TERESA CRISTINA FONSECA SILVA, DEUSANILDE SILVA, and LUCIAN A. LUCIA

ABSTRACT

This century has been witnessing the increasing development of ecofriendly materi­als derived from natural fibers to reinforce composites. In this chapter, wood-based polymers (i. e., cellulose, hemicellulose and lignin) have been chosen as the chief biopolymeric templates for review because together they comprise the most abun­dant resource on the planet, viz., lignocellulosics. Moreover, although wood has long been used as a raw material for building, fuel, papermaking, and various other ap­plications, the potential of wood-based polymers to reinforce composites has shown significant progress. One of the greatest challenges to advancing this area had been the lack of economic and abundant alternatives for natural fiber-reinforced compos­ites. This issue is currently being addressed by the application of lignocellulosics. The chemical, thermal, and physical properties of the biopolymers and resultant composites are influenced by the molecular weight distribution and composition of the biopolymers with the miscibility of the individual components being of great significance and often being the limiting aspect to the optimization of the physi­cal properties of the final blends. To overcome miscibility issues between many naturally occurring polymers and associated composites, chemical modification and graft polymerization of the surfaces of such biopolymers are common approaches.

This chapter will review the overall characteristics as and mechanical properties of the reinforcing biopolymers that come from wood and are used in the final bio­composites. In addition, the methods employed for polymer modification, mainly the chemical methods, will be discussed.

14.1 INTRODUCTION

Broadly defined, biocomposites tend to be composite materials formed from natu — ral/bio-based fibers as the reinforcing elements and petroleum-derived nonbiode­gradable polymers (e. g., PP, PE) or biodegradable polymers (e. g., PLA, PHA) as the matrix material. Biocomposites from plants and related biomaterials are more ecofriendly because the traditional composite structures (reinforced with epoxy, un­saturated polyester resins, polyurethanes, or phenolics) tend to have negative im­pacts on the environment because of their nonrenewable (nonbiodegradable) nature. Biocomposites can be classified as partially or completely biodegradable as shown in Fig. 14.1.

In general, an appropriate selection of biopolymers is usually determined by the stiffness and tensile strength of the resultant composite. Other deciding factors are thermal stability, self-adhesion of fibers, and matrix, dynamic and long-term behav­ior, economics and processing costs.

Although bio-fibers confer some attractive properties to manufacturers such as flexibility during processing, high specific stiffness, and low cost, the formed composite still lacks the necessary thermal and mechanical properties desirable for engineering plastics compared to synthetic polymers. Also, an enhanced miscibility between natural fibers and matrix should provide better biocomposites. Intensive research on these new compositions and processes has triggered an increased level of development and application. For instance, the worldwide capacity of bio-based plastics is expected to increase from 0.36 million metric ton (2007) to 2.33 million metric ton by 2013 and to 3.45 million metric ton by 2020.

Over the last few years, a number of research efforts have focused on investigat­ing the future development of natural fibers as load bearing constituents in compos­ite materials. The use of such materials in composites has increased mainly due to their abundance and relative low cost, their ability to recycle, and fact that they can compete well in terms of strength per weight of material.