Effects of lignin content on the properties of lignocellulose-based biocomposites
Introduction
Biocomposites (biodegradable composites) are obtained by blending together a biodegradable polymer and biodegradable fillers (e.g., lignocellulose fillers). Since both components are biodegradable, the composite as the integral part is also expected to be biodegradable (Mohanty, Misra, & Hinrichsen, 2000). Tailoring new composites within a perspective of eco-design or sustainable development is a philosophy that is applied to more and more materials. Ecological concerns have resulted in a resumed interest in renewable resources-based and/or composable products. It is the reason why material components such as natural fibers, biodegradable polymers can be considered as “interesting” – environmentally safe – alternatives. The classification of these polymers has been shown elsewhere (Avérous, 2004). A large number of the biodegradable polymers are commercially available such as the biopolyesters (biodegradable polyesters). They show a large range of properties and at present, they can compete with non-biodegradable polymers in different industrial fields (e.g., packaging, agriculture, hygiene, and cutlery).
Lignocellulose-based fibers are widely used as biodegradable fillers. With their environmentally friendly character and some techno-economical advantages, these fibers motivate more and more different industrial sectors (e.g., automotive) to replace e.g., common glass fibers. Intrinsically, these natural fibers have a number of interesting mechanical and physical properties (Bledzki and Gassan, 1999, Mohanty et al., 2000, Rouilly and Rigal, 2002, Saheb and Jog, 1999). Table 1 shows that, according to the botanical source, these renewable materials present large variations in their composition. Main elements are cellulose, hemicellulose, and lignin which are known to present a very complex structure. Fig. 1 shows a tentative of schematic representation of wheat straw lignin proposed by Sun, Lawther, and Banks (1997).
For short-term applications, biocomposites present strong advantages. Thus, a large number of papers has been published on this topic. Except some publications based on polysaccharide matrix e.g., plasticized starch (Avérous and Boquillon, 2004, Avérous et al., 2001), most of the studies (Avérous, 2004) are based on biopolyester matrices (Mohanty et al., 2000, Netravali and Chabba, 2003). For instance, aliphatic copolyesters have been used with cellulose fibers (Wollerdorfer & Bader, 1998), bamboo fibers (Kitagawa, Watanabe, Mizoguchi, & Hamada, 2002) or flax, oil palm, jute or ramie fibers (Wollerdorfer & Bader, 1998). Aromatic copolyesters-based biocomposites have been less investigated. In a recent publication, lignocellulose fillers from wheat straw have been associated with different matrices such as aromatic copolyesters (Le Digabel, Boquillon, Dole, Monties, & Avérous, 2004).
This work is focused on the analysis of biocomposites based on treated lignocellulose fillers (TLF) displaying various lignin contents. The lignocellulose fillers (LF) are a by-product of an industrial wheat straw fractionation based on the extraction and the recovery of most of the hemicellulose sugars. Table 1 shows that, compared to wheat straw, LF present higher lignin content (30%) and a lower cellulose/lignin ratio, 1.9 compared to 3.2 for wheat straw. These low cost fillers (LF) are treated to reduce the lignin content and then to increase the cellulose concentration with the aim to analyze the impact of the lignin content variation on the biocomposites properties. By varying the extraction conditions, several fillers fractions displaying different lignin contents are obtained. To elaborate the biocomposites, these treated lignocellulose fillers (TLF) are mixed with polybutylene adipate-co-terephthalate (PBAT), a biodegradable aromatic copolyester. The aim of this paper is particularly focused on the analysis of the properties of both the treated fillers and the corresponding biocomposites. We have investigated the influence of the lignin extraction method, the impact of the lignin content on the fillers-matrix compatibility and on the final properties of these materials.
Section snippets
Materials
The matrix, a biodegradable and aromatic copolyester (polybutylene adipate-co-terephthalate, PBAT) has been kindly supplied by Eastman (EASTAR BIO Ultra Copolyester 14766). Copolyester chemical structure is drawn in Fig. 2. The ratio between each monomer has been determined by 1H NMR. We have measured 43% of butylene terephthalate and 57% of butylene adipate. Molecular weight (Mw) and polydispersity index (IP) are 48,000 and 2.4, respectively. They have been determined by size exclusion
Composition and granulometry of the treated fillers
Table 2 shows the different lignin contents determined by Klason titrations. After treatment the lignin content has decreased. Between TLF1 and TLF3 values, we can notice the effect of the temperature on the extraction. Extracted lignin content increases with the temperature. Between TLF2 and TLF3 values, the effect of the medium (aqueous vs. organic) is noticeable. Aqueous medium is much more efficient for extraction. This is partly due to the reflux temperature which is lower for the organic
Conclusion
Different biocomposites (biodegradable composites) have been produced by incorporation of lignocellulose fillers into a biodegradable aromatic polyester, polybutylene adipate-co-terephthalate. The paper is focused on the analysis of the behaviour of biocomposites reinforced with fillers fractions which present lignin content variation. These materials have been carried-out by extrusion and injection moulding. The lignocellulose fillers are a by-product of an industrial fractionation process
Acknowledgements
This work was funded by Europol’Agro through a research program devoted to materials based on agricultural resources. The authors want to thank Pr. Monties for his great investment in this project.
References (22)
- et al.
Biocomposites based on plasticized starch: thermal and mechanical behaviours
Carbohydrate Polymers
(2004) - et al.
Composites reinforced with cellulose-based fibres
Programs in Polymer Science
(1999) - et al.
Abiotic and enzymatic degradation of wheat straw cell wall: a biochemical and ultrastructural investigation
Journal of Biotechnology
(2000) Novel structures and properties of lignin in relation to their natural and induced variability in ecotypes, mutants and transgenic plants
Polymer Degradation and Stability
(1998)- et al.
Composites get greener
Materials Today
(2003) - et al.
A tentative chemical structure of wheat straw lignin
Industrial Crops and Products
(1997) - et al.
Influence of natural fibres on the mechanical properties of biodegradable polymers
Industrial Crops and Products
(1998) - et al.
Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw fibre composites: thermal, mechanical properties and biodegradation behaviour
Journal of Materials Science
(2000) Biodegradable multiphase systems based on plasticized starch: a review
Journal of Macromolecular Science—Part C, Polymer Reviews
(2004)- et al.
Plasticized starch–cellulose interactions in polysaccharide composites
Polymer
(2001)