Table 4.

Physical and Chemical Treatment for Surface Modifications [93].

Type of Treatment	Name of Treatment	Mechanism of Treatment	Improvement
Physical	Corona	The formation of a high-energy electromagnetic field close to charged thin wires/points induced ionization species (ions, radicals, etc.) and activated for surface modification through introduction of oxygen-containing functional groups	-Existence of hydroperoxide groups that could initiate grafting of the matrix chains led to significant improvement of interfacial shear strength
	Plasma	Similar mechanism to corona. However, the apparatus required a vacuum chamber and gas feed to maintain the appropriate composition of the gaseous mixture.
	Mercerisation	Soaking the fiber in sodium hydroxide.	-Improves adhesive characteristics by removing natural and artificial impurities and promotes rough surface topography Fiber fibrillation (breaking down the composites fiber bundle into smaller fiber) Increase the effective surface area available for contact with the wet matrix Enhances the reactivity
	Heat treatment	Heated and the fiber undergoes physical (enthalpy, weight, strength, color, and crystallinity) and chemical changes (reduction degree of polymerization by bond scission, creation of free radicals, formation of carbonyl, carboxyl, and peroxide groups)	-Increased yield strength
Chemical	Esterification-based treatments	-Use of a variety of chemicals to form ester bonds with the fiber surface To coat the OH groups (hydrophilic character) with molecules that have a more hydrophobic nature Chemical process used for esterification: acetylation, benzylation, propionylation, and treatment with stearate	-Remove non-crystalline constituents of the fibers, thus altering the fiber surface topography
		$Fiber – OH+CH3 – C\left( \equiv O\right)-O-C\left(\equiv O\right)-CH3 \to Fiber-OCOCH3+CH3 COOH(Acetylation$) $\mathrm{React} \mathrm{the} \mathrm{hydroxyl} \mathrm{group} \left(-OH\right)$$\mathrm{of} \mathrm{the} \mathrm{fiber} \mathrm{constituents} \mathrm{with} \mathrm{acetyl} \mathrm{groups} (CH3 CO-$) for full esterification	-Modifying surface of natural fibers and making it more hydrophobics Reducing swelling of wood in water Reducing moisture absorption Enhanced thermal stability
		-$Fiber – OH+NaOH \to Fiber-O^{-}N{a}^{+}+H2O \left(Benzylation\right)$ Two stages. Firstly, immersed in sodium hydroxide solution and preceded with benzylchloride	-Promotes compatibility with polymers containing aromatic rings
		Propionylation -Similar method to acetylation; only had one more methyl group than the acetic anhydride	-Interface stress transfer efficiency improved
		Treatment with stearate $\mathrm{Modified} \mathrm{fiber} \mathrm{with} \mathrm{stearic} \mathrm{acid} \left(C17H35 COOH\right)$	-Formed stable ester bonds with the hydroxyl group
	Saline coupling agents	-Formation of covalent bonds between the Y group and the matrix during curing. $CH2 CHSi\left(OC2H5\right)3\stackrel{H2O}{\to }CH2 CHSi\left(OH\right)3 +3C2H5 OH$ $CH2 CHSi\left(OH\right)3$ $+Fiber-OH\to CH2 CHSi\left(OH\right)2O-Fiber+H2O$	-Reduce the number of hydroxyl groups on the surface of polar materials such as natural fibers rendering them more hydrophobic
	Graft copolymerization	-Two different mechanisms are involved; polymerization on the fiber surface by free radical and free radical formed by cracking the cellulose molecules Graft copolymerization can be divided into three subcategories; treatment with triazine coupling agents, treatment with isocyanates, and treatment with maleic anhydride
		-Triazine coupling agents; treated with three derivatives of trichloro-s-triazine (2-octyloamino 4, 6-dichloro-s-triazine, methacrylic aci, 3-(4,6–dichloro-s-triazine-2-yl) aminopropyl ester, 2-diallylamino 4,6–dichloro-s-triazine)	-Tensile strength increased
		-Isocyanate; formation of covalent bonds between cellulose and isocyanate coupling agent, which hydrophobises the fiber surface Increased interaction between polymethylene (polyphenylene) isocyanate (PMPPIC) and polystyrene that contain benzene ring due to interaction of delocalized $\pi -$ electrons of the benzene rings (Van der Waals type of interactions) of both polymers	-Increase in stress and modulus values of the composites superior mechanical properties and dimensional stability
		-Maleic anhydride; chemical bonds of esoteric nature, as well as hydrogen bonds, are formed between the maleic anhydride functional groups of polypropylenes and the hydroxyl group of cellulose.	-Increased in tensile strength and Young’s modulus
Various chemical	Dimethylurea (DMU)	-Reaction DMU with OH group of the fibers that subsequently almost eliminated any fiber–fiber interaction resulting from intermolecular hydrogen bonds.	-Tensile modulus and elongation increased Better dispersion of flax fibers in the matrix
	Phenol formaldehyde (PF)	-Methylol groups react with hydroxyl groups, forming stable ether bonds, while at the same time, it contains hydrophobic polymer chains.	-Water uptake of composites decreases, and moisture content of treated fiber composites is 50% lower than non-treated fiber composite