Effect of lignin removal on the properties of coconut coir fiber/wheat gluten biocomposite

The increase in fossil energy costs and the environmental concerns result in new opportunities for the industrial production of biodegradable materials based on natural renewable resources. A growing demand for various applications are thus expected such as short-lived applications for agriculture (e.g., plant pot, mulching films to cover soil), food and non-food packaging [1], [2]. Gluten-based material displays interesting functional properties, in terms of viscoelasticity and water resistance. Mechanical properties of gluten-based materials can be modulated according to the process conditions for example temperature [3], [4] or mechanical energy input [5], or to the blend composition by the modification of the plasticizer content [6] or by the addition of natural fibers [7], [8], [9].

Natural fibers, which are essentially composed of cellulose, hemicellulose and lignin, are widely used as a reinforcement to produce biocomposite [7], [8], [9]. The fiber composition depends on the plant from which it is extracted, as well as on the agricultural conditions. It is mainly composed of three compounds which are cellulose, hemicellulose and lignin. Cellulose and hemicellulose are polysaccharides, while lignin is a three-dimensional amorphous polyphenolic macromolecule consisting of three types of phenylpropane units (as shown in Fig. 1) [10], which are forming a complex, highly branched and amorphous structure. Moreover, the local repartition of the compounds is not homogeneous. In general, lignin is mainly located at the surface of the fiber, while the backbone is mainly composed of cellulose.

Adhesion between matrix and fiber is an important parameter affecting the mechanical properties of composite, as a good adhesion ensures a good stress transfer from the matrix to the fiber [11]. This adhesion can result from a physical origin or from a chemical cross-linking. In natural fibers/wheat gluten composites, both types of adhesion are supposed to be effective. Our previous study [8] showed that different reinforcement effect can be correlated with different Pressure Sensitive Adhesive (PSA) properties of the gluten matrix. Additionally, chemical bonding can strongly affect the quality of the interface. Lignin, a polyphenolic compound located on fiber surface, may play a key role on the fiber/matrix chemical adhesion. Indeed, polyphenol/protein interactions have been largely described in literatures [12], [13], [14] and various types of interactions are identified. In a recent study, we have demonstrated that Kraft lignin can strongly interact with wheat gluten [15], and evidenced the role of the phenolic group in this interaction [16]. Therefore, variations in the fiber lignin content should monitor the density of fiber/matrix interactions, and resultantly the biocomposite properties.

A specific phenomenon that can be observed in biocomposites is called the matrix deplasticization [8]. Indeed, the agropolymer used as a matrix (here, a protein), is thermosensitive, and begins to degrade at a temperature lower or close to its glass transition. Therefore, agropolymers need to be plasticized by a small polar molecule (as glycerol) to decrease their glass transition temperature, and therefore allow their industrial processing with a limited thermal degradation. This plasticizer significantly affects the matrix properties, the common observation is that it decreases both Young’s modulus and tensile strength, while increasing the elongation at break of materials [8]. Then, when natural fibers are added, there is a competition between the matrix and the fibers for the plasticizer absorption, which can result in a deplasticization of the matrix, and thus in a different reinforcing effect. Unlike cellulose which is associated in microfibers, lignin is an amorphous polymer, and thus may play an important role on this mechanism. Therefore, lignin might play a key role by increasing the mechanical properties of those biocomposites due to its location on the surface fibers, its amorphous structure, and its reactivity with wheat gluten.

Coconut coir fiber, which is in average composed of 46% of lignin (weight basis) is one of the natural fibers containing the higher lignin content [17]. Lignin can be extracted selectively and progressively by treating the fibers with an aqueous alkaline solution or with an organic solvent [2]. It is thus a medium to conduct a systematic study on the effect of lignin on biocomposites properties. About 55 billion of coconuts are harvested annually in the world, but only 15% of the husk fibers are actually recovered for use [18]. Most husks are abandoned in the nature, which constitute a waste of natural resources and a cause of environmental pollution [19]. Therefore, biocomposites from wheat gluten reinforced with coconut coir fiber would certainly offer interesting routes for the production of environmentally-friendly materials. Use of coconut coir as a reinforcement has been already studied, but only on cement board [20], polypropylene [21], and starch/ethylene vinyl alcohol copolymers [22].

Pretreatments of coir fiber by washing and boiling in order to remove the impurities on the coir surface have been already studied [20]. In terms of surface topology, pretreatments can create voids and produce fiber fibrillation, leading to a better fiber/matrix adhesion and therefore better mechanical properties of coir/cement composite. For biocomposite, the modification of fiber chemical composition and their effect on the properties of materials was studied [23], [2]. The effect of the lignin content has been studied on lignocellulosic fibers incorporated into a biodegradable aromatic polyester, polybutylene adipateco-terephthalate [2]. In that case, lignin removal by chemical treatment increased the biocomposite moduli, suggesting that the lignin/cellulose ratio is an important parameter [2]. However, the effect of chemical composition of natural fiber on properties of biocomposite especially on matrix deplasticization and fiber/matrix interaction has not been clearly reported.

The objective of this work was to study the reinforcing effect of coconut fiber in protein-based biocomposites, by modifying the fiber lignin content. Firstly, coconut coir fiber was pretreated in order to decrease progressively its lignin content. Properties of original and modified fibers were characterized. Then, the glass transition temperature (T_g) of biocomposite was determined by dynamic mechanical thermal analysis (DMTA), in order to investigate the matrix deplasticization. Chemical bonding between fibers and matrix were investigated by Fourier Transform Infrared Spectroscopy (FTIR). Mechanical properties and water absorption of the samples were finally characterized to study the functional properties of materials.