Chapter 16: Lignin Polymers and Composites

Overview

The most abundantly available renewable resources are cellulose, lignin, and plant oils, in this order. Wood is made up mainly of three macromolecular species: cellulose, hemicellulose, and lignin [1]. Minor components include pectin, fats, wax, moisture, and water solubles [2]. Of these three, cellulose is the best known and most abundantly available. The estimated yearly production of cellulose by photosynthesis is 830 million tons [3]. Cellulose is a polysaccharide with an extensive amount of intra- and intermolecular hydrogen bonding, rendering it highly insoluble [4]. The hydrogen bonding and rigid structure of cellulose [Figure 16.1(a)] result in a high degree of crystallinity [4]. The cellulose in wood and natural fibers is found as fibers, consisting of helically wound crystalline microfibrils. The cellulose polysaccharide is constituted of anhydroglucopyranose units [Figure 16.1(b)] with a reducing end group (right side), where a ring opening results in the formation of an aldehyde, and a nonreducing end group (left side) in the form of a secondary alcohol.

Figure 16.1: Chemical structures of (a) cellulose and (b) the repeat unit anhydroglucopyranose. The left end group in structure (a) is nonreducing, whereas the right end group is reducing and an aldehyde can be formed by ring opening.

Hemicellulose is a low-molecular-weight polysaccharide with a degree of polymerization ranging from 70 to 200, with lower degrees of polymerization found in softwoods [5]. Hansen and Bjorkman [6] showed, using the solubility theory of Hildebrand and Scott [7] and Hansen [8], that hemicellulose acts as a compatibilizer...