Interface modification of natural fibre-polymer matrix composites is a common duty before composite fabrication because the chemical nature of fibre and polymer matrix is different, i. e. natural fibre possesses hydrophilic nature, whereas polymer matrix possesses the hydrophobic nature. A number of researchers and manufactur­ers tried to improve compatibility between polymer matrix and natural fibres by various techniques such as using compatibilizer, by matrix modification, by apply­ing hybrid filler and by chemical treatments of filler. Among them chemical surface treatments of natural fibre have been reported to significantly improve the interface bonding of fibre-matrix including bleaching, alkaline treatment, silane treatment, acetylation, grafting with vinyl monomers, isocyanate and different coupling agents (Khan et al. 2009; Rosa et al. 2011). Alkali treatment enriches cellulose content of OBF around 75-80 % due to extraction of pectin and water extractives during treat­ments along with hemicelluloses from the fibre surface. The fibre became less dense and less rigid which provides the fibrils more capability of rearranging themselves along the direction of tensile deformation. The observations have also been reported for mercerized coir, flax, sisal and bamboo fibre (Varma et al. 1984; Sreenivasan et al. 1996; Sreekumar et al. 2011; Sharma et al. 1995; Das and Chakraborty 2006). Alkali treatment creates micropore on fibre and finally developed rough surface topography. Similar phenomenon happened when fibres were treated with NaClO2 (bleaching). Fibre turns into whitened, more floppy and fibrillated after this treat­ment. Cellulose percentage as well as crystallinity also increases due to removal of huge amount of lignin and impurities. However, surface coating is developed when the bleached fibre was modified by vinyl monomers. Sometimes fibrillated fibre becomes defibrillated. Modified fibre behaves like compatible reinforcing fibre with several hydrophobic polymer matrices for producing high-performance composites (Khan and Alam 2013).

The percent of elements present in OBF can be determined by an Elemental Analyzer. The main elements in untreated, alkali-treated and bleached OBF are C, H and O which are mainly for cellulose, lignin and hemicelluloses (Table 10.1). The modified fibre by acrylonitrile (AN) monomer shows trace amount of N. It may come from the CN group of acrylonitrile monomer. It is seen that the percentage of C, O and H in all types of fibre is almost the same. Hence, the fibre is not degraded upon the chemical treatments (Fig. 10.2).

The effects of chemical treatment of OBF on the physical properties are given in Table 10.1. The quality of OBF mainly depends on its physical properties, for exam­ple, fineness, moisture regain and densities (Majumdar 2002). OBF is a multicellular fibre. The unit cell of OBF is formed with plenty of cellulose molecules. These are attached with each other in longitudinal direction to produce long continuous fila­ments. The filaments are sometime attached with neighbouring filaments with inter­molecular hydrogen bonding or any other loosely attached bond to form a mesh-like structure. But attachments by week bonding between the cellulosic filaments are not sufficient to form high stiff fibre. The cementing materials (lignin and hemicelluloses) give stiffness by staying in between the gaps of the cellulosic filaments.

Table 10.1 reveals that the fineness of the fibre increases after chemical treat­ments especially in the case of alkali and bleaching treatments. Hemicelluloses are composed by p-cellulose and y-cellulose carbohydrates. These both types of carbohydrates are soluble in alkali. So, it is supposed that hemicelluloses of the fibre are partially or completely washed out in alkaline medium. On the other hand, bleaching operation is frequently performed in textile industries to remove lignin from cellulosic yarn. A part of hemicelluloses is also removed during bleaching. Splitting of the cementing materials trends to be lesser diameter as well as mesh size. Mukherjee et al. (1993) also reported the reduction of fineness value due to the removal of lignin on bleaching. Further modification through AN-grafting has given surface coating onto OBF. Thus, the fibre diameter is increased slightly. The densi­ties of untreated, alkali-treated, bleached and AN-grafted fibres are 1.15, 1.40, 1.40 and 1.32 g/cm3, respectively, showing that the surface treatments have significant effect on fibre density (Aquino et al. 2007; Bledzki and Gassan 1999).

Hydrophilic nature of cellulosic fibres is a great barrier to achieve strong adhesion with hydrophobic polymer. The presence of hydroxyl (-OH) groups in OBF cellulose is the main cause of moisture absorption of natural fibre which tends to poor wettabil­ity. To achieve better wetting of fibre in matrix, those hydrophilic hydroxyl (-OH) groups of cellulose should be blocked by suitable modification. The surface chemical treatment of OBF significantly decreased moisture absorption, concomitantly increas­ing the wettability between fibres and polymer. As seen in Table 10.1, untreated OBF contains 4-6 % of moisture, while both alkali-treated and bleached fibres contain larger amount of moisture (6-7 %). Due to the removal of hemicellulose, pectin, waxes and fats, fibre becomes floppy, i. e. it contains more pores. Therefore, moisture can easily diffuse in these pores, and hence increases moisture content. On the other hand, moisture affinity is significantly decreased when OBF is modified by AN monomer.

The tensile strength and modulus of untreated and treated OBF are determined from the average of ten fibres of each modification; the values are shown in Table 10.1. Alkali-treated and bleached fibres have shown an appreciable reduction in tensile strength. This decrease may be attributed to considerable delignification and break down of fibre took place during the chemical treatments. The elongation at break in these fibres does not vary much. The AN-grafted OBF brings about a considerable improvement in tensile strength (TS) and tensile modulus (TM). This may be attributed to the fact that AN-grafted OBF may create orderly arrangement of fibrils by surface coating via cross-linking reaction (Rout et al. 1999).