## 7 Physico-Chemical Treatment of the Natural Fibers
### Chemical Treatment of Natural Fibers  
Chemical treatment, on the other hand, is an attempt to change the composition of the material by adding new elements to interact with the matrix by utilizing a hydroxyl group to modify and activate the fiber structure [137]. Chemical treatment of fibers improves interfacial shear and tensile strength, modulus of elasticity, elongation at break/failure strain, fracture toughness, flexural characteristics and impact strength of composites, and thermal stability and long-term moisture resistance attributed to the reduced moisture uptake of natural fibers [138; 139]. Several works are available based on the surface modification of the fiber surface with the chemical treatments in the literature [36; 37; 38; 39; 40]. The roughness of the fibers can be increased by alkali treatment to promote fiber-matrix adhesion. Alkali treatment eliminates components such as hemicellulose, lignin, pectin, fat, and wax from the fiber, exposing cellulose and increasing surface roughness/area, resulting in better interfacial bonding. The cellulose crystallinity increases with alkali treatment by modifying the cellulose structure as the treatment removes such materials that obstruct cellulose crystallinity. However, the harsher treatments lead the material into becoming amorphous [34]. [PERSON] et al. [41] investigated the interfacial adhesion between bamboo fibers and epoxy using scanning electron microscopy and reported that the interfacial bonding could be using NaOH treatment of the fibers. The presence of contaminants prevents bonding between the fibers and matrix. Typically, NaOH treatment is used to remove the wax coating and roughen the fiber surface [141]. It also results in the establishment of a strong interfacial interaction between the fiber and the matrix. The NaOH and acrylic acid-treated fibers showed improved mechanical properties. The treated fibers also showed less water absorption, leading to their construction application where higher strength is required [142]. Acetylation, despite being costly, increases the hydrophobicity and interfacial properties because the acetyl functional group (hydrophobic) is attached, after acetylation, into natural fibers [143; 144]. Fiber-specific processes (mercerizing) for cellulosic fibers (cotton and bast) improve properties such as dye and luster due to increased specular reflection resulting from a change in fiber from flat ribbon to circular shape [145]. Chemical crosslinking treatment improves the wrinkle recovery of fibers because the amorphous cellulose molecules are restricted for movement because of crosslinking [146]. The mechanical stability of the NFRCs increases by the acetylation [144; 147] and grafting treatment [18; 24] of the fibers due to the increased bonding between the fiber and matrix. The grafting copolymerization of natural fibers improves hydrophobicity, stress transfer, and interfacial adhesion properties in the NFRCs [18]. [PERSON] et al. [148] studied the effect of hydrogen peroxide treatment along with silanization on flax/cotton epoxy laminates and found improvement in mechanical characteristics because the treatment enhanced the adhesion between fiber and matrix. Isocyanate treatment of fiber improves mechanical properties and reduces hydrophobicity. This improvement occurs because the isocyanate functional group reacts with a hydroxyl group and forms urethane linkages, reducing hydrophobicity. Thus, fiber matrix adhesion improves, leading to better mechanical strength [149; 150]. Compared to isocyanate, benzoylation treatment effectively reduces hydrophilicity. Moreover, it also improves the thermal stability of fibers. The benzoyl (C\({}_{6}\)H\({}_{5}\)C=O) group reduces hydrophobicity [150]. However, fibers before isocyanate and benzoylation treatment re  
Figure 4: Results of fiber treatments [140]. Reprinted with permission. Copyright 2020 MDPI. quire alkali treatment to remove lignin, wax, and oil coverings.