FIBER CHEMISTRY in PALM

FIBER CHEMISTRY

2.2.1    Cellulose

            Cellulose is a carbohydrate composed of carbon, hydrogen and oxygen, with the latter two elements in the same proportion as in water.  It is a polysaccharide and that means it contains many sugar repeating units (Smook, 1992). Cellulose is the major structural component of plant cell walls. It exists in the cell wall as long, thread-like fibers (microfibrils) (Casey, 1981a).

The chemical formula for cellulose is ( ) n, where n is the number of repeating units or the degree of polymerization (DP). The value of n varies with the different sources of cellulose and the treatment received. Most papermaking fibers have a weight-averaged DP in the 600-1500 range (Smook, 1992). Wood cellulose does not have a particularly high molecular weight and the highest molecular weight celluloses are generally obtained from non-woody sources such as flax and cotton (Roberts, 1996). Figure 2.1 below shows the formula of cellulose.

            Figure 2.1: Formula of cellulose (Fengel and Wegener, 1989).

The properties of cellulosic materials are related to the DP of the constituent cellulose molecules. When the molecular weight is decreasing below a certain level will cause reduction in strength of paper (Smook, 1992). The physical and mechanical properties are in large measure due to the cellulosic fibers (Roberts, 1996).

2.2.2    Hemicellulose

The hemicelluloses are a group of branched polysaccharide polymers which built up by xylan, glucomannan, glactoglucomannan, arabinogalactan and galactan (Gullichsen and Fogelholm, 2000a). Hemicellulose and degraded cellulose are categorized (by chemical means) according to degree of polymerization which are beta-cellulose and gamma cellulose. Beta-cellulose has DP range between 15 and 90 while gamma cellulose is less than 15 (Smook, 1992).

The hemicelluloses are generally water-insoluble, alkali-soluble substances that are more readily hydrolyzed by acid than in cellulose. The hemicelluloses differ from cellulose in structure that they are branched and have much lower molecular  weights (Casey, 1981a).

2.2.3    Lignin

Lignins are polyphenolic cell wall constituents, occurring exclusively in the plant kingdom. They are formed through an enzyme-catalyzed dehydrogenation polymerization of three basic precursors: (1) coniferyl (2) sinapyl and (3) p-coumaryl alcohols (Britt, 1970). The contribution of each of these three monomers to the lignin macromolecules differs depending on the source of the lignin. Gymnosperm (softwood) lignin is based on coniferyl alcohol, while angiosperm (hardwood) lignin is a mixed polymer based on both coniferyl and sinapyl alcohols, and grasses (non-wood) lignin contains all three alcohols (Roberts, 1996). Figure 2.2 below shows the structure of lignin.

Figure 2.2: Structure of lignin (Tejado et al., 2006).

The principal function of lignin is to form middle lamellae, the intercellular material which cements the fibers together (Smook, 1992). Figure 2.3 below shows the three common monolignols (lignin).

Figure 2.3: Three common monolignols (lignin) (Roberts, 1996).