The components of lignocellulosic fibers include water soluble substances, hemicelluloses, cellulose, pectin, lignin, and waxes. Cellulose is a semicrystalline polysaccharide, the large amount of hydroxyl group in cellulose gives natural fibers

the hydrophilic character; when used to the hydrophobic matrix, the result is an incompatibility (poor adhesion) between matrix and fibers and poor resistance to moisture absorption (Yang et al. 2007). The strength of the interface adhesion (fiber/ matrix) depends on the degree of mechanical, chemical and electrostatic bonding, and level of interdiffusion between the matrix and fibers. The compatibility between the two components can be achieved by physical and chemical modification of the fibers and polymer surface, or by use of coupling agents and compatibilizers. Figure 14.4 shows SEM image of fractured surfaces of composite without coupling agent. It can be seen clearly a decohesion zones fiber/matrix. This is a clear indica­tion of the poor adhesion between fibers and the matrix.

Alkali treatment was the standard chemical modification of fibers surface. The fibers are alkali washed using sodium hydroxide to remove amorphous materi­als from the fibers surface such as waxes, pectin, and other non-cellulosic compo­nents. Figure 14.5 shows the FTIR spectrum of hemp fibers, as example, before and after alkali treatment. It was observed in the Fig. 14.5 that a disappearance of the peaks around 1,730 and 1,230 cm-1, which correspond respectively to the carboxylic

ester in pectin and to the C-O stretching in lignin, respectively. The chemical treatment used has eliminated pectin and waxes in the fibers and also it has reduced lignin rates in the fibers surface.