Study on the preparation and properties of lignin-based flexible polyurethane foams with NaCl as a medium

https://doi.org/10.1016/j.ijbiomac.2025.140370Get rights and content

Highlights

  • •
    NaCl optimizes polyurethane foam (PUF) cell size and improves PUF properties.
  • •
    PUF with appropriate NaCl addition has good mechanical properties.
  • •
    PUF with proper addition of NaCl has high resilience and elastic recovery.

Abstract

Currently, most polyols used in polyurethane foam (PUF) synthesis are derived from petrochemicals. However, lignin as the most abundant aromatic biopolymer rich in hydroxyl groups, is a suitable filler for synthesizing polyurethane foam. Therefore, in this study, Kraft lignin (KL) was utilized as a partial substitute for flexible polyethylene glycol 400 (PEG400). After adding 15 % KL, NaCl was introduced as a medium for synthesizing LFPUF and Polydimethylsiloxane (PDMS) was used as a foam stabilizer. Lignin-based flexible polyurethane foam (LFPUF) with high elasticity was synthesized. The aromatic ring structure of KL acted as a reinforcement for the foam, while the flexible PEG400 provided excellent flexibility to LFPUF. Na+ interacts with oxygen atoms in the long chain of PEG to form a cyclic crown ether structure. This promotes proximity and ordering of the hydroxyl groups, thereby facilitating proton transfer and optimising the cell size of the foam. Ultimately, the foam synthesized with 3 % NaCl (LFPUF-3%NaCl) exhibited an average cell size and hole size of 484.1 and 183.6 μm, respectively, significantly smaller than those of the foam synthesized without NaCl (843.5 and 249.3 μm, respectively). The foam resilience and elastic recovery of LFPUF-3%NaCl were 39.56 and 99.03 %, respectively, which was higher than that of the foam synthesized without NaCl (20.36 % and 97.08 %, respectively). In addition, LFPUF-3%NaCl maintained a high elastic recovery of 97.61 % after 20 cycles of compression. The egg drop test demonstrated that the foam effectively provided protective cushioning for fragile items.

Introduction

Polyurethane (PU), also known as polycarbamate, is a versatile polymer characterized by the urethane group (-NHCOO-) in its main molecular chain. It is used in a wide variety of areas including foams, elastomers, coatings and fibers, playing a significant role in industries such as automotive, construction, home appliances and furniture [1]. Due to its broad range of applications, PU has become the sixth largest renewable plastic material globally [2]. Among these applications, polyurethane foam (PUF) is particularly notable for its porosity, low density, excellent resilience, chemical stability and scalability in production. These attributes make PUF ideal for cushioning, insulation, clothing, and soundproofing, capturing approximately 65 % of the PU product market [[3], [4], [5]].
Currently, the raw materials used in PUF products are predominantly petrochemical-based, which are limited in supply and non-renewable. Hence, developing a renewable and environmentally friendly biomass-based raw material for synthesizing PUF is of great importance [6,7]. Lignin, the most abundant natural aromatic biopolymer on earth, is rich in hydroxyl groups and is a suitable filler for synthesizing PUF [[8], [9], [10], [11]]. Furthermore, the aromatic ring structure and inherent rigidity of lignin assist in enhancing the physical properties of the foam [12]. Substituting petroleum-based polyols with lignin can reduce the production costs of PUF, providing a novel approach for lignin utilization and PUF synthesis [13].
Inorganic salts (NaX, where X = Cl, Br, I and OAc) serve as important green and non-polluting auxiliaries that can have a catalytic effect on foam synthesis in context with metal-organic compound catalysts. Due to their advantages of non-toxicity, easy handling and wide availability, researchers have utilized these salts to synthesize solvent-free green polyurethane foam materials, which are also free from ecocompatibility issues [14]. However, there has been limited research on synthesizing FPUF materials using lignin as a polyol component, and even less on the synergistic effects of inorganic salts as organometallic auxiliaries.
PUF can be characterized into three types: rigid, semi-rigid and flexible polyurethane foams, and the detailed information is shown in Table 1. FPUF have a wider range of applications and greater market demand than rigid polyurethane foams (RPUF) and semi-rigid polyurethane foams (SRPUF). Therefore, in this study, Kraft lignin (KL) with its multi-hydroxyl structure was used as a filler to synthesize PUF, leveraging lignin's structural support to produce lignin-based flexible polyurethane foam (LFPUF). The prepared LFPUF have several special properties, for example, low density, excellent mechanical properties, superior elastic recovery and low hysteresis loss [15,16]. Additionally, NaCl was incorporated as a medium into the LFPUF, resulting in NaCl-mediated LFPUF. The reaction mechanism of KL partial substitution of petrochemical polyols and its effect on the cellular structure and physical properties of LFPUF were investigated when NaCl was used as a medium. This approach to polymer material preparation offers a new valuable pathway for the sustainable development of lignin resources, aiming to produce highly elastic foam composites suitable for cushioning materials and home products.

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Section snippets

Experimental materials and reagents

Papermaking black liquor was obtained from Fujian Qingshan Paper Mill. H2SO4 (98 %), polyethylene glycol (PEG, AR, Mn = 400), dibutyltin dilaurate (DBTDL, 95 %), hexamethylene diisocyanate (HDI, 99 %) and deuterated dimethyl sulfoxide (DMSO‑d6) were purchased from Innochem Technology Co. Ltd., China. Polydimethylsiloxane (PDMS, AR), triethanolamine (TEA, 99 %), potassium bromide (KBr, 99 %) and sodium chloride (NaCl, 99.5 %) were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd.,

Characterization of KL

The elemental analysis and molecular mass distribution analysis results of KL are shown in the Table 3. The results of quantitative 1H NMR determination of the partial hydrogen content [mol mol−1] % of the different functional groups of KL versus the ratio of all hydrogen-containing functional groups are shown in Table 4. The results of quantitative 31P NMR determination of hydroxyl content of KL are shown in Table 5.

Synthesis of NaCl mediated LFPUF

In this study, the long-chain PEG400 belongs to the polyether diol category

Conclusions

In this work, the polyhydroxy structure of kraft lignin (KL) was used to partially replace petrochemical polyols, and it was blended with flexible PEG to form lignin-based polyol components. By adding HDI and various additives, LFPUF was synthesized. Different NaCl contents were incorporated to prepare LFPUF, and the foam was subjected to mechanical property tests, FT-IR spectroscopy, XPS, DSC and other analyses.
The results demonstrated that:
  • 1.
    Na+ acts on the O atoms of the long PEG chain to form

CRediT authorship contribution statement

Shibo Han: Writing – original draft, Methodology, Investigation, Formal analysis. Guangying Huang: Data curation. Han Han: Methodology. Xilai Yan: Formal analysis. Jie Xie: Data curation. Hongshen He: Methodology. Ajoy Kanti Mondal: Writing – review & editing. Weijie Lin: Supervision. Fang Huang: Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (22078061), Natural Science Foundation of Science and Technology of Fujian (2023J01463) and Foundation (KF201922) of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences (China) and the Foundation (2023GXZZKF23) of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University

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