, and if heat-set inks are used, to blistering. Excessive mois¬
ture has an inhibiting effect on the drying of the ink, drying time
in direct proportion to the moisture content of the paper.*
ink is generally retarded by a very low or by a very b.gb of the i«per
although the source of the alkalinity must be considered. A fH around ..

** The moisture content of paper for printing can lie measured ter>
«U. S. Government Printing Office. Tech. Bulletin No. 18. "Nev.-spnnt and

H. H. Slater. Prin/m.o Equipment Eugr. ,^ 3 ,

« M. T. Leckey, Paper Trade J. 116 . No. 20 : 223-226 (May 20. 1943)

XIX. PUNTING

1159

dimply by means ot a special hygrometer, called a “sword.** This instru¬
ment, after it has been calibrated <^inst a sling psychrometer, is inserted
between the sheets of paper and held there until a steady reading is ob¬
tained. The moisture content is obtained from tables similar to Tabel II.

T.\BLE II

RCJ.AT10K BeTWC£.V READING OVTAl.SCO WITH SWORO HYGROMETER

AND Moisture in Paper

Sloisture. %

4.0
5.0
6.0
7.0

Static Electricity. Static electricity may Ije generated by paper rub¬
bing against metal, wood, or otlier sheets of j>aj)cr. Static develops most
frequently under conditions of low relative humidity of the atmosphere
(i.e., below about 50%), wlien the paper has a low moisture content (i.e.,
IkIow 3)1^% moisture), when the paper has a high tini.sh, and when the
paper has low*er temperature than the pressroom. Static electricity has be¬
come more of a problem as operating speeds have increased.

Some of the undesirable conditions developed as a result of excessive
static electricity are: (J) fine specks of foreign matter (e.g., j^aper dust)
are attracted to and held on the surface of the paper; (3) the paper sticks
*'*K^^^* causing difficulties in feeding and register and resulting in offset
on the printed sheets. Spraying or misting of ink is said to be the result of
sutic Imilt up in the ink from the break away of the rollers. Controlled
(loUnttes have been utilized in the electronographic press to print with a
gap of 0.00! in. between the printing surface and the paper.

electricity can lie controlled by grounding the paper with tinsel
cord makes contact with the paper as it comes off the press. It can
be minimized by keeping the relative humidity of the pressroom- as high as
poswWe. Paper having a high moisture content is less likely to develop
sutic electricity and, if it does, it can be discharged readily by grounding,
smee the conductivity’ increases at high moisture content. Warming the
paper helps to reduce suiic electricity. A method of discharging static is
to pa^ the paper through a localized alternating ionized field as it conics
off the impression cjlinder on the way to the delivery’ table.**

C oatfd Paters for Relief Printing

Sn* halfirmc illuMration. B.4h rcpiUr coatol and Inachm«^:oat«l [a-
•* M. S PomeU. Pcftr Trodf J. JSO, So. IS: 33-37 (Apr. 13, 1950)

1160

r-ULT* AM) PAPKR

pers are used, machine-coated i)apers having rei)laced sujx'rcalendered pa¬
pers to a large extent.

One advantage of coated paper over ujicoaled pa[)er lies in the greater
surface smoothness obtained from the mineral coating, which overcomc.s
the high and low spots found in ordinary printing papers. 1 he increased
smoothness of coated paper means that less pressure is required in print¬
ing and less ink need be carried on the printing plate. For example, Bckk^'’
found on a laborator}' press that a given degree of lilackness was obtained
on coated paper when only 1.38 g. of ink per square meter was carried on
the press compared with 2.60 g. for newsprint. However, the greatest
advantage of coated paper is that the coating layer furnishes a printing sur¬
face which is far more uniform in density and oil absorption than any un¬
coated paper surface, however smooth. Massey®' states that the ideal
printing paper is the one which most closely approximates a 100% mineral

surface.

By far the biggest difficulty with coated papers is the tendency for the
coating to pick from the surface of the paper, thereby resulting m an un¬
attractive printing and contaminating the ink ith bits of coating. The
basic cause of picking is too tacky an ink, but picking can be the fault of
insufficient binder in the coating. When ink is applied to the surface o
the paper, part of the ink transfers to the paper and part remains on the
printing plate. The pull resistance or tack of the ink exerts a force on the
paper, and it is the relationship between this pull resistance of the ink, the
adhesion between the ink and the coating, and the strength of the coate
paper which determines the tendency to pick. If the pull resistance is less
than the strength of the coating and the paper, no picking occurs. How¬
ever, if the bond between the paper and the coating is ^^ eaker t an t e pu
resistance of the ink, or if the bond between the indmdual pigment par¬
ticles in the coating layer is weaker, or if the coating is strong
the paper itself is weaker, picking will occur. The first t^^ types are ^lled
coating pick, and the last is called a body stock pick. P.ck.ng may resuU
from breakting of the coating m handling (by bending). T is

to weak coating or body stock. . , , j + i of

The tendency of an ink to pick is a relahvely ^

the ink. However, the pull resistance of the ink ^

printing and with a reduction in the thickness of the ink film.
picking can sometimes be reduced by slowing down the press

Lasing the thickness of the ink film. The most

ing is to reduce the tack of the ink by adding waxes or thinners b

5. J. Bekk. Textil-Ru»ischau 1, 3: 7S-78 (Sept., 1946) through B. 1. P- C. . -

5 . pY’LLT wcLm* /. 109. No. 16: 210-213 (0«-

’iR. F. Red Pol-c; TraBe J. 102. No. 22 : 281-283 (May 28, 1936)

XIX. PRINTING

1161

the disadvantage of lowering the printing quality (see section “Pressroom
Difficulties”).

Cracking of the coating, which is sometimes confused with picking,
frequently occurs on heavily coated papers which are printed under so
much pressure that the coating is broken. Cracking occurs if the paper
loses its pliability as a result of low humidity, which is most likely to hap¬
pen in the winter months. Other difficulties with coated papers arise from
imperfections in the coated surface, such as those due to pinholes, “fish
eyes,” brush marks, roll marks, dirt specks, or lumps of undispersed ad¬
hesive and pigment.

Most coated papers have good ink receptivity and clean the printing
plate almost perfectly. However, some coated papers have low ink recep¬
tivity and result in the accumulation of ink on the printing plate, leading
to messy printing. The amount and type of adhesive used in the coating is
the principal factor affecting the ink receptivity, although certain pigments
(e.g., calcium carbonate and satin white) improve this property. Too
much adhesive in the coating results in a coating which is hard and does not
lift the ink cleanly from the printing plate, and the ink which is transferred
to the paper is slow in drying. However, hard-finished coated papers may
be desirable where the printing must be non-scratch. Starch is generally
considered as a soft adhesive but starch-coated papers may be harder than
casein-coated papers because of the greater amount of adhesive required in
the coating. Machine-coated papers sometimes contain stearates and other

soaps (added to overcome dusting and improve the leveling properties),
and if excessive amounts of these materials are present in the coating, dry¬
ing of the ink may be greatly retarded.®^ The ink can be adjusted to in¬
crease the pick-up by reducing the cohesiveness, but this has a tendency to
make the ink too short. Cast-coated papers are highly ink receptive but

fnction-glazed papers have a hard, waxy surface which is quite' ink-re¬
sistant.

With some coated papers, the opposite trouble from that described
above IS encountered, namely, the coating has too high an oil absorption
and tends to rob the ink of binder, with the result that the ink chalks. A
e icate a ance must be maintained, and consequently the drying of inks
on coate paper is more of a problem than it is on uncoated paper The
correct amount of binder for a particular coated paper can be determined
only by tnal and error, but the Dennison wax and oil absorption tests can

slin. M ^ . •I gonoral, coated papers for ordinary relief printing

should pass a Denmson wax of 4, but should be no stronger than a Dennb

son wax of 6 to 7. The n^inimun, pick resistance for coated two-side paper
« M. J. Leckey, Paper Trade J. 116, No. 20 : 223-226 (May 20, 1943)

1162

1‘ULP AND rAPKR

TAI'.LK III

Minimum Pick Resistance for Coated Paper Printed
BY Relief Process on Sincle-Coltir Press

Type of printinfiT

Halftone—few solids .

Tint blocks and halftones .

Gloss ink—light fomi .

Gloss ink—heavy form .

DennUon waui Inkometer

5 7

6 12

«> 12

7 ^

for relief printing on a single-color press is shown in Table III/* Using
a proofpress, Bowles®* found that absorbent coatings resulted in serious
mottle, whereas non-absorbent coatings resulted in little or no mottle when
inks of low pigmentation Avere used.

Paperboard for Relief Printing

A considerable amount of paperboard is printed by the relief process,
and the problems are much the .same as those in the printing of paper.
There are, however, a few special precautions to observe. Paperboards
are often scored and folded, which means that the ink film must be fairly
rubproof. Scuff-proofness is required for resistance to rub during ship¬
ping. If the board is to be used for the packaging of food.stuffs, particular
care must be taken to select odorless inks. Inks with a drying oil base tend
to have a bad odor unless special care is taken in the formulation of the ink.

Printing inks must be adapted to the type of board, for example, white
patent coated board cannot be printed satisfactorily with an ink designed
for chipboard. Sulfite and sulfate-lined paperboards do not print with the
same ease. As a rule, jute board (board lined with waste kraft) prints
more easilv than solid virgin kraft board, primarily because of its greater

smoothness and better ink receptivity.

Much paperboard is printed with high-gloss inks which dry to a hard,

<^lossy film. Gloss inks dry mainly by evaporation and oxidation of die

vehicle with a minimum of penetration, and hence paperboard for g oss in

printing must have a^high resistance to ink penetration. It is for this r^n

that paperboard for gloss ink printing is usually calender-size wi s

algin, methylcellulose, or carboxjTnethylcellulose. In some cases, p^ ^

board may be overprinted with varnish to increase the gloss and prot

the surface of the board against scuffing.

A special process sometimes used for printing the outside liner _

rugated and solid fiber paperboard utilizes a relatively ^

.S with casein, alkali, and dyestuffs. The casern arts “
dye and imparts water resistance to the ink so tha i ui

■ MF. A. WertUOUth, Tra* /. JSH. No. M: 118-121 (Jane IS, 1950)

R. F. Bowles, Penrose Annual 42, 153 (1940)

XIX. PRINTING

1163

off in handling., Starch treated with urea-formaldehyde resin has also been
used in this process with some success. The press consists of two rolls, a
metal engraved roll'for applying the ink and a rubber impression roll for
pressing the paper against the printing roll.

Printing with Aniline Inks

Aniline printing is a variation of the relief process, which uses a rotary
press equipped with rubber plates to print paper continuously in web form.
Aniline printing is widely used when the paper is to be printed and con¬
verted in continuous operation, e.g., when making bread wrap. An aniline
press is a relatively simple press consisting of a motor-driven impression
cylinder, a plate cylinder, and ink fountains. Rubber ink rollers and rubber
plates are generally used. The fountains can be backed away from the
plate, or the plate and fountains can be backed away from the impression
cylinder, using hand wheels.

Aniline inks are composed of organic dyestuffs dissolved in solvents
such as alcohols, ethers, ketones, esters, or ether-alcohols, basic dyes being
generally used because of their brilliance. The ink contains a resinous
binder (e.g., shellac, gum copal, ester gum, phenolic resin, special rosin-
type maleic resins, vinyl acetate resins, nitrocellulose, glyptal, or manila
gum) which serves to add body to the ink and to act as a binding agent for
holding the ink on the paper. Pigments such as titanium dioxide are some¬
times added for opacity, particularly when the ink is to be used on trans¬
parent papers. Many other compositions may be used.

Aniline inks dry principally by evaporation, sometimes with the aid of
gas, electrically heated driers, or infrared lamps. Because of their rapid
drying, aniline inks are used for the printing of non-absorbent stocks.
When printing moistureproof cellophane or dry waxed papers, a small
amount of ethylene dichloride may be added to the ink to increase the wet¬
ting. Special drying apparatus is generally required when printing mois¬
tureproof cellophane. Gas drying with intense heat is best, since it is quick
enough to prevent detrimental action to the stock.^® Paper bags, Christmas
wrappings, bread wrapping, paper milk containers, metal foil wraps, and
gummed tapes are grades sometimes printed with aniline inks. These
papers are generally fabricated on fast machines where the rapid-drying
properties of aniline inks are advantageous in permitting handling of the
paper immediately after printing.

Printing oj Newsprint

The printing of newsprint is a special type of relief printing. News¬
print is generally printed in roll form on rotary pres.ses at very high speeds.

®®R. M. Bates, Paper Trade J. 104, No. 13; 192-195 (Apr. 1, 1937)

1164

PULP AND PAPER

No bearers are required, and the impression cylinder packing is quite soft.
Some weekly newspapers are printed on flat-bed presses in a sheet size of
30>4 X 44 in.

Newsprint is generally printed with a special, non-drying black ink,
called news ink. The ink consists of a simple mixture of mineral oil (semi-
refined naphthenic or paraffinic base petroleum oil) and about 13% carbon
black. A cheap resin (e.g., rosin) may be present in the ink to increase
the tack, improve the wetting qualities, and increase the hardness of the
ink film. Toners (black dyes dissolved in oleic acid) may also be present.

There is no drying of news ink in the sense that ordinary printing inks
dry, since news ink dries entirely by penetration, the ink being attracted
into the pores of the paper by capillary forces and retained there in the form
of a soft, easily smudged film. The absorption, or so-called drying, of news
ink depends upon the absorbency of the paper, the viscosity of the ink, the
moisture content of the paper, and the relative humidity of the atmosphere.

The rate of drying is an inverse function of the square root of the ink vis¬
cosity.Because of the high speed of news presses (800-1500 f.p.m.),
there is only a relatively short time during which the ink must be absorbed
by the paper. When operating at high speed, the paper is in contact with
the plate for about 1/1000 of a second, and 1/10 to 2/5 of a second later
the reverse side of the paper is printed.®*-Hence, rapid drying of the

ink is an essential requirement in newspaper printing.

The surface oil absorption test, in which the time is taken for the ab¬
sorption of a thin film of oil into the paper, bears no relation to the total or
internal oil penetration,and hence this test is not well suited for the evalu¬
ation of newsprint and other porous papers (e.g., mimeograph). For
these porous papers, the rate of penetration of oil into the interior of the
paper is a better criterion of the printing qualities than the surface absor -
ency. In measuring oil penetration, a drop of castor oil is placed on the
paper and {1) the time is taken for the complete absorption of the drop, or
(2) the time is taken for the paper to become translucent. The rate o
penetration is logarithmic in nature.- The rate is directly proportional to
the viscosity of the oil,- and, as would be expected, is inversely
to the temperature of the oil between 25 and 35° C.®* In genera , ig n
absorbency is desired, lack of absorbency being the most common short-

57 A. De Waele, /. Am. Chem. Soc. 48: Convention Issue

58 G. Larocque, Pulp Paper Mag. Canada 48. No. 3. 94-yy, Lonveni

59WG Forster TAPPI Bulletin No. 80 (Aug. 11, 1947)

«.a L LaToiue, P.,p Paper Ma, Canada 3S:

61 Y V Vallandigham, Paper Mill Neivs 68, No. 52. en ri^n 25 1940)

0 . ^InLmentatioii Studies.” Paper Trade /. No. 4: 44-50 ( J- 25. 194J^

63 H. H. Grantham and W. Ure, Paper Trade J. 101. No. 12. 14J t

1935)

XIX. PRINTING

1165

coming of newsprint.®"* Too high oil absorbency, however, results in strike
through of ink vehicle.

Like all printing papers, newsprint should have a smooth surface, high
opacity, and uniform formation, and although newsprint is generally inferior
to other printing papers in these respects, improvements have been made
in recent years. The finish is generally a machine finish, a higher finish
being preferred by English than by American newspapers. The higher the
smoothness, the lower the oil absorbency. The oil absorbency is greater on
the wire side than on the felt side because of the lower smoothness on the
wire side.®®'®®

Newsprint is generally made unsized, although a small amount of alum
may be added. There is little or no mineral loading, the ash generally being
less than 2%, although in some cases, about 4 to 5% calcium carbonate
may be used to improve the oil absorbency. (Because a large quantity of
American newsprint is imported, the amount of sizing and loading in stand¬
ard newsprint paper has been defined by the United States Customs Court
and Treasury Department in such a way as to prevent its competition with
other printing papers.) The basis weight varies from 30 to 35 lb.
(24x36—500), 32-lb. newsprint being most common. High moisture
content (e.g., 10 to 12%) reduces the danger of breaks on the machine,®^
but a lower moisture (e.g., 4 to 6%) is most desirable from the standpoint
of faster drying of the ink. High bulk is desirable. The per cent bulk as
measured by calipering four sheets and dividing by the weight of the paper
varies from about 75 to 98%, 82% being an average.®®

The standard furnish for newsprint is 80 to 85% spruce groundwood
and 15 to 20% unbleached spruce sulfite. Groundwood adds the desirable
properties of high opacity, smooth surface, and high oil absorption.®®’
Sulfite adds the necessary strength required to get the paper through the
press without breaks. The cause of breaks is not so much a lack of tensile
strength as it is a lack of elasticity of the paper, since the tensile strength
in the machine direction is about 11 lb. per inch of width, compared with
the average pull on the paper in the press of about 0.5 to 1.0 lb. per inch.^^

®^ G. Larocque, Pulp Paper Mag. Canada 48, No. 3; 94—99, Convention Issue
(1947)

«5H. H. Grantham and W. Ure, Paper Trade /. 101, No. 12: 143-147 (Sept. 19
1935)

®«A. Voet and J. S. Brand, Paper Trade J. 122, No. 24: 264-269 (June 13, 1946)

®U. P. Grant, Pulp Paper Mag. Canada 46, No. 3: 149-151 (Feb., 1945)

«8A. T. Gardner, Pulp Paper Mag. Canada 46: 78-79 (Aug., 1946)

69 U S. Government Printing Office, Tech. Bulletin No. 18, “Newsprint and News
Inlc (1933)

Ure, Paper Trade J. 101, No. 12: 143-147 (Sept. 19,

^ nUs a"933)^ “Newsprint and

\\(d,

rvt.r AND fArrm

S«inc of ihr irtluT caiisr> of hrralc5 Jir«* ins u fficicwt mnnltirr m fV p»prt,
ftoorly wotimi rolls. <(lmw‘ thin hmips, Sltvf cakwW mt«. Ow

infj to the loss of stren|;:th which occurs on MThHft prints tn-tirr

when it is less than six months oW/**

Intaglio ProccHA

Intaglio printing consists of printing from an engras-rd or mccMcd «ir-
face, which is the reverse of the relief process. In the intaglio process* the
plate is inkeil by rollers, the excess ink removed from the unetched surface
of the plate, an<l the iiajier is then pres*!ed against the plate where it picks iq>
the ink held in the recessed areas of the plate. There are Kveml types erf
intaglio printing, aniong the most important of which are copper and steH
engraving, sheet-fed gravure printing (photogravure), and rotogravure.

Copper OHti Stfcl Enffravtmp

In engraving, printing is done from flat steel or copper plates, using
short, buttery inks. The plate is inked, wiped with a cloth, a sheet orf paper
is placed on the plate, and the plate and paper assembly is then passed under

heaNY rollers.

Pictorial designs can he obtained in engraring In' aitting out fine lines
or dots in reverse directly on a copper plate, using a steel cutting tool,
latched plates can be made by scratching the design on a wax coat»" g Q«^
surface of the plate, using a fine needle, apd then removing the expoaed

metal in an acid liath. ^

The results obtained by engraving are very artistic and indindual m

their effect, but the process is slow and expensn-e. Copper plates are re -
tivclv soft and arc suitable only for a (ew thriosand uiipressiofM , ter
which the printing loses its sharpness. Because of the h«h c o« znd Itmi^
production, engraving is used nwstly for the printing of l^eihea^t^
notes, social stationers-, bonds, and for some illustrations where excee^
fine and delicate lines are desired. In one process, referred to as die sta^
ing. the printing is done at very high pressures to emboss the
tially. This process is u.sed mostly for letterheads name car _
invitations. and greeting cards. Special
in enuring bv graining of the plate surface wrA

esses, taown as aquatint Mid meiiodnt. ate used only for hand pnrting

fTigraving may be of the.drying ml
dissolving resins sneh as nitroceUulose, maleK

ritonite. or namral resins in soh-ents s«h as xylol.- tolnol. and toghdxfihnj
^•G- Larocqnc, Tapft 34, No. 1: 86A 1951)

XIX. PRINTING

1167

iiuncrsl thinncrs. Generally, the inks are very stiff so that they will hold
in place on the paper. Color strength is not so important as it is in relief
printing, since relativel)' thick films of ink are applied.

Rotogravure

Rotogravure is a variation of the intaglio process adapted to high-speed
rotary web press printing. Printing is done from copper cylinders or
from copper plates fitted to permanent plate cylinders; chromium-plated
plates are used for long runs. The design is placed on the printing plate
photographically, the process being in reality a rotary-photogravure process,
the name having been shortened to rotogravure. In addition to its regular
use in the printing of Sunday newspaper supplements, rotogravure printing
is also used for wrapping and magazine papers and for the printing of mail
order catalogues. Rotogravure is the most important of the intaglio print¬
ing processes. It is economical for long runs but is not suited for short
runs because of the high first cost of preparing the cylinders.

In preparing illustrations for rotogravure printing, a negative is first
made, and from this a transparent positive is obtained on sensitized trans¬
parent film. Type matter is made from a glassine paper proof. The posi¬
tives are printed photographically on a thin sheet of sensitized gelatin-coated
carbon tissue paper. First, however, a fine reversed line screen (usually
150-line) is used to put a screen on the carbon paper by passing strong light
through the screen onto the paper. The screen is the reverse of an ordinary
halftone screen in that the lines are transparent and the in-between areas
are opaque. After exposure to the screen, the gelatin-coated carbon tissue
is then exposed to the positive film and glassine prints. This hardens the
coating on the carbon tissue in proportion to the amount of light which
comes through, which means that the gelatin is hardened least in the shadow
areas and to the greatest degree in the highlight areas. The gelatin remains
soft in those areas corresponding to the lines on the screen.

After e.xposure, the carbon paper is wetted with water and squeegeed
onto a polished copper cylinder, gelatin-coated side against the cylinder.
After drying, the back of the paper is moistened with water, and the paper
is removed. The soft gelatin is removed with the paper, but the light-
hardened gelatin remains on the c)dinder, leaving a multitude of little gelatin
squares of varying thickness and hardness adhering to the cylinder. The
thickness of the squares adhering to the copper cylinder is least in the dark
tones and greatest in the light tones. The cylinder is then washed and
etched with ferric perchloride which eats into the copper to a depth de-
[iending upon the thickness of the gelatin on the cylinder. The deepest
etching corresponds to the darkest printing areas. No etching takes place
where the screen lines are located, because they are protected by painting
with asphaltum before etching. These screen lines function on the finished

1K>8

»*ULP AND PAPE*

I)late to support the cl«xtor blade and pre\*cnt tlw Made from scraping tli**
ink out of the etched areas.

Ink is apidied to the cylinder during printing by means of an ink roller
which revolves in an ink bath. A doctor blade scrapes all the ink from the
smooth part of the cylinder, leaving mk only in the engraved portions of the
cylinder. The paper web is pressed against the cylinder by means of a
rubber-covered impression roll which causes the paper to pick up the ink
from the recesses in the cylinder. \ ery thin inks are used, since the ink
must flow readily into the recesses of the cylinder and must penetrate the
l^aper very quickly because of the very high-speed press used. The ink dries
on the paper mainly by evaporation and only slightly by ijcnetralion.

Rotogravure plates differ from relief halftone plates in thafall the dou
are of uniform size and shape and are closer together than the usual relid
halftone. Tone is obtained by differences in the depth of the raps or etch-
in- which determines the thickness of the dots of ink transferred to the
paper. One notable difference which distinguishes rotogravure printing
irom relief (and planographiel printing is that all type and hne tllusm-
tions, as well as other illustrative matter, may be screen^. The ‘nk ten^
to spread slightly on the paper, which results in a lack of fine dttod. hut prm

duces a very sofi, even, almost continuous printing. Because of
ing the screen in rotogravure printing is not so apparent as it is in other
proc^L The printing area is about 60-e and this «sulU in approxi-
mately 75fo coverage of the paper due to ink spread.

Shect-Fcd Gravure Printing (Photogravure)

In contrast to the hand work used in copper or steel “

nossible to place tlie design on the printing plate photographi^ly. _
mrial ratter for sheet-fed gravure printing of book illustmt.ons u^e^v
prepared in this way, using a process similar to ttat ™

As in rotogravure, tone is obtained by differences in the depth of etch ng.

rather than bv the size of the halftone dots. . • j Uar tK#* aliove

In addition to the regular (screen)

Mutations. ™s is done by coating the plate ^

to melt partially the resin partiejes. A tran^rent ^ ^

bon tissue is then placed on the plate and t ep at^s ^ ,o the

veloped plate is then etched in feme perchlonde. The dust adhering

plate furnishes the grained effect.

Paper for Intaglio Printing

partially sqi
le ink- Paper

I tne rcccbbcu —- i—

Tn»F. B. John^n, Tappi 34, No. 1: 82A-86A (Jan-, 1951)

XIX. PRINTING

1169

intaglio printing need not have the high smoothness required for relief
printing, but it should have a fairly smooth surface to permit uniform con¬
tact with the printing surface. The body of the paper must be soft and
resilient to permit its being pressed into the recessed areas of the plate;
paper for engraving is frequently moistened before printing. The surface
fibers must be well bonded so that there will be no fuzz to fill in the plate.
The paper must be reasonably opaque in order to prevent show through,
but it must not contain gritty fillers which tend to scratch the polished

surface of the printing plate.

In rotogravure, the paper should have most of the above requirements,
but since very thin inks are used in contrast to the stiff inks used in engrav¬
ing, drying of the ink is not a problem. Supercalendered papers made from
groundwood and chemical pulp are most frequently used for rotogravure
in weights from 35 to 70 lb. (25 x 38—500). Coated papers are sometimes
used, although coating is not necessary. If coated papers are used, they
should have the highest resilience possible and should be free of abrasive
materials which might scratch the polished non-printing areas of the plate,
causing them to print fine lines on the paper. Uncoated papers made from
sulfite pulp sometimes cause tarnishing of the mirror finish copper cylinder
due to sulfur compounds which may be present.

Planographic (Lithographic) Process

Planographic (lithographic) printing differs from relief and intaglio
printing in that the printing ink is carried on substantially the same level
as the plate surface, and not upon a raised or recessed surface. Certain
areas of the plate are made ink-receptive, and certain areas are kept ink-
repellent, the difference between image and non-image areas being obtained
chemically, instead of by differences in height of plate. Water containing
a small amount of acid is used for moistening the plate during printing to
keep the non-printing areas ink-repellent. There is a type of lithography
known as dry lithography, but so far this has not been very successful be¬
cause of the extremely close tolerances which are required.

In order to understand planographic printing, it is necessary to know
something about the preparation of the printing plate and the reactions
which take place during printing. Planographic printing is a real art be¬
cause of the many variables involved, but the basic operations should be
understood by every paper chemist who works with paper for the plano¬
graphic process. Two principal types of printing plates are used, albumen
plates and deep-etch plates. The method of preparing these plates, as well
as the earlier stone plates, are discussed in the following sections.

1170

rULT AND PAPER

Printing jroin Stone

The principle of lithography was discovered by Senefelder in Munich
in 1798 while trying to make etchings on limestone. For many years after
Senefelder’s discovery, lithography was done with a special porous lime¬
stone.

In preparing a stone for lithography, the stone was first ground flat
and then grained by rubbing with an abrasive such as ground glass. The
image was then drawn on the stone in reverse, using a waxy crayon or ink.
After the ink was hardened by an acid treatment, the exposed areas of the
stone were etched or desensitized by treating with an etch containing
gum arabic acidified with nitric acid. The gum arabic was adsorbed and
absorbed into the pores of the stone where it acted to keep the non-image
areas clean. Finally, the stone was washed in water to remove the acid.
The final result was a stone which was receptive to ink in the image areas,
but repellent to ink in the non-image areas where water was carried m the
pores of the stone. In later years, transfer papers were used for trans¬
ferring subject matter to the stone, which eliminated the necessity of draw¬
ing the image in reverse. Printing from stone is practically a thing of the

past, although it is still used for specialty jobs.

Printing from Metal Plates

Modern planographic printing is done from aluminum or zinc plates
and. more recently, from bimetallic and trimetall.c plates of
or aluminum coated with a very thin film of copper or copp
mium. The metal plate is specially grained with hard abrasives (^, alu^

minum oxide or silicon carbide) to imitate the texture of the
used by the early lithographers. Graining is necessary so that P
will absorb water in the non-printing areas of the plate an a
the printing areas. The grain provides an anchorage for the g

image used in the printing areas. • .i, ^ ii ort work

Planographic printing differs from relief printing in that a

is photogrVd by the lithographer at the printing plant, 'vhej^s m reW

ZeTitde On the other hand, type is not set by the bthographer, but in
stead he usually buys proofs of type from a trade t^graphing

the proof, and stripping into the form. A P"‘^J^rob^ned.
strips similar to the linotype except that a photo^aph g hj.

In L albumen process, the lithographer teproduces he type pho^^g
cally from a negative prepared trom a paper proof of the OP ’ ‘ . je

etch process, the lithographer reproduces the type from a positive

XIX. PRINTING

1171

from a transparent (cellophane) proof of the type. Under cerUin con i-
tions (work up to 200-Hne screen), it is possible to convert a printed proof
to a negative and then to a positive for preparing deep-etch plates.^^

The last step in litliography before preparing the plate is stripping, in
which the negatives (or positives) are pasted onto a layout using goldenrod
coated paper for the albumen process and cellulose acetate sheeting for the
deep-etch process. Once a lithograph plate is made, changes cannot be
made on the press, as in relief printing. All changes must be made on the
negatives (or positives, in the deep-etch process) before the plates are made.

Printing from Albumen Plates (Photolithography). In the al¬
bumen process, the plate, after graining, is cleaned (counter-etched) m
dilute acetic acid and then coated with a light-sensitive mixture (egg al¬
bumen, water, and ammonium bichromate) and dried. An image is then
made on the sensitized plate by exposing the plate under a photographic
negative, to strong light from an arc lamp. After exposure, the plate is
treated with developing ink and then is washed with water to remove the
unhardened albumen coating which carries the developing ink from the
non-printing areas of the plate. The printing areas of the plate are the
light-hardened areas of albumen which retain the developing ink, whereas
the non-printing areas are the exposed regions of the metal plate. As a
final step, the plate is gummed to deposit a thin film of gum arabic in tl^

exposed regions of the non-printing areas. ™

During printing, the non-image areas of the plate are kept wet with an
acidificfl aqueous solution of gum arabic, while the work areas of the plate
are kept wet with printing ink. The gum arabic solution is attracted to the
e.xposed areas of the grained metal plate where it is held in the pores, while
the ink is attracted to the albumen image. The ink and water are prevented
from mixing by their natural immiscibility. The pH of the dampening
solution must be maintained within certain limits to maintain a stable film
of gum arabic on the non-printing areas of the plate and to keep the plate
clean by dissolving a slight amount of metal.” The optimum />H for satis¬
factory cleaning of the plate surface is about 3.8. If operating conditions
make it necessary to lower the /'H beyond this point in order to keep the
plate clean, the albumen image will tend to swell, causing distortion or
blinding of the image, and the halftone dots will be undermined, causing
sharpening of the image.

Printing by Deep-Etch Process. Deep-etch printing is the most
important of the planographic printing processes. The plates used in deep-
etch printing last longer than albumen plates, and they print with sharper
detail. A bimetallic offset plate can print up to 1,000,000 impressions,

uthopraphy 1ST. So. 1: 77-78 (Jan.. 1947)

F. Kendall. Modem Lithography (Mar., 1942)

1172

PULP AND PAPER

which is comparable to the number of impressions obtainable from a relief
plate.

In deep-etch printing, the printing image is chemically etched a few
thousandths of an inch below the surface of the plate so that the process is,
in reality, a compromise between gravure and true planographic printing.
In preparing a deep-etch plate, the grained plate is first cleaned (counter-
etched) and then coated with a light-sensitive coating consisting o ^i-
chromated gum arabic. An image is then printed on the dried coating y
passing strong light from a carbon arc lamp through a corrected positive
which hardens the coating in the non-work areas of the plate. e prin e
plate is then developed in special developer solution containing lactic aci
for about three minutes to dissolve the image and expose the metal in he
work areas of the plate. Thus, in the deep-etch process, the coating on the
non-printing areas of the plate is hardened, which is the reverse o
situation in preparing an albumen plate. This light-hardened coating then
“a steLl so tL the exposed work areas of the plate can be etch^
by a solution which attacks the metal. The etching is rather ^hal “w (usu
ally from 00002 to 0.0003 in.) and, m fact, deep etching consists o
more Zn removing the grain peaks on the plate so that the -r^ ar
comparatively smooth and level with the gram va leys.
plate is washed in alcohol to remove the etching solution and

immediately with an ink-receptive lacquer which prevents oxi a lo ^
immediately wim h ^ developing ink which

WinZnZa'ZasZthe plate, after which the plate is washed
with water to remove the im^^ZnTproperties

vSes, which is similar to the situation in relief printing, but opposite

SulT^iStL plate is moistened
sensitizing solution containing ammoni . _ ^ is to make the

plate more receptive to water " ‘he . . ^ „„„ arabic as an

sorption of hydrophilic materials. number of which is in¬
etch is due to the presence of car oxy ic , puiose has proved

creased by the chromic acid in the eteh. Ca^oxymet

to be a fair substitute for gum arabic^ T

case of albumen plates, because the a sor « {^vorable conditions, only

surface is stable only in the aad regmm Unto
a small amount of the dampening solution is needed,)

» G A H. Elton and G, Macdongall, /. to. tom. Ind. 65: 212- (

XIX. PRINTING

1173

the plate and keep the non-printing areas clean. However, when printing
with a poorly made plate or in times of carelessness, so much dampening
solution may be used tliat trouble results from weakening of the image,
poor drying of the ink, or excessive expansion of the paper.

Offset Printing

Originally, lithography involved printing from a stone or plate directly
onto the paper. This is kmown as direct lithography. Very few stones are
used today, but some direct printing is done from metal plates, notably for
the printing of posters and cigar labels. Most lithography is done by offset
in which a rubber-covered roll is used to transfer the printed design from
the printing plate to the paper. The offset principle of printing was intro¬
duced in about 1906. An offset press consists of the plate cylinder, a sys¬
tem of inking rolls for applying ink, a system of fountain rolls for applying
water, a rublier-covered offset roll which receives the image from the plate
cylinder, and an impression cylinder to which the paper is attached and
which presses the paper against the offset roll. A schematic drawing of an
offset press is shown in Figure XIX-2.

Ink roll

Ink o<it\erts to
ortos

Rubber

blanket

Moisture adheres to
non-tmage oreos

Moisture

Water roll

Paper

Hg. XIX-2. Diagram of the offset press principle.

The offset process has several advantages over relief printing in that
(i) it uses less pressure, (2) it uses the original image plates, (J), it re¬
quires less time in makeready, (4) it consumes less ink, (5) it prints rough
papers, and (d) the individual dots blend together in halftone printing to
produce a print of soft, pleasing quality. Static electricity is less of a prob¬
lem in offset than in relief printing because of the higher moisture content
of the paper and the presence of moisture on the offset roll. As a rule.

1174

rui I* AND FAfCi

static IS not a serious pmlilem in offset unless, ihr rrbmr hufnidrty iHe
pressroom almosjihcrc is less than

The offset process lias the following dtsadvanUges: (/ ) ilrf »rr

sensitive to mechanical ahr.Lsion and chemical attack: PTd;'ss re

ijiiires a |»ajKr of high surface strength : (i) register tronbles are frequent.
(4) printing hy offset rctiuires a high degree ci Atfl on the jart of T*re%*-
inan. I'oniierly offset |>rinting lacked color intcnwty but this has lierti
overcome hy the development ol strong inks. The pnnt^ are n<M as sharp
as those from tv])e. hut gootl results arc oliiained. Very fine screens, iq> to
4a) lines, have Ikcii printed by off^l, and ISO-line acreem are in common
use for line detail halftones on coated papers. Proc* |>rinling can be done
hy single impression i*rcssc? or hy two-, three-, four-, and six-color preutt.
Most offset i^rcsses print single sheets, although web pres«» are in use.

Planographic Inks

l'lan.JKrai.liic inks are very siiniUr in properties lo relW ink, exceji
iha. thev are iioriiially made in heavier body and color concmiratnn.

, U-avier and more highly pigmented inks are re^nirrf lo
„i .he ink and water on the press and to otoin

"tion of pigment in the ink is too low fading of ^
though fugitive colors also cause diis trouble. Fading of colored
times takes place in ,I« tints and not in the mass color, and s« v^ ^
though the permanent colors do not show fade in either strength. )T*

of Extender used in making a tint has some effect on tadmg *

rule, offset inks must be reduced slightl^y '>« P"”™" ^

but recently, offset inks have been offered which can be used jus. a.

"‘''fnk for offset printing must be free of strong) aJMi«

riaU tend to react with the acid in the
compounds which upset the ink-water baUnce.

matter should be absent, since t )„ high coocefftratioii.

roll. Surface-active agents are undesirable i.

since thev tend to cause emulsification ot ink m th

There is always some mixing , contain as mudi ^ ^

'2 lus 'i:n“,ss:7--tr:t’crng"t^;^"

• tilic Tpcults in a condition known as wasmng
tdmiing. since this re. u ^ t, i/i nmt he Fre^ tattv add m

light film of ink is printed where it should not be. tree .

XIX. PRINTING

1175

ink may cause trouble by becoming adsorbed on the grained surface of a
defective plate, causing the plate to pick up ink in the non-work areas.

Many offset inks contain no driers. A great many ink makers do not
add drier to their inks, since inks lose drier in storage, but many ink mak¬
ers add drier and count on loss of drier in storage. Many pigments are
natural driers and when added to inks, driers are not required. As a rule,
offset inks tend to dry more slowly than relief inks, although this is capable
of adjustment by the pressman. In cases of slow drying, it may be neces¬
sary to spray the printed sheets with a slurry of talc, gum arabic, or starch

to prevent offset.

General Requirements of Paper for Offset Printing

. The planographic process is widely used for the printing of paper
labels, children’s books with colored ‘illustrations, posters, billboards, bank
checks, and more recently, for the regular printing of books. There are
some advantages to printing newsprint by offset,^® and several fairl}*^ large
newspapers are printed this way. Some of the common grades printed by
the planographic process are uncoated offset book (ranging in basis weight
from 50 to 100 lb., 25x38—500), coated offset book (ranging in basis
weight from 60 to 100 lb., 25 x 38—500), uncoated lithograph (ranging in
basis weight from 45 to 70 lb., 25x38—500), coated lithograph (usually
60 lb., 25x38—500), litho blanks (coated book-lined card middles), litho
box wrap (coated one side), and litho coated labels (coated one side).

Smoothness. • In the early days of lithography when printing was
done directly from stone or metal plates, it was necessary to use smooth¬
surfaced papers in order to obtain good reproduction of fine prints. How¬
ever, with the invention of the offset process, in which a flexible rubber
blanket is used to transfer the ink from the printing plate to the paper, it
became possible to print fine halftones on rough-surfaced papers. As a
consequence, paper for offset printing need not have a smooth surface like
that required for relief printing. Fine halftones can be printed satisfactorily
on uncoated offset book papers having finishes ranging from antique and
eggshell to machine and English finish. In fact, in the early days of offset
printing a certain amount of grain or tooth was considered essential to
permit entrapped air to leak out, but this is no longer considered necessary.

Ink Receptivity. Paper for offset must have good ink receptivity
in order to permit the uniform removal of ink from the offset blanket, since
otherwise the ink will pile on the plate and blanket, causing fill-up of fine
halftones. The paper must have a fair degree of absorbency in order to
prevent squashing of the ink film lietween the offset blanket and the paper.

Chemical Requirements of Offset Papers. There must be no

75 C. F. Geese, A. N. P. A. Mecli. Pull. No. 323: 141-14-1 (Sept. 9, 194(5)

1176

PULP AND PAPER

gritty or abrasive particles in offset papers, since these might come loose
from the paper and scratch the plate surface, producing a condition known
as scumming where the ink is attracted to the non-work areas. The pres¬
ence in the paper of excess water-soluble chemicals (e.g., free alkali, acids,
alum, or chlorides) also causes scumming by chemically attacking the plate.
The paper must be free of surface-active agents since these tend to leach out

in the dampening solution and cause emulsification of the ink.

Offset paper must not be too alkaline, since alkalies tend to neutralize

the acid in the fountain water, making it necessary to raise the pU of the
water, possibly to the point where trouble occurs. Most uncoated offset
oaoers have a />H around 5.0, and consequently do not cause trouble.
Coated papers containing alkaline pigments are the most frequent cause

^Sur^face Bonding Strength. Because of the thin ink films and stiff
inks used in offset printing, picking is more of a problem t an it is m re le
Snting. Thus, in order to print satisfactorily by offset, it is necessary to

have a stronger paper surface than that used for relief printing.

The surface of offset papers should be free oUoose fibers, the sur¬
face fibers should be firmly bonded to the body of the paper. In the printer s
ter ntlgy, the paper should be hard and tight. Soft papers (e.g.. news¬
Lk for making offset papers is generally beaten more than the stock

'^'^'’'offset° papers are usually surface-sized with starch to ^'^en^hen te
surface of L' paper and prevent

face over a relatively soft, resilient base the press,

that the body of the paper be strong enoug o oil-resistant.

Trouble will occur if the surface of the paper v y d
while the interior of the paper is soft, “ hav

tacky to take on the resistant paper sur Government Printing

paper causes an internal rupture of the , No. 11

Office specifications for however, L offset papers to

Dennison wax on both sides, it s p Dennison wax and

be surface-sized to the extent that they pass a NoJ4 Uen^^

yet undergo body stock is to adjust the surface

Strength of the paper. The remeay p^per.

sizing to obtain a greater penetration o ^ Dennison

Paper for gelatin-plate printing must have a pick test

wax No. 22. Paners Proper moisture control is

Moisture Control with Offset P p • ^ois-

tumSeTu^rrc" o'

XIX. PRINTING

1177

press, causing distortion of the printing image and improper positioning
of the paper. Misregister can result from bad press conditions, as well as
from faulty paper, but most cases of misregister on successive printing on
an offset press can lie traced to distortion of the pa^^er. A change in the
dimensions of 1/50 of 17 o‘during printing is enough to cause trouble.

It is well known that paper is hygroscopic and changes in dimensions
with changes in moisture content. Paper expands less in the machine di¬
rection than in the cross direction, and for this reason, printing papers for
sheet-fed presses are usually cut with the grain or machine direction the
long way of the sheet in order to keep the stretch and shrinkage on the
shortest dimension (i.e., the cross direction). The sheets are fed to the
press with the machine direction parallel to the axis of the press roll; this
is done because the pressman can compensate for expansion and contraction
around the roll (by changing the packing), but he cannot make adjust¬
ments for dimensional change in a direction parallel to the roll axis. Also
the sheet lies against the cylinder better when fed in this fashion. Brown’^''*
found, however, that the resistance to picking in the case of machine-
coated paperboard was less when the sheets were fed with the giain
around the printing cylinder. However, paper is usually fed grain paral¬
lel to the roll axis and consequently the misregister taking place
during printing is directly related to the coefficient of linear expansion in
the machine direction. Weber^® suggests a high ratio of machine direction
to cross direction folding endurance as a measure of the most desirable
funnation for close register in printing. Seasoned paper prints better than
green or freshly made paper because of its greater stability.

Dimensional changes in pa|)er are almost directly proportional to
changes in the moisture content,^^ and hence it is necessary to stabilize the
moisture content of paper if trouble-free printing is to be obtained. The
relation which exists between the moisture content of the paper and the
humidity of the pressroom atmosphere is of great interest to the lithog¬
rapher. Paper shows less dimensional cliange between relative humidities
of 40 to GO^J (corresponding to a moisture content between 5 to 7%’®)
than at higher or lower humidities, greatest stability being obtained at about
45% R.H. A variation of 5% or less in the relative humidity of the at¬
mosphere will cause faulty register.^® Lithographers are aware of the im-
jKjrtance of proper conditioning of paper, and it has long been the custom

^•C. G. Weber, Paper Trade J. 109. No. 16 : 205-209 (Oct. 19, 1939)

B. F. Brown, Paper Mill News 73. No. 28 : 54-56 (July 15, 1950)

” R. F. Reed, Paper Trade J. 107. No. 4: 25-28 (July 28, 1938)

H. O. Ehrisman. Paper Trade J. 103. No. 1: 1-4 (July 2, 1936)

“Air Conditioning in Printing and Lithographic Plants,” Parks-Cramer Com¬
pany Bulletin No. 1029, through W. Dahl, Paper Trade J., pp. 329-332 (June
29, 1939) •

1178

PULP AND PAPER

among lithographers to hang paper in the pressroom until it comes to equi¬
librium with the plant atmosphere. In some cases, the pressroom is air-

conditioned to prevent fluctuations in relative humidity.

The problem of dimensional stability is not so great in relief printing
as it is in offset printing. If paper for relief printing is conditioned so that
its moisture content is in equilibrium with the atmosphere, there will be no
change in dimensions during printing with ordinary inks. However, it
heat is used to set the ink, the moisture content of the paper may be reduced
to 3.2 to 3 . 5 % during drying,which is enough to change the dimensions

and affect the strength of the paper.

In offset printing, it is not enough to condition the paper before print¬
ing, inasmuch as during printing, water is transferred to t e paper y le
dampening solution. Part of this water evaporates, but even if the pr
operLs his press with a minimum amount of dampenmg
water will be transferred to the paper to cause a ■

Ir S the pres;, which gives an idea of the opportunities for ahsorp^n
water% Jpaper. A 60-lh. offset paper

to result fn an expansion of 1/64 in. in the machine direction and 3/64 in.

in the cross direction, enough to cause . . pressroom may

Paoer which is conditioned to equilibrium with the pres

pick up'mi additional 0.5 to 1.0% moisture on "

equilibrium with in moisture within its hysteresis

teresis effect by which paper readjusts itse (desorption) curve.

,i„its from the l^er eomniercial paper is

rvhich corresponds to ^ oV.he paper

pressroom conditions. For example, p f equilibrium moisture

machine to a moisture content of 3 g q-hus, when the

content under pressroom conditions is ^ ® ^ e^ppbrium moisture

paper is conditioned before printing, i moisture), and conse-.

content from the low side (i.e., the a p ^nd still remain

quently can pick up addit.ona 3 "“^ a paper is used in multicolor printing,
within the hysteresis limit.s. P P ^, 1 ! for the first five to

it will show a progressive " equilibrium with the

six printings, even if the paper is nrou„

pressroom after each 'tip H'rouB ‘ (Oct. 7, 1937)

:: '."^iV^u tsep..

82 C. G. Weber and M. N. Geib, ^tper i rui
• 1936)

XIX. PRINTING

1179

It is possible to precondition paper before printing at a humidity higher
than that existing in the pressroom and then condition at the same humidity
existing in the pressroom so that the paper approaches equilibrium rom
the high side, i.e., by the desorption of moisture. Such a paper can be
printed with no permanent gain in moisture content, since it is alrea y at
its maximum moisture content within its hysteresis limits.*® This does not
mean that the paper cannot pick up moisture during printing; it simply
means that the sheet can be reconditioned before each passage through the
press so that it can be started through the press with the same starting
moisture content, and the change in moisture content will be the same each

time.

The best solution to the moisture problem in offset printing is to ad-
just the moisture content of the paper before printing so that it loses mois¬
ture to the atmosphere at the same rate that, it absorbs moisture from the
press, thus preserving the proper moisture balance throughout the run.
In order to obtain this result, the moisture content of the paper at the start
of the run should be about 0.5 to 1.0^ higher than it would be if the paper
were in equilibrium with the pressroom atmosphere. Recent bulletins of
the Lithographic Technical Foundation recommend that offset paper should
contain, at the time of printing, between 5.5 to 5.8% moisture when used
in air-conditioned pressrooms at 45% relative humidity, and between 6.0
to 6.5% moisture when used at a relative humidity of 50%, but these values
depend somewhat on the grade of paper. This corresponds to conditioning
of the paper at a relative humidity of 5 to 8% above that of the pressroom.®^
The ideal situation is for the printer to buy from the manufacturer
preconditioned paper which has been wrapped in a moisture-vaporproof
wrapper,*® and then use this paper in an air-conditioned pressroom. If it
is impractical to buy preconditioned paper, the printer can condition the
paper himself by storing at a relative humidity of about 5 to 8% above the
relative humidity of the pressroom. The value of proper preconditioning
is illustrated in Figure XIX-3,*® where the pick-up of moisture during a
nine-color press run is shown for (a) an unconditioned paper, (fo) a paper
conditioned at pressroom humidity (45%), (c) a paper conditioned at a
relative humidity 8% above that of the pressroom (53%), and (d) a. paper
conditioned at a much higher humidity (65%) and then reconditioned at
8% above that of the pressroom (53%). The unconditioned paper picked
up 0.9% moisture during the press run and consequently changed appre-

83 R. F. Reed, Paper Trade J. 107, No. 4: 25-28 (July 28, 1938)

8«C. G. Weber and M. N. Geib, Paper Trade J. 103, No. 12: 210-214 (Sept. 17.

1936)

88 H. R. Russell, Paper Mill News 73, No. 22: 11, 14 (June 3, 1950)

88 R. F, Reed, The American Pulp & Paper Association Superintendent's Year¬
book, 1948, pp. 210-216 “(through courtesy of Lithographic Technical Founda¬
tion) ”

1180

PULP AND PAPER

ciably in dimensions. The sheet conditioned at pressroom conditions (45%

relative humidity) picked up O.dfo moisture
appreciably. On the other hand, the sheet

conslt t iTsture content during printing. The fourth sample vhich

Fig. XIX-3. Effect of condhiontag of pa^ier ^ % «ois

i„g a nine-color press ( 0 ^ T gy, above Pre^room ho-

at pressroom humidity (45%) . (c) relative humidity and then at 53%.

midity (53%) ; and (d) conditioned at 65% relative

• V. -n in Table IV, where it can be seen

during a f f^^ned whh paper having a moisture content

that the best register is obtained w p k „

about 0.5% above that corresponding ’ (jj^re-vaporproof wrappers

Preconditioned paper purchased t.„p„ature equilibrium has

should not be taken from the ntmlphere, it will

been reached. If the unwrapped pape ^ p^per take

cool the surrounding air, with t e resu ^ tem-

on more moisture and become ? obtained, namely, the

peratiire than the atmosphere, the opposit ^ be mam-

edges become tight. The temperature of the pressroo

n r 7 109 No 16 : 205-209 (Oct. 19, 1939)

87 c. G. Paper (Mar., 1947)

88 R. F. Reed, Paper Ind. 28, No. U. i/h

XIX. PRINTING

1181

tained constant (as well as the humidity), l)ecause paper changes in tern
perature more slowly than air. A rise in pressroom temperature in plants
w’ith automatic humidity control means that more moisture will be a e
to the air, and since the paper tends to cool the air close to it, the paper will
pick up additional moisture, thus causing wavy edges. When the tempera¬
ture of the pressroom is decreased, the opposite occurs and the pile of paper
develops tight edges. Wavy edges can sometimes be corrected in the print
shop by exposing a pile of the wavy paper to infrared lamps to dry out the

edges of the paper.

T.XBLE IV

Relatio.s of Pape* Co.nditionixg to Register i.v Offset Printing

Approximate moisture . Average misregister

Paper number content,* % 7-color press run, in,

j 0 - 0023

2 +2 +0.024

3 _2 -0.093 T

4 +0.5 ± 0.002

• Moisture content measured above or below that corresponding to eauilibrium in

the pressroom. .. , ,

* Plus values refer to the number of inches outside of first color, whereas minus

values refer to the number of inches inside of first color.

Roll paper for web offset does not present as serious register problems
as sheet paper. Roll paper should be slightly on the dry side in order to
reduce shrinking of the exposed edges and to reduce edge cracks and web
breaks.**

Curl in Offset Papers. Special care must be taken in the manufac¬
ture of offset papers to prevent waviness or curl, since curl causes trouble
by interfering with the action of the feeder mechanism and register guides,
thereby causing the sheet to feed unevenly and affecting the register. A
higher degree of flatness is required in offset papers than in papers for the
relief process, liecause the 100^ impression in offset causes buckling or
fanning out if the paper is not entirely flat. Fanning occurs at the edge of
the sheet away from the grippers where the paper is not held securely.

Curl may be inherent or built-in curl, or it may be caused by uneven
expansion or contraction of the paper due to changes in moisture content.
Too much moisture on the plate in offset printing will cause curl. Curl is
worse with thin, hard papers than it is with bulky, soft papers, and is great¬
est during periods of low humidity. It is a serious problem when printing
papers treated on one side only, e.g., gummed labels. The subjects of di¬
mensional stability and curl are discussed further in the chapter on the
properties of paper.

**R. F. Reed, Paper Trade /. 130, No. 12 : 36-38 (Mar. 23, 1950)

1182

PULP AND PAPER

Coated Papers for Offset Printing

Within recent years, the printing ot coated papers by offset has in¬
creased,^" and now considerable offset printing is done on coated stock.
Coated papers are preferred for four-color process printing by offset. Bril¬
liant effects are obtained on coated papers, since the coating tends to keep
ink on the surface of the paper where it has a greater brilliance. On the
other hand, coated papers have lower strength, poorer folding qualities,
lower water resistance than uncoated papers, and consequendy are not de¬
sirable for all offset printing. Furthermore, offset printing on coated
stocks frequently produces a slurred impression caused by a slight slipping
of the rubber blanket resulting from excessive ink and excessive pressure
used in the areas of heavy lettering or halftone shadows."^ Printing on
uncoated paper results in a softer appearing print than that obtained on
coated paper because of the grain of the paper, and because the offset d
is not so Tharply outlined. This softness is desirable on certain multicolor

'’""‘printing of coated paper by offset presents certain difficulties whkh

are not so critical in relief and gravure printing. Coated ^ °

must be stronger than the corresponding papers for relief printing

to withstand the 100% impression of the off.set roll and

null exerted hy the thinner, more viscous ink hlms used.- &ated paper

for offset shotill pass a Dennison wax of 6 or

5 for relief printing). The high strength ° ^ ^

that high percentages of b.^der must farther ag-

'glatfodt'Thl Ibe ink oft'en contains moisture picked up on the

'’"“In addition to the moisture transferred to tl.

in the ink, additional moisture is picket up causes softening and

olTsct roll. In certain co^ec coating tends to work its way into

loosening of the coating. T „ ;,il- increasing the viscosity and caiis-

Ihe ink where it attracts water o i ^ abrasive on the plate,

lug piling."^' The loosened coating teas Coated papers

causing It 10 scum, i.e., atcep susceptible to these troubles than

containing casein as the „„d consequently, casein-

eoateil papers conlainiiig starch as the . • . ,papers can

coaled papers are preferred for off.set pru ^

"’‘"’t:d :!;:rs'’:re"i;:.W r::foal. w'ater-soMHe substances which
Reed: /V.er fWe /. 1

« M. II. Bn, 1 . 0 , Paper Iradc I. IM . N". 24.

•a R. P. I.o.«. Wederil IMoprap^ W No <)

9.1 M. ZiicluT, fnf>pt 33 . No. J . o/ v i •

XIX. PRINTING

1183

work into the ink and cause emulsification of water in the ink. If too highly
waterproofed, casein-coated papers may he so non-receptive that moisture
accumulates on the blanket and works its way into the ink. This latte
trouble accurs most often on multicolor presses where the second color is
applied before the moisture added in the first impression has a a c ance

to be absorbed. i^i-f

A />H of 8 0 for the coating layer is about optimum from the standpoin

of ink drying. A neutral or slightly acid coating tends to retard drying^^

as illustrated in Figure XIX-4, which shows the relationship between the

Fig. XIX-4. Effect of pH of coated paper and relative humidity of
atmosphere on the drying time of offset ink.8« Abscissa, relative humidity in
per cent; ordinate, drying time in hours.

/>H of the coating, the relative humidity of the atmosphere, and the drying
time of the ink. On the other hand, a highly alkaline coating tends to lower
the pH of the fountain solution. A />H of 6.0 to 7.0 for the coating layer is
usually obtained when clay is used as the pigment, and this is generally
satisfactory. Coated paper containing calcium carbonate as one of the pig¬
ments often causes trouble because of the sensitivity of the carbonate to the
acid in the fountain water. Some calcium carbonate can be used, however,
if the coating is sufficiently water-insoluble. Lithopone has been known to
cause trouble in coated offset papers by reacting with the acid in the foun¬
tain water to liberate hydrogen .sulfide, which combines with lead in the
printing ink to cause a blackening of the ink. Coated papers containing
9* R. F. Reed, Paper Trade J. 120, No. 25: 76-«0 (June 21, 1945)

1184

PUI-P AND PAPER

satin white usually cause trouble and are rarely used for offset printing.
The presence of surface-active agents in coated papers sometimes causes
trouble by lowering the surface tension of the water on the plate to the
point where ink will emulsify in the water.

Phologelalin (Collotype) Printing

Photogelatin printing, sometimes known as collotype, albertype, or
i)hototype, is a planographic printing process similar to lithography, whic^
is used for short runs (100-4,000 copies) when special effects are desired,
for example, for postcards, posters, special displays, and art work. Collo¬
type printing differs from ordinary planographic printing m that no screen
is used. This results in illustrations having a true gradation m tone,
hence, the process is known as continuous tone printing.

Early collotype printing was done with glass plates having ^ Y'
mate sensitized albumen coating. More modern printers ^

minum plates. The plate is prepared by exposing to a ^Uhr^h

a film negative which hardens and waterproofs the light-sensitive co g
on tte phte according to the amount of light passing through the nep^
The plate may then be heated to further harden the albumen 8

c;s 2 23 «

The paper should pass a Dennison wax N o.

Silk-Screen Printing (Serigraphy)

Silk-screen printing is very old, haying been used
tians and Chinese. It is a stencil printing "mv over a frame.

A rubber blade is used to force the mk trough * g^phed

of the paper. Stencils may be cut by hand or they may^be^P ^

or painted on the screen. If photographic m

coating must first be applied to the silk screen. rt ^ g „ for display

of practically any shape can be printed.

Pr0ssrooni Difficulties

Difficulties in the pressroom may arise or faulty

or unfavorable pressroom conditions such as poor ink distribut

XIX. PRINTING

1185

rollers. Either the ink, the paper, or the press may be ^

for the trouble, or the trouble may be caused by a combina •

For example, lack of detail in the fine work may be caused by » ‘aulty o
worn plate, soft or improper makeready, lack of smoothness m the p p ,
or an ink which is too short or too soupy. On the other hand, cert

troubles may be fairly definitely related to a spectfic ^

example, ghosting, a very light repeat of form showing up n solid areas,
which is generally caused by faulty distribution of ink resulting from de-

analyzing printing troubles, it is helpful to look for patterns which
will give a clue to the cause of the trouble. If the trouble occurs at t e
same location in successive sheets, rather than at random over the surface
of the paper, the chances are that it is due to faulty press adji^tment. If
the trouble occurs more or less regularly on every second, third, or four
sheet, it is likely that the paper was sheeted at the paper mill either two,
three! or four rolls at a time, and that one of the rolls of paper was inferior.
A gradual fading or darkening of color in a particular direction on the

printed sheets is an indication of faulty ink distribution.

Printing problems have increased greatly in recent years because of
the increased complexity of printing presses, the increase in rhulticolor
printing, and the speeding up of printing presses. Some of the common
printing troubles and their remedies are discussed in the following sections.

Strike Through

Strike through is the staining of the reverse side of the printed paper
by oil vehicle or ink which has penetrated through the paper. Strike
through may be caused by an ink, a paper, or a press condition. If the ink
vehicle is too low in viscosity or if the pigment in the ink is poorly dispersed,
strike through may result. If the paper is too absorbent, strike through will
result. Too much impression on the press in relief printing will tend to

force the ink into the paper and cause strike through.

Strike through may be corrected by changing to a heavier bodied ink
or to a faster drying ink, or by using less ink on the press. Sometimes the
addition of magnesia or talc to the ink will help. Changing to a paper of
lower porosity also helps.

Sometimes it appears that the ink has penetrated completely through
the paper, whereas what actually has happened is that the oily vehicle has
separated from the ink and penetrated beyond the pigment. This lowers
the opacity of the paper and makes the ink visible on the back of the sheet.
In cases where this situation has occurred, it is possible to measure the
relative penetration of ink and vehicle into the paper by progressively peel¬
ing off layers of the printed paper, by using Scotch tape. If this is done

1186

rULP AND PAPKR

uticler a binocular niicrovSCo|>c, the i >01111 beyoinl which no ink )»articlpv ha\<‘
penetrated can he readily detected. '1 he distance penetration can W-
measured by checking the caliper of the paper before and after the various
layers have been peeled off with the Scotch tape.

Sho 2 v Through

Show through is the visibility of printing on the reverse side of the
paper. It may be caused by paper which is too low in opacity, or by pajier
which is poorly formed; or, as explained above, show through niay be
caused by excessive penetration of ink vehicle into the paper, causing a re¬
duction in the opacity of the paper. Show through is basically a paper
problem, although using less ink on the press will sometimes help the situ¬
ation. The real remedy for show through is to increase the opacity of the

paper and to improve the formation.

Offset

Offset is the unintentional tvanster of wet ink from the printed paper to
the back of the following sheet, due to top tack on the snrtace ol the mk
film. Offset is more prevalent in halftone printing than m so i s ‘‘'k
dinarily is more prevalent in offset printing than in relief printing. Offs
is very common when non-drying inks (e.g.. news inks are used and «hen
overprinting one color on another. Offset is very serious when thick ink

films are applied, as when forcing halftones.

Offset res-nlts when the oil absorption of the paj^r is too low. Od,

conditions causing offset are: too much moisture in the
inks'! DOor fountain adjustment, too much ink necessita • u

:,akeierdy: too much ini necessitated by lack of --- -en^ -n «he mk.
rollers set too heavily on the form causing mk to ^ ^

draw closely together and attracting ink frmn one of

delivery of the printed sheets, forced production and roi g _

delivery 01 i 1 „^-,,rrine in the smashing ot printed matter tor

the printed paper. Offset occurring m 11 , ^ and press

books is due to poor drying of the in - j without

problem, rather than a paper problem. ^ '

offset if the proper ink and proper vapor-set

Increasing the rate of drvmg of the mk by ti

inks has helped in overconting offs . ^ offset. Offset may

less ink on the press will genera y le p p compounds in the ink

be overcome by «'■" during drying: usmlly.

which tend to exude to the sunace oi i Spravmg

about to 1 oz. of offset compound per f-""^of mk^^ T,,

starch or gum ^ 7lhher'il'’dri- form or after suspending m

Starch or gum may be spray eci eiiiic

XIX. PRINTING

1187

water but must be used uncooked so that the starch or gum particles he
surface of the printed sheets where they hold the sheets apart and
prevent contact of the wet ink film with the back of the next sheet. Either
potato starch or a special grade of corn starch is used. Another remedy is
to resort to slip sheeting, by which a blank or waste sheet of paper is in¬
serted between the printed sheets to protect the wet printed surface from
offsetting on the back of the next printed sheet. Slip sheeting is too ex¬
pensive to use on most printing, but it is sometimes used on especially fine
printing jobs. Use of a bottomless box at delivery will assist in preventing

offset.

rklJ/yrfi itn

Collecting is a condition resulting from the collection of foreign matter
on the press plate. This matter may be loose fuzz from the surface of the
paper, excessive trimming dust from the paper, or pigment removed from
coated or heavily loaded paper. Such foreign matter tends to transfer to
the printing plate during inking, thus leading to an impression composed
of a small dot surrounded by a white ring. In extreme cases, collecting

may make it necessary to shut down the press and wash up.

Collecting may be caused by paper which is insufficiently sized with
rosin or starch, iiy too tacky an ink, or, in the case of offset prindng, by a
tacky offset blanket. Collecting occurs most frequently when printing the
wire side of the paper becau.se of the greater number of loose fibers (whisk¬
ers) on this side.

The remedy for collecting is to avoid loose fuzz or dust on the paper
by brushing the edges of the piled paper before printing. Fuzz and dirt on
the press and in the atmosphere should be held to a minimum. If the ink
i.s too tacky, a wax-reducing compound may be helpful. Collecting is often
taken care of by using a tacky first form roller, which will pick up any
specks and move them back into the ink system or cover them up on the
printing roller so that the specks will not repeat.

Cakivg or Fill-up

Caking or fill-up is the tendency’ toward a filling up of the fine work- on
a halftone plate. Anything which prevents the proper removal of ink from
the printing jilate will cause caking or fill-up. One cause is imi)roper dis¬
persion of pigment in the ink vehicle. Insufficient vehicle in the ink will
also cause fill-up, as will an ink which is too heavy or too soft.

The remedy for fill-up is to clean the press and then change the ink.
If the ink is too thin, heavy' varnish may be added. If the ink is too heavy,
the addition of a reducing varnish, a Xo. 0 or No. 1, may help. In some
cases, it may be necessary to regrind the ink. Running less ink on the press
is sometimes lieneficial.

1188

PULP AND PAPER

Piling

Piling is the uneven collection of ink on the press plate, rollers, or, in
the case of offset printing, collection on the rubber offset blanket. Piling is
closely related to fill-up. It may be caused by ink which is too short or by
paper which has too low an ink receptivity. In offset printing, piling may
be caused by the emulsification of a large amount of water in the ink, caus¬
ing the ink to become putty-like in consistency.

When piling occurs in offset printing as a result of emulsification of
water in the ink, the addition of a heavy varnish may help by causing the
ink to repel the water more effectively. Another remedy is to change to a
new ink of longer body and better receptivity for the paper. Too much
drier in the ink will sometimes cause piling.

Crystallisation

Crystallization results in the poor bonding of top colors to the base
or first-down colors, for example, poor bonding of a red to the first-down
• yellow when producing an orange. When crystallization occurs, the last-
down colors are easily rubbed oflf the base color.

Crystallization may be due to the presence of too much drier in the
first-down color which causes it to dry out before the second color is printed.
Lack of penetration of the first-down color into the paper may also be a
factor.

One remedy for preventing crystallization is to retard the drying of the
base color by the addition of oily or w'axy materials. If the base ink has
already crystallized, the second ink may be modified with a wax (beeswax)
or similar material which causes it to take on the crystallized surface. Use
of a thinner varnish or the addition of more drier to the later colors is often

helpful.

M ottling

Mottling is unintentional irregularity or unevenness of the printed

matter. There are a number of causes of mottle.

Mottling in the gray areas of halftones results from fill-up of the small
spaces between the halftone dots causing irregular printing of ink from
these spaces. This type of mottle can be controlled by using a minimum

amount of ink.

Mottling in solid areas is generally caused by an ink which is too soft
(an ink with too weak cohesive forces), or by an ink which has too much
l)ody. Too soft an ink gives a smeary mottle, whereas an ink with too
much l)ody gives a dry-looking mottle. Mottle in the solid areas of the
printing may also be caused by the uneven absorption of ink by the pape ,
due to variations in ink receptivity of the paper. Too much ink on t e

XIX. PRINTING

1189

plate, uneven distribution of ink (due to inferior ink or improperly set
rollers), and too much or too little pressure during printing are other causes
of spotty or mottled printing. Mottling caused by too soft an ink can be
corrected bv building up the body of the ink by adding No. 5 varnish or by
mixing with another heavy-bodied ink. Improving the wetting of the pig¬
ment or the addition of talc to the ink is beneficial if the ink is too greasy.
Using less ink is sometimes helpful.

M isregister

Misregister is a condition-whereby the printing does not fall in the
proper place on the paper. It may be caused by a faulty press adjustment
or bv paper which is not suited to the operation. It is fairly common on
paj)ers with wavy or feather edges, or on papers which have uneven edges
because of poor trimming. Pick-up of moisture by paper during offset
printing will cause misregister, due to expansion if the paper is not prop¬
erly conditioned ahead of time. Misregister is difficult to overcome in the
offset process because the printing plate is a single sheet of metal, and
therefore it is not possible to shift plates as in relief printing, which means
that the responsibility for good register lies chiefly with the paper. The
s|)ecial precautions necessary in preventing misregister in offset printing
have alreadv been discussed.

Misfit is a special type of misregister in which some images coincide
and others do not. It is caused by uneven expansion or contraction of the
paper.

Chalking or Poivdering

Chalking is a condition in which the pigment in the dried ink film tends
to dust or powder, thus producing a dull surface. Chalking is caused by
loo much absorption of vehicle and varnish into the paper, leaving the pig¬
ment in the ink without sufficient binder. The main cause of chalking is
too low a viscosity of the vehicle. However, chalking also results if the
drying of the ink is too slow, since this means that the varnish continues to
penetrate until eventually too little is left with the pigment. Paper which
has too low a moisture content is likely to cause chalking. The trouble
occurs most frequently with heavily filled and coated papers, particularly
where the coating is highly absorbent.

To overcome chalking, a hard-drying varnish (e.g.. Overprint A—
No. 1) may be added to the ink, or a heavier bodied ink may be used. The
addition of more drier to the ink is sometimes helpful because it reduces
the time in which the vehicle remains in a liquid state. In the case of coated
papers, changing to a paper which has a stronger, harder sized coating will

reduce chalking. In offset printing, a size or tint base may be printed on
the paper to help prevent chalking. *

1190

PULP AND PAPER

Rubbing or smudging of the printed sheet during trimming and han¬
dling is a trouble related to chalking. The basic cause of this trouble is the
same as that for chalking, i.e., insufficient binder in the ink film. The rem¬
edies are the same as given above. A wet ink film will also cause smudging.

Picking

Picking is a lifting of the surface of the paper by the ink. Picking is
a common trouble which occurs mostly with coated papers, although it also
occurs at times on uncoated papers. Generally, picking shows up as a white
spot on the inked background, the white spot being where the ink has lifted
some of the paper surface. White spots do not always indicate picking,
however, since similar spots may be caused by bits of paper which have be¬
come stuck in the wet ink, or by spots which did not receive ink because
of a damaged printing block. In some cases, the surface of the paper is not
pulled loose, but is simply raised in the form of a blister. Picking is very
undesirable, since it not only detracts from the printing in the place where
the paper has been picked, but the particles lifted from the paper tend to
lodge on the printing plate or be carried into the ink system. Picking is

especially noticeable in solid printing areas.

Picking is predominantly a paper and ink problem. It may be caused
bv the ink drying too fast, by an ink which is too tacky, or by a weak paper
or weak coating on the paper. Inks which have very high tack are likely to
pick all but the most tightly bound paper surfaces (see relief printing inks).
In multicolor printing, where one ink is printed over another while .still
wet (trapping), there is likely to be picking after the second or third im¬
pression, due to the weakening of the paper by the first impression; or if the
first-down colors set too fast, picking may result as the second or third
colors are printed. In order to avoid these difficulties, softer and softer
inks are used in each succeeding printing. Too low a temperature increases
[licking by increasing the tack of the ink. Picking is most likely to occur
when the press is first started before the ink has warmed up. An in>.
wliich prints ])erfectly at 70^^ F. may pick at 60'^ F. Pressrooms which op¬
erate only one shift are likely to e.xperience trouble with picking when t le
|jres.ses are fir.st started in the morning. High printing .speed increases t le
tendency toward picking. Large printing areas increase the tencenc) to
tear the paper liecause of the greater area from which the ink mus
nulled. Too rapid oil alisorplion may create picking due to the sud en in¬
crease in tack of the ink resulting from the light varnishes [lenetratmg m o
the [laper. Too thin an ink film may he the cause of pick also, since

cohesion of the ink may be too great in the thin film.

The remedies for picking are to change to a paper with a s rong

face, to reduce the tack of the ihk by adding reducing agents (e.g.,

XIX. PRINTING

1191

compounds or kerosene), or to change to another less tacky ink. Raising
the temperature of the pressroom often helps. Reducing the tack of the
ink is a common remedy for picking, but if the strength of the paper is too
low, the ink may have to be reduced to the point where mottle and weak
printing result. Picking of coated papers can be prevented by using suffi¬
cient adhesive in the coating formula, as already pointed out in the discus¬
sion of coated papers for relief and offset printing (see also Ch. XVIII).
Increasing the amount of.ink on the plate will sometimes eliminate picking,
particularly if the trouble is caused by the ink drying too fast. If picking
occurs in the non-printing areas when printing by the offset process, it may
be caused by an oxidized or tacky rubber offset blanket. In this case, the
blanket must be corrected, or a new blanket installed. Powdering the
blanket with sulfur or washing with carbon disulfide are temporary means
of reducing a tacky blanket.

Tinting and Washing

Tinting (bleeding) is the printing of a light film of color on the areas
of the paper which should be unprinted. This phenomenon occurs in litho¬
graphic printing where it is generally caused by a small amount of ink or
pigment dissolving in the water. The only remedy for this trouble is to
change to a non-bleeding ink, or to add a heavy, non-bleeding varnish to
increase the body of the ink. Bleed is sometimes due to a poorly washed
organic pigment in which there is a small percentage of free dyestuff. The
addition of barium chloride compound will mordant this dye, removing
the bleed.

Washing is a trouble often confused with bleeding. Washing, how¬
ever, is generally caused by a breakdown of the ink on the offset press re¬
sulting from too much water or too strong an acid solution being carried
on the dampening rollers. Washing can generally be corrected by adding
a longer varnish to the ink or Ijy changing the ink.

Scumming

Scumming or greasing is a trouble occurring in lithographic printing
caused by the formation of scum on the printing plate, making it difficult
to keep the work open and clean. Scumming can be caused by any one of
a number of different conditions.

Scumming is a frequent cause for complaint in the printing of coated
papers. Excessive alkali in the coating causes scumming {!) by removing
the adsorbed gum arabic film from the non-printing areas of the lithographic
plate, thus permitting the adsorption of fatty acids in the ink vehicle, or
(2) by reacting with fatty acids in the ink vehicle to form soaps which
destroy the delicate balance between the ink and water systems. Coated

1192

PULP AND PAPER

papers containing calcium carbonate are most likely to contain excessive
alkali, and hence most likely to cause scumming. Reed**® has shown that
the scumming tendency of a coated paper correlates fairly well with the
foaming tendency of a water extract of the coated paper, scumming in this
case probably being caused by colloidal substances in the extract which
tend to emulsify the ink and water on the plate. Extracts from starch-
coated papers do not foam as much as extracts from casein-coated papers,
but starch-coated papers are not generally used in lithography because of
their lack of water resistance. Coated papers lacking in water resistance
cause scumming by the coating pigment transferring to the blanket where it
acts as an abrasive on the plate so that the plate picks up ink in the non¬

work areas.

In uncoated papers, the presence of excess alum or chlorides may cause
scumming by resulting in deterioration of the printing plate surface. Ex¬
cessive grit will cause scumming by acting as an abrasive on the plate. An¬
other possibility is the extraction of surface-active agents from the paper.

So far as the ink is concerned, an ink which is too short in body or an
ink which contains too much greasy compound will cause scumming. The
remedy in this case is to use a longer varnish and to eliminate the greasy

compound from the ink. -j-* r

If scumming is caused by excess alkali, increasing the acidity of

fountain water will be helpful.«« This remedy can be used in the case of

■ uncoated and certain coated papers, but cannot be used with coated papers

containing calcium carbonate. If scumming is ^used by excess.ve wear of

the plate, changing to a less abrasive paper will be helpful.

Duplicator Processes

A discussion of printing would be incomplete without
closely related field of duplication. Considerable amounts o p p
Hied each year by government agencies, educational .nsU«
and church organizations, and by business and ^ „„„

nnerations involving one or more of the major duplicati g p
o‘n the market. These processes may be subdivided «

(n stencil duplicators, ( 2 ) hectograph or gelatin dupl eu‘ors, 10j p ^
Locators (4) lithographic or offset duplicators, and (5) pho ly

ssr M.. .«'»'• .< i

nprs such as special master sheets or copy papers. C P P ^
very well-known jiroduct used for copying, are discussed in C ap er ^
''"7he fiindammital economy of duplicating -e.ho * -p. ^
ease of operation, and flexibility have been primarily responsible

:l 1: P»Tr IZt J.

XIX. PRINTING

1193

increasing usage. Future developments in duplicating machines and ma¬
terials, as well as greater demands on the part of the consumer for higher
quality, will call on the ingenuity of the paper chemist for the development
and production of improved papers. Although considerable research has
been done by individual paper manufacturers in the development of dupli¬
cator grades, very little material has been published on the specific require¬
ments of papers for this type of work. As a result, the properties of papers
used in the various processes are subject to fairly wide variations when the
products of individual producers are compared.

Stencil Duplicators

Stencil duplicators are among the older methods of producing multjple
copies of typewritten or drawn material. There are many models of stencil
duplicators. The mimeograph is the best known of these processes and has
lent its name to grades of paper normally used for stencil duplication work.
The mimeograph process was invented in 1884 and the name “mimeo¬
graph” is the trade name for the process of A. B. Dick.

The process consists, briefly, of preparing a stencil and subsequently
forcing ink through the stencil openings onto the copy paper as it passes
through the duplicating machine. The stencil consists of a fibrous sheet
of strong tissue mounted on a tympan backing sheet. The stencil is treated
with an ink-resistant mixture composed of nitrocellulose, waxes, fatty acids,
and plasticizers.

In cutting a stencil, the stencil is backed up with a second sheet,
a sheet of unimpregnated tissue called the stencil cushion sheet, which is
placed between the stencil sheet and the backing sheet to cushion the stencil
paper. Auxiliary equipment usually supplied by the manufacturer, such
as various graining or shading tools and styli, allow the operator consid¬
erable freedom in composition and artistic effects. Standardized forms can
be mass produced from dies on a printing press, thereby eliminating much
tedious hand labor when such forms are applicable.

The duplicating machine consists of a perforated revolving impression
cylinder, an ink-feeding mechanism on the inside of the cylinder, a felt
blanket or wick which covers the perforated face of the cylinder, a paper
infeed, and a delivery. In setting up the cut stencil, the cushion sheet is
removed and the stencil hung on the hooks of the cylinder. The upper
clamp is closed, the backing sheet is torn off, and the stencil then is laid
smoothly on the 'pad around the cylinder. If the stencil is full length, it is
clamped under the bottom clamp. If not, a sheet of paper is placed over
the pad to cover the non-printing areas, and the tail of the stencil runs free.
During printing, the ink is forced through the openings in the stencil onto
the copy paper as it comes in contact with the stencil.

1194

PULP AND PAPER

Inks used in this type of duplicator are i)rincipally the non-drying oil
types very similar to news ink. Drying of the ink is thus dependent on
penetration of the ink into the copy. Certain water emulsion inks are also
being used in stencil duplicating work at the present time. The type of ink
used has a great influence on the properties of the paper required for satis¬
factory performance with this process. The maximum number of copies
obtainable depends upon the stencil, copy, paper, and other factors, but is
generally between 2,000 to 5,000 copies per stencil. Speeds vary from 165
to 250 per minute.

Mimeograph papers are a grade of writing or book paper used for
making copies on a mimeograph machine. Mimeograph papers usually
have a grainy or toothed surface to aid in preventing offset until the ink
has had a chance to penetrate the paper. Antique wove paper made from
sulfite and soda stock is frequently used. The paper must have a closed
formation and possess good absorbency for the ink vehicle. It must have
good opacity, be free of fuzz, and, in most cases, be sized for pen and ink
writing. Mimeograph papers are manufactured in a full range of colors
and in weights from substance No. 16 up through ledger weights, sub¬
stance No. 20 being most common. The paper is usually loaded or filled
with clay or other mineral fillers so that the ash content ranges from 2 to
12%. Bulky, highly oil-absorbent fillers have been used in mimeograph
grades to improve the copying characteristics, although the cost of such

fillers frequently limits their use.

Hectograph {Gelatin) Duplicators

The hectograph jtrocess is very old. It was the only method o* '•'■P*'
eating Wore the introduction of stencil duplicators. The process .s st.l
used where the number of copies required is very sntall and when speaal
color effects are desired. Gelatin duplicators employ gelatin W";®
receive copy of the image and transfer it to suitable copy paper. i
chines range from flat-pad hectographs to modern rotary
nloying specially prepared rolls of paper or fabric coated with g .
Inodifed Uati.. mixtures. The basic process is, however, the same in all

A master copy of the material to he duplicated is prepared on a special
master paper by printing, typing with special ribbons or “ »

by meam of special writing inks or pencils. The 'f,.

iion the master is placed, face down on the —^^"'rthe

is allowed to remain for a few seconds, ^ jhe same

irelatin After removal of the master, copy sheets are place

ar^ of gelatin in succession, either manually or mechanically, to obtain

XIX. PRINTING

1195

desired copies. From 75 to 150 legible copies can be obtained by this
method, depending upon the copy. Inks give the brightest work and long¬
est runs (150 copies), pencil work being used only for short runs of about
75 copies or less. The speed varies from about 12 per minute for hand-
operated flat-pad hectographs to 75 per minute for rotary machines.

Reprints can be made by reusing the master as many as eight times.
As many as eight colors can be reproduced in one operation by using differ¬
ent masters. The gelatin can be reused after it has been allowed to stand
for from four to twenty-four hours between each use.

The papers required for this process can be divided into master pa-
l)ers and copy papers. Master papers are normally strong, chemical wood
jnilp or rag content sheets, rather heavily tub-sized with starch or starch-
glue mixtures. Some parchment is used for pencil work. The master must
possess good resistance to ink penetration and must be strong enough to
withstand the forces encountered in mechanical stripping from the gelatin
mass. Master papers are manufactured in weights ranging from substance
Xo. U to No. 24.

Copy papers for the gelatin process are manufactured in weights from
substance No. 13 through ledger and index weights. They are made in a
full range of colors for coded business systems. The principal require¬
ments for copy papers are sufficient surface strength to prevent peeling or
tearing when the copy is stripped from the gelatin, freedom from curl, both
before and after duplication, sizing for pen and ink writing, freedom from
surface fuzz, and sufficient opacity to prevent show through. Papers for
this process are usually loaded with clay, talc, or other mineral fillers, ash
contents of 5 to 10% being common. Titanium dioxide is frequently used
to improve color and opacity. Most gelatin process duplicator papers are
tub-sized with starch or modified starch mixtures. Some special coated

])apers are produced for short run work where extremely bright copies are
desired.

Spirit Duplicators

The spirit or liquid duplicator is a reproduction process in which a
master copy is perpared by means of a special carbon paper. The carbon
paper used in making the master copy is a special one-time carbon con¬
taining a high concentration of a spirit-soluble dye, usually a basic dyestuff,
of high tinctorial strength. The master is prepared by making a reverse
carbon copy on the back of the master paper, after which the master is at¬
tached to the impression cylinder of the duplicating machine. Then, the
copy paper, slightly moistened with a special duplicating fluid consisting of
a mixture of alcohols, is brought into contact with the master copy. The
solvent on the copy paper dissolves sufficient dye from the master copy to
prrKluce the impression as a positive image on the copy paper. About 300

11‘>0

rULP AND PAPKl

to 500 legible coj>ics can Ik? obtained by tUia pr^jcess. The sfjeed i* alxnit
75 per ininnie. Carlwjns arc available in jniq lc, rod, blue, and gri'cn.
ruri>le carbon gives the be^t results. There arc no \cr\ g -od black car¬
bons f»)r spirit duplication U'causc there arc nt» true basic black dyes.

Master pajK-rs h)r spirit duplication arc specially prepared, coated pa¬
pers. .Special blocking agents arc often added to the coaling to prevent
bleeding of the oil and dye from the carlwn into tlic pajicr during standing
or storage of |>rcpared masters.

Copy papers for spirit duplication are made in a range of grades, from
groundwood content sheets to sulfite Ixmds. The i»per is usually loaded or
filled with talc, clay, or other mineral fillers with ash ojntcni* varying from
5 to 15%. Titanium tdioxide is frequently used as an o|>aafying agent.
W^cights range from substance Xo. 16 through ledger and index grades.
A full range of c^ilors is usually stocked. The copy sheets are normally
tub-sized with starch or other sizing materials to improve the surface char¬
acteristics. The pajXT need not be as absorbent as other duplication |ja|»ers,
but some absorliencv is necessarv to carrv a controlled amount of du])1icat-
ing fluid to the master sheet to protluce an adequate transfer of the dye¬
stuff to obtain a clear copy. Most grades are sized for jien and ink
writing. The relationship l.>ctwecn the solvent uscrl in this process and the
paper is extremely inijxirtant in the production of clear cojjics and kmg
runs. Incompatibility of solvent and paper can produce blurred copies and
rapid breakdown of the master sheet.

Lithographic {Ogsei) Duplicators

Offset duplicators are small offset presses, especially de.signed for of¬
fice use. Special paper plates can be used for reprcidudng written, typed,
painted, or drawn image placed on the plate, or regular metal lithographic
plates can be used. Parchment papers laminated to a -waterproof backing
sheet and special pigment-coated papers are used for paper plates. Equij>
raent for the preparation of photo-offset plates is usually supplied as auxil¬
iary equipment bv the manufacturer.

The discussion of lithography and offset printing in the pre^^ous^
of this chapter applies in whole Or in part to this form of duplicauon. Con¬
sequently, there is little need to repeat the discussion oi ink and paj^r re-
quirements or press problem! at tlbs point. Howerer. me
ference between offset duplicating and regular lithography is that
is generally used in the fountain solution in the former to
plate moist and prevent dry-ing. Special inks are required tor office _

chines because of this glycerine in the lountmn oe,

Off.e, dunlicators operate at speeds up to about .',000 mipressions per

XIX. PRINTING

1197

hour, but average production is about 4,000 to 4,500 per hour. The maxi¬
mum number of copies is about 25,000.

Letterpress Duplicators (Multigraph)

Letterpress duplication is done on small rotary relief presses. The
process is not, strictly speaking, a duplication process, being more closely
related to printing. Hand-set type, stereotypes, electrotypes, or rubber
plates can be used. The plates can be inked by regular ink rollers, or print¬
ing can be done through an inked ribbon wrapped around the printing cylin¬
der to which the paper is pressed b}’^ means of an impression roll.

Letterpress duplication is suitable for the reproduction of typewritten
material used for form letters, reports, and advertising matter. It is used
for reproducing a large number of copies of material simulating type¬
written matter. The speed of operation is 1,000 to 5,000 copies per hour;
the maximum number of copies varies from 50,000 to 200,000. The prop¬
erties of the paper should be the same as those for relief printing. Bond
and writing papers are most frequently used.

Automatic Typing

Individually typewritten letters can be produced in quantity by auto¬
matic typing at low cost. A master roll or tape working on the principle
of the piano roll automatically types the letters, leaving space for address,
heading, or special information. These spaces are then typed in by hand
later, a near-perfect match being possible. The speed is about 110 to 125
words per minute, and any number of copies can be produced.

Photo-Copying Processes

Photo-copying duplication processes such as direct process and blue¬
print are used to produce multiple copies of original material. The ex¬
pense of these processes and the time consumed in making copies usually
limits this form of duplication to relatively specialized fields where only a
limited number of copies are desired, for example, copies of plans or draw¬
ings prepared by architects, engineers, or draftsmen. In the blueprint
process white lines are produced on a blue background, whereas in the

(irect (diazotype) process, black, brown, red, or blue lines are produced
on a white background.

Reproduction by the photo-copying processes is done from original inl^
or penci rawings on transparent tracing paper. In making prints, the
ransparent tracing paper bearing the original drawing is placed on top ol
e ig -sensitive reproduction paper in very close contact and then the

kZ Tb ^ vapor

the kver oMkht through the transparent paper and acts upon

la>er of light-sensitive chemicals on the surface of the paper. No light

1198

PULP AND PAPER

j)cisscs where there ure lines or figfiires on the tracing paper. After ex¬
posure, the tracing pajier is removed and tlie printing paper is developed
to bring out the design.

The tracing paper used in making reproductions must have high trans¬
parency in order to permit passage of light during printing. Creamy or
yellow tracing papers are undesirable, since they tend to block out the light;
good tracing papers should be colorless or have a bluish tint.”^ Other re¬
quirements of a good tracing paper are good receptivity to drawing inks,
high strength (particularly good tear and fold), and good erasing qualities.
The paper must have good formation and even caliper. The common
weights are 12, 14, 16, and 18 lb. (17 x 22—500). Both uhimpregnated and
impregnated grades are used commercially. The lines of the drawing on
the tracing paper must be heavy enough and sufficiently continuous so that
no light penetrates through them during printing to weaken the line on the
final print.

Blueprint Process. Blueprint paper is a special grade of paper
which has been treated on the felt side with a solution of sensitizing chem¬
icals, i.e., potassium ferricyanide and iron salts such as ferric ammonium
oxalate. The solution is applied to the paper on a coating machine, where
conditions are controlled to obtain the necessary penetration of chemicals
into the paper, after which the paper is dried in the dark.

In making reproductions by the blueprint process, the sensitized blue¬
print paper is exposed to light through the original drawing on tracing
paper. This converts the ferric salt to ferrous salt which reacts with the
ferricyanide to produce ferrous ferricyanide which in turn is converted into
insoluble blue ferric ferrocyanide. The exposed paper is w’ashed by spray¬
ing and by immersion in water to remove residual unexposed chemicals.
The paper is then treated with a solution of oxidizing agent (c.g., peroxides
or bichromates) and washed to remove excess oxidizing agent. The result
is a negative print containing white lines on a blue background.

Rag or rag content sulfite papers are generally used in making the base
stock for blueprint papers. The paper must have a fairly smooth surface,
good folding endurance, good tearing resistance, and above all, must have
a high degree of wet tensile strength and wet rub resistance to withstand
the soaking in water during developing. The paper is generally surface
sized with glue which is hardened with formaldehyde in order to develop
sufficient wet strength. The paper must be free of all chemicals (e.g., re¬
sidual formaldehyde left over from surface sizing) which w^ould react with
the chemicals in the sensitizing solution or with the oxidizing agents use
in developing. The absorbency and sizing of the base paper nwist be con¬
trolled to obtain the correct amount of penetration of the sensitizing chenii-

H. Klein, Paper Trade J. 129, No. 11: 338-341 (Sept. 15, 1949)

XIX. PRINTING

1 199

cal in the treating operation. high opacity is rc<(ihre<l, and generally the
j)apcr is filled with clay or titaniinn pigment. The color of the ])aper shouhl
be yellowish or pinkish in order to nia.sk the light blue tint left in the un¬
developed areas and provide the maximum visual contrast with the blue
background.®® The basis weight is usually 24 lb. (17 x 22—500), although
weights from 12 to 30 lb. are used.

Ordinary blueprints are negatives, but positive blueprints can be made
having blue lines on a white background. These are made by first forming
a negative print, called a brown print, in the same manner as described
above for a blueprint. The brown print, which contains relatively trans¬
parent white lines on a dark brown background, is then used as an inter¬
mediate negative for making blueline prints on regular lilueprint paper.
Paper for making brown print negatives must be light in weight (12 to
16 11)., 17 X 22—500), but must be very strong. The paper must be highly
transparent so that the light can readily penetrate through the transparent
lines of the negative. The brown developed areas must, however, be opaque
to prevent the passage of light. In order to obtain a high degree of opacity
in the brown areas of the negative, the sensitizing solution is allowed to

penetrate deeply into the paper and a long exposure time is used during
printing.

Direct (Diazotype) Process. Direct (diazotype) papers are newer
than blueprint papers and have replaced blueprint papers to a large extent.
Diazotype papers are made by treating the surface of the paper with an
acidic solution (usually a of 2) of light-sensitive chemicals, an attempt
lieing made to obtain only a slight degree of penetration into the paper.

During printing, the sensitized paper is exposed to light through the
original draw ing, after which the paper is developed. Tw^o developing proc¬
esses are used, the ammonia and the semi-wet proces.ses. In the ammonia
process, all the color-producing agents are present in the sensitized coating
on the paper, and developing consists of treating the light-exposed paper
to fumes of moist ammonia, which neutralizes the acid present in the coating
and penults coupling to take place. In the semi-wet process, the paper is
nioistened with a strongly alkaline developer solution which contains part
ot the color-producing components. The final result in either case is a
positwe print having blue, black, brown, or red lines on a white background.

sulfite diazotype process is generally made from

a mixture of^st '1 ^^e paper is surface-sized with starch or with

oped brthe se resin if the paper is to be devel-

1 ' j *Tu-wet process. Glue-sized papers are not used because

g ue pro uces yellow-brown backgrounds in the tints,®® and because high

IIS' ^1^-"' No. 11 : 338-341 (Sent 15 1Q4Q1

. Klein, Paper Trade J. 129, No. 13: 381-382 (SeV 29, 1949)

1200

PULP AND PAPER

wet tensile strength is not required, since the papers are not immersed
during developing. The paper should be well sized, and it is desiral)le that
the paper retain its sizing after developing so that notations can be made on
the print with ink. Sizing due to rosin is generally destroyed during de¬
veloping because of the alkaline developing solutions normally used. High
opacity and high brightness are desirable properties, and generally diazo¬
type papers are filled with titanium. A pinkish cast is desirable when blue
lines are produced whereas a blue-green cast is desirable when black lines
are produced; acid- and alkali-resistant dyes are required. The paper
should be free of iron. Dirt spots are objectionable since these are not
hidden as they are in blueprint papers. As with blueprint papers, high
folding endurance is desirable so that the paper can stand up under the
repeated handling which it normally receives in use.

Lithoprints. Lithoprints are made from gelatin on which an exposed,
undeveloped blueprint is transferred. The resulting print is black on white.
One advantage of ^his process is that clean copies can be made from dirty
originals.

CHAPTER XX

LAMINATING AND PASTING

The operations of laminating, combining, gluing, and pasting are im¬
portant parts of the paper industry. These operations involve the sticking
together of two or more sheets of paper by means of a laminant or glue,
with the purpose of producing a laminated structure possessing either in¬
creased strength, improved appearance, or greater utility in some other
respect.

Laminating Processes

Laminating of paper products is usually carried out in a separate con¬
verting plant. Some of the more familiar laminating operations carried out
as part of the paper industry are:

(1) Lamination of two to five plies of paper, usually either kraft or jute, into a
thick paperboard which is suitable for solid fiber shipping cases. This process is gen¬
erally known as solid fiber laminating.

(2) Combination of kraft or jute liner stock with a corrugated sheet of straw,
kraft, or semichemical to produce corrugated paperboard for use in manufacture of cor¬
rugated shipping containers. This process is known as corrugating.

(S) Lamination of a thin sheet of high-grade paper to chipboard to produce a
more attractive board for fancy boxes. This process is called sheet lining.

(4) Lamination of a special sheet such as glassine paper or greaseproof paper to
another dissimilar sheet or to chipboard to produce a final product which has special
properties. In this process, one or more of the units may be some material other than
paper, such as lead foil, aluminum foil, copper foil, cellophane, cellulose acetate, Plio¬
film, or polyethylene sheeting. A frequent combination is 0.00035-in. aluminum foil

aminated to a 30-lb. sulfite sheet used for greeting cards, box coverings, seals and
labels, and as a liner for cigarette packages.

All the above are continuous processes in which the paper is fed in roll

form through the laminating machine. The first step in the operation is

the application of adhesive to the paper, after which the various plies are

pressed together to produce the laminated board. The combined board is

t en passed over driers and finally cut to the desired size at the end of the

mac me. There is another process called sheet pasting in which small

sheets of paper or paperboard are fed by hand through a set of rolls which

apply paste to each sheet individually, after which the pasted sheets are
combined by hand.

As previously mentioned, it is not the function of this book to discuss

1201

1202

PULP AND PAPER

engineering details. This policy will be followed in this chapter, and no
attempt will be made to discuss the various types of laminating machines.
There are, however, certain general mechanical principles with which the
chemist should be familiar, because if these principles are not understood,
a satisfactory job of laminating cannot be done. In fact, the method of ap¬
plying and setting the adhesive is as important, or more important, than
the properties of the adhesive itself. The amount of glue applied to the
paper, the temperature of the glue, the amount of pressure applied to the
glued assembly, and the time allowed for the setting of the glue are all ex¬
tremely important factors. All laminating machines should be kept in the
best possible condition. The glue applicator rolls should be smooth and ab¬
solutely true, because otherwise, uniform glue lines will not be obtained.
The pressure rolls should also be true and smooth to provide uniform pres¬
sure across the laminated sheet. Some means of temperature control
should be provided in order to control the viscosity and penetrating prop¬
erties of the adhesive.

Types of Adhesive Used

Many types of adhesives are used in laminating paper. These include;
(1) aqueous adhesives such as animal glue, starch, silicate of soda, and
casein; (2) hot-melt adhesives such as asphalt and wax; (J) lacquer type
adhesives such as cellulose nitrate dissolved in organic solvents; and (4)
emulsion type adhesives such as polyvinyl acetate emulsified in an aqueous
medium. Special adhesives, whjch may fall in one or more of the above
groups, are water-resistant adhesives, heat-seal adhesives, and pressure-
sensitive adhesives. In addition, adhesion between plastic bodies may be
obtained by heat-molding or by solvent welding.

Aqueous versus Non-Aqueous Adhesives

Up to fairly recent times, nearly all adhesives used in the paper in¬
dustry belonged to the class of aqueous adhesives, represented by the
starches, vegetalde gums, casein, blood glue, sodium silicate, and animal
glue. However, within recent years, the newer lacquer adhesives, emulsion
type adhesives, and hot-melt adhesives have gained ground, principally for
specialized uses. With these new adhesives, it is possible to obtain special
properties such as w'ater-proofness, alkali-proofness, water-vapor-proofness,
gas-proofness, or grease-proofness, which cannot be obtained with the older
types of glue. On the other hand, there are certain disadvantages to these
new glues. Many of them are too expensive for ordinary use. Further¬
more, some have undesirable odors which make them nnsuited for food
I)ackag'>m- use of all these adhesives will be discussed In greater de¬

tail later in this chapter.

XX. r.AMINATIXG AND r.\STING

1203

11 'alcr-Rcsistanf . Itiht'sh't’s

For many jnirptiM*.';, it is necessary or tlesirable to use an adhesive
which produce.^ a water-resistant bond. Some of the laminated products
requiring water-resistant adhesives are solid fil>er paperboard for use in
overseas shipping containers, solid-fil>er paperboard for use in beer cases,
ami paperlx)ard for beverage bottle cartons. In addition, water-resistant
adhesives are required for gummed labels used on medicinals, toiletries, etc.

Lacquer or hot-nielt adhesives are the most satisfactorv tN'j'jes when
maximum water resistance is desired, because these non-aqueons adhesives
provide complete protection against water. Some of the resin emulsion
adhesives also produce highly water-resistant bonds, .\mong the natural
adhesi\es which produce water-resistant glue lines are the casein glues,
blood glues, soyiiean glues, and other protein glues. (In the other hand,
starch adhesives, as a rule, do not produce water-resistant bonds, although
they an l>c made water-resistant by using urea-formaldeliyde resins in com¬
bination with the starch. Sodium silicate has very poor water resistance
liecatise of its high alkalinity, which tends to destroy the sizing in the paper,
thm proflucing a waler-.sensitive area in the pafier just below the glue line.

Heat-Seal Adhesives

Heat-.seal adhesives arc usually applied as a coating which in later

combined to another surface by melting the adhesive with heat. I'here are

two types, the direct contact ty|>e. and the delayed action tvpe. In the direct

wintact t>-pe. heat is app!ie<I after the coate<l adhesive' surface has been

brmight into contact w ith another ,«iper surface. In the delaved action tvne

Jh. a. h.s,vc surface is hoa.c! and ac,i^,,e,I .he ,vvn surfaces have

Iiern brought into contact.

Ifeat-se^ adhesives are available in emulsion, hot-melt, and lacquer
l>r«. raraffin .s uscl as a heat-seal adhesive in waxed bread wramatr

Ik wrapiaxl loaf between two heated platens. This melts the wax, which

uk kl** cooled. Cooling is necessarv

rv’pe a^i^ lacquer type and emulsion

. pe ad^sues tend to fuse immediately u,H.n the application of heat

(/) Plastjcued cellulose derivatives such *♦!, i n i

(^1 Rubber derivatives sikH as cvclir«t kka, ^ cellulose nitrate.

U) PolrivAotylaKs. ^ “""I rubbers.

1204

rULF AND PAPER

{ 4 ) Polyvinyl resins such as polyvinyl chloride ac.etate copolymer, polyvinyl-
idene chloride and copolymers, polyethylene, and acrylics.

(5) Polyamides.

Usually, a plasticizer is necessary in the torinulation to obtain the best heat-
seal properties. One patent' calls for a coating on cellopliane consisting of
polyvinyl acetate, cellulose nitrate, and diethylene glycol monoinethyl ether.
The subject of heat-seal adhesives is discussed further later in this chapter,
and also in the chapter on coating with resinous materials.

Pressurc-Sensitivc Adhesives

The adhesives on Scotch tape and masking tape are examples of pres¬
sure-sensitive adhesives. Pressure-sensitive paper tapes are made of sev¬
eral layers composed of the base paper, a primer coat on the paper, and a
layer of pressure-sensitive adhesive. On the other side of the base paper,
it is necessary to have a layer of some material which is repellent to the ad¬
hesive in order to prevent the adhesive from sticking to the paper when the
tape is wound into a roll. A layer of cellulose acetate is usually satisfactory

for this purpose.

Among the materials which may be used in pressure-sensitive adhesives
are rubber and rubber derivatives, synthetic rubbers (GR-S, neoprene, and
poly isobutylene), alkyd resins, ethylcellulose, rosin derivatives, acrylic
resins, other vinyl resins, and the natural resins. Rubber has been widely
used, and one satisfactory formula consists of a mixture of chlorinated rub¬
ber, a plasticizer such as castor oil, and a solvent such as butyl acetate.
Another formula is represented by 20% of rubber, vinyl resin, or ethyl-
cellulose ; 40% hydrogenated methyl abietate; 40% rosin, dammar resin,
or coumarone-indene resin. Another formula is represented by a mixture
of poly\4nyl chloride-acetate copolymer and Buna N rubber, together with

methyl ethyl ketone as a solvent.

Solvent Welding

The solvent welding oi plastic surfaces is not widely used in the lam-
nating of paper products, but it is of interest to a student of adhesive action.
Solvent welding can be used only for the laminating of
tic bodies. In this method of laminating, a solvent is used t
sheets of plastic materials which can then be welded into a. strong o
the solvent diffuses away from the joint. This method of wddmg^“
factory only if substances of similar nature are welded, s
substances will not form a strong bond, even though the ^

sol«nt for both. For example, acetone will weld two sheet c Mo^
acetate, but will not produce a strong bond between a shee

1J. A. Mitchell, U. S. 2,374,767 (May 1, 1945)

XX. LAMINATING AND PASTING

1205

acetate and a sheet of cellulose acetate-butyrate, even though acetone acts
as a solvent for both substances.^

Heat IVelding

Heat welding is another method of combining surfaces when mutually
compatible polymers are involved. . In this process, heat is used to soften
the surfaces so that they can be welded into a continuous joint. Only
plastic bodies which soften under heat can be used for heat welding.

Theory of Adhesion and Adhesive Properties

Before discussing the subject of adhesion, it is necessary to understand
what constitutes an adhesive. Actually, it is very difficult to define an ad¬
hesive, since almost any substance will act as an adhesive under certain
circumstances. A classical example is the strong adhesive effect which is
obtained with a thin film of water between two pieces of clean plate glass.
In general, however, an adhesive may be defined as any fluid which will
“wet” the two surfaces to be combined and which can then be converted
into a tough mass by cooling, evaporation, etc., so that the two surfaces
are held together with appreciable strength. A good adhesive should have
a modulus of elasticity comparable to that of the materials laminated.

Paper is a particularly easy material to combine w’ith adhesives because
of its porous structure which permits a high degree of interlocking or me¬
chanical adhesion. As a result, the lamination of paper is not a particularly
difficult proposition. However, laminating some of the newer materials,
such as waxed papers, impregnated papers, synthetic resin sheeting, and
coated papers often involves considerable difficulty. The introduction of
these new materials has greatly increased the complexity and difficulties of

laminating problems. The lamination of these materials involves a form
of adhesion known as specific adhesion.

opectfic Adhesion

There is no universal glue w'hich wdll work on every surface. On the
contrary, all adhesives are more or less specific for a particular surface.
A strong bond can be formed only if there is a similarity in molecular struc¬
ture between the adhesive and the surface to be joined. For example, highly
polar adhesives (starch, animal glue, etc.) make excellent adhesives for
paper because paper is highly polar. On the other hand, strong bonds can¬
not be produced when polar adhesives like starch are used on non-polar
surfaces such as those presented by waxed paper, unless the continuity of

rf r 7 destroyed with solvents. As another example

effect of polarity, it can be shown that increasing the number of polar

New York"*N.^*^(T 947 )^^^^^ Adhesives, Reinhold Publishing Corporation,

1206

I'ULr AND I’APKR

hyclioxy] groups in polyvinyl acetate through hydrolysis will increase the
adhesiveness of this material for polar surfaces.

Specific adhesion is due to molecular attraction (electrostatic, co¬
valent, or residual binding forces of the van der Waal’s type) between
the adhesive and the surfaces joined. The Itonds which are produced are
often tremendously strong, and it is possible (with very thin films of ad¬
hesive) to obtain bonds which are stronger than the tensile strength of the
adhesive itself. This is apparently due to fields of force from the opposing
surfaces being superimposed upon the ordinary cohesive forces within the
adhesive film in such a way that they strengthen the cohesiveness of the glue.

In order for adhesion to take place, the adhesive molecules must come
into close contact with the surface of the materials to be adhered so that
molecular forces can come into play. Molecular nearness is accomplished
by formulation of the adhesive so that the molecules of adhesive are in a
highly mobile state at the time of application. This is done by applying
the adhesive in a solvent, in a molten state, or in an unpolymerized form
so that it can be subsequently polymerized in place.

Adsorption phenomena probably play an important part in specific
adhesion. In fact, Bancroft^ believes that the degree of adhesion can, in
many cases, be measured by the amount of adsorption of adhesive on the
material laminated. Considerable work on the fundamentals of specific
adhesion has been carried out at the U. S. Forest Products Laboratory.

One of the important differences between specific and mechanical ad¬
hesion is that adhesives which act by specific adhesion are effective on non-
porous, as well as porous, surfaces, whereas adhesives which function by
purely mechanical means are effective only on porous surfaces. Thus, spe¬
cific adhesion is all-important in the lamination of metal foils, coated cello¬
phane (MST, MSAT), cellulose acetate, coated glassine, and other similar
non-polar and non-porous materials. It is probably a factor with such
semiporous materials as glassine, uncoated cellophane (PT), and vegetable
parchment. Wet-strength papers are more difficult to laminate than ordi¬
nary papers, and this may be due to lower specific adhesion. Some of the
newer transparent plastic sheetings (e.g., Saran, Pliofilm, and particular!)
polyethylene) are very difficult to laminate and require specialized adhe¬
sives having a high degree of specific adhesion.

MecJianicaJ Adhesion

Mechanical adhesion differs greatly from specific adhesion. It is a
much simpler concept and involves a keying of the adhesive into the pores
of the laminant. Mechanical adhesion is particularly important on paper,

3 W. D. Bancroft. Applied Colloid Chanislry, p. 74, McGraw-Hill Bcx.k Com-

pany, New ^ork, N. Y. (1921)

XX. LAMINATING AND PASTING

1207

inasmuch as paper is liiglil}'^ jiorous an<l poriiiits considerable penetration
and solidification of the adhesive around the fibers. A ver}' strong bond
can be formed by this mechanical embedding of adhesive in the pores of the
paper. Glued wooden joints also appear to be held together by mechanical
adhesion.^ McBain and co'workers® in England have been among the prin¬
cipal proponents of the mechanical theory of adhesive action.

In cases in which mechanical adhesion predominates, it is necessary
that the adhesive have the proper viscosity to flow into the pores of the
laminant or a good bond will not be obtained. The adhesive should then
solidify in place to produce an interlocking system. Penetration is the im¬
portant factor, and either too much or too little can result in an inferior
glue joint.

Effect of Adhesive Properties

Most adhesive materials are high polymers. Each adhesive has an op-
dmum molecular weight for best adhesive action, and this is usually in the
intermediate range of molecular weight. Too high a molecular weight is
undesirable because it leads to high viscosity, low solubility in solvents, and
reduced adhesive strength. On the other hand, too low a molecular weight
is undesirable because it means that the adhesive is likely to have insuffi¬
cient cohesive strength to hold the,assembly together. Thus, starches are
modified to products of intermediate viscosity before conversion into ad¬
hesives, By the same token, animal glues of medium jelly strength are
usually selected for adhesive purposes. Thermoplastic polymers also have
their best adhesive action at an intermediate degree of polymerization where
the resin has good specific adhesion for the surfaces to be joined and enough
cohesive strength to form a strong assembly. According to Del Monte«
the range of polymerization best suited for good adhesive action for a num-
ler of different synthetic resin adhesives is as follows:

Type of adhesive

Poijrvinyl acetate polymers
Polyvinyl chloride-acetate
Polyethyl acrylate .^

Polyisobutylene .

Polyamides .

Chlorinated rubber
Cellulose nitrate ...

Degree of polymerization

60-200

100-150

80-150

50-150

50-100

125 cps. vi.scosity grade
150-300

molecular ‘he resin increases in

lolecular weight during the curing cycle, tending to i,ass through the state

dusnbi RkS-cI^ Scientific and In-

= J. W. McBain and W B Lee E"glsi>d

®J. Dei Monte, The Tech,M3: 60Cy-620 (1927)
New York, N. Y, (1947) ' rriccj, Reinhold Publishing Corporation,

1208

PULP AND PAPER

of maximum adhesiveness into the state of maximum cohesiveness. This
may, in the case of strong assemblies, lead to a weakness at the interface
between the adhesive and the surfaces joined. The number and type of sub¬
stituent groups and the number and type of side chains are other important
factors which govern the polarity, solubility, chemical compatibility, and
viscosity characteristics of resinous adhesives.

Viscosity of Adhesive. The viscosity of the adhesive must fall
within certain definite limits, depending upon the type of adhesive and the
surface to which it is applied. If the adhesive is too viscous, it will make

poor contact with the paper surface, whereas if it is too fluid, it will be
sucked into the pores of the paper, thereby producing a starved joint. In
either case, an improper film results, and the bond is weak. The viscosity
is also important in regulating the amount of adhesive transferred from the
glue rolls to the paper. Low viscosity permits orientation of the adhesive

in the glue joint.

The viscosity of an adhesive can be controlled in one of three different
ways. These are: ( 1) by changing the degree of polymerization of the
adhesive material itself, (2) by changing the type or amount of solvent used
in formulating the adhesive, or (3) by changing the temperature. In prac¬
tical operation, the viscosity is controlled by regulation of the amount of
solvent in the adhesive. The adhesive may be diluted with solvent to
produce a more fluid paste, or part of the solvent may be withheld dur¬
ing preparation if a higher viscosity is desired. This, of course, changes
the solids content of the paste, as well as the viscosity of the paste. With
starch adhesives, and other adhesives available in several degrees of con¬
version, it is possible to select products which give quite a range of viscosity
at the same solids content. In the case of resins, it is possible to app y
high-viscosity materials in a low-viscosity form by emulsifying in v^ter.
Low-viscosity pastes are best suited for highly sized papers, whereas high-

viscosity pastes are best suited for slack-sized papers.

Increasing the temperature of the paste nearly always lowers the vis¬
cosity. This is particularly true of the colloidal adhesives such as stare i,
glue, casein, and vegetable gums with the notable except.on ^

fose, which tends to form a gel at high temperatures. The
viscosity relationship is, of course, of paramount importence

‘’''""Tintype of adhesive flow is important. The practical adhy™ user
recognizeslis fact in a qualitative fashion hy

either long or short. A short paste is one witi a iij, ^ ‘ ^

high mobility, whereas a long paste is one with a low >ne^

low mobility. The paste mixer examines his paste f fl

watching how the paste runs off a stick or paddle.

XX. LAMINATING AND PASTING

1209

Tackiness of Adhesive. Tackiness is a measure of the adhesive
quality and refers to the stickiness or gumminess of the paste. Tackiness
implies a combination of factors such as plasticity, adhesion, and cohesion.
A tacky paste is usually one with a low yield value and a high mobility. Al¬
though there is no satisfactory method of measuring tackiness quantitatively,
an ex|)erienced paste mixer can obtain an indication of the tackiness l)y
working the paste between his thumb and forefinger and measuring the
tendency of the paste to string out when the fingers are pulled apart.

The practical glue user prefers a tacky adhesive, or one which will
string out and exert a definite pull. Actually, however, tack is not an es¬
sential requirement for a good glue, since some of the strongest adhesives
arc non-tacky. For example, many of the modern synthetic resin adhesives
show no tackiness at the time of application, yet they form very strong
bonds on drying. On the other hand, corn S}Tup and molasses are examples
oi substances which are extremely tacky, but which have low adhesive

strengths. Tackiness is of most importance with starch and dextrin ad¬
hesives.

The practical advantage derived from the property of tack is that
highly tacky adhesives hold the surfaces together while the glue is still in a
wet or Iluid condition. It the adhesive is non-tacky, the surfaces to be
joined must be held in contact until the glue has dried or set. After the glue
lias finally set, tack is no longer an important factor.

In general, tackiness increases as the concentration is increased. With

most glues, tackiness increases as the temperature is raised to a certain
jx»int. Starch or dextrin glues generally have very little tackiness unless
borax is present. The addition of clay increases the tackiness of some glues.

Pressure-sensitive adhesives have a lingering tack. These adhesives

have extremely high viscosity and de'rive their adhesive strength from

plastic flow.^ '1 he strength of the bond produced with these adhesives de¬
pends upon the rate of pull.

Other Properties of Adhesive. There are many properties of the

adhesive which are important from a practical standpoint. Some of these

are solids content, wetting properties, type of solvent, alkalinity, and setting

rate. 1 he properties of the adhesive which promote penetration are long

wet lite, small angle of contact, low viscosity, and low surface tension.® The

properties which inhibit penetration are high viscosity, short wet life high

^urtace tension and large angle of contact. The characteristics of individual
adhesives will be discussed later.

' Chem.. Anal. Ed. IS. No. 3 : 201-206

•F. Camps-Ornipins, Pafar Trad, J. no. No. 8: 107-109 (Feb. 22, 1940)

1210

PULP AND PAPER

Effect of Sheet Characteristics

A rough surface is beneficial in increasing the amount of mechanical
adhesion. On the other hand, a rough surface is undesirable when specific
adhesion is involved, because thicker adhesive films are required on rough
surfaces. Excessively rough surfaces are undesirable in all cases since
they increase glue consumption and there is the possibility of trapping air
bubbles in the glue line.

If adhesion is due primarily to specific adhesion, it is necessary only
that the adhesive wet the surfaces to be joined. If the attraction between
the adhesive and the surfaces to be joined is low, specific adhesion will be
poor and a weak bond will be formed. An}1:hing which improves the wet¬
ting of the adhesive for the laminant will improve the adhesion. Papers
containing a small amount of moisture are more easily laminated with

aqueous adhesives than absolutely dry papers, l)ecause the paper with the
higher moisture content is wetted better by the adhesive. Surface-active
or wetting agents are sometimes used in acpieous glues to increase the net¬
ting, particularly for i)apers which are highly sized with rosin or wax. Ihe
addition of asphalt or wax emulsions to aqueous adhesives helps in the

lamination of asphalted or waxed papers.

Ordinarily, starch or animal glue adhesives are not satisfactory for
laminating highly waxed papers, because of poor wetting. However, it is
iKjssible to produce a fairly good bond by emulsifying a small amount of
carlmn tetrachloride, benzol, or ethylene dichloride in the adhesive. About
one-third t|uart of benzol per gallon of adhesive will greatly improve the
adhesion of starch adhesives for paperboard winch has been given a wax
,h on Ihe calenders. The solvent acts to dissolve the protective him o
:, thus permitting the starch to make better contact with the surface of

wax

'mo"’ solid surfaces are contaminated with films of
vre-ise oxides etc. These films lend to interfere with adhesion and, n

toiiie cases, must l^. removed for best res.ilts Although

no practical imiKirtance on i«per products, they are sometimes an injr

taiit factor in the lamination of metal foils. A

overcome the difficulties of honding to metal surfaces is to api ly . I

-n.e’'Sro 5 ol'dfIirfrsloThnl^ is prohahly the most im-

inohde m lii'nid ,«rl of the adhesive. The ^

XX. l-\ If IN ATI .VC AND PASTING

1211

lou hijii A rc»ults^tii a s4ar\rtl joint. C'aiiif»s-Cainpins* lists tlu*

fnllowifig \alur> for the porosity of |iapers coniiiKHily in laminating.

The purosiiy is ex|»resse<l as ilie nuiiilier of cubic feet of air. siiacc per cubic
feet of air-free l^aprr.

Glassioe .. 0.15

BteidL bcsA paper (km* hulk) .. . 0.5

Boad, Ihjuk paper ..... 07

. 0.9

Oiiip^jard, nrmhkiidnl ... 1,1

Wmtef'fifiiJiiFuttnlrutief kralt ... 1.4

Slnm-^RSih FourdrtnifT kraft .*.. 1.6

I>rjr* 6 iiiili Fcwdrmkr krah ....... ^ 2.0

^ high dmjiity (0.1—1.0 in above table) inhibit penetration of the

•olid portion of the adhesive, but tend to remove the liquid fraction of the

adhesive fr«n the glue line more rapklly than low-density papers liecau.se of

the greater contact lietween the adliesive and |)a|ier surface. This |iartly

explains why a given adhesive w ill set faster on smooth-surfaced jute than

on Djugh‘surfaced kraft In order to oliuin best results in laminating, the

imtrating profienies of the adhesive must be matched with the porosity
of the taper.

surfaces must be cunxiderrcl in laminating. If these sur-
faces wy in s^ichness. porosity, etc., the problems in laminating are
greater than if similar surfaces are involved. Some of the combinations' fre¬
quently enentintered are as follow s;

.. to a hard Mn^. .uch a* the lamiiutioti of xlatsinc ,«ipcr

rjTa* " ■anwfafiurr of trantparmt wrapper* for foodstuffs.

M l a ^ "*T* lamination of metal

T»t a “ *** inanofacture of randy or eura wrappers.

paper lo .wlftte paper m the manufartiwe of heavy wrapping or packaging ,aper.

above cnmlnnatuitis [iresents a different tirotilem and reouire.s
a 'hit*rent type of adhesive.^

_ miportant properties are moisture content. <legree of sizing, and

*»- W-r. Increasing the moisture content increas^ the
tMil. 1 .2!!* ° **? r”‘* lnnp<Tatiir. of thr iKi|wr i> impor-

™«« 1! wock which

ft«n .he nnu .«i ««k ,hich h.. i«p ^ „„

Trmde /. no. No. 8 :

107-109 (Feb. 22 .

1940)

1212

PULP AND PAPER

Application and Setting of Adhesive

When the adhesive is first applied, there is some penetration of ad¬
hesive into the pores of the paper. This is desirable, since it increases the
area of contact and strengthens the layer of paper just below the glue line.
Too much penetration must be avoided, however, since this leads to a

starved joint.

In order to secure good adhesion, pressure must be applied to the as¬
sembly to bring the adhesive into contact with the paper surfaces and con¬
trol the amount of penetration. Pressure is necessary when rough-surfaced
papers are used, but excessive pressure should be avoided. Too niuc^
pressure must not be used on porous papers, since this forces too much o
the wet glue into the paper, thereby leaving too little to function as a glue
film. Too much pressure must not be used on non-porous surfaces, since
this results in glue squeezing out at the sides of the assem y. or ^es
results, the press rolls on the laminating machine should be independent of
the glue applicator rolls in order to provide independent control of pressure

to make an arbitrary but useful distinction between two
fairly distinct setting stages. The first stage occurs when the adheswe
forms an initial or wet bond which is sufficiently strong to withstand al or¬
dinary handling. The second stage is represented hy the formation of t
final or dry boL, which is often much stronger than the ^per itself. It is
the wet bond which determines the setting rate, since the final or dry bo
nay n t r formed until some later time. The length of time which inns
elaiise before the laminated product can be handled determines the speed at
S fi^flaminating machine can be operated. If th^a es.ye is s e -
ting, the machine can be operated at high s,«ed whereas if the adhesiv
slow setting, the machine must be operated at a ow speed
There is no relation between the strength of the imtu

the strength of the final or dry bond. In '* ^^eak

tacky adhesive will produce a strong we but

dry tioiid. For example, are'strong enough to

their dry bonds are often quite , ” , , „ good envelope

stick rag content papers to the point of " native

dextrins are used. On the other ham , pa. .1 extremely

starch have relatively low wet bonding [lapet

strong dry bond and, for this reason, are used in the

’“''“'Adhesives are set hy one or more of the

gelation due to cooling (e,g hot Uporation'(e.g., or-

by gdation due to chemical reaction

XX. LAMINATING AND FASTING

1213

(e.g., catalyzed resin adhesives). These methods of setting are discussed
briefly in the following sections.

Setting of Adhesive by Solvent Loss. Most adhesives are set by
loss of solvent, either to the atmosphere or by absorption into the paper.
.•\s a general rule, the rate of development of him strength in the glue line
is directly related to the rate of solvent removal. In cases where the adhe¬
sive is applied to relatively non-porous surfaces, there may be a high reten¬
tion of volatile solvent left in the glue line, which is undesirable because it
reduces the cohesive strength of the adhesive film.

As solvent is removed from the glue line, the adhesive film increases
in viscosity (and tackiness) and eventually sets to a plastic-like solid.
Resin emulsion glues and lacquer type adhesives set rapidly because they
readily release solvent to the paper stock. Starch adhesives set more slowly
because they tend to release water only gradually. Dextrin adhesives and
silicate adhesives set fairly readily because they increase greatly in viscosity
upon the removal of only a small amount of water.

Increasing the temperature of the paste increases the rate of solvent
removal because of more rapid absorption of solvent by the paper. On the
other hand, high moisture content, high density, and a high degree of sizing
in the paper tend to reduce the rate of solvent removal. A long air draw
before the pasted sheets are brought into contact with each other tends to
^ increase the setting rate by increasing the loss of solvent to the atmosphere.
Too long an air draw must not be used, however, since the adhesive film
may become too dry to maintain satisfactory contact with the second ply.

Setting by Gelation Due to Cooling. Setting by cooling can be

used for starch glues which have a strong tendency to gel on cooling. The

glue IS applied to the paper at a high temperature (160-190° F.) at which

It is quite fluid, but as soon as the hot glue contacts the relatively cool paper.

It kJns to gel. The next ply of paper is brought into immediate contact

so that the bond starts to form immediately. This process is suitable only
tor highly sized paperboards.

Setting of the adhesive through cooling is used in hot-melt adhesives.

" r j '"olten adhesive, paraffin, or microcrystalline wax, is

apphrf to the paper and then is chilled immediately to set the glue joint
Setting by Chemical Reaction. The mechanism of setting hv cheni-

to the

^per and then are set by a combination of time, temperature and *H
nerally speaking, these resins form a wet bond long before the chemical

""'“oU^tar^: - fi-l’sta"

and ainnionium^^mirc7 7’’™ typ:77: wiiuTf ^’’7 T'"’

1214

AM» fAMl

siiKC ii intIucrKo tlw ctirms M the r«i^ Kar fhr rrvti-caitalviji

cuiiiltinattnfi just nKntHM>rfl. the lai^T UmtM ^avr a f»H Mr*«r 7

Setting by Gelation Due to Heating. McM arr aei

rai.idlv at InRh tcnn<^r.Hure than at km lemi^fariife, due !« the gTe*i
\nss ol !M.lvriit at the higher tetni*eratnre, Hrmwr, with rtarch
(or cornig;iting. heat is* us*etl in a different way to bring aJcnji a rapid
due to gelation of the starch. In thi' pr«CT^<, imoccked _ i* u »*
the adhesive, and lieal >ul»>e*|uentl> b applied to the
n«nie<i.ate gelling of the starch. Thb form, a strong isrl.boirf
the plies together until the glue film fose> mmsiure awl <^in » ,

drv l«nd. Ai-tunlK. tlic drs U«kI .. prohaWy w4

a omsiderahle time after tlw Imard is removed from tite machnte. Tim
process will lie descrilie^l in greater detail later.

Propi Ttiti of .4dli//frr fiJm

•I iK ihickncs, oi ihf rU« film is imponant
cost of latotnating and l«a«« of iu cfltrt on 1*^.

pr«luc. .VS a mle. thin fiWs of »<«-■"

than thick fiim.s. ^ i reduced down to a »mV

hlv increases a. the thickness of the glue him ii,--.

r ' ♦! f Jn inch when the gUic is applied to ojitically flat surfaces. H
honth of an inch, wh^ ^ g ^ involved, relatively .thick film* fww* .

ever, when rougli-suHaced papers are inv^vo^^^

be applied to obtai.t good contact betn-e« the adhrsn* ^ >

Z connection, it has been found d». strongs

tai.«d hy ad^« «o

one surface only. Del M^te I™* ^

which are ind^^ent of ^jhickne*.. fSte «-
show a marked drop in strengt ^ ^ ^

planatkm for the greater stretch E ho«c»cr.

likelihood of flaws in thin glue lii^ than in , ^ ^ joro-

o*er factors involved.=such as the rf« and thr

iron, opposing surfaces on the ordin^ e”*

effect of dimensional changes m the^ue pheskal propertif

In addition to film thickness, there are many ^

of the glue film which are ot imiiorunce. .,a..ricrts- and creep rale

tensile strength, cwnpressive in^iTtadiialion of "cal

These factors ate often of paranmunt imponance in the laim

»•/. Ckem. I»d. 43: 165 (192■t^

.. /. Pkyr Re,. 5ee.

1* T \\. McBain and W. B. lece. - ^.^21 -

.3 K. Sebumnan. /. -f/•rfW P*-«-BcW,M
1. I rtel ilonte. rw re.fmefo?, ef Adhrttirr. f. «»•

Y.»k. X. V. (ItW)

XX. LAMINATING AND PASTING

1215

foils and plastic films, but are of lesser imiiortance with jiaper where the
strength of the glue film is ordinarily much greater than that of the paper.
For example, a pull of 2 p.s.i. is sufficient to tear the surface of paperboard,
\vhereas the utlimate tensile strength of dried silicate glue lines is in the
neighborhood of 750 p.s.i.^’’ As a rule, there is enough margin of safety
in the strength of glue lines so that any rupture takes place within the paper
itself. However, in sealed waxed papers, the tensile strength of the wax
film is lower than that of the paper, and in this case, a weak film is desirable
because the package must be capable of being easily opened. Another ex¬
ample where a weak bond is desirable is in glues for palletizing miiltiwall
bags or paper boxes in a unitized stack where the individual units must be
separated later without tearing the paper. Masking tapes require weak
bonds so that the tape can be removed later. These tapes may be removed
under small loads if the pull is steady and slow, but even then, they are
sometimes stronger than j^aper.

In making strong as.semblies with metals or plastics, one of the most
important properties is the modulus of elasticity (stress-strain character¬
istics) of the glue, or the ability of the glue to distribute loads from one sur¬
face to the other. In general, a modulus of elasticity comparable to that of
the materials being laminated is desirable. Creep is important with adhe¬
sives which tend to migrate under stress. This condition often arises in

hot-melt adhesives which contain a high percentage of plasticizer and are
soft.

Dimensional changes in the adhesive film are a factor in adhesion be¬
cause thej’^ lead to stress concentrations between the adhesive film and the
adhering surfaces. Dimensional change may result from loss of solvent,
chemical changes in the adhesive, changes in temperature, changes in mois¬
ture content, or variations in applied pressure.

The tensile strength of the glue film is quite important, but often of
lesser importance than the specific adhesion between the glue and the sur¬
faces of the laminant, How’ever, in laminating wuth thermosetting resin
adhesives, the resin, wdiich increases in molecular w^eight during the curing
cycle, passes through a state of optimum adhesiveness to a final state of
optimum cohesiveness. Thus, when the adhesive is fullv cured and has its
maximum cohesive strength, adhesive failure in strong laminants (but not
with paper products) is likely to occur at the interface.

Cuvling aud IVaypiug

Curling and warping are frequent causes of complaint in laminatino-
operations. Curl is sometimes noticeable at the time of laminating but
more often, it is not apparent until the laminated product has been cut into

2i ■P”/’" Trade 1. ,10. No. 12: 167-

1216

PULP AND PAPER

sheets and dried. In extreme cases, the laminated product may curl into
a cylindrical tube which defies all attempts at flattening.

Curling and warping are the result of uneven expansion in the various
plies of the laminated product, generally caused by the uneven absorption
of moisture from the wet glue. If the two outside plies of the assembly are
of different composition, curl is more likely to occur than if the same paper
is used in both outside plies. As a rule, aqueous adhesives give the most
trouble. Non-aqueous adhesives, such as the hot melt and lacquer adhe¬
sives, are not troublesome because the solvents used do not swell the fibers
in the paper. When aqueous adhesives are used, the less water used in the
glue, the less the chances of excessive curl, and for this reason, high-solids
adhesives (e.g., 40 to 50% solids) are generally preferred when curl is a
problem. Plasticizers in the adhesive are helpful in preventing curl, and
when properly plasticized adhesives are used, trouble with curl can be
greatly reduced, and often eliminated entirely. As examples, it might be

pointed out that glassine has been pasted to 0.02 chip using an adhesive of

11% solids, and litho has been pasted to chip, using an aqueous glue o
23% solids, and in both cases, absolutely flat assemblies were produced be
cause the adhesives were properly plasticized. ^ ^ ^

(

sugge.stions for reducing curl are as follows:

a smooth surface and are neither too soft nor too
. . . • 1 -_. .....1 --fQss direction strengths

absorbent.

from one ply, run that

lat the pressure is even, that the dry
tension on the paper at any time.

Aqueous Adhesives

in water constitute

on paper products. Among the most un-
dextrins, starches, casein, vegetable gums,

1217

XX. iJlIflNATING AND PASTING

ftudnini silicate, aniniaJ (lur. tUh and soy flour. These do not include

the ctiralsion type adhesives, which are discussed in a separate section.

The manufacture and fonnulatkin of aqueous adliesi>'es is a very coni-
plicatrd business about mhich the axTrage pei>on outside the business knows
very little. Incrcdienu in great x-ariety arc used to iiKxlify the Itase ma¬
terial. including wetting agents, penetrating agents, fluidifying agents, set¬
ting agents, preserxatixes, extenders, and xxaterprooting agents. Clays, for
cxam|jlt. are sometimes added to increase the solids content of the liquid
ghir, which hastens the drx'ing rale. Clay also prex'ents excessive pene-
traticNi €ji adhrsixe into the fiaprr and is used in adhesives for porous pai^ers

St^ch and Dextrin Adhesh\'s

Starch and dextrin adhcsix*e» constitute one of the largest class of
aqueous adhesives used in the (ia|)er industry. Starch is use<i in prac-
ticafiy ait degrees of modification, ranging frtHii unnuKlitied native starch to
highly CMivened dextrins. EnxxTne-convened starches are used to some
extent The same general |irncedurr is followed for eniyme conversion as
m other starch applications, except that high percentages of enzyme must be

used because of the high solids. The procedure is x’ery similar to that used
in the cimversiiin of starch for pigment coating.

starch ailhesives conuin from 5 to 20% liorax which is added to

* ** ^ IMiste Too much Imrax, however, increases

the cohesivr rtrmgth to the point where tlie paste liecomes rubliery. Al¬
kalis (eg., sodium cartxmate) may lie added to re<luce the viscosity and

m thrh^. Special non-ionic surface-active agents (eg. Vanesta) may

to mhiUi gelling tendency; these are jarticuUrly etTectiye with
Ihm hoili^facid-hydrolyzed) and enz>*me<onvened starche.s in which
U^5 to I 00% of surface-active asent will nrevent ^

-mrlMi I - - ..^v; -'WKn-speea riackagine

■itsw), oomnirum, up u. wlkU may be deairable. The dextrin

1218

PULP AND PAPER

must not be too Iiiglily OMivcrted, however, since this means that it will
produce a relatively weak, brittle film. As a rule, the conventional white
and canary dextrins are suitable for carton sealing because they combine
quick setting properties with adequate film strength.

Starch Adhesives for Solid Fiber Combining and Sheet Lining.
One of the large uses for starch adhesives is in the lamination of solid fiber
l)oard. In this process, two or more plies of heavy kraft paper or chipboard
(which ordinarily run from 0.009 to 0.030 inches in caliper) are laminated
into a thick board suitable for fabrication into l)Oxes or shipping cases.

Sheet lining is similar to solid fiber laminating. In this process, a light¬
weight sheet of paper such as bond, kraft, coated paper, or glassine is pasted
to one or both surfaces of heavy paperboard. In general, the same type of
starch adhesive and the same equipment are used for both sheet lining and
solid fiber laminating.

.Several different methods are used to apply paste in solid fiber lami¬
nating. The olde.st method, but one which is still used in many places, is
to apply the paste between two sheets just before entering the press section,
or, in other words, by flooding the nip. In this method, the excess paste is
removed by the pressure exerted on the sheet at the press section. Since
the pressure exerted at the press rolls also affects the amount of paste forced
into the paper, this method of application is lacking in control, do over
come this difficulty, the newer machines use se^iarate glue stations for each
glue line applied; each station is equipped with a set of glue rolls which
applies a film of paste of predetermined thickness. The pressure exerted on
the laminated ]>a]^er is then regulated independently at the press section.

d'he conventiotial canary dextrins are not suited for laminating paper-
board becau.se of their low strength and hygroscopicity. The dextrins used
for solid fiber laminating are modified to an intermediate degree so that
they produce a paste of the desired viscosity when cooked at about 25 to
40% solids 'I'he tvpe of adhesive and concentration at which the adhesive
is used depends upon the type of hoard to be laminated, the type ot laminat¬
ing machine, and the temperature of the j.aste. The adhesive generally con¬
tains from 10 to 20% borax on the weight of the starch. W etPng agents

and alkalies are sometimes added to help in the wetting

naners On the other hand, if the paper is too porous, the addition ot clav

(„■ lientonite to the adhesive may be beneficial in reducing penetration.

Ordinarily, starch adhesives arc cooked and cooled m the
of cooking starches. As a starting ,>oint, the directions supp -d ^ the
manufacturer .should be followed, but it is freiiuently \

slight adjustments in order to satisfy local conditions. le coo nig ^
should be equipped with a direct steam line, since
highly modified that they tend to “burn on steam coils. oi

XX. LAMINATING AND TASTING

1219

are useful, however, for cooling the paste and for controlling the tempera¬
ture of the paste. It is hest to withhold part of the water during cooking
and then to add as much as necessary of the remaining water after cooking
to obtain the desired concentration and viscosity.

The paste is ordinarily applied at or near room temperature (85—
120® F-)- However, there has been a trend in recent years, when laminat-
ing highly sized kraft and chipboard, to use the paste at high temperatures
(160—180® F.) in order to obtain the maximum tack and to obtain rapid
setting of the paste through cooling when it contacts the relatively cool
paper. By using higher temperatures, it has been possible to obtain in¬
creased production and to decrease the consumption of adhesive.

It is desirable to use a paste of the lowest possible solids compatible
ith the production of the driest and most rigid board. Ilowever, the
solids must not be so low that excessive moisture is added to the jiaper,

TABLE I

Effect of Paste Coxcentration on

CoXSU.Ml'TIOX OF AuIIESIVE

Ter cent solids in
starcli paste

Tounds of dry starch
used per 1,000 sq.ft,
of glue line

2.1-2.3

2.5- 2.75_

3.0-3.2

3.5- 3.7
4.0-4.2

4.5- 4.7
4.8-5.2

since this causes curling anti leads to shrinkage. Shrinkage after the land-
na e oard has been cut to a definite sheet size is very undesirable. For
example, if laminated paperlioard is cut to the dimensions of a standard
box, excessive shrinkage will reduce the dimensions and result in a box
which IS too small. The two most important factors in determining^ the
amount of mo.sture added to the hoard are the thickness of the gh.e“fit
and the sohds content of the glue. Pastes of high solids produce hoard of
ower motsture content than pastes of low solids, provided a glue film of
le same thickness is applied in both cases. For this reason, rastes with
sohds content as high as 30 to are used when board of ver -1w u oi'

: prX:i.::rditf ■ ^--j

produce board of low moisture content by applying thin glue films

of nioXr~d1:X Xf™ ‘

Th^ai Zt f 1 “ "f laminating

nes, 0 X 1 r “ “““’’I’"™' Jopetident primarilv ti|Hm the thick-

glue line and the concentration of the atihesive. Table I shows

1220

PULP AND PAPER

the effect of concentration on consumption of adhesive when well-sized
chipboard or kraft papers are used. The consumption will be slightly
higher for slack-sized papers.

The consumption of glue varies with the number of glue lines in the
laminated product which, in turn, depends upon the number of plies and
the type of laminating machine. If the laminating machine is of the type
which has multiple glue stations (i.e., a glue applicator box for each glue
film) and the glue is applied to only one surface, the number of glue lines
will always be one less than the number of plies. Qn the other hand, if the
machine is of the older type, in which the glue is fed between the two plies,
the number of glue lines is determined as follows:

Number of plies

Number of glue lines

Water-Resistant Starch Adhesives for Solid-Fiber Combining.
Water-resistant starch glues are becoming increasingly important for solid-
fiber combining and for pasting multiwall paper bags. In this process, the
starch is used in combination with water-soluble urea-formaldehyde con¬
densates to produce a film which is insoluble in water and has a high de¬
gree of wet strength. Laminated products can be obtained which show no
paper-fiber failure even after a week of soaking in water.

Starches vary considerably in their reactivity with urea-formaldehyde
resins. Starches with high gelling tendency are the most reactive, due to
their higher molecular weight. Figure XX-1 shows the results obtained
with several different types of starch when prepared with different amoun s
of urea-formaldehvde resin on the basis of the starch. The vertical ordina e
shows the per cent delamination which occurred when the laminated boar
was soaked for twenty-four hours in water at 80*^ F. As can be
is considerable difference in the water resistance o the ^

„p„„ the type of starch «se<l. The acid-hydrolyzed
viscosity dextrin produced the greatest water-resistance with the smal

'"""Tn general. K to iirea-fornialdehydc resin, based on the

the .starch, is u.sed. The resin is nsnally added to

cookiiiL^ although in some cases, the two may be cooked toget

carefnii; controlled conditions. The reaction

.aciil catalyst, and usually alum or aninioiiiiim J , ^orated

will take place under alkaline conditions, and consequently the

XX. LAMIXATIXG AND PASTING

1221

starch adhesives cannot be used. A formula is (see also p. 1227) ;

Step 1. Cook tlie starch at the proper concentration, which is usually between
20 to 25% solids. (This should be a special starch containing no alkali.) Cool the
paste to room temperature.

Step 2. Add sufficient urea-formaldehyde resin to give 8 to 25% resin on the
basis of the starch. This may be commercial resin syrup or a syrup made by dissolv¬
ing dry resin in a small amount of water. Adjust the />H to about 6.0.

Step 3. Add 1 to 1.5% of ammonium chloride based on the total weight of paste,
i.e., starch, resin, and water.

The above paste will not be as tacky as regular starch adhesive because

It contains no borax, and as a result, the laminating machine will have lo
run at a slightly lovv’^er speed.

After the paper is combined, heat should he applied to speed up the re¬
action between the resin and starch. Unless a high temperature is applied
the reaction is rather slow, and the laminated product will have to be aged
for wveral days before maximum water resistance is developed. Aging
» ould always be carried out in a wartn room, Wcause it has been found

that aging m a cold warehouse is not satisfactory and actually may prevent
the final development of water resistance.

denJaId'^^[i“''^ ''ith urea-formaldehyde resin-starch glues should be

ig I) sizet with rosin. If the paper is made from kraft, the

1222

PULP AND PAPKR

pulp should Ik; well washed and treated with sufficient alum to prevent any
eventual bleed of alkali from the fibers into the glue line.

Starch Adhesives for Corrugated Board. Corrugated hoard man¬
ufacture is an important part of the ])aperboard industry. The familiar
corrugated shipping case is to lie seen everywhere and promises to become
increasingly popular because of its durability, light weight, and other good
properties.

Regular corrugated board is made with one corrugating medium and
two liners. Another combination sometimes used consists of one corru¬
gating member and one liner, which is known as single-lined board. If two
corrugating members and three liners are u.sed, the board is called double-
doul)le board. The liners are made from heavy-weight kraft or jute paper
and are usually 0.012, 0.016, or 0.030 in. in caliper. The liners should
have the qualities of toughness and durability. The corrugating medium
is made of straw, chestnut, kraft, waste ])aper, or semichemical pulp, and
is usually about 0.009 in. in caliper. Good corrugating medium should
have a high degree of stiffness, uniform caliper, and uniform moisture con¬
tent. Nine-point corrugating medium is generally made in the following
weights per 1,000 sq.ft.;

Strawboard .

Kraft .

Chestnut .

Waste paper stock

34 lb.

24 to 26 lb.
32 to 34 lb.
26 to 28 lb.

Corrugated board is fabricated on a special machine, the corrugating
machine, which consists in its simplest form of a set of corrugating rolls,
glue applicator rolls, and a hot plate section. Before entering the corrugat¬
ing rolls, the corrugating medium is preconditioned by steaming and is t len
further heated at the steam-heated corrugating rolls. This treatment with
moist heat makes the paper soft and pliable so that uniform corrugations are
produced, and prevents sticking of the paper to the corrugating rolls. le
number of corrugations per foot of board and the height of the corrugations

are standardized as follows:

A-

Type of flute

ITeight of flute?, in.

1/8

5/32

Number of flute?
per foot

A flute is used where high top-to-bottom compression and where maxi-

mum cushioning is desired, e.g.. for Iroard us«l m the

cartons and glass jars. B flute is used where high end-to-end p

XX. LAMINATING AND PASTING

1223

is desired; it is used for the packing of canned goods and where the box is
stitched instead of being taped.

On leaving the corrugating rolls, the corrugated medium flufts out to
meet a glue applicator roll which applies adhesive to one side of the jiaper.
Long metal strips (fingers) are set to control the amount of this fluffing
out, and the setting of these fingers is one of the most important parts of
the oiieration of a corrugating machine, since the improper setting causes
damaged flutes and poor adhesive application. Immediately after the ap¬
plication of glue, the first liner is brought into contact with the glued side
of the corrugating medium, and then (on pressure type machines) high
pressure is applied for a fraction of a second to set the adhesive. The re¬
sulting board, now known as single face, then passes to the double-liack
glue station where adhesive is applied to the flutes on the other side of the
corrugating medium and a second liner applied. Only a slight amount of
pressuie can lie applied at thi.s point (because of danger of crushing the
corrugations), and consecpiently, adhesion is more of a problem than it is

for the single face. If the flutes are uneven, only the high flutes will receive
glue and the adhesion will be spotty.

After leaving the double-liack glue station, the combined board enters

the hot plate section where the board is held against heated platens by
means of heavy belts. Setting of the adhesive on the double-back side takes
place at this point. At the end of the machine, the board is slit and flajv
scored. The blanks are then taken off the machine and piled in such a way
as to minimize warping and sweating of the board.

The wet glue liond must be fairly strong as the board comes off the
machine to withstand the handling at this point. Later, the adhesive sets
to produce the final dry bond. This bond makes an important contribution
to the over-all strength of the finished lioard, aftecting such properties as
bending moment, column compression, tensile strength, puncture resistance
and water resistance- Improperly applied adhesTve Lnlting in Starved
or flooded joints and poorly made corrugations, masked flutes, and deep
hnger lines reduce the strength of the bond.^' ^

Some of the tests used to measure the properties of the finished board
are mullen puncture, crush, impact, score tear, adhesion, co.npression,
rop, an rum test, the last four tests being made after the board has been
ahncated into a box. End-to-end and top-to-hotton, compression tests c.
^e sealed but empty case are widely used as a quality control standard
E. her maxunum compression or compression at M in. deflection can be
d. The crush test is widely used on the board before fabrication into a

” B P o' « ■E''’" Coiilainers 49-52 fOct 1047'

Lfy .»■ 1% No 4; 45 !^,

1224

PULP AND paper

box; good C flute board will pass a crush test of 35 to 50 p.s.i., B flute
board being soniewbat higher, and A flute board being somewhat lower.

The two principal adhesives used in corrugating are starch and sodium
silicate. Asphalt is used in a small number of cases for making a si)ecial
water-resistant board. Starch- and silicate-combined board can be distin¬
guished from each other by adding a few drops of phenophthalein solution
and iodine solution to the glue line. Iodine will stain starch blue, whereas
phenophthalein produces a strong red color with silicate, but not with the
starch.

The conventional starch or dextrin glues used in solid fiber laminating
are not suitable for making corrugated board because of the slow setting
rate of these adhesives. Furthermore, these adhesives contain so much
water that they wet the corrugated medium to an excessive degree and
produce soggy board. To overcome these objections, a special process
has been developed for making corrugated board.**-^" This process involves
the use of a two-component starch adhesive consisting of ungelatinized
(uncooked) starch suspended in an aqueous solution of gelatinized
(cooked) starch of sufificient viscosity to keep the ungelatinized starch
granules in suspension. The paste ordinarily contains a ratio of four parts
uncooked starch to one part cooked starch, and a total solids content of
about 20%. The theory of this unique paste is that (when the paste is ap¬
plied to the tips of the corrugations in the usual manner and the board is
passed through the hot plates) the heat in the hot plates will gelatinize the
uncooked starch in the paste, resulting in a tremendous increase in the vis¬
cosity and tack of the glue film. This rapid increase in viscosity brought
about by heating of the glue film produces a wet bond sufficiently strong
to hold the board together while it is cut into sheets at the end of the ma¬
chine. Heat is absolutely essential in order to gelatinize the uncooked star^
in the paste, and the starch film must reach a temperature of at least
to 150° F. This is an important difference between this adhesive and sili¬
cate, inasmuch as silicate-combined board will set up in the stacks even
thouo^h the w'et bond is weak on the board coming off the machine, v it
starch paste, however, practically no bond will be formed if there is too
little heat on the hot plate section of the machine. Insufficient heat is in i
cated by a very weak bond and a glue line which has a white granular ap¬
pearance caused by the presence of ungelatinized starch. . , , . • .

The preparation of the above starch adhesive must be carried oi
the corrugating plant under the direction of a man who has been especia }
trained for the job. A number of different ingredients are used.

18 J. V. Bauer, U. S. 2,051.025 ( Aug. 12, 1936)

19 1 V Bauer, U. S. 2,102,937 (Dec. 21, 1937)

20 j: v! bZt, U. S. 2,212.557 (Aug. 27. 1940)

XX. L.AMIXATING AND PASTING

1225

modified (pearl) starch is used as the ungelatinized (uncooked) starch
component because it gives the greatest increase in viscosity on heating.
Borax is added to increase the viscosity developed by the uncooked starch
on heating. Sodium hydroxide is added to lower the gelatinization temper¬
ature of the uncooked starch, so that the starch will be readily gelatinized
in the hot plate section of the corrugating machine. Enough sodium hy¬
droxide should be used to lower the gelatinization temperature of the
starch to 140® F. For the carrier or gelatinized starch portion, a partially
modified starch is generally used, and this is gelatinized with heat or with
sodium hydroxide. The principal function of this carrier is to prevent the
settling of the raw starch granules and to keep the adhesive mixture from
soaking too rapidly into the paper.

A formula for starch adhesive suitable for domestic corrugated board
is given below. This grade of board is generally made with two liners of

well-sized kraft and a corrugating medium of unsized chestnut, straw,
kraft, or semichemical.

No. 1 portion (carrier or gelatinized portion) . 448 lb. cold v/ater

117 lb. slightly modified starch
16 lb. sodium hydroxide in 30 lb. water

No. 2 portion (uncooked starch portion) . 1520 Ib. cold water

16J4 lb. borax

«... ^ , ,r unmodified starch

wetting agent, such as sulfonated castor oil (about 3 pints), is sometimes added

if the paste is to be used with highly sized kraft liners. A preservative such as for¬
maldehyde is also added.

The No. 1 and No. 2 portions are prepared separately. The No. 1
portion, which is highly viscous or gummy, is added to the No. 2 portion,
which consists of a thin slurry of starch in water. After mixing the two
components vigorously for fifteen to twenty minutes, the paste is ready for
use. The paste is thixotropic and tends to thicken on standing, and con¬
sequently must be agitated continuously to break down the false body. Ac¬
curate control of the viscosity is essential. The consumption of paste is

approximately 12 lb. of liquid paste per 1,000 sq.ft, of double-face board
(two glue lines).

Water-resistant starch adhesive suitable for corrugating can be pre¬
pared by using urea-formaldehyde resin in combination with starch. In
preparing this paste, the starch in the carrier portion (No. 1 portion) is

not gelatinized with sodium hydroxide, but rather it is gelatinized with heat.
A satisfactory formula is given:

No. 1 portion (carrier or gelatinized portion) ,. 117 lb. sligl.tly modified starch

11 / Ik j- , . water

(Cook with live steam Ys 190” fT

1226

PTn.1* AND I’APKR

No. 2 portion (nngclatinized p(jrtion ) . 585 lb. unmodified starcli

1100 lb. water
52 lb. alum

(Mix for 15 minutes)

After preparing each portion separately, the Xo. 1 portion is added to
the No. 2 portion. Then, after cooling to 80 to 85° P'., 105 Ih. dry urea-
fornialdehyde resin dissolved in 150 11>. water is added. The pa.ste is
finally mixed, and the />H is checked and adjusted to 4.0 to 4.5. If regular
alkaline starch paste or silicate has been used in the system previously, the
pipelines in the paste system should be thoroughly cleaned with water
containing a small amount (tf alum, so that all residual alkali in the system
will he neutralized.

I'he above is known as weatherproof i)aste, since it jwoduces a board
which is resistant to water. The board, which is generally made from two
highly sized kraft liners and a highly sized kraft corrugating medium, is
used mostly for overseas shi])ping cases and for domestic cases where the
box must stand up under conditions of high moisture content. Properly
made weatherproof board can withstand at least twentj'-four hours’ immer¬
sion in water without delamination. The consumption of this paste is
higher than that of regular paste, being in the neighborhood of 20 to 22 11).
of liquid paste per 1,000 .sq.ft, of double-face board (two glue lines).

Within ver}" recent times, experiments have been carried out on the
use of water-soluble ketone-aldehyde resins for making water-resistant
corrugating pa.ste with starch. It has been claimed that 5% of this resin
based on the starch will produce a water-resistant bond. Heating to
140° F. is required to cure the resin-starch bond, and resorcinol to the ex¬
tent of 5^ on the weight of the resin is helpful in some cases in de\eloping
a rapid cure. These resins are interesting hecau.se they can be cured at
/>H values between 10 to 11 and hence can lie used in regular alkaline pa.ste.

Starch Adhesives for Bag Pasting. Starch adhesives are used for
the pasting of kraft grocery bags and also for the pasting of 100-lb. multi¬
wall kraft shipping hags. Several types of starch adhesive are used, de¬
pending upon the tvpe of equipment and whether the paste is used on the
seams or on the bottoms of the bags. One important requirement for all
bag pasting adhesives is that they do not string or throw when applied b>

the disk applicator, since this spots the hags with glue.

On a Potdevin, or similar hag machine, where the complete grocery
bag is made in one operation, the seam paste must be rapid setting so t t
the seam will not slip or move when the bottom of the bag is pasted a ew
seconds later. This means that a high-solids berated dextrin is required

for pasting the seams. ,

Generally speaking, the same type of seam pastt is used for brth

arocerv bags and multi wall kraft shipping bags. This same paste a

XX. r.AMlNATiNG AND PASTING

1227

used for holding together the plies in a multi wall bag (where the glue is
applied in spots to hold the various plies together). A typical seam paste
is;

90 lb. converted dextrin
10 lb. borax

0.5 lb. sodium hydroxide
30 gal. water

Bottom pastes do not have to be fast setting, since the adhesive has a
chance to set while the bags are being stacked. Consequently, low-solids
pastes are used for pasting the bottoms. Unmodified (pearl) starch is often
used, and in some cases special starches thicker than pearl starch are used
for this purpose. These lower the pasting costs because of the low solids
at which they are used, and the lard-like consistency of this type of paste
makes it easy to squeeze the paste into the corners and folds. On bag ma¬
chines wdiich paste the seams and bottoms in two separate operations, it is
possible to use the bottom paste for both operations. A typical bottom
paste is;

100 lb. thick-boiling starch Wfif®

75-80 gal. water
0.5 lb. soap

0.5 lb. sodium hydroxide

1 he addition of sodium hydroxide to the seam and bottom paste is de¬
sirable only if highly sized and dense paper is used. If the paper is weakly

it is desirable to leave out the sodium hydrox¬
ide and, in extreme cases, to add an anti-penetrating agent such as ben¬
tonite or vegetable gum. The paste should be as rapid-setting as possible

in order to minimize penetration and prevent staining of the outside of the
bag.

Water-resistant adhesives are required for pasting the seams and bot¬
toms of multiwall bags used for packing potatoes and chemicals. Starch-

eli\ de resin glues have proved satisfactory for this purpose
(see pages 1220—1222). A formula is given below:

100 parts special dextrinized starch (or a mixture of thin-boiling starch and dextrin)
o lU parts powdered urea-formaldehyde resin
2~3 parts ammonium chloride

^‘ '™y heated together to a temperature of

ys to 200 F. for fifteen to twenty minutes and then cooled to room tem¬
perature This paste forms a highly water-resistant glue line within a pe¬
riod of three to four days after the bags are pasted. The paste has a tend¬
ency to thicken on standing. Hill and Sliwinski^’ recommend the addition

E. H. Hill and V. X. Sliwinski, Tappi 32, No. 7: 297-299 (July, 1949)

1228

PULP ANU PAPER

of 5% sodilun chloride and 1% hydrogenated vegetable fat for increasing
the pot life.

Starch Adhesives for Pasting Bristols. Unmodified starch is used
for pasting wedding bristols, announcement cards, menus, etc. In this
j)rocess, the starch (pearl) is cooked at high concentration to jjroduce a
solid gel, which is then applied to the paper in excess. The paste is fed
to the paste rolls in large chunks or fed to a grinder which disintegrates the
paste before pumping it to the paste rolls. The paper is run between press
rolls to squeeze out the excess paste, after which the laminated sheet is
run over driers.

Unmodified or thick-boiling starches are used because they produce
maximum stiffness and keep the costs low. The pasted sheets are gen¬
erally plated to produce either a vellum or antique finish. One of the im¬
portant final requirements is a high degree of snap, which can be judged
by flexing one corner of the sheet to test the speed of recovery.

Starch Adhesive for Tube Winding and Carton Sealing. Pre¬
pared starch adhesives are used for making paper tubes and paper cans.
In making these products, glue is applied to a strip of paper which is then
wound, either in a spiral or a straight winding, into a tube. The tube is
then cut to size on leaving the mandrel. Borated dextrin-type adhesives
ranging in solids from 33 to /O^c are used for this purpose. A fast-setting
rate is one of the requirements. Mixed animal glue and dextrin adhesives
are sometimes used, i.e., when the label is stuck to the tube with the same

paste used for making the tube.

Considerable quantities of dextrin adhesives are used for case sealing
and carton sealing. Usually these glues are sold by the adhesive manu¬
facturer in liquid form. A large number of products is available. Fast
setting is the most important requirement, since the machines are operated

at very high speed.

Sodium Silicate

Sodium silicate is widely used as an adhesive for the combining of cor-
ruo^ated paperboard and for the lamination of solid fiber board. It is also
used for laminating aluminum foil to paper. Sodium silicate is a relatively
inexpensive material and is sold in tanks cars in ready-to-use form

When selecting silicate, there are several factors which must be con¬
sidered, among the most important of which is the ratio of SiO^ to . a, ,
since the ratio of these two ingredients affects the viscosity, gelation ch^-
acteristics and specific adhesion of the silicate. Silicate is a\ ai a e co

ranging from 2Na,0 • ISiO. to ■ 4.0SiO,. Silicates hatnng

a ratio o'f about 1 to 2.8 (soda to silica) have the greatest coheire
whereas silicates having a ratio of above 1 to 3.o ^

lowest cohesive strength (in the wet state), but have the highest g

XX. LAMINATING AND PASTING

1229

At ratios of silica to sodium of over 4.2 to 1, the solution is unstable and
tends to form a jelly-like semi-solid through slight evaporation of water.
The most common silicate for laminating is Na-O • 3.3Si02, which is gener¬
ally sold in liquid form at 42® Be. (approximately 39% total solids). The
sodium oxide (Xa 20 ) content is determined by titrating the silicate (after
dissolving in hot water) with 0.2 A' hydrochloric acid, using methyl orange
as the indicator. The silica content (SiOg) is determined by adding addi¬
tional acid after titration in several treatments with intermediate evapora-
ions and finally digesting on a steam bath. The silica is determined after
ignition by weighing the residue.

All solutions of silicate above the ratio of NaaO * l.SSiOo can be con¬
centrated to viscous licpiids. The solutions are truly viscous, but as the

Fig. XX-2. Effect of sodium oxide to silica ratio on t!ie vis¬
cosity of silicate solutions of different concentrations.

solution is concentrated (as in the drying of adhesive films), the viscosity

increases gr^tly and the solution changes from a fluid to a tough, plastic

mass. If silicate is reacted with acids, a gel of high mechanical strength and

rigidity is obtained. The gel is irreversible and cannot be redispefsed by
neutralization of the acid.

The highly siliceous silicates (i.e., those with high ratios of silica to
^iiim oxide) show the mo.st striking vi.scosity change for small changes
in concentration. This is shown in Figure XX-2,*" where the effect of

Company, New

1230

PULP AND PAPER

sodiiini oxide to silica ratio on viscosity is plotted at several different con¬
centrations. The high-silica silicates not only tend to increase in viscosity
more rapidly, but also produce more elastic gels. The viscosity is greatly
affected b)^ temperature, particularly in the case of the highly siliceous
(least alkaline) grades. The viscosity temperature relationship is prac¬
tically a straight-line one, the slope increasing as the ratio of silica is in¬
creased. At high concentrations silicate acts almost as a thermoplastic
material.^®

Clays are sometimes added to sodium silicate to increase the thixo¬
tropic properties and increase the setting rate. The addition of clay in¬
creases the solids content and thus reduces the amount of water which
must be evaporated to set the adhesive. On the other hand, the addition of
clay makes it possible to use a more dilute liquid phase which improves the
wetting properties of the adhesive for the highly calendered and hard-sized
papers which have come into general use.^^ Clay also acts to control the
j^enetration of adhesive into the paper l)y helping to contain the residual
silicate in the glue line.

Dilute solutions of silicate freeze fairly readily, in which case the crys¬

tals appear on the surface while the silicate at the bottom becomes more
concentrated. Thawing and remixing, however, return the silicate to its

original state.

Use of Silicate in Solid-Fiber Laminating. Silicate produces a
strong, hard, elastic bond, although the bond may be brittle if the glue line
is too thin or overdried. Silicate yields a water-resistant film, but the high
alkalinity destroys the sizing on the fibers adjacent to the glue film, thereby
causing them to let go when wet. The alkali in silicate has a tendency to
migrate into the hoard under conditions of high moisture, and in some cases
where the hoard has been stored at high humidity, the alkali has been known
to migrate to the surface and produce staining. This alkali destroys the
size, discolors the fibers, and attacks alkali-sensitive inks and dyes. It is
generally believed that this diffusion is a sort of hydrolysis which leaves
most of the silica in [ilace and permits the sodium to diffuse to the surface
of the l)oard.=>^ Coating the outside liners of the board with reactive chemi¬
cals overcomes the troubles with staining.-'^ Precoating of the paper with
aluminum chloride before laminating is said to increase the water resistance

of the combined hoard.*'*'* ... n i f

1'he coirsiimption of silicate in .solid-fiI)cr laminating is genera y a )OU

6 to 8 lb. of .solid silicate or, in other words, alioiit 18 to 20 lb. of n\\w
»E. R. Holler. J. G. Lander .md R. Morclionse, Pafcr Trade J. HO, No. 12:

e- L n' WifeRaArV’vVo* J. >27. No. 2 : 304-307 (July 8, 1048)

25 j. D. Carter, U. S. 2,015,359 (Sept. 24, \9^S)

2«J. D. Carter. U. S. 2,414,360 (Jan. 14, 1947)

XX.

LAMINATING AND PASTING

1231

silicate per thousand square feet of glue line. This adds more weight to the
hoard than is the case with most adhesives and is, in general, an advantage,
since weight is a factor in pricing the hoard. I he silicate also adds con¬
siderable stiffness to the board. The maximum adhesive strength in fiber
board laminated with silicate is apparently not obtained until about four
hours after lamination,

Use of Silicate in Corrugating. Until recent times, silicate was
about the only adhesive used for making corrugated paperboard. It is still
widely used although it has been replaced in some cases by starch (see
previous section). Silicate has the advantages of high solids content, rapid
setting rate, and strong adhesive qualities.

The decided increase in viscosity which silicate undergoes upon losing
only a small amount of water accounts for the fast setting rate. The quick
set of silicate adhesives with small water loss is also a factor in preventing
softening of the paper. The finished product has, therefore, a smooth sur¬
face and the flutes maintain the round tip formation which results in maxi¬
mum strength and caliper. The operating performance of silicate is largely
determined by the water loss required for initial set, since this determines
the maximum operating speed.The water loss required for set varies
from about 1.35 to 1.50 lb. of water per 10 lb. of silicate.In some

^ ^ Q

cases, a low viscosity silicate is used on the single facer and a high viscosity
silicate is used on the double backer. Silicate-clay mixtures are sometimes
used where exceptionally fast setting is desired.

One objection to the use of silicate for corrugating is that it forms
hard, glassy deposits on the machine. However, there are additives which
the manufacturer recommends to lessen this problem. An advantage of
silicate is the high compression strength of the combined board resulting
the presence of shoulders of dried silicate between the corrugating

medium and the liner. Enough glue should be applied to produce these
reinforcing shoulders.

The consumption of silicate in corrugating runs from 18 to 23 lb. of
liquid silicate per thousand square feet of double-faced board, depending
upon the type of paper used. The size of the flute is also a factor; for ex¬
ample, when 18 lb. of silicate are consumed for A flute board, a comparable
consumption for B flute board would be 22 lb. The desired degree of pen¬
etration of silicate into the liner is no more than 0.002 in.^^

H. M. Hale Secotid Progress Report, Project L-20/-4, Forest Products I ih-
oratory, Madison, Wisconsin (Mar. 11, 1919)

Chem. 41: 81-86 (1949)

(May 9.%46) 122, No. 19; 201-207

31 ?■ : 23&-240 (May. 1949)

176^(M21"’h 110 No. 12: 167-

1232

PULP AND paper

Protein Adhesives

At one time proteins were the most important adhesive material used
in the paper industry. Today, this is no longer true because protein ad¬
hesives have been replaced to a considerable extent by cheaper materials
such as starch, dextrin, and sodium silicate adhesives. Casein, animal
glue, albumen, soluble dried blood, and soybean protein are now rarely used
except for specialty papers.

Protein glues have some very definite advantages over other adhesives.
They produce stronger bonds than starch or dextrin glues, which is impor¬
tant for some purposes, although of only minor importance in the lamina¬
tion of paper products, because nearly all glues are stronger than paper.
Protein glues tend to produce more flexible and continuous films than either
starch or sodium silicate, which is useful for the lamination of stock for
paper cans and bottles where oil resistance is desired. Another advantage
is that proteins can be formulated so that they produce highly water-re¬
sistant glue lines, but this has become of less importance since the recent
development of the starch-urea formaldehyde glues. Some of the more
important protein glues are discussed in the following sections.

Casein Glues. Casein dispersed in alkali makes a strong adhesive.
The most important alkali used for dispersion is sodium hydroxide, al¬
though lime is frequently added when water resistance is desired. An ex¬
ample of a water-resistant casein glue suitable for gluing wood veneers is

given below:

Casein .. parts

■\Yater . 200-230 parts

Hydrated lime . 20-30 parts

Water . 100 parts

Sodium silicate . ^0 parts

Casein adhesives can be rendered highly water-resistant by treatment
with a tanning agent. The agent can be mixed into the adhesive, if care is
taken to prevent coagulation. Under these circumstances, water resistance
develops gradually upon drying of the adhesive film. Formaldehyde, para¬
formaldehyde, and copper salts are some of the materials which can be
added to casein glue to increase the water resistance. Rubber latex, urea-
formaldehyde resins, and other similar materials may be added to casein

glues. . , ^

Soybean Glues. Soybean protein can be used m the same way a

casein. For some purposes, soybean protein is preferred because o i s

lower viscosity and greater tolerance for formaldehyde.

Soy flour is used to some extent in the adhesives industry, but mam y

as a filler, since it has very little tack of its own. Usually the flour is is-

XX. LAMINATING AND PASTING

1233

perved in sodiuni hydroxide, and for special purjxises. a little carbon disul¬
fide may be added.

Blood Glues. Soluble dried blood obtained from the meat v>acking
indu^ry can be used as an adhesixT after dispersing in w'ater with ammonia.
Blood glues are heal gelling and can be readily converted into a water-
resistant form by heat alone. Blood glues have l>een used in plywood man¬
ufacture ahere a a'aier-resi^iant bond is desiretl. Recently, however, blood
glues have been replaced to a large extent by the urea- and melamine-
formaldehyde resin glues.

Animal Glues. Animal glue is a protein adhesixe which is still l)eing
used to a large extent in tlie [xa{>er industrx’. In pre|>aring animal glue for
adhesive purposes, the dry glue (in eillier ground or flake form) .should l>e
stirred into cold water and allowed to soak for .several hours, .\fter soak¬
ing, the glue should be melted by heating in a xvater- or steam-jacketed tank
to a temperature of 50 to 60* C. High temixrraturcs and long heating
should be ax'okled. Ordinarily, aliout 2 to 4 ]iarts of xx'ater are used for
each |jart of glue. dc^«cnding on the jelly test of the glue.

One of the large uses of animal glue is for tiu: prcxhiction of gummed
P*|*t^*» which is described in a later section. Animal glues are also used
to a large extent as booklnnding adhesives, for folding and set-up boxes,
fur tube winding an<i cans, and enx'elojjes. A typical liookbinding adhesix'e
can he made from 25%' glue, 25*^ glycerin, and 50% water, together with

a small anxiunt erf preservative and teqiineol for masking the oilor of the

glue

For making water re^isUnt glues using animal protein, a small amount
of formaldehyde, hexamethylenetetramine, or [jaraformatdehyde should Iw
added About I to 5% fomuldehyde on the dry weight of the glue is or-
dinarity used. I’rea. rakium nitrate, glycerine, sorbitol, or similar coin-
posmds are sometimes added to animal glues to plasticize the glue film and
imprmre the rrmoistentng pro^ierttes.

Fish Glues. Fish glues, which are derived from the heads, skins, and

other parts of fish, wxre once wklely used. These glues are now n.sed very

seldom, although they find some use in frfintoengraving and in the manufac¬
ture of gummed labels.

Other Aqueous Adhesh'es

^TVre are many caber water-soluble materials which can l>e used for
a^ive purposes. Among these are the water-soluble cellulose derivatives
|»I.Tv»nyl alcohol. vegeUl4e gums cereal flmirs, and XAater-vrfuble svnthetic

alcohol makes a verx* desii
m-lacky. but a strong bond

1234

PULP AND paper

is highly resistant to water and requires no heating or curing period. The
commercial product contains a high percentage (50%) of clay which re¬
duces the cost. Good results can be obtained with thin films, which helps
to further reduce the cost. Polyvinyl alcohol can be mixed with starch, but
the starch must not contain borax, alum, or formaldehyde since these ma¬
terials cause gelling. Polvwinyl alcohol is incompatible with sodium silicate.

Among the vegetable gums, gum arabic is the most widely used adhe¬
sive. This material can be dissolved in 2 to 3 times its weight of water to
produce a strong adhesive. Gum arabic is used as a remoistening adhesive
on cigarette papers. However, the high cost of gum arabic in comparison
with other adhesives limits its use.

Gummed Papers and Paper Tapes

Gummed paj^ers are widely used for the manufacture of stamps, seals,
labels, sealing tapes, presized wallpaper, mending tapes, etc. Gummed pa¬
pers and paper tapes have a surface coating of water-soluble adhesive which
is remoistened with water just before use to produce a tacky quick-setting
adhesive. Other special types of gummed paper contain a coating of water-
insoluble adhesive which can be activated by organic solvents, by heat, or
by pressure.

Gummed papers are made on a gumming machine which consists of a
fountain roll, an applicator roll, and a pressure roll. The thickness of the
adhesive film applied to the paper is determined by the pressure between
the applicator and i)ressure rolls. The adhesive is generally applied hot.
After coating, the sheet is passed through a drier which may be the festoon
type or the tunnel type. In the tunnel type, tension on the sheet prevents
curling, whereas in the festoon drier, curling is held to a minimum by care¬
ful control of the humidity and temperature. The adhesive film should
never be dried below a moisture content of 4.5%. After drying, the paper
can be pissed over a breaking machine to crack and break the gummed
layer. The film is generally broken in two directions by passing it over the
breaking bar twice. This reduces the tendency toward curl, but even so curl
is a problem with gummed papers. Animal glue, dextrin, casein, fish glue,
and gum arabic are some of the adhesives used in making gummed paper.
Animal glue and dextrin are the most important. About 6 to 10 lb. of ad¬
hesive is generally applied (17 x22—500).

Remoistening Gums. Highly

age stamps, envelope gums, and paper labels. Tapioca and potato dextrins
are preferred. Ordinary corn dextrins are considered inferior, but excel¬
lent dextrins can be produced from the waxy varieties of corn. The re¬
quirements for a good dextrin are low viscosity, high solubility, low reduc¬
ing sugar content, fast drying rate, and high film gloss. Low reducing

XX. LAMINATING AND PASTING

1235

sugar content is desirable in order to prevent discoloration due to the reac¬
tion of the sugar with the degraded protein-like products in the paper. Fast
drying is desirable because it permits rapid machine operation. The glue
should not string or cotton, since this interferes with the operation of the
machine.

A high percentage of plasticizer (glycerine, corn syrup, etc.) is gen¬
erally used to reduce curling and brittleness and to improve the remoisten¬
ing properties. Too much plasticizer must not be used, however, as this
causes blocking of the film. Urea is very effective as a plasticizer when
used in high percentages.^^ Wetting agents such as sulfonated castor oil
are added, and sometimes oil of wintergreen or other essential oils are used
for flavoring. Borax can be used to improve the tackiness. Preservatives
may be added in some cases.

Stamps and labels are made of sulfite paper, usually in a l5asis weight
of 50 to 60 lb. Paper lighter than this is likely to result in strike through
of the adhesive, whereas heavier paper is expensive and likely to be hard to
paste. In general, labels for bottles are cut so that the machine direction is
horizontal. The paper should be fairly well sized in order to prevent ex¬
cessive penetration of the glue. A smooth surface is important where the
paper is to be printed.

Sealing Tapes. The requirements of an adhesive for sealing tape are
many. The glue film must be non-blocking, but must be capable of absorb¬
ing water evenly. The film must he tacky and quick setting when mois¬
tened. However, the film must not set too rapidly, since a certain amount
of slip is desirable in order that the tape may be moved slightly after it is
put in place. The film must be thick enough so that it will be softened no
more than one-half of its thickness, regardless of how much water is used
for moistening.'*® On the other hand, the film must not be so thin that there
will be inadequate bonding when the tape is applied to porous papers. As
a final requirement, the adhesive strength of the glue must be exceptionally
high. This last property can be measured on a gummed tape tester which

records the force necessary to break the bond produced by the moistened
tape.

Animal glues fulfill the above requirements better than starch or dex¬
trin adhesives and hence are most widely used for making strong paper
tapes. Generally, a mixture of hide and bone glues is used. Kraft paper is
generally used because of its good strength. Three grades of base paper
are used: light weight (35 to 40 lb.), medium weight (60 lb.), and heavy
ueigit (90 to 120 lb.). The amount of adhesive varies with the weight of
the paper, being about 15 lb. or less for the light-weight grades and 18 lb.

''qj’24.^939) ■ 2.144.610, and 2.145,195

3® Fibre Containers 60-62 (June, 1945)

1236

PULP AND PAPER

or more for the heavy-weight grades. The finished tape is usually double-
wrapped to protect the glue from excessive moisture.

A special patented grade of sealing tape, called Sisalkraft, is a gummed
product consisting of two plies of kraft paper (a 50- and a 30-lb. sheet)
laminated with asphalt which is reinforced with sisal fibers. The sisal fibers
embedded in the asphalt run either lengthwise or crosswise, depending
upon the type of strain which the tape is subjected to in use. About 25 to
30 lb. of animal glue are applied in the gumming operation and the paper
is embossed. Special care is required in drying of the glue film to prevent
delamination of the asphalt; the drying temperature must be kept below

200° F.

Emulsion Type Adhesives

Emulsion type adhesives have become increasingly important in recent
years. Among the most important of the emulsion type adhesives are rub¬
ber latex, synthetic rubber latices, synthetic resin emulsions, and asphalt
emulsions. In general, emulsion adhesives consist of a resinous base ma¬
terial dispersed in water and a plasticizer. Compounding of the adhesive
can be regulated to control the drying rate, flexibility of film, and similar
conditions. In some cases, a solvent for the resin is used, particularly
when the adhesive is to be used on impervious surfaces (e.g., lacquer-coated

papers or on cellulose acetate sheeting).

Emulsion adhesives have many good features. They can be used at
very high solids content, which means that their setting rate is high and
only a small amount of water is added to the product. Setting is rapid be¬
cause the emulsion breaks as soon as it contacts the paper surface. Emul¬
sion adhesives can be made to produce water-resistant bonds and, in some
cases, water-vapor-resistant bonds. Reduced warp and greater film flexi¬
bility are other advantages. .

Resin-emulsion adhesives can be made to work where other adhesives

are unsatisfactory. Resin-emulsion adhesives combine the desirable fea¬
tures of both aciueous and non-aqiieous base adhesives. Thiine^^ points out
that acetate sheeting can be readily laminated to chipboard with a resin-
emulsion adhesive because the water in the emulsion wets the paper at t ie
same time that the organic solvent in the emulsion ivets the acetate, t lere y
resulting in good adhesion on both surfaces. In contrast, a lacquer type
adhesive would adhere well to the acetate sheeting only, whereas an aqueous
base adhesive would adhere only to the paper and not to the a^tate sheeting.

Emulsion adhesives are used for carton sealing, tuhe winding, and to

the combining of special papers. High-solids (^Sfe) «^"'-™''TThe
hesives have hcen suggested for the comhining of corrugated board.* The

"• .S. F. Tliuiic, F»rc Cmlaiiicrs

30 W. F. Gillespie, Fibre Coulanters 59-62 (Sept., 194/)

XX. LAM1N-\T1NG .\XD PASTING

1237

chief disadvantage of emulsion type adhesives lies in their high cost, but
on a dry solids Ijasis, they are less expensive than the lacquer types. Very
thin films should l)e used, since their use promotes fast setting. Emulsion
type adhesives are sometimes used in combination with starch, de.xtrins,
animal glue, casein, soyljean protein, and other aqueous adhesives.

Rubber Latex

Kublier latex is an emulsion type adhesive which is widely used for
laminating metal foils to pai>er. WVappers for gum and candy are fre¬
quently made by laminating a thin sheet of aluminum foil to a paper base
with rublier latex. The advantages of rubber latex lie in its strong adhesion
{or metal and its highly flexible film. Rubber latex tends to form a skin on

the surface which cannot be readily dispersed again.

A small amount of casein, glue, or technical gelatin is sometimes added
to rubl)er latex to act as a stabilizer and prevent coagulation of the latex
by the action of the applicator rolls. C'lays are frequently added to rubber
latex adhesives and. in special cases, vulcanizing agents, such as sulfur and
zinc oxide may l>e added in emulsified form. Materials added to rubber

latex should liave the same surface charge (negative) as the latex particles
to |)revent coagulation.

\\ hen used by itself, rubber late.x forms a water-resistant film. If
mixed with .starch adhesive, the final bond has only slight water resistance.
If latex IS to l)e mixed with a starch adhesive, the starch should contain
addetl alkali in order to prevent coagulation of the rubber particles.

Synthetic rublicr laticcs (butadiene-styrene copolymers, polychloro-
prene, and butadiene-acrylonitrile copolymers) have been introduced in re¬
cent years, and promise to l)e more useful adhesives than natural latex.
The.se materials are often of complex character and fre(iuently contain cur-

ing agenl.s which are added to increase the water resistance or strength of
the final Ixind.

Asphalt Emulsions

Bituminous emulsions are sometimes used for laminating. These
emulsions are similar to the u.sual asphalt emulsions, except that the bitu¬
men IS plasticized so that it is of a tacky nature. The disper.sed particles
or asphalt are aU.ut 1 to 4 microns in diameter.

A.-phalt enitil.sions are usually mixed with starch or other adhesives •
T inex,»nsive extender. Borated .starch

'>ut plain oxidired

to the'* Some asplialt emulsions do not add much

to the water r^rstance of the starch film or even produce a very water-

resvstant film hy ,hem.se1ves. Recently, however, new asphalt cLdstns

'V n pr need for which increased water resistance is claimed.

1238

PULP AND PAPKR

Resin Emulsion Adhesives

Resin emulsion adhesives consist of a resinous material dissolved in
an organic solvent and emulsified in an aqueous medium. Emulsions can
be produced in this way which are stable against aging, dilution with water,
and agitation. The large number of resinous materials which are available
makes it possible to produce resin emulsion glues having widely different
properties, i.e., emulsions can be selected for particular properties such as
water resistance, grease resistance, and solids content. Plasticizers can be
added to control the hardness and fle.xibility of the film. Special solvents
may be added to aid in the penetration of lacquer coatings, cellophane, cellu¬
lose acetate, or similar surfaces.

Polyvinyl acetate emulsion without solvent phase is used as an adhe¬
sive, often in comlnnation with starch. P'or best results, the laminated
sheets should be heated at 100 to 125° C. to bond the resin. Acrylic acid
esters in emulsion form are also used as adhesives, often in pressure-sensi¬
tive or heat-sensitive self-sealing tapes. These resins have better aging
characteristics than natural rubber latex and retain their tack better. They
are compatible with synthetic rubber latices and other synthetic resins and
often act as plasticizers and modifiers for the dry film.

Certain of the rosin derivatives, such as the hydrogenated methyl
ester of abietic acid (Hercolyn) and methyl ester of abietic acid (Abolyn),
are used in emulsified form as adhesives. They are recommended for use
with starch. Pentaerythritol abietate is sometimes used with rubber latex
to improve' the stability. The rosin-derived alkyd resins (Keolyn resins)
can be emulsified and blended into latex type adhesives to improve the
strength, flexibility, and adhesion. These resins can be emulsified by dis¬
solving hi xylene and then emulsifying with the aid of potassium hydroxide
and oleic acid and stabilizing with ammonium caseinate. There are many
other materials which can be used as emulsion type adhesives, but the above

serve as typical examples.

Lacquer Type Adhesives

Lacquer type adhesives are useful for bonding materials which are
impervious to water and cannot be laminated with the aqueous or emulsion
type adhesives. Moistureproof cellophane and varnished or lacquered pa¬
pers are examples of materials which cannot be bonded with aqueous ad¬
hesives because the adhesive will not wet the surface, and there is no way
for the -lue line to drv out. Lacquers work satisfactorily on these surtaces,
becauseVe solvent in the lacquer will penetrate these surtaces. Lacquer
adhesives are also useful for the bonding of porous to non-porous surtaces,

such as the pasting o{ paper labels on glass or metal

Lacquer 'type adhesives are useful for laminating nvo paper surtac

XX. 1_\MIXATING AND PASTING

1239

whore a niiniimun «*f diMonion or curling

is desired.

The solvents iised in

lac<|uer adhesives do not swell the filK'rs, and ct>nsequently, there is no dis-
torlum of the ]>aj^>er anti no j>rol)leni of swelling, curling, or warping. 1 his
feature is particularly useful where the j>aper must he pasted under close
tolerance (such as the jasting of map jxapers on a globe), or when no curl¬
ing can lie tolerated (such as in i)lu>tographic mounting). The high cost
of lactpier adhesives limits their use to specialty uses. Bleeding of the ink
on the surface of the jiaper sometimes results from the solvent in the ad¬
hesive.

The .same general considerations apply in the formulation of lacquer
tyjie adhesives as prevail in the use of these materials for protective coat¬
ings. Most of these factors will Ik; discussed later and the reader is referred
to the cltapter on coating with resinous materials. As a rule, solvents
with high evajKirating and ditfusing rates are desiretl for adhesive purposes
in order to increase the rate of setting. This is in contrast to the delicate
iialance of solvent mixtures retjuired in laetjuers for surface coating, where
iIr* him cliaracteristics arc of primary imi)ortance.

Modified vinyl acetates can lie u.sed as solvent adhesives. These pro¬
duce dried films which are tough and flexible and have good adhesion to
non-|x>rous surfaces. Polyvinyl acetate dissolved in methyl acetate, acetone,
hydnxarlKnis, or similar .solvents is particularly effective on cellulo.se
acetate >heeting. Polyvinyl chloride acetate can also be used, and a typical
tormulation is:“

20 parts |Kj|yvinyl cliluridc acetate
32 parts nietliyl ethyl ketone
32 parts cycloiiexanoiie
16 larts propylene t».xtdc

ThcM; a<Ihesives are used for sealing cups for hot drinks.
.\n example of a nitrcKrellulose lacquer adhesive is

4 5 parti nitrocellulose
I f«rt plasticizer

ino part* of mixture containiiift solvent an<l 60% non-solvent

Kithcr synthetic or natural resins can Ik; added. In some cases, a nuich
higher percentage of plasticizer may lie recpiired. Xitrtxrellulose adhesives
are suitable for joining moistureproof regenerated cellulose sheeting.

A very tacky, high-vi.scosity cement can Ik; made by rlissolving rubber
in gasoline, naphtlia, Ijenzene, carlion tetrachlorirle, trichloromethylene, or
similar solvents. The viscosity deiiemls uikki the degree to w hich the rub-
l>er is iuasticate«l and the amount r»f adde<l filler. Dammar, manila, ester

“J. I>cl SUmtr. Thf Tfchnology nf Adhesh-cs. p. 124. Reinhold
poration. Xrw York. Y. (1947)

*■ A. C Ritman, U. S. 1.969,477 (1935)

Publishidfr

Cor-

1240

PULP AND PAPF.R

gum, coumarone-indene are examples of resins which are sometimes added
to rubber cements.

An example of a laccpier made from rosin is:’’*

Rosin . 29-707o

Venetian turpentine . 70-29%

Diethylene glycol ether . 1-20%

A suitable adhesive for many papers can be made by dissolving manila
resin in alcohol and an aromatic thinner. Shellac dissolved in alcohol has
been used for pasting labels. Hydrogenated methyl esters of rosin (Her-
colyn resins) can be used in solution form in combination with a film¬
forming resin and non-film-forming resin to produce adhesives which vary
from non-tacky adhesives to pressure-sensitive adhesives. Glycol esters of
hydrogenated rosin (Staybelite resins) can be dissolved in suitable solvents
to produce a very tacky adhesive. These resins can be mixed with rubber,
ethylcellulose, chlorinated rubber, polybutene, and nitrocellulose lacquer
adhesives. The coumarone-indene resins can be used in solvent type ad¬
hesives of the rubber cement type or the pyroxylin cement type. The
phenol-modified grades are recommended.

Hot-Melt Adhesives

Hot melts are discussed in the chapter on coating with resinous ma¬
terials (see p. 1307). Laminating wdth hot melts is similar in many ways.
The materials used in hot-melt adhesives are waxes or wax-like solids. A
typical mixture is composed of a wax, a synthetic resin, and a plasticizer,
but more complex mixtures are often used. Hot-melt adhesives are very
fast-setting.

Hot-melt adhesives are particularly useful for laminating materials
which are impervious to water or organic solvents. They can be used for
combining glassine, chipboard, kraft, cellophane, cellulose acetate sheeting,
metallic foils, etc. The most important use for hot-melt adhesives is in the
manufacture of water-vaporproof laminants, where the hot melts not only
function as the adhesive, but also serve as a continuous moisture-resisting
film. Products laminated with hot melts combine the advantages of an im¬
pregnated and a surface-coated paper. A continuous wax barrier is pres¬
ent, as in the case of a coated paper, yet the film is protected from scratching ^
and marring because it is sandwiched between two protective layers of
laminant. The film of adhesive must be unbroken to be effective as a
water-vapor-resistant barrier, and no fibers may extend through the film
to act as a wick for transferring moisture across the barrier. The wax
which penetrates the paper is ineflFective so far as moisture vapor-resistance
is concerned, since only the wax which exists as a continuous film is effectn e

38 J. E. Glegg, U. S. 1,948,3.34 (Feb. 20, 1934)

XX. LAMINATING AND TASTING

1241

in holding back water vapor. Products laminated with hot-melt adhesives
are resistant to oil, grease, water, dilute acids and alkalies, as well as to
water vapor. Some of the products commonly packed in hot-melt laminated
products of the heat-sealing type are coffee, potato chips, tea, and dehy¬
drated sops. Hot-melt coatings for subsequent heat sealing are also used
for making papers labels.

Most hot melts are supercooled liquids, which means that they exhibit
after-flow if the laminated product is stored under warm conditions. This
causes the film to migrate into the paper, leading to a loss in water-vapor
resistance. At very low temperatures, products laminated with hot melts
tend to become brittle. The strength of hot-melt bonds is dependent upon
the rate at which the disrupting force is applied. For example, if subjected
to a continuous tension, sheets laminated with hot melts rupture at lower
tensions than if the force is applied quickly.

Applying Hot Melts

Most hot-melt laminating is carried out at a temperature below
350° F., and usually in the range of 150 to 250° F. The melt is applied in
a molten state to one of the sheets, and then is combined with a second sheet
by the application of pressure in a set of squeeze rolls. The laminating op¬
eration is continuous and requires careful control of the temperature and
speed of the machine in order to obtain a continuous film of melt between
the two plies. For best, results, the temperature of the melt in the pan
should be about 15 to 20° F. above the melting point of the melt, depending
upon the speed of the machine and the distance between the waxing and
combining rolls. The temperature of the glue line should be slightly above
the setting point of the melt by the time the comliined sheet reaches the
squeeze rolls. If the glue film is hotter than this, it will be too fluid and
will tend to soak into the board to an excessive degree. If the glue film is
much colder, it will not be liquid enough to produce a good bond with the
second ply. The film of adhesive tends to solidify when it comes into con¬
tact with the second ply, due to the cooling effect, and this prevents further
penetration of melt into the paper. In some cases, the press rolls may be
cooled to speed up solidification.

Papers Used for Hot-Melt Laminating

The type of paper used in hot-melt laminating is an important factor.
High-density papers are desirable because they absorb less melt and tend
to keep the melt on the surface of the paper. However, high-density pa¬
pers are more difficult to laminate than porous papers. For very hard,
non-porous surfaces, melts are required which have a high degree of ad¬
hesion for the surface to be laminated. Tensile strength of the melt may
have to be sacrificed to adhesive quality, but even so, the rupture of the

1242

PULP AND PAPKR

laminated product usually occurs at the interface between the wax him and
the laminant. Melts of lower adhesive strength may be used on ])orous
papers, because the laminated products usually rupture within the paper
anyway. When a hard surface is to be laminated to a porous surface, the
hot melt must be sufficiently tacky to adhere to the hard sheet. The rup¬
ture of such a laminated sheet usually occurs at the interface between the
wax and the hard-surfaced sheet. Wdien combining two dissimilar sheets,
best results are obtained by applying the hot melt to the den.ser and harder-
surfaced sheet, and then allowing a short time interval before combining
with the porous pai)er. This prevents absorption of the melt into the porous
paper and insures the formation of an unbroken film. It also aids the melt
in adhering to the flense sheet.

JFax Adhesives

Paraffin is sometimes used for laminating, but tends to produce brittle

films and relatively weak bonds. Blends of paraffin with polyisobutylene
resins may be used when improved bonding and flexibility are desired.

Microcrystalline waxes are better suited as adhesives than paraffin be¬
cause of their greater tackiness, greater film flexibility, greater grease re¬
sistance, greater water and water-vapor resistance, and greater adhesive¬
ness. The adhesion of microcrystalline wax (as measured between two
strips of cellophane) varies from 45 to 320 g. per square inch, whereas
paraffin wax has zero adhesion under the same conditions.-^^ The plastic
types of microcrystalline waxes are preferred, particularly when non-porous
materials, such as cellulose acetate foil, are to be laminated. In general,
the adhesive strength of microcrystalline waxes increases with the tensile
strength of the wax,^" but is not affected by the melting point and hardness
of the wax.^^ Oil in the wax tends to lower the strength.

The consumption of wax in laminating depends upon the type of paper
used. It varies from about 2j/^ lb. per 1,000 sq.ft, for glassine papers to
4 to 10 lb. per 1,000 sq.ft, for porous papers.

Asf'halt Adhesives

Asphalt or bitumen products are sometimes used for laminating. As¬
phalt has a high degree of ductiliW and tackiness and a high viscosit}- in
the molten state, but has a black color and a bad odor. The bad odor pre¬
vents the use of ordinary asphalt on paper for packaging foodstuffs, but
there are specially processed asphalts with reduced odor. One method of
treating asphalt is to blow steam or air into the molten product to produce

39A. Kinsel and H. Schindler. Pap,r Trade J. No. 5: <Feb 3 19J9)

49 F. W. Padgett and R. B. Killingswortli, Paper Trade J. J—, No, 19. 20

41 A^^Kins’cl and H. Schindler, Paper Trade J, 128, No. 5: 45^7 (Feb. 3, 1949)

XX. L.\MIXATING -KXD r.\STIXC

1243

l»k*uii a>phah. Tlu* trcatcti |)riKluct ]»rtKluces a U»uglu*r, more flexible
film, and lehS suMreptible lo tem|»eralure change than natural asphalt.

Asplialt is used for combining solid fil)er and corrugated j>aperboard,
and for the laminating of inutli-ply wrapping and packaging papers. A
layer of asplialt lieiween two of the plies in laminated Ixiard .serves to in¬
crease the water-vapor resistance f>f the Ixmrd. An asphalt duplexed
sheet was used during W’orkl \\’ar 11 for making highly water-resistant
overseas shipping cases. The asphalt layer was put close to the surface of
the lioard where it functionetl as a l»arrier to the penetration of water, .\s-
phalt-laniinated jiajjerlKXinl is widely used for cookie boxes.

.Xsplialt is applied at a temjjeratnre around 275® F., or about 100® F.
aUive its .s<jftening ]>oiut. The e(|uipment most generally u.sed consists of
a heated pick-up, a .set of rubl)er-covered press rolls, and a number of cool-
iug rolls. The pick-up roll revolves in the molten asphalt and picks up the
asphalt and apjflies it to the i)af)er. The asphalt is u.sually applied to the
liner slock, and then the filler stock brought into contact a few seconds later.
If tlie filler is warm when brought into contact with the asphalt film, a
stmnger liond is prcxluced. I he high temperature of application (made
necessary by the high vis<'osity of asphalt) results in excessive absorption
of asphalt by the f>a|)er. The high tem|K*rature also teiuls to dehydrate the

|a}XT.

One disadvantage of asphalt is that it tends to bleed from tbe glue
line of tlic combined lajard on .standing. This is particularly bad when
thin sheets are used, as in the ca.se of asphalted crepe papers. The bleeding
resistance of asphalt-laminated pa|)ers is measured by sandwiching the test
>hert lietweeii two pieces of smooth white i)ai>er and holding at 150® F. at

a pressure of 0..s p.s.i. for five hotirs.*® The white pa[)er is then examined
for .staining.

.Asphalt is sometimes use<l in combination with other hot-melt ma¬
terials, such as f>etr(»leum waxes, [XMitaerythriufl resins, rosin e.sters, hydro-
carlion teqiene resin.s, and coumarone-iiulene resins. These materials add
toughness and tack to the asplult an<l. in .some cases, improve the flexibility
awl water-vajK»r resistance. Reinforcement.s, e.g,, jute, si.sal, cotton, or
burlap, in the form of cord or fabric may l>e incor[)orated in the a.s|>halt

layer when making laminated f)ai)ers. Many grades of asphalt-laminated
papers are creped.

Kcsin Adhcswcs

The natural and .synthetic resins are frequently used in blends with
H-axes. a.sf>halt, and other h.it-melt adhesives. The^ resins are added to
increase the toughness of the adhesive and, in some cases, to increase the

« S«T T.APPI Standards

1244

PULP AND PAPER

water-vapor resistance. Certain hot-melt adhesives are entirely composed
of resins. These are used for pasting paper labels on metal cans and for
the laminating of metal foils. A wide variety of different formulas are used,
and a few typical examples are given below.

Ethylcellulose is spmetimes used in hot-melt adhesives. When prop¬
erly compounded with other ingredients, ethylcellulose produces a fast¬
setting adhesive which has good heat-seal properties. Ethylcellulose hot
melts are suitable for bonding paper, cellophane, metal foil, glassine, or
other similar materials. A formula for a hot-melt adhesive containing

ethvlcellulose is

Ethylcellulose (10-20 cps.) . 5-20 parts

Wax (carnauba, spermaceti, montan, beeswax, etc.) .. 0-20 parts

Mineral wax . 15-30 parts

Plasticizer . 0-10 parts

Resin . 40—65 parts

Low-viscosity polyvinyl acetate makes an effective hot-melt adhesive,
Plasticizers are generally added to increase the flexibility, to lower the heat¬
sealing temperature, and to reduce the melting point. Natural resins such
as dammar, may be added to lower the melting point. A suitable formula

is:

80 parts \nnyl chloride acetate resin
20 parts modified alkyd resin
Plasticizer such as dibutyl phthalate

The coumarone-indene resins are well suited for hot-melt adhesives
because of their low melting point (110-130° C), thermal stability, and
good tack. The hydrogenated and phenol-modified grades are useful be¬
cause of their high degree of compatibility with synthetic resins. Cou¬
marone-indene resins can be mixed with rubber derivatives, ester gum,
shellac, waxes, paraffin, oils, and most organic solvents except the alcohols.
The unmodified grades can be used in combination with nitrocellulose.
The phenol-modified grades can be used in combination with polystyrene
and cellulose acetate. The chlorinated diphenyls make good plasticizers.
A suitable formula combining coumarone-indene resin with ethylcellulose

Ethylcellulose (20 viscosity) .

Coumarone-indene resin (Nevilla R-7) .

Rosin derivative (Staybelite) . ^

Dibutyl phthalate ..*. *

« Tech. Bulletin, "Ethocel Hot Melts for Paper Coatings,” Plastics Division, The

Dow Qiemical Company, Midland, Michigan xt -n t i Plttehnrph

« “Neville Resins and Plasticizers,” The Neville Co., Neville Island, Pittsburg ,

Pennsylvania

XX. LAMINATING AND PASTING

1245

Rosin is not well suited for hot-nielt laminating because of its bad
odor and poor stability. However, certain of the rosin derivatives, such as
hydrogenated methyl ester of abietic acid (Hercolyn), the ethylene glycol
esters of hydrogenated rosin (Staybelite), and the ethylene (or diethylene)
glycol esters of rosin (Flexalyn), make effective adjuncts to hot-melt adhe¬
sives. These can be used with nitrocellulose, ethylcellulose, polyvinyl com¬
pounds, and rubber to increase the tack and improve the flexibility of the
film, Biddle'® suggests the following formula;

8 parts ethylcellulose

42 parts hydrogenated methyl ester of rosin
50 parts hydrogenated rosin

Another formula is

10 parts ethylcellulose

63 parts ethylene glycol esters of hydrogenated rosin (Staybelite)

27 parts paraffin

The rosin-derived alkyd resins (Neolyn resins) are also suitable for hot-
melt adhesives. Some of these resins have enough flexibility and sufficient
strength to be used by themselves, but they are generally used in combina¬
tion with other film-forming resins, such as vinyl chloride-acetate copoly¬
mers. A suitable formula requiring an application temperature of about
200 to 250® F, is as follows

80% rosin-derived alkyd (Neolyn)

10% vinyl chloride-acetate (Vinylite VYHH)

10% di-2-ethyl hexyl phthalate

Products with higher melting points can be made by eliminating the
phthalate and using a Santicizer resin.

The rubber derivatives, cyclized rubber and chlorinated rubber, and
some of the synthetic rubbers, can be used in hot-melt adhesives. The
rubber adds water-vapor resistance, flexibility, and adhesiveness to the
film, but increases the viscosity of the melt. The butyl rubbers are effec¬
tive in increasing the bonding strength of wax melts.

A. Biddle, Paper Trade J. 122, No. 23 : 252-258 (June 6, 1946)

<®Tech. Bulletin, “Ethocel Hot Melts for Paper Coatings," Plastics Division, The
Dow Chemical Company, Midland. Michigan

*^“The Neolyns. A New Series of Resins,” Synthetics Department. Hercules
Powder Co., Wilmington, Delaware

CHAPTER XXI

INTERNAL TREATMENT OF PAPER
WITH RESINOUS MATERIALS

The following material is concerned with the internal treatment of
paper with plastics and resinous materials. It has been subdivided into
sections, treatment of paper with latices and emulsions, saturation of paper
(with asphalt, oils and waxes), and paper plastics.

Treatment of Paper with Latices and Emulsions

Latices and emulsions can be applied to paper by internal application
(beater addition) or by surface treatment (tub sizing or saturation). Nat¬
ural rubber latex, reclaimed rubber dispersions, synthetic rubber latices,
synthetic resin emulsions (e.g., acrylic, polystyrene, vinyl, and alkyd), and
oil emulsions are some of the emulsion type materials used.

The results of beater addition depend upon the type of resin used,
method of application, and the amount of resin absorbed Ijy the paper. If
a high percentage of resin is added, the paper takes on the properties of the
resin, resulting in a product of high toughness, wearing, and tearing re¬
sistance. Some of the common products made by internal treatment of
l)aper with resins are artificial leathers, shoe parts, wrapping papers, gas¬
ket papers, and j^aper base tapes. Artificial leathers are usually coated
after treatment. Internal treatment of paper with resins is used when a
finished product is desired having such properties as toughness, softness,
flexibility, stretch, initial or edge tear, and strength at sewm seams. The
tensile strength and stiffness of the paper may be reduced by treatment.

Properties oj Beater-Treated Papers

Beater addition is the simplest method of incorporating resinous ma¬
terials in paper, inasmuch as no s])ecial equipment is requited. Howe\er.
there are several serious disadvantages to lieater apjtlication, particular!}
when large amounts of resinous materials are added. Among the most seii-
ous disadvantages are excessive white w’ater losses, foaming, fouling o
e(|uipment, slow drainage on the machine, and two-sidedness of the paper.
Some of these difficulties have Iteen overcome by the intfoduction of new
types of resin latices, Imt there are still many cases where beater addition

is not satisfactory. • ,

Beater-treated [tapers have radically different physical [troperties iron^

[tapers saturated in the dry condition. In general, beater-treated [tapers are

1246

XXI. INTERNAL TRE.\TMEXT WITH RESINS

1247

tougher, stronger, and better able to retain their strength after creasing
than dry-saturated papers. These advantages are derived troni the fact
that a greater number of tiber-to-resin bonds are produced by beater addi¬
tion, compared with the number produced by the saturation of dry paper.
W’hile saturation of dry paper results in a coating of resin on the fibers
which tends to flow away from the fibers during molding, beater addition
produces resin bonds which remain in place and are further strengthened
during molding.' In the case of beater-treated papers, the fibers are free
to slide past one another during molding, thus re-forming the sheet into a
more homogeneous material.

Types of Resins Used in Beater Treatment

Both elastomer (rubber) latices and resin emulsions are used for
beater addition. The jirincipal use for elastomer latices in beater treatment
has been for the manufacture of tough, leather-like products in which rela¬
tively large amounts of elastomer are used. A typical grade is fiberboard,
a very tough and pliable board which is used in shoes as stiffeners, counters,
innersoles and heels. About 20^o rubber latex is generally incorporated
in the board to impart the necessary pliability, wet strength, and resistance
to wear. The board is made on a wet machine where the sheet is formed
continuously on a cylinder mold until a layer of the desired thickness is
obtained. This layer is then cut loose and laid out in sheet form, after which
it is pressed, kiln-dried, and finally calendered. Pressing is necessary to
cause the rubber particles to adhere to one another, and thereby bring out
the rubber-like characteristics in the paper. Pulp treated with small
amounts of elastomer latices has improved strength properties and increased
wet strength.*

Among the resin emulsions used for beater addition are the poly¬
styrenes, alkyd, polyalkylacrylate, and phenolic resins. The use of resins
for beater addition is discussed in a later section on paper plastics.

Methods of Applying Latices in Beater Treatment

Best results in beater treatment are obtained by using unbeaten or
lightly beaten pulps, because this gives the maximum number of resin-to-
lilicr bonds. The retention of resin in the paper increases with increased
Iteating up to a certain point, after which it levels off. The type of pulp is
imiKirtant, rag stock being preferred over sulfite or sulfate for the devel¬
opment of maximum physical properties. However, a considerable amount
of chemical wood pulp is used because of the lower cost, and it is also quite

' loem’ Spiwak and A. E. Moyer, Tap pi S3, No. 4: 87A-90A (Apr.

1V5U/ *

' S ? W. W. Pockman. J. R. Galloway and E P

GtJ (FA '’•'5th Annual ^feeting of TAPPI, New York

1248

rULt* AND TAPER

common to use a mixture of rag with either sulfite or sulfate. Yost and
Aiken-’* found in the case of rubber (nitrile) latex that bleached pulp (sul¬
fate) had a greater tolerance for the latex than unbleached pulp when the
pulp was unbeaten, but that beating increased the tolerance with the un¬
bleached pulp but not with the bleached pulp.

One of the principal objections to-beater addition has been the high
percentage of material lost in the white water, a serious problem when or¬
dinary anionic latices are used in an open system. Three principal methods
are used to improve the retention of latex in the paper. These are: (7)
alum precipitation of the latex, (2) pretreatment of the fibers to impart a
positive charge which attracts the negatively charged latex particles, and

(2) the use of positively charged latices.

Originally, alum precipitation of the latex was the only means of in¬
creasing the retention of latex. This method consists of mixing an anionic
latex with the stock in the beater, followed by the addition of sufficient alum
to lower the pH to the isoelectric pioint of the latex. One disadvantage of
this method is severe fouling of the beater, because of the deposition of latex
particles. The retention is fairly good with rag and sulfate stock, but is
generally poor with sulfite stock. The poor retention with the latter is
due to premature coagulation of the latex resulting from the acid character
of sulfite fibers. This leads to the formation of relatively large coagulated
particles which are not retained well in the stock. Buffering the stock to
a pH of 8.0 to 8.5 with alkaline salts aids in retention.

An important advance in beater treatment with elastomer latices was
the development of a method for the pretreatment of the fiber to change its
electrostatic charge from negative to positive. The best-known pre^s of
this type is the Bardac process, developed by the *\mencan Cyanamid Lorn-
tZy, L which a melaLe-formaldehyde acid coUoid is used to impart a
positive charge to the fibers before the latex is added.® By using tht. jnoc-
ess, relatively large amounts of latex can be retained in the stoA vviAou
serious white water losses or fouling difficulties. Between 1 to a Jo of
melamine resin is used to impart the desired charge to the fibers, and ttas

is followed by 10 to 50% of resin latex, such as J ’

aervdate, vinyl, etc., to obtam the desired properties in the P ^

■ It was first thought that the melamine-treated pulp in iK ,x)s mdy

charged condition'attracts and adsorbs negatively charged ^

(e g" GR-S and Hycar). Subsequent theory, however seems to mdi^
tha^'ffie positive fiber does not direcUy adsorb the emulsion particles, but

1. D. M. Yost and W. H.

Beater Addition of Nitrile Rubber Latices, Tappi Paper Tiasncs

S^Tacuse, N. Y. (Oct. 19-20, 1950) on/toc Can Pat 450.523 (Aug. lA

2b L. H. Wilson. C. S. ^faxweU and C. G. Landes, Can. i^at

1948)

XXI. INTERNAL TREATMENT WITH RESINS

1249

instead inactivates a small amount of the negatively charged emulsifying
agent acting as the latex stabilizer. This reduces the stability of the latex
emulsion particles, causing them to be deposited on the fiber surface. Emul¬
sions containing an excess of emulsifier are not deposited, even though part
of the emulsifier is adsorbed or neutralized by the fiber. Only that part of
the melamine resin acid colloid adsorbed on the fiber is effective in fixing
the emulsion particles at the fiber interface; any unadsorbed resin colloid
merely coagulates the emulsion in the aqueous phase. A general procedure
for incorporating rubber latex in kraft paper using the Bardac process is
given below. All data are expressed on a dry basis.

(1) Of prime importance in the Bardac process is the hydrogen-ion concentra¬
tion, since slight alterations affect the adsorption of the acid resin colloid which, in
turn, affect the retention and loss of the latex or resin dispersion. The /»H of the pulp
system should be adjusted between 5.0 to 5.5 with 5% acetic acid before beating (as¬
suming the pulp to have an initial />H of 8.0, it will take approximately 1% of acid
on the pulp) after which the pulp should be beaten to a Canadian freeness of about
600 to 650.

(2) 2% of melamine resin (Parez Resin 607) should be added to the pulp from
a 6% solution previously prepared and stored for at least twenty-four hours. In order
to obtain the best possible adsorption of the resin colloid, it is important to provide
adequate contact time between the resin colloid and the pulp. Although most of the
adsorption takes place witliin the first few minutes, thirty minutes to one hour contact
has been found to give best results. The />H should be between 4.5 to 5.0 during the
contact period. No further additions of acetic acid arc required to adjust to this pH,
since the addition of the acid resin colloid will generally suffice to lower the /"H to the
desired value.

(S) After the prescribed contact time, the />H of the system should be raised to
between 6.5 to 7.0. It is preferable to use sodium aluminate (about 1% on the fiber),
but sodium bicarbonate can be used (approximately 2% on the fiber). The alkali
should be added in the form of a 10% solution. (Coagulation of the disperse phase of
the latex is accelerated by low pH values and adsorption is retarded by high />H
values.)

(4) About 20% latex (dry solids to dry fiber) is next added to the stock, after
the latex has first been compounded with stabilizer and anti-oxidant and the total
solids reduced to 10% by weight with tap water. The pulp plus acid colloid plus com¬
pounded latex appears as a milky slurry, but upon circulation, the system should clarify
completely to a water-white appearance. The circulation time may be as short as

. ^1 as long as two hours, depending upon the concentration of the
melamine resin adsorbed, the ^H, the nature of the stabilizers and emulsifiers in the
latex, and the concentration of disperse resin or rubber phase on the fiber. Jordaning

or any violent agitation should be avoided in order to prevent a disturbance to the
fiber-resin bond.

(5) The pH should finally be adjusted between 5.5 to 6.0 with a 5% alum solu¬
tion in order to coagulate any small amount of residual latex which has not been ad¬
sorbed after a reasonable contact time.

In addition to the Bardac process, there are two other somewhat sim¬
ilar processes—the neoprene latex (du Pont Process) and the phenolic

1250

PULP AND PAPER

resin (Snyder Process). In the first of these, latex particles (up to 50%
of the fiber weight) are deposited on the fibers by a controlled flocculation
jjrocedure using alum or other polyvalent salt as the jirecipitant. The latex
is stabilized (with naphthalenesulfonic acid-formaldehyde condensation
product) and sensitized with alum before addition to the beater, but the
pH is maintained on the alkaline side. Coagulation in the heater is ob¬
tained by lowering the /jH of the stock to about 4.5 with alum. Vulcanizing
agents and antioxidants can be mixed with the stock prior to the addition
of the latex. In the other (Snyder) process, a special phenolic resin is
used to precipitate fairly high percentages of latex on the fiber. The latex
is added first to the beater, followed by the phenolic resin and then by a
precipitant consisting of an acidified alum solution.

i\nother more recent method of obtaining a high retention of elastomer
latices in heater addition is by the use of positively charged (cationic)
latices. These latices have a natural affinity for the fiber and are added to
the pulp by an exhaustion procedure. The original work on cationic latices
was done in England in the production of a positively charged natural rub¬
ber latex called Positex.^ Some of the other elastomers now available in
positively charged latex form are Neoprene,^’'* nitrile rubbers, and GR-S.

Protective colloids, anti-oxidants, vulcanizing agents, and plasticizers
are generally emulsified with the latex prior to addition to the beater. The
natural anti-oxidants in natural rubber latex are mostly lost in the white
water, but these are compensated for by the addition of more effective syn¬
thetic anti-oxidants emulsified with the rubber.

External Application of Resins to Paper

The two principal methods of external application of resins to paper
are the tub-sizing method and the off-machine saturation method. In the
first method^ the tuh-sizing method, the paper is treated on the paper ma¬
chine in a partially dried condition. A relatively small percentage of resin
is added to the paper by this method, and while the physical and chemical
properties of the paper are greatly improved, the final product must still le
regarded as paper. In the second method, the off-machme saturation
method, the treatment is carried out in a separate operation and the paper
is dry at the time of .saturation. Ordinarily, a high percentage o resin i^
added hv this method so that the final product takes on the properties o

the resin.

>C. M. Blow. Positex Pamphlet No. I, "Poeltex in the Paper Indu^ry.” The
British Rubber Dev. Bd.. Market Building. Mark Lane. London, . ■ „ „

. R"H.'’wath! H H. Abernathy. W M tIp'h C Yolk

Hartsfielcl, paper presented at tlie 35th .Annual Meeting of TAFJ^l, we

• R G. Partridge*Pnprr Trade J. 127. No. 6: 45-4g (Aug. 5, 1948)

XXI. INTERNAL TREATMENT WITH RESINS

1251

After saturation, the paper is passed between squeeze rolls to remove
excess saturant, after which heat is applied to drive oflf the water and set
the resin. Heat increases the adhesion between the resin and the fibers and
increases the strength. The maximum increase in burst, tensile, and elon¬
gation is obtained by heating to the softening point of the resin.® However,
too high a temperature for prolonged periods results in thermal degrada¬
tion of the paper and oxidation and degradation of the resin. Stepwise
drying is desirable successiveU' to eliminate the water, set the resin, and
finally sinter the resin. The temperature should be slightly lower than the
melting point of the resin at the first few driers and then should be increased
to about 20 to 30° higher than the melting point at the last few driers. In
general, saturation prodtices a softer, more flexible sheet than lieater ad¬
dition’ because of the greater continuit}' of resin.

Types of Paper Used for Saturation "

The paper used for saturating with latices or emulsions is generally
a kraft, sulfite, or a rag paper of high alpha cellulose content, although pa-
])ers made from soda pulp have been used. In some cases, the paper may
contain an inert filling material, e.g., ground leather. The paper should be
very loosely felted and should have an apparent density varying from 4.0
to 7.5.® It must be made with very little beating, pressing or calendering,
and must be unsized.

The moisture content of the paper at the time of saturation is one of
the most important variables. Papers with a normal or higher-than-normal
moisture content are required, since it is almost impossible to saturate dry,
non-porous papers such as well-beaten kraft.® On the other hand, almost
any type of paper can be saturated in the wet state, inasmuch as latex par¬
ticles will penetrate along the water-filled pores, because of Brownian move¬
ment. The presence of water in the pores permits uniform infiltration of
latex particles, and hence saturation of a wet sheet tends to be more uniform
than saturation of a dry sheet.

Types of Resins Used for Saturation

Elastomer latices, including both natural and synthetic rubbers, are
used extensively for the saturation of paper for making artificial leathers.
The latex is generally applied at a solids content of 5 to 35%, and the paper
is immersed for a period of seven to ten seconds. The pick-up is about 20
to 100% of dry solids on the weight of the paper, 30% being an average

* 1947^ Trade J. 125, No. 7; 73-80 (Aug. 14,

■ A. F. Owen. Tech. Assoc. Special Report 359, Minutes of the Plastic Committee

p. 49 (July 31, 1945)

* R. T. Nazzaro, Tappi 33. No. 1: 23-25 (Jan., 1950)

®M. J. Viltengl, Paper Mitt 61, No. 1: 15-18, 20 (Jan. 1, 1938)

1252

PULP AND PAPER

figure. This amount of saturant greatly increases the tear and wearing
resistance of the paper and greatly increases the stretch.

Natural rubber latex imparts an unusual combination of physical and
chemical properties to the paper, but it presents problems in stability, aging,
non-uniformity, and odor. In many respects, synthetic rubber latices are
superior to natural rubber latex, particularly when oil resistance and re¬
sistance to aging are important. Some of the synthetic rubbers suitable for
saturating paper are the chloroprene polymers (neoprene), the butadiene
acrylonitrile copolymers (Buna N-GR-A, Hycar OR-23, Hycar OR-15,
and Chemigum 200), and the styrene-butadiene copolymers (Buna S-GRS
and Dow Latex 512-K, a high styrene type). The copolymers of acrylo¬
nitrile and butadiene (Hycar OR-25) are particularly well suited for satu¬
rating because of their small particle size."^" They generally result in an
appreciable increase in elongation, flexibility, tensile strength, tear resist¬
ance, chemical resistance, and grease resistance. Polychloroprene (neo¬
prene) latex gives high tensile, good tear, and good burst strength, but
produces papers somewhat lower in flexibility and elongation than some o
the other synthetic rubber latices.- Reclaimed rubber dispersions, widely
used during World War II, have a larger particle size than the rubber
latices and are, in general, typified by inferior physical properties, poor

"‘^'"TheTat.SntT paper with synthetic resin emulsions is becoming
increasingly important. Resins impart quite different properties to paper
tothe Lstomers or rubber latices. The results depend upon the type o
resin, amount added, type and amount of plasticizer

in general, the synthetic resins impart to paper a '’>f "

light, and chemical resistance and a lower degree o e y

Amon<» the synthetic resins used for saturation are the alkyds, acrylics,

fiifrSissfg

excess resin. Proper y P^* . ^^er latex,« but vinyl resins

r^SrpCr uve poorer water r—

‘"af ohtar„ed with natural rubber or

» E. G. Partridge, Paper Trade 1.127. No. 6: 45^8 (Aug. 5. 1948)

/dm. _ , r ito Mn n* 33-36 (Sept 28, 1944) _ , t

f Y KaoTcdtA. S."ulb R. Worden and W. Abramowi.a, Paper Trade .

Ly'No. 3: 42-47 (July 20, 1944)

XXI. INTERNAL TREATMENT WITH RESINS

1253

polyvinyl acetate. Oliner and O’Neil'* found natural rubber to be most
effedive, and the vinyl polymers next, followed by the polyymybdene
chloride resins. Alkyd resins have a fast rate of cure, good color, hig i wa
resistance, and good electrical properties, but they produce papers of ow
strenrth Resin emulsions are sometimes used m combination with e a -
tomer latices. A comparison of results obtained by saturating paper with

several different commercial latices and emulsions is shown m \

An important factor in saturation is the degree of adhesion which is

obtained between the rubber or resin particles and the paper fibers,
hesion depends upon the structural features of the polymer, the amount
and type of modifying agent used, and the degree of pressing and curing o

TABLE I

Effects of Resins Incorporated in Paper by

Latex

Saturation

Properties of

Paper

Type of latex

Sat.

bath,

total

solids

Per cent
loading

Tensile
lb./in.
width

Elonga¬
tion, %

Burst,

p.s.i.

Fold,

MIT

Int.
tear, g.

Wet
tensile,
lb./in.
width

Copolymers of acprloni-

trile and butadiene

7.7

28.3

112

298

4.6

Hycar OR-25 .

Hycar OR-15 .

9.9

37.0

15.9

10.0

40.7

18.6

7.7

(44.8)

434

290

5.6

Acrylic acid ester

Hycar PA ..

Polyvinyl chloride poly¬

10.0

38.7

20.6

3.8

29.2

853

281

1.2

mer plasticized

Geon PX-8 .

12.6

38.1

26.2

3.5

39.2

459

190

8.9

the saturated paper. Among the elastomers, polychloroprene produces pa¬
pers of very high tensile strength because of its strong intermolecular cohe¬
sion, but for the same reason, lateral adhesion to the fibers is poorer than
for other elastomers.^® Commercial styrene-butadiene copolymer (e.g.,
GR-S) has fair adhesion to paper, but the adhesion decreases as the styrene
ratio is increased.^^ Polyvinyl acetate, properly plasticized, exhibits better
adhesion to paper fibers than GR-S. Alkyd resins have poor adhesion to
the fibers. In general, it may be stated that resins exhibiting high inter¬
molecular forces exhibit poor adhesion to paper fibers unless these forces
are weakened by the introduction of plasticizers.

Poor anchorage of the resin to the fibers and migration, which are
serious problems with certain resins, can be overcome by special com¬
pounding of the latex to render the latex heat sensitive so that the resin is

A. E. Oliner and F. W. O’Neil, Paper Trade J. 125, No. 7: 73-80 (Aug. 14,
1947)

Results presented by R. L. Steller of B. F. Goodrich Company at 1948 Plastics
Conference, Appleton, Wisconsin (Aug. 16-17, 1948)

18 R. T. Nazzaro, Tappi 33, No. 1: 23-25 (Jan., 1950)

11 Idem.

1254

PULP AND PAPER

set in place upon drying. 'I'here are two principal methods; (i) by stabiliz¬
ing the latex to tolerate normally coagulating salts, e.g., magnesium sulfate,
at room temperature, but maintaining sensitivity to the salt at the higher
drying temperatures, or ( 2 ) by adding special complex salts of zinc
or other elements with ammonia or amino compounds which are decom¬
posed at the temperatures of drying (Nieley process). With the latter
method, the high temperature of drying evaporates the ammonia and liber¬
ates the metallic salt which coagulates the resin and prevents migration.

In addition to the use of plasticizers for improving the adhesion of
resin to the fibers, plasticizers are also incorporated in resin emulsions for
the purpose of improving the flexibility and aging properties of the resin.
The plasticizer should be emulsified with the resin or added later m emul¬
sified form. One difficulty is that plasticizers, particularly the highly polar
types are attracted by the fibers as well as by the resin, thus robbing the
resin of its plasticizer and resulting in a loss of flexibility upon aging. Mi¬
gration and absorption of plasticizer are particularly serious with certain

resins (e.g-, pol)^inyl acetate). ^ • e *

In addition to plasticizers and heat-sensitizing agents, latices for satu¬
ration often contain stabilizers, anti-oxidants, and curing agents. These
should be compounded with the latex prior to saturation, and unless water

soluble, must be added in emulsified fonu. One of *e troute—
tered in saturating with rubber latex is the tendency of the saturated shert
to stick to the squeeze rolls. This can often be overcome by the addition o
soap or other surface-active materials. Softening agents for the paper are

sometimes added with the latex. ^;c1■r^hntion of

The amount of resin picked up by the paper and

resin in the paper are determined ';y

.ribution of tl^ dispers^a tides beiause larger particles tend to

“po"es‘of the sheet and inhibit penetrahon a P—

. 1 • 4. t^Via i:»iprtrnc;tntic charge on tne aisperijcu [jcxi

to as plating ou ■ ^ stabilitv and the manner in which the

important heca.ise it affects both th 1

latex penetrates the paper. to exhibit plating

migrate during the drying process. Cat on,c latices

out. Non-ionic latices tend to be unstable.

Saturation of Paper with Asphalt, Oils, and Waxes

Certain grades of paper are These mateHals tend

molten wax to produce products or specia resistance. Wax-

i:e!:;:d

terials, but a few of the other grades are discussed below .

XXI.

IXTEXXAL TtEATMEXT WITH RESINS

1255

Asfkalt-Saiuraitd Papers

.\sph»h is olnaiiKil from the processing of certain iMJtroleiun oils, or
from lUturaUv uccurruig asphalt deposits resulting from the partial oxida-
tioo of letroJeum. Urge natural deposits of asphalt occur on the island
of Trinidad and in \'enczuela- Asphalt is shipi>ed in i>aper cartons, steel

drums, and tank cars.

Asphalt \’aries from a semi-solid, stic^^ V
materuL It consists of a dispersion of solid material in an oily medium.
Some asphalts have softening points as high as 300® F., but most
used in the paper industry haxe a softening jicint lietween 140 to 200® h.
Asphalt differs from paraffin in that it has no sliarp melting |X)int and has
a modi higher viscosity. It is very riscous when applied just above its
M^tening point, but liecomes quite fluid when heatetl to alnnit 100 1’.

above its softening point. Unrefined asplialt is generally used for impreg¬
nating and coating roofing }ia|Mrrs.

Vegetable pitch (^tcarine), which is a soft black, tarry substance ob¬
tained a> the final residue in the manufacture of distilled rc<l oil and stearic
add, U sumetinics uied in the manufacture of nxifing ijai>ers by heating

and mixing with the other ingredients.

In the saturation of pajier with asplialt, jiajicr felt at a moisture content
of 3 to is run through a hath containing asphalt at a temperature of
about 350 to 400* F. After leaving tlie bath, the felt is fiassed over steam-
heated rolls to drive the saturant into the sheet and remove any excess from
the surface. Alwut 125 to 135^ saturant on the weight of the .sheet is ap¬
plied in the ease of floor fek. and 140 to \(iS% in the case of roofing felt.

In the c aK of roofing felts, a surface coating of asphalt is applied to the
saturated sheet and spread evenly by means of steam-heated rolls. Then,
wiitlr the coating is still hot, the sheet is passed under a hop^ier where par¬
ticles of sand, tak, slate flour, shell, or other granular material is sifted on
the surface and firessed into the soft asphalt hy means of chilled metal rolls.

Paper fehs for saturation must have sufficient strength to hold up in
the process of saturating. In addition, the |ia[ier must have a low moisture
content and must be sufficiently porous to absorb a high percentage of as¬
phalt. Waste rags are generally used for making saturating felt and these
are cooked under mild conditions and lieaten in such a way that the fillers
are shorlened without too much filirillation and hydration. The color of
the rags it of no importance, inasmuch as they arc used predominantly for
their strength and not for their appearance.

The saturating properties of roofing felt can lie measured by the kcro-
iroe test (which measures the total saturating cajiacity) and the oil pene¬
tration or xylene |«nclration test fwhich measures the rate of saturation),
llie kmuroe test is reported as the percentage of kerosene w'hich is re-

1256

PULP AND PAPER

tained by the paper after displacement of air. in carrying out this test,
strips of paper are cut and dried in an oven for one to two hours, after
which they are cooled, weighed, and then immersed in kerosene at 25° C.
under a vacuum of 28 in. of mercury. After soaking for fifteen minutes,
the strips are carefully drained and weighed. Another method is to add
kerosene from a burette until the paper is saturated. The kerosene number

is then determined as follows:

/ weight of saturated paper

\ weight of dry paper

-1

X_i-^^r^TT-XlOO

\ weigiiL Ui u., . specific gravity of kerosene at 25 C.

The kerosene number varies from about 115 to 235. The saturating capac¬
ity of paper for a particular bituminous saturant can be determined by t e
relationship between the specific gravity of the saturant at 25° C. and the

kerosene number. . •if..

The saturating rate of the paper is measured by the time required fo

clear paraffin oil (Saybolt viscosity 253 sec. at 72° F.) to soak throug a

sample of paper floated on its surface. The rate can also be measured by

the time required for xylene to rise by capillarity m a strip of the paper o

a distance of 3 cm.

Tracing Papers

The primary requirement of tracing paper is a high degree of trans
parency so that printed or written nratter can be seen through a srngle th.d-
^ c nf the oaoer Hi^^h-grade tracing paper can be made from hig y

Zien stock without the addition of added niaterkls.

in this way has good transparency and, m additio , p

rViJr'in Viati excellent writing and erasing qualities.

Wh.ch has exed en » impregnating paper w,th a

Other tj^s ot tracing p f j

substance having “ “J “ yeiretable drying oils produce papers with good
transparent sheet ^^le u y^^g^ ^^^P ^ ^

writing and erasing qua i _ sufficient oil is used to give good

paper with poor — because of its tendency

transparency. .-t-t-u Furthermore non-polar materials, such

to crystallize and become brittle. > nencil and bind

as inLral oil or paraffin, tend to wet *^/j;X.rera ure difficult«
the particles of graphite to the paper so firmly that

Oiled Papers

Fruit wrap, meat wrap, and metal wrapping
treated with paraffin oil to >ntproveffie water res.s anc^^

iH^onhl fr A Oil-treated papers are better than wax^d papers or
' is H L, Vincent, Pn,. Wr /. «. No. 36: 304-306 (Dec. 39, 1932,

Idciti,

1257

XXL IXTCXNAL TXEATMEXT WITH lESlNS

wTa|)f 3 inc diilled pork because 6t the higher gas permeability of oil-treated
papers.

Paraffin od is loosely defined, but generally refers to a Ipw-viscosity
od derive<l from a paraffin type or a raid-continent crude oil. Un-
ld(e paraffin axx, h b not a true member of the paraffin family because
(e%en m Ptamsyhanb ods) there are minor percentages of ring type hydro-
carboitt, ff y** as the naphthenes. Crude mineral oil is the fraction left after
the lighter fractions (Le., the gasoline, naphtlu, and kerosene).
To prepare purified white mineral od, the crude oil is treated with sulfuric

1 ^ washed with alkali to renwe the unsaturated and aromatic hydro-
carbom. The purified od b colorless, odorless, and tasteless. Refined oil
can he treated for unsaturaied hydrocarbons by digesting the oil with an-
equal amount of 95^ sulfuric arid for ten minutes. If the oil turns dark
brown or black, it cortains a high percentage of unsaturated hydrocarbons.

LxiW'viscosity mineral oib (about 70 Sayboh at 100® F.) are the
grades gmerallv used for the impregnation of fruit wraps. High-viscosity
oils (15(^300 ^ybolt) are used for tracing jiapcr and meat wrap. Oils
used lor food wrajipers should be free of all objectioiuiblc odor. Odors
frequently occur in oil-impregnated kraft papers, particularly in the bleached
grwkf. where the trouble teems to be caused by bleaching residues left in
the puper which hasv a caul>lic effect on oxidation of the oil. One remedy
b to add an anti-oxidant to the oil.

Some of the methods used for applying oil to paper arc dipping, spray-
iiy. and roll coating. Pafter will pick up between 16 to 20% of its weight
of oil when dipped.** Under these conditions, the sheet is definitely oily in
fcH and appearance. This amount of oil is desirable in wrappers for ap¬
ples, where one of the main functions of the wrap is to furnish a protective
film of oil but most grades of fruit wrap contain only 5 to 8% oil. In
tome cates, biphenyl b dissolved in the oil for treating fruit wrap in order
to pre v e n t stem roi and Mae mold.**

Trr aim rm m-ith oil reduces the opacity of the paper. This is caused
by the fillmg in of the pores of the paper with oil and, in the' case of low-
Ttseosily oils, hy a penetration of oil into the fine pore structure of the fiber.
This penetration of oil into the fiber structure further reduces the light
mirring and acroonfs for a slow rise in light transmission as the treated
sheet stands

Stfnril Papers

Stencil papers most have high tearing resistance, a high degree of hard¬
ness, and high tdffnesa. The paper must be oil resistant and waterproof.
In order to obt ai n these properties, the paper U impregnatedi with linseed

*• j. C Dam. Pa^ ViB Ktm dt, Ma 24 : 74-76. 78 (June 16. 1945)

« t. a Mbylcy md W. R. Bsiber. U. S 2.I73,4S3 (Sept 19, 19»)

1258

PULP AND PAPER

oil. Certain other grades are made by impregnating with a mixture of
gelatin, pigment, and oleic acid.

Paper Plastics

Cellulose is the raw material used in the' manufacture of plastics such
as cellulose acetate, ethylcellulose and cellulose nitrate, which are used for
making a wide variety of molded and extruded products. This field is too
large to be considered in this book. It is the basis of a large and fast-grow¬
ing industry, the plastics industry. There are, however, aspects of the
plastics industry which are the immediate concern of the pulp and paper
chemist One important field is the manufacture of resin-treated paper
laminates, another is pulp molding, and still aaother is the use of cellulose
pulps as fillers in molding compounds. Resin-treated papers are produce
in the form of thin sheets and heavy boards, whereas pulp-reinforced p as-

tics are sold in the form of granular molding compounds.

\ distinction is made between ordinary resm-treated papers described
in th^r vions sections and resin-treated paper plastics on the basts that
resin-treated paper plastics are molded or laminated after tntpregna ton.
Ordinary resin-treated papers are sometimes treated by pressing or ea ing,
te they are usnally not subjected to the high curing temperatures and

iwp<;qures which are necessary with paper plastics. ^

One of the common types of resin-treated paper plastics is a gra e

known as paper laminates. This product consists o several plies of re ^

treated pa^ which are molded or laminated together to "S

structure in which the paper resistance to light and chemicals,

furnishes stiflfness, water resistance, and ^ ,^dio

Paper-base laminates are widely used m le |rcuit-breaker panels.

sub-panels, socket bases, a Enable surface

Two types of paper plastics method

tics and resm-impregnated paper p , jjesin-tilled papers are

by which the resin is incorporated m t PP_ ^ ,3

made by adding resin ‘ P" [ " ,„,„rolled volatile content which
formed on the ™^hin and d ^ C.« Re«n-

can be measured as the los^ . ^

G. K. Dickerman, J'j ^^6 No’ 16: 199-201 (Apr. 15, 1948)

J. c. Parsell, Paper Trade /. 126, wo. lo.

XXI.

INTERN'AL TREATMENT WITH RESINS

1259

impregnated jjapers are made by applying a solution of resin to paper in
web form.

Kcsiti-FUlcd Paper Plastics

Resin-tilled ijaper plastics are referred to as preforms. There are
twu types of preforms, flat and shaped. Flat preforms are made in the
form of flat sheets, whereas shaped preforms are vacuum-fitted to a
shape closely conforming to the shape of the final molded product.
Shaped preforms are used for deep forms and where high strength con¬
tours (e.g., automobile panels) are required. Long-fibered pulps such
as rag, which normally do not flow very well, can be used in shaped
preforms because of the reduced amount of flow required during molding.

Among the resins used in making resin-filled papers are the phen-
olics, ureas, nieljimines, vinyls, jiolystyrenes, and acrylics. The resin
may be added to the pulp as {!) a solution or emulsion, which is sub¬
sequently precipitated on the fibers, or (2) as a finely divided pow’der.

Phenolic resin emulsions are available which can be added directly
to the beater and precipitated by the use of alum and sulfuric or phos¬
phoric acid so that retentions over 907o are obtained.®* Papers con¬
taining 45 to 557c of such a resin are highly plastic in nature and are
suitable for low-pressure molding. These resins can be used in com¬
bination with synthetic elastomers such as Hycar, GR-S types, neo¬
prene, and vinyl polymers to produce products of very high impact
strength and elongation. Their principal use is irT the manufacture of
hardlxwird and pai)er-base laminates.

Polystyrene emulsions for beater addition are available commer¬
cially.** The emulsion, which has a particle size of about 0.2 micron
and a />H of 9.5, is added to the stock and precipitated on the fibers
with alum at a pH of 4.0 to 4.5. The amount of resin solids added
varies from 25 to 50% of the weight of the paper, and retention is be¬
tween 90 to 93%. A hard durable laminate is obtained when the treated
l>aper is molded in a press at 500 p.s.i. and 300° F. for ten minutes.

Finely divided, water-insoluble, pulverized resins generally have a
higher molecular weight than the water-soluble and emulsified resins
and do not penetrate the fil)ers excessively during laminating or molding.
These resins can be added up to 50% of the fiber weight, but high pres¬
sure (2000 p.s.i.) is required for molding. The addition of soybean
protein to phenolic resin is said to reduce the tendency of the resin to
agglomerate into sticky particles and also to improve the flow during
molding.**'**

•♦F. H. Snyder. T.. Spiwak and A. E. Moyer, Tappi 33, No. 4: 87A-90A (Apr.,

W. B. Moreh.iuse, Paper Trade /. m. No. 21: 465-408 (Nov. 24, 1949)

*• P. K. Potter. U. S. 2.436.328 (Feb. 17. 1948)

»* P. K. Potter, u. S. 2.436.329 (Feb. 17, 1948)

1260

PULP AND PAPER

Lignin has been tried as a partial or complete replacement for
phenolic resins. Lignin produces plastics of lower toughness and con¬
siderably higher water absorption than phenolic resins, although plastics
with good physical properties can be obtained when up to /5% of the
phenolic resin is substituted with soda-process lignin.^* The lignin can
be added as an aqueous solution and precipitated in the stock with sul¬
furic acid, or lignin which is sold in the form of a powder of about 10
microns in diameter can be added directly to the stock at the beater. In
one process,lignin is added to the beater in the proportion of 40%
lignin and 60% fiber. The pU is adjusted to about 4.5 and glue is
added to increase the retention, which can be as high as 9S%.^° Alum
lowers the strength of the final product, but increases the water resist¬
ance. About \S% of phenolic resin may be added later at the size press
to improve the hardness and water resistance of the final molded prod-
uct, A paper containing 40% lignin added in the atove manner ai^
\S% phenolic resin added at the size press can be molded at 750 to 8

lb. pressure at 150 to 155°

Some of the problems encountered in beater addition are poor re

tention of resin, foaming, and reduced speed on ^
a closed system, resin retention generally runs from 75 to 95%^ L g
fibered stocks are preferred from a strength standpoint, but hbers w
are too long tend to lower the retention. Retention can ,be measured by
:iem ext?aln, but this is likely to give low results if the resm has

been partially insolubilized by heating.

Resin-Itnpregnated Paper Laminates
In making resin-impregnated paper laminates, the l«^r

prel^ted pap- is then molded or laminated with other plies of impreg-

mSs of Applying Resin. The -sin may be
machine equipment jr iMnay bejp j^e wet sheet

on'lheTap- machine is carried * ^om 65?

supported by a wire^ ^y^e'pS-^ -sb depends upon the moisture

.. s. L. Schwarlr. J. C. Pew and H. R. Meyer, Pu,p Paper Map. Canada 46.

10: 771-779 (Sept., rOct 2 1947)

::g“ a li: S80-S82 (June, 1948)

alK Bauer, Paper Trade J. 125. No. 3: 28-30 (July 17, 1947)

XXI.

treatment with resins

1261

resin solution, temperature of the bath, speed of tlie machine, and pres¬
sure at the squectc rolls. Only water-soluble or water-dispersible resins
can be used. The fibers arc saturated with resin and somewhat em¬
brittled when this method is used.”

.■\nother method of adding resin is by a modified surface sizing
operation carried out on the paper machine. This method is sometimes
used to enrich sheets containing resin added at the beater. It has been
suggested that lignin be applied in this way by adding an aqueous sus¬
pension of purified lignin (5-10 microns in size) at a concentration of 15

to 20% solids.”

Most impregnation is done on off-machine equipment. *1 his con¬
sists of treating dr>- paper with a solution or emulsion of resin. The
paper is generally dipped into the resin solution and then is passed under
and between two metering rolls, after which the jiaper is dried to remove
sc^vent and, when thermosetting resins are used, partially to polymerize
the resin. The machine may lie equipped with sky rolls so that the sheet
can be brought out of the resin bath to increase the penetration by releas¬
ing air from the paper.** Most of the following discussion refers to the
treatment of f^per with resin solutions on off-machine equipment.

In one process, thin tissue jiajier is impregnated with an aqueous
solution of phenolic resin. The treated paper contains a high percentage
of resin and is sold to plywood manufacturers, where it is used alternat-
ingly between layers of plywood to build up an assembly suitable for
laminating. When pressure and heat (280-320® F.) arc applied to the
assembly, the resin in the paper acts essentially as a resin glue line.
The laper in this case acts principally as a carrier for the resin. Crepecl
pa|jer is sometimes used when curved shapes are to be laminated.

Resins Used for Paper-Base Laminates. Both thermoplastic and
thermosetting resins can Itc used for paper-ljose laminates, but thermoset¬
ting resins arc more widely used localise of their lower viscosity in or¬
ganic solvents, and because they impart the necessary physical properties
to the laminate. There are several principal classifications of laminates:
medianical (structural), electrical, punching, and decorative.

Phenolic resins are widely used, particularly in medianical lami¬
nates where water, heat, and electrical resistance are re<iuired. For
(Winching grade laminates, phenolic resins specially modified with plas¬
ticizers or reacted with drying oils arc used to yield laminates having
good plastidty and elastidty. The phenolics used in electrical grades

«E. S. Baocr, Pa^rr Trad* J. 125, Ko. 3 : 28-30 (July 17, 1947)

** Cooperative Report of the U. S. Forest Products Lahjratory, Paper Ind. 27.

Na 11: 1683-1689 (Feb.. 1946)

"F. W. Efan. Paper Trade /. 121. No. 14: 137-140 (OcL 4. 1945)

1262

nn.f ANn fApm

arc Koncmlly ratal) zc«l wuh amm<mb« amines, nr 44licr lrs% condiiclivr
catalysts in place of the >tr«Mijj alkaline catalysis normally

I’rca and luclaininc resin? arc used for making decorativr lanimatcs
Melamine resins are used where translucent, light*colored jwodwet* with
good heat and water resistance are required. Acrylics. alkyd». fioly*
esters, polystyrenes, and polv>invl resins arc examples of tlirrm<n^»U»ik

• resins sometimes used.

In the thennosetting field, both water-soluble and solvent-solulile
resins are used. As a general rule, the phenolic resins are applied from
alcohol solution, whereas the urea and melamine resins are ap|ilied from
water solution. Water-soluble resins are generally used fr»r imt»r^-
tion on the paper machine, whereas alcohol-soluble resins arc nwA widely
used for off-machine treatment. In some cases, no soUrnt is used, as,
for example, with polyester resins, which arc used as partially jwilymer-

ized liquids. '.i.

The primary purpose in impregnation is to saturate t s el wi

resin so that the final laminated product take.s on the c^raciemtic* cH

the resin." If too little resin is present (less than about 30/e). the w^

resistance of the laminate n-jll be low and it will he a

«et On the other hand, the mechanical projx-rtws of tlw laminate are

better if small amounts of resin are used. Resin adds

strength to the laminate, bn. maximum all-ron^

obtained when the amount of resin is jus. sufficient to fill all the s-«d.

"’om ^Hhe important functions of the resin is to

,he laminated product. Low molecular weigh. "

setting resins give the maximum water resistance because
Wher! thev‘a.; subscueutly polimerired in tfw
1 Because of Uieir polar nattire. these r«ins

smbilitv and a high degree of w^.er .rdgtt

to emirittle the fiters. On the offier hand.

(solvent-soluble) resins do not produce as

product as the water-soluble resins because ' ^ ^

L fiber as weU and there -

bers and hence less cross Iroking. Comparati

papers treated wi.Ii water-soluble and soIventsoluUe resms

'in Figure XXI-l-*' ___ . propoitiooal

Bennett paper given at .\nnijal

2f>-23. 1950 ) ^ „ ..

Straka, Paper Trade /. 126, No. 1

XXL

INTERNAL TREATMENT WITH RESINS

1263

to the viscosity.-'" Low-viscosity varnishes ixMietratc better and result
in products of better water resistance than high-viscosity varnishes.^”
To obtain a high degree of alkali and water resistance in a laminate to
contain 50% varnish, Kline^" recommends the application of 20% pen¬
etrating (low-viscosity) varnish and 30% regular (high-viscosity) var¬
nish, However, if maximum impact strength is desired, the varnish
should be of the viscous, non-penetrating type in order to keep the var¬
nish on the surface of the fibers as much as possible.^® Resins of high
degree of polymerization require less polymerization on curing, and
consequently shrink less during curing than resins of low D.P.

NO OF DAYS IMMERSED IN WATER

AT 25* C.

Fig. XXI-1. Effect of (a) solvent-soluble and (b) water-soluble
phenolic resins on the water resistance of paper laminates.

Paper Used for Paper-Base Laminates. The tensile strength,

stiffness, flexural, and impact strength of the laminated product are de¬
rived chiefly from the paper. The electrical properties of the laminate
are also dependent upon the electrical properties of the paper, so that
paper with low power factor, good dielectric strength, and high dielectric
constant are required for electrical insulation work. However, the most
important factor affecting electrical insulating properties is the amount
of resin absorbed by the paper.

Paper for impregnating must be uniform. The paper should be

free of fiber bundles and slime spots, since these resist penetration by

38 G. A. Albert, Paper Trade J. 101, No. 11: 127-131 (Sept. 12, 1935)

39 M. P. Seidel, Tappi 32, No. 8 : 374-377 (Aug., 1949)

^9H. Kline, Paper Trade J. 119, No. 13: 128-130 (Sept. 28, 1944)

1264

PULP AND PAPER

the resin and tend to cause blisters in the laminated product. The finish
should be low, since the adhesion between plies increases with increasing
tooth on the paper. Thus, low-finish papers are used where a high de¬
gree of bonding between plies is necessary, for example, when the lami¬
nate is to be milled or drilled parallel to the laminations. High-finish
paper may be desirable on the outside surface of the outside ply to in¬
crease the surface finish of the laminate, although this finish can be regu¬
lated during pressing.

The paper should have a /jH around the neutral point, because too
high a retards the curing of most resins, and too low a /»H results in
a brittle product. The paper should be free of bleach residues, since
these reduce the electrical insulating properties and cause degradation
of the paper during curing. The paper may be colored with direct dyes
or a few selected members of the acid dyes. As a rule, however, colored
pigments are preferred because they stand the heat of molding better

and are more permanent than soluble dyes.

The paper is generally unsized. Internal sizing has relatively little
effect on the penetration of varnish, but reduces the absorption of aque¬
ous resin solution. If sizing agents are present in the original paper, they
are decomposed under the heat and pressure used in lamination.

The moisture content at the time of impregnation should be low
and the moisture uniformly distributed, since moisture affects the pick-up
of varnish.'*^ High moisture content at the time of molding
the ease of molding, but if the moisture content is more than the
dimensional stability and moisture resistance of the laminate are redu« .

Thin papers are desired, since they absorb resin more uniformly
than thick papers. Thin papers also make it possible to “btam a higher
degree of orientation if the sheets are cross-laminated.
hand, thick sheets produce laminates with greater ^

vertical direction, because of the greater number of fibers ly g
nearly upright position in thick papers.

The most important property of impregnating papers ^
sorbency of the paper. High absorbency is desirable when g
trical resistance and minimum water absorption are required, bee
these properties depend upon the absorption of a l>'Sh Percentage o res ^
On the otlier hand, when a laminated product of high impact stre ^
°nd good punching cp.alities is desired, the paper should -e a lowj^

sorbency to reduce the amount of absorbed resin. ^ P

f ' ic tn he added tlie sheet should have a high densi y
of resin is to be added, ti e^ j^e density should not be so

tion requirements. DicKennan j,*'

<. A. H. Croup, Pafer Trade J. 118. No. 20: 175-178 (May 18, 1944)

XXI. INTERNAL TREATMENT WITH RESINS

1265

gravity for a laminated product made from sulfite paper, since laminated
products of lower specific gravity are lacking in compressive strength

and water resistance.

Almost all types of paper have been used in making paper lami¬
nates, but the most important are rag, kraft, sulfite, and alpha. Ground-
wood and soda pulps are too weak for general use. Kraft papers are
used where strength is necessary and color is of no importance. Sulfite
pulps of high alpha cellulose content are used where good electrical char¬
acteristics are desired or where the laminate is to be used for decorative
purposes. Alpha pulps are used for maximum electrical resistance
(e.g., radio insulation panels). Rag papers are used where minimum
color is desired and where the laminated product must have excellent
molding, machining, or punching qualities. As a rule, rag papers show
better bonding between plies than sulfite or alpha papers.

TABLE II

Water Absorption of Laminates Made from Various Types of Paper

45% Phenolic Resin

Type paper absorption*

0.005 Hard-finish kraft . 5.7

0.009 Kraft . 4.1

0.006 Sulfite . 2.3

0.004 Alpha . 1.7

0.004 Rag . 1.3

0.010 Cotton linters . 1.8

• Per cent gain on soaking twenty-four hours, sample 1x3 in.

Pitzer'*® lists the water absorption of laminates made from different
papers containing 45% phenolic resin, as shown in Table II. These re¬
sults show that kraft papers produce laminates of much higher water
absorption than alpha paper, even when the same amount of resin is ab¬
sorbed. Furthermore, kraft papers absorb less resin than rag and alpha
paper for the same method of treatment, and this further increases the
water absorption. Recently, however, kraft papers of greater absorptive¬
ness have been produced. Dickerman*^ rates alpha, unbleached sulfite,
and unbleached kraft as follows:

Property

Absorbency ..
Color stability
Electrical ....

Tensile .

Toughness ...

Alpha

Sulfite Kraft

unbleached unbleached

1 3 2

1 2 3

1 3 2

3 1 2

2 3 1

♦*G. K. Dickerman. Paper Trade J. 118, No. 26 : 239-242 (June 29 1944)
J. C. Pitzer, Tappi 32, No. 8 : 382-384 (Aug., 1949)

Private communication from G. K. Dickerman

1266

PULP AND PAPER

Two or mor£ different grades of paper may be used,in making a lami¬
nate. For example, a high-grade paper such as alpha or rag stock may
be used as the top sheet and strong kraft used in the middle plies of
decorative laminates.

Lamination and Molding. After being treated with resin, the
sheet may be passed between squeeze rolls to drive the resin into the
paper. In cases where maximum penetration is desired, the treated paper
may be wound into rolls and allowed to stand before it is dried. This
increases the time for penetration and allows the resin to become equal¬
ized in the sheet.

After impregnation, the paper must be dried to remove solvent. It
is common practice to dry the sheet to an accurately controlled percent¬
age of contained volatile matter. During drying, a partial polymeriza¬
tion of the resin occurs which serves to control the amount of resin flow
in the subsequent molding stage. All thermosetting resins have a criti¬
cal drying temperature beyond which they cannot be heated without
prematurely curing the resin. The drying should not be so rapid that
moisture is trapped within the sheet. After drying, the paper is ready

for laminating or molding.

In making paper laminates, a number of sheets of resin-impreg¬
nated paper are placed in a pile and then laminated under heat and pres
sure. The number of sheets used depends upon the thickness require
in the final laminate. The various plies are selected with an eye to the
best utilization of raw materials. For example, it is common practice
to use several layers impregnated with phenolic resin as the core of e
laminate, and then to use paper impregnated with hght-colored urea-
tor melamine-) formaldehyde resin as the surface layer A htgiy pg
mented sheet may be used between the surface ply and the inner p ies m
prevent the dark-colored phenolic resin from bleeding ‘.'’''“8^ *° *
surface sheet. Pigmented sheets prevent show through in cases wlier

the surface ply is fairly transparent.

The sheets may be cross- laminated by laying alternate sheets with

their machine direchons at right angles. Parallel laminated products
show excellent strength in the machine direction, but ^ '

in bolh the lengthwise and crosswise directions, al houg i y ^

slight weakness on the bias.« All laminates hav^ a line of w'^nes ^ ,

^ ^ n1ip<; but in general, laminated plastics are mecl }

tween “te pbes, but i section)

.stronger than pulp-hlled plastics (iiescrine

because of the support tiiriiished by the paper web. S.idel point

n' I f 11'i Nn 2fi-230-242 (June 29, 1944)

n :: h :- 549:55, cnov ., ,950,

IKTOSAL TMATMENT WITH lESIXS l-W

titti €%m wiA mm which hive been nude is free is possible of 6ber
onmtitiun, there will be coosidenble orienfilion in the finished laminate,
dne prolably to tensaon and cnn^iression exerted on the pilfer during the

treafinc opcntkm

Lmiaiting is done on large, hydraulic t>‘pe presses e<|uij»ped with
ftcMn-heilrd platens. If paper plastic tubes are to be nude, the i^per is
in tn tube form and then is cured in an osm or on a mandrel under

heal and pressure

During the early stages of molding, the resin softens and flows,
fiBinf in the sxads in the paper and bonding together the various plies.
Dpon further heatim;. the mass becomes relatively homogeneous, ami
thr resin begins to pol y mr ri ie and harden, growing from a product of
low molecular weight b> oar of high molectilar weight. 1 he final jioly-
Bwntcd resin is msululilr in water and is generally resistant to most
chcmtcals I'ndrrruring resultf in |ioor adhesion lietween fitirrs and

Fw XXI*Z £i«c« oi aoMint timr on vstrr absurptMi uf iwiwt laminates.

resin, and produces a Untinate of low tensile strength ()\\ tlie other
hmtd. oscminng produm brittleness and results in a porous resin stnic>
lore •• The efleci of moMmg time on the water resistance of tlie laminate
(mmnwrd by the |«r cent wrtglH incrcase'‘cNi soaking in water) i.s shown

• Figure XXI-2. where H can be seen that overcuring results in a slight
mcfcaar in water a ba orpt i on -

molding and laminattng characteristics defiend upon tlie heat
awd preisun nard and the type and amount of resin present in the paper,
Incrcassng the amourn of plastic mstrral in the sheet reduces llie amount
of presauee m}mfvd However, the tjest all*round properties in the
tel tesnasr are olMatncd when there is just enough resin to fill all tlie
* *** sheet. More resin than this reducrt the strength, whereas

* amount is iMt sufficient to impnn the required water, chemical.

•C J Siraka rrw4f /. m. No 14: Itt-IM (Apr. I, IMS)

PULP AND PAPnr

12fj8

or electrical resistance to the laminate. In sonK cases, more resin ift used
in the outer plies than in the inner plies. Increasing the amount of
moisture in the sheet increases the flow of resin, but reduces the dimen¬
sional stability of the laminate.

The earliest laminating processes used high temperatures and rela¬
tively high pressures (1.000 p.s.i.). Since then, new resins have l^een
introduced which can be laminated at lower pressures. In some casea,
pressures as low as 50 p.s.i. can be used, although ordinarily pressures
in low-pressure laminating run from 200 to 300 p.s.i. All the common
thermosetting resins, including the urea-formaldehyde, mclamine-fomid-
dehyde, phenol-formaldehvde, and resorcinol-formaldehyde, can l»e
molded under low pressure. Low-pressure laminating is be¬

cause it permits molding of very large sheets.

Pulp-Reinforced Plastics

A pulp-rcinforccd plastic is a plastic molding comfK>sition which
contains filler pulp added to increase the strength. A complete stu y o
the properties of pulp-reinforced plastics would 1)C far remosed from

papermakiug and consequently cannot be tak^

completelv ignored, for the plastics industry consumes a la ge amount d
Lerest to the %lp chemist to know something of the requirements .

PriL‘'”of "fLous Fillers, Fibrous fillers 1-*™^

function in molded plastics. i nc^ ^ and

Characteristics Thev increase the strength, heat, chemicd and

liberated by the resin nol^swell in tins

w^y, tol^Tproduct would shrink and not conform to the mold di-

Among the fiUers commonly used are
ous lengths, and fo, opaque moldings, whereas

purified wood pulps or alpha ceUuto ^ g ^ obtained from
colored, translucent, or %ht-resistant moK^J^-* ^

the molding of urea and melamine resins. k\ o^ ^ ^

compressive strength and -ter rcsts^ce. “* Ub-

(erred for high sheanng strength. Work at the

« Pni>rr Trade /. IW, No. 23 : 225-227 (June 194:)

47 C H. Pottenger, Paper l raae j.-l

XXI. INTERNAL TREATMENT WITH RESINS

1269

oratory^® indicates that sulfate pulps and hardwood neutral sulfite pulps
produce strong plastics, but groundwood pulp plastics are lacking in
toughness, although still tougher than wood flour-filled plastics.

Lewis and Ingle^® ^ive the following analytical constants for pulp
suitable for molding: minimum alpha cellulose content of 85.5^, maxi¬
mum pentosan content of 4%, maximum wood resin content of 0.2%,
and a maximum solubility in 1% sodium hydroxide of 4%. The hemi-
cellulose content should be held to a minimum, since hemicelluloses in¬
crease the water absorption of the molded product. Furthermore, hemi¬
celluloses tend to cause yellowness in light-colored urea-formaldehyde
plastics.®® Resins and tannins are undesirable because they lower the
color, especially if solvents are used in which the resins are soluble.

O
tn

1 j I m

h- X
~ h-
? O

t UJ

S ^
— I-

-j (/>

20,000

15,000

10,000

^ 5000

Maximum strength

Modulus
of elasticity"

Proportional limit

Elongation

J—I_I_I_ » I I

30 40 50 60

RESIN CONTENT, %

2000 ^
3-i

UJ w

1500 oo
o
CO o

1000

Z)
Q

5^ O

2 9

I o
0 o

-J

1.42 <
1.40 ^
1.38 y

1.36 o

1.34 Q-
cn

30 40 50 60

RESIN CONTENT, %

Fig. XXI-3. Effect of amount of phenolic resin on properties of kraft-filled plastics.

High lignin content is undesirable in pulps for light-colored molded
products, but not objectionable in dark-colored products. Lignin func¬
tions as a thermoplastic resin, but it has a lower heat resistance and a
higher water absorption than phenolic resins. One process calls for en¬
richment of filler pulp with lignin obtained from waste cooking liquor.

Effect of Resins. The resin may be added to filler pulp as a dry
powder or as a varnish. After the resin is thoroughly mixed with the
fiber, the product is ready for molding. In one process, alpha cellulose
is impregnated with an aqueous solution of urea-formaldehyde resin,
dried in vacuum, mixed with coloring agents, and then blended with the

Sept ^ ^ Meyer, Pulp Paper Mag. Canada 46, No.

50 (Aug. 21, 1941)

i acfyim

12/0

PULP AKD PAPFJl

resin aiul other ijigriMients.** The resulting comi>osiiion can lie molded
at a tcniperalure of al)out 140* C. and a pressure of 3,0(X) to 6,000 p.t.i.

to produce a translucent product.

I ncreasing the resin content of the molding composition decreases
the pressure rc«iuired in molding. However, excessively higli resin con¬
tent is likely to result in resin pockets in tlie final product. In most
cases, the resin content varies from 35 to 50^ of the weight of the mold¬
ing composition.

The resin acts as the jirincipal lionding agent. Fiber-to-filier bond¬
ing is only of minor importance; Ideating the pulp actually decreases the
tensile strength of the molded product.” Increasing resin content re¬
duces water absorption, hut also tends to reduce the trrtighness and
strength of the plastic, although tensile^ strength is not always adversely
affected.” The effects of increasing amounts of pliOTolic resin on the
])roperties of kraft pulp plastics are shown in Figure XXI-3.

«S. L.’ PewTnd

Handanger an^R^ H. Mosher, %fodfrn Plastics 20. No. 11: 76-79 (Joly.

1943)

CHAPTER XXII

COATING WITH RESINOUS MATERIALS

This cha|.>ter is concerned with coated |■>a|)e^s, |)articularly those j)a[>ers
coated with resinous inateriais. These |japers are made by applying a fluid
mixture of resinous coating com|X)und to the surface of the paper and then
drsing. The base paper may l»e either supercalendered sulfite, kraft, glass-
ine, or cellophane. The coating mixture may l)e either a lacquer, an
aqueous dispersion, an emulsion, latex, organosol, plastisol, or a hot melt.
Cellophane and glassine make ideal lase .stixrks l>ecause their smooth, non-
porous surface makes it possilde to obtain gotxl results with small amounts
of coating.

Papers coated with resins are usrtl primarily for wrap|)ing and [ackag-
ing where tlie function of the paper is to protect the contents. C oated
wra|)ping j)apers liavc many ads’antages over uncoated wraj)ping pa|>ers,
liecause several different functional pro{>erlies can !)e combined in a single
sheet of coated paper. For example, unco:ue<l glassine has g(H)d physical
strength and a high degree of water resistance, hut it is not resistant to
water vapor. Coated glassine ha> the desirable properties of the uncoate<!
product and, in addition, is quite resi.**tant to water vapor.

Coated j«pers are sometimes use<l for their improved apjx'arance
(e.g., coQte<l fancy box |ia|)ers). ’I hese iKqjers In-long in the protective
coating field for two reasi»ns: (J ) liecause the same materials and pnx’e.sses
are used, and {2) liecause one pur|X)se «*f the coating is to protect the ]>aper
from soil. In many cases, the coating is Ixith prolectivie and decfirative, as
in the case of cartons. laliels, shelf pafjer, lamp shade stock, and hottle-cap
stock, where the coating is primarily protective hut also serves a decorative
purpose. 5kNne coatings are purely protective, for example, coatings ap-
|4ied to the inside surface of pajxr cartems to prevent the loss of moisttire
and essential aromatic conqwunds and to retain the crispness, firmness, and
other qualities of the packaged product. The various reasons for coating
papf** with resiiKHis materials may Ije summed up as follows: (/) to im-
pro\-e the decorative value, (2) to provide i)rotection against oils, greases,
water, water vapor, ga.-ses. clteiiiicals. solvents, abrasion, and heat, (3) to
proside heat- or pressiire-sealahle coatings for sealing tajjes, bread wrap-
|<rs, gaskets, etc., and (4) to proside electrical insulation.

borne of the general properties retjuired in w rapping |>a|>er.s are high
j>irei^h (particularly resistance to tear an<l puncture), g<xKl creasing
qualities (partkubrly wlirn used on automatic madiiiiesj, gfxxl color,

1271

1272

PULP AND PAPER

cleanliness, a high degree of resistance to the penetration of liquid and
gaseous materials, good printing qualities, and good aging properties. If
the wrapper must stand stretching, creped paper may work best.

Before starting this chapter, the reader is informed that there is a sub¬
sequent chapter on the properties of resins. The properties of resins are
discussed in a separate chapter because of the multiple uses of resins in
such operations as coating, impregnating, laminating, and saturating, which
makes it impossible to assign a particular resin to any one chapter. Fur¬
thermore, the number of resins being used is so great that a description of
each resin in a chapter on coating, for example, would make that chapter
far too bulky. The rule which has been followed has been to discuss the
general properties of resins in the chapter on properties of resins, and to
discuss the more specific properties in the chapter pertaining to the use of

these materials.

Fundamental Properties of Films

Success in protective coating depends upon the ability to produce a
satisfactory film of resinous or w'axy material on the surface of the paper.
The coating must meet the following requirements: good adhesion, freedom
from tackiness, freedom from blocking, good flexibility, freedom from color
and odor, freedom from toxicity, high ^gloss, resistance to deterioration on
aging, resistance to water, oils, water vapor, etc., and good heat seal.

Adhesion of Films

The coating must adhere strongly to the base paper. Adhesion is par

ticularly important if the coating is to be heat-sealed.

Most resins adhere fairly w'ell to paper, but may not adhere so we
cellophane and other smooth materials. The degree of adhesion depends
upon the tj-pe and magnitude of the bonding forces between the e^.n?
and the paper surface. The attractive forces which may be

order of increasing

hydrogen bonding, and covalent bonding. Polar resins have a '’•gh deg e
of specific adhesion for cellulose, whereas non-polar resins are depe ^
upon mechanical adhesion. According to McLaren and Hofrich ,
order of increasing adhesion which various substituent groups exhibi fo
^HTfolfows: alkyl, nitrile, chloromethylene,

1 A LirHrnwl Thev found poor adhesion to exist between c
butyral, and ethylcellulose, pol.vvinyl

a^S^llulose, and excellent between cellulose, polywtnyl

1 A. D. McLaren and C. H. Hofrichter, Paper Trade J. 125, No. 19: 96-100 (Nov.
7, 1947)

XXII. COATING WITH RESINOUS MATERIALS

1273

alcohol, the urea- and melamine-formaldehyde resins, and the phenolics.
Adhesion of resinous coatings for paper can be improved by saturating the
paper with rubber or resin emulsions.

Another method of obtaining satisfactory adhesion between cellulose
and a material which normally will not adhere to cellulose is to precoat the
paper with a substance which is adherent to both the paper and the second
coating. Cheyney^ has patented a process of precoating paper with poly¬
vinyl chloride and then overcoating with vinylidene chloride.

Adhesion takes place in two stages: (1) migration or movement of the
polymer to the interface and (2) sorption of the polymer at the interface
of the paper. The degree of adhesion is related to the average molecular
weight and chain length of the resin,^ a degree of polymerization in the
neighborhood of 50 to 300 being most effective.^ According to McLaren,”
the lower the tack temperature of the resin,, the better the adhesion. The
density, strengh, and adhesion of resinous films can often be improved by
heating the coated paper,®

Film Flexibility

The continuity of the film is an important factor when coated papers
are used for protective purposes, e.g., as wrapping or shelf papers. If the
film is cracked or checked, it will not be effective in holding back water,
gases, or oils. Resins are commonly classified as film-forming or non¬
film-forming.

The important property is the degree of film continuity after the
coated paper has been put to use, which usually means after it has been
subjected to a considerable amount of handling. Many coated papers are
used under conditions where they are flexed and creased many times during
their life. Sometimes flexing takes place at relatively low temperatures,
e.g., as in wrappers for frozen foods. The question of flexibility is of great
importance when the coated paper is used on automatic wrapping machines.
An important factor is the amount of plasticizer in the film. The thickness
of the film is also important, since thin films are more flexible than thick

films. The effect of aging on film flexibility is important, particularly when
plasticizers are used in the coating.

No method of testing film flexibility is completely satisfactory. One
simple method involves creasing the paper in a creasing device and then
smearing the coating with a penetrating substance, such as water or oil con-

^L. V. E. Cheyney, U. S. 2,392,972 (Jan. 15. 1946)

^ 7 ^’ K. Schneider and G. W. Seagren, Ind. Eng.

Chem. 33, No. _1: 72-74 (Jan., 1941)

*H. Mark, Physical Chemistry of High Polymeric Material, Interscience Pub¬
lishers. Inc., New York, N. Y. (1940)

” A. D. McLaren Paper Trade J. 126, No. 22 : 263-264 (May 27, 1948)

■H. L. Bender, Ind. Eng. Chem. 29, No. 11: 1130-1134 (Oct., 1937)

1274

PULP AND PAPER

taiiiing a soluble dye, to see if any penetration takes pkice at the creases.
The humidity at which the paper is conditioned has an important effect on
the test results.

Hardness oj Films

Most resinous materials exhibit a transition point at which the material
changes from a hard substance to a soft, sticky substance. This is known
as the .softening point. The softening points of some of the common resins
are listed below. The three-dimensional resin polymers do not exhil)it a

softening point.

N on - ft Im-f orin i n R
resins

SofteninR

temperature

Shellac
Dammar
Manila
Kauri .
Rosin .

70-80’ C.
65-70’ C.
70-90’ C.
90-125’ C.
80-85’ C.

Film-forming

resins

Cellulose acetate .

Cellulose acetate butyrate

Ethylcell ulose .

Polyvinyl acetate .

Cyclized rubber .

Softening

temperature

220-250’ C.
205-225’ C.
165-200’ C.
45-85’ C.
50-75’ C.

In substituted products; the softness depends upon the substituting
anent and tlie degree of substitution. Cellulose propionate produces
sLr film than cellulose acetate. Ethylcellulose has its mintnium softemng
rmhtt at a decree of substitution of about 2.4. Asphalts, the higher alk)
iL '^Ight acrylic and methacrylic resins, and " oto-m
have too low a softening point to be used as surface coatings by _

Film hardness is related to the durability of *he coatmg^ Durabto)

can be measured by a special abrader equipped with

faces and a numerical counter to ^ complete disruption

n-. —-

her of rubs. • • + ^ ertfr it exhibits a condition kno^^■n as

■Rlockino. If the coating is too sott, it exiiiDits ,,.cctirp

blocking, in which adjacent coated when

so that the film continuity and g oss -e dest^yed

the coated paper is stac e in pi s. ,n.,i pressure in the roll.

into rolls when thermal e.vpansion sets »P CTubiecting a stack of

mocking can be determined in the lakiratory by subjecting

XXII. COATING WITH tESINOUS M.\TERIALS

1275

coatni sheets to a definite pressure ami tenij^erature. One inethcxl calls
for stacking twelve slieets of tlie lot specimen between two pieces of plate
gbss under a pressure of 1 p.s.i. for twenty-four to seventy-two hours. The
shrets can be pbced in desiccators maintained at constant relative humidity
and the whole as.senibly pbced in an oven maintained at a temperature l>e-
low the softening ^loint of the coating. The results are reported as; (/)
no blocking, if the coating retains its gloss and the sheets slide readily over
each other; (2) slight blocking, if the sheets do not slide readily; (J) com¬
plete blocking, if the surface of the coating is marred, or if the coating pulls
fibers, or if a complete seal is formed. Most coatings are fonmilatetl to be
non-bkicking at 125 to 1J0° F. If the coating does not meet this require¬
ment. it must be refonnubted. or it can be dusted with starch, talc, or mica
to |ire\'ent blocking.

Heat Seal. For certain purposes, coated pa|>ers must have heat-seal¬
ing properties. Packaging papers are frequently heat sealed by heating two
plies of the coated pa|»er with a hot iron for a few .seconds at a teni|)erature
of alxHit 275 to 350® F. The coatings should fuse and form a strong liond
under these conditions. Heat-seal adhesives are cliscu.ssed in Chapter XX.

Slippage. Another im|)ortant projierty retpiired in certain coatings is
slipfoge, or the ability of the coating to slip over hot metal surfaces. Slip-
foge is sometimes imiiartetl to the cixiting by adding paraffin wax to the
coating formula.

Gloss

Gloss is important wlicrever the coating is used for its decorative ef¬
fect. Gloss is particularly inqiortant in overprint varnishes for labels,
covers, etc. On the other hand, gloss is relatively unim|K»rtant in wrapping
papers. Gloss is olitained by the use of the jiroper resins in the coating.
Coated fapers can lie given a numerical rating for gloss by comparing to a
stambrd of 100 obtained by covering a sample of the imc«*;ited pajier with
a jiiece of pbte glass.

Gas Pertnfability of Coated Papers

Many articles are susceptible to oxygen and must lie .sold in packages
having a low degree of gas permeability. Coffee, for example, is deterio¬
rated by oxygen and should lie packaged in containers jicrmitting the entry
of less than 3 cc. of oxygen per 100 g. of coffee.' Wrapfiers for certain
iwlished metallic articles should have a low permeability to oxygen in order
to reduce corrosion of the metal. In packaging tea, butter, shortening, and
starch, wrafifiers must be chosen which are impervious to gases in order to
present the absorption of foreign odors. Spices must be packaged in such
a way as to reuin the vobtile odors and essential oils, which are a valuable

* L C Cartwrifht. Tradr /. 124. So. 25: 275-277 (June 19, 1947)

1276

PULP AND PAPER

part of the product. On the other hand, some gas permeability is desirable
in wrappers for cheese in order to permit the passage of carbon dioxide

liberated by the cheese.

Gases penetrate through films by one or both of the following methods;
(1) pore type penetration and (2) solution permeation (whereby the gas
dissolves in the sheet, diffuses through, and evaporates at the other side).
Pore type penetration is a factor once the gas reaches the fibrous part of
the coated paper, but is of relatively little importance in the penetration of

surface films.

Permeability depends upon the type of gas and the temperature.
Davis® found that the permeability is highest with carbon dioxide and de¬
creases in the following order: carbon dioxide oxygen nitrogen. This
is the reverse of that expected from Graham’s law, but can be explained by

differences in the resonating nature of the gases.

Gas permeability can be measured by using the specimen as a barrier

and applying pressure to one side. The gas then permeates from the high-
pressure to the low-pressure side, thus changing the pressure of the gas.
Either the change in volume or the change in pressure of t e gas ^
measured. Increasing the applied pressure results in a great increase m he
rate of transmission if'the penetration is principally through the pores of the
paper On the other hand, increasing the applied pressure does not change
fhfrate of penetration to so great an extent if solution permeation is in-

2'°'''Davis’ has described another slightly different method for measuring
>,fermeaMW in which two or more gases are used, so that the total press^e
fs eoual on Lth sides of the test sheet. The proportion of gases is
to maintain a significant difference in the partial pressures eUveen e w

sides of the sheet.

Water-Vapor Resistance
Water-vapor permeability is a special case of

of area and unr^ tlnilnfatlom^^

differential. . rpnnired in packages containing ce-

High water-vapor ,vhich tend to cake and harden

r. s. 7 123 No 9: 97-104 (Aug. 29, 1946)

8 D. W. Davis, Paper Trade /. ino. v.

» Idem.

XXII. COATING WITH RESINOUS MATERIALS

1277

water-vapor resistance.’® Furthermore, there is no relation between the
water-vapor resistance and the sizing. The lack of correlation between
sizing and water-vapor transmission can be explained by the fact that liquid
water passes through the pores of the paper, whereas water vapor is trans¬
mitted through the fibers, as well as through the interfiber pores. Paper
can be sized against liquid water {1 ) by coating the individual fibers with a
substance which lowers the contact angle with water (e.g., rosin) or (2) by
filling in the pores of the sheet with some hydrophobic substance (e.g.,
wax). However, these methods do not appreciably increase the water-
vapor resistance of the paper. The only means by which the paper can be
made water yaporproof is by coating the surface of the paper with a con¬
tinuous film of a hydrophobic substance (wax, rubber, etc.) which com¬
pletely covers the surface of the sheet, as well as any fibers protruding
above the surface.

The above explains why a paper which has been surface-coated with
wax will be relatively water vaporproof, whereas a sheet impregnated with
wax will not. It further explains why a paper which has been impregnated
with wax will absorb nearly as much water as an unimpregnated paper (al¬
though the rate of absorption will be lower) unless a surface film of wax
is present. Water which comes into contact with one part of a sheet will,
in time, become uniformly distributed throughout the rest of the sheet.
Consequently, if moisture should pass through a break in the surface coat¬
ing, it will spread throughout the entire sheet. This makes it necessary to
maintain a continuous surface film of protective coating, because otherwise
the protective qualities will be lost.

Moisture appears to move through surface films in the condensed state,
i.e., as moisture having a density approaching that of liquid water.’^ Solu¬
tion permeation plays an important role, i.e., the water dissolves in the sur¬
face film of coating material and passes between the individual molecules
until it reaches the other side of the film. In severe cases, the presence of
water causes whitening of the film, resulting in a spongy, porous film. Be¬
cause of solution permeation, a film may not be resistant to water vapor,
even though it is impervious to air.

The transmission of water vapor is directly proportional to the area
exposed and the time of exposure. For homogeneous plastics, the amount
of water passing through the film is inversely proportional to the thickness
of the film.’2-i3 Even with coated papers, there appears to be a linear rela-

10 A. Abrams and W. A. Chilson, Paper Trade J. 91, No. 18: 175-180 (Oct. 30,
1930)

lip. T. Carson, Paper Trade J. 125, No. 19: 118-128 (Nov. 7, 1947)

12 P. M. Doty, W. H. Aiken and H. Mark, Ind. Eng. Chem., Anal Ed 16 No 11 •
686-690 (Nov., 1944)

“CL. Comar and E. J. Miller, Ind. Eng. Ghent., Anal. Ed. 15, No. 12: 737-740

A ^ r ■ ^

5 s♦ I* ./ ■''-■tirjft

t I

WV'T'S ■?>.,«

rvt^' AND< rArsB

• ^ - 3 u^. ‘*4 ^

^ tinn^ip'liciwrm ihc Uiickncwi <rf ihe aoltng aimI the rrsutance to vapor

iSl —t ■ A - - 1 ^ ^ ' " - - ^ ^ -— lIT^

r '1 A prtacuniimU

5^1. humidity ol ihr atr.** An inerrax in humidity on,one side"o< the sh^ i

tn il^ mftmii&sinfi ni£. The effect of humklitv"

T'\

V ^

l^ftmilarly STynt on him* oJ hi^ ahaorptioit. Some films whi^
highly vijwr|*roerf under ordiiwry eondttioos show a hi^ rale of trana
Sion at hiifh humidilieiu These fihn* ahso^ equilibrium w

air on the high humidily side of the she<i.!and iM with the a%^rsge ot me
ism wdew’* i Hi the other hand, film* of low water alworptkxi (e.g.. poly-
rthylrtie) show a uniform swriaiksn in moiahire coolefrt thfough«*J «*«
hhn.'* and henw are lem aJIecied hy ehang^ in relive humidity. Tnd^
erriam nfrumauiice*, iwimeabilUty is directly propwtional I? ^

nmMifr airtmw* tmM iht <ilm.« •' h«rt <•»» rH»rt«>»»»P *>».»« ^•»>’'
Md Ihrouihaiit the cnmplete rangr of
«»•• lomi • *mt rrtatinmJiip «<*

dtHerenlAl below a relalive humidily of 75^. revi ve ham ity »-

(omUM Mill inniirtitm* *«•»*. i^nwpbilitvt" 4««l1» pro(iortKi«iil

^ p«r*«ir« diBmiili.1 H«ro»t. «h« '

,•««« liiawTW. »«4l«. pot • *«*« «»

(nml. prfmwWily iiWTm«» withitntrrM*d lnn|wr»t»«. b*«^

_ -tl

Ihr »»pot iwrwutr diUrrrow

» bIk <4 wpot inuwmiMion nrt* »«»m«t ihr r«>pr««i

Thr<»rtiertly. • pw . Aikrii" (o«n4 lh.1

«J Ihr iri^ilulr irmpml»»c^*«W I. . «««« iw Ai^

,h, M innmxMan ^ ^ rM-^ "«««h *

0* F. 1. .h«« 1/51 j>'• J’?!-

„ „wh, M f K* “

iajar^^

: aL ‘w." Miit '*•"'

fDer. II. IHIli

Charch V7'.?„

^ ri kw w B-i. cB'fjf- ^"

«t jrt' “

firratah

m _

8m«t*e

7»^l

‘^i^iV r I

XXll. ttJATIXC WITH RESINOUS MATERIALS

1271 )

HMSsktt rate by sofiwiing the ctoling, i^articularly when K>\v-incIting-|xmU
niRteruls, c.g, parartiii, are user!.

There are nuiitrrous methods for measuring water vajwr ijerineabillty
ci paper and paperboard products. In one method, a sample of the test
material of known area is clamped over the lop of a small container which is
panially filled with water or a saturated salt solution. The specimen is
sealed to the rim of the conuiner, using a mixture of paraffin and niicro-
crystalKne wax to make a va|ior*tighl seal. 1 he assembly is placetl in a
cabinet under controlled humidity and air cirailation, and left there for a
dpecified length of time, usually twenty-four hours. The loss in weight is
detemtined and reported as the grams of water va|x>r |^r 100 sq.in. or |)er
square meter of surface jier twenty-four hours. In a variation of this
method, the container i$ fiUed^with a desiccant, e g., anhydrous magnesium
perchkintr, and the a>sembly is placed in a cabinet or room maintained at a
relative humidity of 50% and a temperature of 73® F. In this case, the
gain m weight is reported. In a statistical study of this method, Rentier’*
found that tlie greatest source of rariahility in results is a$signal>le to the
day on which the sanqdes are tested and the position of the samples in the

test cabinet.

Pierce and Helms’* describe a variation of tlie aUne method. In their
method, the assemldy is scaled in an impenncalile cell and stored at the
proper temperature while a controlled current of drie<1 gas is jiasseil through
the cell to remove the water vapor escaping from the asi^emhly. The escap¬
ing water vafior is absorlied by a desiccant which is weighed at frequent
intervals

Most of the methods used for measuring water-vapor permeability
are based upon a gravimetric measurement of the gain or loss of water.
This is vrrv slow, because enough time must elapse to result in the transfer
of sufiioem water to cause an appreciable change in weight. Van den
Akker” describes twro methods using an electric hygrometer for measuring
the chaiige tn relative humidity resulting from the transfer of moisture.

HTst method, called the dynamic method, involves the passage of dry
air of known relative humidity over one side of the test sheet while the
•hret is held in a diffusion cell under carefully controlled conditions of
irmperaturr and humidity. The relative humidity lielow the test sheet is
mainiainrd at a omstant high value by means of a saturated salt solution.
The relative humidity ci the air passing over the top of the sheet is meas¬
ured at regular intervals and plmted againvt time. The tnttiaJ rate of change
n taken as an indication of the water-vajior permeability, and ln-cause this
mrthod not call f«^ equililirium. the methoil is very rapifl. According

» « w 6: 65 f Auk. 7. 1947)

j A. > M dm Akker. r»wrf/ /. 126, No. I; 6-10 (Jan. 1, 1948)

1280

PULP AND PAPER

to Van den Akker,^® this method can be used for determining water-vapor
permeabilities as low as 0.001 g./lOO sq.in./day (at 0° F. arid a relative hu¬
midity differential of 73^) in from four to five hours. This method is
suitable only for the testing of relatively non-hygroscopic films.

In the second method, called the sweep-gas method, dry air of known
initial humidity and known rate of flow is swept over the upper surface
of the sheet. The air in the compartment below the sheet is maintained at
a definite high value by means of a saturated salt solution. This method is
based upon the principle that the air leaving the upper compartment changes
in relative humidity according to the volume rate of air flow, the permeabil¬
ity of the specimen, the area of specimen, and the initial relative humidity of
the entering air. The volume rate of flow of air in the upper chamber is
regulated so that the relative humidity of the air leaving this chamb^ is
carefully maintained about 3% higher than that of the incoming air. This
difference in humidity is accurately determined by an electric hygrometer.
The water vapor permeability is determined as follows

W.Y.F. = Kir^-rx)pQ

where K\s a constant, is the relative humidity of air leaving the chamber
Ti is the relative humidity of air entering the chamber, p is t le ensi y o

s turated water vapor at the tetnperature used and Q -
of flow of air over the specimen. This method ts an equd.bnum (s^
state) method, and consequently is more tune-consuming J

method. It is, however, free from errors due to sorp p .

in testing y absorptive, it tends to lower the hu-

m!dit”he air on the iigh hurntdity side of the ‘est. Suffeient time ™s.

be allowed for the the rate

determine when a steady rate pnn«;tant slone is obtained,

values should be plotted against time until a co ^ P

.-V steady rate of moisture transmission yL i„<,„ases with

for two to three weel<s.“ In some cases, the ^ is

time,” and consequently the test ® ™ be exposed

brought about in the material bei exposed in use, e.g.,

in the test apparatus in the saine should be tested with the inner

a sheet designed for packaging j thf toting apparatus. The

surface exposed to the low hiimi i y si ability to retain their

utility of packaging materials depends upon their ability

28 J. A. Van den Akker, 232-238\May, 1949)

29 W. A. Wink and L. R. /fn ^0 16- 160-163 (Oct. 19, 1944)

30 G. J. Brabender, Paper I’/L' 19 . ^8-128 (Nov. 7, 1947)

SI F. T. Carson, Paper Trade /. 125, No. IJ . i lo-*

XXII. COATING WITH RESINOUS MATERIALS

1281

water-vapor resistance upon creasing. Most permeability tests are made
on flat sheets. However, a comparison of values made on papers creased
in a special creasing platen with values obtained on flat sheets gives a useful
indication of the eft'ect of creasing on water-vapor resistance.

Coating materials differ widely in their water-vapor resistance. When
the General Foods test cabinet is used, the permeability is considered to be
low if 0.15 g. or less of water vapor is transmitted, while the permeability is
considered to be high if over 1.00 g. of water vapor is transmitted per 100
sq.in. per twenty-four hours. Cyclized rubber, polyethylene, nitrocellulose-
wax combinations, vinylidene chloride copolymers, and certain of the syn¬
thetic rubbers are some of the materials noted for producing highly water-
vapor resistant coatings. Pierce and Helms^^ obtained the values in
Table I showing the relative rates of transmission of water vapor at 0° F.
for several different packaging materials. Such low temperature com¬
parisons are useful for frozen food wrappers. Metal foils have the greatest
water-vapor resistance, but even they are not absolutely resistant to the
penetration of water vapor under all conditions.

TABLE I

Water-Vapor Transmission of Setoral Packaging Materials

Material Gain, &/100

sq.in./24 hr.

0.017 Paperboard . 0.907

0.0018 Waxed paper (creased) . 0.317

0.0018 Waxed paper . 0.042

Rubber hydrochloride film . 0.016

0.00035 Aluminum foil . 0.012

0.017 Paperboard (wax-coated) . 0.004

0.0009 Moistureproof regenerated cellulose . 0.003

0.0007 Aluminum foil . norm

Machinery for Coating with Resins

Many different types of coating machines are used for applying plastic

coatings. Three important types of coaters, knife, roll, and air, are de-
scribed below.^^

Kmje Coaters

Knife coaters are of several types, commonly called the floating knift
coater, the blanket type knife coater, and the rubber spreader. These var)

No. 16: 174-176 (Apr. 17,

” w IS taken in part from a talk given by M. A Gaio of Tohn

1282

PULP AND PAPER

j)riiicipiillv in the wav in which the ])aper Aveb is supported, but they all use
a knife for spreadin.sj tiie coating on the paper. 1 he larger the radius of the
knife, the greater the amount of coating which is applied. The paper should
leave tangent to the radius of the knife and perpendicular to the cut of the
knife. The angle of the knife should be inclined from the vertical in the
direction of web travel. A disadvantage of knife coaters is their tendency
to cause streaks when particles become lodged under the knife. The knife
used may be a solid knife or the Mayer-type applicator with a revolving

wire-covered rod at the apex of the knife.

In the floating knife coater, the coating is applied onto the face of the
paper in front of the knife. The thickness of the coating is controlled by the
angle of the knife, radius of the knife, and tension on the web. The paper is
unsupported, and hence only heavy papers can be coated by this method.

Dams are used to keep the coating on the paper.

The blanket type knife coater uses a rubber blanket as a support or
the paper web and the knife. Weak papers can be coated by this method
better than by the floating knife because of the support furnished to le
paper hy the rubber blanket. More uniform coatings are also obtained

cause of the more uniform tension supplied by the blanket.

The rubber spreader employs a knife mounted above a rubber roll.
The ruliber roll furnishes uniform pressure on the web against the knife.

Roll Coaters

There are a large number of roll coaters, some of the more important
n{ which are mentioned below.

The doctor kies roll is one type of roll coater. It employs an app
roll and a knife for removing excess coating. This type o coa er

low solids, low-viscosity coatings. .

The roll kiss coater uses two rolls, a furnish roll reviving in the c«t-

ing mixture and an applicator roll directly above. The paper pa..

the top (applicator) roll. aonlicator roll and a backing

The pressure roll coater uses two rolls, m pi produced

roll. The paper is passed between the ‘wo 'S by passing the

in the coating by this type of coater can ^ ■ j ^ ^ pipe

paper over a smaller smoothing roll. It ‘'™ “

can he used for applying coating to the top side 11

The reverse roll coater employs three rolls, a do . P

metal casting roll, rdUmder slight pressure. The

between the casting rol and the bad g • between the doctor

amount of coating applied is P, j the web

and casting rolls and the speed of the 3 , „„ coater,

speed It has been shown, in the case of a laborator)

XXII. COATING WITH RESINOUS MATERIALS

1283

that the wet thickness (0 film applied is determined by the following

relationship

/ = kDR

where D is the doctor roll clearance, R is the ratio of speeds of casting roll
to backing roll, and k is a constant. The thickness is independent of web
speed so long as R, the ratio of casting roll to backing roll speeds, remains
constant. Any decrease in casting roll speed below a critical value of R (a
value less than 1 ) results in an irregular coating. This coater will handle
a wide range of coating viscosities and is particularly applicable for viscous
coatings.

Air Kitijc Coater

111 this type of coater, the coating is applied to the web in excess by
means of a coating roll. A slotted air blade is then used for distributing the
coating and blowing off the excess. The air slot can be adjusted for width,
for angular position, and for position in relation to the backing roll. The
thickness of the coating depends upon the amount of coating applied and
the amount removed, llie latter being controlled by changing the air pres¬
sure.

Coating with Lacquers

I-acquers consist of a resin dissolved in a suitable solvent, c.g., shellac
<lis.solvcd in alcohol, although commercial lactiuers are generally more com¬
plicated than this. -\ typical composition consists of a film-forming resin
di.ssolved in a suitable solvent, a plasticizer to make the film flexible, and
a non-filin-forming resin to increase the hardness and gloss of the film.
Because a high piercentage of solvent is used, lacquers are sometimes re¬
ferred to as solvent coatings. Lacquers freijuently contain dyestuffs, pig¬
ments, or metallic powders. Coatings for heat-sensitive record papers con¬
tain special salts which react to produce fine black lines on the coating when
written ujxin with a heated needle.

Solvent coatings are used for decorative wrappers, food wrappers, and

bottle-cap linings. They may Ijc applied as overprint varnishes. Coatings

can be produced which are water-resistant, water-vapor resistant, grease-

resistant, alkali-resistant, and heat-sealing by proper formulation of resin,
plasticizer, and solvent.

Solvent coatings produce dense films, are easy to apply, and are quickly
dried, even in heavy films, ^[ost of the common resins can be applied in
solvent fonn. which is not true of other methods of application. On the
other hand, solvent coatings create a fire hazard, and are expensive to u.se,

r- Annual

Aiming ot lAPPI. New \.irk City (Feb. 20-23, 1950)

1284

rri.r and rAiTR

unless equipment is available for recovering the solvent. Other disadvan¬
tages are that huhhle-free cf>aiings are difificult to obtain in heavy coatings,
and solvent retention may cause trouble with undesirable odors in the fin¬
ished product.

Methods of Applying Solvent Coatings

Solvent coatings can be applied by roll coaters, spray coaters, dip
coaters, and knife coaters. Drying is very rapid (1-2 minutes) and can l>c
done in conventional type ovens, radiant transfer ovens, or indirect fuel
heaters. Drying temperatures vary with the tyi)e of lacquer. Certain of
the thermoplastic resins have a minimum drying temperature below which

the coating will show poor adhesion and poor gloss.

Penetration into the paper should lie held to a minimum to keqi as
much coating as possible on the surface of the paper, since excessive jxjne-
tration embrittles the sheet” and increases the comsumption of coating ma¬
terial. The amount of coating absorbed depends upon the initial density
and porosity of the paper, since there is practically no swelling of
to the solvent. The amount of internal sizing has relatively httle effect on
penetration. Albert""^ reports that varnishes (phenolic) penetrate papers
with greater difficulty than oils, of the same \nscosity.

Solvents for Lacquers

The principal function of the solvent is to dissolve the resin and con¬
trol the viscosity of the solution. Most resins can be * ""y.

of solvents so tliat a choice must be made, depending upon the re ute de
sired In preparing lacquers, it is best to add the film-forming resi
solvent and stir until dissolved. The non-film-forming resm j '

liter thorough mixing, the plasticizer and other modifiers 1« add^-
In some cases, heat is necessary to incorporate the »i ^

In applving overprint spirit varnishes to pnnted pape ,
print ng r^aV bleed Tnto the alcohol solvent, thereby r^ulting m t^.^^
aLndlhe ^dges of the print and betis een the

when the pigment in the printing ink is not alcohol P • yemedv

to use printing inks made with ^ ^-hich do not

is to use a lacquer or overprint varnish containing sohents ^hicn

dissolve the pigment. , , solvent has

The boiling point or -"-S* " u!r' ’ "latiUty of

a pronounced effect on the viscosity of the . Low-bofling

Uie solvent is an important factor in the drying of the film.

35V. A. Ryan, Paper 1 f•’^-^3WaaMt

s'i: H. E. L- Boldeschevieler. Ind. Ena. Chem. 34, No.

(july, 1942)

XXII, CO.\TIXG WITH RESINOUS M.\TERIALS

1285

solvents are desired from the standpoint of speeding up drying, but too
rapid evaporation leads to blushing on account of condensation of water in
the fihn. If this occurs, the trouble can be corrected by the addition of a
small amount of high-boiling solvent. However, too much high-boiling sol¬
vent leads to the retention of solvent in the coating, resulting in bad odor
and a loss of film strength.

Most commercial formulations are made with the aid of mixed solvents,
since mixed solvents are often more effective than either solvent alone. In
some cases, two non-solvents make an excellent solvent mixture when used
together, e.g., chlorinated hydrocarbons and alcohol, which separately are
non-solvents, but which make a good solvent mixture for cellulose esters.
Many fonnulations contain a cosolvent, which (although not a solvent by
itself) improves the solubility of the resin in the true solvent. For example,
solutions of nitrocellulose (RS) in a ketone solvent often contain alcohol
(cosolvent) which improves the solubility and lowers the viscosity.

Diluents are often added to solvent coatings. These cheapen the for¬
mulation, but have the disadvantage of lowering the water vapor resistance
of the film. Diluents (such as naphtlia and toluene) can be used in nitro¬
cellulose lacquer when esters or ketones are used as the true solvent. They
can l)c used in cellulose acetate lacquers when acetone or nitromethane are
used as solvents, but not when alcohol is the solvent. Only a small amount

of diluent can l)e used if the formulation contains a high percentage of nat¬
ural resin.

W hen using mixed solvents, it is desirable that the solvents produce
a constant-evaporating mixture, because otherwise the initial balance be¬
tween solvents will change during drying, thus changing the properties of
the film, 1 he fact that the components of a solvent mixture have the same
boiling points does not necessarily mean that the mixture will be constant¬
evaporating.” This is due to the fact that evaporation depends upon the
diffu.sion rate of solvent in the coating film, as well as upon the difference in
partial pressures lietwecn the interface and the body of the gas. The diffu¬
sion rates are dictated largely by the molecular weight of the solvent and
the affinity l)etween the solvent and the resin in the coating.As a general
rule, alcohols evaporate more slowly than esters, and non-solvents leave the
film more rapidly than the true solvent. The rate of evaporation of the true
solvent is often affected by the presence of non-solvent in the mixture.

rlasticizers

Ucquers generally contain plasticizers, which are low-melting solids
or high-l)Oihng liquids whose function it is to improve the flexibility and

” No. 12: 1395-

••W. K. Lewis and L. Squires, Ind. Eng. Chem. 29, No. 1 : 109-114 (Jan., 1937)

1286

PULP ANH PAPF*

tonj^lim-ss nl' till' film ami, in vmnr cav«‘N, io iinpri»v<* ilir Ivat M'iilinn
<Tti<'.s. All resins <!<* mil miniro |(laMicizrrs. as. i<>r example, ]>oh« tliyUne
aiul cyclizetl rubber, which can Ix' used alone. < >n the otlvr hand, the nat¬
ural resins, nitrocellulose, and many other film-forming resins nmst l»e wv-d
with plasticizers. Some resins exert a partial plasticizing effect on the
other resins in the mixture and consequently can be considered as plas¬
ticizers. This is particularly true of the low molecular weight jMslymers.

Plasticizers are related to solvents, but the difference lies in the fact
that plasticizers are non-volatile compounds and impart relatively j^erma-
nent flexibility, whereas solvents are highly volatile and their softening
effect is temporary. Plasticizers function by weakening the secondary
forces (van der Waals forces) l>etween chains of thermoplastic resins so
that these chains are freer to move in relation to one another. This soften¬
ing effect increases the ease with which the resin can be formed into films
and imparts flexibilitv to the film. From the mechanical standpoint, plas¬
ticizers reduce the tensile strength and increase the elongation and elasticity
of the resin film. At high concentrations, the plasticizer forms a continuous
phase, hut the plasticized resin, under these circumstances, is likely to lx* a
soft gel or a liquid. The amount of plasticizer used depends the type

of resin and the results desired, and normally varies from 10 to -..V/c . As
a general rule, the highly pohanerized resins tolerate more plasticizer an

the low molecular weight resins without becoming tacky,
n Two tvpes of pla.sticizers are recognized, the solvent type and the non-

solvent t^•pe. In general, solvent type plasticisers are
thev detract from the chemical resistance and strength of the film. On die
other hand, they have the advantage of Ingh retention

BaSLrs mL h^ Lsen with care for such properties as .e^cnOrt^

odor, toxicity, volatility, compatibility with the resins u . . ^

ance low temperature flexibility, and solubility in fats oils wate . ^

casting a

for cloudmess, streaUiness, cr^ stalli.-

e of the plasticiser is i"nK>«ant pa^ci^tlyjnjhe

case of paper coatings where a large surface molecular weight

the mass of the coating. In general, plasticizers ^c-

and low vapor pressure show the highest e^ ot consideration

nuer coating is to be baked (i.e., heated to 300 to -W h-I.

Ist be gi«n to the effect of high f'so to

plasticizers show negligible \ apor _ ■ .g—neratures. in which

200° F.. but tend to evaporate very rapidly at ig . ,he other

^ase there may be a substantial loss of plasticizer on baking. On the

XXII.

COATING WITH RESINOUS MATERIALS

1287

hand, there is often an affinity between resin and plasticizer which tends to
hold a low-boiling plasticizer in the coating. Loss of plasticizer when the
coating is brought into contact with water is another important factor.

This depends upon the water solubility of the plasticizer. .

Some of the materials commonly used as plasticizers are dibutyl
phthalate, tricresyl phosphate, castor oil, camphor (where odor is not im¬
portant), stearates, phthalyl glycolates, dibutyl sebacate, dibutyl phthalate,
butyl phthalyl butyl glycolate, and others.

• Modifiers

Waxes and oils are sometimes used as modifiers in solvent coatings to
increase the water and water-vapor resistance of the film and to lubricate
the coated surface. In certain coatings the wax is believed to migrate to
the surface, forming a thin but highly water-resistant surface film. If too
much wax is used, a condition known as wax blush occurs, due to the ap¬
pearance of an excess of wax on the surface. Treatment of the surface of
the coated paper with a 5 to \0% solution of wax in a suitable solvent (e.g.,
naphtha) is said to accomplish the same purpose as adding wax to the coat¬
ing formulation."*®

Paraffin, paraffin oil, stearin, and soap may be added to resinous coat¬
ings to increase the slip. Dyes may be added for tinting. Pigments may be
added for color and opacity. In general, the hiding power of the pigmented
coatings depends upon the pigment volume concentration in the coating
mixture. In the case of baking varnishes containing titanium pigment and
urea-alkyd resin binder, Armstrong and Madson^^ obtained maximum hid¬
ing power at a j)igment concentration of 25% by volume.

Use of Non-Film-Forming Resins

The function of non-film-forming resins in lacquers is to impart luster
to the film, to increase the resistance to oil, grease, and water, to increase
the solids content, and to increase the wax tolerance. Certain resins in¬
crease the hardness of the film, whereas others have a plasticizing effect.
Too much non-film-forming resin cannot be used, since it tends to lower
the block resistance and flexibility of the film.

Among the natural resins used are 'manila, shellac, zein, dammar,
pontianak, rosin, and rosin derivatives. Shellac is widelv used for makiner
food wrappers, shelf papers, and holiday wrapping papers. Shellac coatings
have high grease and oil resistance, but low water resistance. Zein is some¬
times used as a substitute for shellac.

Rosin derivatives are becoming increasingly important. The rosin-

F. K. Shankweiler, Pulp Paper Mag. Canada 37, No. 8: 445^50 (July, 1936)

G. Armstrong and W. H. Madson, Ing. Eng. Chem. 39, No 8 - 944-947
(Aug.. 1947)

1288

PULP AND PAPER

derived alkyd resins (Neolyn resins) can be used in nitrocellulose lacquers
or vinyl resin lacquers to increase the adhesion and gloss of the coatings
A suitable formula for the use of this rosin derivative in lacquer is

Non-volatiles

RS Yi sec. nitrocellulose (dry basis) . 21 parts

Rosin-derived alkyd resin (Neolyn 24) . 7 parts

Dibutylphthalate . 7 parts

Solvents «

Toluene . 35.7 parts

Butyl acetate . 13.0 parts

Ethyl acetate . 6.5 parts

Ethanol . 6.5 parts

Butanol . parts

Figure XXII-shows the effect of rosin-derived alkyd resins (Ne¬
olyn resins)’ on nitrocellulose lacquers. Other rosin derivatives, such as the
methyl ester of rosin (Abalyn resin) and the hydrogenated methyl ester of-
rosin (Hercolyn resins) can be.used in nitrocellulose lacquers to improve
the gloss. When used in ethylcellulose films, these resins decrease the water
sensitivity. These derivatives are classed as resin plasticizers.

In recent years the trend has been away from natural resins toward
the use of synthetic resins. Some of the most important synthetic non-film¬
forming resins used are the solvent-soluble grades of urea-formaldeh)i de,
melamine-formaldehyde, phenol-formaldehyde, coumarone-indene, terpene,
modified and unmodified alkyds, and the aryl sulf onamide-formaldeh)^ de

resms.

The phenol-formaldehyde resins are used in solvent coatings where
color is not important, but where good chemical, heat, and electrical resist¬
ance are necessary. The modified phenolic resins are most widely used.
The solvent-soluble grades of urea- and melamine-formaldehyde resins are
used in solvent coatings where color is of importance. Melamine resins

have the greater stability, but are more expensive.

The alkyd resins, particularly the modified grades (see chapter on

Resins), are used in solvent coatings. These resins have good electri
properties and a high degree of grease resistance, but their water resistance
is rather poor. One use for alkyd resins is in baking varnishes for coa
pie plates and other dishes used for baking food products. The resin is ap-
plied in solvent form and then is cured at 280 to 350° F. to J ’

durable, heat-resistant coating on the surface of the board. The pap
for this product should be as nearly neutral as possible in order

42 “The Neolyns, A New Series of Resins,” Synthetics Dept., Hercules P
Co., Wilmington, Delaware

1289

XXII. CO.\TINC. WITH RESINOUS M.'\TERL\LS

the least embrittlement on heating.^* Usually sulfite or sulfate papers are

used. .

The ar>’l sulfonamide-formaldehyde (SantoHte) resins can be used m

nitrocellulose lacquers in a ratio of 1 part resin to 2 parts nitrocellulose to
increase the moisture resistance and increase the hardness of the film. In
cellulose acetate lacquers, this resin increases the retention of the plasticizer
and imparts oil and gasoline resistance to the film. Iji ethylcellulose lac¬
quers, the resin increases the flexibility and toughness of the film. In vinyl
copolymer lacquers, the resin improves adhesion, gloss and light stability
without materially changing the hardness and flexibility of the film. Some
practical formulas are given below.**

Nitrocellulose lacquer for brushing

RS (34 sec.) nitrocellulose .•. 1 part

Aryl sulfonamide-formaldehyde resin (Santolite MHP) . 2 parts

Butyl beniyl phthalate (Santicizer 160) . 0.5 part

Butyl acetate . 3.6 parts

Butanol . 1-2 parts

Xylene . 7.2 parts

Cellulose acetate lacquer

Cellulose acetate (Hercules LL-1) . 6 parts

.\ryl sulfonamide-formaldehyde (Santolite MHP) . 6 parts

Methyl phthalyl ethyl glycolate (Santicizer M-17) . 3 parts

Acetone . 41 parts

Ethanol .. S parts

/-nitropropane . 17 parts

Methyl cellosolve acetate . 16 parts

Toluene . 6 parts

Polyvinyl fesin lacquer

Polyvinyl resin (Vinylite VYHH) . 70 parts

.Aryl sulfonamide-formaldehyde (Santolite MHP) . 10 parts

Dioctyl phthalate . 20 parts

The final solids are 42% by weight in a solvent composed of equal parts of methyl
ethyl ketone, cyclohexanone, and toluene.

The coumarone-indene and phenol-modified coumarone-indene resins
are used in solvent coatings where low cost and a high degree of chemical
resistance are retjuired, and where poor odor and color are not objection¬
able. A recommended formula is :**

** V. A. Ryan, Paper Trade 7, 113, No, 8: 87-91 (Aug 21, 1941)

«*'I^ical Bulletin 0-0-112. "Santolite MHP,” Development Dept., Organic
Uicminls Ui>ision, Monsanto Chemical Co., St.Louis, Missouri

PlasticUers,” The Neville Co.. Neville Island,

Pittsburgh, Pennsylvania

PULP AND PAPEA

Nitri»ccllulobc (*/j ^cc.) ....

I’licnol-modificd coumap'W-iitflcw r«^in ( NeritUr Pail« Hard) .

I >ibutyl phthalatc .....

M-Ilutyl acetate ...

H-Dutanol .........

Toluol ...

Ktliyl acetate ...

l.V» jisrtfc

(iaf11

48 jiart >,
jiart%
52 parti
224 parti
<i7 part I

Use of Film-Fonning Resins

Filtn-fonning resins are used in lacquers to increase the toughness and
lle.xibility of the filtn, to improve the resistance of the film to moisture and
cold check, and to reduce the tendency lots'ard blocking. A few of the
common film-forming resins used in lacquers arc nitrocellulose, ethylcellu-
lose, cellulose acetate, cellulose acetate-butyrate, |>olyethy1enc, vinyl chlo¬
ride, vinvl acetate, vinvl chloride-acetate, poh'S'inylidene cnpolNTners, poly-
styrene and methacrvlate resins. Only the film-forming resins which
dissolve in the common solvents (alcohols, esters, aromatic or aliphatic
hvdrocarbons. ketones, etc.) and which produce continuous and transparent

films are practical for solvent coatings.

The viscosity of the film-forming resin is an indication of film strength
and, in general, the higher the vi.scosity, the stronger and more flexible the
film (see Fig. XXII1-3). On the other hand, high-\nsc<*.sity resins require
a high proportion of solvent which increases the cost and the difficult) of

application. Thus, a compromise must often be made.

In working out a lacquer formula, the proper film-forming resin is
selected on the basis of the properties desired. Plasticizer is then added
to determine the ma.ximum amount of plasticizer which the resin will tol¬
erate without fonning too soft a film. Then, using alxmt one-half this
amount of plasticizer, sufficient non-film-fomiing resin is added to obtain
the desired film properties. The amount of added resin should be less than
■ that which causes blocking or tackiness of the film. If more complete in-
fomiation is desired, ternarv- diagrams can be constructed (see hig-

XXII-1). r ■ ■ '

In the following sections, the general use of film-forming resms m so

vent coatings is discussed. These are by no means the only film-forming

resins which can be used, but instead represent a few ot the more

resins selected to illustrate the ^-arious methods of formulation and appb-

edition •

Use of Nitrocellulose. Xitrocellulose is widely used m l^uer coat-

ings because of its relative cheapness, its

resins, its solubilitj- in a wide range of solvents, its g ^

properties. The solvents used with the RS t>Te of '

LL. but.1 acetate, acetone. »d eth,d -

XXII.

COATING WITH RESiNoUS MATERIALS

1291

toluene). Alcohols act as cosolvents and lower the viscosity, whereas hy¬
drocarbons act as thinners and cheapen the mixture.

The -second grade of nitrocellulose is used where lacquers of maxi¬
mum solids content are desired. The 5-6-second grade is preferred if max¬
imum folding qualities are required in the finished coating. The interme¬
diate ^-second grade is often used, and in some cases, mixtures are used.
Nitrocellulose lacquers do not lend themselves well to brushing, but other
forms of application can be used.

The weight of nitrocellulose applied varies from about 2 to 5 lb. per
ream (24x36—500) for glassine paper to about 5 to 10 lb. per ream for
label paper and carton board."® The water vapor resistance obtained with
glassine coated with different formulations of nitrocellulose lacquer are
shown in Table II in comparison with waxed papers."® The advantage of
incorporating wax in the formula is apparent.

TABLE II

Water Vapor Transmission of Glassine Coated with Nitrocellulose
. Lacquers in Comparison with Other Materials

Type of material

Moistureproof regenerated cellulose .

Glassine .

Glassine coated with ordinary nitrocellulose lacquer .

Glassine coated with high resin nitrocellulose lacquer.

Glassine coated with nitrocellulose lacquer containing wax

Glassine coated with wax (full waxed) .

Sulfite (dry waxed) ...

Sulfite (full waxed) .

Water vapor
transmission,
g./lOO sq.inV24 hr.

0.1-0.5

0.2 J

0.2-4}.6 \

5.8 J
0 . 2 - 0.8

The alkyd resins (both drying and non-drying oil types), the polyester
or plasticizing type of alkyd resin, and rosin esters are examples of some of
the non-film-forming resins which are used in nitrocellulose lacquers.
These resins tend to reduce the hardness of the coating, improve the heat
seal and enhance the gloss and depth of finish. With certain of the plas¬
ticizing type resins, no other plasticizer is needed. A ternary diagram
showing the effect of rosin-derived alkyd resin (Neolyn 41 resin) in nitro¬
cellulose (RS-J4 sec.) lacquer is shown in Figure XXII-1"^ The areas in
the figure may be classified as follows;

A flexible, non-blocking, non-tacky
B brittle, non-blocking, non-tacky
C flexible, blocking, non-tacky
D brittle, blocking, non-tacky
E flexible, blocking, tacky
F brittle, blocking, tacky

*®F. K. Shankweiler, Pulp Paper Mag. Canada 37, No. 8; 445-450 (July, 1936)

1292

rt’Lr AND fArni

Jn this cliaKraiu, the titost useful area is in the lower Icft-liand curuf f (if
area . / (shatletl area), since this rcj>rescnts the area of c<nnfM>sitif»n in which
the greatest aniouni of resin can be incorporated without losing block re¬
sistance and Hexihility. The various (omiulas were t( stestj Ijy apiilying th''
lacquer to a sujiercalendered sulfite paper (22 lb. 24x36-- 500), ueing a
knife coater. The coating weight was 8 lb. per ream, and the cowid pai^ r
was dried for fifteen to twenty minutes at room temperature, after w’hich
it was force-dried for two minutes at 120* C. A small amount of ethyl-
cellulose may lx: added to nitrocellulose lacquers to reduce cold check.

and non-tacky (above).

Use of Ethylcelltilose. Etfiylcellulose is used in lacquers where a
high degree of film flexibility and good heat-seal properties are desir
Ethylcellulose is soluble in practically all t}-pes of lacquer solvents^cept
petroleum thinners. It is soluble in straight alcohol and can be a^ili^
a mixture of ethanol and butanol (70-30). However, it is generally used

4T Courtesy ‘-Hercules R&ins for Paper Lacquer,” Synthetics DQ>t. Hercules
Powder Co., Wilmington, Delaware

XXII.

COATING WITH RESINOUS MATERIALS

1293

in solvent mixtures containing 60 to 80% aromatic hydrocarbon, and 20 to
40% alcohol. No more than 40% alcohol should be used, since the greatest
strength and flexibility are obtained when the aromatic solvent is the last
to evaporate from the film.'‘« The addition of alcohol to ethylcellulose m
toluene results in a reduction in viscosity, the greatest reduction m vis¬
cosity being obtained with the low molecular weight alcohols. The mini¬
mum viscosity is obtained when alcohol constitutes about 20 to 30% of the
total solvent. The following represents a suitable solvent mixture for ethyl-
cellulose in the ethoxy range of 48.5 to 49.5%.^®

Toluene .

Xylene ..

Ethyl alcohol .

H-Butyl or amyl alcohol

40 parts
30 parts
15 parts
15 parts

High-solvent naphtha may be substituted for the toluene. In the medium
ethoxy range (45.0—46.5%), slightly more alcohol should be used. Resins
are added to improve the gloss, adhesion, solvent resistance, and rubbing
properties of the film, and normally, about 15 to 30% plasticizer is added

(see Ch. XXIII).

Use of Polyvinyl Acetate and Polyvinyl Chloride. Polyvinyl ace¬
tate is used in lacquers where good adhesion, good heat-seal properties, and
good flexibility are desired. It is used in protective overlacquers for
printed paper labels, posters, and decorative papers, and is also used as a
heat-seal adhesive. Polyvinyl acetate is soluble in most solvents, including
alcohols, ketones, and aromatic hydrocarbons (see Ch. XXIII). The lac¬
quers can be applied by dipping, roll-coating, knife-coating, or brushing.
Spraying is not practical with vinyl resins, except in the low-viscosity
grades, because of the tendency to string or cobweb. The film is usually
dried and then baked for a few minutes at 225 to 250° F. to release the
solvent. Forced convention ovens or infrared lamps can be used. A small
amount of nitrocellulose (10 to 25%) may be incorporated in the lacquer
to raise the softening point.

Polyvinyl chloride coatings have excellent oil and solvent resistance
and are used for paper oil containers and paper gaskets. Polyvinyl chloride
is, however, relatively insoluble in most solvents, and even when dissolved,
must be used at low solids. This fact has limited its use in solvent coatings.

Use of Polyvinyl Chloride-Acetate Copolymers. Polyvinyl chlo¬
ride-acetate copolymers produce lacquers with low viscosity and rapid
evaporating properties. They are used for coating decorative papers, heat¬
sealing papers, paper drum linings, grease-proof papers, and milk bottle
caps. The films are tough and have a high degree of water, grease, solvent

^®T. A. Kauppi and S. L. Bass, /nd. Eng. Chetn. 30: 74 (1938)

Technical Bulletin, “Dow Coating Materials, Ethocel,” The Dow Chemical Co.,
Midland, Michigan (1947)

1294

PULP AND PAPER

and chemical resistance, and are quite flexible at low temperatures. The
films are also quite resistant to water vapor. Because the films are re¬
sistant to alkali, the resin can be used on alkaline wallboards and soap
wrappers. Supercalendered sulfite, glassine, greaseproof, and parchment
are some of the papers used for coating.

The copolymer containing 87% vinyl chloride and 13% vinyl acetate
is one of the most suitable grades for paper coating lacquers. This resin
is soluble in a wide variety of solvents, but mixtures of ketones and aromatic
hydrocarbons are the most practical. Methyl ethyl ketone is widely used,
although for brushing, slow-evaporating solvents (methyl n-amyl ketone,
mesityl oxide, or cyclohexanone) may be required. The aromatic hydrocar¬
bons can be slower evaporating than the ketone because of the solubility of
the resin in hydrocarbons at elevated temperatures.®" Low-viscosity grades
require a high percentage of ketone. The ratio of ketone required also de¬
pends upon the solids content of the lacquer, and for the highest practical
solids (about 20%), a ratio of 50-50 ketone to aromatic hydrocarbons is
required. Figure XXTI-2®^ show^s the equilibrium viscosities of the me¬
dium viscosity polymer in different mixtures of methyl ethyl ketone and
toluene at various solids content. Above about 20% solids, the lacquer
becomes quite thixotropic. Figure XXII-3®’ is a phase diagram showing
the type of flow behavior obtained at different concentrations of resin and
different ratios of solvent (methyl ethyl ketone) to diluent (toluene) at

20° C.

In preparing a lacquer from this resin, the resin is added slowly to the
solvent under vigorous agitation. If considerable diluent is to be used, the
resin should first be mixed with the diluent and then the solvent ad e .
Fleat is generated during mixing, but additional heat may be desirable to
obtain complete solution. In order to avoid discoloration m the presence
of iron, about 0.1 to 0.3% of propylene oxide based on the weight of the

resin may be added. _ ,

Another useful copol)'mer is that composed of 62% viii) c ori e a

38% vinyl acetate. This resin is compatible with nitrocellulose

vinvl resins in the presence of a small amount of plasticizer. If used wi ^

nitrocellulose, the alcohol content of the lacquer should be kept to a mini-

"’""’in order to obtain niaximnm water vapor resistance with polyvinyl

chloride-acetate copolymers, a small amount (1 to 5%) of para "'
added. The wax is not compatible with the resin, but can be d.ss^ed in

hot toluene and added to the resin solution where it is held in fin

soC. W. Patton, Oficial Digest, New York Paint and Varnish Production (Ma.v,

M C™r\esy “Vinylite Resins for Surface Coatings^' Ba^Iite Corporation. Carbide
arfd Carbon Chemicals Corp., New York, N. Y. (1942)

PER CENT RESIN BY WEIGHT AT 20® C

XXII.

COATING WITH RESINOUS MATERIALS

1295

PER CENT METHYL ETHYL KETONE IN THINNER

BY WEIGHT

Fig. XXII-3. Flow behavior of poly¬
vinyl copolymer in mixtures of methyl
ethyl ketone and thinner (toluene). Cour¬
tesy Bakelite Corporation.

Fig. XX11-2. V’iscosity of polyv'inyl
copolymer resin in mixtures of methyl
ethyl ketone and thinner (toluene). Cour¬
tesy Bakelite Corporation.

PER CENT METHYL ETHYL KETONE
IN THINNER BY WEIGHT

pension. Resin solutions containing wax work best on non-porous papers
which do not absorb too much of the wax.

For best results with polyvinyl copolymer lacquers, it may be neces¬
sary to apply several different coats. The first coat serves for anchorage of
the coating to the paper, the second coat serves to build up the film, and the
final coat acts as a sealing coat.°^ Drying of the coating must be carried out
slowly at temperatures no greater than 160° F.

Use of Polyvinylidene Chloride Copolymers. Lacquers made from
polyvinylidene chloride copol 3 miers form very dense and flexible films which
have a high degree of resistance to water, water vapor, and grease, and have
good heat-sealing properties. The resins have been used for coating paper

H. W. Foelsch, Paper Trade /. 129, No. 25: 501-502 (Dec. 22, 1949)

1296

PULP AND PAPER

milk containers. However, when suitably polymerized for high acid and
alkali resistance, this resin®® is too inert for solvent coatings.

The best solvent for pol}winylidene chloride copolymers is methyl ethyl
ketone. Diluents such as the aromatic hydrocarbons or chlorinated hydro¬
carbons may be added, and secondary solvents of the acetate type may be
used. Table III shows the maximum concentration of diluents which can
be used with methyl ethyl ketone solutions.®® Diluents reduce the viscosity
and improve the solvent release during drying of the film. To dissolve, the
dry resin pow’der should be added slowly to the solvent or solvent mixture
with vigorous agitation. Warming the solvent helps to speed up solution.
If the lacquer is to be stored in an iron container, the container should be
given a phosphoric acid wash, or a small percentage of phosphoric acid
should be added to the solution to prevent discoloration.

TABLE III

Maximum Concentration of Diluent That Can Be Added to Methyl Ethyl
Ketone Solutions of Polyvinylidene-Acrylonitrile Copolymer

Diluent

Acetone
Ethyl acetate
Butyl acetate
Nitromethane
Toluene
Xylene

Skellysolve (60-70“ C.)

Methanol

Isopropyl alcohol

Carbon tetrachloride

Chlorobenzene

Dioxane

Percentage by volume of methyl
ethyl ketone that can be replaced
without gelation at 25“ C.

75%

60%

50%

25%

30%

20 %

10 %

25%

15%

10 %

30%

90%

The lacquer may be applied by brush, dip, knife, or roller application,
but cannot be sprayed except at low concentrations because of the rapid
solvent release in the early stages of drying. All solvent must be removed
during drying to obtain the maximum resistance to water vapor. For
ing a coating composed of about 14 lb. of resin per 3,000 sq. ft. (coating 0.
mil in thickness), the paper should be passed through an oven ^vhlch
gradually increases in temperature from 95 to 300° F. in approximately two
minutes; then the coating should be subjected to a flash bake at 500 ^. or
approximately ten seconds, and finally passed over a cooling roll. ^
coating is 1 mil or over in thickness, a slower drying schedule must be foi¬
es Technical Bulletin, “Dow Coating Materials. Saran F-120,” The Dow Chem¬
ical Co., Midland, Michigan (1946)

XXll. COATING With RfiSlNOtfS MATERIALS

1297

lowed (or higher boiling solvents used) in order to prevent blistering.. In¬
frared lamps can be used for removing the last traces of solvent. Poly-
vinylidene chloride copolymer coatings have water-vapor resistance ap¬
proximately as follows

Thickness of
coating, mil

M.V.T. g./lOO sq.in./24 hr.
at 100* F. and 95 R.H.
diilerential

0.6

1.0

0.25

0.15

Use of Polyethylene. Polyethylene produces films having good flexi¬
bility at low temperatures, and hence is well suited for frozen food wrappers.
Heat sealing can be readily accomplished, particularly if the surface is pre¬
wetted with mineral oil. The films have high water vapor resistance but
low resistance to other gases. A comparison of the gas permeabilities of
polyethylene and vinyl resin films are shown in Table IV.®* Film thickness
was 0.002 in. in both cases and measurements were made with a vacuum
on one side and gas at atmospheric pressure on the other. Properties of
polyethylene are discussed further in Ch. XXIII.

TABLE IV

Gas Permeability of Polyethylene and Vinyl Resin Films

(cc./lOO sq. in./24 hr.)

Resin

Oxygen

Nitrogen

Carbon

dioxide

Moisture

vapor

Polyethylene .

... 320

1500

0.5

Vinyl, containing 20% plasticizer ...

2.0

The grade of polyethylene most widely used in lacquers is the resin of
medium molecular weight, i.e., about 20,000.®® The resin is dissolved and
applied hot. If the temperature is lowered, the resin precipitates from so¬
lution but solution can again be obtained by reheating. Figure XXI1-4®*
shows the solubility of commercial polyethylene resin in several different
solvents at different temperatures. At 90° C., a 35% solution can be ob¬
tained in toluene. A phase diagram for polyethylene-xylene system is
show'n in Figure XXII-5.®*

Polyethylene lacquer can be applied by reverse roll coaters or by
spreader knives. Hot knives or heated applicator rolls are desirable to
maintain the temperature over 90° C. Immediately after application, be¬
fore cooling occurs, the coated paper should enter an oven at high tempera-

«Technics Bullrtin 7, “Polyethylene Resins,” Bakelite Corp., Carbide and
Larbon Corp., New York, N. Y.

Sold as DYNH by Bakelite Corporation

1298

PULP AND PAPER

ture. All solvents must be removed before the coating cools to the point
of precipitation in order to prevent poor film formation. Maximum film
strength is obtained by baking at temperatures of about 150® C. A coating
about 1 mil in thickness is obtained by applying 15 lb. of coating per 3,000-
sq. ft. ream.®®

Use of Rubber and Rubber Derivatives. Natural rubber is not used
to any extent in solvent coatings because of its poor solubility characteris¬
tics. However, some of the rubber derivatives and synthetic rubbers are
used in solvent coatings.

Fig. XXll-4. Solubility of poly¬
ethylene resin in different solvents at
different temperatures.

CONCENTRATION,WT. PERCENT

Fig. XX11-5. Phase diagram for poly¬
ethylene resin-xylene system. Figs, 4 and
5 courtesy Bakelite Corporation.

Cyclized rubber produces flexible, heat-sealable, and highly water
vapor resistant coatings when applied in very thin films from either aromatic
or aliphatic hydrocarbon solvents. Table V shows the drying character¬
istics of cyclized rubber films deposited from different solvents.®^ The re¬
sults also show comparative viscosities in the different solvents^ The effect
of film thickness on water vapor resistance is shown m Figure XXli-b tor

both coated glassine and coated label stock. ,• u f.v

Chlorinated rubber is readily soluble in both aromatic and aliphatic

hydrocarbons and can be applied to paper from these solvents. Chlonnatea
rubber produces films with a high degree of chemical
must be added if high water vapor resistance is desired.
chloride, on the other hand, is not widely used because of i s po

50 J. K. Honish, Paper Trade J. 128, 15: 125-1^

57 H. R. Thies, Paper Trade J. 108, No. 8: 79-84 (Feb. 23, 193 )

XXII.

COATING WITH RESINOUS M.\TERIALS

1299

However, a mixture of rubber hydrochloride with vinylidene chloride co¬
polymer dispersed in benzene has been suggested.

Some of the newer synthetic rubbers are soluble in a wide range o^
solvents and produce coatings having interesting properties. Latham®®

Fig. XXII-6. Effect of film weight on the water-
vapor resistance of cyclized rubber coatings.

describes a mixture of polyisobutylene wdth polyethylene. A relatively
new butadiene-styrene product, called S-7 resin, has excellent properties

for lacquer type coatings (see Ch. XXIII). This material may be used in

TABLE V

Comparative Viscosities and Drying Times of Cyclized Rubber

IN Various Solvents
Films 0.001 in. thick

Solvent

Viscosity, Ford cup

Drying time,

74® F.—#4 orifice

79® F.—25% R.H., min

Benzene

1.70

1.5

Skellysolve U

1.13

3.25

Skellysolve C

121

3.00

Xylene

1.68

13.50

Varnish makers’ naphtha

2.0

53.0

Turpentine

6.0

28.5

unmodified or modified form, the latter types being most useful. A typical
formula is (see also p. 1349)

S-7 resin .

Modifying wax
Modifying resin
Toluene .

100

500

**F. H. Manchester, U. S. 2,386,700 (Oct. 9, 1945)

5»G. H. Latham, U. S. 2,369,471 (Feb. 13, 1945)

Bulletin No. 701, “Pliolite,” The (joodyear Tire and Rubber Co., Akron, Ohio

1300

PUI-P AND PAPER

Wax increases die resistance to water vapor and raises the blocking |:»oint
of the lilin, but no more than l5^c wax should be used, since this lowers
the heat seal. Microcrystalline waxes (melting points of about 170^ K.)
are recommended. Glassine pa|>er coated with 1 lb. j>er 1,000 sq. ft. of the
above formulation transmits between 0.2 to 0.3 g. water vajx>r jjer 100 sfj.
in. per twenty-four hours when one side of the coated pa|>er is subjected
to 20^ relative humidity and the other side to 95% relative humidity at
100° F. Heat seal, slip, anchorage, and resistance to creasing are good.

Coating with Emulsions

Recent advances in emulsion technology liave made available many
good film-forming resins in emulsion form. With these new emulsions,
it is possible to produce coatings having high gloss, good scuff resistance,
and good resistance to water, oils, and water i^apKir. The emulsions used
in the paper industry include rubber latices, wax emulsions, and resin
emulsions. Wax emulsions are used mostly for the internal and surface

sizing of paper.

Rubber (natural and synthetic) latices produce results quite different
from that of the synthetic resin latices. Rubljer latices deposit films which
are definitely rubbery in nature; that is, the film is resilient and can be
stretched many times its original length, and when released, returns to
approximately its original dimensions. Because of these characteristics,
rubber latices are better suited for impregnation and saturation than they
are for coating. On the other hand, the s>*nthetic resin latices yield films
which are much harder, less resilient, show' less elongation, and do not re¬
cover so well from stretching. In fact, some resin latices (e.g., vinyl
resins) deposit films which are powder}- in nature unless the resin is plasti¬
cized. How'ever, when properly compounded, synthetic resin latices pro¬
duce hard, flexible, tough films which are resistant to grease, chemi» s,
moisture, and abrasion. Consequently, s}mthetic resin emulsions are often

used for coating. . . , . i

\11 the rubbers, wdth the exception of the isobutylene t}'pe, are a\ai -

able commercially in the prepared latex form. These are n«de by emuU.jm

poh-merization, whereby the proper tjpe and amount of mongers a«

charged to a reactor together with water, emulsifier, catohjt, and modifier

and the reaction carried out at the proper temperature and for the pr^r

time to obtain a latex of the desired physical and chemical property

adled to fhe resulting latex. Since isobuts-lene rubbers are not made
emulsion polvmerization, they are not as-ailable in latex form, although
aqueous dispersions of the rub^r l^ve ^en made

XXII. COATING WITH RESINOUS MATERIALS

1301

greater ease of application, compared with solvent coatings. High-viscosity
materials can be applied in emulsion form at high ^oljds “ntrat. O
other hand, low-solids emulsions can be used when light-weight coating
are desired by diluting the emulsion with thickening agents. Emulsions can
be sprayed without stringing, which is an advantage over the solution type
of application. The early emulsions had the disadvantage of lack of s
bilitVagainst freezing and electrolytes, but recently highly stable types have
been produced. The drying time of emulsions is slower than that of solven
coatings, and emulsions also tend to cause curling and warping o t e pape

FoTwulatiofi oj Aqucous Ewiilsio'^is

Emulsions can be purchased in prepared form, or they can be made
at the converting plant by emulsifying solid resins. The properties of the
emulsion depend upon the type of resin, amount of emulsifying agent used,
and amount of agitation available. In preparing or diluting emulsions,
clean water of low dissolved salt content should be used. There are two
types of aqueous emulsions, those in which the resin is dispersed directly in
an aqueous medium and those in which the resin is dissolved in an organic
solvent before dispersing in an aqueous medium. Some of the materials
used in formulating emulsions are discussed below. These materials are
usually present in the emulsion as purchased, although in some cases they
may have to be added by the user.

When making an emulsion from a solid resin, the type of solvent used
for dissolving the resin prior to emulsification is very important. If
the emulsion is to be used on porous surfaces, low-boiling solvents should be
used. On the other hand, high-boiling solvents should be used on relatively
non-porous surfaces in order to avoid blushing on drying. The ratio of
lacquer phase to water phase should be as high as practical. There should
be a high ratio of lacquer to resin, particularly if low temperatures are used
in drying.

The most important factors in the stability of the emulsion are the type
and the amount of emulsifying agent used. As a rule, the emulsifying
agent is carried in the water phase, although in some cases it may be carried
in the lacquer phase. Normally, 2% or less is used on the weight of the
water present since emulsifying agents generally reduce the water,resistance
of the film. However, certain emulsifying agents (e.g., ammonium oleate)
can be used which decompose during drying and do not reduce the water
resistance. If ammonium oleate is used as the emulsifying agent, oleic acid
may be added to the lacquer phase and ammonium hydroxide added to the
water phase. During drying, ammonia is removed and the small amount of
oleic acid remaining blends with the resin. One disadvantage is that am¬
monia causes yellowing of certain solvents. Emulsifying agents tend to be

1302

PULI* AND PAPER

absorbed by the paper, which reduces their effect on the water sensitivity
of the film.

Plasticizers are added to emulsions to increase the elongation and
-flexibility of the film. This, in turn, increases the tear resistance of the
film, but lowers the tensile strength. As a rule, plasticizers reduce the
chemical resistance. Anti-oxidants are added to prevent deterioration of
the physical and optical properties of the film upon aging. Stabilizers are
frequently necessary to stabilize the emulsion against coagulation. Thick¬
ening agents (methylcellulose, carboxymethylcellulose, casein, etc.) are
sometimes added to control the viscosity of the emulsion and to aid in
keeping the coating on the surface of the paperThe water vapor resist¬
ance of the coating can generally be greatly improved by incorporating a
fairly high percentage of wax emulsion in the formula; for best results, the
coating must be heated above the melting point of the wax during drying.
Other materials which may be added include wetting agents and vulcanizing

agents.

The following is a general description by which a rosin-derived alkyd
resin (Neolyn resin) can be emulsified.®^ This procedure is, of course, not
suitable for all resins, but it illustrates a general method of emulsion prepa¬
ration.

(1) The resin is dissolved in the solvent (in this case, xylene).

(2) Oleic acid is added to the resin solution. ., • •

(S) Potassium hydroxide dissolved in water is added dropwise with rapid stirring.

(4) A solution of stabilizing agent is added (in this case, ammonium casemate).

(5) Water is added slowly with agitation until the system reverses itself to e

desired oil-in-water type.

(<5) Diluent water is then added.

The composition of the final resin emulsion is:

Resin .

Solvent .

Oleic acid .

Primary water .

Potassium hydroxide
Ammonium caseinate
Secondary water ..
Casein pres'ervative

40%

10 %

0.25%

23.5%

0.25%

1.5%

24.5%

0.015%

Emulsions can be prepared from the glycol esters of

;„T:hetis dispersed with the aid of potassium hydroxide, oje.ac.d, and
mlfated castor oil to produce emulsions ranging from 7

OIL. H. Silvernail, Tappi32/No 6: pept of Hercules Powder

62 “The Neolyns, A New Series of Resins, Synthetics Liept.

Co., Wilmington, Delaware

XXII. COATING WITH RESINOUS MATERIALS

1303

1 1 In thP first method, a minimum of emulsifier is

Two methods may be used. Inti ^ second

added and a colloid ndll used for <>-l- '"1 ‘^ ha, the resin is spon-
method, a relatively large an.ount of soap ,s used so

taneously emulsified.

Applying Emulsions as Surjace Coalings

Emulsions have the Tliirreduces the

ings in that they tend to swell the fibers in th p P

effective pore size of the paper and, ” ; determine the pene-

i„, Theviseosityoftl.—, also

ltdTtfha: been reported that a given -in P— P=‘P- ^ "’•=

resistance, it is necessary that the dispersed droplets ,n ““'ftlr-

on dtng. The solvent in the lacquer phase is responsible for th ato

flow, and consequently most of the 1 tItordTr for this

until the water is evaporated or al^orhe b> f P 1^

irut itsd ntti:^;::::';;:. ' v.iug of ,he comed pape,

to a high temperature is effective in fusing the coating.

Txpes of Aqueous Eintdsions

Polyvinyl acetate emulsions can he readily prepared by dissolving the
resin in a water-immiscible solvent, incorporating an emulsifying g .
and then emulsifying in water of low mineral content under violent agit
tion The emulsion can be improved by passing through a homogenizer o

colloid mill. From 0.,S to ^.O^o of emulsifying agent
triisopropanolamine oleate, or sodium laiiryl sulfate) gives the b -

A suitable formula is

^ , , S0 0% oolyvinyl acetate resin

82% Lacquer phase . ^ tricresyl phosphate

43.5% toluene
1.5% oleic acid

iQ«r Wnter ohasc . ammonia (28%)

18% Water phase . 92.0% distilled water

Polyvinyl acetate-chloride copolymer resins can be prepared in
sion form by the same general procedure."" Emulsifying agents such as
the sodium salts of alkyl naphthalene sulfonic acids or ammonium oleate can

63 C. B. Hollabaugh, Paper Trade J. 101, lio. 25; 347-353 (Dec.

6* Technical Bulletin, “Vinylite Vinyl Acetate Resins, p. 10, Bakelite Co p..

Carbide and Carbon Corp., New York. N. Y. (1947) rorh,^n

63 Technical Bulletin, “Vinylite Resins,” Bakelite Corp., Carbide and Carbon

Corp., New York, N. Y. (July, 1948)

1304

PULP AND PAPER

be used. If the resultant film is to be air-dried, the concentration of resin
in the lacquer phase should not exceed 25%. However, if the film is to be
dried at a high temperature, a gel type of emulsion may be used containing
a high percentage of resin dispersed in a non-solvent thinner. In this case,
the dispersed particles of resin do not merge into a continuous film until
the film is heated to a high temperature.

Prepared emulsions of polyvinyl acetate resin are sold commercially.
An analysis of one commercial product is as follows

Total solids, % by weight. 50

Viscosity, centipoises . 3,000

Free monomeric vinyl acetate,

% by weight . ].5

/’H . 4.5 to 5

Viscosity of contained resin®. 100 to 150

Average weight, pounds per gallon

at 60° F. 9.25

Particle size, average maximum. Not over 10 microns

® The viscosity in centipoises at 20° C. of 86.1 g. vinyl acetate polymer plus benzene
to make one liter.

Vinyl acetate emulsion can be applied by brush, spray, dip, knife, or
roll coaters. The dispersion has good stability to agitation, to brushing,
and to dilution with water, and does not show the objectionable stringing
that is characteristic of solution coatings of polyvinyl acetate. The pYi
may be adjusted to 7.0 or slightly above by the addition of ammonia or
triethanolamine, but excessive amounts of alkali, acid, or inorganic salts
should be avoided. Plasticizers may be stirred into the dispersion, and
their use in moderate amounts tends to improve the water resistance of the
dried film. Concentrations of 10 to 30% (based on the resin) are sug¬
gested. Air-dried films are water sensitive, but a short bake (5 minutes
at 300° F.) helps to render them water-resistant.

Films of vinyl acetate emulsion are quite resistant to blocking and sur¬
face marring, even at high temperatures. The block point is generally
about 400° F. (1 p.s.i. for 1 minute), and the print point (temperature at
which the coated paper shows the pattern of a piece of cheesecloth placed
on the surface for 15 minutes under a pressure of 1 p.s.i.) is about 200 to
220° F.®® The high blocking resistance naturally hinders heat sealing of
the dried film, but excellent bonds can be obtained if the surfaces to be ad¬
hered are coated and then pressed together while still damp. Force drying
or baking and the application of pressure improve the strength and resist¬
ance of the bond. If it is not practical to seal the surfaces within two or
three minutes of application of the film, addition of ethylene glycol or other

«6 Technical Bulletin “Vinylite Resins,” Bakelite Corp.. Carbide and Carbon Corp..

New York, N. Y. (July, 1948)

XXII. COATING WITH RESINOUS MATERIALS .

suitable material will extend the bak^d can be

which have not dried more than a few " with water or

reactivated for heat sealing by merely wetting the surtace

weak ammonia. ^r,Lr^Mnv1 acetate powder which can be

There is available a commercial polyvinyl acetate p

dispers' d in water to form an enrnlsion suitabie fc-

powder is readily dispersible in water in ^ ^ pastes To prepare

r^itoe) to enhance heat-sealing properties or to adjust the with little

XitfctrMetsins are available as emulsions in several differe^
forms (see Ch. XXIII). Synthetic rubber latices are also aval (

Pol” ii'idene chloride-polyvinyl chloride co,mlymer em^on
rSaranWsee Ch. XXIII) can be applied to paper by knife or roll coater
to prodice a coating which is heat-sealing, water-repellent, oil-resistan , an
water vapor-resistant.®® A suitable formula is:

100 parts commercial latex (57% solids)

24 parts dibutyl phthalate emulsion (60% solids)

1.7 parts thickening agent

The coating must be dried in a festoon or tunnel drier because of its tacky

Ethylcellulose emulsions may be prepared by emulsifying ethylcellulose
lacquer in water. The ethylcellulose is first dissolved in xyUne or naph ha
and a solution of emulsifying agent in water is then added slowly to the
lacquer with thorough stirring. At first, a water-m-o. emukion is ob-
tained, but this reverses to an oil-in-water emulsion at t le pom o w
saturation. Soft water should be used, and the emulsion should be passed

through a colloid mill. A suitable formula is given below.®®

Lacquer phase.Ethylcellulose . 2^

Xylene .

Butanol .

Water phase .Water (^H 8.0 with

Emulsifying agent.. ■ • 0.5

6T Technical Bulletin “Vinylite Resins,” Bakelite Corp., Carbide and Carbon Corp..

New York, N. Y. (July, 1948) .t' j r I'yy vr a. ^Amr R

6* G. W. Stanton and W. A. Henson, Paper Trade J. 122, No. 6 : 68-72 (Aug. 8,

1946) • n

69 Technical Bulletin, “Dow Coating Materials, Ethocel,'’ The Dow Chemical Co.,

Midland, Michigan (1947)

1306

PULP AND PAPER

Acrylic resin emulsions are available commercially in solids content
langing from 25 to 50%. ITie solutions vary from water white to slightly
yellow in color. Films produced from acrylic resin emulsions range from
hard, brittle films with good surface slip to elastic, flexible films which are
quite tacky, depending upon the grade of resin used. The films have good
resistance to aliphatic hydrocarbons, mineral oils, alkalies, and acid. The
film is fairly \\ater resistant if baked or force-dried at temperatures above

110° c.

Aqueous emulsions of nitrocellulose lacquer have been suggested for
paper coating.'® These produce thin, continuous, non-porous films on
papei. Polyvinyl butyral emulsions produce coatings of excellent proper¬
ties, but have the disadvantage of high cost. Polystyrene emulsions are
suitable for coating paper if the coatings are force-dried at 100 to 150° C.

Organosols and Plastisols

Organosols and plastisols constitute a different class of emulsions.
Organosols consist of a dispersion of resin in a plasticizer and a non-solvent
liquid (i.e., diluent). Plastisols consist of a dispersion of resin in a non¬
solvent plasticizer without diluent.

Vinyl chloride-acetate copolymers are used in making organosols and
plastisols. The resins used for this purpose contain a very high ratio of
vinyl chloride and have a high molecular weight. Other resins may be
added, such as the rosin-derived alkyds (Neol}^ resins), to improve the
adhesion characteristics and the gloss of the film. Organosols may be
prepared by mixing the ingredients in a slow speed agitator or by milling
in a ball mill for twenty to forty hours, depending upon the form of the
original copoU'iner. Resin, plasticizer, diluent, and stabilizer are mixed
together until a smooth creamy paste containing particles about 0.2 micron
in size is obtained. General!}’, from 30 to 100% plasticizer on the weight
of the resin is used.

One advantage of organosols and plastisols is that relatively cheap
diluents, such as aromatic, naphthenic, and aliphatic hydrocarbons are used.
Another advantage is that dispersions of high solids can be prepared using
resins which normally form solutions of very high viscosity. Organosols
containing up to 60 to 80% non-volatile components have been prepared,
and plastisols up to 70 to 100% solids are used with no volatile ingredients
at all. These high solids permit the application of very heavy coatings.

The diluents and plasticizers in organosols and plastisols exert very
little solvent action on the resin at room temperature. However, when the
film is heated to high temperature, the diluents and plasticizers act as
solvents for the resin and cause the resin to fuse into a continuous coatmg.

70 C. B. Hollabaugh, Paper Trade J. 101, No. 25: 347-353 (Dec. 19, 1935)

XXII. COATING WITH RESINOUS MATERIALS

1307

Plastisols have higher fusing temperatures dle^t

higher molecular weight of the resms used, and because th ^

o solvate the resin during lieating. In baking orpnosol “f "’8^’ “
lake for thirty to sixty seconds at 200 to 250” F. is nsed, followed by a bake

oven is steaiiheated and is equipped for the recovery of Jf? J

heaters. The coatings have good abrasion resistance and ' ^

because of the high molecular weight of the resms used As a rule, th
heat-seal properties are not so good as with other types o coa mgs.

Coating with Hot Melts

Hot-melt coating involves the application of molten wax
the surface of the paper, followed by cooling to produce a smooth, hard Wm
on the paper surface. Waxing of paper is an example of a hot-melt coat
inv process which has been used for years. Recently, advances have le
made in the number of materials suitable for hot-melt application

Hot-melt coatings are of two types; (?) those consisting o \vax a one,
or a mixture of wax with a small percentage of modifying material, sue as
polyisobutylene or cyclized rubber; and (2) those consisting prmcipa y o
a film-forming resin and relatively small percentages of modifying agen s,
such as wax, non-film-forming resin, and a plasticizer. Hot-melt coatings
have the advantage of low cost, low fire hazard, and quick dry ing, an e

coatings are heat sealing. . , , . u

Heating of the melt in the bath is done by electrical heaters or by

circulating oil. Controls must be adequate to maintain the temperature
within 8 to 10° F. Temperatures differ widely, depending upon the type
of melt used. For the sake of convenience, it is possible to make an arbi¬
trary distinction between high- and low-temperature melts, depending uj)on
whether the melt is applied at a temperature higher or lower than 300° F.
Most hot melts are applied at a temperature between 225 to 350 F. The
temperature must be high enough to keep all the ingredients in solution,
since otherwise a streaky coating will be obtained. Temperatures as high
as 400° F. are sometimes used, but in general, high temperatures are to be
avoided, since they cause drying out and a weakening of the paper. In
cases w'here high temperatures are required, it is desirable to run the ma¬
chine at high speeds and to pass the paper over cooling rolls immediately
after coating to prevent too long a contact at high temperature.

Methods of Applying Hot Melts

Hot melts can be applied by roll, knife, dip, spray, or extrusion coaters.
One of the most popular types of roll coaters is the gravure coater. One

1308

PULP AND PAPER

model suitable for low-viscosity melts consists of a heated gravure roll which
revolves in the hot melt, a doctor blade for removing excess melt from the
roll, and a pressure roll for holding the paper web against the gravure roll.
Another model suitable for higher viscosity materials contains a third roll,
a hot metering roll which contacts the pick-up roll and controls the quantity
of melt applied to the paper. The melt is applied in the form of a pattern,
and hence must be smoothed out by passing the coated paper under a heated
smoothing bar. The smoothing bar remelts the coating in the high spots
and causes it to flow into a smooth film.

After the coating is smoothed, it must be cooled rapidly. Low-tem¬
perature melts need to be chilled to produce a glossy surface,’'^ but high
temperature melts generally have gloss characteristics of their own. If the
coating is cooled too slowly, it tends to crystallize or separate and form a
non-glossy coating. The rate of cooling affects the size of the wax crystals,
and this influences the water vapor resistance of the coating.

Paper for hot-melt coating must not be too porous, because this results
in excessive penetration and the loss of protective qualities. The amount of
melt required for different papers has been given by Viner and Miller as
follows

Type of paper

Pounds of melt
per ream

Glassine

Book

Wrapping or bag

2-3

4-6

Generally speaking, about 6 to 8 lb. of hot melt per ream are required for
good quality supercalendered papers. For heat sealing, about 8 to 10 lb. of
coating are required. Special papers for packaging frozen foods may re¬
quire 10 to 30 lb. of coating.

Waxing with Paraffin

Waxing with paraffin dates from about 1866 and hence is one of the
oldest hot-melt coating processes. Paraffin is popular because of its rela¬
tively low cost, low melting point, low viscosity, and ability to produce films
which are hard, glossy, non-tacky, and water-resistant.

Paraffin wax occurs naturally with crude paraffin oils, and is removed
.from distillate stocks to lower the pour points of these oils. It is derived
from the waxy overhead fraction known as the wax distillate, which is ob¬
tained in the first distillation of the crude oil. Separation is based upon the
differences in solidification points between the wax and other components.
The first step consists of chilling, after which the mixture is filtered through
a filter press where the solid wax is removed. This wax, which is known

71J. W. Viner and B, C. Miller, Paper Trade L 118, No. 24; 23L222 (June 15,
1944)

XXII. COATING WITH RESINOUS MATERIALS

1309

1 w wav has a very high oil content, and must be put through another
as ^ I This consists of chilling to produce a

^id X rd trsl“ ; raising the tentperature to su.at ^t the .1.

The remaining wax which “ 'i™ * ^ because of the 1

ISrahlf T^r:;L-rr « .e wax and lowers

flip <;trenefth and hardness of the \\ax. ^ u*^u

Refifed waxes are made by additional sweating operations which re¬
solve rwa^tto various fractions. Fully refined waxes are made by a

solvent pressing operation, which is substantially a ® ^ .j
wax from a suitable solvent. This produces a wax o veo lou oil cont

(less than O.S^l.). The final steps in the treatmeiit “"“"j b

dude a steaming oiieration to remove odor and a final deco y

fitring though fullers earth or bone char. In some cases senii-refined

waxes are sold, but these are poorly defined and samples o t

have shown oil contents ranging from 0.5% to as high as o.Ofo, 1 S

true meiiiher of the paraffin family, l.ing composed

of a mixture of straight-chain saturated hydrocarlxms to

The ^ fraction's have different melting points, the lowest molecuUr

weight fraction having a melting potnt of about 30 C. being iqui ,

dinary room temperature. The nielting point ^ j jg

generallv between 120 to 130“ F., although paraffin ranges from 110 to
150“ F.'in melting point. (The melting point of paraffin ts determined as

the temperature at which melted paraffin first shows a 1™“'"“'" [ ,

tem,«rature change when cooled slowly under presertbed
Melted paraffin has a very low viscosity, the vtscosity being very nearly
same as the viscosity of the oil from which it was derived (as s lOw y
comparative viscosities at 210“ F.). Paraffin ts crystalline m nature and
forms films which tend to crack and check. Chlorinated ^ “

manufactured. These range from pale viscous liquids of about >

rine content to yellow solid resins of about 70% chlorine content.

A considerable amount of paraffin wax (almost 80^ of the tota pro
duction) is used in the paper industry. Machine-finished papers, super-
calendered papers, coated (titanium dioxide) papers, glassine, and grease¬
proof are some of the grades used for waxing. The waxed products are
used for bread wrappers, drinking cups, bottle-cap stock, and paper car¬
tons. The stiffening effect of paraffin on paper is related to the modu us
of rupture of the wax.” Paraffin produces a brittle film at low temperature,
and hence is not suitable for frozen food wrappers if the wrapper is to be
folded, although plasticizers (e.g., aluminum stearate) can be added to im-

”L. E. Hoag, Tappx 33, No. 7; 343-345 (July, 1950)

1310

PULP AND PAPLR

piovc the flexibility of the tilin. 1 he film softem at a relatively bnv tem¬
perature, and hence the waxed product shotild not 1)C stored in a warm place.

Difficulty is sometimes encountered with a loss in hardness and the de-
\ elopment of odor in the wax caused by a chemical reaction between the
wax and the oxygen in the air. To minimize these efifects, the temfK-ra-
ture in the wax bath should l)c as low as jx)ssible (not over 215® F.), and
low temperature steam should Ik? used for heating. In addition, the wax
bath should be designed so that a minimum of area is exposed to the air.
I'hc system should be cleaned regularly. Paraffin can l)e stored indefi¬
nitely in licpiid form at temperatures below 150® F., for about three to four
weeks at 150° F., for several days at 200® F., but only for a few hours at
250° Anti-oxidants may be added to the wax. The efficiency of anti¬
oxidants can be tested b}* passing oxygen at a controlled rate through a
samjile of the wax at 160° C. The results can l)e expressed as the numl)er
of hours required to yield a peroxide number of 50. In some cases, dark-
colored insoluble products are produced in the wax which are caused by
a reaction between the molten wax and metal. Copper is particularly bad
in this res])ect. and consequently copper equipment should not be used.

Dry Waxing. There are two types of waxing processes, wet and dry
waxing. In dry waxing, the wax is applied in such a way that it is con¬
fined entirely Avitliin the sheet. Dry waxing is in reality^ a saturating proc¬
ess.

The wax can be applied by squeeze roll coater or by spraying. The
amount of wax applied depends upon the speed of the machine, density of
the paper, temperature of the wax, and pressure at the squeeze rolls.^*
According to Ferguson,^® the speed of the pa|ier should be the same as the
peripheral speed of the rolls. Soft rubber squeeze rolls should be used
where a high percentage of wax is desired. The temperature of the wax
should be relativety high, and the squeeze rolls should Ije heated to drive
wax into the paper. A heated roll ma}’ be used after the squeeze rolls to
drive wax farther into the sheet. A chilled finishing roll may be used to

cool and set the wax.

The paper used for dr}- waxing should have a low, but even, finish.
The moisture content should be between 6 to 8%. Over 10^ wax must be
applied to obtain unifonn results. The upper limit is between 20 to 25^
wax, although most commercial papers contain about 16 to 20^.

Dry waxed papers do not present a continuous film of wax because
most of the wax exists within the sheet, and very- little is left on the surface
of the paper. Consequently, uncoated fibers protrude above the surface of
the sheet. These fibers are free to absorb moisture and, as a result, dry

735. Bondv, PopsT TthiIc y, 129, ^o. 12i /8 (SepL 22. 1949}

B Killinesworth, Paper Mill News 66, No. 25: 70, 72 (June 19,
75 E. E. Ferguson, Paper Ind. 28, No. 7 : 979-984 (Oct, 1946)

1943)

XXII.

COATING WITH RESINOUS MATERIALS

1311

waxed papers have very little resistance to water vapor. On the other
hand, dry waxed papers have a high degree of resistance to liquid wate

hecaiise the pores of the paper are filled with wax.

The absorptiveness of paper for paraffin can be measure jy p
a weighed sample of the paper to be tested between two sheets of waxed
blotting paper, and inserting between two steel or glass plates, anc ea ing
^e asseli^ at 105« C. for one hour. The difference in weight before and
after treatment is a measure of the absorptiveness of the paper or p
The amount of paraffin absorbed is reported as a percentage o t e un\\ax
paper to the nearest one per cent. Waxed blotters can be P-pared by^n-
mersing samples of blotting paper in molten paraffin (71 C.) }

minutes and then drying in an oven (105° C.) for one hour.

A rapid test for measuring the wax absorptiveness of pap^ can )e
carried out by soaking samples of the paper in a bath of hot paraffin (

132° F.) for twenty seconds and then, after cooling, scraping t e ar ene
wax off the surface of the paper, using the edge of a microscope slide. e
percentage of wax absorbed is determined by the difference in weight 3e-

tween the before and after waxing.

■ Wet Waxing. In wet waxing, the wax film is kept as much as pos¬
sible on the surface of the paper. The final product is called self-sealing
paper because the paper can be readily sealed with heat on automatic wrap¬
ping machines. Wet-waxed papers are more water-repellent than dry-
waxed papers, and furthermore, have a high degree of water-vapor resist¬
ance because of the film of wax on the surface of the paper. The wax oa

may be 40 to 50% of the weight of the base paper.

Roll or spray coaters can be used, or if both surface and internal wax

is desired, the sheet may be submerged in molten wax at 150 to 215° F.
Wire-wound steel rods are also used. After waxing, the paper is passed
over refrigerated rolls to set the wax or, if minimum penetration and maxi¬
mum gloss are desired, the wax may be applied at low temperatuj*e and the
waxed sheet quenched by dipping in water or brine at about 40° F. This
produces a higher gloss than passing over chilled rolls, because of the more
rapid cooling of the wax. Water is removed by blowing, suction, scraping,

or squeezing.

Waxing stock for self-sealing papers should have a high machine
finish to prevent excessive penetration of w'ax. The moisture content is
usually higher than that for dry waxed papers and is usually between 7 to
9%. The paper should be clean, since dirt in the paper shows up more after
waxing. Glassine papers are used where the highest transparency and
greatest protective qualities are desired. Waxes with melting points lower
than 128 to 130° F. should not be used for wet waxing because of the
danger of excessive softening of the wax film in warm weather.

When waxing printed papers, trouble is sometimes experienced with

1312

PULP AND PAPER

bleeding of ink into the hot wax. The amount of bleeding depends upon
the type of pigment and vehicle in the printing ink.^® If the ink contains
a drying oil base which has been well oxidized, i.e., dried for a period of
tw'enty-four to forty-eight hours, the ink will be insoluble in the wax. Pa¬
pers printed with moisture-set inks can be waxed about two hours after
printing. If the ink has been modified with wax-soluble resins, it will tend
to bleed. Inks dispersed in petroleum hydrocarbons will tend to dissolve
off the sheet.

Waxing of Paper Cartons. In addition to the waxing of paper, wax is
also used for the waxing of fabricated paper cartons. This may be done by
(i) prewaxing of the package before filling or (2) waxing of the package
after filling by shower method, single-dip coating, or by double-dip coating.
In the prewaxing process, the paper carton (with glued bottoms) is im¬
mersed into the wax bath and then drained. Later the package is filled
and the top is sealed. Applying wax to the filled carton is a more foolproof
method, and generally a dipping is used. In the single-dip method, the en¬
tire package is submerged in molten wax. In the double-dip method, the
package is coated half at a time. The time of contact of the paper with the
wax should be carefully regulated. If submerged in the hot wax too long,
the air in the box is greatly expanded, causing bubbles to form in the wax
film. Too short a period of contact is also bad, because this results in a
poorly bound coating, due to the congealing effect of the cold package on
the film of wax. In the double-dip method, there is less danger of bubble
formation, since the wax is applied to only half of the package at a time.

Determination of Paraffin in Waxed Papers. The amount of paraf¬
fin in waxed papers can be determined by extracting folded strips of the
paper with carbon tetrachloride, treating the extract with alcoholic potas¬
sium hydroxide, and shaking with petroleum ether in a separatory funnel.
The weight of material in the ether extract gives the amount of paraffin in
the paper. This test is based upon the assumption that the unsaponifiable
• material soluble in petroleum ether represents the paraffin content of the
paper. The test is therefore not suitable for papers containing materials
other than paraffin which has these characteristics.

Waxing with MicrocrystalHne Wax

Microcrystalline wax is a special grade of hydrocarbon wax derived
from petroleum oils. Microcrystalline waxes were first introduced ^out
1927, and since then, have supplemented paraffin wax for many uses. ey
vary from plastic products to hard, and even brittle products. ^

Petrolatum is the raw material source of microcrystalline waxes, t is
obtained from the heavy lubricating oils after the solvent refining of paraffin

'’’® E. F. Carman, Paper hid. 27, No. 9: 1367, 1371 (Dec., 1945)

XXII.

COATING WITH RESINOUS MATERIALS

1313

oil. In other words, petrolatum is removed from heavy lubricants to lower
the pour point of these oils, and is therefore comparable to slack wax m
paraffin operations. In preparing microcrystalline wax, the petrolatum is
subjected to a de-oiling operation to produce waxes of controlled me ting
points and different degrees of hardness. The waxes are dark tan to brown
in color, but white or lemon-colored waxes can be obtained by decolonzation

with activated clays or bone char.

At one time, microcrystalline waxes were believed to be amorphous,
but are now known to contain very fine crystalline material much finer than
the crystals in paraffin wax, hence the name “microcrystalline wax.” The
exceeding small crystal structure is believed to be due to the presence of
a crystal inhibitor, which is not present in paraffin wax.^^ Microcrystallme
waxes generally contain a high proportion of oil, which has a strong af¬
finity for the wax and controls the hardness and penetrating qualities^ of
the wax. Microcrystalline waxes differ further from paraffin wax in having
higher molecular weight, higher melting point, and higher viscosity in the
molten state. The melting point varies from 150 to 200° F., and the waxes

are generally classified as follows;

High melting ..
Medium melting
Low melting ..

175“ minimum
166“ minimum
155“ minimum

In general, the lower melting point waxes are the cheapest.

Microcrystalline wax is not hard enough to be used by itself for most
coating purposes, but it is often used in combination with paraffin to increase
the flexibility and toughness of the film. Papers coated with paraffin-mi-
crocrystalline w^ax retain their water-vapor resistance even after consider¬
able handling. There is practically no difference in the water-vapor re¬
sistance of paraffin and microcrystalline wax coatings so long as neither
film is folded, but paraffin-coated papers lose their vapor resistance as soon
as they are folded, whereas papers coated with microcrystalline wax do
not.'^® Coatings containing microcrystalline wax have improved heat-seal¬
ing properties and increased oil resistance. Because of its greater oil re¬
sistance, microcrystalline wax is used in coatings for butter, lard, and
cookie cartons. The grease resistance increases with the melting point of
the wax, and since microcrystalline waxes have higher melting points than
paraffin, they are only about one twenty-fifth as soluble in fats as refined
paraffin (melting at 130° F.). Microcrystalline wax is used mostly in wet
waxing, but is sometimes used in dry waxing when grease resistance is
desired.

” B. H. Clary, Paper Ind. 27. No. 11: 1679-1682 (Feb., 1946)

Idem.

1314

PULP AND PAPER

The ductility of the film is roughly proportional to the amount of mi¬
crocrystalline wax in the blend. In making frozen food wrappers, it is
customary to use 25% microcrystalline wa.x and 75% paraffin, but up to
50 to 60% microcrystalline wax is sometimes used. However, above about
25% microcrystalline wax, the effects of increased tackiness and tendency

to block become apparent.

Use of Resin-lVax Blends as Hot Melts

Resins are used in combination with waxes to improve the properties
of the film. In some cases, only a small amount of resin is used, in which
case the resin acts as a modifying agent for the wax. In other cases, the

melt is predominantly resin.

■ Film-forming resins (e.g., ethylcellulose and polyethylene) are widely
used to impart blocking resistance and toughness and to increase
resistance of the film, although certain film-forming resms lack sufficien
heat stability to be used as hot melts (e.g„ nitrocellulose and polyvtnyl cldo-
ride) Non-film-forniing resins (e.g., modified rosin derivatives, modified
phenolics, and modified alkyds) are sometimes added to increase gloss,

transparency, water and grease resistance, am a esion o e .
idizing or polymerizing resins are, in general, unsuitable for hot melts b

wax with some d« “re Ire

show no decomposition at tempei aiures jv

Resta-wax meta may contain added ingredients such as oils, metallic
soaps, non-vdatile solvents and 1,^^^^^^^

'fir ‘TlereiSSs must be co^ahble w^ both the^^r^n a^d^^x,

Tnd r: "rr !"ma? be used in’eonjunction with paraffin

:: increase the hardness tensile stren^K celffilose deriva-

Use of Ethylcellulose. . ^,1,cellulose. Ethylcehillose has

vides water and water paraffin by itself, and conse-

Ethylcellulose is not compati l solvent. Hy-

Cuently a third ingredient is for this purpose.

drogenated vegetable oils (e.g., ^ increase the hardness, to

add gloss, and m some cases, lo f

XXII. COATING WITH RESINOUS MATERIALS

1315

of the film. Resins of high acid number should not be used. Plasticizers are
added to ingredients except the ethylcellulose are

are given below.^®

Formula

Ethylcellulose .

Resin .

Plasticizer .

Paraffin wax .

Vegetable wax .

Microcrystalline wax

Coating for

Soap wrap,
shelf paper,
and label

Bread wrap

Frozen food
wrap

Low moisture
vapor trans¬
mission

High grease
resistance

papers

None

. None

None

a\nother formula for a hot-melt coating is as follows

Ethylcellulose (10 centipoises) ...
Amberol (maleic rosin type phenol-

formaldehyde resin) .

Paraffin wax (140“ F.) .

Opal wax No. 10 .

Blown castor oil .

Triphenyl phosphate .

11.5

33.3

19.0

20.0

15.7

0.5

In preparing this mixture, all ingredients except ethylcellulose should
be melted together and heated to 130 to 150° C. The ethylcellulose should
then be sifted in as fast as it will dissolve. Bleeding an inert gas into the
mixture from the bottom of the pot helps to maintain best color. The higher
the percentage of ethylcellulose in the formula, the higher the viscosity of

the melt.

Ethylcellulose hot melts can be applied by roll, knife, or spray coaters,
or by dipping. The melt is usually applied at a temperature of 230 to
275° F. At temperatures low-er than 230° F., the ingredients are likely to
separate. On the other hand, prolonged heating at high temperatures
causes some degradation and loss of film flexibility. Anti-oxidants help to
stabilize the melt. On a good quality supercalendered paper, about 5 to
8 lb. of ethylcellulose hot melt will produce a coating of good gloss and pro¬

tective qualities.

Use of Polyethylene. Polyethylene is not widely used alone in hot

70 Technical Bulletin. “Ethocel Hot Melts for Paper Coatings,” Plastic Division,
The Dow Chemical Co., Midland, Michigan (1948)

Private communication, p. 49, Rohm and Haas Co., The Resinous Products
Div., Philadelphia, Pennsylvania (Apr., 1948)

1316

rULP AND PAPER

melts because of its high viscosity. However, polyethylene of relatively low
molecular weight (i.e., about 12,000) can be used alone or in combination
with waxes. The coating is usually applied by extrusion coater in a film
about Yi mil in thickness. Coverage is high because of the low specific
gravity. The film has a very high degree of flexibility and shows an enor¬
mous elongation at a load just below the breaking point. The water vapor
resistance is high, but the resistance to carbon dioxide and oxygen is low.
The coated paper is admirably suited for use as a locker jjaper for wrayjping
frozen foods. The coating can be heat sealed at 325 to 350° F.

Papers coated with polyethylene-wax blends show good strength, ex¬
cellent flexibility, good heat-seal properties, good electrical properties, and
high water-resistance. The addition of 2 to 5% of polyethylene resin to
paraffin increases the blocking temperature, reduces rub-off, increases the
tensile strength, and improves the strength of heat-seal bonds. The water
vapor resistance is not appreciably reduced. The resin increases the vis¬
cosity of the wax and necessitates the use of a slightly higher temperature.
Ordinarily, the minimum temperature w’ill be approximately 212° F. Anti¬
oxidants are recommended to prevent oxidation. In the case of micro¬
crystalline w'axes, about 1 to 2% polyethylene is helpful in reducing tacki¬
ness of the film.

Use of Other Resins. Many of the vinyl resins are not suitable for
hot melts because of their tendency to decompose at high temperatures.
However, pol>winyl acetate can be used if compounded with natural resins
(e.g., dammar) and suitable plasticizers. As a rule, the acrylics and poly¬
styrene resins are not suitable. However, «-butyl methacrylate has been
used in combination with paraffin, hydrogenated castor oil, and a plasti

cizcr.

Cellulose acetate is not particularly well suited for hot melts because
of its low compatibility with resins and plasticizers, although some cellulose
acetate has been used. Cellulose acetate butyrate has been suggested for
hot melt coatings.®" Esters having a high butyral content (oO^o) can e
applied at temperatures in the range of 325° F. The following formula has

been found satisfactory

40-60%

Cellulose aceto-butyrate ..

Resin (e.g., chlorinated diphenj’l 10-30%

or rosin-maleic resin) .

Plasticizer (e.g., dioctyl phthalate, ^ ^ 15-25%

dibutyl sebacate, or w-butyl stearate) .

Wax (e.g., paraffin) .

r 1 j tr TT VKn'an U S 2 387,773 and 2.387,774 (Oct. 30,
81 M. Salo and H. F. Vman, ^ ^ lOiA’i

1945);

82 W. S. Penn, World’s Paper Trade Rev. 128, J\o. /.

XXII. COATING WITH RESINOUS MATERIALS

1317

Use of Rubber Derivatives in Hot Melts. Natural rubber is not well

suited for hot-melt coatings, although it has been suggested in
with paraffin As mentioned earlier, cyclized rubber can be used in
Le with paraffin to improve the flexibility, toughness, and gloss of ffie
film, and to strengthen the heat seal.- This product is soluble m paraffin
to a slight extent at room temperature and is complete y so u e in a p
portions at a temperature above the melting point of the wax.» It raises
L melting point of the wax approximately 3° F. for every 5% of rubber
and also increases the viscosity of the melt. Generally by the time su
dent cyclized rubber is incorporated in paraffin to obtain patly improve
flexibihty, the mixture is so viscous that it is difficult to handle with con-

ventional waxing equipment. , k f ,i mKh^rc are

Of the synthetic rubbers, the isobutene rubbers and butyl rubbers are

most interesting. These can be used in hot melts in combination with wax

to improve the flexibility and heat-seal properties of the coating.

Carbon Paper

Carbon paper is made by coating paper with a mixture consisting prin¬
cipally of a wax and a pigment. The color is obtained from the pigment,
usually a carbon black of low oil absorption, plus toners, usually lake pig¬
ments or oil-soluble dyes, which are added to increase the blackness. T e
wax, which acts as a binder for the pigment, must have a viscosity low
enough to permit slight penetration into the paper, but^ not so low that
the wax strikes through or sweats oil. Waxes with a melting point between
105 to 120° F. and a viscosity of about 60 or 70 Saybolt at 210° F. are gen¬
erally used. The principal wax used is carnauba, although some ceresin,
beeswax, candelilla, ozokerite, ouricury, and synthetic waxes are also used.
Special grades of microcrystalHne waxes may be added to soften the coat¬
ing and improve the printing qualities. In addition to the above ingredients,
non-drying oils (mineral oils) are used to soften the coating and control the
amount of coating transferred to the copy. Oleic acid is sometimes used as
a solvent for oil-soluble dyes.

All carbon papers must be free of offset, flaking, wrinkles, curl, or
other defects, and must give a good impression on the copy paper. The
coating must be hard enough not to smear in hot weather, but the exact de¬
gree of hardness depends upon the intended use, that is, whether the paper
is designed as a pencil carbon, a typewriter carbon, or a one-time carbon.
The amount of coating varies from a very thin coating used in making one¬
time carbons to a very heavy coating used in making high-grade typewriter
carbons for multiple use. The latter, in which the paper may be reused
up to 40 to 50 times, must have a coating of very high color value and the

A. Abrams and C. L. Wagner, U. S. 2,054,112 and 2,054,113 (Sept, 15, 1936)
®^H. R. Thies, Paper Trade J. 108, No. 8; 79-84 (Feb. 23, 1939)

1318

PULP AND PAPER

coating must he compounded so that only a small amount is transfcrrefl to
the co])y sheet. A simplified formula for a typewriter carbon would he as
follows:

Carnauba wax . 34%

Ozokerite . 6%

Beeswax . 7%

Petrolatum . 6%

Mineral oil . 25%

Carbon black . ^3%

Toners .

Oleic acid .

Ill the coating of carbon paper, the molten wax mixture is applied to
the paper at a temperature of about 200° F. Coating is done on a carbon
paper coater which consists of an inking roll which is supplied with coating
directly from a bath or from a heated fountain. The inking roll revolves in
a direction opposite to that of the paper web. Excess coating is scraped ofif
the paper by an equalizer rod and the sheet then passed over a water-cooled
cylinder to chill and harden the coating. Crystallization of the wax occurs
upon chilling, and this process continues over a period of several days. For
this reason, it is customary for carbon manufacturers to age their coated

paper for two to seven days before shipping.

It is absolutely essential that the coating be absorbed evenly by die
paper. Therefore, the paper must have a smooth surface, uniformly high
density, good formation, and above all, be free of pinholes. Further re¬
quirements are high strength, low basis weight, and freedom from flaws
such as slime spots and dirt specks. Because of these rigid requiremen :>,
the base stock for carbon tissue is difficult to make. The best gra es are
made from new cotton or linen rags, or from manila hemp, whereas he
cheaper grades are made from sulfate and sulfite pulps. The stock is beaten

for a considerable period of time, often up to thirty hours

velop maximum strength. The stock is only hghtly sized. Calcmm ^

UonL is often used as a filler, but the ash should not be o^ 5%. Th

basis weight is usually 4, 5/., 7, or 10 lb. c

4-Ib paper is used when a large number of copies is to be made,

=rn ror;“r:::ue desert

other grade known as x22l500).T^^^ used mostly in

.sales books which are intended for pen and Pend o^k.^ the base

slocf lirmusTbe drsetno^to ke^p the wax coating on the surface

errade than in the typewriter grades.

CHAPTER XXIII

RESINS

The general proi»erties oi resins are of great importance to the papei
chemist because of the wide use of resins in the pai>er industry. Some of
these Uses include: (J) the use of rosin as an internal sizing agent, (2) the
application of resin emulsions and latices at the beaters and the calender
>uck$, (i) the lamination of paper with resin solutions, resin emulsions,
and hot melts, (4) the coating of pai>er with lacquers and varnishes, and
(3) the impregnation of pai)er with resin solutions and latices. In this
chapter, the origin and metho<i of manufacture of resins is discussed, to¬
gether with their general solubility and compatibility characteristics. For
sjiecihc infonnation on the use of resins, the reader is referred to the chap¬
ters on coating with resinous materials, internal treatment of paper ith

resinous materials, j)aj)cr plastics, and laminating.

Tliere arc several important classes of resins, and these have been di-
videtl in this chapter into the following: natural resins, synthetic and semi-
s\-nthetic film-forming resins (cellulose derivatives and vinyl polymers),
sjTithetic non-film-forming resins, and rubber and rubber derivatives.
The synthetic resins offer a promising field for development and have al¬

ready rqdaccd the natural resins for most uses.

The synthetic resins are divided into the film-forming and non-film-
forming tvpes. Film-forming resins are those which form films of sufficient
flexibility and cf»hesion to l»c self-supporting. Non-film-forming resins are
those which fonn hard, brittle films which are not self-supporting. The
clas‘iificatif>n is arbitrary and difficult to make, since there is considerable
overlapping in proj>erties. For example, many resins which are considered
to lie film-forming will not form continuous films unless they are com-
pouivletl with plasticizers and modifiers. On the other hand, some of the
non-film-forming tj'pes produce films of a fair degree of continuity when
properly compounded with modifiers and plasticizers.

The important properties of resins are their chemical characteristics,
softening temperature, flow properties, and film-forming characteristics.
If sold as a liquid product, the volatile content is important. The solids can
be determined by e^•aporation of liquid resins in an open cup, but this
method has the disad\*antage that some of the volatile resin-forming material
is lost. Another method is to evaporate a sample of the liquid resin at
alxMit the sante temperature used in curing the resin, with a relatively large

1319

1320

PULP AND PAPER

surface area exposed. Still another method is to cure the resin by heat or
by the addition of a small amount of acid so that all the resin-forming ma¬
terial is solidified.

Natural Resins

Natural resins are obtained, for the most part, as exudations from
living trees or plants. The resins are collected and sold with a minimum of
chemical and physical treatment. Many natural resins are named after
the geographical location of the source of the resin, e.g., East India, Manila,

and Congo resins.

At one time the natural resins were the only available resins, but re¬
cently they have been largely replaced by the synthetic products. However,
some natural resins (e.g., rosin) are still being used in large quantities by
the paper industry. Most of the natural resins are used in varnishes and
lacquers for coating paper, although rosin is, of course, most widely used

for sizing. , , j

Natural resins have many disadvantages, although these disadvantages

do not apply to all natural resins. Some of the common disadvantages are
lack of adequate supply, fluctuating market price, bad odor, and tackiness o
film. On the other hand, natural resins generally have good compatibility
with a wide variety of other resins and waxes, and excellent solubi ity, par¬
ticularly in alcohol.

Rosin

Wood and gum rosin are the two major classes of this important
uct of the SoutLn pine. Both are alnrost exclusively obta.ne^ fro^^
loneleaf (Pinits palustris) and slash {P. canbaea) pines. Gum rosin

a nfoduct of the L tree, while wood rosin is obtained from virgin s urops.

Though the development of refining techniques, particularly as aPP>>ed “
lo^d rosin these robins have become readily interchangeable for all impor-

tant commercial applications. fmm living tree

Gum rosin is a component of oleoresm obtained from he hvmg «

bv wounding The oleoresin usually contains approxima e y Jo .

2 S;r;enfine, and 12,^ water. T^^rosin ^ revered as a reside after

trZ cfreted re—ir^^ matter, both prior to and during

'"""wtod rosin is extracted from virgin stump wood ava^ab. 00 *
of acres of cut over land terpenes to process econom-

Ve'^able mamna. from

under pressure after the

XXIII. RESINS

1321

stumps have been chipped and shredded. The solvent extract is fraction-
ated to Weld recovered solvent, turpentine, monocyclic terpene hydrocar¬
bons, and pine oil, the crude rosin remaining as a molten residue. T iis
rosin is a dark ruby red color and must, therefore, be rehned for general

commercial acceptance.

Two refining procedures are employed commercially, solvent refining,
and adsorbent earth refining. In the former, a gasoline solution of the
crude rosin is extracted with furfural to yield a gasoline solution of the p^e
rosin and a furfural solution of the dark-colored constituents. The effi¬
ciency of the solvent extraction determines the grade of rosin recovered.
In the adsorbent earth process, a gasoline solution of the crude rosin is
passed through active earths, such as fuller’s earth, which selectively adsorbs
the color bodies, giving an effluent of pale rosin solution. In both instances,
the pale rosin solutions are evaporated to yield the refined finished product.

Rosin is graded for color by comparing with standard color cubes.
Major sales of both gum and wood rosin are in grades ranging, on an al¬
phabetical scale, from G to X, the latter being the palest grade available.
In order of decreasing color, these grades are G, H, I, K, M, N, WG, WW,
and X. The particular grade chosen for the sizing of paper depends upon
the brightness requirements of the paper. ^ White papers are sized with G

rosin or higher.

The composition of gum and wood rosins is approximately 90% rosin
acids and 10% resenes. The rosin acids are present in approximately the
following proportions for both rosin types:

Abictic and abietic type acids . 50%

Dehydroabietic acid .

Dihydroabietic acid .. • • 15-16%

Tetrahydroabietic acid . 16-17%

Dextropimaric acid . 1^%

Abietic acid has been investigated most extensively. It has the structural
formula shown in Figure XXIII-l. The chemistry of the neutral or un-
saponifiable portion of gum and wood rosins has not been developed com¬
pletely. These materials apparently consist of high-boiling hydrocarbons,
sterols, and alcohols combined as esters with small quantities of wax.

The acid number of rosin can be determined by dissolving the rosin in warm neu¬
tral alcohol and then titrating with 0.5 N sodium hydroxide to a pink end point with
phenolphthalein. The acid number is the number of milligrams of potassium hydrox¬
ide consumed per gram of rosin.

1 ml. 0.5 N NaOH = 28.06 mg. KOH

The saponification number of rosin can be determined by dissolving a small sample
of rosin in 0.5 K alcoholic potassium hydroxide, and after boiling for 2 hours, titrating
with 0.5 N hydrochloric acid to a pink end point with phenolphthalein. The saponifi-

1322

PULP AND PAPER

cation number is the number of milligrams of potassium hydroxide consumed by 1 g.
of rosin.

The cslcr numbrr is the difference between the saponification number and the
acid number.

The amount of unsaponifiable matter in rosin can be determined by boiling a sam¬
ple of the rosin with an excess of 0.5 AT alcoholic potassium hydroxide for 2 hours in
a reflux condenser. The alcohol is evaporated and water is added, after which the
water solution is extracted several times with acid-free ether. The amount of un¬
saponifiable matter is collected in the ether layer where it is evaporated and weighed.
Unsaponifiable matter may also be determined by weighing the amount of rosm un¬
reacted upon when boiled under reflux with sodium carbonate solution. The un¬
saponified rosin is extracted with ether as in the preceding method.

Vi\/\

C
H;:

\/\y

CH:

Dirt is determined

Fig. XX111-1. Abietic acid.

Other tests sometimes made on rosin are ash and dirt content,
as the matter insoluble in toluene.

Rosifi Devivatives

Rnsin will undergo most of the normal reactions of organic acids and
1 fi ^ mch as iu and ester formation, substitution and addition reactions.

acids are readily Pob--ed and polymerized -ins.J-c ^

rZ are produced commercially (Poly-pale and

Hydrogenation of

200» C. tends to isomerize mixture, termed by

ture, it is probable that the an j ri e recently identified as

ltabilL° rosi^'lXC prevent oxidation, which results in darken-

XXIII. RESINS

1323

ing of rosin with concurrent undesirable changes in its chemical and physi-
''a wide variety of rosin derivatives are available commercially m both

solid and solution form.'-

The ethylene glycol and diethylene glycol esters of rosin Flexalj n
resins) are important commercial resins which are available as solid resins,
as solution of 80^0 solids, and as emulsions. These resins are soluble in a -
most all solvents except alcohols, and are completely miscible with nunera
and vegetable oils. They are compatible with most other resins, including
nitrocellulose, ethylcellulose, rubber, and chlorinated rubber, and are also
compatible with a wide variety of waxes, asphalts, and gums. ey are
used as plasticizers and softeners in resin-coated papers, and as tackifymg
agents in adhesives, being particularly well suited to making pressure-
sensitive adhesives. The emulsions can be used with water-soluble ma¬
terials, such as starch and casein, to increase the toughness and adhesion,

and reduce the water sensitivity of the dried film.

The glycol esters of hydrogenated rosin (Staybelite resins) are very

stable derivatives, having an oxygen pick-up of less than 0.5 to 1.0%.
compared with over 6.5% pick-up for rosin." These resins are soluble in
aromatic hydrocarbons, aliphatic hydrocarbons, esters, ketones, glycol
ethers, chlorinated solvents, drying oils, and terpenes. They are insoluble
in alcohols. They exhibit wide compatibility with non-film-forming resms,
waxes, plasticizers, and film-forming resins. Some of the film-forming
resins with which they are compatible are butyl methacrylate, ethylcellulose,
nitrocellulose, rubber, rubber hydrochloride, cyclized rubber, polyvinyl bii-
tyral, and polybutene. Some of the non-film-forming resins with which
they are compatible are dammar, ester gum, rosin, nianila, polystyrene,
shellac, and urea-formaldehyde resins. In addition, they are compatible
with asphalt, beeswax, carnaiiba w’ax, ceresin, opalwax, ozokerite, and
paraffin. Some of the plasticizers which can be used with these resins arc
blown castor oil. butyl stearate, dibutyl phthalate, mineral oil, tricresyl
phosphate, and vegetable drydng oils. The resins are used in lacejuer, hot
melt, and latex form. Their greatest use is in adhesives where their in¬
herent tackiness is of great value. They are also used as plasticizers and
modifiers for other resins.

The methyl ester of rosin (Abalyn resin) and the hydrogenated methyl
ester of rosin (Hercdlyn resin) are important commercial liquid esters of
rosin. These resins are miscible with ethers, organic esters, ketones, al¬
cohols, and aromatic and aliphatic hydrocarbons. They are compatible with

1 “Synthetic Resins,” Hercules Powder Company, Wilmington. Delaware

“Hercules Synthetic Resin Solutions,” Hercules Powder Co., Wilmington, Dela¬
ware

^“Staybelite Esters,” Hercules Powder Co., Wilmington, Del. (1947)

1324

PULP AND PAPER

and act as solvents for many film-forming resins such as nitrocellulose,
ethylcellulose, chlorinated rubber, polychloroprene, vinyl acetate-chloride
copolymer, and natural rubber, but are incompatible with cellulose acetate
and vinyl acetate. Some of the non-film-forming resins with which they
are compatible are the alkyds, coumarone-indenes, ester gum, rosin, and
urea-formaldehyde resins. They act as softening agents for the natural
resins such as manila, kauri, and shellac. They are compatible with veg¬
etable waxes and asphalt, but are incompatible with paraffin when used in
minor proportions. They are sometimes used in combination with starch,
casein, and animal glue to increase the toughness and flexibility of the film.

Another important group of rosin derivatives are the rosin-derived
alkyd type resins (Neolyn resins). These are amber-colored resins which
range from soft, balsamic materials to solid products having a softening
range around 75° C. These resins are soluble in toluene, ethyl acetate,
acetone, carbon tetrachloride, and chlorinated solvents, but are insoluble m
aliphatic hydrocarbons, fats, oils, and waxes. They are soluble in alcoho
at high resin concentrations only. These resins are compatible with nitro¬
cellulose, vinyl acetate-chloride copolymers, polyvinyl chloride, chlorinated
rubber, methyl and ethyl methacrylates, urea-formaldehyde resin, an
others. They are incompatible with waxes and have only a very linii e
compatibility with ethylcellulose and cellulose acetate. They are use as
adhesives and as coatings, being applied as varnishes, hot melts, or emul¬
sions.

Shellac

Shellac is a resin of insect origin. India is the main source of supply.
Chenfcllly speahing, shellac is a polyester formed by the sel -estento^on
of a mixture of hydroxy acids. These hydroxy acds are >>oth monoto

and dibasic but nearly all contain more than one hydroxy group,
hi treatment of sheL. ester linkages are established between chams, re¬
sulting in the formation of a three-dimensional polymer.

sliellac is sold in a number of different grades, including hard brown
or oranglrs (TN, Superfine, ASO, DC). It is sold in bleached and

Shetoc Ts soluble in alcohol and is used in

cohol solvent. Shellac is insoluble m b“ ^ ^

by the use of alkalies. One type of '^ ^ suit-

which has been solubilized with ammonia and borax and use

able dyestuff. .

Dammar Resins

Dammar resins are pale, hard resins which are “Stained from ^

lively fresh exudations of trees w*ich grow

/c. E. Barnes, hid. Eng. Chem. 30. No. 4; 449-tSl (Apr., 1938)

XXIII. RESINS

1325

lands East Indies and Malay States. Alter collection, these r^ins are
cleaned and classified according to quality. The resin has a low ac.d

Dammar resins are soluble in coal tar solvents, petroleum solvents, and
hydrogenated petroleum solvents. They are compatible with some waxes,
ethvlcellulose, and some oils. The poorer grades form solutions of higher
viscosity than the better grades, probably because of the presence of im¬
purities. , ji • j

Dammar resins are thermoplastic. They show good adhesion an

have high water resistance. They are used in both hot melt and lacquer
coatings where moisture-proofness and heat-sealing properties are desired.
They are also used for decorative coatings.

Manila Resins

The manila copals constitute another important class of natural resins.
They are characterized by high acid number and pale color.

Manila resins are soluble in alcohol, but are insoluble in both aromatic
and aliphatic hydrocarbons. They are compatible w'ith ethylcellulose and

some vegetable oils.

East India Resins

East India resins are commonly divided into the pale and black grades.
The former have light color, whereas the latter are much darker. Both are
soluble in petroleum and aromatic solvents. Films produced with East
India resins have good water resistance and heat-seal properties.

The Batu resins are similar to the pale resin. They have excellent

water resistance.

Congo Resins

The Congo resins are the most insoluble of all natural resins. There
are no solvents for these resins in their natural state, so that the resins must
first be processed thermally in order to make them soluble, after which they
become compatible with a wide range of solvents and drying oils.® The
solubility of the final product depends upon the degree of heat treatment.
The treated resin is hard and dark in color and has a high acid number.

Treated Congo resins are soluble in petroleum and coal tar solvents,
and are compatible with ethylcellulose and vegetable oils. In addition to
the regular heat-treated product, other grades are available in which the
resin has been esterified with glycerine to form Congo resin esters.

Kauri

Kauri resin has high acid number. It is soluble in alcohol, but insol¬
uble in aromatic and aliphatic hydrocarbons. It must be heat-treated to
render it compatible with oils.

»C. L. Mantell and R. W. Allan, Ind. Eng. Chem. 30, No. 2: 262-269 (Mar., 1938)

1326

PULP AND PAPER

Accroidcs mid Elcvti

There are many other natural resins, hut most of these are of rela¬
tively little importance to the paper chemist. The accroides and elemi
resins are others which are sometimes used. The accroides are soluble in
alcohol but insoluble in aromatic hydrocarbons. They are compatible with
cellulose acetate. Elemi resins are soluble in alcohols and esters, and are
compatible with ethylcellulose.

Zein

Zein is a protein (prolamine) obtained from corn gluten, a by-product
of the wet-milling corn industries, in which it constitutes about 70% of the
total protein in the gluten. Zein is extracted from the gluten with aqueous
isopropyl alcohol and sold as a fine, slightly yellow powder.

Zein is soluble in methanol, ethyl alcohol (95%), and isopropyl al¬
cohol (91%). Some water must be present to obtain solubility in alcohols,
although water-free solutions can be prepared in glycols. Zein tends to
denature in alcohol solution and, in time, will set spontaneously to a gel,
particularly if the water content is high. Cosolvents (e.g., aliphatic esters,
chlorinated hydrocarbons, and lower ketones) can be used to prepare an¬
hydrous systems. Diluents (e.g., aromatic and aliphatic hydrocarbons)
can be used if desired. Zein is compatible with acid resins such as rosin,
Manila, or shellac, and also with some of the neutral resins, e.g., ester gum.
Other compatible resins include the phenolics, urea resins, and coumarone

indene resins. . c,iK

Zein produces oil- and scuft'-resistant coatings. It is a satisfacto y

stitute for shellac. The coatings have low gloss, although this can e im-

proved by the addition of resins. Zein is used in coatings for food wrapper

Ld cartons, and in decorative solid^olor printing ’37"’

LL of its high molecular weight. It will dissolve ^ J,,

about 118 and will dissolve in aqueous solutions of sodium htdroxide
LdTum soaps. However zein is ordinarily not applied in this manner.

Cellulose Derivatives (Film-Forming Resins)

There are two broad classes of synthetic (and semi-synthetic) resins

which belong to the film-forming group. These are the cdlutee ^ "

and the vinyl polymers. Some of the most important cellulose
of solvent-soluble grade are discussed below.

NitTOCcU ulosc

XXIII. RESINS

1327

tJKilltv with -i lar'^e number of plasticizers, and its solubility m a

Nil 0 ellulcse is made by treating pare cellulose w,.b

f," and^nlinric acids until between .1,2 and >2^

is added in the form of nitrate, corresponding to a mixture
trinitrates. The following percentages of nitrogen correspon

oretical derivatives.

14.16% nitrogen

'Irinitrate . . 11 . 13 % nitrogen

Dinitrate . 6.77% nitrogen

Mononitrate .

The de-ree of nitration is the important factor governing the solubility
of the final Vodurf- The commercial products used for lacquers are ni¬
trated to the point where they are soluble in alcohol acetone,

tate. In general, nitrates of low substitution are soluble in a CO 1 ,

nitrates of higher nitrogen content are soluble in acetone. le ng r
Xgree of substitution, the lower the water sensitivity. There are two prin¬
cipal tyires, the RS (regular soluble) type, which is soluble m esters and
ketones, and the SS type, which is soluble in alcohol (see Ch. X. ^

Xitrocellulose is classified according to its viscosity in standard solvent
mixture, using the falling hall viscosity method. Commercial products have
viscosities ranging from M second to 1,000 seconds and above by this
method. The viscosity depends almost entirely upon the average molecular
chain length, and hence is determined by the type of cellulose used in t e
preparation of the derivative. As with all film-forming resins, the viscosity
is an indication of the film strength. The viscosity also controls the total

solids at wliich the resin can be applied in solvents.

Xitrocellulose is compatible with the natural resins, modified alkyd
resins, rosin derivatives, polyester resins, paraffin wax, and other synthetic
resins. Plasticizers such as dibutyl phthalate and tricresyl phosphate are

generally used.

/tlhtJnc/* ArfifntP

Cellulose acetate is made by heating cellulose under pressure with a
mixture of'acetic anhydride and glacial acetic acid, and a small amount of a
catalyst such as sulfuric acid, phosphoric acid, chloroacetic acid, or zinc
chloride. During acetylation, the sulfuric acid combines with the cellulose
to produce a cellulose acetate acid sulfate, and it is only toward the comple¬
tion of the reaction that the sulfate is gradually replaced by acetyl.® The
reaction produces an ester with a high degree of substitution, so that it is
necessarv to dilute the reaction mixture with dilute acetic acid in order to
hydrolyze the product to one of lower substitution. When the product

® C. J. Malm. L. J. Taughe and B. C. Laird, Ind. Eng. Chem. 38, No. 1: 77-82
(Jan., 1946)

1328

PULP AND PAPER

reaches the desired degree of substitution, it is precipitated by the addition
of water, after which the product is neutralized and washed.

Cellulose triacetate produces weak films, but products of lower acetyl
content produce very strong films. Commercial products have an acetyl
content ranging from 38.0 to 41.5%, corresponding to a mixture of the di-
and triacetates. The following shows the acetyl contents for the theoretical
esters.

Triacetate .. • 44.8% acetyl

Diacetate . 35.0% acetyl

Monoacetate . 21.1% acetyl

The commercial acetate has a softening range of 435 to 480° F.

Cellulose acetate has lower solubility in solvents and is less compatible
with resins and plasticizers than nitrocellulose. On the other hand, cellu¬
lose acetate produces films which are less flammable, more heat- and light-
stable, and more solvent-resistant than nitrocellulose. The solubility de¬
pends on the degree of substitution as indicated by the acetyl content. The
triacetate is not soluble in acetone, but is soluble in the less polar solvents,
such as chloroform, nitrobenzene, nitromethane, and pyridine. Esters in
the commercial range are soluble in acetone and methyl ethyl ketone. Co¬
solvents such alcohol and diluents such as toluene are sometimes added.
Among the resins with which cellulose acetate is compatible are the phenol-
ics, alkyds, and some of the natural resins. Many different plasticizers can

be used.

Other Cellulose Esters

It is possible to prepare a large number of cellulose esters containing
fatty acid radicals of different lengths. As the length of the substitute
group is increased, the derivative becomes more soluble in non-polar sol¬
vents. Thus, cellulose tripropionate is soluble in benzene, whereas ce u ose
triacetate is not. Other manifestations of increased size of the substitu e
groups are reduced tensile strength, increased flexibility, increased softness,
lower water sensitivity, and lower melting point. The densities of cellu os
esters range from 1.28 for the triacetate to 1.16 for the tnbutyrate, com¬
pared with 1.52 for regenerated cellulose.^

Mixed cellulose esters can be prepared by using mixed este > g
agents Certain mixed esters have improved properties over t e pure e
rivatives for example, cellulose acetate butyrate and cellulose acetate prop -
onate haU higher water resistance, more flexibility,
ity with resins than cellulose acetate. The softemng po.n s are
400 to 435° F. for cellulose acetobutyrate) than that “

(435 to 480° F.) Because of the lower softening point, cellulose

T c. J. Malm, L. B. Genung and J. V. Fleckenstein, l«d. Bng. Chem. 39. No. :

148&-1S03 (Nov., 1947)

XXIII. RESINS

1329

tyrate is more suitable for hot-melt application than cellulose ^

mixed esters are not used to any extent in paper coatmg, however, because

of their higher cost.

Ethylcellulose

The most important cellulose ether, from the standpoint of the coatmg
chemist, is ethylcellulose. Ethylcellulose is sold as white porous granules
which can be used in hot-melt or solvent form to produce odorless, color ,

FTHOXY TYPE DESIGNAT fOjj.

STANDARD

MELTING POINT RANGE

SOFTENING POINT RANG

266

248

220

338

320

302

284

-H-1-^-1- ' --T

45 46 47 48 49

lETHOXY CONTENT - PER CENT|

200

210

Fig. XXIII-2. Effect of ethoxyl content on the softening

and melting points of ethylcellulose.

and tasteless coatings. The films are tough and flexible and have good
heat-seal properties. Ethylcellulose is better suited for hot melts than
cellulose acetate or nitrocellulose because of its better heat stability and
better compatibility with film-modifying agents.®

The commercial grades of ethylcellulose used in lacquers and hot-melt
coatings have a degree of substitution in the neighborhood of 2.25 to 2.60
ethoxyl groups per glucose unit (46.8 to 48.5^ ethoxyl content). At

® J. H. Long, Paper Trade /. 109^ No. 2; 38-40 (July 13, 1939)

1330

PULP AND PAPER

about this degree of substitution, ethylcellulose has its maximum solubility
in mixtures of alcohols and hydrocarbons (see Ch. I). Products in this
range have low softening points and the films remain flexible at low tem¬
peratures. The effect of ethoxyl content on the softening and melting points
is shown in Figure XXIII-2.® The low ethoxyl types are not widely used
because of their low solubility and reduced compatibility with resins and

plasticizers.

COO

700

600

2 500
O

-» 300

UP TO 20 CPS'

50-100 CPS

UP TO 50 CPS.

UP TO 10 CPS.

200

too

L-

UP TO 100 CPS.

APPROXIMATE LOAD-ELONGATION CURVES
FOR ETHOCEL VISCOSITY TYPES

1. STANDARD-ETHOXY ETHOCEL
solvent: 80 TOLUENE- 20 ETHANOL

2.MEDIUM-ETHOXY ETHOCEL
— solvent:60TOLUENE- 40 ETHANOL

ALL FILMS 0.040 MM. ±0.002 MM.

Fis. XXIlI-3.

10 12 14 16 16 20 22 24 2 6 26

eloncationj percent

Effect of viscosity on toughness
ethylcellulose films.

11360

9940

6520

710 0 -
a

5680 A

4 26 0 3

2 840

1420

30 32 34 36 38 40

(load-elongation) of

Ethylcellulose is available commercially iu different viscosities ranpng
from about to 165 cps., measured on the BrookHeld v^come^r - a
toluene-ethanol solvent. The viscosity influences the strength and tong

ness of the film, as shown in Figure XXIII-3.’

Resins are sometimes added to ethylcellulose o improve Ae g>° ^

hesion solvent resistance, and rubbing properties of the film. Et y

:ihyds and the viny. ^

..CfnirUbsy Dow Chemical Company. Midland. Michigan (1947)

xxni. RESINS

1331

improved by incorporating a third ingredient which is a good mutual sol
vent, such as a fatty acid of a drying oil or rosin.'” Ethylcellulose is con -
patible with montan and beeswax, but not with paraffin, unless a third in-
aredient (e.g., hydrogenated vegetable oil) is present. use as a
melt acid Vesins should not be added, since they cause deterioration.

Ethylcellulose is easily plasticized with the common plasticizers, and
normally, from 15 to 30% plasticizer is used. The plasticizers “
monly used are the phthalates, phosphates, stearates, cas or o ,
chlorinated biphenyls. Esters such as dioctyl phthalate and bu y s eara
are best for low temperature plasticizing. Methyl phenol is said to improve

the heat and light stability.

Vinyl Polymers (Film-Forming Resins)

The vinyl polymers are very important to the paper chemist because
of their ability to produce tough, adhesive films which have high gloss and
good heat-seal properties. There are a large number of vinyl polymers,
and the field is growing rapidly. Most of these resins are suitable for lac¬
quer and hot melts and, in addition, certain of the vinyl resins are avail¬
able commercially in emulsion form. The water-soluble polyvinyl alcohol

is discussed in the Chapters XI and XVIII.

Vinyl polymers are obtained through resinification of materials con¬
taining the vinyl group (CH 2 =CH-). Polymerization of vinyl compounds
is entirely different from that which occurs with phenolic, alk)'d, or urea-
formaldehyde resins, since it involves the self-addition of unsaturated hydro¬
carbon derivatives, whereas these other resins involve a condensation re¬
action. Polymerization of most vinyl compounds results in the formation
of straight-chain products which have none of the cross linkages found in
the non-film-forming resins. However, both thermoplastic and thermo¬
setting vinyl resins are available.

Polyvinyl Acetate

Polyvinyl acetate is an important member of the vinyl polymers. It
can be prepared by passing acetylene through acetic acid in the presence of
the proper catalysts. The polymerization of the acetate to high molecular
weight polymers proceeds rapidly in the presence of the correct catalyst
and proper temperature.

Polyvinyl acetate is a thermoplastic colorless, non-toxic, odorless resin
which is sold in either powder or solution form. The commercial resins
vary in molecular weight, most products having a degree of polymerization
of 60 to 230, although products of much higher D.P. are possible. Solution
viscosity, film strength, toughness, and softening temperature all increase
with increasing molecular weight.

low. Koch, Ind. Eng. Chem. 29, No. 6 : 687-690 (July, 1937)

1332

PULP AND PAPER

Polyvinyl acetate has excellent solubility characteristics and is soluble
in esters, alcohols (containing a small amount of water), ketones (e.g.,
methyl ethyl ketone), ethers, aromatic hydrocarbons, nitroparaffins, and
chlorinated hydrocarbons. Ketones and hydrocarbons make the best sol¬
vents. Diluents such as xylene may be used in some formulations.

Polyvinyl acetate is compatible with a wide variety of resins, including
rosin, dammar, phenolic resins, chlorinated rubber, alkyd resins, and nitro¬
cellulose. Polyvinyl acetate can be used with a number of plasticizers.

Polyvinyl acetate is stable to heat and light and is resistant to weak
acids, alkalies, and salt solutions, but is hydrolyzed by strong acids and al¬
kalies. It has a low degree of solvent resistance and is swelled and softened
upon long exposure to water. The film is softened upon exposure to tem¬
peratures above 150° F. and exhibits cold flow at even low temperatures.

Polyvinyl acetate is used in solvent coatings, particularly if pod heat¬
sealing properties are desired. Hot-melt applications are sometimes used.
PolyAunyl acetate is also available in emulsion form containing up to 60 to
65% solids. An excellent bond can be produced with these emulsions by
evaporating the water in the film and curing under heat. The emulsions
are often used as laminating adhesives, sometimes in combination wit
starch, casein, or rubber latex. Modified vinyl acetate resins containing
varying percentages of hydroxyl groups are sold in solution form and are

used principally as adhesives.^^

Polyvinyl Chloride

Vinyl chloride can be prepared by passing hydrochloric acid

acetylene in the presence of a catalyst. The vinyl chloride

way can be polymerized to products having a wide range of molecular

Polyvinyl chloride is not so useful to the paper converter “
acetate because of its lack of solubility in the common solvents. It's. how¬
ler sold^ in some of the chlorinated hydrocarbons. Polyvinyl chloride

films have a high degree of solvent resistance. different

Polyvinyl chloride polymers are available as latices _ .finVed

forms (Lon resins) : (7) unplasticized form, (2) -‘^^’totSer
c 1 i->ln<;ticized during manufacture with nitrile type

r latrUd^ has iHe

pirsrwiLXurrsrofi^ p--. rr"

r«in will fuse at about 250° F., compared with about 300 F. for

plasticized resin. • „ u

1. Sold under the trade name Vinylseal by Bakelite Corp., Carbide an
Chemical Corp., New York, N. Y.

XX III. XESINS

1333

m •

Vinvl chloride (a gas) and vinyl acetate (a low-boiling liquid) can be
cofiolwriied to produce vinyl chloride-aceute copol>iner. Polymerization
can be carried out simultaneously in the presence of acetone and the i>oly-
mer precipitateti in the form of a powder by the addition of water. Resins
fiimied in this manner combine the high chemical resistance, water resist¬
ance. and toughness of polpinyl chloride, and the excellent solubility and
Miftness cluracteristics of pol>Tinyl acetate. The proportions of the two
monomer a can be \aried to secure the desired combination of propertiM.
One of the first copolymers made contained 87^ vinyl clilonde and 13%
rinvl acetate. Today, copedyiners ranging from 3% vinyl acetate and %
vinvl chloride to 38*^ vinyl acetate and 62% vinyl chloride are available,
llie copol>mers are as-atlable in different viscosities. They are witlely used

in boquer oontings (see Qi. XXII).

Polyrinvl chloride-acetaie copolymers arc soluble in kt tones, tve ic

ketones, estm, chkirinatcd hydrocarbons, nitro|xiraftin, M-bnlyl acetate,
dioxane, and certain chlorinated h)'drocarlions. Methyl ethyl ketone, nitro-
methane. and mr%ityl oxide are some of the solvents which have lieen used
commercblly. Tlic re^in is insoluble in alipliatic hydrocarlions. Hie resin
ii f^'flfed by ari'niatic hsdrocarlxKis and some grades are soluble in aro-
ntalic hydrocarbons (e.g.. toluene and xylene), jiarlicularly at high teni-
irralures Toluene is often used as a diluent for part of the ketone stdvent.
Copolymers containing high percentages of vinyl chlori<le (over 90%) re¬
quire strong solvents, such as methyl ethjl ketone, cyclolw xanone, or mesityl
oxide. If these grades are used as lacquers, a high ratio of plasticizer of the
solvent ts'pe must be used. Tricresyl phospliate, biitoxyglycol phtbalate,
and ethyl hexyl phthalate are some of the suitable plasticizers.

Polyvinyl chloride-acelate co|>olyniers arc sometimes used as organo¬
sols In this way, high-iolids coatings can l*c appliesl in the presence of
kfw-cost thinners and dispersants. These resins arc also available in emul-
akm f<wm. These emulsions are used as laminating adhesives, particularly
for metal foils, and for treating pa|>er to improve the water and grease
resistance.

Polp'inyl chloride-acetate copolymers arc ccrtiipaliblc with rosin, cer¬
tain of the phenolic resins, chlorinated di|ihcnyls, alkyds, coumarone-in-
deoes. chlorcqaTaffins. methacry lates, ester gum, accroides, elcnh gum, and
other vinyl resins. They are not compatible with drying oils, non-drying
oik, certain of the phenolics, and certain of tlve natural resins.

Several modifications of polyvinyl chloride-acetate copolymers have
tfiprared in recent years. One of these is actually a tripolyTTier in which
a wnaO quantity of unsaturated dilosic acid is reacted chemically with the
mDOQoic rs durine nolvmertzation. Amjther modification consists of re-

1334

puur AKD PAPE*

moving part of the acetate and chloride grou|» from the ct3pol>'incr and re^
placing with hydroxyls.*’ Better compatibility with alkyl mint, oleo
r«*si’nnn«; varnishes, and waxes is claimed.**

Potyinnylidctif Chloride and Copolymers

l*olyvinyUdene chloride copolymers arc vinyl type resins, although
iheir proi,)crlies are quite different from other sHny! resins. Copolymer.>
of piilvvinylidene chloridc-acrylonitrile arc sold commercially under the

trade name of Saran.’*

Polyvinylidene chloride copolymers arc soluble in mesityl oxide, cyclo¬
hexanone. and methyl ethyl ketone, the latter being the most widely used

Diluents can Ikj used (sec Ch. X\ll).

Polyvinylidene copolymer is compatible with the phcnolks, polysty¬
renes, certain of the acrylates, ester gums, chlorinated paraffins, and otlter
vinyl resins. A small percentage ( 1 - 15 ^ ) of solvent tyiJC plasticizer nwy
he used. Plasticizers lower the waler-vapor resistance, but may lie desirable
when low-tcnnierature flexibility is desired. There are cmly a
few plasticizers which produce clear films with a high degree of flexibility.

Phthalates improve the adhesion to metal foils.

Pol^rvinylidene copolymer resins are a\-ailable in latex form. The com¬
mercial iatex consists of a coiloiaal dispersion of alxmt 60-^ solids ^tam-
ing dispersed particles ranging from 0.008 to 0.15 micrOT in size.
co!it>- is aboui 22 cps. at 579fc solids. The pH of the latex » close to , V
neutral point, but the stability is relaUvely unaffected by chaises /v
from 2 to 12.- The latex is coagulated h>^ eleetrobt^
centrations, although ordinar>- hard water has relatively little efto. '
ditional plasticizer is sometimes required with tte

most effective plasticizers are the organic esters ot phthalic. glycolic, selaoc,
and phosphoric acids.

Polvi'inxl Aldehydes

A lar« number of polvvHnvl aldehydes are of interest to the paper
coated tLsc are prepared by hydrolyzing

alcohol and tlien rracting the dcohol w^ a uk

product usually products are

high water absorption of this rc>in. .vmt

Corp. Carbide ai^ Caibon Chemical Corp.

York, N. Y. _ — . . j-.- York Paint and Varmsh Production (May-

IS C W. Patton, Official Digest. New ^oric Pamt auu.

1948)

«Dow Chemiml Trade /. 12J. So. «: >*-7? 8.

IS G. W. Stanton and w- A. Hensno, ro,

1946)

XXlll.

1335

|ial%%invl ftjniiaL p^hviml iKitvraK

lnih^xirs wM hartkr, kss soluWe rfsins ihan the higher aWch>*dc. .

Pc4v%imi bmxTal Im ^%»n the mast promise of tlie resms in thi>
^ It is u4ubic in esters .ml aWoh.^ ami is comiatible with a number
of^ural ami s)-nthetic resins. It is suitable for use in Muers or xn^oi
n»h», and is abo avaiWde in emulsion form. Films formed from po > vm>
l«iml are tough, flexible, and resistant to gases, but are imt Nrry resistan

to uater, acids, and alkalies.

PoIyslyrtHf

Styrene is a clear cokirless liquid which conuins a beiuene nucleus and
a rinvl grouping PoIvstsTene. which is one of the oldest synthetic resms,
U fonwd by the ,«lvn;er;«tion of styrene. The resin is brittle and low m
tfrength. bit has a high degree of chemical resistance, good electrical prop-
enies. and good water resistam-e. I Hchlorostyrene lias lieller heat an
rhemical resioance than iwlystyrene. Various copol>aners of styrene with

other resins show' giaid all‘round properties.

l\iJysl%Tme resins are available in a wide range of melting points, vis-

oiMiies. hardness, and solulwlity. Tliese range from low-melting, friable
pniducii to high-melting, tough products, with imilccular weights ranging
from 6.000 to alwut 85.000. They are used in lacriuer and hot melts and arc
also ayiilalJe in emulsion form. Pc4y5tyrene resins are wiluble in esters,

ketones, aromatic h>drocarbons (

phatic hydrocariions. ami chlorinated hydnicarlions. In the low-viscosity
grades, the toluene solution can lie diluted with large volumes of cheap
hytirocailjons such as troluoil. Solvent coatings linish well and

relmsc the solvent readily.

Pols'styrene r^ins are conqialible with the phenols, melamine resins,
cocmarone-indenes. and toluene-sulfonamides, but arc inconqiatiblc with
the alkyds, rinyU, acrybtes. cellulose derivatives, and polybntene resins.
Tl>ey are compatible w'ith the common plasticirers such as the phosphates,
(duhalates. telacatrs. and aliietates In general, however, jilastidreri are
iMjl yf f y useful, siocc low Concentrations result in films w’hich are brittle,
whereas high concentrations result in Aims which are very soft and tacky.
The commercial products are sometimes modific<1 with plasticizers added
by the supplier. Internal |4a«ticization of stirene results in |)ennanent
in flexibility and there is one product (Dow 512-K) which is very
ttsefol in fwper coatii^. Straight styrene latices arc available (Lustrex).

Acryiie and MetkacryiU Polymers

-Vi'

Acrylate and methacrylate resins arc made by the polymerization of
esters of acrylic and ntrthacrylic acids, respectively. Acrylic acid can lie

1336

PULP AND PAPER

produced by a series of reactions whereby ethylene and hypochlorous acid
are reacted to produce ethylene chlorohydrin, which reacts with sodium cy¬
anide to form ethylene cyanohydrin. Upon the elimination of water, acrylic
nitrile is formed, which in turn can be readily hydrolyzed to acr 3 dic acid.
This acid may be reacted with alcohols directly to form the ester, or eth¬
ylene cyanohydrin may be used. Esters of methacrylic acid may be ob¬
tained by reaction of the ketone cyanohydrin with the alcohol, e.g., by reac¬
tion of methyl alcohol and acetone cyanohj'drin.

The resins are sold as water solutions of the polyacrylic salts, as sol¬
vent-soluble resins, or as emulsions. According to Staudinger,^® ethyl
acrylate polymers vary from viscous colorless oils at a molecular weight of
2 200 to tough, somewhat fluid substances at a molecular weight of 7,800,
and to tough elastic solids at a molecular weight of 175,000. The properties
of the resin depend upon the properties of the alcohol used m making the
ester; for example, polymethyl acr\date is tough, pliable and elastic, poly¬
ethyl acrylate is softer and more elastic, and polybutyl acrylate is very soft

and almost sticky at the same degree of polynierization.

Commercial acrylate and methacrylate resins are suitable for use in
hot melts, saturants, and lacquers. The films are soft and some%^at tacky,
and hence the resins are useful in pressure-sensitive adhesives. The resins
are soluble in esters, ketones, chlorinated hydrocarbons, furfural, dioxane,
and other solvents. They are compatible with a large number of resins,

including the cellulose derivatives. .

Acrylic resin is available in emulsion fonn suitable for use as adhesives

and for coating and saturating. It produces very clear fil>“
hard and brittle to soft and flexible. Recently non-ionic grades hav® bee
produced which have high mechanical stability, a high tolerance for in¬
organic salts, and good stability at low pH values.

Polyethylene Resins

Polyethylene resins are one of the newer resins, having
commercially in the United States only since 1942. P°'>-^‘hylene ^sins are
obtained from the polymerisation of ethylene at high ^

rures. The resins are re^sins

have molecular weights in the range of 1-, lline and have rather

substances resembling paraffin. The resins crystalline

sharp melting points, usually around 1 - to -...nee excellent water

Polyethylene resins have excellent chemical resistance, excel

resistance h^h water-vapor resistance (but low resistance to most oth
»H, Staudinger and E. Trommsdorf, Ann. 502, 207 (1933) from C. .d- 2 •

4213 (1933) j T 19S TMn 15* 125-127 (Apr. 14, 1949)

ir j. K. Honish, Paper Trade J. 128. No. 15. l4J5-i-/ P

XXIII. RESINS

1337

gases), and very good electrical properties. The resms are odorless, tas e-
less, non-toxic, and have a low specific gravity. They have good heat seal-
ability and are very flexible even at low temperatures. Their low le ectnc
loss makes them valuable as electrical insulators m high-frequency equip¬
ment. Polyethylene coatings are used on paper bags for the packaging o
food products and hygroscopic materials and for wrappers for frozen foo s.

The coating is generally done by the extrusion process.

Polyethylene resins are insoluble in all organic solvents at room tem¬
perature, but they begin to dissolve in a number of solvents at temperatures
of 50 to 60° C. (see Ch. XXII). The resins are used in lacquers or in hot
melts, generally mixed with paraffin. They are available commercially as
water dispersions. These resins are compatible with coumarone-indene
resins, some of the synthetic rubbers, ester gum, chloroparaffins, ethylcellu-
lose, phenolic resins, and paraffin. Plasticizers are rarely needed with the
polyethylene resins because of their inherent flexibility.

Synthetic Non-Film-Forming- Resins

The synthetic resins in the non-film-forming group represent a large
and important group of industrial resins. These are made from industrial
chemicals which are built up into resinous compounds by polymerization or
condensation. Polymerization can produce linear molecules or it can pro¬
duce branched molecules, depending upon the type of monomer and the
conditions under which polymerization is carried out. Even a small amount
of cross linking (often less than 1%) results in a very marked decrease in

the solubility of the resin.

Polymerized resins are characterized as either (1) thermoplastic resins,
which soften on heating, but can be resolidified and resoftened indefinitely,
or (2) thermosetting resins, which are changed on heating into insoluble,
infusible products. Film-forming resins are generally thermoplastic. Non¬
film-forming resins are generally thermosetting, although there are many
members of this group which are thermoplastic. Thermosetting resins are
often sold in the partially polymerized state so that the final polymerization
can take place after the resin has been applied. Three stages are recog¬
nized in the polymerization of thermosetting resins; A-stage, where the
produce is soluble and fusible; B-stage, where the product is less soluble and
difficult to fuse; C-stage, where the produce is insoluble and infusible.

Mark^® lists the important characteristics of high polymers as follows:
(1) average molecular weight, (2) chain length distribution, (3) chemical
nature, and {4) intermolecular forces between molecules. The solubility
characteristics depend upon the size and shape of the molecule and upon the
degree of branching. Linear resins tend to be soluble in all stages, whereas

18 H. Mark, Ind. Eng, Chem. 34, No. 11: 1343-1348 (Nov., 1942)

133S

pn.r AND pArni

the cyclic rci.ins form iiisoluhlc prolucts in ihe higher stages ol
tion.’^' Linear molecules tend to associate with »»nc another, luit thi^ cohe¬
sion is'broken by solvents, which force thetnscive'i Vietween the linear chains.
Cyclic chain molecules, on the other hand, are hound together by primary
valence forces which cannot lie ruptured by solvent^ In highly concen¬
trated solutions, linear molecules may orient theruMrlves tf. form micelles
leading, in extreme cases, to gel formation.

Phenol-formaldrhydc Resins

rheuol-formaldehyde resins are prepared by the interaction and jxjly-
merization of iihenol and formaldehyde. Tliey were among the earliest of
the synthetic resins, having l)een actively investigated by Baekland in 1905
to 1W7. Phenolic resins have good heat and cliemical resistance and gwjd

electrical properties, but tlie color is poor. i

The properties of phenolic resins can Ije regulated by varying (7 j the

tvpe oC phenolic body used (e.g., phenol, cresol, xylenol, or resorcinol),
(/) the tvpe and amount of catalyst, and (3) the ratio of reaettnts. By
changing anv of these factors, it is possible to obtain a slow- to mt-setting
resin a resin with [loor to go<«l dielectric jiruiicrties. a resin with jKwr o
good' alkali resistance, etc. For e.saniple, straight phenol-formaldehj^
produces a resin with the highest mechanical strenph, whereas «e» -
formaldehyde produces a resin with the best electrical
cinol reacts more readily with formaldehyde than phenol
nets which are noted for their good adhesive qualities. Rcsorcinol-torm

tins are particularly well suited for ’-P--
Other aldehydes, aside from formaldehyde, may be used in speaal ca

''■■pheimfe ret frlay belong to the t^r

- “ tHr tt: ^r^nttibJS^-^- --

to possess a linear structu e, j j :« tv,p farlv stages of

eve ic structure." Linear molecules are produced m he ear^ s^
cyclic struc y ^ structure by the

polymerization, but these ® ,:,n,,„es The chains contain phenol

lomiation of comparatively tew cross

molecules attached to one another by CH= ro“ps-

conditions are required in the final cun ^ u , ^

resins. The pH under add coalitions

leads to the formation of resins ^formaldehyde than

.\lkaline-catalyzed resins contain a hig e pe

,,VV R RevnoUs. «.->».

i943)

XXIII. RESINS

1339

acid-catalyzed resins. They contain methylol groups (CH.OH) in the

Phenol-fonnaldehvde re.sins are made in both the water-soluble (ap¬
proximate molecular weight 150 to 200) and alcohol-soluble grades (ap¬
proximate molecular weight 200 to 500). Water-soluble but acid-precipit
able grades are also produced. Both the water- and alcohol-soluble gra es
are used in making paper laminates. Around 1910 to 1920, a new series
of phenolic resins was developed which were soluble in oleoresinous var¬
nishes. These resins provided the first competition to the natural resins,
and since then, have replaced the natural resins for protective coatings and
plastics. These resins are made by reacting alkylated or arylated phenols
with rosin or similar materials. These resins are soluble m esters and
aromatic solvents, and some are soluble in alcohols. Both reactive and non¬
reactive t>'pes are sold, the latter undergoing further condensation when
cooked with dr)’ing oils in the varnish kettle.

Urea-Formaldchyde Resins

Urea-formaldehyde resins are produced by the condensation of urea
and formaldehyde under controlled conditions of temperature, />H, and
concentration. The reaction can be controlled to produce (1) dry crystal¬
line water-soluble products, (2) at}ueous syrups, or (5) products soluble in
organic solvents. These resins are relatively new, having been produced
on a large scale only since 1930. The resins are inferior to the phenolic
resins in chemical and electrical resistance, but have better color.

Monomethylol and dimeth)dol urea are the first products formed in the
reaction, being formed liy the condensation of one and two formaldehyde
molecules, respectively, with each urea molecule. Dimethylolurea, which
contains two hydro.xyl groups, can react further by (2 ) condensing to pro¬
duce methylene urea, (2) by reacting with another urea molecule, or (3)
by reacting with an amine group of a molecule of monomethylol urea. The
reactions become very complex and result in the formation of long-chain
linear molecules or in the building up of branched molecules by cross link¬
age, depending ufxin the conditions under which the reaction is carried out.
Ratios of formaldehyde to urea of greater than 1 to 1 are necessary to per¬
mit the formation of three-dimensional structures.*® Too high a ratio of
formaldehyde to urea results in a molecule which is difficult to polymerize.

During poljmerization. the molecular weight increases, and the vis¬
cosity of the medium increases. In the early stages, the resin is soluble
in water, but if the reaction is carried far enough, the resin becomes in-

»» H. Kline, Paper Trade /. 119, No. 13: 12^130 (Sept. 28, 1944)

**T. S. Hodgins and A. G. Hovey. Itul. Eng. Chem. 31, No. 6: 673-678 (June,
1939)

1'340

Pin.P AKD PAPE*

soluble in water. The resin jcisse'' through se\'eral which Aui»’n**

classifies as follows:

/f-j/uyc nicthylol dcri\-alives, analogous to oDiK^neri.

B-siagf linear molecules in intermediate stafre of polycoodcnsaticin. are

soluble or dispersible in water.

C'Stagc insoluble, infusible, hard stage with molecules in two- or three-dimen¬
sional network.

In the B-stage, the resins have definite colloidal pro|)crtics and IWiave
as protective colloids. They precipitate from solution ufion dilution with
large quantities of water. In this stage, the resins arc very reactive an<l
cannot he stored more than a few months. Both the A-stage and B-stagf
low molecular weight polymers will react with cellulose.

A-stage resins are sold as aqueous solutions or as spray-dried |X)wdcrs
which are readily dispersible in water. These can be further j^lymerized
during use hy reacting under acid conditions, high temperature (250 F.),
or a combination of both. Alum and ammonium salts of strong acids fe.g.,
ammonium chloride) are catalysts commonly used to increase the rate o

polymerization. j r i

■ Within recent years, a new type of water-soluble modified urea-fo^h

dehvde resin has been developed in which the conventional resin is modified

bv introducing electrolytic groups at intervals along the J*"* '"'

creases the size of the molecule, but maintains the water srjubihty. O

type is modified with sodium-sulfonate to produce a resm which is anionic

in character and is precipilable with cations." Another type is
nature. These resins are used in making wet-strength papers On XII).
Urea resins condensed in aqueous media are not compatible with non-

aqueons solvents. However, if the reaction is carried ^

monohydric or polyhydric alcohols, the alcohol t^es

forming products of unusual solubility characteristics. y, ^

cation takes place, resulting in the introduction of

amoles are the bntvlated and isobutylated ureas which are pf'Pa'”

butLol and isobutanol, respectively. The modified resins

nearly all common organic solvents, including '' ^

and mixtures of alcohol and petroleum solvents.

are compatible with ^J™' ' ■ ; „«jified in the pres-

r:\n he used in varnishes. It debirea, me

ence of the "t" ‘ TU™ iiTC Wghly fiexible

sate containing up to alia a rebiu.

W. .\uten, Pafer We J- If . ^9^’

R. W. Auten and J- H R^SJett^id C J.

25 T.

Eng.

Me^ke,

XXIII. RESINS

1341

Mclamine-ForntaidcJiydc Resins

Melamine-formaldehyde resins are similar to urea resins in general
properties and beliavior. They are newer than the urea resins, having been
produced in the United States in quantity only since 1939. Melamine resins
have greater heat and light stability, and greater moisture and chemical
resistance than urea resins^ but are considerably more expensive.

Melamine differs from urea in being more resistant to hydrolysis and
more resistant to heat, and these characteristics are carried over into the
respective resins. Melamine contains six amino groups (in comparison
with two for urea), which means that it can react with as many as six for¬
maldehyde molecules to form hexamethylol melamine. Resins can be
formed from this monomer by esterification, by condensation (through
elimination of water and formation of ether linkages), or through the for¬
mation of methylene linkages.

Water-soluble melamine resins are used in the preparation of wet-
strength papers. In this process, the resin is condensed into a positively
charged (cationic) colloid which can be added directly to the pulp to
produce a high degree of wet strength.®® (See Ch, XIL)

Solvent-soluble grades of melamine resin are produced by the intro¬
duction of alkoxyl groups in the same manner as previously discussed for
urea resins. The modified melamine resins are superior to the equivalent
urea resins in curing speed and chemical resistance. At normal curing
temperatures of 250 to 300® F., the melamine resins require approximately
one-third less curing time than the ureas.®^

Acetone-Formaldehyde Resins

Within recent times, new water-soluble resins of the ketone-aldehyde
(acetone-fonnaldehyde) type have been developed. These resins cure at
/>H values alxtve 7, preferably in the range of 9.5 to 10.0, and consequently
offer promise for applications where it is necessary to cure the resin under
alkaline conditions. A small amount of resorcinol is said to promote the
cure of these resins.

Alk\d Resins

Alkyd resins are formed by the interaction (polyester formation) of
polybasic acids and polyhydric alcohols. The most common type is made
from phthalic anhydride and glycerol. There are, however, a number of
special t>’pes of alk)M resins. Both thermopalstic and thermosetting prod¬
ucts can be made. For example, the reaction of glycerine and phthalic an¬
hydride produces a thermosetting resin, whereas the reaction of a glycol

*• H. P. Wohnsiedler and W, M. Thomas. U. S. 2,345,543 (Mar, 28, 1944)

” R. \V. Autm. Paper Trade J. 127, No. 5 : 332-338 fjuly 29, 1948)

1342

PULP AND PAPER

<111(1 phthtilic anhydride produces a thermoplastic resin. Alkyd resins maji
he divided into the following types; thermoplastic, thermosetting, unmodi¬
fied, oil-hiodified, non-oxidizing, semi-oxidizing, and oxidizing. The modi¬
fied grades are more important than the unmodified grades.

Phthalic acid reacts readily with glycerine at 200® C. Esterification
takes place rapidly with the formation of acid phthalate esters. First, clear
straw-colored solutions are produced, but if the heating is continued at high
temperatures, an infusible, insoluble resin is formed. Commercial alky(l
resins vary from liquids to hard, brittle solids, depending upon the type of
acid and alcohol used and the conditions under which the reaction is ar-
ried out. They are available in both water- and solvent-soluble gra es.
One type is soiuble in water, but precipitable by acids. As a class, they
have good insulating and adhesive qualities and good grease resistance, but
are low in water resistance. The important properties of alkyd resins are

color, softening point, and acid number.

Modified alkyds are made by the substitution o' '“ndias c

acids or fattv-acid oils for part of the phthalic anhydn e. is p

resins of reduced thermosetting properties, improved solubility in orgamc

solvents, and increased flexibility. By varying the amount of oi or o 1 fat

acid it is possible to obtain products ranging from hard brittle so i

soft olastic products. Oil-modified alkyds are classified as long or short,

SenS^pon the relative ratio of oil or oil acid to phthalic anhydride,

maldehyde resins. These resins have „ooq ^

used with urea-foi malde ) toughening effect. Drying-oil

finishes, principally for their plasticizm, tvne coatings where they

alkyds are used in either air-dry.ng JlP^jr’kl-drving and

produce harder finishes hey „

baking alkyds may be used a nfm-drving oil-modified alkyd resins

quer formulations. Both drying an ■ j j available

Ire used in nitrocellulose lacquers, (,,etone and

which are soluble in esters (etiy chlorinated hydrocarbons,

ethyl ethyl ketone) turpentine. Some

SlTaTetlul" aliphatic hydrocarbons. The long-oil types are very

hu™:r: ay:dt^XeX-->' am. reduced color com-

XXlll. RESINS

1343

-.1 „,l„.r alkvds. Alkyds modified with pentaerylhritol have good
Tmer and chemical resistance and good curing proiierties.

Polyester Resins

.He

brittle resins, such as nitrocell , . , ^.-ides. The commer-

Tliey are made in both "..bbery semi-solids. They are

cial products range from viscou^i 1 solutions in an

siioolied as liou ds containing 1(X)7<. solids or concern
supplied 1 _ water-white to pale yellow.

c.ster solvent. 1 he color ra k nolyiiierircd state so that the

Polvester resins are sold iii tlie partially poiyni ..ndergo

mostiv (or molded products where their short curing cycle has bee

definite advantage. They are also used in Ihe produc a” d

mites where they add chemical, heat, and oil resistance to the Pr°d«ct and

provide good electrical proiierties, good strength properties, a g
nwiisionfl stability. The temperature of curing generally runs fi om 2

to 305® F.

Other Resins

One tvi« of ester resin which has enjoyed widespread use for many
years is esier gum. This product, which is made by the reaction of rosin
'and glycerol, resembles shellac in its proiKirties and has liecn widely used

in varnishes. _ j r

An interesting new group is the maleic resms, which are made from

maleic anhydride and related materials such as glycerol and pcntaerythntol.

Tliesc resins arc characterized by high melting point, hardness, and rapi

solvent release. , ., i i

Another important group of resins arc the polyamides made by

copolymerization of long-chain organic acids with cthylenediamine. These

rcsins arc available in a wide range of properties.

One of the interesting new resins is a rosin-maleic anhydride con¬
densation product sold under the trade name of Mersize.*® This resin
has been widely used as a partial replacement for rosin in the internal sizing
of paper. Maleic rosin type phenol-formaldehyde resins have been used
for paper coating. .Another resin is made by the condensation of terpene

*• Monsanto Oicmical Company, St. I.ouis, Missouri

1344

PULP AND PAPER

polybasic acid (Petrex acid) and maleic anhydride. These resins are sold
under the trade name of Petrex resins.

Couinarone-Indene Resins

Coumarone-indene resins are mixtures of polymers obtained from the
polymerization of the distillates from coke ovens, consisting mostly of a
mixture of coumarone and indene. The distillates are polymerized either
catalytically (acid conditions) or thermally to yield resins of the desired
properties. The resins obtained are thermoplastic and range from viscous
liquids to brittle solids. They range in color from a pale yellow to a dark

brown. , • u

Coumarone-indene resins are characterized by low acidity and a high

decree of chemical resistance. They are non-saponifiable and have an acid

number less than 1, which makes them useful m coatings requiring a higi

degree of alkali resistance. The resins have a dark color and bad ^or, and

these two factors have limited their use in the paper industry. They are

used however, in varnishes to extend phenolic resins, and in hot melts.

Coumarone-indene resins are compatible with a great number of resins

and waxes, including the phenolics, some of the alkyds P*"®" ^ ^

asphalt, some of the synthetic rubbers, rubber

(partly), shellac, chlorinated diphenyls, polystyrenes, rosin, and ester g .

They are not compatible with nitrocellulose and cellulose acetate.

GoZone-i^dene resins are soluble in most organic so vents except

the alcohols Some of the common solvents are turpentine, esters, ethers

ketones aliphatic hydrocarbons, aromatic hydrocarbons, and chlonna
ketones, aliphatic ) , ^,i,ons are exceedingly strong solvents

hydrocarbons. Aromatic nyuruc dissolved

and benzene gives the lowest viscosi y so u i . ^ heated to

cold by stirring for several hours or e^e ^

a temperature not in excess of 218^^CJo obta^^

Solubility in pafhffi™ ‘ solutions may be improved by

point of J'; with aromatic solvent. The resins are so d

replacing part ot the napni Flash solvent and can be

coLierclally at « and 70% spirits. Cou-

thinned to normal working solids with ^ (i„duding

marone-indene resins are soluble in raw f „f\he modi-

dehydrated castor oil), but are not soluble in castor oil.

fied grades are soluble in .j y ^^dsion form and can be

Coumarone-indene resins we available emulsions,

used in combination with Paraffin wax ...h natural

and synthetic rubber emul.ons. eoagulation of the

:“ubter."ca?be done by adding a solution of 10% sodium hydroxide.

XXIII. RESINS

1345

Phenol-modified coumarone-indene resins can be prepared by
• ■ lint Ae presence of phenols or cresols. The phenol-mod.fied

S we soluble in alcohols and show greater compatibdity with a number
rirtti the unmodified types. These modified resms are compatible

with nitrocellulose Ucqiiers. The alkylated phenol-m^.hed '

hU m oracticallv ever>- solvent except w-ater and methanol. They are tl

:t,‘:ipatib.e of the-coumaroiie-indene resins, c-

tS-lcellull, trocelhilose. and cellulose acetate lacquers. Hydrogen¬
ated Imarone-indene resins are used when improved color ami odor

desired.

Terpcne Resins

The teniene resins are producial by the polymerization of pinenes, both
alpha- and heta-pinene living used. They are thermoplastic and are char-

acterized by a high degree of cbemical and grease resistance.

The resins are soluble in high-boiling alcohols, esters, ketones, aliphatic
and aromatic hvdrocarlions. They are compatible with paraffin, otter
waxes, rosin, asphalt, and rubber derivatives. Some of the terpene resins
act as heat and light stabilizers for nitrocellulose, ethylcellulose, vinyl resins,

and niblier derivatives.

Silicone Resins

The silicones contain atoms of silicon surrounded by atoms of oxygen
and carlwn and. as such, are both organic and inorganic in nature. Ihe
resin has a high degree of heat and chemical resistance and excellent elec¬
trical resistance. It is non-inflammable (flash point, 600® F.).

Silicone resins are soluble in toluene, ketones, and many other organic
solvents. They are compatible with the cellulose derivatives, phenolics,
urea resins, and some of the natural resins. They are not widely used as
paper coating resins l)ecause of their high cost and because of the relatively
long curing schedules and High temperature required. They have been
suggested for sizing of paper, and recently, papers treated with silicone resin

have been sold for use as lens cleaning tissue.

Chlorinated Polyphenyls

The chlorinated polyphenyls, sold under the trade name of Aroclors,^®
lielong to the class of semi-synthetic thermoplastic resins. The commercial
products range from water-white, mobile liquids and pale yellow viscous oils
to stick 7 or hard dark brown resins, depending upon the degree of chlori¬
nation.

The resins are thermoplastic and non-drying, and the softer types

** Monsanto Chemical Company, St. Louis, Missouri

1346

PULP AND PAPER

have a definite plasticizing effect on other resins (e.g., nitrocellulose and
ethylcelhilose). They have good electrical characteristics, high dielectric
strength, and low power factor. Paper for electric condensers is sometimes
made by submerging the paper in a bath of chlorinated biphenyl resin. The

resins are also used in coating and for laminating.

These resins are soluble in most common organic solvents. They are
very soluble in aliphatic hydrocarbons, aromatic hydrocarbons, and in es¬
ters. The hard crystalline resins are less soluble than the oils and soft
resins. They are compatible with ethylcelhilose, nitrocellulose, rubber, rub¬
ber derivatives, paraffin, coumarone-indene resins, rosin, dammar, poly¬
styrene, styrene-butadiene copolymers, and vinyl polymers. They are in¬
compatible with cellulose acetate.

Aryl Sitljonamide-Formaldehyde Resins

The aryl sulfonamide-formaldehyde resins are made by the condensa¬
tion of formaldehyde with aromatic sulfonamides. These resins are sold
under the trade name of Santolites.^» They range from soft, yellow, sticky
products to hard, water-white, brittle solids. They are highly soluble m
esters, ethers, alcohols, ketones, and aromatic hydrocarbons, but are m-
solub/e in aliphatic hydrocarbons and vegetable oils. The solvent release
from high molecular weight alcohols is low. They are available commer¬
cially as solid resins, or as an SOfo solution in normal butyl acetate They
are compatible with the vinyl resins, cellulose derivatives, chlonna e ru¬
ber A-stage phenolic resins, polyvinyl chloride, polyvinyl acetate, po y-
vliwl copolymers, and some of the natural resins. They are not compati e

with paraffin. They can be used in lacqners at high f

duce films having good moisture-resistance and good heat-seal prope •

Their excellent compatibility with plasticizers and other resins reduces e

well as other resins, but are not compat.ble w.th drying .

Rubber and Rubber Derivatives

Rubber is used in the paper Svid in

is tlie natural milky substance secre ed by ‘he n-bber ,
organic solvents. Rubber latex is the more important to P P

30 Monsanto Chemical Company, St. Louis, Missouri

XXIII- RESINS

1347

is,. Within recmt years, synthetic rubbers and rubber derivatives have re-
placed the natural product for many uses.

Xatural Rubber Latex

Rubber la,ex is a milky liquid which contains extremely small Far-

particles cl rub^r su,..d^^^^

707„ Most of the latex comes from the Far East and ts sh.ppea

r nirr:;:

■.Xnr-. T,::’particles consist ^ .

,..„te distens^e rubier in the c.„er o the pa^

:7;:X arrbS^^^ ^ U- ^ins semm is subject to b-

‘tcrial decomiiosition and must be protected from spoilap. Amn.oma added

at the time of tappiiiR provides sufficient preserving ,

The rubber particles in latex are relatively easily ^

110 If tbe latL is diluted, soft water should be used, because hard wate
tends to clsl creaming, i.e„ rising of rubber particles to the surface of the
htex A crude test to determine the stability of rubber latex is based upon
L length of time a small sample can lie rublied between the fingers beffire
a coagidated film is produced.” Casein or animal glue are sometimes added

''oteTuton^^^^^ are frequently incori»rated into crude rubber latex
just liefore use in order to obtain the desired effects. Sulfur and accelerm
ors are added to vulcanize the rublier, and anti-oxidants are frequently
added to improve the aging properties. These materials must first be emu -
.sifieil Wore being mixed with the latex. Rubber latex is used as a sat-
iirant in the manufacture of artificial leathers to increase the tear, to t,
flexibility, and resistance to wear of the paper. It is also used as a lamina -

ing adhesive.

Natural Rubber and Rubber Derivatives

Xatural rubber is dispersible in a number of different solvents. The
solvent causes swelling of the rubber micelles, forming solvated units which
immobilize a high percentage of solvent. The viscosity is proportional to

M T Yittenel ^ - 15-16, 20 (Jan. 1, 1958)

“•A. R. Kemp, Ind, Eng, Chem, JO, No. 2: 154-158 (Feb., 1938)

^Am, Dyestuff Reptr, 25, No. 5: 109-111 (1936)

1348

PULP AND PAPER

the swelling power of the solvent,and in general, the viscosity is quite
high and the solution is unstable. The low solubility of natural rubber in
organic solvents and the instability of rubber to high temperatures has
prevented its widespread use in the paper industry.

Chlorinated rubber is made by passing chlorine into a solution of
natural crepe rubber until the rubber molecules are substantially saturated
at the double bonds and some hydrogens are replaced. Commercial chlori¬
nated rubber, which contains approximately 63 to 65% chlorine, is sold in
the form of a white, amorphous powder. It is available in several different
viscosities. Chlorinated rubber can be dissolved at high concentration in

solvents, from which it produces transparent, glossy, and non-tacky films
which are highly resistant to water, acids, and alkalies. Some of the sol¬
vents for chlorinated rubber are aromatic hydrocarbons, chlorinated hydro¬
carbons, esters, and aliphatic hydrocarbons. The film is brittle by itself,
but plasticizers can be added for flexibility. Chlorinated rubber is com
patible with alkyd resins, modified alkyds, coumarone-indene resins, dam¬
mar, and paraffin.

Another rubber derivative, known as cyclized rubber, is prepare • y
reacting natural rubber with strong acids or acid-forming salts, such as
stannic chloride or chlorostannic acid. The final products range from
rubbery materials to hard, brittle solids. Cyclized rubber has gwd heat-
seal nroiterties and excellent resistance to water vapor. It is so u t e in
wide variety of solvents, including the aliphatic hydrocarbons, aromahc
hydrocarbons, and the terpenes (see Ch. XXII). It is compatible wUh
dammar, rosin, ester gum, and modified phenolics. It can be used m lac-

nuers and hot-melt compositions. vu u /^rr,r'1^1nri^

Rubber hydrochloride is made by treating rubber with hydrochloric

acid. Films of rubber hydrochloride are flexible, toug , transparen , a

resistant to water and greases. However, the product is not widely used

for paper coating because of its poor solubility and high

j., „i. —1. - i-i-

soluble in ethers, esters, and alcohols. It is unsuit

of its instability at high temperatures. It has been i ^ i-^^^ates.

coatings where good heat sealing is necessary, anc m p ^ ^

prodllrs thlioplastic derivatives of excellent a hesive properties. The
products are soluble in benzene, toluene, and gasoline.

Synthetic Rubbers

Natural rubber has Sitity after ex-

susccptihility to oxidation, which causes

3 .1. Williams, M. Eng. Chm. 29, No. 2: 173-174 (Feb.,

1349

xxnl. RESINS

posure .0 f' 5 ^^"“;." ™Sers afe’ nTaT^sceplleTo

During World War II. a™*e..c rubbers b came

portance, and today are being use^ m so v unstable unless

emulsion polymerization. Synthetic rti ^ example Nazarro” has

cr “H£rIS

‘"Cong C u“ s’of s'mthetic rubber suitable for use in the paper in-
„ , nre Hyebioro..e (—1. » —^ '

niers fBuna X-GRA, aiemigum 200, Hvcar UK , i ) J

Rutidiene-stvrene copolvmers have proved to he one of the mos
facto" hi,tes for natimal rubier. They are soluble m ketones am

sfer and are also available in emulsion form. A new pol^ner of the
esters, ana , . , • -ic a «;iihstitute for cychzed rubber

liutadiene-styrene type which is suitable as a si ^ toluene or as

is S-7 resin. This is offered as a 30% unmodified sohition

a compounded material ready prepared for paper coating. in benzene

also available in latex form.’* The S7 resin is readily 7““'

toluene xylene, naphtha, and ketones, When aromatic solvents are used,

the sohition mav be diluted as much as 50 to 75% by weight oheap
,«trolcum thinners. The product is compatible with waxes, modified p
nolic resins, couinarone-indene resins, modified styrene resins, and este
gum. Butadiene-styrene copolymers can be made m different ratios of
butadiene and sivrctte ranging from pure ixilybutadiene (a soft, wea ru i
l*r) to pure poiystyrene (a hard, brittle resin). Increasing the styrene
content increases the hardness, tensile strength, and stiffness, but <>ccrcases
the elongation. GR-S rubber latex may contain styrene content up to SO^o.

Isobutylene, obtained from cracked petroleum, can be polymerized to
form a series of materials ranging from oily substances to rubber-hke com¬
pounds. The outstanding characteristics of these polymers are toughness,
chemical inertness, and low-temperature flexibility. The polymers are
soluble in aliphatic and aromatic hydrocarbons, and also in some chlori¬
nated hydrocarbons. They are compatible with paraffin, linseed oil, as¬
phalt, hvdrogenated coumarone-indene resins, terpene resins, and melamine-
formaldehyde resins. They are suitable for use in pressure-sensitive ad-

“ R. T. Nazarro, Pofer Trade 1.121, No. 19 : 90, 94 (I^v. 21, 1945)

Pliolite Latex 170 sold by Goodyear Tire and Rubber Co., Inc., Akron. Olno

1350

PULP AND PAPER

hesives, hot-melt coatings, and hot-melt adhesives. Styrene-isobutylene
copolymers are also available commercially. These resins combine plastic
and rubber-like properties. They are soluble in aliphatic, aromatic, and
chlorinated hydrocarbons, and are compatible with wax. Butyl rubber is
made by copolynierizing isobutylene with a diolefin (e.g., isoprene). This
permits the cross linking of chains during vulcanization. Butyl rubber is
soluble in carbon tetrachloride and slightly soluble in aliphatic and aromatic
hydrocarbons. It is compatible with paraffin, linseed oil, alkyds, nitrocel¬
lulose, and some rosin derivatives. It is suitable for hot-melt coatings and
laminating adhesives.

Butadiene-acrylonitrile copolymers are soluble in ketones, xylol, cyclo-
he.xane, nitropropane, ethylene dichloride, chlorobenzene, butyl acetate, and
1 -chloro-l-nitroethane. They are compatible with rosin, rosin derivatives,
phenolics, nitrocellulose, rubber derivatives, and the polyvinyls. These
polymers are available in solid and latex form. The latex contains dis¬
persed particles having dimensions of 600 to 3000

The isobutene polymers constitute another class of synthetic rubber.
These polymers are soluble in alcohols, esters, and ketones, and are com¬
patible with paraffin. They can be used in hot melts and in adhesive for¬

mulations.

The polychloroprene polymers (neoprene) are made by converting
acetylene to vinyl acetylene and then to chloroprene by reacting with hydro¬
chloric acid. The products are soluble in benzene, trichloroethylene, methyl
ethyl ketone, furane, toluene, ethylene dichloride, carbon tetrachloride,
and coal tar naphtha. These polymers are not widely used in solvent form
in the paper industry, but have found some use m latex form, where su¬
perior aging properties are desired. Neoprene latices are compatible with
natural rubber latex and with some synthetic resin emulsions, especia y
rosin derivatives. They are incompatible with vinyl resins. T e
particles have a .smaller particle size than those m natural rubber la ..
Neoprene latex is available containing particles with a positive
The olefin polysiilfide polymers are prepared by t le reacaon o

dichloride with sodium polysiilfide. These polyniers are ^

outstanding resi.stance to aromatic and aliphatic

alcohols, esters, and ketones. They are, however, solulile in chlorinate
hvdrocarlions. The polymer has an undesirable odor.

37 D M Yost and W. H. Aiken. "The Effect of Certain Latex Yariahle^m^^^^^^

Adchtion of Nitrile Rubber T.atices.

Conference, Syracuse, N. Y. (Oct. 19-20, 1950).

Corollary Reading

A„.er... <or Tes.ta. Ma.eriaU, c. r.,cr,o.r,, A.S.T.M., PbUa-

Bra^'p^^E” T*. Ch^is,ry cf Ug«in, Academic Press, New

R. H., and W. Henderson, .Vedrrn raHr-<»ah»9. 2''<i «•■■ B'^^l-wen,

Oxford, 1941. _

1 ^: 2 , t ed., Orapman . HaU, London,

(iraff, J. H.. p«lf O’"! P^t" ‘'//rrojroPj, Instilnle o( Paper Cliennstiy, Appleton,
GranU' H W

l»ie K Kuiulsridr mtd Zelliyolk, Springer. Berlin. 1940. . ^

Ciund t ;/nl=. kciinv, 2nd ed.. Akad. Verlagsgesellscliaft, Leipaig. 92 . a.cni-
U,ry »/ H eed (trans. by P. Oesper). Academic Press. New \orW, 1951.

Heuser,' E.. The Chemistry Cellulose. Wiley, New York, 1944.

Hunter U Papcrmikinq, Knopf, New \ork, 1943. - 0 .

il mid A. V. Tobolsky, Physieol Chemis,ryj

Polymers, Volume II. 2nd cd.), Interscience New , , jy

Meyer, K. H.. Natural and Synthetic High Polymers (High Polymers, \ olume ,

2nd ed.). Interscience, New York, 1950. r - riarpn-

Norman. .\. G.. The Biochemistry of Cellulose, the Polyuromdes, Lxgmns, etc., Claren

don Press. Oxford, 1937. t * -

Ott. E ed.. Cellulose and Cellulose Derivatives (High Polymers, Volume V), Intcr-

sciimcc New York. 1943. Second edition in preparation.

Sutcrincister, E., The Chemistry of Pulp and Paper Making. 3rd cd., \\ ilcy, cw

York, 1941, ,

Stephenson. J. N.. cd., Pulp and Paper Manufacture. ^JcGraw-Hill, New

Volume I, Preparation and Treatment of Wood Pulp, 1950.

Volume II, Preparation of Stock for Paper Making, 1951.

Volume III, Manufacture and Testing of Paper and Board, in preparation.

Volume IV, .Auxiliary Paper Mill Equipment, in preparation.

Voet, A., Ink and Paper in the Printing Process. Interscience, New York, in prep-

aration. , a

West, C. J., Pulp and Paper Manufacture, Bibliography 194&-I9o0, Technical Associa¬
tion of the Pulp and Paper Association. New York, 1951 (Bibliography 1935-
1945. 1947; etc.). This is a listing arranged by subject matter of all domestic and
foreign publications (books, articles, patents, etc.) in the various fields related to

pulp and paper manufacture.

Witham. G. S.. Modem Pulp and Paper Making. 2nd ed.. Rcinhold, New York, 1942.
Wise, L. E.. Wood Chemistry. Reinhold, New York, 1944.

Wolfe, H. J.. Printing and Litho Inks, 4th ed., MacNair-Dorland, New York, 1949.

1351

AUTHOR INDEX

Abernathy, H. H. (see Walsh, R. H.)
Abramowitz, W. (see Kao, J. Y.)

Abrams, A., and Brabender, G. J., 1278

— and Chilson, W. A., 1277

— and Wagner, C. L., 1317
Adams, D. 0., 952, 953

Adams, F. W., and Bellows, J., 826
Aiken. W. H. (see Doty, P. M.; and
Yost, D. M.)

Albert, C. G. (^^^ also Callighan, 0. W^.)
Albert, C. G., 1040, 1045, 1098
Albert, G. A., 849, 1263, 1284
Allan, R. W. (see Mantell, C. L.)

Allison, H. J., Jr. (see Rowland, B. W.)
Andella, D. J., 1137

Andersson, 0., Ivarsson, B., Nisson, A,
H., and Steenberg, B., 820
— and Steenberg, B., 819
Annis, H. M., 1105, 1108, 1109
Applegate, P. D. (see Smith, J. W.)
Arendt, A., and Wathelet, E., 936
Armstrong, W. G., and Madson, W. H.,
1287

Arnold, K. A., 1048, 1049 (ref. 66), 1072,
1079, 1081

Aronovsky, S. I., Nelson, G. N., and
Lathrop, E. C., 850

Asdell, B. K., 1049
Auten, R. W., 1340, 1341
— and Rainey, J. L., 1340
Ayers, L. R. (see McKee, R. C.)

Baird, P. K. {see also Doughty, R. H.;

Heinig, M.; and Seborg, C. O.)

Baird, P. K., and Irubesky, C. E., 811,
847

Ball, R. W., and Lane, O. P., 911
Bancroft, W. D., 1206
Barber, W. R. (see Mispley, R. G.)
Barclay, E. H. (see Bearse, N. I.)
Bardsley, J. H., and Morin, L. J., 1149,
1150, 1154

Barnes, C. E., 1324

Barrett, W. P. (see Hodgins, T. S.)

Bass, S. L. (see Kauppi, T. A.)

Bates, R. M., 1163

Bauer, E. S., 1260, 1261

Bauer, H. F., Bauer, J. V., and Hawley,

D. M., 1235

Bauer, J. V. (see also Bauer, H. F.)

Bauer, J. V., 1224
Bearse, N. L, 1092
— and Barclay, E. H., 1088, 1094
Bekk, J., 1160

Bell, R. W. (see also Whittier, E. 0.)

Bell, R. W., and Gould, S. P., 1018
Bellows, J. (see Adams, F. W.)

Bender, H. L., 1273
Bennett, C. B., 1262

Berberich, O. P., Engelhart, L. F., and
Zucker, M., 1106, 1149
Bernstein, I., 1143

Bicking, C. A., 998, 999, 1000-1002 (ref.
25)

Bicking, G. W. (see Whittier, E. 0.)
Biddle, A., 1245

Bingman, R. T. (see Willets, W. R.)
Black, W. C, 1112

Blaisdel, C. A., and Minor, J. E., 947
Blow, C. M., 1250

Boldeschevieler, E. L. (see McArdle, E.
H.)

Boiler, E. R., Lander, J. G., and More¬
house, R., 1215, 1230, 1231
Bondy, W. S., 1310

Bosworth, A. W. (see Van Slyke, L. L.)

Bowles, R. F., 1162

Boyce, D. H. (see Wehmhoff, B. L.)

Brabender, G. J. (see also Abrams, A.)

Brabender, G. J., 1280

Bracewell, R. J. (see Mosher, R. H.)

Bradner, D., 1101

Brand, J. S. (see Voet, A.)

Breyer, F. G., 1146
Brotherton, M. (see McLean, D. A.)
Broughton, G., Egan, L. W., and Sturken,
R. C., 1283

1353

1354

AUTHOR INDEX

Brown, B. F., 1177
Brown, 0. J., and Smith, W. R., 1140
Brownlee, K. A., 993, 997, 1006
Bruno, M, H., 1182

Buchanan, M. A. (see Van den Akker,

J. A.)

Buchdahl, R., Polglase, M. F., and
Schwalbe, H. C., 1150
Buckman, S. J., and Henington, V., 978
Burbidge, H. G., 996
Burton, J. 0., and Rasch, R. H., 931

Cagle, J. H. (see also Davidson, G.)
Cagle, J. H., 1095
Calkins, C. R., 951, 953, 956
Callighan, O. W., and Albert, C. G., 1118
Campbell, A., 951

Campbell, W. B, (see also DeLuca, H.

A.)

Campbell, W. B, 833, 834
Camps-Campins, F., 1209, 1211
Canaud (see Chene, M.)

Carlson, F. I. (see Snyder, L. W.)

Carlsson, G. E., 1149

Carman, E. F., 1312

Carpenter, L. A. (see Heritage, C. C.)

Carson, F. T., 847, 848, 1277, 1280

— and Worthington, F. V., 832, 833
Carter, J. D., 1230

Carter, R. E. (see Hewett, P. S.)
Cartwright, L. C., 1273
Casey, J. P., UlS

— and Libby, C, E., 1091, 1094
Catlin, J. B., and Strieby, J. G., 997
Chamot, E. M., and Mason, C. W., 942
Chapman, S. M., 851, 853, 854, 855, 1153,

1154

Charch, W. H., and Scroggie, A. G., 1278
Chene, M., Deissenberg, C., Canaud,
Martin-Borret, and Chiaverina, 942
Cheyney, L. V. E., 1273
Chiaverina (see Chene, M.)

Chilson, W. A. (see Abrams, A.)
Church, R. J* (see Carter, R- E.)

Clark. F. M., and Montsinger, V. M., 953
Clark, J. d’A.. 810, 811, 812, 814, 825, 828,
829. 830, 830 (ref. 62),832, 837,860,910

Clary, B. H., 1313
Clewel, D. H., 877

Close, J. W., 1029
Cobb, R. M., 847, 848, 1091
— and Lowe, D. V., 1090, 1156
—, Lowe, D. V., Pohl, E., and Weiss, W.,
813

Comar, C. L., and Miller, E. J., 1277, 1278
Cottrall, L. G., and Gartshore, J. L., 834
Craig, W. L. (sec also Hughes, D. A.)
Craig, W. L., 1023, 1024
Cramer, G., 1153, 1155
Crawford, E. A., and Strain, M., 808
Croup, A. H., 1264

Csellak, W. R. (see Pigman, W. W.)
Cushing, M. L. (see Gussman, L.)

Cyr, H. M. (see Kress, 0.)

Dahl, W. H., 938

Danielsen, R., and Steenberg, B., 802
Davidson, G., 1092, 1095, 1107
— and Cagle, J. H., 1124
Davidson, H. R., 1052
Davies, J. D., 1098
Davis, D. W., 1276

Davis, M. N., 851, 890, 894, 896, 906, 909,
916

Davis, M. N., Roehr, W. W., and Malm
Strom, H. E., 856
Dean, J. C., 1257

Dearth, L. R. (see also Wink, W. A.)
Dearth, L. R., Van den Akker, J. A., and
Wink, W. A., 890
Deissenberg, C. (see Chene, M.)
Delevanti, C, Jr., and Hansen, P. B., 951,
952 (ref. 342), 954, 956
Del Monte, J., 1205, 1207, 1214, 1239
DeLuca, H. A., Campbell, W. B., and
Maass, 0., 951

Dickerman, G. K., 809, 1092, 1094, 1258,

264, 1266

and Riley, R. W., 1071, 1081, 1100
:hm, R. A., 1136

hne, W. P., and Libby, C. E., 944
ty, P. M., Aiken, W. H., and Mark,

l’ 1277, 1278

ughty, R. H. (see also Seborg, C. U.,

nd Simmonds, F. A.)

ughty. R. H., 811, 812, 813, 828, 849

ref. 112), 894, 910 „ ^

Seborg, C. O., and Baird, P. K., 848

ref. 112), 849

AUTHOR INDEX

1355

Drake, R. L., 1145
Durant, L. G. (see

Stocker, F. W.)

Egan, F. W., 1010, 1261

Egan, L. W. (see Broughton, GO

Egerton, L. {see also McLean, D. A.)

Egerton, L., 955
Ehrisman, H. O., 936, 1177
Elliot, F. D. (see Erickson, D. K.)
Elton, G. A. H., and Macdougall, G.,

Emanueli, L., 849 • t? ^

Emley, A. L. (see Kimberly, A. E.)
Endicott, H. S. (see Race H H.)
Engelhart, L. F. (see Berbench, 0. P.)
Engstrom, G. C. (see Julian, P. L.)
Erickson, D. R., 1031
_ and Elliot, F. D., 1147
Eriksen, L. H. (see WHlets, W RO
Erspamer, A. E., and Rice, W. .»

Fay, H., 935

Ferguson, E. E., 1310 t ^

Fleckenstein, J. V. (see Malm, C. J.)
Foelsch, H. W., 1295

Foote, J. E., 847, 849
Foote, W. J., 897, 927
Forni, P. A., 870

Forsaith, C. C., 981, 987, 990, 991. 992

Forster, W. G., 1164
Friel, J. J., 1090

Frisch, N. M. (see Gussman, L.)
Fronmuller, D. (see Lewis, H. F.)
Frost, F. H., 1088
Fuwa, T. (see Wilson, R. E.)

Gaegauf, H., and Muller, M., 911

Gagliardi, D. D. (see Hager, O. B.)

Galloway, J. R* (see Walsh, R. H,)

Gardner, A. T., 1165

Gartshore, J. L. (see Cottrall, L. G.)

Gebhardt, G. W. (see Young, C. H.)

Geese, C. F., 1175

Geffken, C. F. (see Voet, A.)

Geib, M, N. (see Weber, C. G.)
Genung, L. B. (see Malm, C. J.)
Geohegan, K. P., 942
George, A. J., 1106

Getty, E, 1070

Giertz, H. W., 945, 947 .

Gillespie, W. F. {see also Sapp, }■

Gillespie, W. F.. 1236

Glegg, J. E., 1240

Gloster, A. J. (see Work L. T.)

Godlove, 1. H., and Laughlm, E. R.,

1151

Gold, L. (see Kao, J. Y.)

Gould, S. P. (see Bell, R. W.; and Whit-

tier, E. 0.) _ « ^

Graff, J. H. {see also Hecht'nan, J. h J

Graff, J. H., 860, 861, 960, 961 (ref. 375),

963, 969. 973. 974, 975 976

Schlosser, M. A., and Nihlen, E. K.,
964

Grant, J. P-,

Grant, N. S., 998 L'ST

Grantham, H. H., and Ure, W.. 1164, 1165

Green, H., 1075, 1078, 1145
^ and Weltmann, R. N., 1209
Green, R. E., Sams. R. H., and Wills.

J. H., 1223

Greenfield, E. W., 954

Griffin, R. C., and McKinley, R. W., 827

Grim, R. E., 1041, 1042

Griswold, W. E., 1106

Gussman, L., Frisch, N. M., and Cushing,

Hager, 0. B., Gagliardi, D. D., and
Walker, H. B., 838 ■

Hale, H. M., 1231 ■

Halladay, J. F., 854, 1096, 1115
Hansen, P. B. (see Delevanti, C., Jr.)
Hanslanger, R. U., and Mosher, R. H.,

1270

Harrison, V. G. W., 901

— and Poutter, S. R. C., 904

Harrison, W. F., 1143

Hartsfield, E. P. (see Walsh, R. H.)

Hauser, E. A., 1041

Hawley, D. M. (see Bauer, H. F.)

Haywood, G., 1012

Hechtman, J. F., and Graff, J. H., 974

Heikkla, M. A., 1059, 1062

Heinig, M., and Baird, P. K., 851

Helms, J. F. (see Pierce, S. W.)

Hemphill, R. J. (see Race, H. H.)

Henington, V. (see Buckman, S. J.)

1356

AUTHOR INDEX

Henson, W. A. (see Stanton, G. W.)
Heritage, C. C., Schafer, E. R., and Car¬
penter, L. A., 823

Herrmann, D. B. (see Taylor, R. L.)
Hewett, P. S., Carter, R. E., and Church,
R. J., 1025, 1112

Hewitt, S. (see Hodgins, T, S.)

Hicks, T. G., 938
Hieronymous, R. H., 1027
Hill, E. H., and Sliwinski, V. X., 1227
Hirschkind, W., Pye, D. J., and Thomp¬
son, E. G., 947
Hoag, L. E., 1309
Hoaglund, S., 1077, 1091
Hodgins, T. S., and Hovey, A. G., 1339
—, Hovey, A. G., Hewitt, S., Barrett,
W. P., and Meeske, C. J., 1340
Hofrichter, C. H. (see McLaren, A. D.)
Hohf, J. P. (see Turner, H. D.)
Hollabaugh, C. B., 1303, 1306
Honish, J. K., 1298, 1336
Hopkins, R. J. (see Miller, H. F.)

Houtz, C. C. (McLean, D. A.)

Houtz, H. H., and Kurth, E. F., 947
Hovey, A. G. (see Hodgins, T. S.)
Hughes, A. E., and Roderick, H. F.,
1064, 1066, 1099, 1113, 1114, 1120, 1121
(ref. 200a)

Hughes, D. A., and Craig, W. L., 1022
Hunter, R. S., 871, 873, 883, 887 (ref.
175), 902

Ingle, G. H., and Lewis, H. F., 1269
Irubesky, C. E. (see Baird, P. K.)
Isenberg, I. H., and Peckham, C. L., 964
Ivarsson, B. {see also Andersson, O.)
Ivarsson, B., and Steenberg, B., 821

Jacobsen, P. M., 829

James, A. L., 1029

Jarrell, T. D., 935

— and Veitch, F. P., 949

Jeuck, F. J., and Rietz, C. A., 1148

Johnson, F. B., 1168
Johnson, L. K., 948, 950
Jones, D., 935

Jones, G. W. (see Van den Akker, J. A.)
Judd, D. B., 905, 908, 917, 92^ 929
Julian, P. L., and Engstrom, G. C., 10^/

Kane, M. W., 829
Kantrowitz, M. S., 945, 1157
Kao, J. Y., Gold, L., Stull, A., Worden,
R., and Abramowitz, W., 1252
Kauppi, T. A., and Bass, S. L., 1293
Keim, C. R., 1029

Kemp, A. R. {see also Taylor, R. L.)

Kemp, A. R., 1347

Kendall, F., 1168

Kerr, P. F, (see Ross, C. S.)

Kerr, R. W., and Schink, N. F., 1020
Killingsworth, R. B. {see also Padgett,
F. W.)

Killingsworth, R. B., 1310

Kimberly, A. E., and Emley, A. L., 948

Kin, M. (see Upright, R. M.)

Kingsbury, R. M., Simmonds, F. A., and
Lewi's, E. S., 895, 896 (ref. 191)

Kinsel, A., and Schindler, H., 1240

Kirkpatrick, W. A., 1065, 1105, 1106, 1113

Klein, H., 1198, 1199

Klemm, P., 936, 945

Kline, H., 1263, 1339

Koch, W., 1331

Kohman, G. T. {see also McLean, D. A.)
Kohman, G. T., 950, 951, 954
Kregel, E. A. (see Sears, G. R.)

Kress, O. {see also Laughlin, E. R.)
Kress, O., and Cyr, H. M., 1059

— and Loutzenheiser, E. J., 862

— and Morgan, H. W., 899, 901

— and Silverstein, P., 933
Kreyling, R. L., 1231
Kubella, P., 923
Kumler, R. W., 1088

Kurth, E. F. (see Houtz, H. H.)

Lafontaine, G. H., 1091
Laird, B. C. (see Malm, C. J.)

Lalk, R. H. (see Stilbert, E. K.)

Lander, J. G. (see Boiler, E. R.)
Landes, C. G. {see also Lukens, A. R-»
and Wilson, L. H.)

Landes, C. G., 1109, 1110, 1113, 1116 (ref.
188)

Lane, O. P. (see Ball, R. W.)

Lane, W. H., 849

author index

1357

r T 8A4 865 938, 1154, 1155,

Larocque, G. L., wh. wjj,

1156, 1164, 1165, 1166

Latham, G. H., 1299 Vv

Uthrop, E. C. {see also Aronovsky, b.

uirop. E. C.. and Naffziger, T. R.. 843

Laucks, I. F., 1027 t H t

Laughlin. E. R- (see also Godlove I. H.)

Uughlin. E. R., 862. 892. 893, 987, 911,
912, 971

_ and Kress. O., 893. 897

Launer, H. F., 944

Leckey, M. J., 1158, 1161

Lee, H. N., 862, 1108 ^ \

Lee, W. B. (see McBam, J. W.)

Lee’te. J. F.. 936

Lester. I. L.. and Lyons, S. G., 1038 1072
Lewis, E. S. (see Kingsbury. R. M.)
Lewis, H. F. (see also Ingle, G. H.; and

Van den Akker, J. A.)

Lewis. H. F., and q, .

Lewis. L. C.. 873. 893, 904. 911, 913 914

Lewis, W. K. (see also Squires, L.)

Lewis. W. K., and Squires, L., 1285

Libby, C. E. (see Casey, J. P-; and

Dohne, W. P.)

Long, J. H., 1329
Long. R. P.. 1182

Loutzenheiser, E. J. (see Kress, 0.)
Low'e, D. V. (sec Cobb, R. M.)

Lukens, A. R.. Landes, C. G., and
Rochow, T. G., 1052, 1064, 1121
Lyne, L. M., 844

Lyons, S. C. (see also Lester, 1. L.)
Lyons, S. C., 1046, 1047, 1099

Maass, O. (see also DeLuca, H. A.)

Maass, O., 916

MacArthur, C., 1178

Macdougall, G. (see Elton, G. A. H.)

Madson, W. H. (see Armstrong, W. G.)

Malm, C. J., Genung, L. B., and Flecken-

stein, J. V., 1328

—, Taughe, L. J., and Laird, B. C., 1327
Malmstrom, H. E. (see Davis, M. N.)
Manchester, F. H., 1299
Mantell, C. L., and Allan, R. W., 1325
Mark, H. (see also Doty, P. M.)

Mark. H., 1273, 1337
Marshall. C. E.. 1043, 1045

Martin. S. W.. 1056

Martin-Borret (see Chene, U.)

Mason. C. W. (see Chamot. E. M.)

Mason. S. G. (see also Tuck. N. •

\Uson, S. G., 817. 838. 840

Massey, P. J-. 1160 „ v

Maxwell, C. S. (s« <■'« Wilson, L. •)
Maxwell, C. S., and Reynolds, W. F.,

McArdle. E. H., and Boldeschevieler. E.

McBaifV W., and Lee, W. B., 1207

McCarron, R. D., and Rowland, B. W.,

1047, 1065 , T 13 ^

McCoy. J. S. (see Niederl. J. B.)

McIntyre, J. W., 947, 948

McKee, R. C., Root, C. H., and Ayer ,

L. R.. 832 „ T Q r

McKee, R. H., and Shotwell, J. b.

933. 934, 935 r k -R

McKinley, R. W. (see Griffin, R. C.)

McLaren, A. D., 1273
— and Hofrichter, C. H., 1272

McLean, D. A., 955, 956
—, Egerton, L., and Houtz, C. C., 955
Egerton, L., Kohman, G. T., and

Brotherton, M., 956
Meeske, C. J. (see Hodgins, T S.)
Meyer, H. R. (see Schwartz, S. L.)

Meyer, W. W., 1040. 1041, 1042
Milham, E. G., 1016, 1017, 1087, 1094
Miller, B. C. (see Viner, J. W.)

Miller, E. J. (see Comar, C. L.)

Miller, H. F., and Hopkins, R. J., 956
Millman, N., 1038

Minnear, F. L.. and Withrow, J. R.. 965
Minor, C. A., and Minor, J. E., 826, 837
Minor, J. E. (see Blaisdel, C. A.; and
Minor, C A.)

Mispley, R. G., and Barber, W, R., 1257

Mitchell, J. A., 1204
Montsinger, V. M. (see Clark, F. M.)
Morehouse, R. (see Boiler, E, R.)
Morehouse, W. B,, 1259
Morgan, H. W. (see Kress, 0.)

Morgan, J. D., 1270
Morin, L. J. (see Bardsley, J. H.)
Mosher, R. H. (see also Hanslangcr, R.
U.)

P H.. and Bracewell. R. J., 798

1358

AUTHOR INDEX

Moyer, A, E. (see Snyder, F. H.)

Muench, C. G., 850
Muller, M. (see Gaegauf, H.)

Naffziger, T. R. (see Lathrop, E. C)
Nazzaro, R. T., 1251, 1252, 1253, 1349
Nelson, G. N. (see Aronovsky, S. I.)
Niederl, J. B., and McCoy, J. S., 1338
Nihlen, E. K. (see Graff, J. H.)

Nisson, A. H. (see Andersson, O.)

Nolan, P. {see also Van den Akker,

J. A.)

Nolan, P., 928
Nordman, L., 925

Odell, I. H. (see Work, L. T.)

O’Leary, M. J. (see Weber, C. G.)

OHner, A. W., and O’Neil, F. W., 1251,

1253

O’Neil, F. W. (see OHner, A. W.)
Owen, A. F., 1251
Owen, M., 833

Padgett, F. W., and Killingsworth, R. B.,
1231, 1240
Parsell, J. C., 1258
Parsons, S. R., 828, 909, 914, 925
Partridge, E. G., 1250, 1252
Patton, C. W., 1294, 1334
Pearson, E. S., 1003
Peckham, C. L. (see Isenberg, I. H.)
Penn, W. S., 1316
Pennell, M. S., 1159
Perot, J. J., 850

Peterson, F. C. (see Upright, R. M.)
Pew, J. C. (see Schwartz, S. L.)

Phillips, J., 1057, 1091, 1093

Pierce, S. W., and Helms, J. F., 1279,

1281 ^
Pignian, W. W., and Csellak, W. R., 895

Pike, C. H., 951
Pike, N. R., 841, 864
Pitzer, J. C., 1264

Pockman, W. W. (see Walsh, R. H.)
Pohl, E. (see Cobb, R. M.)

Polglase, M. F. (see Buchdahl, R.)
Pottenger, C. H., 1268
Potter, P. K., 1259

Poutter, S. R. C. (sec Harrison, V. G.
W.)

Prior, P. H., 993, 1141

Pye, D. J. (see Hirschkind, W.)

Race, H. H., Hemphill, R. J., and Eiuli-
cott, H. S., 953, 955
Rafton, H. R., 1090
Rainey, J. L. (see Auten, R. W.)

Ranee, H. F., 815, 822, 840
Rasch, R. H. (see Burton, J. O.)

Ratliff, F. T., 909
Reed, E. O., 937

Reed, R. F., 1160, 1177, 1178, 1179, 1180,
1181, 1182, 1183, 1192
Rfitz, L. K., and Sillay, F. J., 842, 859
Renner, M. S., 1279
Reynolds, W. B., 1338
Reynolds, W. F. (see Maxwell, C. S.)
Rice, W. D. (see Erspamer, A. E.)

Rietz, C. A. (see Jeuck, F. J. )

Riley, R. W. (see Dickerman, G, K.)
Ritman, A. C., 1239
Roberts, D., 833

Rochow, T. G. (see Lukens, A. R.)
Roderick, H. F. {see also Hughes, A. E.)
Roderick, H, F., 1068
Roehr, W. W, (see Davis, M. N.)

Root, C. H. (see McKee, R. C.)

Ross, C, S., and Kerr, P. F., 1039
Rowland, B. W. {see also McCarron, R.

D.)

Rowland) B. W., 1042, 1043 (ref. 54),
1081, 1082 (ref. 118), 1082, 1092, 1094,

1099

—, Allison. H. J., Jr.. 1042
Ruff, H. T., 1022. 1066
Ruhlemann, E., 828
Russell, H. R., 1179
Rutt, A. H., 933
Ryan, V. A., 1284, 1289

Salo, M., and Vivian, H. F., 1316
Sams. R. H. (see Green, R. E., and Wills,

J. H.)

Sanders, C. E. (see Squires, L.)

Sapp, J. E., and Gillespie, W. F., 822, 834

Sawyer, R. H., 911

AUTHOR INDEX

1359

Schafer, E. R. (see Heritage, C. C.)

Schindler, H. (see Kinsel, A
Schink, N. F. Kerr, R. WO
Schlosser, M. A. (see Graff, J. H.)

Schneider, W. K. (see Young, C. H.)

Schreiber, A. P., 808
Schumman, R., 1214
Schwalbe, C. G., 947 , , ,, p ^

Schwalbe, H. C. (see Buchdahl,

Schwarts, S. L. («e also Turner H D.)
Schwartz, S: L., Pew, J. C., and Meyer.

H R. 1260, 1269,1270
Scribner, B. W., and Wilson, W.K., 931
Scroggie, A. G. (see Charch, W. H )

Seagren, G. W. (see Young, CH)

Sears, G. R., and Kregel, E. A. 1043, 1052
Seborg, C. O. (see also Doughty, R. H.)
Seborg, C. 0., Doughty, R. H., and Baird.

P. K., 936, 937 „ . , „ xr

Simmonds, F. A., and Baird, P. K-,

935

Seidel, M. P-, 1263, 1266

Shaffer, R. W., 975

Shankweiler, F. K., 1287, 1291

Shaw, M. B. (see Weber, C. G., ami

Whittier, E. O.)

Sheets, G. H., 1042, 1049, 1050
Shewhart, W. A., 983, 998 „ „ ^

Shotwell, J. S. G. (see McKee, .

Sillay, F. J. (see Reitz, L. K.)

Silvernail, L. H., 1302
Silverstein, P. (see Kress, O.)

Simmonds, F. A. also Kingsbiiiy,

M., and Seborg, C. O.)

Simmonds, F. A., 847
—, and Doughty, R. H., 843
Simmons, R. H. (see Wehmhoff, B. L.)
Simpson, J. R., 1013

Singleterry, C. R., 1091, 1092, 1108, 1109

Slater, H. H., 1158

Sliwinski, V. X. (see Hill, E, H.)

Smith, J. F., 861

Smith, J. W., and Applegate, P. D., 1078
—, Trelfa, R. T., and Ware, H. O., 1018,

1079, 1097

Smith, S. F., 825, 864, 865, 867, 868 (ref.
156)

Smith, W. R. (see Brown, O. J.)

Snyder, F. H., Spiwak, L., and Moyer, A.

E., 1247, 1259

Snyder, L. W., and Carlson, F. T., 839

Sooy, W., 938

Spencer, H. S., 843 tt ^

Spiwak, L. (see Snyder, F. H J
Squires, L. (see also Lewis, W. K-)
Squires, L., Lewis, W. K., and Sanders,

C. E., 1285

Stanton. G. W., and Henson, W. A.. 1305,
Stifdlnger, H., and Trommsdorf, E., 1336

Steele, F. A., 911, 912 917 923
Steenberg, B., (see also Andersson, O. .
Danielsen, R.; and Ivarsson, B)

Steenberg, B., 817, 820, 823

Stilbert, E. K, Visger, R. D., and Lalk.

R. H., 1032

Stocker, F. W., and Dur^n'; L- G., 971
Strachan. J-, 797, 817, 826, ^ "

Strain, M. (see Crawford, E. A.)

Straka, C. J., 1262, 1267

Strieby, J. G. (see also Catlm. J. B.)

Strieby, J. G., 998
Stringer, W. E., 1017
Stull, A. (see Kao, J. Y.)

Sturken. R. C. (see Broughton, G.)

Stutz, G. F. A., 1121
Sutermeister, E., 1057, 1086, 1114

Sweatt, H. B., 1028

Taughe, L. J. (see Malm, C. J.)

Taylor, G. G., 875
Taylor, R. L., Herrmann, D. B., and

Kemp, A. R., 1278
Tennent, G. R., 863
Thies. H. R., 1298, 1317
Thomas. A. W., 877

Thomas, W. M. (see Wohnsiedler, H. P.)

Thomlinson, G. H., 1260 ^

Thompson, E. G. (see Hirschkind, W.)

Thune, S. F., 1236

Todd, J. E. (see Van den Akker, J. A.)
Tongren, J. C., 947, 948
Toulouse, J. H., 982, 993
Trelfa, R. T. (see Smith, J. W.)
Trommsdorf, E. (see Staudinger, H.)
Tuck, N. G. M., and Mason, S. G., 832
Turner, H. D., Hohf, J. P., and Schwartz,
S. L., 813

Tuwiner, S. B. (see Work, L. T.)

1360

AUTHOR INDEX

Upright, R. M., Kin, M., and Peterson,
F. C, 1030, 1106

Ure, W. (see Grantham, H. H.)
Urquhart, A. R., 933

Vail, J. G., 1229

Vallandigham. V. V., 1156, 1164
Van den Akker, J. A. (see also Dearth,
L. R.)

Van den Akker, J. A., 826, 928, 952, 953,
954 (ref. 355), 1279, 1280
—, Lewis, H. F., Jones, G. W., and
Buchanan, M. A., 896, 946, 948
—, Nolan, P., and Wink, W. A., 895, 946
—, Todd, J. E., Nolan, P., and Wink, W,
A., 878

Van Slyke, L. L., and Bosworth, A. W.,
1016

Veitch, T. P. (see Jarrell, T. D.)

Vincent, H. L., 1256
Viner, J. W., and Miller, B. C., 1308
Visger, R. D. (see Stilbert, E. K.)
Vittengl, M. J., 1251, 1347
Vivian, H. F. (see Salo, M.)

Voet, A., and Brand, J. S., 1113, 1144,
1165

—, and Geffken, C. F., 1145

Waele, A. De, 1164
Wagner, C. L. (see Abrams, A.)

Walker, H. B. (see Hager, 0. B.)
Walker, W. C,, 1143
Walsh, R. H., Abernathy, H. H., Pock-
man, W. W., Galloway, J. R., and.
Hartsfield, E. P., 1247,1250
Ware, H. 0. (see Smith, J. W.)

Wathelet, E. (see Arendt, A.)

Watson, J., 1147

Way, S., 851

Weber, C. G., 1177, 1180

—, and Geib, M. N., 1178, 1179

—, Shaw, M. B., and O’Leary, M. J.,

841, 864

Wehmhotf, B. L., 852
—, Simmons, R. H., and Boyce, D. H.,

1154

Weiss, W. (see Cobb, R, M.)

Wells, S. D., 836

Weltman, R. N. (see also Green, H.)
Weltman, R. N., 1075
Weymouth, F. A., 1162
Wheelwright, W. B., 1007, 1009
Whitehead, J. B., 953
Whittier, E. 0., Gould, S. P„ Bell, R. W..
Shaw, M. B., and Bicking, G. W., 1016,
1017, 1086

Wicker, D. B., 904, 908, 910, 911, 912

Wilcock, D. F., 1045

Willets, W. R., 1049, 1057, 1082 (ref.

119), 1082, 1119, 1120, 1128
—, Bingman, R. T., and Eriksen, L. H.,
894, 895

Williams, L, 1348

Wills, J. H. (see also Green, R. E.)
Wills, J. H., 1230
—, and Sams, R. H., 1231
Wilson, A. D., 1068

Wilson, L. H., Maxwell, C. -S., and
Landes, C. G., 1248
Wilson, N. F., 970

Wilson, R. E., and Fuwa, T., 933, 935
Wilson, W. K. (see Scribner, B. W.)
Wing, H. J., 1278

Wink, W. A. (see also Dearth, L. R.; and
Van den Akker, J. A.)

Wink, W. A., and Dearth, L. R., 1280
Wise, L. E., 945, 947
Withrow, J, R. (see Minnear, F. L.)
Wohnsie^er, H. P., and Thomas, W. M.,
1341

Wolff, H.- J., 1144
Wood, R. W., 901
Worden, R. (see Kao, J. Y.)

Work, L. T., and Odell, 1. H., 1045, 1059
—, Tuwiner, S. B., and Gloster, A, J.,
1055

Worthington, F. V. (see Carson, F. T.)

Yost, D. M., and Aiken, W. H., 1248,13^
Young, C. H., Gebhardt, G. W., Schnei¬
der, W. K., and Seagren, G. W., 1273

Zucher, M. (see also Berberich, O. P.)
Zucher, M., 1182

SUBJECT INDEX. VOLUME II*

Abalyn resins (see also Rosin, deri^^es)

Abalyn resins, adhesives, from, 1238

Abietic acid (see also

Abietic acid, formula, 13^^ ,

Abietic acid esters (see Rosw. dcrwa-

Abrasive paper (see ■

Absorbency of paper, effect on resm,
treated paper base laminates, 1264-

1265

\bsorption coefficient (see Kubelka and

Munk K Value) _

Accroides resin, description, loio
Accuracy, definition, 981

Acetone-formaldehyde resins, properties,

1341

Acetyl content, of cellulose acetate, 1328
Acid dyestuffs (see also Dyestuffs)

■\cid dyestuffs, use in pigment coating,

1070 . , , . ,

•\crylic resins, adliesives from, emulsions,

1238

emulsions of, 1306
properties, 1335-1336
properties of 1306
saturating paper, 1252-1^3
Adhesion, mechanical, 1206-12U/
resin to paper fibers, 1253
solvent welding, 1204-1205
specific, 1205-^1206, 1210
strength obtained, 1212
theory, 1204-1216 .

Adhesives (see also Casetn. Corrugating
adhesive, Dextrin adhesives, Emulsion
type adhesives. Heat-seal adhesives,
Hot-melts, Lacquer adhesives. Lami¬
nating. Pigment coating adhesive,
Resin adhesives. Sodium silicate,
Starch adhesives, etc.)

Adhesives, acrylic acid emulsion, lZ3o
adsorption, 1206
animal glue (see Animal glue)
application to paper, 1212
aqueous types, 1216-1236
asphalt (see Asphalt)
bituminous emulsion, 1210, 1237
blood, 1233
bookbinding, 1233

casein, 1232 , \

corrugating (see Corrugating adhesive)

definition, 1205

S'.Specif?-'--

ethylcdWose,

film, properties, 121‘W2'5. 232

gummed tapes, thickness, 1
sealing tape, thickness, 1235
stress-strain characteristics, 1215

thickness, 1214
thickness, effect on curl,

fish glue, 1233 lono

flow properties, IZUb, izuv _
heat-s% (tee Heat-seal Mestves)

hot melt

lacquer tyP«,.1238-1240

microcrystallme wax, 1-42

miscellaneous, 1233-1234

modifying agents used, 121/

moisture in film on gummed papers,

1234

molecular w'eight, 1207-1208
nitrocellulose, 1239

non-curling types, IZlo

paper cup, 1239
paraffin wax (see

penetration, into paper. 1206-1207. 1212
into paper of hot-melt, 1241
plasticizers in, effect on curl, 1
nohwinyl acetate emulsion, 1238-/,
polyvinyl alcohol, 1233-1234
polyvinyl chloride-acetate resm, 1239,
1244, 1245

pressure-sensitive, 1204

resin emulsion adhesives for, 123

tackiness, 1209
properties, 1202-120 d
protein, 1232—1233
remoistening, 1234-1235
resin (see Resin adhesives)
resin emulsion, 1238
rosin, 1240

rosin derivative, 1323, 1324
rosin derivative lacquer, 1240
rubber, 1239
rubber latex, 1237
sealing tape type, 1235-1236
setting, by chemical reaction, 1213-1214
by gelation, 1213
by heat, 1214
by solvent loss, 1213
setting rate, 1212-1214
effect of paper, 1210-1211, 1213

* In some instances, Volume I covers topics discussed in Volume II. It is sug¬
gested that the reader also check the index to Volume .

1361

1362

SUBJECT INDEX

Adhesives (contd.)

effect of paper porosity, 1210-1211
emulsion type, 1236
solids content used, 1216
solvents for, 1206
soybean protein, 1232-1233
soy flour, 1232-1233
starch (see Starch adhesives)
strength, of bond, effect of viscosity
(see also Adhesion), 1208
of film, 1241

of microcrystalline wax, 1242
surface-active agents, 1210
tackiness, 1209
effect on setting rate, 1212
temperature of, 1208
effect on curl, 1216
effect on setting rate, 1213
types, 1202-1205
for gummed papers, 1234-1235
vegetable gum, 1234
viscosity, 1208

water-resistant, 1203, 1213-1214, 1220-
1222, 1227-1228, 1232-1233. 1236-
1237

Aging of paper (see also Permanence of
paper)

Aging of paper, effect on flexibility of
resin coated papers, 1273
effect on properties, 944-949
on printing, 1166, 1177
on stretch, 831

measuring permanence by, 945
Aging of pulp, effect on brightness, 895
Air, in coating mixtures, cause of foam,

1085, 1086

in paper, effect on dielectric constant,
951

effect on opacity, 910
effect on waxing, 1312
volume, 811-812, 846-847
specific inductive capacitmice, 952
Air brush coaters (see Pigment coating
and Air knife coater)

Air knife coater, 1283
Air permeability (see also Porosity of
paper and Gas, permeability)

Air permeability, relation to water-vapor
resistance, 1277
variables, 848-849
Alabastine, properties, 1061

Albertype printing, 1184 • , n-i

Albumen, planographic printing with, 11/1
Alkyd resins, adhesives from, hot-melt,
1244

emulsions from, _1302
lacquers from, 1288, 1291-1292
oil-modified types, 1342
organisols and plastisols from, 1306
properties, 1341-1343
rosin-derived, 1324, 1342-1343

emulsion adhesives from, 1238
saturating paper, 1253
urea-resin modified, 1,140
varnishes from, use in printing ink, 1143
Alpha cellulose, requirement for iterma-
nent papers, 945

Alpha protein (see Soybean protein)
Alpha pulp, beating, effect on light scat¬
tering, 925
on opacity, 909
odor, 950

plastics from, 1269

resin-treated paper laminates from, 1265
Alum, addition to resin-treated stock,
1248, 1260

effect on clay-water dispersions, 1050
in paper, effect on laminating, 1222
effect on offset printing, 1192
resin precipitation in stock with, 1248,
1260

spots in paper of, microscopic examina¬
tion, 977

use in starch corrugatiiig adhesive, 1226
in washable wallpaper, 1125
Alumina, effect on opacity, 912
Aluminum, presence in paper ash, 941
Aluminum pigment, use in printing ink,
1140

Aluminum silicates (see Clay)

Ammonia, developing direct process
papers, 1199

Ammonium chloride, catalyst for urea-
formaldehyde resins, 1030
Ammonium hydroxide, casein preparation
with, 1017-1018 . .

Ammonium oleate, use as emulsifying
agent for resin emulsions, 1301, 1303
Angle of contact, water on coated paper
to measure adhesive penetration, 1091
Aniline ink (see Printing ink, aniline)
Aniline printing (see Printing processes,
aniline)

Aniline sulfate stain, preparation and use,

976

nimal glue, adhesive, for sealing tape,

1235, 1236

for tube winding, 1228
adhesives from 1228, 1233, 1235, 1-36
pigment coating, ^028
rubber latex, 1237
water-resistant, 1233
blending with casein in pigment coat¬
ing, 1033

effect on permanence of paper,
grease content, effect in pigment coat¬
ing, 1028 --

preparation for adhesive purposes,
nthraquinone, stabilizer for electnca
impregnated papers, 95o
nti-foams, use in pigment coating mix¬
tures, 1070, 1085, 1086-1087
nti-oxidants, for paraffin, 1310

SUBJECT INDEX

1363

for resin emuhions, 1302
for rubber l^tices, 1250, 1254
Antique finish, definition, 903
use in printing papers, 115
Anti-tarnish papers, Porosity, 850
reducible sulfur content, 930
requirements, 930
Aquatint printing, 1166
Aroclors (see Polyphenols) ,

aI?1 suHonamide-iormaldehyde resms,

lacquers from. 1289
AsS^r^iwoscopic appearance 941

Ash content, of carbon tissue, 1

of newsprint, 1165 rhan®--

of paper, conversion factors for chaiio

ing to filler, 942
determination, 939
effect of moisture, 935 936
effect on electrical properties, 955

Aspha'if (Ao Bitummous
Asphalt, adhesives for Sisa^kraft 1236
adhesives from,,l224, 1236, 1242-U45
compatibility with resins 1243
corrugating adhesive, 1224
oroperties, 1242—1243, 1255 ^

Asphalt-saturated paper, fiber analysis o ,

properties, 1255-1256

tensile strength, 829 i9c;<;_i256

Asphalt saturating, paper
A stain (see SKtermetsten s A stam)

Tufe^r’bSg^f.reug.h, impor-

effect of moisture, 820-821
pasting of, bottom pastes, 12 -

seam pastes, 1226-122/
starch adhesives, 1226^1228
water-resistant adhesive, 122 /- 12 -

porosity, 850
printing, 1147-1163
strength, 815-B29
stress-strain in, 817 .

Bakelite (see Phenolic restns)

Banknotes, engraving, 1160 „ loiQ

Bardoc process, description, 12 _

resin application at beaters by, 1248

Bariurn wrbonate in paper, detection, 940
Barium in paper, detection, 942
Barium sulfate (see also Barytes

Blanc fixe) toronfm-

Barium sulfate, decomposition temper

ture, 942 ^

in paper, detection, 94U

SSr’suffite composite pigment with,

1059

Bark specks in Paper, microscopic ex-
BarytrSnfof photographic paper,

preJi?ation (see also Barium sulfate),

1059-1060 „ - .

Rfjcir dvestuffs (see also Dyestuffs)
lasfc dyestuffs, use in pigment eoatmg,

1070 ^

Basic ream size, reasons for, 8U5

table of, 805 , cny sng

Basis weight, calculation 807-808
calculation of density from, 810
conversion factors, ouo
definition, 805-806
description," 8(^09
effect of humidity, 808
effect on Kubdka ^nd Mimk ^^and^5
values, 916-917, 922, 9-4, 9_ ,

929

on light reflectance, 908
on opacity, 908-929
on strength, 859
on tearing resistance, 836, 859
on tensile strength, 8-7, 8 -

on uniformity, 857, 859
hanging stock, 1126
measuring, 807-808
of carbon tissue, 1318
of newsprint, 1165
statistical analysis, 999-1005
uniformity, 857-858
variability, 858, 999-1005
Bast fiber (see Strazu pulp)

Bausch and Lomb glossmeter, descrip-

Bausch ' and Lomb opacimeter, opacity
measurement, 905

Beater, fouling with resins, 1246, 1-48,

resfn addition (see ako

application), 1246-1250, 1-58-1-60
Beating, amount required, for carbon tis¬
sue, 1318

for offset papers, 1176
for printing papers, 115/
effect of nonpolar liquids on light scat¬
tering, 925 •

effect on brightness, 89-^925
on bursting strength, 835
on coated papers, 1089
on color, 874
on curl, 866
on density, 811, 812
on expansion of paper, 864
on folding endurance, 841
on light scattering and absorption, 9-5
on moisture sorption of paper, 935

on opacity, 909-910 10,17

on resin application at beater, 124/,

1249

on smoothness, 855

1364

SUBJECT INDEX

Beating {contd.)
on stiffness, 826

on strength of plastics reinforced with
pulp, 1270

on stretch of paper, 831
on tearing resistance, 837
on tensile strength, 812, 813, 829, 830
on weight factor of fibers, 965
on zero span tensile strength, 830
Beer’s law, absorption coefficient, 928-929
Beidellite (see also Clay)

Beidellite, />H, 1036
Bekk hardness tester, description, 844
Bekk smoothness tester, description, 852
Bentonite, effect on water retention of
coating mixtures, 1084
exchange capacity, 1041
properties, 1035

Binder (see Pig^ment coating adhesive)
Biphenyl, treatment of oiled fruit wrap,

1257

Bird applicator, description, 1103
Bituminous emulsions (see also Asphalt)
Bituminous emulsions, adhesives from,
1210, 1237

Blackening, effect of calendering, 894,
1098-1099

Blanc fixe, effect on calendering of coated
papers, 1099-1117

preparation and properties (see also
Barium sulfate), 1060^
properties for pigment coating, 1034
Bleachability, effect of wet pressing, 894
effect on fiber staining, 962 _

Bleachability stain (see Cookmg stam)
Bleached pulps, detection by fiber analysis
(see Fiber analysis)
yellowing of, effect of hemicelluloses,

947

Bleaching, ash residue from, 940
effectiveness measured by brightness
measurements, 891-892
effect on brightness, 892, 895, 896^8
on moisture absorption of paper, 93a-

936

on opacity, 915-916
on permanence of paper, 94a, 94/
on resin application at beater, 1248
on spectrophotometric curve, 891-892
on staining of fibers (see ^ber an¬
alysis, Fibers, Graf C stam,

etc.)

on weight factor of fibers, 964-965
of clay, process, 1037
Bleach residues, effect on acidity of paper,

944

effect on electrical properties of paper,

954

on paper, 948 , • *

on resin-treated paper base laminates.

1264

Bleach scale in paper, identification, 979
Bleach stain, in fiber analysis, 975
Bleeding of asphalt laminated papers, 1243
Blocking, of gummed papers, 1235
of resin coated papers, 1274-1275, 1286,
1290-1292, 1300, 1304
of resin-wax coated paper, 1316
testing for, 1274-1275
Blood, adhesives from, 1233
Blotting paper, basic ream size, 80S
pore size average, 847
Blueprint papers, properties, 1198-1199
Blueprint process, description, 1198-1199
Bond papers, ash content, 939
basic ream size, 805
bursting strength, 832, 833
density, 811, 812
dirt count, 860

effect of moisture, 864, 865, 866
engraving, 1166

folding endurance values, 841, 842
gloss, 902
opacity, 90S

pu, m

pore size average, 847
porosity, 1211
stiffness of, importance, 826
variability, 858

watermarking for rag content, 959
Bonds, types involved in resin-paper com¬
binations, 1272

t)rpes involved in specific adhesion, 1206
Book papers (see also Pigment coated
papers and Printing papers)

Book papers, ash content, 924
basic ream size, 805
brightness, 924
bulk, 846

bursting strength, 832
coated (see also Pigment-coated papers)
formula, 1069
thickness of coating, 1046
composition, 1148
cutting, 803
density, 811
gloss, 902

hot-melt coating, 1308
moisture, effect, 864, 865
moisture sorption, 934
opacity, 904
porosity, 1211
scattering coefficient, 924

thickness, 810 _ i. •

Borax, amount used in starch adhesives,

1217, 1218 . ,

amount used in starch corrugating ad¬
hesive, 1224-1225

casein dispersed with, 1018 ^

effect on tack of starch adhesive, 12Uy
Bouyoucas hydrometer, measuring c ay

particle size, 1044

SUBJECT INDEX

1365

Roicboard (see also Paperboard)

bursting strength, importance.

835

folding grade, 841
optimum moisture contwt, Voo

stiffness of, importance, 826

Box coverings, flint glazing of, 110
pigment coatmg. lOW
Bread wrap, opacity, 904 914
opaque, requirements, ll-o

wa!x Selilt^for, strengtli o^bond, 1215

Breaking length (see also Tensile
sirengtb)

Breaking length, calculation. 827
Brightness, defimtion, pO, 871, 8»o
effect of beating, 925

of dyestuffs, 892-893
of pigments, 893
instruments, 888-890
K/S value, effect, 917-923, 926
of barium sulfate, 1060
of calcium carbonate, lObl,
of calcium sulfite, 1060
of clay, variables affecting, 1()37, 1038
of coated paper, effect of calendering,
1098-1099

of coating ravvstcKk, 1089

of diatomaceous silica, 1062

of paper (see also Pigment-coaled

papers, brightness)
of paper, 888-898
effect of aging, 944-949
effect on opacity, 906
loss during calendering, 894
pH for optimum, 948
relation to light reflection, scattering
power, and opacity, 922, ^2/,
929-930

two-sidedness in, 861
of pulp, variables, 895
of pulp mixtures, 896-898
of titanium dioxide, 1054
of zinc sulfide, 1059 .

Bright stain, description and use in hber

analysis, 972-973 .

Bristol paper, basic ream size, 8l>o

pasting of, 801, 1228
strength, 815

British gums (see Dextrins)

Brittleness, of paper, determination, 823
Bronze inks, type used, 114()

Bronze specks in paper, microscopic ex¬
amination, 977, 978 .

Brookfield viscometer, description, lU//-

1078

Brown print, description, 1199
Brush-coated papers, formula, 1069
Brush coaters (see Pigment coating)
Brush finishing of coated papers, 1101

M ullen

Brush marks in pigment coated paper,

cause, 1095, 1115 .

Brush surface analyzer, measuring

smootliness witli, 851
Bulk factor, meaning, 846
Bulking factor, meaning, 846
Bulking thickness of paper, 846
Bulk, of newsprint, 1165
of paper, 846, 1088-|1089
Burst factor, calculation, 832
Bursting strength (see also
tester)

Bursting strength, 831-835
and tensile strength, 833, 834
determining machine and cross direc¬
tion from, 803 m.; oi?

effect of moisture m paper, 936-93/

of pH of paper, 944
of sizing procedure, 944
of solid fraction, 813
points per pound, 832
statistical analysis of results, 992 994

values, 832-833
variability, 858
Burst ratio, calculation, 832
Butadiene-acrylonitrile copolymers, prop¬
erties, 1350

Butadiene-styrene copolymers, lacquer
from, 1299-1300, 1349
properties, 1349

latices, pigment coating with, 1U31-1U32
properties, 1031-1032
saturating paper with, 1252—1253
Butyl carbitol test for printing papers,

1156

Cable wrapping paper (see also Electrical
papers)

Cable wrapping paper, 956-957
ash content, 954
Caking in printing, 1187
Calcium carbonate, adhesive demand in
pigment coating, 1064-1()65, 1067-1068
conversion factor used in ashing of
paper, 943

decomposition temperature, 942
dispersing agents for, 1051
effect on air resistance of filled papers,
849

on calendering of coated papers, 1099-
1117

on drying of pigment-coated papers,
1098

on ink receptivity of coated papers,
1113-1114

on optical properties of coated papers,
1120-1121

on solids content permissible in pig¬
ment coating, 1071-1072
on water retention of coating mixture,
1084

1366

SUBJECT INDEX

Calcium carbonate {could.)

foam in coating mixtures caused by,
1086

ground, electron micrograph, 1054
manufacture and properties, 1053
in paper, detection, 940, 941
determination in presence of clay, 943
particle size of, effect in pigment coat¬
ing, 1121

pigment coating formulas using, 1069
precipitated, effect on clay dispersions,
1050

electron micrograph, 1052
preparation and properties, 1051-1053
properties, 1050-1051
properties for pigment coating, 1034
types, 1050-1051
uses, 1050

in carbon tissue, 1318
in newsprint, 1165
Calcium in paper, detection, 941
Calcium sulfate, decomposition tempera¬
ture, 942
impurities, 941 *

preparation and properties, 1061
titanium dioxide extended pigment,

1054, 1057

Calcium sulfite, adhesive demand, 1061
impurities, 941

preparation and properties, 1060-1061
Calendering (see also Supercalendering)
Calendering, curl controlled by, 867
effect of water added, 938
effect on brightness, 894
on color, 874
on density, 813

on flow properties on paper, 822

on opacity, 910

on printing papers, 1154—1155

on smoothness, 855

of coated papers (see Caloidertiig of
coated papers)
of coating rawstock, 1088
Calendering of coated papers, adhesive
effect, 1015, 1116-1117
dusting during, 1100

effect of adhesive penetration, 109-^
of mechanical treatment of coating
mixture, 1066

of pigment particle size, 1065
of soap, 1070

of water retention of coating mix¬

ture, 1084
effect of clay. 1037
of pigment, 1037, 104/, ^

of oigment particle size, 1065
effect on brightness, 1098-1099
on gloss, 1117
on ink receptivity, 1113
on smoothness, 111^1117

variables affecting 1098-1102

Zalender sizing, effect on laminating. 1210

effect on printing, 1162
Caliper (see Thickness of paper)

Canary dextrins (see Dextrins)
Candlepower, definition, 870
Capacitance, definition, 951
Capacitor paper, conducting particles al¬
lowed, 955
requirements, 953

Carbon black, effect on driers used in
printing inks, 1143

use in carbon paper manufacture, 1317-
1318

use in electrical paper, 957
use in printing ink, 1140
Carbonizing paper, description, 1318
Carbon papers, curl, 869
manufacture, 1317-1318
properties, 1317-1318
rotogravure plates prepared with, 1167
spirit duplicator, 1195-1196
Carboxyniethylcellulose, pigment coating

with, 1031

use in planographic printing, 1172
in resin emulsions, 1302
Carnauba wax, use in carbon paper manu¬
facture, 1317, 1318
Carson curl tester, description, 869
Carton sealing, adhesives for, 1228
Cartons, waxing, 1312
Casein (see also Pigment coating ad¬
it esivc )

Casein, adhesives from, 1232
analysis, 1016-1017

blending with other adhesives in pig¬
ment coating, 1033

decomposition in alkaline solutions, 1018
dissolving in water, 1017
effect on calcium carbonate, 1051
solids content permissible in pigment
coating, 1071
zinc oxide, 1059
fat content, 1016-1017
foam caused in pigment coating mix¬
ture, 1086

importance in coating, 1015
in paper, qualitative test, 1020
manufacture, 1015

mixing with starch for paper coating,
1032-1033
spoilage, 1017

spots in paper, detection. 977
stabilization of starch and butadiciic-
styrene latex, 1032
types, 1016

use in resin emulsions, 1302
rubber latex adhesive, 1237
special ink for paperboard, 1162
satin white, 1057
wallpaper coating adhesive, 1124
water resistant, treatment necessary.

1019

SUBJECT INDEX

1367

Castor oil, use with rcsiii-wax blends,

1314—1315 _ lie/'

Castor oil test, for printing papers, 11 M>-

Cellophane, adhesion of resins,
coating with resin, 1271
laminating, 1206, 1210, 1238
printing, 1163

tearing resistance, 838 ^

water-vapor transmisioii, 1278, 1281
of coated grade, 1291
Cellulose, birefringence effect on opacity.

derivatives of, description, 1326-1331
pigment coating adliesive, 1030-1031
dielectric constant, 951
dielectric strengtli, 951, 952
electrical properties, 951
esters of, properties, 1327-1328
hydroxyl groups, effect on dielectric
constant, 951
light reflectance, 913
moisture content, effect on dielectric
constant, 951

molecular chain length, effect of light.

947

permanence, 945
plastic manufacture, 1258
specific gravity, 846
specific volume, 811
Cellulose acetate, lac(|uer from, 1289
manufacture, 1327
properties, 1328
softening temperature, 1274
solvents for, 1285

Cellulose acetate butyrate, hot-melt coal¬
ings from, 1316
properties, 1328-1329
softening temperature, 1274
Cellulose acetate sheeting (see also Plas¬
tic films)

Cellulose acetate sheeting, laminating with
resin emulsion adhesives, 1236
laminating with wax adhesives, 1242
Cellulose nitrate, D. P. for best adhesion,
1207

Cellulose propionate, properties, 1328
Chalking in printing (see Printing)
Qieesc, packaging, 1276
Chemical properties of paper (see also
Paper)

Giemical properties of paper, for photo¬
copying processes, 1200
(Thipboard (sec also Paperboard)
Chipboard, laminating, 1211-1216
porosity, 1211
printing, 1162

Chlorinated diphenyl, impregnated elec¬
trical papers, 955

Chlorinated naphthalene impregnated elec¬
trical papers, 955

Oilorii\ated paraffin, properties, 1309

Chlorinated polyphenols, properties, 134a

1346 ,, .

Chlorinated rubber (see Rubber, cnlort-

nated)

Chroma of color, definition, 876
Chromaticity diagram, 883, 886
Chromaticity of paper, specification, 876
Cigarette papers, porosity, 850
Clark stiffness tester, description, 824-
825

Clark stretch tester, description, 831
Clay, adhesive demand in pigment coat¬
ing, 1019, 1064-1065
adsorption of enzyme on, 1023-1024
amount in various paper grades, 939
analysis, 1035-1036

base exchange properties, effect on hy¬
dration, 1041-1042
brightness, 1037, 1038
coating grades, particle^ size, 1045-1046
conversion factor used in ashing paper,

943 . .

deflocculation (see Clay, dispersing

agents)

dehydrated type, 1042
dispersing agents for, 1047-1049
eft'ect of adhesives, 1082
dispersions, in water, 1047-1050
thixotropy, 1049
effect of cations, 1050
of dicyandiamide, 1050
of heating, 1038-1039, 1042
of ignition on weight, 942
of waxing on paper containing, 1128^
on brightness of coatings, 1045, 1047
on calendering of coated papers, 1099-
1117

on expansion of paper, 864
on flow properties of pigment coat¬
ing mixture, 1081

on ink receptivity of coated papers,
1113-1114

on opacity, 911, 913, 914
on solids content permissible in pig¬
ment coating, L072
on starch adhesives, 1209
on water retention of coating mix¬
ture, 1084

electrokinctic charge, 1049
electron photographs, 1043
English type, adhesive demand in pig¬
ment coating, 1064-1065
electron micrograph, 1043
Iiarticle size, 104^1046
exchange capacity, 1041
effect of dispersing agents, 1049
filler grades, particle size, 1045-1046
filling with, effect on opacity of paper
after printing, 1158
flocculation, 1050
formula, 1035

1368

SUBJECT INDEX

Clay (contd.)

grades used in wallpaper coating, 1124-
1125

hydration, 1041-1042
effect of dispersing agents, 1049
effect on drying of pigment-coated
papers, 1098

effect on flow properties of coating
mixture, 1081
impurities, 941, 1036, 1050
in paper, detection, 940
determination in presence of calcium
carbonate, 943

ion adsorption by, effect on dispersion,
1049

ion exchange properties, 1039-1041
latex adhesive with, 1032
microscopic appearance, 941
nature and properties, 1034—1038
particle size, distribution, 1044-1046
effect in pigment coating, 1121
effect on adhesive demand, 1127
on flow properties of coating mix¬
ture, 1081

on hydration, 1041-1042
on optical properties of coated
papers, 1121
measurement, 1044
/•H of exchange neutrality, 1040-1041
pH of water dispersions, 1049
pigment coating formtdas, 1069
processing, 1037
shape of particles, 1043-1044
silicate for dispersion, 1047-1049
types, 1035-1036
uniformity, 1034

use in adhesives, 1217-1218, 1230, 1234,
1237

polyvinyl alcohol, 1234
rubber latex, 1237
sodium silicate, 1230
viscosity of-water dispersions, 1048-1050
Coal specks in papers, microscopic ex¬
amination, 977

Coated papers (see Resin-coated papers.
Pigment-coated papers^, 1130
Coating (see Resin coating, and Pigment
coating)

Coating of carbon paper, 1317-1318
Coating rawstock, brightness, effect on
coated paper, 1118-1119
composition, 1087-1088
effect on flow properties desired in coat¬
ing mixture, 1081

on penetration of coating mixture,

1093-1095

on properties of coated paper, lllo

1120 . ... .
on water retention required in coat¬
ing mixture, 1083
for carbon paper, properties, 1318
hardness, effect on strength of coated

paper, 1109
imperfections, 1089

moisture content, effect on properties
of coated paper, 1094
opacity, effect on opacity of coated
paper, 1120

penetration of coating mixture, 1090-
1095
pH, 1094

effect on water resistance of starch-
resin coatings, 1025
porosity of, importance, 849
properties, 1087, 1088-1090
for knife coating, 1013
varnish papers, 1127
wetting by coating mixture, 1090
Coating with pigment (see Pigment coal-
ing)

Coating with resins (see Resin coaling
and Waxing)

Cockling, causes, 869
Coefficient of variation, folding endur¬
ance, 842

Coffee, packaging, 1275

Coil winding paper (see also Electrical

papers)

oil winding paper, description, 956
ollecting in printing (see Printing)
ollotype printing (see Printing processes,
photogelatin)
olor (see also Light)
olor(s), dominant wavelength, 883-886
excitation purity, J885-886
Munsell system, 875-876
of paper, effect of illuminant, 874-875

effect on formation, 856
effect on gloss, 901
factors affecting, 873
hue, 875

measurement, 872-888
shade, effect on brightness, 891
two-sidedness of, causes, 862
physical measurement, 876-880
primary, 881
psychology of, 873

specification of, I. C. I. system, 881-

886

Munsell system, 875-876, 883

standard coordinate system, 882-885
three-filter colorimetry, 887-888
specks in paper, microscopic exami¬
nation, 977, 978
visual efficiency, 886^87
:olor curve (see Spectrophotometnc

:olor”ffiters, colorimetry with, 887-888
wavelength range, 880
:olor lakes, coating with, 1063
effect on driers in printing inks, 1H5

wallpaper coating, 1125
:olor matching, effect of wavelength and

illuminant, 875

SUBJECT INDEX

1369

requirements, 886
spectrophotometer for,
spectrophotometer versus visual rating,

880^81

visual, 874-875 . .

Colored paper, spectral reflectivity curves,

878—879

Combining of paper (see Laminating of
paper)

Compressibility of paper, 844-846
Compression test of corrugated paper-

board, 1223 171 ;

Condenser papers (see also Electrical

papers)

Condenser papers, ash content, 954

description, 955 . u- u o??

dielectric strength, reason for high, 95Z

Condenser tissue (see Condenser papers)
Conditioning of paper (see Paper, con¬
ditioning)

Congo resins, description, 1325
Contrast gloss (see also Gloss)

Contrast gloss, measurement, 900
Contrast ratio (see also Opacity of
paper)

Contrast ratio, definition, 904-905
effect of basis weight and of reflec¬

tivity, 929

Control (see Qualify control and Sta¬
tistics)

Cooking stain, description and use in fiber
analysis, 974

Copper, effect on paraffin, 1310
Copper number, permanent papers, 945
Corn gluten, properties, 1028^1029
zein from, 1326

Corn syrup (see also Glucose)

Corn syrup, effect on moisture in paper,
936

Correlation coefficient (see Statistics)^
Corrugated paperboard, effect of silicate
adhesive, 1231

effect of starch adhesive, 1224-1226
flutes used, 1222-1223
effect on adhesive consumption, 1231
liners, 1222
properties, 1222-1224
testing, 798
types, 1222

weatherproof, formula, 1225-1226
Corrugating adhesives (see also Starch,
corrugating adhesive and Sodium
silicate)

Corrugating adhesives, asphalt, 1224
distinguishing different, 1224
resin emulsion, 1236-1237
Corrugating machine, description, 1223
Corrugating medium, steaming, 938, 1223
stiffness, 826
weights, 1222

Corrugating process, description, 1222-
1223

Cosolvents, for lacquers, 1285* 1291
Cotton linters, weight factor, 964-965
Cotton pulp, condenser tissue from, 955
Coumarone-indene resins, adhesives, 124U

hot-melt adhesives, 1244 ^

compatibility with other ingredients,

1244

effect in printing ink, 1143
lacquers, 1289-1290
phenol-modified, 1345
properties, 1344

Cover paper, basic reatii size, »U5
Creasing, effect on resin coated papers,

1273-1274 . -

effect on water-vapor resistance ot
resin-coated papers, 1280—1281
Creep, of paper, 816-817
Creped papers, asphalt, 1243
stress-strain of, 817-818
stretch of, 831
tearing resistance, 838
Creping, effect on softness, 844
Cross direction of paper, air resistance,

849 . -

effect on properties, 801-804

expansion, 863, 864, 868
folding endurance, 840
stress-strain characteristics, 8-il,
tearing resistance, 836
tensile strength, 828
testing, 803

Crown filler, properties, 1061
Crush test for corrugated board, 1223-
1224

Crystallization in printing (see Printing)
C stain (see Graff “C” stain)

Cup paper, adhesive for, 1239
Curl, axis, 803
causes, 866-868
discussion, 866-869
effect of laminating, 1215-1216, 1219
effect on printing, 1181
of gummed papers, 1234
of printing papers, 1158
types, 866—868

Cyanine-glycerine reagent, description and
use in fiber analysis, 974-975
Cyclized rubber (see Rubber, cyclized)
Cylinder flat-bed press, description, 1133-
1134

Dammar resins, adhesives from, hot-melt,
1244

description, 1324-1325
softening temperature, 1274
Data (see Statistics)

Davidson-Pomper tester, description, 1107
Daylight lamps, color matching by, 874-
875

Decalcomania paper, use in lithography,

1170

1370

SUBJECT INlnUC

Uccp-ctdi process (see Pltmcgrtfkt^
frinting, dff^-ftch frocfss)

Dctnked slock, use in coating mw stock.

11 ) 8 ;

Dcnnis^Mi wax test, values rctiuired (or
Coated papers for offset printing.

1182

values rc()uireil for cvated papers for
relief printing, 1161-1162
values required for offset papers, 11/6
values required for phrrtt^elatin print¬
ing. 1184

Density of paijcr, air volume from, 84/
discussion. 810-814
effect of fillers, 814
on brightness. 804
on bursting strength, 813
oTiiCcrtidcnser tissue, 955
on curl, 866

on electrical properties, 954
on hardness,, 845
on laminating. 1210, 1211, 1227
on laminating with hot melts, 1241-
1242

on light absorption and scattering,

916

on opacity, 909-910
on resin coating. 1284. 1299
on resin-treated paper base laminates,
1264-1265

on saturating, 1251, 1255-1256, 1264-
1265

on stiffness, 825
on tearing resistance, 837
variability! 858

variables affecting,,810-811
Dextrin adhesives (sec abo Starch ad¬

hesives) , .

Dextrin adhesives, for tul3c wining, 1«28
setting rate, 1213
solids content, 1218
strength, 1212

types, 1217-1228. 1234-1235
Dextrins, bag adli«ives from, 12-/-!^
foam caused in pigment coatings from.

1086

properties. 1217, 1218

fypes, 1021 _

tvpes used as re mob tening gums, 1234-

1235

Diatomaceous silica, prep^tion and prap-
erties. 1061-1062 ^ "

Diatron coatef, description, 1103-1104
Diazotype papers (see Direct process

Dibutyl‘phthalate, effect cm nitrocellulose
lacquers, 1_^1-1292
Dicyandiamide. effect on clay, lOaO
Dielectric constant 951-953
Dielectric constant of paper, effect of
density, 954
effect of porosity, 849

effect of impregiuntt. 951-9S2
IHeleclric Iom (»ce i'trwrr jaetof )

Diekctrk tircngth, 952-953, 955
Die ^lampmc, 1166

Dictltylcne glycui. u»e in vafM/r »rt {irtill¬
ing ink. 1147

Dilatant flow (see also fiscosity ant]
Ptgmrmi Ci’OttHg rntstwret)

Dilatant Aow, 1075

cause of trouble in roll coating. 10*^»-
1097

IHIuenu, for lacquers, 1285
(or ofganivjls, 1.106
for poljrvinvlidcnc cr»p':4ymcf laciitnT»,
1^

Dimensional changes of adhesive films.

1215

Dimensiorul stability of paper, 863-869
effect in printing, 1176 1184
of dnring, 863,Ji67
of moisture, 1176-1181
of prestraining, 822
of relative humidity. 1176-1181
grades requinng. 863
of lamiirated papers, 1219
of resin-treated laminales. 1262
Dimethylolurea (see abo Urea formalde-
kyde resins)

Dimethylolurea, preparatkw, 1339
use with polyvinyl alcoliol, 1030
Direct dyestuffs, use in pigment coating,

1070

Direct process, 1199-1200
Direct process papers, 1199-1200
curl, 869

Dirt in paper (see also Specks tn paper)
Dirt in paper, 859-861

raicrosco|w examinatirm, 97/-9/9
in pulp, determmatjon, 8^0
effect on brightness. 895
specks, in coating rawstock, 1089
Dispersing agents for clay,

Distribution hnictions of standard

sco'er, 8ffi-883. 884-885

Dominant wavclaigth, definition. 883-»a

meaning, 881

,w lat« 512-K (See But^-strem

copolynuri, laticej oj)
ragoos blood, printing, 1135
raw, effect on marine coating. 1013
rawing paper, finn^ 903 .

riers, use in printing uucSt 1 * 4/—J l *
rying. <rf coated papers, effert on w-gw

1025 . , ..

effect of water retentitem of coating
mixtuTc, 1083
madiine coating
of corrugated board, 1223, li.4
of gunn^ papers. I2M
of ink (« Prisiting mk. dryv^
of madiine coated papers. I09/-I«»

SUBJECT INDEX

1371

of paper, curl controlled in, 863, 867
effect on briglitness, 89.^

dimensional stability, 86 /

light stability, 94^947
strength. 938

stress-strain characteristics,

stretch, 834

tensile and bursting strength, bM
moisture content in offset printing,
1178-1179 _ ,

moisture determination,
shrinkage during, effect on bursting
strength, 834

of pigment-coated papers, 1094-10^3,
1097-1098

effect of solids content of coating
mixture, 1071

of printed papers, 1186^11^
of printing inks (see also PnHbH<7 n'J.
drvift^ of), 1142—1148, 1163, 11 > .
ir68,‘1174-1175, 1183

of pulp, effect on 7 'f,!';"; ,95

of resin-coated paper. 1^284 1-85, l^yo.

1297-1298, 1304-1305, 1306-1307
of resin saturated paper, 1251, 1253-1254
of resin-treated papers, 1266
of sealing tape, 1236
Dry waxing of paper (see IVaxing)
Duplicator papers, color of ink used, 1151
properties, 1195

Duplicator processes, 1192-1200
Dvestuffs, absorption of light. 872-873
effect on brightness, 892-893, 928
on K/S values, 927-9^
on light absorption, 870
on opacity, 906, 911, 912-913, 927-

928

on permanence of paper, 949
on resin-treated paper base laminates,
1264

opacifying effect, 912-913

sp€cks in paper, microscopic examina-

tion, 977, 978

use in carbon paper manufacture, IJI/

1318

use in duplicators, 1194, 1195

use in pigment coating, 1070

use in stains for fiber analysis, 972-976

East India resins, description. 1.325
Edge tearing resistance, 838-839
Eggshell finish, use in printing papers,

1152 . .

Electrical papers (see also Coil rcutaiMf/
paper. Condenser papers. Insuhltug

papers)

Electrical papers, ash, 954
effect of porosity, 849
moisture content, 954

of calcium car-

properties, 955-957
rec|uirements, 930—9ol
resin-treated. 1258
rcsiiV'trcatcd laminates,

tests, 954-955
types, 955-957
uses, 950, 955-957

electron micrographs,

bonate, 1052, 1054
of clay, 1043
of satin white, 1058

Electron microscope
electron)

Electrostatic charge, on clay, 1049
on fibers, effect on penetration ot pig¬
ment coating adhesive, 1094
effect on resin application at beater,

1248-1250 , ^

on resin emulsions, effect on beater
application, 124^1250

(see Microscope,

1138

Elemi resin, description, 1326 ^

Elmendorf tearing tester, description.

835-836 . ,

Emulsifying agents, use for resin emul¬
sions, 1301-1303

Emulsions (see also Latices, P^offtn,
emulsions, Resin cmulstons, Restns,
latices of, and Rubber, late.v of)
Emulsions, acrylic resin. 1336
coating with, 1300-1307
coumarone-indene resin, 1344
polyvinylacetate. 1332 _

poljrvinyl chloride-acetate resin, 1333
preparation, 1300—1307
rosin derivative. 1323
Emulsion type adhesives (see also Restn
emulsion adhesives)

Emulsion type adhesives, description,

1236-1239
acrylic resin, 1336
polyvinyl acetate, 1332
polyvinyl chloride-acetate resin, 1333
English clay (see Clay)

English finish, definition, 903
use in printing papers, 1152, 1153
Engraving (see also Printing processes,
intaglio)

Engraving, 1166-1167
moistening of paper, 1158
Enzyme (s'), adsorption on clay, 1023-1024
inactivation, 1022-1023
starch conversion for pigment coating.

1022-1024

types for converting starch. 1022
Envelope papers, properties, 799
Ester gum, properties, 1343
Ethoxyl content of ethylcellulose, 1329-
1330

1372

SUBJECT INDEX

Ethylcellulose, coating of paper, 1314-
1315

emulsions from, 1305
hot-melt, 1244
adhesives from, 1244
coatings from, 1315
lacquers from, 1292-1293
properties, 1329-1331
softening temperature, 1274
solvents for, 1292-1293
use in nitrocellulose lacquers, 1292
use with paraffin for waxing, 1314-
1315, 1331

Ethylene dichloride, use in aniline print¬
ing ink, 1163

Excitation purity, 885-886

Fade-Ometers, measuring permanence of
paper, 945

Feculose, pigment coating adhesive, 1021

Felt side, alignment of fibers, 802
characteristics, 800-801
fiber composition, 861, 862
oil absorbency, 1157
picking during printing, 1157
printing of paper, 1151
sizing, 862

stren^h of coating applied, 1109
surface bonding of fibers, 1157

Festoon drying of pigment-coated papers,
1097

Fiber(s), adhesion of resins, 1253-1254
deposition of resins in Bardoc process,
1248-1249

dielectric strength, 952
dispersion in fiber analysis, 959-961
specific inductive capacitance, 952
Graflf “C” stain, 967-969
Herzberg stain, 965-966
miscellaneous stains, 976
statistical analysis, 987-988, 992
strength of, effect on tearing resistance,

836-837

zero span tensile strength for meas¬
uring, 829
swelling, 863

Fiber analysis (microscopic), 959-965
stains, 959-962, 975-976
weight factors, 963-965

Fiberboard, fiber length, 850-851
flexural resistance, 825
impact strength, 843
manufacture, 1247
modulus of rupture, 814
properties, 850

Fiber bonding, amount in plastics rein¬
forced with pulp, 1270
effect of pressing on resin-treated
papers, 1247

of resins applied at beater, 1246-1247
on bursting strength, 835

on Kubelka and Munk K and 5" val¬
ues, 925

on opacity, 909-910
on softness, 844
on tearing resistance, 837
on tensile strength, 828-829
in coating raw stock, 1089
measuring by zero span tensile strength,
830

printing papers, 1157
Fiber contact, effect on opacity, 909-910
Fiber counting, statistical analysis, 963
Fiber dimensions, effect on opacity and
specific surface, 914—915
Fiber length, effect on bursting strength,
835

effect on fiberboards, 850-851
on impact strength of fiberboards, 843
on resin-treated papers, 1259, 1260
on sheet strength, 814
on tearing resistance, 837-838
on tensile strength, 829
on two-sidedness, 862
standard deviation, 987-988
statistical analysis, 987-988, 992
Fiber orientation, effect on bending qual¬
ities of boxboard, 841
effect on cable wrapping paper, 956-957
on machine and cross direction ten¬
sile strength, 828

on printing, 1177
on stress-strain of paper, 822
measuring by zero span tensile test,
830

variables affecting, 802-803
Filled papers, ash content, 939
coating to supplement filling, 1109
grit in, difficulties in offset printing,
1176

disadvantage in gravure printing, 1169
effect on offset printing, 1192
S value calculation, 927
Fillers, amount in coated papers, 939
amount in coating rawstock, 1087-1088
in hectograph papers, 1195
in mimeograph paper, 1194
in printing papers, 1148
in spirit duplicator papers, 1196
detection in paper, 940
effect on coating rawstock, 1088
effect on permanence of paper, 949
impurities, 940-941
refractive index, determination, 941
S value calculation, 927 _
types giving highest opacity, 911
use in coating rawstock, 1089
use in printing ink, 1140
Filling, effect on acidity of paper, 944
effect on air resistance, 849
on brightness, 893
on curl, 866, 867
on density of paper, 814

SUBJECT INDEX

1373

. on Kubelka and
on moisture in paper, 935-930
on opacity. 910-912,
on light absorption, S/o-o/J
on permanence of paper^94y
on printing papers, 115/
on smoothness, 855
luminescent pigments for, lUO-
opacity obtained compared to coating,

911

titanium dioxide extended pigment for,

1057

two-sidedness of, 862 .

Fill-up in printing (see
Filter paper, ash content, 939
copper number,^931
manufacture, 939
porosity, 850

Fines, distribution in sheet, 861, 8o^
effect on opacity, 915
effect on properties of paper, oo^
loss through wire, 861
Finish of paper, 902-903
coating rawstock, 1088
condenser paper, 955
effect on bulk, 846
on light reflectance, 871
on printing, 1152-1155, 11/5

on resin-treated paper base laminates,

1264

mimeograph paper, 1194
newsprint, 1165

types. 903 iiwiKt

types used for printing, 115^-1133

Fish glue, adhesives from, 1233

Fish paper, 957 . „ r /

Flax fibers, stains especially for (.see
fiber analysis, microscopic')

Flexalyn resin (see Rosin, derivatives oj)
Flexibility of paper, effect on softness,
844

relation to folding endurance, 8^
Flexibility of resin coatings, 1273-1274,
1286, 1287, 1291-1292
Flexural rigidity of paperboard, calcu¬
lation, 823

Flint glazing, description, 1101
Flocculation, of clay, 1050
of coating mixture, effect on flow, 10/5
of pigment, effect on properties of pig¬
ment-coated paper, 1121-1122

of titanium dioxide, 1056

Flock coated papers, 1130 noo

Flour bag papers, requirements, 112o-li4y
Fluorescence (see also Luminescence)
Fluorescence of semichemical pulp, 896
Foam, in coating mixtures, causes and
types, 1085-1086
spots in paper, identification, 9/8
Foaming, effect of resins applied at beater,

1246

of coating mixture, 1085-1087

Foils, metal, laminating, 1206. 1-11, 1237

metal, water-vapor transmission, 1

Folding endurance, 839-843
direct process papers, 14UU
effect of heat on stability of, 945
of moisture in paper, y»5o—Vo
of solid fraction, 813
significance, 841
uniformity, 858
variability, 858
variables affecting, 841
Food papers, requirements, MJ
Food wrappers, polyethylene foi coating,

1297

waxed, 1309, 1313^1315
Formaldehyde, casein coatings tieate

with, 1019

urea resins from, 1339
use in animal glue adhesives, 1^33
in casein glues, 1232

in washable wallpaper, 1125

Formation, definition, 856-857
effect on coating rawstock, 109U
effect on folding endurance, 841
measurement, 856-857
two-sidedness, 861

wild, 856 . . - ryt

Fountain solution in printing (see 1 lano-
graphic printing and Printing proc¬
esses, offset)

Freeness, correlation with brightness of
paper, 925

French chalk, 1053 c- • \

Frequency distribution (see^ Stahstics)
Friction calender, description and use,
1100-1101

Friction glazed papers, ink receptivity,
1161

w'ax emulsions used in, 10/0
Fruit wrap, oil-treated, 1257
Fuzz, effect on printing. 1176-1187
printing papers, 1157

Gas, penetration into paper, 1276
permeability of resin coated papers,
1275-1276, 1297

Gelatin, saturation of paper, 1258
use in duplicators, 1194
use in printing plates, 1167
Gelatin duplicators, description, 1194-1195
General Electric brightness tester, descrip-
• tion, 888-889

General Foods Cabinet, vvater-vapor re¬
sistance measurement in, 1281
Georgia clay (see Clay)

Glare, relation to gloss, 898, 902
Glassine, brightness, 894
coating with resin, 1271, 1300
density, 811, 812

effect of dyestuffs on transparency, 913
hot-melt coating, 1308

1374

SUBJECT INDEX

Glassilie {could.)
laminating, 1211, 1216
light transmission, 904
porosity, 1211

resin-coated, watcr-vapor resistance.
1291

specific volume, 811
waxing, 1311

Glazed finish, definition, 903
Gloss (see also Contrast gloss)

Gloss, 898-902
measurement, 898-901
of pigment-coated papers (see Pig-
ment-coated papers, gloss)
of resin-coated papers (sec Resin-
coated papers)
of waxed papers, 1311
variables, 899-900

Glossy paper, reflectance of light, 871,
898-902

Glucose (see also Corn syrup)

Glucose, effect on color stability of
groundwood pulp, 947
Glue (see Adhesives)

Gluing of paper (see Laminating of
paper)

Gluten (see Corn gluten)

Glycerol, alkyd resins from, 1341-1342
effect on moisture in paper, 936
Graff “C” stain (see also Fiber analysis)
Graff “C” stain, description and use, 966-
969

Gravure printing (see also Printing proc¬
esses, intaglio)

Gravure printing, 1168
Greaseproof paper, air volume of, 847
effect of moisture, 864, 866
Grease resistance, of waxed papers, 1313
Grease spots in paper, identification, 977,
979

Green liquor, calcium carbonate coating
pigment from, 1051

Ground coating for wallpaper, 1124-1125
Groundwood fibers, analysis of, special
treatment necessary, 963
stains for, 962

Groundwood papers, brightness, effect of
/>H, 948
expansion, 864
folding endurance, 841
light absorption, 946
moisture absorption, 935
odors, 950

permanence, 945-947
smoothness, 855-856
specific volume, 811
Groundwood pulp, brightness, 895
coating rawstock from, 1089
dispersion in fiber analysis, 960
dominant w'avelength, 896
effect on brightness of sulfite pulp, 896-
898

on Kubclka and Munk S value, 924
on opacity, 915-916
fiber analysis (see Fiber analysis)
fiberboards from, 850
function in newsprint, 1165
heat stability, 946, 947
opacity, 914-915
plastics from, 1269
spectrophotomctric curve, 895-896
stains especially for, 975-976
use in coating rawstock, 1087
use in printing papers, 1148
sveight factor, 964
Glycerine (see Glycerol)

Gum arabic, adhesives from, 1234
coating adhesive, 1124
spraying of sheets during printing to
prevent oflFset, 1186-1187
stabilizing satin wliitc, 1058
use in planographic printing, 1170 1172
Gummed labels, printing of, 1181
Gummed papers, adhesives for, 1233
curl, 869

manufacture, 1234-1236
Gum (see also Adhesives)

Gum (vegetable), adhesives from, 12.34
Gum (wood), solid fraction of papers
from, 813

Gurley' densometer, air resistance of paper
by, 848

Gurley softness tester, description, 844-

845

Gurley smoothness tester, description,
852-853

Gurley S-P-S tester, air resistance of
paper by, 848

smoothness of paper wdth, 852-853
Gurley stiffness tester, description, 824
Gypsum (see Calcium sulfate)

Hacker proofpress, 1148-1150
Halftone, dot density, 1136
dot dimensions, 1114, 1135
reproduction (see Printing, halftone re¬
production)

Halloysite, eflfect of heating (see also
Clay, effect of heating), 1039, 1042
Handling of paper, effect on acidity, 944
Handling properties of paper, 797
Hanging paper, basic ream size, 805
composition, 1126
delayed flow of, 817
weights, 1126

Hardness, of paper, 844-846
of pulp, 844

of resin coatings, 1274, 1290
Hardy spectrophotometer, description,

878-880

Head box, secondary, 863
Heating, effect on paper, 945

SUBJECT INDEX

1375

Heat-seal adhesives (see also Adhesives

HeatludhSwS 1203-1204, 1275,1297,
1304-1305

ethylcellulose, 1244

resin emulsion adhesives for, l^ioo

polyethylene, 1297

types, 1205 i o •

Heat-seal coated papers (see also

coated papers), 1275, 1297, 1304-1305,

1313

Heat-seal waxed papers, 1313, 1316 ■

Heat tests for measuring stability ot
paper, 945

Heat welding, 1205 .

Hectograph duplicators, description, 1194-

1195 . ,

Hcmicelluloses, effect on electrical prop¬
erties, 956

effect on plastics reinforced with pulp,
1269

on staining in fiber analysis, 965
on stiffness of paper, 826
on yellowing tendency of bleached

pulp, 947

Hemp, stains especially for (see Fiber
analysis, microscopic)

Hercolyn resin (see also Rosin, deriva¬
tives of)

Hercolyn resins, adhesives from, 1238,
1245

Hercules Hi-Shear viscometer, descrip¬
tion, 1078-1079

Herzberg stain (see also Fiber analysis)
Herzberg stain, description and use in
fiber analysis, 965
Hot melts (see also Resins)

Hot melts, adhesives, 1240-1245
creep, 1215

migration of film, 1241
resistance to water-vapor, 1240-1241
resins, 1243-1245
setting, 1213
temperature, 1241
coating, 1307-1318
cellulose esters for, 1328-1329
ethylcellulose, 1329-1331
temperature of application, 1307, 1310,
1315, 1316, 1317
types, 1307
Hue, definition, 876

Humidity (see also Moisture and Press¬
room, relative humidity of)
Humidity, effect on basis weight, 808
effect on cable wrapping paper, 957
on curl, 868

on expansion of paper, 864-866, 868,
1176-1181

on moisture in paper, 932-935, 936-
937

on permanence of paper, 947-948
on printing, 1177-1181, 1183

on strength of paper, 93^937
on stress-strain of paper, 818
on water-vapor resistance of resin-
coated papers, 1278—1281
Hunter glossmeter, 899-900
Hunter reflectometer, description, 88y-b9U
gloss measurement, 899-900
Hydration (see Beating)

Hydration, of clay, 1041-1042

Hydroxyethylcellulose, pigment coatm

M'ith, 1031
preparation, 1031

Hygrometer for measuring moisture iii

paper, 1159 .

Hysteresis in moisture sorption by paper,

932-933, 1178-1180

lllitc, excliangc capacity, 1041
properties, 1035
Illumination, definition, 870
for color measurement, 876-877
Illuminants, matching colors under dif¬
ferent, 874-875

Ilmenite ore, titanium dioxide from, 105 d
I mpact strength of paper, 843
Impregnants for paper, effect on dielectric
constant, 951-952

Impregnated papers (see also Resin-satu¬
rated papers and Resin-treated
papers)

Impregnated papers, dielectric strength,

953

electrical, 951, 952, 955
fiber analysis of, 960

Impregnating of paper (see Saturation
of paper)

Impregnating papers, formation, 857
requirements, 930-931
Index of refraction (see Refractive index)
India resins, description, 1325
Indicator powder, water retention of coat¬
ing mixtures measured with, 1083
Infusorial earth (see Diatomaceous silica)
Ingersoll glarimeter, description, 898-899
Ink, printing (see Printing ink)

Ink receptivity (see also Oil absorbency
and Oil resistance)

Ink receptivity, effect on printed opacity,
907

effect on printing, 1155-1157
of coated papers, effect of weight of
coating, 1110, 1111
measurement, 1112-1113
variables affecting, 1112-1114, 1161
requirements for offset printing, 1175
testing, 1155-1156

Insulating boards, coated, drying, 1097
effect of drying on color, 946
Insulating papers (see also Electrical
papers)

1376

SUBJECT INDEX

Insulating papers, porosity, 850-551
Insulating tissue (see also Electrical
papers)

Insulating tissue, description, 956
Intaglio printing (see Printing processes,
intaglio )

Ion adsorption, on clay, 1039-1041
Ion exchange, of clay, 1039-1041
Iodine stains (see also Fiber analysis,
microscopic)

Iron, effect on paper, 948
in clay, 1036
in satin white, 1058

Iron specks in paper, microscopic exami¬
nation, 977-979

Isobutene polymers, properties, 1350
Isobutylene-styrene copolymers, proper¬
ties, 1349-1350

Jute board, printing, 1162
Jute paper, porosity, 1211

K and N test for measuring ink receptiv¬
ity, 1112-1113, 1155

Kantrowitz-Simmons stain, description
and use in fiber analysis, 973
Kaolin (see Clay)

Kaolinite, formula, 1035
Kauri resin, description, 1325-1326
softening temperature, 1274
Kerosene test for roofing felt, 1255-1256
Ketone-aldehyde resins, use in starch cor¬
rugating adhesive, 1226
Kieselguhr (see Diatomaceous silica)
Knife coaters, description, 1281-1282, 1297
types used for resin coating, 1281-1282
Kollergang, effect on light scattering, 925
effect on tensile strength of spruce

pulp, 830

Kraft paper (see also Sulfate papers and
Sulfate pulp)

Kraft paper, porosity, 1211
sandpaper, 1129

Kubelka and Munk K value, calculation,

923 926

factors affecting, 916, 923
Kubelka and Munk K/S value, determi¬
nation for pulp mixture, 917-923
relation to brightness, 917-923
Kubelka and Munk S' value, calculation,

923, 926

factors affecting, 916, 923
relation to brightness, opacity, and hght
reflectance, 922, ^7, 929-930
relation to opacity, 909
Kubelka and Munk theory, 916-917, 9^.9-

930

Label pai»crs, coated (sec PigmcniHoalcd
papers)
curl. 869
cutting, 803-804
heat sealing, 1241
pebbling, 826
printing, 1173
proi)erties desired, 1235
water-vapor resistance, 1299
lacquer adhesives, curl obtained, 1238-
1239

description, 1238-1240

Lacquered papers (see also Resin-coakd
papers)

Lacquered papers, laminating, 1238
Lacquers (see also Resin coating, lacquer
and Varnishes)

Lacquers, composition, 1283-1300
cosolvents, 1285, 1291
diluents, 1285

effect of solvent on properties, 1284-
1285

flow properties, 1294-1295

modifiers, 1287, 1300

plasticizers, 1285-1287,1288, 1290,1291-

1707 1701

resins used, 1287-1299

solvents used, 1284-1285, 1286-1300

uses, 1283

Lake colors (see Color lakes)

Laminated paper, adhesive film in, prop¬
erties, 1214-1215
asphalt, bleeding of, 1243
curling, 1215-1216
effect of adhesive, 1219
dimensional stability of, effect of ad¬
hesive, 1219
moisture in, 1219
plies used, 1220
reinforcement with fibers, 1243
strength of bond, 1212, 1213, 1214-1215
effect of hot melts, 1241-1242
effect of thickness of adhesive film,

1214

weatherproof, asphalt adhesive for, 1243
Laminated papertward (see Laminated

Laminates (see Paper laminates, restn-

treated ) . aj

Laminating (see also Adhesives, Ad¬
hesion, Corrugating, Bag papers,
pasting of. Heat-seal adhesives and
Laminating of paper)

Laminating, hot-melt, adhesives for, 124D-

1245

hot-melt, temperature, 1241, 1243
machines for, 1201—1202, 1212, 1218-
1219, 1243

factors affecting curl, 1216
glue stations, 1220

SUBJECT INDEX

1377

of acetate sheeting, 1^236
of cellophane, 1206, 1210, 1238
of metal foils, 1237 _ i j, t, \

of paper (see Lamnattng of paper)
paper properties, effect, 1210 1211
of plastic films, 1206
pressure applied, 1212

with hot-melt adhesives, 1241
setting rate, effect of air draw, 1212,

1213, 1214 ,017 1998

starch adhesives for, 1217-l^o
surface involved, 1206, 1210—1211 _
Laminating of paper (see also Laminated
t>ai>er and Laminating) ■

Laminating of paper, 1201-1202,1218-1228

effect of porosity, 1207, 1210-1211, 1212,

1214, 1227, 1241-1242

of smoothness, and temperature, 12lU

1211

moisture added, 1219
moisture and curl, 1216
glue line, 1214

and adhesive consumption, 1219-122U
hot-melt adhesives for, 1240-1242
pressure used, 1218
sodium silicate adhesives, 1230-1231
solid fiberboard, 1201
water-resistant starch adhesives, tem¬
perature effect, 1221
Lampblack, use in printing ink, 1140
Lampen mill, effect on light scattering,

925 „ .

Latex (see Rubber, latex of, and Resins,

latices of) , , d i

Latices (see Emulsions, Resins, and Rub-

ber)

Laughlin stain, description and use in
fiber analysis, 971-972 .

Leather, artificial, amount of resin used,

1251

artificial, manufacture, 1246-1247
ground, use in saturating papers, 1251
Ledger papers, folding endurance, 841, 842
Letterpress duplicators, description, 1197
Letterpress printing (see Printing proc¬
esses, relief)

Leveling index of pigment coating mix¬
ture, 1078-1079

importance in roll coating, 1096
Light (see also Color)

Light, absorption by dyestuffs and pig¬
ments, 872-873

• absorption by paper, 870-871, 946
effect on pH of paper, 948
nature, 870

reflectance (see also Reflectance and
Brightness)

reflectance, 871-872, 898-902
effect of dyestuffs, 927-928
on gloss, 902

on opacity,

opacity measurement by, 904-90/
standards for, 872
scattering of, effect of

effect of titanium dioxide, 911
source of, effect on opacity, 912-913
spectral energy distribution, 874-«/5
transmission of, opacity measurement

by, 907—908
types, 903—904
wavelength of, 870
effect on color matching, 875
on fading of paper, 946
on light scattering and absorption,

on opacity, 912-913
most detrimental, 947
trichromatic coefficients and distn-
bution functions, 882-883, 884-885

Lignin, beater application, 1260
effect of light, 946 .

in paper, effect on light absorption, 870
effect on moisture absorption, 933, 935
on permanence, 946
on power factor loss, 953
in pulp, effect on adsorption of dye¬
stuffs in staining, 972-973
effect on brightness, 895
on electrical properties, 956
on plastics reinforced with pulp,

1269

on staining in fiber analysis, 965
plastics from, 1260, 1261
pulp enrichment for plastic manufac¬
ture, 1269

Lime, calcium carbonate manufacture,
1051-1052

making satin white, 1058
use in casein glues, 1232

Linen pulps, condenser tissue from, 955-
956

Line reproduction (see also Printing),
1134-1135

Linseed oil, impregnated electrical papers,

955

printing ink from, 1139, 1141, 1142, 1143
saturation of paper, 1257-1258
types, 1142

varnishes from, use in printing ink, 1143

Lithographic duplicators, description,
1196-1197

Lithographic printing (see also Printing
processes, offset and Planographic
printing)

Lithographic printing, description, 1169-
1173

Litho papers, 1175
ash content, 939

coated (see Pigment-coated papers)
Lithopone, manufacture and properties,
1059

1378

SURTECT IXDEX

Lithoprints, dcscriptiun. 1200
Lofton-Mcrritt stain, description and use
in fiber analysis, 973-974
Luminescence, pigments having, 1062
types, 1062

zinc sulfide pigments having, 1059
Luminous flux, definition, 870
Luster, relation to gloss, 898
Lustrex, 1335

Machine coating (see Pigment coating)
Machine direction, air resistance, 849
curl alignment, 868
effect in printing, 1177
effect on properties, 801-804
expansion of paper, 863, 864, 868
folding endurance, 840
stress-strain characteristics, 821, 822
tearing resistance, 836
tensile strength, 828
testing, 803

Machine finish, definition, 903
printing papers, 1152
Machine speed, effect on two-sidedness,
861

MacMichael viscometer, description,

1076-1077

values desired for brush coating, 1080
Magnesium carbonate, effect of ignition on
weight, 942

reflectance standard from, 872
Magnesium oxide, reflectance standard
from, 872, 876, 905
Makeready for printing, 1138-1139
effect of thiclmess of paper, 1151
Malachite green reagent, description and
use in fiber analysis, 975
Maleic resins, properties, 1343
use in vapor-set printing ink, 1147
Manila hemp, sandpaper, 1129
use in cable wrapping paper, 957
Manila papers, stretch, 832
Manila resin, adhesives from, 1240
description, 1325
softening temperature, 1274
Map papers, expansion, 863
pasting, 1239

Martinson coater, description, 1103
Masking tapes, adhesives, 1215
Mayer coater, description, 1013, 1282
Melamine-fonnaldeh 5 'de resin, in Bardoc
process, 1248-1249
laminates from, 1262, 1266, 1268
properties, 1341
Mersize, manufacture. 1343
Metal foils (see Foils, metal)

Metallic coated papers, 1130

Metallic pigments, use in printing inks,

1140

Methacrylic resins (see Acrylic resins)
Methylcelhilose, blending with other ad-

hcNivts in pigment roating, lO.U
pigmait c<iatmg mth, lO.H)
u>e in rc>in ••muKiMns, l.Vi2
Methylene Iduc. effect <>n <*paiity, 912 913
Mezzotint printing. 1166
Mica coated papers, 11.10
Microjet coater, description, 1014
laboratory’ model. 1103
Microcrystalline wax, adhesives from.
1242

properties, 1312-1313
use in resin coatings, 1300
waxing of paper, 1313-1314
Microscope, examination of fillers, 941-
942

electron, description, 958
optical, description, 958
paper examination with, 957-980
smoothness measuremait with, 851
types, 957-959

Microscopical analysis^ fibers (see Fiber
analysis, microscopic)

Microtome, paper sectioning w'ith, 979
Mimeograph ink, description, 1194
Mimeograph paper, bursting strength, 832
dirt count, 860
gloss, 902
properties, 1194

testing printing qualities, 1164
\’ariability, 858

Mimeograph process, description, 1193-
1194

Mineral oil, properties, 1257
saturation of paper, 1256
Misregister in printing (see also Print¬
ing), 1189

M. I. T. folding tester, description, 839-
840

variability of results, 842-843
Modifiers, for lacquers, 1287, 1300
Modulus of elasticity, of plastics rein¬
forced witli pulp, 1269, 1270
Moisture (see also Humidity)

Moisture, in clay, 1037
in fibers, effect on staining in fiber an¬
alysis, 965
in newsprint, 1165
in paper, determination, 932
effect of fillers, 935-936
of humidity, 932-935
of printing, 1178-1181
of temperature, 933-934
on curl, 868

on electrical properties, 954
on folding endurance, 841
on expansion, 864-866, 868
on laminating, 1210
on misregister in printing, 1189
on permanence. 947-948
on printing. 1158-1159, 1176, 1181-
1184, 1189

on properties, 932-936, 938

SUBJECT

INDEX

1379

on resin-treated paper base laini-

on saturating.
on smoothness, o55
on stiffness. 8^ oso c'>i

on stress-strain. 818.
on tearing resistance, Ovto
ffluilibrium. 1177-1181
for waxing. 1310, 1311

hysteresis in.absorption of. effect

offset printing, 11/^1180
measurement, 11^8-1159
rate of sorption. 934-9^ .

values desired for offset printing.

1178-1181

resistance to (see II ater-iutpor resist-
09u:c)

vapor resistance (sec H ater-vapor re¬
sistance)

Molding of resin-treated papers, l-oO-

1268 1270

Montmorillonite (see Beii/om/c)

Mottling in printing (see Printing)
Mullen tester (sec also Bursting strength)
Mullen tester, calibration, 832
description, 831-832

modification for measuring
strength. 833

Multi-ply papers, determiiiiiig
and cross direction of, 803
ratio of machine to cross direction,
ratio of machine and cross direction
stiffness, 825-826
two-sidedness, 862-863
Mtinsell svstem, color expression l>y, 8/5-
876

Multigraph, description, 1197

tensile

machine

Naphtlialcnesulfonic acid-fornialdchydc,

use in latices, 1250

Xeocarminc stain, description and use for
filler analysis. 976

Neolyn resin (sec also Rosin, derivatives
of, and Alkyd resins)

Neolyn resin, adhesives from, 1238
hot-nielt a^csivc, 1245
Neoprene (sec Polychloroprene)

News ink (see Printing ink, nnvs)
Newsprint, ash content, 924
basic ream size, 805
brightness, 924
composition. 1165
effect of humidity, 864, 865, 866
ink film thickness in printing, 1141-1142
moisture sorption, 933, 934, 935
offset printing, 1175
oil alisorption required in printing, 1164-

1165

opacity, 915-916
performance, 817

printing, 1163-1166
properties, 1164-1166 .

reejuirements,
scattering coefficient, 9-4
smootlmess, 855
stretch, 832
tensile strengtlt, 829
testing, 1164-1165

[Newtonian flow, defimtion, 10/3, 10/-t
Nicol prism. Hardy spectrophotometer,

gyolggO

Ingersoll glarimeter, 898-899
Nicley process, resin latex stabilization,

1254

Nitrocellulose, adhesives from, 1239
description, 1326-1327
emulsions from, 1306 „

lacquers from, 1288, 1-89, 1-90 129-
oroperties of films, 1306
solvents. 1285, 1290-1291
types. 1290-1291

use in polyvinyl acetate lacquers, 1293
Non-tarnishing papers (see Anti-tannsh
papers)

_ - f • * A

Ochre, effect on brightness, 893
Odors, absorption by paper, 949
in oil-impregnated papers, 1257
in paper, detection, 949-950
sources, 950

in paraffin, 1310 no/- 1107

Offset duplicators, description, 1196-119/

Offset in printing (see

Offset jiapers (see Printing papers)

Offset printing (see Printing processes,

offset)

Oils, alkyd resins from, 1342
compatibility with polyvinyl chloride-
acetate resin, 1333

drying types, use in printing, 1142-1143
effect on pigment coating mixture, 1085
in microcrystalline wax, 1313
in paraffin, 1309

use in carbon paper manufacture, 1317-
1318

use in news ink, 1164
use in printing ink, 1142-1143, 1144
Oil absorbency (sec also Oil penetration.
Oil resistance, and Ink receptivity)

Oil absorbency, effect of calendering, 1154
effect of moisture, 938
meaning, 1155
of newsprint, 1164-1165
testing, 1156
tw'o-sidedness, 1157

Oil iienetration (see also Oil absorbency,
Oil resistance, and Ink resistance)
Oil penetration, coated papers, effect of
adhesive, 1024

1380

SUBJECT INDEX

Oil penetration {contd.)
effect of calendering, 1154
meaning, 1155
of roofing felt, 1255-1256
Oil resistance, effect of porosity, 849
relation to air resistance, 849
Oil spots, in coated papers, 1116
Oil-treated papers, 1256-1257

Olefin polysulfide polymers, properties,
1 oou

Oleic acid, saturation of paper, 1258

use in carbon paper manufacture, 1317-
1318

use in preparation of resin emulsions,
1301, 1302

Oleoresinous varnish (see Vatnishes,
oleoresinous) ’

Opacity of paper (see also Contrast ratio,

TAP PI opacity, and Printing opac¬
ity)

Opacity of paper, blueprint process, 1199
coated, effect of weight of coating, 1110,

nil

coating rawstock, 1089
definition, 903

effect of dyestuffs, 906, 911, 912-913,
927-928

of fillers, 910-912, 1158
of oil treatment, 1257
on printing, 1157-1158
on show through in printing, 1186
for intaglio printing, 1169
measurement, 905-908
newsprint, 915-916

relation to brightness, scattering power,
and light reflection, 922, 927, 929-
930

variability, 858
variables affecting, 908-916
Optical properties of paper, for photo¬
copying processes, 1200
Organosols, preparation and use, 1306-
1307,1333

Overprinting (see also Varnishing, Resin
coating, lacgncr and Lacquers)
Overprinting, inks used, 1144
of paperboard, 1162

Oxygen, effect on permanence of paper,
946

Ozokerite, use in carbon paper manufac¬
ture, 1318

Palletizing, adhesives for, 1215
Paper, absorption of light, 870-871
acidity of, determination, 943
source, 944

aging (see Aging of Paper)
air content of, effect on dielectric con¬
stant, 951

effect on opacity, 910
air volume, 811-812, 846, 847

ash content (see also Ash content, of
paper)

ash content, 938-939, 943
basis weight (see Basis weight)
brightness (see Brightness)
carbon (see Carbon papers)
chemical properties, 930-950

effect on electrical properties, 954-
955

effect on permanence 944-949
color of (see Color of paper)
conditioning, 799-800, 808
effect on dimensions, 864-866
effect on moisture content, 932-935
copper number, 931

cross direction (see Cross direction of
paper)
defects, 861

effect on resin-treated paper base
laminates, 1263-1264
density (see Density of paper)
dielectric constant, effect of density, 954
value, 951, 952

dielectric loss (see Paper, Poiver factor)
dielectric strength, 952-953
effect of density, 955
effect of thickness, 955
dimensional stability (see Dimensional
stability of paper)
dirt in, definition, 859
determination, 859-^60
microscopic analysis, 977-979
distinguishing between felt and wire
sides, 800-801

electrical properties, 950-957
felt side (see Felt side)
fiber analysis (see Fiber analysis, micro¬
scopic)

fiber orientation (ste Fiber orientation)
filled (see Filled papers)
formation (see Formation)
high alpha, 931

index of refraction, effect on gloss, 898
light reflectance (see Light, reflectance
and Reflectance)

machine direction (see Machine direc¬
tion of paper)

microscopic analysis (see also Fiber
analysis, microscopic)
microscopical analysis, 957-980
moisture content (see also Moisture in
paper)

moisture content pf, 932-938
moisture sorption, effect on dimensions.

864-866, 868
odor, absorption, 949
sources, 950’

opacity (see Opacity of paper)
optical properties, 869-930
factors affecting, 869
paraffin content, 931-932
permanence of (see Permanence)

SUBJECT INDEX

1381

pH, determination, 943
effect on Permanence 948
effect on printing, 115^11/0
effect on resin-treated paper base
laminates, 1264
physical properties, 797-869
pore volume, 846-847 , . .

porosity (see Porosity of paper)
power factor, 953

effect of acid washing of sulfate pulp.

956

effect of temperature, 956
printing (see Printing papers)
properties (see Properties of pal'ei )
qualitative analysis, 940-943
quality control, 997-1005
reducible sulfur, 930 „ .

reflectance of light (see a.ho Brightness,
Light reflectance,

reflectance of light, 871-872, 898-90-
effect on opacity, 929
relation to brightness, opacity, and
scattering coefficient, 922
sampling, 799
sectioning, 979-980

sizing agents, 931-932 , ^

smoothness (see Smoothness of paper)
specific gravity, 846
specks in, microscopic examination, v//-

979

stiffness (see Stiffness of paper)
storage, 938

strength (see Sheet strength, stress-
strain, Bursting strength, Tensile
strength, etc.)

transmission of light (sec Light, trans¬
mission of, and Opacity)
two-sidedness (see Tivo-sidedness)
uniformity, 857

unsatisfactory appearance, 859, 861

watermarking, 801^

wire side (see Wire side)

Paperboard (see also Boxboard and
Chipboard)

Paperboard, absorption of tastes and
odors, 949

bursting strength, 993-994
curl, 867

effect of overdrying, 938
finish, 903

grades used for printing, 1162
laminated (see also Laminating of paper
and Laminated paper)
laminated, effect of silicate adhesive on
weight, 1231

laminating with resin emulsion ad¬
hesives, 1236
odor, 949, 950
printing, 1162-1163, 1177
smoothness, 854
stiffness of, calculation, 823
stress-strain characteristics, 823

thermal conductivity, 850
water-vapor transmission, 1-oU, i^oi
Paper cartons, waxing, 1312
Paper laminates, resins for, lZ5y-l/ou
Paper laminates, resm-treated (see a so
Plastics, paper, Resin-coated papers,
and Resin-saturated papers)

Paper laminates, resin-treated, amount of
resin used, 1267-1268
application of resins, 1260-1-64, 1-66
cross laminating, 1266-1267

description, 845, 1258
effect of curing on sizing agents 1Z04
effect of paper properties, 1263-1 Zoo
equipment for making, 1260-1261
manufacture, 1260-1268

molding, 1259, 1260, 1267-1-68
molding, effect on water resistance,
1267-1268

preparation, 1260-1268
pressing, 1259, 1260. 1267-1268
properties, 1261, 1262
strength, 1266^1267
water absorption values, 1265, 1-6/
Paper plastics (see Plastics, paper)

Paper strength (see Sheet strength.
Bursting strength. Tensile strength.

Paper tapes, manufacture (see Sealing
tapes)

Paper testing (see also Properties of
paper)

Paper testing, 797-798
precautions necessary, 799
Paraffin (see also ff'a.r and I Taxing of
paper)

Paraffin, adhesive for bread wrap, 1203
adhesives from, 1242
strength of bond, 1215
carbon paper manufacture, 1317-1318
chlorinated, 1309

compatibility with polyisobutylene, 1242
emulsions of, 1302^
impregnated electrical papers, 955
in paper, analysis, 1312
manufacture, 1308-1309
properties, 1308-1309
specific inductive capacitance, 952
use in polyvinyl resin lacquers, 1294-
1295

use in resin coatings, 1275, 1287, 1300,
1302, 1314-1316
uses, 1309-1310

use with ethylcellulose, 1314-1315
use with rubber derivatives, 1317
waxing of paper, 1308-1312
Paraffin oil (see Mineral oil)

Parchment paper, fiber analysis of, special
treatment necessary, 960
Pasting of paper (see Laminating of
paper)

Pearl finish, properties, 1061

SUBJECT INDEX

1382

Pearl wliito, properties, 1061
Pcntaerytliritol, resins from, 1343
Permanence of paper (see also Aging of
paper)

Permanence of paper, effect of dyestuffs,
949

of fillers, 949
of heat, 945, 946
of light, 946
of 948

of relative humidity, 947-948
of lack of, 944
measurement, 892, 944
Permanent papers, pH, 944-948
precautions in using, 948
properties, 945
requirements, 930-931
Permeometer, measuring air resistance,
849

Petrolatum, description, 1312-1313
Phenol-formaldehyde resins (see Pheno¬
lic resins)

Phenolic resins, beater treatment with,
1250

emulsions of, beater application, 1259,
1260

impregnated electrical papers, 955
lacquers from, 1288
laminates, from, 1261, 1263, 1265, 1266,
1268

plastics reinforced with pulp from, 1269
properties, 1338-1339
varnishes from, use in printing ink, 1143
Phenol-modified coumarone-indene resins,
properties, 1345

Phloroglucinol stain, preparation and use,
975-976

Phosphorescence (see Luminescence)
Photocells, color measurement with, 877-
878

spectral sensitivity, 876, 877-878
Photo-copying processes (see also Blue¬
print papers, and Direct process
papers)

Photo-copying processes, description 1197-
1200

Photographic paper, ash content, 939
coating applied, amount, 1110
coating with blanc fixe, 1060
requirements, 930-931
Photograph mounting, adhesives for, 12.39
Photogravure (see Gravure printing and
Printing processes, intaglio)
Photolithography (see Planographic
printing)

Phototype printing (see Printing proc¬
esses, phofogelatin)

Photovolt brightness tester, description,

890

Phthalic anhydride, alkyd resins from,
1341-1342

Picking of ]iriniiiig inks (see Prlnfinti.
picking)

Picking, in ])rinling (see Printing, Print¬
ing papers, Pigment-coated papers,
and Dennison wax test)

Pick testers, types, 1106-1107
Pigments (see also Pigment coating, pig¬
ments for)

Pigments, effect on brightness, 893
effect on optical properties of coated
papers, 1118-1123
metallic, use in printing inks, 1140
printing ink, 1139-1141, 1143
properties desirable for pigment coating,
1034

types used in pigment coating, 1034
use in printing inks, 1143
use in resin coatings, 1287
Pigment-coated papers (see also Book
papers and Printing papers)
Pigment-coated papers, advantages for
printing, 1160

alkalies in, effect on scumming in offset
printing, 1191

amount of adhesive present, determi¬
nation, 1110

amount of coating, 1109-1111
determination, 1110
effect on optical properties, 1118-1120
amount of pigment present, determina¬
tion, 1110
ash content, 939

ash content of coating, method of de¬
termination, 943
bread wrap grades, 1128
brightness (see also Brightness, of
paper)

brightness, effect of amount of coating,
1045

effect of clay type, 1045, 1047
effect of pigment, 1047
variables affecting, 1117-1123
brush finishing, 1101
brush marks, 1115
cause, 1095

calendering (see also Calendering of
coated papers)
calendering, 1098-1102
effect of pigment, 1037, 1047, 1065,
1116-1117

cast, ink receptivity, 1161
cracking. 1161
curl, 868

defects, 1102, 1114-1116, 1160-1161
distribution of adhesive, 1090-1095
drying, 1094-1095, 1097-1098
dusting of, effect on printing, 1187
early uses, 1007

effect of added ingredients, 1069-1070
of adhesive, 1015
of blanc fixe, 1060
of butadiene-styrene latex, 1032

SUBJECT INDEX

1383

of diatomaceous silica, 1062
of drying, 1094-1095, 1097—1098
of latex adhesive, 1032
of moisture in offset printing, 118 j
of penetration of adhesive on prop¬
erties, 1092
of satin white, 1057
of soaps, 1070

of thickness of coating applied on
smoothness, 1097
of titanium dioxide, 1057
examination, 1091

finish of, effect of cast coating, 1101-
1102

variables affecting, 1098-1102
flexibility, effect of plasticizers, 1069-
1070

flint glazing of special grades, 1101
flour bag grades, 1128-1129
formula, 1069

friction glazing of special grades, 1100-
1101

gloss, 902

effect of calcium carbonate, 1053
of pigment, 1047
of weight of coating, 1110, 1111
measurement by Ingersoll Glarimeter,
899

variables affecting, 1117
grades, 1102-1103

grease spots in, effect of casein, 1017
index of refraction, 899
ink receptivity, 1161

effect of drying of coating, 1095
of pigments, 1113-1114
of sizing of rawstock, 1093-1094
of weight of coating, 1110, 1111
measurement, 1112-1113
optimum, 1112

variables affecting, 1112-1114
moisture content, effect on blackening
during calendering, 1099
effect on calendering, 1099
effect on strength, 1105
number of coatings applied, 1110
offset, 1175, 1182-1184
oil penetration, effect of adhesive, 1024
oil spots, 1116
cause, 1085

opacity, effect of weight of coating,

1110, nil

variables affecting, 1117-1123
pK, 1183

effect on varnishing, 1127
picking during printing, 1160
pick testers for, 1106-1107
pinholes in, causes, 1115
effect of pine oil, 1069
foam as a cause. 1085
pits in coating, 1115-1116
pore size average, 847
printing qualities, 1160-1162

effect of penetration of coating ad¬
hesive, 1084

properties, 1024, 1102-1103, 1182-1184
effect of calcium sulfite, 1061
of soaps, 1070

of titanium dioxide, 1054-1055
ridges in, cause in roll coating, 1096-
1097

rotogravure, 1169
scumming in printing, 1192
smoothness (see also Pigment-coated
papers, surface pattern of)
smoothness, measurement, 854

effect of calendering, 1098-1099
variables affecting, 1116-1117

special grades, 1129-1130 _

streaks in, cause in air knife process,
1096

strength (see also Pigment coating ad¬
hesives, pigment bonding strength,
Pigment coating, pigments for, ad¬
hesive demand, and Dennison war
test)

strength, 1063-1066
effect in offset printing, 1182
of adhesive penetration, 1092, 1093
of calendering, 1100
of pigment type, 1064—1066, 1067
of solids content of coating mix¬
ture, 1072-1073
of spoilage of adhesive, 1085
of starch preparation, 1022
of water retention of coating mix¬
ture, 1084

of weight of coating, 1110, 1111
on picking during printing, 1160
meaning, 1104
measurement, 1103-1107
relation to ink receptivity, 1113
two-sidedness, 1109
variables affecting, 1107-1109, 1110,

nil

varnish grades, 1127-1128
wallpaper, 1127
supercalendering, 1098-1100
surface-active agents in, effect in offset
printing, 1184

effect on scumming in printing, 1192
surface irregularities, 1115
surface pattern, cause, 1096-1097
effect of coat weight applied, 1097
thickness of coating, 1046, 1089, 1090,
1110

types used in printing, 1159-1160
varnish grades, 1126-1127
water resistance of, effect in offset
printing, 1182-1183
effect of casein, 1019
of />H of coating rawstock, 1025
of polyvinyl alcohol, 1030
of starch, 1024-1025
on scumming in printing, 1192

SUBJECT INDEX

♦

Pigment-coated papers (contd.)
measurement, 1112
requirements, 1111
waxes in, effect on printing, 1161
wax pick test (see also Dennison wax
test. Pigment-coated papers,
strength of, and Printing papers,
strength of)

wax pick test, 1104-1107, 1109
effect of different adhesives (see Pig¬
ment coating adhesives, pigment
bonding strength)

requirements for offset printing, 1182
Pigment coating, air brush, 1014, 1072
air knife, 1095-1097

amount of coating applied, 1009, 1010,
1090, 1109-1111

effect on coated paper, 1118-1120
on opacity after waxing, 914
on smoothness, 1097
knife coaters, 1013
offset rotogravure coater, 1012
anti-foams, 1070, 1086-1087
effect on oil spots, 1085
brush, discussion, 1010-1011
flow properties desired, 1072, 1079-
1080, 1095
solids content, 1072
calcium carbonate for (see also Pig¬
ment coating, pigment for)
calcium carbonate for, effect in offset
printing, 1183

effect on scumming in offset printing,

1191-1192

casein demand of different pigments,
1064, 1067-1068

casein for (see casein and Pigment coat¬
ing adhesive)
cast, description, 1101-1102
clay for (see also Clay and Pigment
coating, pigments for)
clay for, adhesive demand, 1019
effect in offset printing, 1183
coat weight (see Pigment coating,
amount of coating applied)
color for (see Pigment coatmg mix¬
ture)

color lakes for, 1063, 1125

conventional, 1008

curl during, 868

double coating application,

drying methods, 1094-1095, 1097-1098

dyestuffs used, 1070

effect of high solids content of coating

mixture on drying, 1071
on acidity of paper, 944
on gloss, 902
on light absorption, 8/4
on opacity, 914
on smoothness, 855
formulas, 1068-1070
pvaliiation. 1103—1104

knife coatcrs for, description, 1013
solids content, 1072
laboratory procedure, 1103-1104
leveling the coating, 1095-1097
lithopone for (see also Pigment coat¬
ing, pigments for)

lithopone for, effect in offset printing,
1183

machine coating process, amount of
coating applied, 1090, 1109
clays used, 1046
discussion, 1008-1010
drydng, 1097-1098

effect of solids content of coating
mixture on drying, 1071
sizing desired in rawstock, 1094
smoothing of paper, 1088
solids used, 1011
machine types used, 1010
materials used, 1007-1008
Mayer coater, 1013
minor ingredients in formula, 1069
mixture for (see Pigment coating mix¬
ture)

moisture in paper going to driers, 1013
offset roll coater, effect of wax emul¬
sions on rolls of, 1070
offset rotogravure print coater, descrip¬
tion, 1012

opacity obtained compared to filling, 911
paper for (see Coating rawstock)
penetration of coating into rawstock,
variables, 1090-1095
pigment colors used, 1063
pigments for (see also Blanc fixe. Cal¬
cium carbonate. Clay, Pigments,
Satin white, Titanium dioxide.
Zinc oxide, etc.)
pigments for, 1008

adhesive demand, 1015, 1019, 1052,
1060, 1068-1066, 1067-1068, 1127

reduction by mechanical treatment,
1066-1068

amount used, 1068-1069
brightness of, effect on brightness of
coated papers, 1120
color for (see Pigment coating mix¬
ture)

cooking starch with. 1022
effect of particle size on adhesive de¬
mand, 1064—1065

of properties on solids content per¬
missible, 1071—1072
of starch on dispersion, 1021
on adhesive demand of particle size,
1046

on brightness, 1118-1123
on calendering, 1099
on coating mixture, 1081-1082
on drj'ing of coated paper, 1098
on dusting during calendering, 1100
on flow properties, 1081-1082

SUBJECT INDEX

1385

on gloss, 1117 „

on ink receptivity, 1113-1114, 1161

on offset printing, 1183-1184

on opacity, 1118-1123

on solids content permissible, 10/1-

1072

on water retention, 1084
flocculation of, effect on properties of
coated paper, 1121—1122
functions, 1033-1034
hiding power, 1118
luminescent, 1062-1063
mixing, 1066, 1069
mixing with adhesive, 1066—1068
particle size, effect on optical prop¬
erties of coated papers, 1121
properties, 1034
types, 1034

wallpaper coating, 1124-1125
pine oil use, 1069

planographic roll coater for, descrip¬
tion, 1011

plasticizers used, 1069-1070
processes, 1008-1014 _
solids content permissible in different,
1072

rawstock for (see Coating rawstock)
roll coaters for, adhesives used, 1083
description, 1011-1013
effect of amount of coating applied
on smoothness, 1097
flow properties desired, 1078-1079,
1080-1081, 1096-1097
flow properties measurement, 1078-
1081

shearing stress obtained, 1096
solids content, 1072
water retention desired, 1084
rotogravure roll coater, description,

1011-1013

satin white for (see also Pigment coat-
mg> pigments for)

satin white for, effect in offset print¬
ing, 1183-1184
size press application, 1009
soaps used, 1069, 1070
special processes, 1123-1130
speed of brush coaters, 1010
starch for (see Pigment coating ad¬
hesive, and Starch)
sulfonated castor oil in, 1069
titanium dioxide extended pigment, 1057
wallpaper, 1123-1126
wax emulsions used, 1070
waxes, effect on printing of paper, 1161
weight, (see Pigment coating, amount
of coating applied)
w’etting of rawstock by coating, 1090
wire-wound coater for, 1013
Pigment coating adhesive (see also Ani¬
mal glue, Casein, Polyvinyl alcohol,
Resin latices. Soybean protein. Soy

flour. Starch, etc.)

Pigment coating adhesive, amount used,
1068-1069

amount, determination, 1110
effect on ink absorption, 1161

on chalking of printing ink, 1189
on picking during printing, 1191
animal glue, hardening with alum or
formaldehyde, 1028
properties, 1028

blending, properties obtained, 1032—1033
carboxymethylcellulose, properties, 1031
casein (see also Casein)
casein, amount required with calcium
carbonate, 1067-1068
amount required with different pig¬
ments, 1064, 1065, 1067
effect of aging, 1085
effect on flow properties of coating
mixture, 1082-1083
knife coating with, 1013
offset printing, 1182-1183
pigment bonding strength, 1018-1019
water resistant, 1019
wetting of rawstock, 1090
cellulose derivatives for, properties,
1030-1031

corn gluten, properties, 1028-1029
dextrin, types, 1021
early types, 1015

effect in waxed bread wrap, 1128
of pigment particle size on adhesive
demand, 1127

on calendering, 1015, 1099-1100
on cast coating, 1101-1102
on coating mixture, 1082-1083
on curl, 868

on dusting during calendering, _ 1100
on flow properties of coating mixture,
1082-1083
on gloss, 1117

on ink receptivity of coating, 1113
on offset printing, 1182-1183
on opacity after waxing, 914
on optical properties of coated paper,
1122-1123

on printing qualities of paper, 1161
on removal of coating in analysis of
coated papers, 1110
on sizing desired in rawstock, 1094
on smoothness of coating, 1115-1116
on strength of coated paper, 1106,
1108, 11(»

on water resistance of coating, 1111-
1112

on water retention, 1084
functions, 1014

hydroxyethylcellulose, properties, 1031
latex (see also Butadiene-styrene co¬
polymers)

latex, knife coating with, 1013

1386

SUBJECT INDEX

I’iRim'iit coatiiiK adhesive (contd.)
mcthylcellulose (see also MethylceUu-
lose)

methylcellulosc, properties, 1030
mixing different, 1032-1033
mixing with pigment, 1066-1068
penetration into paper, 1015, 1090-1095
effect of water retention, 1084
pigment bonding strength (see also Pig¬
ment coating adhesive, strength, and
Pigment coating, pigments for, ad¬
hesive demand)

pigment bonding strength, 1018, 1019,
1020, 1022, 1024, 1028, 1029, 1032,
1122-1123
comparison, 1019
factors important, 1015
polyvinyl alcohol, pigment bonding
strength, 1019, 1029
properties, 1029-1030
preparation, 1066
properties necessary, 1014—1015
resin latice's (see also Butadiene-sty¬
rene copolytners)

resin latices, properties, 1031-1032
solids content permissible with different,

1071

soybean protein (see also Soybean pro¬
tein)

soybean protein, pigment bonding
strength, 1019, 1028
soy flour (see also Soy flour)
soy flour, blending with casein, 1033
properties, 102^1027
spoilage, 1085, 1086
starch (see also Starch)
starch, amount required with calcium
carbonate, 1067-1068
conversion in presence of clay, 1023-
1024

cooking, 1021-1022
effect of aging, 1085

of solids content on strength, 1093
on offset printing, 1182
pigment bonding strength, 1019, 10—4
effect of enzyme conversion, 1022-
1023

properties of oxidized types, 1021

roll coating with, 1083 ,

soap used to prevent dusting during
calendering, 1100
types used, 102(1-1021
use of small amount of casein. 1100
water resistant, 1024-1025
strength (see Pigment coating adhe¬
sive, pigment bonding strength, Pig¬
ment-coated papers, strength of, and
Pigment coating, pigments for, ad¬
hesive demand)
types, 1008

viscosity of, effect on solids content^per-
missible in coating mixture, 1071

walljiajtcr coating, 1124
water retention fsee Pigment loalnig
mixture, water retention}
zein (see also Zein)
zein, properties, 1029
Pigment coating color (sec Pigment coat¬
ing mixture)

Pigment coating mixture, agitation of,
cause of foam, 1085
anti-foams used, 1086-1087
application to paper, 1090-1097
classification of pigment in air brush
coater, 1014
colloidal stability, 1085
converting starch in presence of pig¬
ment, 1023-1024

cooking starch and pigment together,

1022

effect of added ingredients, 1069-1070
of calcium sulfite, 1061
of oil, 1116
of satin white, 1057
of soaps, 1070
of zinc oxide, 1059
evaluation, 1103-1104
foaming, 1085-1087

flow properties (see also Dilatant flow.
Pigment coating mixture, leveling
of. Pigment coating mixture, vis¬
cosity of. Thixotropic flow, and
Viscosity)

flow properties, Brookfield viscometer
for measuring, 1077-1078
discussion, 1073-1083
effect of casein, 1018
of clay hydration, 1042
of dilatancy, 1080-1081
of high yield value, 1074-1075
of solids content, 1071
of standing, 1085
of starch adhesive, 1023
of thixotropy, 1080-1081
on penetration, 1093, 1094
on sizing desired in rawstock, 1094
on type of rawstock required, 1083
on water retention, 1084
Hercules Hi-Shear viscometer for
measuring, 1078-1079
importance in various coating proc¬
esses, 1095-1097

leveling index for measuring, 1078-
1079

MacMichael viscometer for measur¬
ing, 1076-1077, 1080
variables affecting, 1081—1083
viscometers used for measuring, 10/6-
. 1080

formula, 1008

laboratory evaluation, 1103-1104
leveling index, importance in roll coat¬
ing, 1096 _

leveling of (see also Pigment coating

SUBJECT INDEX

1387

inirturc, floz^’ properties of) .
leveling of, effect of water retention,

1083

mechanical treatment, 1047, 1066-1068
milling, 1047, 1048-1049
mixing adhesive and pigment together,
effects obtained, 1082
penetration into paper, 1090-1095

effect of properties of coating mix-

• ture, 1093
of rawstock, 1093-1095
on dusting during calendering, IlOU
on ink receptivity, 1113
on strength of paper, 1108
pn, 1018, 1023, 1093
preparation, 1008, 1066-1087
properties for air brush coater, 1014
rheology of (see Pigment coating mix¬
ture, flow properties of)
screening to remove foreign matter, 106b
soap used with starch, 1083
solids content, 1068—1069
advantages of high, 1070-1071
effect on amount of coating applied
in offset rotogravure coater, 1012
on flow properties, 1081
on leveling index, 1078-1079
on penetration into paper, 1093
on results obtained, 1072-1073
high, 1071
low, 1071

machine coating, 1011

obtainable wdth calcium carbonate.

1068

■ spoilage, 1085, 1086
surface tension, 1084—1085
effect on smoothness obtained in roll
coating, 1115
thickness of film, 1090
viscosity (see also Pigment coating
mixture, flow properties of)
viscosity for air brush coater, 1014
viscosity for gravure coater, 1013
water retention, importance in brush
coating, 1095
measurement, 1083
variables affecting, 1084
wetting of rawstock, 1090
Pigment coating rawstock (see Coating
rawstock)

Pigment colors, types used in coating,
1063

use in printing inks, 1140
Pine, groundwood, fiberboard from, 850
Pine oil, use in pigment coating, l069
Pinholes in coated papers, causes, 1085,
1115

Pitch, vegetable, properties, 1255
Pitch specks in paper, microscopic ex¬
amination, 977, 978

Planographic printing (see also Printing
processes, offset)

Planographic printing, albumen process,

deep-etch process, 1171-1173

description, 1169-1174 11791171

fountain solution ^sed, 1171, 1172 1173

graining of plates, 1170, 1171, 1 7

plates used, 1170—1173

stone, 1170 .

Planographic roll coater (see Ptgt

coating) , , , ,

Plastic films (see also Cellulose acetate

sheeting) .

Plastic films, laminating, 12Ub
Plastic flow, 1074

1233 .

for aqueous adhesives, 1216 ^

for coated papers, effect on calendering,

1100

for ethylcellulose, 1331
for gummed papers, effect on curl, o69
for hot-melt adhesives, 124^1245
for lacquers, 1285—1287, 1288, 1290,
1291-1292, 1293
for nitrocellulose, 1327
for paper toweling, 826
for pigment coating, 1069-1070
for polyethylene, 1337
for polystyrene resins, 1335
for polyvinylidene chloride copolymer
resins, 1334

for polyvinyl resin, 1332

for remoistening gums, 1235

for resin emulsions, 1254, 1302, 1304

for resin emulsion adhesives, 1238

for resin organosols and plastisols, 1306

fnr rnsin derivatives. 1323. 1324

for tissues, 844
for waxed papers, 1309—1310
Plastics, paper, 1258^1268 (see also Paper
laminates, resin-treated, and Resin-
saturated papers)
paper, resins used, 1259-1260
properties, 1258
pulp-reinforced, 1268-1270
Plating of pasted bristols, 1228
Plating-out of resin in saturating of paper,
1254

Plywood, resin-treated papers for, 1261
Points, measuring thickness of paper, 809
Points per pound, bursting strength, 832
Poise, definition, 1073
Poiseuille, equation, 1073
Polyamides, D. P, for best adhesion, 1207
Polychloroprene polymers, beater applica¬
tion, 1250

latex of, beater treatment, 1250
saturating paper, 1252-1253
properties, 1350

Polyester resins, properties, 1343
Polyethyl acrylate, D. P. for best ad¬
hesion, 1207

1388

SUBJECT INDEX

Polyetliylene, coating of paper, 1315-1316
hot-melt coating, 1315-1316
lacquers from, 1297-1298
gas permeability, 1297
molecular weight, 1297
properties, 1335, 133^1337
resin coatings of, water-vapor resist¬
ance, 1278
solvents for, 1298

use with paraffin for waxing, 1315-1316
Polyisobutylene, D. P. for best adhesion,
1207

use in wax adhesives, 1242
Polymerization of resins, 1337-1338, 1339
Polyphenols, chlorinated, properties,
1345-1346

Polystyrene resin, emulsions of, beater
application, 1259
properties, 1335

Polyvinyl acetate, adhesives from, 1238,
1239

D. P. for best adhesion, 1207
emulsions of, 1303-il305
adhesives from, 1238
hot-melt adhesives from, 1244
lacquers from, 1293
properties, 1331-1332
properties of films, 1304-1305
softening temperature, 1274
solvents for, 1239, 1293
Polyvinyl alcohol, adhesives from, 1233-
1234

properties, 1233-1234
types used in pigment coating, 1030
Polyvinyl aldehyde, properties, 1334-1335
Polyvinyl butyral, properties, 1335
Polyvinyl chloride, lacquers from, 1293
latices, 1332
properties, 1332
saturating paper, 1252-1253
Polyvinyl chloride-acetate copolymers, ad¬
hesives from, 1239
D. P. for best adhesion, 1207
emulsions from, 1303—1304
hot-melt adhesive, 1244, 1245
lacquers from, 1293-1295
organosols from, 1306, 1333
plastisols from, 1306
properties, 1333-1334
solvents for, 1294—1295
types, 1294—1295

Poly^dnylidene copolymer resin, lacquer
from, 1295-1297
latices, 1334
properties, 1334
solvents for, 1296

Polyvdnyl resins, coating with, 1273
description, 1331-1337
lacquer from, 1289, 1293-1297
gas permeability, 1297
Pore radius of paper, values, 847
Pore volume of paper, 846-847

Porosity of paper (see also Air permea¬
bility)

Porosity of paper, 846-851
coating rawstock, 1088-1089, 1090
effect in printing, 1141-1144
of density, 812
on dielectric constant, 849
of moisture, 938

on laminating, 1207, 1210-1211, 1212,
1214, 1241-1242
on oil resistance, 849
on smoothness, 853-854
filter paper, 850
measurement, 1311

relation to water-vapor resistance, 1277
variability, 858
variables affecting, 848-851
Positex, beater application, 1250
Posters, printing, 1173
Potassium ferricyanide, use in blueprint
process, 1198

Potassium pyroantimonate, starch treat¬
ment, 1025

Potdevin bag machine, description, 1226
Power factor, 953

Power factor of paper, effect of moisture,
954

Precision, definition, 981
Preforms, resin-treated, 1259
Preservatives, in starch adhesives, 1225
Pressboards, description, 957
Pressing, effect on brightness, 894
effect on flow properties of paper, 822
on opacity, 911
on smootlmess, 855
on tensile strength, 829
of fiberboard, effect on strength, 1247
Pressroom difficulties (see Printing, dif¬
ficulties)

Pressroom, relative humidity of (see also
Humidity)

Pressroom, relative humidity of, effect
on drying of ink, 1183
relative humidity of, effect on moisture
content desired for offset printing,
1177-1181, 1183

effect on register in offset printing,
1176-1181

effect on static electricity, 1159, 1173-
1174

temperature of, effect on paper, 1180-
1181

effect on picking, 1190
Pressure-sensitive adhesives (see also Ad~
' hesives) 1204

Printability, effect of hardness of paper,
845-846

Printed papers (see also Printing. Print¬
ing inks, and Printing papers)
Printed papers, bleeding in overpnntmg,
1284

chalking of ink, 1189-1190

SUBJECT INDEX

1389

contrast, 1150-1151

defects (see also Prmttng, dtfficiiUtes)
defects, 1184-1192
drying, 118^1187
ink film thickness, 1141-1142
offset during printing (see also frint-
ing, offset during)
offset during printing, lleo-llo/
powdering of ink, 1189-1190
print quality, 1148-1151
show through, 1157-1158, 1186

smudging, 1190
spots on, cause, 1190
strike through, 1185-1186
waxing difficulties, 1312
Printers, coated papers demanded by, lUU/
Printing (see also Engraving, Overprint¬
ing, Planographic printing. Printing
processes, and Varnishing of paper)
Printing, backing of ink away from foun¬
tain, cause, 1144

bleeding during, causes and remedies.

1191

breaks during, causes in newsprint,
1165-1166

caking during, causes and remedies, 1187
chalking during, causes and remedies,
1189-1190

effect of coated papers, 1161
coated papers, 1007
coated papers for varnishing, 1128
collecting during, causes and remedies.

1187

color, 1136—1137

copy for, 1135, 1136

crystallization during, causes and rem-

cdics 118S

difficulties, 1173, 1174, 1175, 1181, 1184,
1185-1192

effect of static electricity, 1159
drying of heat-set inks, 1145—1146
drying of ink in, effect of wax emul¬
sions in coating, 1070
drying of vapor-set ink, 1147-1148
dusting of paper, 1157
effect of absorbency of paper, 1144
of moisture, 1158-1159, 1176, 1181-
1184, 1189

of porosity of paper, 1141
of temperature of ink, 1144 •

electrotypes, 1137-1138
fanning of paper, 1181
feeding of paper, difficulties, 1181
effect of static electricity, 1159
feeding of paper to offset press, 1177
fill-up during, causes and remedies, 1187
forms of reproduction, 1134-1136
fountain setting, 1141
ghosting in, cause, 1185
grain effects in gravure, 1168
halftone reproduction (see also Half¬

tone and Printing, tone reproduc¬
tion in) _ not ii-}/:
halftone reproduction, 1135-1136

copy for, 1135—1136
halo effect, 1150
impression required, 1139, 1141
papers used, 1152-1153
planographic, 1170-1171
rotogravure, 1167

screen angle rotation in color, 1136-

screen sizes used, 1135—1136, 1152

^ ,,71

screen size used in offset process, 11/4
size of dots, 1114
spreading of dots, 1156
tone in, 1150
wax test required, 1162
handling of paper, 1151
history, 1137

humidity during (see also Humidity
and Pressroom, relative humidity

humidity during, effect on drier in ink,
1143

impression obtained, 1138-1139
deep-etch process. 1171-1172
ink absorption, effect of calendering,
1154

ink for (see Printing ink)
inti'oduction, 1131-1132
line reproduction,^ 1134-1135
papers used, 1152
rotogravure, 1167
linting of paper, 1157
maker eady, 113^1139
effect of thickness of paper, 1151
misfit in, 1189

misregister in, causes and remedies,
1189

effect of moisture in paper, 1177, 1181
moisture pick-up in offset, 1174-1175,
1176, 1178-1181, 1182-1183
mottling, causes and remedies, 1144,
1150, 1153, 1162, 1188-1189
multicolor, 1136-1137
difficulties, 1183

moisture pick-up in offset, 1178
order of printing, 1145
picking during, 1190
trapping of ink (see Printing, trap¬
ping of ink)
newsprint, 1163-1166
number of impressions, 1138, 1171-1172
offset during, causes, 1186
effect of oil absorbency, 1156
proofpress testing, 1149
remedies, 1186-M87
paperboard for, 1162
paper for (see Book papers. Pigment-
coated papers, Printing papers, etc.)

1390

SUBJECT INDEX

Printing (contd.)

picking during (see also Printing pa¬
pers, picking of and Pigment-
coated papers, picking during
printing)

picking during, causes and remedies,
1160, 1190-1191
coated papers, 1160
effect of ink drying, 1142
effect of tack of ink, 1144-1145
troubles caused by, 1157
types, 1160

piling during, causes and remedies, 1175,
1188

plastic plates, 1138
plates, 1137-1138, 1167-1168
damage, 1152
effect on misregister, 1189
rubber, 1163

plates used in engraving, 1166
in offset process, 1174
in photogelatin printing, 1184
in planographic, 1170-1173
in rotogravure, 1167, 1168
powdering during, causes and remedies,
1189-1190

press for (see Printing press)
pressure used, 1139, 1150, 1153, 1154
effect on mottling, 1188-1189
on offset, 1186
on strike through, 1185
on wax test required, 1162
processes for (see Printing processes)
punching of paper, cause, 1153
register in offset, 1176-1181
rubber plates, 1138

scumming in offset, causes and rem¬
edies, 1176-1182, 1191-1192
sheetwise, 1151

show through during causes, 1186
slip sheeting, offset prevention by, 1187
speed, effect on ink requirements, 1144
effect on strength of coated papers.

1107

effect on trapping, 1145
spraying of sheets in, offset prevention
by, 1186-1187

spraying with moisture to set vapor-
cipt ink 1147

static electricity, 1159, 117^1174
effect on offset, 1186
stereotypes, 1137-1138
strike through during, causes, 1185-1 liw
thickness of ink film, 1141-1142, 1150,
1152-1153

effect on offset, 1186
rotogravure, 1168

tinting during, causes and remedies,

1191 . , r, •

tone reproduction (see also PrtfiUng,
halftone reproduction)
tone reproduction, 1150

dccp-ctch pr<Kcss, 1172
gravure, 1168
|4anographic, 1170, 1171
transfer of ink to iiapcr (sec also Print
ing ink, transfer to paper)
transfer of ink to paper, 1141-1142
trapping of ink, 1145
effect of crystallization, 1188
effect of waxes in ink, 1144
effect on picking, 119fJ
troubles (see Printing difficulties)
tumble, 1151

type reprfxiuction, papers used, 1152
planographic, 1170-1171
rotogravure, 1167
t)T)e setting, 1134-1137
wallpaper, 1125

washing during, causes and remedies,
1174, 1191

waxed paper, difficulties, 1311-1312
waxy or greasy compounds used, 1144
work and turn, 1151
Printing ink, aniline, 1163
anti-skinning agents in, 1143
application to paper in proof press test¬
ing, 1149, 1155
bleeding in overprinting, 1284
chalking, 1189-1190
cobalt driers, 1142
cold-set, 114^1147
color value, 1140
composition, 1139-1141
compoimding, 1140-1141
crystallization during printing, 1188
driers, 1142-1143

effect on crystallization during print¬
ing, 1188

effect on drying, 1143
offset, 1175

drying of, 1142-1143, 1145-1148
aniline, 1163

effect of moisture in paper, 1158, 1183
of of coated papers, 1183

of of paper, p58

of relative humidity, 1183
of type of paper, 1156-1157
of water in offset ink, 1174
of waxes in coated papers, 1161
on offset, 1186-1187
news, 1164
. offset, 1174-1175

pigment coated offset papers, 1183

effect on odor in paper, 950
■ on waxing of printed ^papers, 1^-
on picking during printing, 1160
fatty acids in, 117^1175
effect on scumming, 1191
film thickness, effect on picking, HW.

1191

floW' properties, 1144—1145. 1168
foreign matter present 1190
hectograph, 1194-1195

SUBJECT INDEX

1391

liigh gloss, diaracteristics, 1162
papers requiring, lli>/
intaglio, 1166-1167
lead driers, 114-

S:'of. coated papers IIM

of smoothness of paper, 11 S.J

milling of, 1^41
mimeograph, 1194

mixing with water on offset press, 11 /+-
1175, 1176. 1182-1180
moisture in offset, effect on piling dur¬
ing printing, 1188

news, 1164
drying, 1164
odor, 1147, 1162
offset, 1174^1175
order of printing, llo/-ll4o

overprinting, 1284 iicciie?

penetration into paper, 1144, 115i-ii3/,

1164-1165

effect on chalking, 1189
on opacity, 1157-1158
on picking, 1190

on strike through and show
through, 1185-1186

rotogravure, 1168
pigmentation used in offset,
pigments in, effect on driers, 114o
planographic, 1174-1175
powdering, 1189-1190 .

properties of, effect on chalking, ll»y
effect on collecting, 1187

on crystallization during printing,

1188

on fill-up in printing, 1187
on mottling during printing, 118»-

1189

on offset, 1186
on picking, 1190-1191 ^
on piling during printing, 1188
on scumming in offset printing,
1191-1192

on strike through and show
through, 1185-1186
on tinting and washing, 1191
relief, driers used, 1142-1143
cold-set, 1146-1147
oils used, 1142—1143
solvent heat-set, 1145-1146
types, 1139—1141
vapor-set, 1146-1147
resins, 1143-1144, 1146, 1147-
rheology (see Printing ink. flozv prol>-
erties)

rotogravure, 1167, 1168
flow properties, 1168
scuff resistance, 1162

shellac, 1324
smudging, 1190
solvent heat-set, 1145-1146
special types for pai>erboard, 1162

spreading on paper in rotogravure print¬
ing, 1168 . iii4_ii45

tack, effect on trapping, 1144-1145
measurement, 1145
variables a ffecting, 114^,1145
temperature, effect on
thickness of ink film, 1141-114-, IKO,

1152-1153

effect on offset, 1186

rotogravure, 1168 iiidius

transfer to paper, 1141-1142, 1144-1145,

1148 . . ,

types used for relief printing of paper-

board, 1162
vapor-set, 1147-1148
vehicle addition, 1141 . , _ . .

viscosity (see Printing mk, floiv prop-

water pick-up in offset, 1174-1175, 1176,
1182-1183

waxes added, offset Prevention by, 1186

Printing opacity, definition, 905 90/

Printing papers (see also

Duplicator papers, and Pigment

coated papers)

Printing papers, absorbency of, effect on
ink required, 1144

absorbency of, effect on stsake through .j
and show through, 1185
ash content, 939
beating required, 1157
brightness, importance, 1150-1151

bulking, 846
butyl carbitol test, 1156
calendering, effect on printability, 1154-

1155 „ . .

castor oil test (see also ^i-

pers, oil absorption) 1156, 1164

chemical requirements for offset print¬
ing, 1175-1176

coated (see also Pigment-coated pa¬
pers)

coated, advantages, 1160

hriffhtness, variables afiectiiig, 111 /"

gloss, 1117
grades, 1102

ink receptivity, 1112-1114, 1161
opacity of, variables affecting, 1117-
1123

picking, 1095
smoothness, 1114-1116
variables affecting, 1114—1116
testing, 1102-1123

wax test required for relief process,
1161-1162

color for best contrast, 1150-1151

conditioning, 1177—1181

effect on moisture pick-up during

offset printing, 1179-1 l 8 l

curl, 1180, 1181
%

1392

SUBJECT INDEX

Printing papers (confd.)
cutting, 803, 1151, 1177
damage, 1152
defects, 1152
delayed flow, 817

difficulties with (see Prinfinff, difficul¬
ties, Printing, picking. Printing,
show through in. Printing strike
through in, etc.)
dimensional stability, 1176-1184
effect of heat in drying of heat-set inks,
1146

finish, 1152-1155
effect on static electricity, 1159
gloss, 1117
grades, 1148

effect on ink drying, 1156-1157
halftone screen sizes used on various,

1152-1153
offset, 1175
gravure, 1168-1169

halftone screen sizes used on various
grades, 1152-1153
hot smashing, 1155
imperfections, 1151-1152
ink absorption of, effect on mottling
during printing, 1188-1189
ink receptivity, 1155-1157
proofpress for testing, 1149, 1155
requirements for offset printing, 1175
intaglio, 1168-1169
moistening, 1158
moisture content, 938, 1176-1181
effect on chalking, 1189
of misregister, 1189
on newsprint, *1165
on printing, 1158-1159
on static electricity, 1159
on vapor-set inks, 1147
requirements for offset printing, 1176-
1181, 1182-1184
offset, 1175-1184
conditioning, 1176-1181
curl, 1180, 1181

difficulties w'ith coated, 1182-1184
effect of moisture on properties, 936-
937, 1182-1183

effect on scumming during prmting,
1191-1192

moisture content, 938
picking of, 1176
requirements, 1106, 1175-1184
strength, 1176
surface sizing, 1176
wax test required, 1106, 1176
oil absorption, effect on offset, 1186
meaning. 1155
newsprint, 1164
testing, 1156

oil penetration, meaning, 1155
opacity, 1157-1158

opacity required in intaglio printing,
1169

photogelatin, 1184

of. effect on drying of ink, 1183
effect on printing, 1158
picking of (see also Pigment-coaled
papers, picking and Printing, pick-
ing)

picking of, 1176
causes, 1157

correlation with wax pick test, 1105
effect of ink drying, 1142
effect of sheet strength, 1157
proofpress testing for, 1149
testing, 1149

pick testers for, 1106-1107
porosity importance, 849
printability, 1148-1151
testing, 1148-1150, 1152
properties affecting ink transfer and
drying, 1148

proofpress for testing, 1106-1107, 1148-
• 1150, 1155

punching during printing, cause, 1153
requirements. 1131, 1148-1166, 1168-
1169,1175-1184
resiliency, 1153-1154
rotogravure, 1169
sizing, effect on collecting, 1187
smoothness, 1152

smoothness required for intaglio print¬
ing, 1169

for offset printing, 1175
softness, 1153-1154

softness required for intaglio printing,
1169

splitting, 1176, 1190
stiffness, 1152

strength (see also Dennison wax test.
Pigment-coated papers, strength
of. Pigment-coated papers, wax
test, and Printing papers, picking
of)

strength, 1063-1066, 1106, 1157, 1161-
1162. 1182

effect in offset process, 1176
effect of heat in drying of heat-set
ink, 1146^1158

strike through in, effect of titanium di¬
oxide, 1055

temperature, effect on curl, 1180-1181
effect on static electricity, 1159
testing.-797-798, 1103-1107, 1148-1149.
1155, 1162

thickness of, effect on makeready, 1151
trimming, 1151, 1152
cause of misregister, 1189
two-sidedness, 1151
types used in aniline printing, 1163
uniformity, 1151-1152
wax test (see Dennison wax test and
Printing papers, strength of)

SUBJECT INDEX

1393

w-ire marks of, effect on printability,
1154

wrapping o/preconditioned, 1179, 1180-

1181 „ . . . ^ .
Printing plates (see Prmimg. plates)
Printing press, adjustments (see also
Printing difficulturs ) ,,«e

Printing press, adjustments, IIoj
newsprint, 1163-1164
offset, 1173-1174

speed, effect on picking, 1190 . . .

Printing processes (see also FniiMni/)
Printing processes, aniline, llo3
collotype, 1132
gravure, 1168 _

intaglio, description, 1166-1169
offset (see also Planographc printing)
offset, advantages and disadvantages,
1147, 1173-1174
description, 1173-1174
effect of moisture in paper, ll/o-llol,
1182, 1183-1184 _ »

fountain adjustment, 1178-1179, 1183,
1102

misregister, 1181 ....

misregister in, effect of conditioning
of paper, 1181

moisture equilibrium of paper during.

1177-1181 ,, ,

picking during, effect of tacky blanket,

1191

pressure used, 1150
printing of tint base, 1189
register in, 1176-1181
scumming in, 1176

causes and remedies, 1182, 1101

1192

photogelatin, 1132, 1184
press speeds, 1132
relief, description, 1133-1139
embossing during, 1150
press types, 1133-1134
rotogravure, 1167-1168
silk screen, 1132, 1184
special, 1132
types, 1132

Printing qualities, effect of smoothness,
1114

Print point of resin-coated papers. 1304
Probable error in testing paper, 858
Probability curve, 984-985
Proof press, testing printing papers, 1106-
1107, 1148-1150, 1155
Properties of paper (see also Basis
weight. Bursting strength. Folding
endurance. Paper, Paper testing.
Tearing resistance. Tensile strength,
Thickness, etc.)

Properties of paper, chemical, 797
classification, 798
effect of basis weight, 809

of fines, 862 * • i... iidA

of heat in drying ot heat-set inks, 11

of light, /99 I' *’ ■« sni

of machine and cross direction, 801-

804

of moisture, 799-800 700700

of papermakmg vanablcb, 798 799

of temperature, 800

electrical, 797
functional, 798-799
fundamental, 798
handling, 797
microscopical, /97
optical, 797

physical, 797 , _

relation to use requirements, /9/

variability, 858-859 ,

•rotective coating (see
»roteins, adhesives from, 1^32-l.-33
’seudoplastic flow, 1074, 1075
hilp, brightness, 895-896
dirt in, 860, 861
effect on brightness of paper, p5
electrical properties, effect of composi¬
tion, 956

mix1ng^of,^effect on brightness, 896-898

plastics from, 1268-1270

quality measured by brightness, 895-

staining (see Fiber analysis)
swelling, effect in plastics, 1268
weight factors, 96^-965

Pulp fibers, area, 828 ,. i .

carboxyl groups in, effect on dielecti ic

loss, 956

distinguishing byastains, 959
tensile strength, 828
Pulpwood, 994—996
Purified pulp, brightness, 895
Purity of color, effect on brightness, 8911-

891

meaning, 881 . . 1 ^07 1 -lo

Pyroabietic acid, composition, lo22-132

Qualitative analysis, paper, 939-943
Quality, definition, 997
[Quality control (see also Stcittsttcs)
Quality control, normal level, 997-;-998
Quality control chart, construction for,
998-1005

t _r__inoi_inn'l

Rafton mill, description, 1067
Rag papers, alpha cellulose content, 931
ash content, 924
brightness, 924
bursting strength, 833
coating rawstock from, 1087
expansion, 864

1394

SUBJECT INDEX

Rag papers (conid.)
folding endurance, 839, 841, 842
opacity, 905
permanence, 945

permanent, requirements, 945-946
scattering coefficient, 924
smoothness, 856
watermarking, 959
Rag pulp, brightness, 892
coating raw stock from, 1089
fiber analysis (see Fiber analysis)
printing papers from, 1148
resin-treated papers from, 1247-1248,
1265

saturating papers from, 1255
weight factor, 964
Ream weight (see Basis ivciyht)

Reel curl, cause, 868
Refining (see also Bcatiiuj)

Refining, of paraffin, 1309
of rosin. 1321

Reflectance (see also Light reflectance)
Reflectance (R/f), definition, 871
Reflectance (Ro), definition, 871
Reflectance (Roo)i definition, 871
Reflectivity, definition, 870
Refractive index, of fillers, determination,
941

of paper, effect on gloss, 898
effect on opacity, 906
Register, in printing, 1189
Relative humidity (see Humidity)

Relief printing (see Printing and Print¬
ing processes, relief)

Remoistening adhesives (see Adhesives)
Resorcinol-formaldehyde resins, proper¬
ties, 1338

Resenes, amount in rosin, 1321
Resin adhesives (see also Adhesives,
Resins, Resin emulsion adhesives,
etc.)

Resin adhesives, curl obtained, 1216
hot melt, 1243-1245
setting, 1213-1214, 1236-1237, 1238
solvents for, 1239-1240
thermosetting, properties, 1215
Resin-coated papers (see also Heat-seal
coated papers, Resin coating, and
JVaxed papers)

Resin-coated papers, amount of coating
applied, 1315
blocking, 1274-1275

effect of pla.sticizcrs, 1285-1287, 1290-
1292

fiber analysis of, special treatment
necessary, 960

film properties, 1272—1281, 1286-1287
gas permeability, 1275-1276, 1297
gloss. 902, 1275, 1308
hardness, 1274 1275
heat-.scal i)roperties, 1275, 1286, 129/
properties, 1271-1272, 1272-1281, 1283-

1284, 1300-1301

properties of films, 1300-1301, 1303,
1306, 1307, 1308
slippage, 1275, 1287
testing, 1273-1281

thickness of film, effect on water-vapor
resistance, 1277-1278
water absorption, 1278
Resin coating (see also Heat-seal coated
papers, Resin-coated papers, and
Waxed papers)

Resin coating, adhesion of resins, vari¬
ables affecting, 1273
amount of coating applied, 1308
bonding of resins to paper, 1272-1273
comparison with saturation, 1277
decorative, 1271, 1275
effect of film thickness, 1273
effect of moisture, 1277
emulsions, 1300-1307
equipment used, 1304, 1305
formulation, 1300-1307
penetration into paper, 1303
polyvinyl resins, 1303-1305
equipment used, 1281-1283, 1284
hot-melt, 1307-1308
effect of paper, 1308
equipment used, 1307
resin-wax combinations, 1314-1316
lacquer (see also Lacquers, Varnishes,
and Varnishing)
lacquer, 1283-1300

amount of coating, 1291, 1292, 1295,
1296, 1298, 1299, 1300
paper for, 128^1289, 1291, 1294
methods of application, 1284, 1296
penetration into paper, 1284
paraffin used, 1275, 1302, 1314-1316
purpose, 1271

resins producing high water-vapor re¬
sistance, 1281

thickness of film, variables affecting,
1282-1283

Resin emulsion adhesives (see also Emul¬
sion type adhesives)

Resin emulsion adhesives, description,

1236-1237, 1238

Resin emulsions, application to paper,

1246

emulsifying agents for, 1301
formulation, 1300-1307
plasticizers for, 1238
Resin-filled paper plastics, 1259-1260
Resin-paper laminates, impact strength,

843

Resin-saturated papers (see also Impreg¬
nated papers. Paper laminates, resin-
treated, Plastics, paper, Resins, satu¬
ration of paper, and Resin-treated
papeis)

Resin-saturated papers, migration of resin,

1254-1255

1395

SUBJECT INDEX

properties, 1247, 1250-1251, 1252, 1253,

^ 1261, 1262-1266 , ^ ,
Resins (see also Adhesives, A Ik yd lesms,
Cellulose acetate. Coumarone-indene
resin, Ethylcellulose, Hot-melts,
Melantme-formaldehyde resins^ lyf
tTOccllulosc, Phcfiolic tcsius, ^ o y
ethylene, Polystyrene, Polyvinyl ace¬
tate, Polyvinyl chloride-acetate resins,
Polyvinyl resins, Resin adhesives,
Resin emulsioiis, Rubber, Urea-for¬
maldehyde resins, etc.)

Resins, affinity of solvents, 1-85

analysis, 1319-1320 Pnhpr

application to paper (see also

laminates, resin-treated, r lasncs,
paper, Resin coating, Resins, beater
application, and Resins, saturation
of paper)

application to paper, methods, 1246,

1258-1259 ^ ^ ^

beater application (see also Kulwer,
latex of, beater application)
beater application, 1246—1250, 1258—1-60
alum precipitation, 1248, 1260
Bardoc process, 1248-1249
disadvantages, 1246—1247, 1260
Du Pont process, 1249-1250
effect of electrostatic charge, 1248-50
effect of pH, 1248, 1260
properties obtained, 1246—1247, 1-63,
1265-1266

retention, 1247—1250, 1259, 1260
Snyder process, 1249-1250
types used, 1247, 1259-1260
classification, 1319—1320, 1337—1338
compatible with alkyd resins, 1342

with aryl s.ulfonamide-formaldehyde

resins, 1346

with cellulose acetate, 1328
with chlorinated polyphenols, 1346
with coumarone-indene resins, 1344,
1345

with ethylcellulose, 1330-1331
with nitrocellulose, 1327
with polyethylene, 1337
with polystyrene resins, 1335
with polyvinyl acetate, 1332
with polyvinyl chloride-acetate resin,
1333-1334

with polyvinylidene chloride copoly¬
mer resins, 1334
with rosin derivatives, 1323-1324
with rubber derivatives, 1348
with synthetic rubbers, 1349, 1350
D. P. for best adhesion, 1207-1208
D. P. of, effect on adhesion for paper,
1273

effect on curing required in paper
base laminates, 1263
effect on hardness, 1274

emulsions of (see also Emulsions and
Resins, latices of)

emulsions of, _

. beater application, 1247
coating with, 1300-1307
film-forming, 1326-1337
classification, 1319
lacquers from, 1290-1300
waxing of paper, 1314
films of, properties, 12/2-1^81
heat-seal (see also Heat-seal coated pa¬
pers) , 1203-1204
hot-melts (see also ^^t melts)
hot melts from. 1307, 1314-1316
in fibers, stains for, 980
in pulp, effect on plastics reinforced

with pulp, 1269

lacquers (see Lacquefs and 1 arnishes)
latices of (see also Emulsions. Resins
emulsions of, and Rubber .latex of)
latices of, amount picked up in satura¬
tion, 1251-1252
electrostatic charge, 1254
particle size, 1254
penetration into paper, 1254
pigment coating with, 1031-1032
resin-filled paper plastics from, 1259—
1260

stabilization, 1254

types used for saturating paper, 1252—
1253

methods of application, 1246, 1258-1259
migration in saturated papers, 1253-
1254

modifiers, 1287, 1300, 1302
natural, 1320^1326
lacquers from, 1287
use in rubber cements, 1239
non-film-forming, 1337-1346
classification, 1319
use in lacquers, 1287-1290
waxing of paper, 1314
plasticizers for, 1285—1287, 1288, 1290,
1291-1292, 1293

polymerization, 1337—1338, 1339

pressure-sensitive, 1204

saturating types, 1251-1252, 1261-1263

saturation of paper (see also Impreg¬
nated papers. Paper laminates,
resin-treated. Plastics, paper, Resin
coating, Resins, beater application,
Resin-saturated papers, and Resin-
treated papers)

saturation of paper, 1250-1254, 1260-
1264

penetration obtained, 1254, 1262—1263,

1264

properties obtained, 1247, 1250-1251.
125^ 1253, 1262-1266
softening temperature, 1274

1396

SUBJECT INDEX

Resins (contd.)

solvents for, 1284-1285, 1288, 1289, 1290,
1292-1293 (see also Solvents)
solvents used for paper laminates, 1262
substituent groups of, effect on bond¬
ing, 1272-1273
synthetic, types, 1326-1350
thermoplastic, 1337
drying of coatings of, 1284
thermosetting, 1337
suitability for laminates, 1261, 1262,
1266

types for beater treatment, 1247, 1259-
1260

for heat seal adhesives, 1203-1204
for hot melts, 1314-1316
for high water-vapor resistance in
coated papers, 1281
for lacquers, 1287-1299
for paraffin, 1314-1316
for pressure-sensitive adhesives, 1204
for resin-filled paper plastics, 1259-
1260

for resin-treated laminates, 1261-1263
use, 1272, 1319
use in aniline ink, 1163
in asphalt adhesives, 1243
in heat-set inks, 1146
in ink, 1166

in printing ink, 1143-1144, 1146, 1147
in sandpaper, 1129
in vapor-set printing ink, 1147
varnishes (see Varnishes)
viscosity of, effect on lacquers, 1290
Resin specks in paper, cause of dirt, 861
microscopic examination, 977, 978
Resin-treated papers (see also Impreg¬
nated papers, Paper laminates, Plas¬
tics, paper, Resin-coated papers, Resin
coating. Resin-saturated papers, Res¬
ms, beater application, and Resins,
saturating of paper)

Resin-treated papers, molding of, effect
on strength, 1247, 1267-1268
properties, 1246-1247, 1258, 1263, 1265-
1266, 1271-1272
types, 1246-1247, 1258-1259
Resin-wax coated papers, formulas, 1315,
1316

properties, 1314-1316
Resorcinol, use in starch corrugating ad¬
hesive, 1226

Retention, of resins applied at beater,
1247-1250, 1259, 1260
Rheology (see Pigment coating mixture,
flow properties of) ^

Roll coaters (see also Pigment coating)
Roll coaters, description, 1282-1283
for hot melt, 1307-1308
for resin coating, 1282-1283
Roofing felts, manufacture, 1255
saturating properties, 1255-1256

Rosin, adhesives from, 1240
analysis, 1321-1322
composition, 1321
derivatives, 1322-1324
emulsion adhesives from, 1238
emulsions from, 1302-1303
hot-melt adhesive, 1245
lacquer adhesives from, 1240
lacquers from, 1287-1288, 1291-1292
esters of, 1323-1324
grading, 1321
gum, description, 1320
manufacture, 1320-1321
properties, 1320-1322
reactions, 1322-1323
softening temperature, 1274
spots in paper, detection, 977
use in news ink, 1164
wood, descriptiton, 1320
Rosin size (see also Sizing value)

Rosin size, amount used in coating raw-
stock, 1087-1088, 1094
effect on air resistance, 849
on bursting strength, 944
on moisture in paper, 936
on permanence of paper, 948-949
removal from paper in fiber analysis,

960

Rotary press, description, 1133-1134
Rotogravure papers, requirements, 1169
Rotogravure printing (see also Printing
processes, intaglio)

Rotogravure printing, description, 1167-
1168

Rubber, adhesives from, 1239
beater application (see Resins, beater
application)

chlorinated, D. P. for best adhesion,

1207

lacquers from, 1298-1299
properties, 1348

cyclized, hot-melt coatings from, 1317
lacquers from, 1298-1299
properties, 1348
softening temperature, 1274
solvents for, 1299

derivatives, hot-melt adhesives, 1245
hot-melt coatings from, 1317
lacquers from, 1298-1300
solvents for, 1298-1300
dispersions of, saturating paper, 1252
emulsion coatings from, 1300
emulsions of, 1300

fiber analysis of paper impregnated with,

960

hydrochloride, properties, 1348
water-vapor transmission, 1281
lacquers from, 1298-1300
latex of (see also Resins, la f ices of, and
Resins, emulsions of)
latex of, adhesives from, 1237

SUBJECT INDEX

1397

anionic, beater treatment with, 1248-

1250

beater application, alum coagulation,

1248, 1250

amount used, 1247, 1248-1250

compatibility with rosin derivatives,

1238

composition, 1250, 1254
compounding for beater application,

1249, 1250

properties, 1346-1347

solids content used for, saturation,

1251

synthetic, adhesives from, 1237
compatibility with acrylic resin
emulsions, 1238
properties, 1347-1348
shipment in special cases, 1032—1033
solvents for, 1239

specks in paper, identification, 979
synthetic compatibility with rosin de¬
rivatives, 1323-1324
hot melts from, 1245, 1317
latices of, application to paper, 1246,
1250

properties, 1250, 1349, 1350
saturation with, 1252
properties, 1348-1350
types used in latices for saturating
paper, 1252-1253

Safety papers, requirements, 930-931
Sampling, rules, 982-984
Sandpaper, manufacture, 1129
Santicizer resin, hot-melt adhesive, 1245
Santolites (see Aryl sulfonamide-formal¬
dehyde resins)

Saran (see P olyvinylidene chloride
resins)

Satin white, adhesive demand in pigment
coating, 1018, 1064-1065
detection, 940
effect of heat, 1058
on calendering of coated papers, 1099-
1117 _

on clay dispersions, 1050
on drying of coated papers, 1098
on ink receptivity of coated papers,
1113^1114

electron micrographs, 1058
impurities, 941

properties and manufacture, 1034, 1057-
1058

unsuitability of latex with, 1032
Saturated papers (see Impregnated
papers, Resin-saturated papers, and
Resin-treated papers)

Saturating papers, porosity, 849
properties desired, 1251, 1255-1256,

1263-1266

Saturation of paper (see also Resins,

saturation of paper) ^oka

Saturation of paper, asphalt, 1256-1^^0
comparison with resin coating, 1
effect of paper properties, 1261, i260

1256, 1263-1266

effect on water-vapor resistance, 12//

oil, 1256, 1257

resin, 1250-1254, 1260-1-64 *

effect on adhesion of coatings, l2/o
Scale wax, properties, 1309
Scattering coefficient (see Kubelka ana

Munk S value) ^ r .

Schering bridge, measuring factor

and dielectric constant, 953
Schopper folding tester, description, 839
variability of results, 842-843
Scotch tape, 1204

Scuff resistance, resin-coated papers, 12/4
Scumming in printing (see also Prin
mg), 1191-1192

Sealing tape, manufacture, 1236--1230
Sedimentation, measuring size of clay par¬
ticles by, 1044
Self-sealing paper, 1311
Selleger reagent, description and use in
fiber analysis, 970-971

Semichemical pulp, fluorescence, 896
modulus of rupture of papers from, 814
plastics from, 1269
Semi-micro analysis of paper, 941-942
Serigraphy (see Printing processes, suk-

screen) _ , • r.

Shaffer stain, description and use in fiber

analysis, 975

Shake, effect on smoothness, odd
S heathing papers, porosity, 850
Sheen glossmeter, description, 899
Sheet lining (see also Laminating, af
paper ), 1201

Sheet strength, coating rawstock, 1088,
1089

effect of basis weight, 859
of heat in drying of heat-set inks,
1146

of humidity of atmosphere, 936-937
of pH, 944

of resins applied at beater, 1246—1247,
1258-1260

of rosin size, 948-949
on offset printing, 1176
on picking of paper during print¬
ing, 1157

meaning of, 814-815
permanence, 944—949
Shellac, adhesives from, 1240
description, 1324

impregnated electrical papers, 955
lacquers from, 1287
softening temperature, 1274
use in pigment coating, 1070
Shewhart quality control chart, construc¬
tion of, 999-1005

1398

SUBJECT INDEX

Shives, in paper, 859, 860
detection, 977

Shoes, paperboard for, manufacture, 1247
Show through in printing (see Printing)
Silica, presence in clay, 1036
Silicone resins, properties, 1345
Silk-screen printing (see Printing proc¬
esses, silk-screen)

Single face board (see Corrugated board)
Sisal fiber, use in sealing tape, 1236
Sisalkraft, description, 1236
Size press, resin addition, 1260
Sizing agents (see also Rosin size).
Sizing agents, determination in paper,
931--932

effect in coating rawstock, 1088
Sizing value, blueprint papers, 1198-1199
direct process papers, 1200
effect of sodium silicate adhesives, 1230
on curl, 866, 867-868
on penetration of adhesive in pigment
coating, 1092, 1094
on resin-treated paper base laminates,
1264

mechanism, 1277

relation to water-vapor resistance, 1277
two-sidedness of, 862
Slack wax, properties, 1309
Slime, effect on odor in paper, 950
Slime spots in paper, identification, 977,
978-979

Slippage of resin-coated papers, 1275,
1287, 1300

Slip sheeting in printing, 1187
Smashing of printing papers, 1155
Smoke, filtration by paper, 850
Smootliing roll (see Breaker stack)
Smoothness of paper, 851-856
coated, variables affecting, 1116-1117
coating rawstock, 1088-1089
effect of fillers, 855
of pressure in printing, 1153-1154
on gloss, 898, 901-902
on laminating, 1210-1211
on opacity, 910
on printing, 1152-1155, 1175
on softness test, 845
newsprint, 855

requirements for intaglio printing, 1169
requirements for offset printing, 1175
testing, 851-853
variables, 852-856

Soaps, use in pigment coating, 1069, 1070,
1083, 1086, 1100, 1161
use in resin coatings, 1287, 1301, 1302-3
St)da pulp, breaking length, 829
density of paper from, 811
opacity, 914

scattering coefficient, 924
use in coating rawstock, 1087
weight factor, 964

Sodium acetate, use in washable wall¬
paper, 1125

Sodium aluminatc, use in beater applica¬
tion of resins, 1249

Sodium carbonate, casein preparation
with, 1017-1018
use in starch adhesives, 1217
Sodium chloride, use in water-resistant
starch adhesives, 1228
Sodium hexametaphosphatc, dispersing
clay with, 1047-1049
Sodium hydroxide, casein preparation
with, 1017-1018
clay dispersion with, 1048
soy flour preparation with, 1026-1027
starch bag adhesives, 1227
starch corrugating adhesive, 1224-1225
Sodium pyrophosphate, clay dispersing
with, 1047-1049

Sodium silicate, adhesive films, strength,

1215

adhesives, 1228-1231
analysis, 1229
casein glues, 1232

clay dispersion in pigment coating,
1023-1024

effect of clay, 1230
on clay, 1047-1049
on flow properties of casein coating
mixtures, 1019

on enzyme conversion of starch in
presence of clay, 1023-1024
paperboard coating, 949
solids content, effect on viscosity, 1229
spots in paper, detection, 977
viscosity, 1229-1230, 1231
Sodium tetraphosphate, clay dispersing

with, 1047-1049

Softness of paper, coating rawstock, 1089
discussion, 844

requirements for intaglio printing, 1169
Softeners (see Plasticisers)

Solid fiberboard, laminating of (see Lavii-
nating of paper)

Solid-fiberboard, weatherproof (see also
Laminated paperboard), 1220-1222,
1243

Solid fraction (see Density of paper)
Solvent coating (see Lacquers, Resin
coating, lacquer, Varnishes, and Var-
nisliing)

Solvents (see also Resins, solvents for)
Solvents, for acrylic resins, 1336

for aryl sulfonamide-formaldehyde res¬
ins, 1346

for cellulose acetate, 1328

for couniarone-indene resins, 1344, 1343

for cyclized rubber, 1299

for ethylcellulose, 1292-1293, .

for lacquers, 1284-1285,1288, 1289,1-90,

1292-1293

SUBJECT INDEX

1399

{or melaniine-formaldchydc resins, 1341
U»r natural resins, 1324-la20
for nitrocellulose, 1285, 1-90-1291, 13-/
tor phenolic resins, 1339
for polyethylene, 1298, 1337
for polyphenols chlorinated^ 1346
for polystyrene resins, 1335
for polyvinyl acetate, 1293, 1332
for pob'^inyl butyral resin, 1335
for polyvinyl chloride-acetate copoly¬
mers, 1294-1295, 1333
for poly^’invlidene copolymer resins,
1296-1297, 1334

for resins in emulsion, 1301, 1302, 1303,
1305, 1306

for rosin derivatives, 1323, 1324
for rubber derivatives, 1298-1300, 1348

for shellac, 1324

for silicone resins, 1345

for synthetic rubbers, 1_349, 1350

for terpene resins, 1345

for urea-formaldehyde resins, 1340

for zein, 1326

organic, use in starch adhesives, 1210
Solvent welding. 1204-1205
Soybean protein, adhesives from, 1232-
' 1233

foam caused in pigment coating, 1086
manufacture, 1027

use with resins in resin-filled paper plas¬

tics. 1259

Soy flour, adhesives from, 1232-1233
analysis, 1026

blending with casein for pigment coat¬
ing. 1033

manufacture, 1026

pigment coating with, 1025-1027, 1033
wallpaper coating adhesive, 1124
Specific adhesion. 1205-1206
Specific inductive capacity (see Dielectric
constant )

Specific surface, effect on light scatter¬
ing. 925

effect on opacity, 909, 914, 915
SjKxific volume, determination, SIO
values. 811

Si)ecks in paper (see also Dirt in paper)
Specks in paper, equivalent black area
of. 859

microscopic examination, 977-979
Si)ectral energy distribution of light,
874-575

Spectrophotometer, color matching, 878-
880, 881

description, 878-880

Siiectrophotometric curve, correlation
with brightness, 890
groundwood, 895-896
standard observer, 881-883
unbleached pulps, 892
visual efficiency. 886-887
white papers, 891

Spcctropluitoinctry, abridged, 880
Specular gloss, nieasiircment, 900
Spices, packaging. 1275-1276
Spinning paper, tensile strength, 8-9
Spirit duplicators, description, 1195-1IVO
Splitting of paper, effect on tearing resist¬
ance, 836

Spoilage (see Adhesives, Pigment coating
adhesives, etc.)

Spots in paper, microscopic examination.
977-979

Snruce, area of individual fiber, 828
solid fraction of papers from, 813
statistical analysis, 996
tensile strength of fibers, 828
Stability of paper (see Permanence of

paper)

Stabilizers, for resin emulsions, I30w
Stainers in wallpaper coating, 11_25
Stains for fiber analysis (see Fiber an¬
alysis, microscopic)

Stamp paper, properties desired, 1235
Standard deviation (see also Statistics)
Standard deviation, of folding endurance,
842-843

Standard observer, meaning, 881-882

Stand oils, types, 1142
Starch, amylose content, pigment bond¬
ing strength, 1020 |

beater addition of, effect on coating
rawstock, 1013
effect on density, 813-814
effect on folding endurance, 841
blending with other adhesives in pig¬
ment coating, lO.p

coating with (see also Pigment coat¬
ing adhesive)

coating with, effect on flow properties,
1082-1083
soap used, 1083
wallpaper, 1124
rooking. 1021-1022
corrugating adhesive, 1222-1226
setting of, 1214

effect on solids content permissible in
pigment coating, 1071
effect on stiffness, 826
effect on tearing resistance, 837
enzyme conversion, 1022-1024
in coated papers, distrilmtion, 1091, 1093,
1094

in paper, microscopic examination, 980
mixing with casein for pigment coating,
10.32-1033

oxidized, effect on calcium carbonate,
1051

properties, 1021

pigment coating adhesive, 1020-1025
spots in paper, detection, 977
.si»raying of paper during printing to
prevent offset, 1186-1187

1400

SUBJECT INDEX

Starch (contd.)

surface sizing with, effect on hardness,
846

effect on permanence of paper, 949
types of, reactivity with urea-formalde¬
hyde resins, 1220-1221
types used in pigment coating, 1020-1021
use in special ink for paperboard, 1163
use with butadiene-styrene latex, 1032
viscosity of, effect of enzyme, 1022-1023
water-resistant, treatments for making,
1024-1025

Starch adhesives (see also Starch, cor¬
rugating adhesive and Dextrin ad¬
hesives)

Starch adhesives, adhesion properties,
1205

bituminous emulsions addition, 1237
consumption of, effect of temperature
and solids content, 1219-1220
for bag pasting, 1226-1228
for bristols, 1228
for tube winding, 1228
preparation, 1218-1219
rubber latex addition, 1237
setting, 1213

solids content, 1217-1218, 1219-1220,
1228

solvents added to improve adhesion on
waxed papers, 1210
strength, 1212
tackiness, 1209, 1217, 1221
types, 1217-1228

urea-formaldehyde resins used, 1220-

1222, 1226, 1227
viscosity, 1208, 1217, 1218
water-resistant type, 1213, 1214, 1220-
1222, 1225-1226, 1227-1228
Static electricity in printing, 1159, 1173-
1174

effect on offset, 1186

Statistical analysis, of fiber counting, 963
of fiber length, 987-988, 992
of folding endurance, 842-843
of jjaner, uniformity measurement by,

857

Statistics, coefficient of variation, 989
correlation coefficient, 1005-1006
examples of use, 991-995
frequency distribution, 984-989 .
construction in quality control, 998-
1005

grouping and analyzing data, 984
maximum error, 990
mean, 985-986
significance, 992-995
methods of comparison, 982
probability, 991
probability curve, 984-985
probability integrals, 987
probable error, 990
range, 986

significant figures, 990-991
standard deviation, 986-989
standard error, 989
tests of significance, 991-995
theory, 981
variability, 984-989
variance, 989, 995-996
Staybelite (see also Rosin, derivatives of)
Staybelite resin, adhesives from, 1240
hot-melt adhesive, 1245
Stearine, properties, 1255
Stencil duplicators (see also Mimeo-
graph)

Stencil duplicators, description, 1193-1194
Stencil papers, manufacture, 1257-1258
Stencils, for mimeograph, 1193
Stereotypes, preparation and use, 1137-
1138

Stiffness of paper, 823-826
determining machine and cross direction
by, 803

effect of resins applied at beater, 1246-
1247, 1258

effect on printing, 1152
effect on tearing resistance, 837
for bristols, 1228
grades requiring, 826
variability, 858
variables affecting, 825-826
Stokes equation, clay particle size meas¬
urement by use of, 1044
Strain, definition, 815
in paper (see Stress-strain in paper)
Straw pulps, filter papers from, 850
weight factor, 964
Stress, definition, 815
Stress-strain, of glue films, 1215
of paper, 814-823
effect of moisture, 865-866, 867
on curl, 867-868
on expansion, 865-866, 867
on tensile strpgth, 827
relation to folding endurance, 840
to tearing resistance, 838
to stretch, 831
testing, 818
variables, 818-823
of paperboard, determination, 823

Stretch of paper, 830-831
effect of moisture, 936-937
effect of resins applied at beater, 1246-
1247, 1258

grades requiring, 831
Mullen tester, 832

relation to bursting and tensile strength,

833, 834
testers, 830-831
variability, 858
variables, 831

Strike through in printing (see also
Printing), 1185-1186

SUBJECT INDEX

1401

‘Stripping, for planographic printing, 1170-

1171

StjTcne resin (see Polystyrene resin)
Suction on wire, effect on two-sidedn«ss,

861-862 ^ ^

Sulfate papers (see also Krajt paper)
Sulfate papers, printing, 1162
Sulfate pulp, bleaching of, effect on bright¬
ness, 892

breaking length, 829
brightness, 895
coating rawstock from, 1087
condenser tissue from, 955-956
density of paper from, 811
electrical properties, 956
fiber analysis (see Fiber analysis)
ion exchange properties of, effect on
electrical properties, 956
kraft, odor of, 950

microscopical analysis (see Fiber analy¬
sis. Graff "C” stain, Lofton-Mcr-
ritt stain, Paper miscroscopical
analysis, etc.)
plastics from, 1269

resin-treated papers from, 1247-1248,
1265

scattering coefficient, 924
tearing resistance, 838
weight factor, 964
Sulfite papers, permanence, 945
requirements for rotogravure, 1169
smoothness, 856
specific volume, 811

Sulfite pulp, bleaching of, effect on bright¬
ness, 892

breaking lengtli, 829
brightness, 895
coating rawstock from, 1087
density of paper from, 811
effect of beating on light scattering, 925
of drying on light stability, 947
of groundwood on brightness, 896-898
of light wavelength on K and S val¬
ues, 925-926
on opacity, 915-916
electrical properties, 956
fiber analysis (see Fiber analysis)
function in newsprint, 1165
microscopical analysis (see Fiber analy¬
sis, Graff “C" stain, Lofton-Mer-
ritt stain. Paper, microscopical
analysis, etc.)
moisture absorption, 936
opacity, 914

optimum humidity for greatest perma¬
nence, 947

resin-treated papers from, 1247-1248,
1265

scattering coefficient, 924
weight factor, 964

Sulfonated castor oil, use in pigment coat¬
ing, 1069

use in remoistening adhesives, 1235 _
Sulfonated oils, corn gluten dispersion

with. 1029

Sulfur dioxide, effect on paper, 948
Sulfur, in paper, effect on printing plates,
1169 . . ,

stabilizer for electrical impregnated

papers, 955

Sunlight, bleaching effect, 946
color matching in, 874-875
Supercalendered finish, use in pi inting
papers, 1152, 1153

Supercalendering (see also Calendering)
Superealendering, effect on brightness,

894 . ^ ■

of coated papers, variables affecting,
1098-1100

Supercalenders, types used on coated
papers, 1098

Surface-active agents, in paper, effect on
printing, 1176
use in adhesives, 1210
in resin emulsions, 1254
in starch adhesives, 1217
Surface sizing, direct process papers,
1199-1200

duplicator papers, 1195
effect on folding endurance, 841
on moisture in paper, 936
on odor in paper, 950
on permanence of paper, 949
on printing papers, 1157
on smoothness, 855
offset papers, 1176

penetration reiiuircd in offset papers,
1176

resin addition, 1260, 1261
spirit duplicator papers, 1196
Sutermeisters “A” stain, description and
use in fiber analysis, 969
S value (see Kubelka and Mnnk S value)
Swelling of paper, by pigment coating
mi.xtures, 1092

Sword hygrometer for measuring moisture
in paper, 1159

Styrene-butadiene latex (see Bttladienc-
stvrene latex)

Taber Abrader, measuring softness with,
845

Taber stiffness tester, description. 824

Table roll, effect on tw'O-sidedncss, 861-
862

Tabulating cards, cutting, 803
thickness, 809-810

Tack of adhesive (see Adhesives, tacki¬
ness)

Tack of printing ink (see Printing ink,
tack of)

SUBJECT INDEX

HU2

Tackoincters, measuring lack of printing
ink. 1145

Talc, effect of ignition on weight. 942
effect on calendering of coated papers,
1099

in paper, detection, 940
microscopic appearance, 941
use in printing ink, 1189
Tall oil. sulfonated, effect on coating mix¬
ture, 1070

Tannins in pulp, effect on plastics rein¬
forced witli pulp, 1269
Tapes (see Paf'cr liif>cs and Sca!m>f
lal>cs)

Tapioca dextrins. properties, 1217
TAPPI opacity, 905, 906
Tastes, absorption by paper, 949
Tear factor, calculation, 826
Tearing resistance, description. 835-838
effect of basis weight, 836, 859
of moisture in paper, 937
of resins applied at beater, 1246-1247
of solid fraction, 813
initial, 838
uniformity, 858
variability, 858
variables, 836-838
Tear ratio, calculation, 836
Temperature of room, effect on moisture,
933-934

Tensile strength, description, 826-830
determining machine and cross direction
by, 803

effect of beating, 812-813
measuring on Mullen tester, 833-834
newsprint, 829
of newsprint, 1165

of paper (see also Stress-strain of
paper)

of paper, effect of moisture, 936-937 _
icffect of resins applied at beater, 1247
effect of tension in drying, 822
relation to folding endurance, 841
of plastics reinforced with pulp, 1269,
1270 „

relation to density, 812-813
tester for, 818, 826-827
measurement of stretch, 830-831
theoretical value, 828 ^
time dependence of, 815-816
values. 828, 829
variability', 858
variables, 827-829
zero span, 829-830
calculation, 830
values, 830

Tensile tester, edge tearing resistance
measurement, 839
Terpene resins, preparation,
properties, lv345

Thermoplastic resins (see Resms)
Thermosetting resins (see Rcsifts)

Thickncs> tif pajKrr, 80*^ 810
calculation of sjjcciffc volume from,
810

effect on bulk, 846
on condenser pajiers, 952, 955
on curl, 866, fib?
on light ahsor|.»tion, 87,V 874, 916
on light scattering, 916
on pr>rosity, 848
on stiffness, 824-825, 826
on two-sifledness, 861
Thixotropic flow, 1075-1076, 1077
effect in roll coating, 1080-1081
importance in various coaling proc esses,
1096

measurement <»ii Mac Michael viscom¬
eter, 1077

Thixotropy (sec also Pif/wrnI coalimj
tnixture, flow properties of)
Thixotropy of starch corrugating adhe¬
sive. 1225

Thwing basis weight scale, use of, 807
Tinius Olsen stiffness tester, description,
825

Tinting, blueprint papers, 1199
direct process papers, 12fX)
in printing (sec Printing)
of coated papers, 1070
of paper, effect on brightness, 891, 893,
896

effect on contrast of printing, 1150-
1151

effect on opacity, 913
tracing paper, 1198
Tissue, makeready, 1139
for carbon paper, properties desired,
1318

softness of, importance, ^

Tissue paper, basje ream size, 805
light transmission, SK)4
Titanium dioxide, adhesive demand in pig¬
ment coating, 1064

coating with, effect on properties of
coated paper, 1118-^1123
effect of ignition on weight, ^2
of waxing on paper containing, 1128^
on calendering of ccated papers, 1117
on light reflectance, 913
on opacity, 911, 913, 914
on opacity after printing, llo8
on optical properties of coated papers,
1118-1123

extended types, 1054, 1057, 10a9

fflling with, and opacity of paper, llao

impurities, 941

in clay, 1036

light absorption, 872 _

particle size, 1055-1056

prejjaration, 1055

properties, 105^1057

properties for pigment coating, 1U84

types, 1053-1034

SUBJECT INDEX

1403

o$c in aniline printing ink, 1163
wallpaper cuatinf;. 1124
Xitaniiann in paper, detes'lkm, 940-^Ml
Toledo basis weight scale, use.

Top colors in wallpaper coating. 1125
Toxicants, effect on odor in paper. 950
Tracuig papers, manufacture, 1256
properties, 1256

properties desired for use in plwrto-

ci^yinf. 1197-11**®

Trade sires for paper, listing, 805
Transparency, effect on formation, 856
relatkai to brightness, 893-894
Transparency of paper, deffnitiun, 903-904
effect of density, 910
Transparency ratio, definition, 904
Trkhromatk coefficients, of standard <4>-

icrver, 882-883. 884-885
Trirraning, of printing papCTS. 1151, 1152
Trlslimulus system, description, 881
Tristiniulus values, (germination. 881
Tube winding, adhesives fur, 1228
Tynipan sheet for printing, 11.49
Type (see also Printing)

Tvpe, reproduction. 1134
setting, 1134

for planographic printing, 1170-1171
Typing, autonutic. 1197
Typogra{4tic printing (see Printing f>roc~
relifi)

Two-sidedness. 861-863

raus^ 861-862

coating raw stock, effect cm C(Uted pa|>er,
1090

effect on curl. 867-868
in surface bunding of 6hers, 1157
of fflled papers. 862. 893
of oil abMrbency, 1157^
of fuprr, effect on coating process, 1109
effect on printing. 1151

intramirroccope, desiripturii, 958
Uniformity of paper (seealsii I'ariaMity)
Uniformity of paper, coating rawrstixk,
1090, 1094-1095
importance. 857

Uniformity of printing jupers, 1151-1152
Urea, plasticizer for dextrins. 1235
Urea-formaldehyde resins. lamirutes from,
1262. 12^. 1268

plastics reinforced with pulp from, 1269
pruperttes. 1339-1.440
use in starch bag adhesives, 1227-1228
use in starch corrugating adhesive.
1225 1226

use in water-resistant starch adhesives,
1^1222, 1226, 1227
use with polyvinyl ^ohol, 1030
use with starch in pigment ccating.

1024-1025

Value of color, tleliuitioii, 87t»

N'andercook proofpress, 1148-1150
Variability, definition, 982 ^
of lolding endurance* test, 842
V'ariance (see S’ldtiJ/icr)

Varnish papers, effect of adhesive in coat¬
ing on. 1017
wax test, 1106

Varnished papers, grades. 112(>-1127
requirements, 1126-1128
N'arnishcs (s<?e also Laeguers and licstn
coating, lactfuers)

Varnishes, baking types, 1287, 1288
from urea resins, 1340
ol<?oresiiious, phenolic, 1339
use in printing ink, 1143-1144
printing, 1132

tyx>es used in printing, 1142, 1143-1144
viscosity of, effect on jienetration into
pai>er, 1262-1203

Varnishing (see also Resin coating, lac¬
quer and Lacquers)

Varnishing, bleeding of ink, 1284
of paper, types, 1132

Vinyl compounds, polymerization, 1331,
1333

N'inyl resins (see Polvtnnyl acetate,
Polyi'inyl alcohol, Roly’vinyl chlo¬
ride, etc,)

X'iscometers, types, 107^1080
N'iscose, removal from impregnated paper,
960

Viscosity (see also Pigment coaling mix¬
ture, ftoxv properties of.)

Viscosity, coefficient, 1074
definitiun, 1073

of adhesives (sec Adhesives, viscosity
of)

of coating mixtures (see Pigment coat¬
ing mixture, flow properties of)
of sodium silicate solutions, 1239-12,40
of starcdi adliesives, 1217-1218
viscometers for measuring, 1076-1080
Visibility function, determination, 887
value from green filter, 887
N'isual efficieticy, 886-887
correlation with brightness, 886, 888,
890-892
meaning, 881

Vulcanizcil fiber, electrical insulation
from, 957

V'ulcanizing agents, use in rubber latex
adhesive, 12.47

Wallbuards (see Fiherhoard)

Wallpaper, coating of, 1007, 1074, 1123-
1126

pine oil in, 1069
soy flour, 1026-1027

1404

SUBJECT INDEX

Wallpaper (contd.)
testing, 1126
washable grades, 1124
Warren air knife, description, 1014
Warren M. P. tester, description, 1107
Washing in printing (see Printing)
Water, absorption by resin coatings, 1278
effect on brightness of pulp, 895
effect on odor in paper, 950
properties for preparing emulsions. 1301
Watermarking, of bond papers, 959
of paper, 801

Water-resistant adhesives (see Adhe¬
sives)

Water resistance, of blueprint papers,
1198-1199

of plastics reinforced with pulp, 1269-
1270

of resin-treated paper laminates, 1262-
1263, 1265, 1267

Water-vapor permeability (see Water-
vapor resistance and Water-vapor
transmission)

Water-vapor resistance, effect of laminat¬
ing with asphalt, 1243
effect of lamination with hot-melts.

(see also
testing of

1240-1241

measuring, 1279-1281
paperboards, 1280, 1281 ^

variables affecting, 1276, 12/7-1281,

1299

waxed papers, 1281, 1310-1311, 1313,
1316

Water-vapor transmission
Water-vapor resistance)

Water-vapor transnnssion,
paperboard for, 798
values for different materials, 1281,

1297, 1299 .

values for resin-coated glassine, 1291,

1299

Wavy edges, causes, 868

Wax (see also Paraffin and Microcrystal-

Hhc

Wax, carbon paper manufacture, 1317-
1318 ** ■

compatibility with ethylcellulose hot-
melt adhesives, 1244
with rosin derivatives, 1323 , 13-4
hot melts from, 1307, 1314-1316
paraffin (see Paraffin)
types used with paraffin, 1314
use in printing ink, 1144
in resin coatings, 1287, 1300. 1302,
1314-1316

with ethylcellulose, 1331
Wax blush, in resin coatings, U&/

W^ax emulsions, sizmg \'i

effect on laminating, 121^0
use in pigment coating, 10/0
Wax in printing ink, effect on crystalliza¬
tion during printing, 118o

offset prevention by, 1186
to prcA’cnt collecting, 1187
Wax pick test (see Dennison vox test,
Pigment-coated papers, strength of,
and Printing papers, strength of)
Wax pick test, coated papers (sec Denni¬
son wax test, Pigment-coated papers,
strength of. Pigment coating, pig¬
ment for, and Pigment coating ad¬
hesive pigment bonding strength)
Wax-resin coating, formulas, 1315. 1316
Wax spots in paper, identification, 979
Waxed papers, adhesion on, 1205, 1210
analysis, 1312

fiber analysis of, special treatment neces¬
sary, 9(M
formation, 856
gloss, 902
laminating, 1210
printing, 1163, 1311-1312
properties, 1309, 1312, 1313, 1314, 1316
properties obtained by addition of resins,
1314, 1316, 1317
quenching, 1311
self-sealing, 1311
uses. 1309, 1317-1318
water-vapor transmission, 1281, 1310-
1311, 1313, 1316
wax content, 1310, 1311
Waxing (see also Paraffin)

Waxing, effect of vapor-set inks, 1147
effect on opacity, 913-914
of cartons, 1312
of opaque bread wrap, 1128
of paper, dry, 1310-1311
equipment used, 1310, 1311, 1315,1318
microcrystalline wax 1312-1314
paraffin, 1308-1312
resins used, 1314—1316
wet, 1311-1312

Waxing paper, basic ream size, 805
requirements, 1310, 1311
tjT>es, 1309

\\'eight factors for pulps (see Fiber an-
anlysis, microscopic)

West coast pulps, smoothness of papers
from, 856

Wet machine, fiberboard manufacture,

1247

Wet strength, coating rawstock, 1088
Wet strength paper, pU determination,

944

fiber analysis, special treatment neces¬
sary, 960
laminating, 1206

sandpaper, 1129 .

Wettmg agents, for dextrin adhesives.

1235

use in starch corrugating adhesive, 1^5
Wet waxing of paper (see Waxing of
paper)

SUBJECT INDEX

1405

Wheelwright process of coating paper,
description, 1009
Wild formation, meaning, 856
Williams porosity tester, description, 848

849 . . .

Williams smoothness tester, description,

853

Wilson stain, description and use in fiber
analysis, 969-970
Wire marks, 803

effect on printing, 1154
effect on smoothness, 854, 855
Wire side, alignment of fibers on, 802
characteristics, 800-801
fiber composition, 861, 862

printing, 1151, 1187

. sizing, 862 , j •

Wire-wound coating rods, description,

1103

Witherite, barium sulfate from, 1060
Wood, density, statistical analysis, 994-
996

Wood flour, plastics from, 1268
Wood specks in paper, identification of,
979

Wood veneer, adhesives for, 1232, 1233
casein adhesives for, 1232
Wool fiber, stains especially for, 976
Wrapping. papers, ash content, 939
basic ream size, 805
coated with resin-wax mixture formula,

1315

gas permeability, 1275-1276
hot-melt coating, 1308
oiled, 1256-1257
printing, 1163
properties, 1271—1272
resin-coated, 1271

water-vapor resistance requirements,

1276

waxed, 1309, 1315

W^ ratten color filters, wavelength range,

880 rr * f

Wrinkling, of coating rawstock, eftect oi
adhesive penetration, 1092
of paper, causes, 868

Writing ink, effect on permanence ot

• paper, 949

Writing paper, ash content, 924
brightness, 924
formation, 857
scattering coefficient, 924
smoothness, 856

X-rays, 958-959

Yankee drier, finish obtained, 903
Yellowness, measurement by three-filter
colorimetry, 888
Yield value, definition, 1074
Young’s modulus, calculation, 823—824

Zein, description, 1326
lacquers from, 1287
properties, 1029
Zinc, in paper, detection, 942
Zinc chloride, effect in iodine stains for
fiber analysis, 965, 966
Zinc hydrosulfite, bleaching of clay, 1037
Zinc oxide, manufacture and properties,

1059

properties for pigment coating, 1034
Zinc salts, resin latex stabilization, 1254
Zinc sulfide, composite pigment, 1059
effect on permanence of paper, 949
detection, 940
luminescence, 1062

manufacture and properties, 1058-1059
sublimation temperature, 942

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