*4.2.2 Chemical modification of fibres*

Natural fibres possesses high polarity in nature due to the presence of numerous hydroxyl groups on the fibre surface which makes them incompatible with the synthetic hydrophobic matrix resulting in poor interfacial bonding between the cellulosic fibres and the matrix producing bio-composites with lesser physical and mechanical strength. Chemical treatments of fibres are an important step for processing of bio-composites and enhancing the compatibility of fibres to the synthetic matrix. Bi-functional groups are introduced chemically into the fibres leading to activation of hydroxyl groups. The activated hydroxyl groups further react with the synthetic matrix thereby enhancing the interfacial adhesion and compatibility between the fibres and matrix [76, 82]. Reviews and chemical methods have been reported in the past to increase the functional behaviour of hydroxyl groups present in the polysaccharides. These methods have their own limitations for particular industrial aspects and are used for diverse industrial applications [83–94]. Mercerization, benzoylation, acrylation, acrylonitrile grafting, permanganate treatment, peroxide treatment, and isocyanate treatment, bleaching, acetylation, silane and peroxide treatments, etherifications viz. cynoethylation, quaternization, carboxymethylation and various coupling agents are commonly used for lignocellulosic fiber activation [76, 87]. Mercerization of fibres using alkali at different concentration and time period for activation and production of modified fibres with good adhesion properties have been applied for modification of fibres [95, 96]. Transverse strength of flax fibres may be increased sufficiently by the alkaline treatment which led to produce better adhesion properties between flax fibres and epoxy matrix [97]. Introduction of acetyl groups into the cellulosic fibres increases plasticity leading to hydrophobic character and mechanical strength to the reinforcing material [76, 98]. Coupling agents are frequently and successfully used to reduce the interfacial adhesion of fibres to the matrix. Various organosilanes mostly trialkoxysilanes are variably used as coupling agents and the process is referred as Silanization. The reactive alkoxy groups present in the silanes chemically bond with the hydroxyl groups and the formation of polysiloxane structures occurs [58, 99]. Maleic anhydride is another coupling reagent used to increase the interfacial adhesion of biocomposites [100]. Partial removal of lignin on the henequen fibres increases the adsorption of the silane couplings and interaction among the fibre and the matrix [101]. The mechanical properties including tensile, flexural, impact strengths and tensile modulus of the biocomposites were improved several times on Jute fibre polypropylene composites using m-isopropenyl-α-α-dimethylbenzyl-isocyanate (m-TMI) as the coupling agent using grafting process. Further, the tensile modulus of the composites prepared from virgin polypropylene increases manifold [102]. The use of the clay in the bio-composites formulation led to reduced mechanical properties. Intriguingly, techniques such as pre-coated fibres with nanoclay and maleated polyethylene mixture enhance the synergetic effect of the clay and bamboo fiber and further significantly increase the tensile strength, bending modulus and strength of the high density polyethylene bamboo composites [103].

Maleated coupling agents are widely used to strengthen composites containing fillers and fibre reinforcements. The maleated coupling provides efficient interaction of maleic anhydride with the functional surface of fibres and matrix. Agrofibre polypropylene composites were studied by introducing maleated coupler

#### *Biocomposites*

that provides the flexural and tensile strengths by more than 60% with Epolene™ G-3015 increment in comparison to composites without coupler [89]. Maleic anhydride grafted rice husk [104], hemp fibres unsaturated polyester composites coupled with 3-isopropenyl-dimethylbenzyl isocyanate [105], maleic anhydridegrafted polypropylene jute fibre composites [106], coir fiber and m-isopropenylα-α-dimethylbenzyl isocyanate grafted polypropylene composites [107] may be implemented in production of superior biocomposites having high mechanical properties and strength.
