Extensive research has been underway to study the potential of different natural fibers as reinforcement for biodegradable (a synthetic or renewable) polymer ma­trices in order to develop the components for different body parts of automobiles.56 Natural fibers are incorporated into door panel trims, package trays, trunk trims and other interior parts.5

The interest in using biocomposites based on natural fibers and biocompatible polymer matrices has grown because they are lightweight, biodegradable, nontoxic, nonabrasive during processing, have low cost and are easy to recycle. Natural fiber reinforced materials offer environmental advantages such as reduced dependence on nonrenewable energy/material sources, lower pollutant and greenhouse emis­sions. Lacarin et al. have compared the environmental impacts of the biocomposites and the glass/PP composite for the different steps of the life cycle.7 The energy consumption to produce a flax-fiber mat (9.55 MJ/kg/1), including cultivation, har­vesting and fiber separation, amounts to approximately 17% of the energy needed to produce a glass-fiber mat (54.7 MJ/kg/1).2 Environmental aspects reveal that natural fibers display an increase of about 15% of the performance of the composites, while focusing on economical aspects they cost about seven times less than glass fibers (Table 10.1).

Depending on their performance when they are included in the polymer matrix, lignocellulosic fibers can be classified into three categories: 1) Wood flour particu­late which increase the tensile and flexural modulus of the composites, 2) Fibers of higher aspect ratio that contribute to improve the composites modulus and strength when suitable additives are used to regulate the stress transfer between the matrix and the fibers, 3) Long natural fibers with the highest efficiency among the lignocel — lulosic reinforcements.

Natural fibers may be classified by their origin as either cellulosic (from plants), protein (from animals) or mineral. Plant fibers may be further categorized as: seed hairs (e. g., cotton), bast or stem fibers (e. g., linen from the flax plant), hard (leaf) fibers (e. g., sisal), or husk fibers (e. g., coconut).

Cellulose is one of the most abundant renewable and biodegradable biopoly­mer resource with high mechanical performance. It is a hydrophilic glucan polymer consisting of a linear chain of p-1,4-bonded anhydroglucose units that contains alcoholic hydroxyl groups. Cellulose represents the main structural component of plant cell walls. These hydroxyl groups form intra and intermolecular hydrogen bonds inside the macromolecule and among other cellulose macromolecules, re­spectively, as well as with hydroxyl groups from the surrounding air and polymer matrices. In terms of primary walls, cellulose fibrils have been found to be preferen­tially deposited perpendicular to the axis of cells during their initial state of growing. Due to the great stiffness and strength of cellulose fibrils, it is much easier to expand the cell wall perpendicular to the orientation of cellulose. The secondary cell walls consist of different layers that are deposited on the primary cell wall in a charac­teristic manner (strictly parallel). The interaction between the stiff cellulose fibrils and the plant matrix polymers in the cell wall is one of the key issues to elucidating the mechanical performance of plants.8 The most efficient natural fibers considered include threads with a high cellulose content coupled with a low microfibril angle, resulting in high filament mechanical properties. Due to their hollow and cellular nature, natural fiber preforms have up to 40% lower density. They act as acoustic and thermal insulators, and exhibit reduced bulk density.

The absolute mechanical data of natural fibers are inferior relative to E-glass and Carbon fibers. But when they are used in composites, their mechanical properties are even higher than E-glass reinforced composites (Table 10.2).9

So, natural fibers have lower densities and they can be found to be cheaper than glass fibers, although their strength is usually significantly less. Because of their good specific modulus values, natural fibers can be preferable to glass fibers in ap­plications where stiffness and weight are primary concerns. Theoretically, tensile and flexural moduli of composites are strongly dependent on the modulus of the components and display slight sensitivity to interfacial adhesions. In natural fiber reinforced biocomposites, the inclusion of a rigid phase such as cellulose fibers, contribute to increase the polymer matrix stiffness.

In fact, not only the modulus, but also the tensile and flexural strengths are sen­sitive of the fiber/matrix interfacial adhesion, and interface is a determent factor in transferring the stress from the matrix to the fibrous phase. So, in order to create a good and strong fiber/matrix interfacial adhesion between fibers (highly polar) and common polymer matrices (nonpolar), a proper strategy to improve fiber/matrix compatibility is required. Today, for optimization of a strong fiber/matrix interfacial adhesion, generally two approaches are considered as effective: the fiber surface modification and the use of an appropriate compatibilizing agent. Generally, the me­chanical properties of natural fiber reinforced biocomposites have been improved by using surface modification treatment of the fibers such as dewaxing, merceriza — tion, bleaching, cyanoethylation, silane treatment, benzoylation, peroxide treatment, acylation, acetylation, latex coating, and steam-explosion.910

Poly(lactic acid) (PLA) is currently the most popular polymer derived from re­newable resources, which is fermented to lactic acid. The lactic acid is then, via a cyclic dilactone, lactide, ring opening polymerized to the desired polylactic acid. This polymer is modified by certain means, which enhance the temperature stability of the polymer and reduce the residual monomer content. The resulting polylactic acid can be processed similarly as polyolefins and other thermoplastics although the thermal stability could be enhanced. The polylactide is fully biodegradable. Accord­ing to our current understanding, the degradation occurs by hydrolysis to lactic acid, which is metabolized by microbes to water and carbon monoxide. By composting together with other biomasses, the biodegradation occurs within two weeks, and the material fully disappears within 3-4 weeks. PLA is a thermoplastic, aliphatic polyester, which is useful in the packaging-, electrical — and automotive industry, for example, applications where biodegradable materials started competing with cheaper synthetic plastics.

It was widely reported that tensile and flexural modulus of PLA could be im­proved by increasing the cellulose content or cellulose based reinforcements in PLA based composites [1112]. Regarding the impact properties, it was shown that tough­ness results were impaired for PLA composites reinforced with cellulose fibers12- while small improvement were obtained with the addition of cotton or kenaf fibers6. Different natural fibers have been employed in order to modify the properties of PLA. Up to now, the most studied natural fiber reinforcements for PLA have been kenaf61112, flax1314, hemp15, bamboo16, jute17, wood fibers 18. Besides conventional natural fibers, recently reed fibers have been tested in appropriate PLA composites in order to improve the tensile modulus and strengths19. Other innovative methods over the last few years to improve the mechanical properties of PLA based compos­ites have been utilization of continuous hybrid fiber reinforced composite yarn ob­tained by the microbraiding technique20. Naturally derived microbraided-yarn was fabricated by using thermoplastic biodegradable PLA resin fiber as the resin fiber and jute spun yarn as the reinforcement. Using jute spun yarn/PLA microbraided — yarn, continuous natural fibers reinforced biodegradable resin composite plates was molded by hot press molding with various molding conditions.