Natural fibres are usually hydrophilic in nature and also regulate moisture phenomena. Their moisture content can reach from 8% to 12.6% and their degrading temperature is almost 180°-200°C.1,2 The development of natural fibre-reinforced composites has increased the extensive array of applications for which they are suited. One of the most important characteristics of a fibre is its mechanical enactment due to interfacial bonding between the fibre and the polymer matrix.2 Interfacial compatibility can be achieved by ensuring physical and chemical compatibility between the fibre and matrix. If this is not done, it leads to poor dispersion and adhesion and reduces the overall quality of the composite. This can be rectified through strategy modifications. Mechanical interlocking, electrostatic adhesion, interdiffusion, and chemical reactions are generally accountable for the interfacial bonding of natural fibre-reinforced composites; structural property associations can be identified by conducting indirect and direct interfacial assessments such as the microbond test (MT), single fibre fragmentation test (SFFT), and single fibre pull-out test (SFPT).
Effective inherent and superior interfacial bonding and stress transfer throughout the interface can be created through physical treatments such as heat treatment, solvent extraction, and corona and plasma treatments. Physicochemical processes such as UV bombardment, y-ray, and laser treatments improve interfacial bonding. Chemical modifications can also improve the compatibility and bonding between the lignocellulosic molecules and hydrocarbon-based polymers3-5 by changing the surface tension and impregnating the fibres.6
Lee and Wang found that fibre alkalization and the addition of coupling agents can be used to increase interfacial compatibility. Biocomposites treated w'ith an alkali solution and silane coupling agent show improvement in the modulus of elasticity, tensile strength, and moisture resistance.5,7 Polylactic acid (PLA), chitin, chi- tosan, starch, cellulose, collagen, lignin, polyhydroxyalkanoate, soy-based resins, and natural rubber are fully biodegradable and sustainable biopolymers. Bio-based materials have become increasingly appealing to most researchers and engineers in recent times as they offer an eco-friendlier alternative to traditional materials.8-10
Azwa et al. added a 10 mm thick water-based acrylic clear electrometric coating to bamboo/polyester composites to provide an additional barrier to the moisture penetration. Additionally, it provides durable protection against microorganisms, chemical exposure, rotting, UV rays, and abrasion."
Blends within Natural Fibres
The key components of natural plant fibre include cellulose, hemicellulose, lignin, pectin, waxes, and other low-molecule substances. Cellulose is the major structural element found in the form of lean rod-like crystalline microfibrils, aligned along the length of fibre.12
It is a semi-crystalline polysaccharide, consisting of a linear chain of hundreds to thousands of p-( 1 -4)-glycosidic bonds associated with D-glucopyranose along with the existence of a large number of hydroxyl groups. Hemicellulose is a low-molec- ular-weight polysaccharide that functions as a cementing matrix amongst cellulose microfibrils, which are present along with cellulose in almost all plant cell walls. Cellulose is crystalline, strong, and resistant to hydrolysis, while hemicellulose has a random, amorphous structure with poor strength. Furthermore, it is hydrophilic and can be simply hydrolyzed by dilute acids and bases.1314
Lignin is one class of complex hydrocarbon polymers - that is, cross-linked phenol polymers - and it gives a plant its rigidity. It is relatively hydrophobic and aromatic in nature. Pectin is a structural heteropolysaccharide enclosed in the primary cell walls of plants, and it gives plants their flexibility. Wax and water-soluble ingredients are used to protect the fibre and its surface. The distinctive chemical structure makes the natural plant fibre hydrophilic in nature.15
Weak interfacial adhesion formation may arise due to a fibre’s hydrophobic nature, limited processing temperatures, non-polar poor moisture resistance, inferior fire resistance, the formation of hydrogen bonds within the fibre itself, the development of bundles in the fibre, uneven spreading in a polar matrix during complex processing, and inadequate wetting of fibre by the matrix.16-18
Interfacial adhesion can be improved with various physical treatments, and one method used to change the surface structural properties of fibre is electrical discharge. Hosting surface crosslinking and altering the surface energy or creating reactive free radicals, groups, and thereby stimulating the mechanical bonding to the matrix. Chemical modification can permanently alter the natural fibre cell walls by grafting polymers onto the fibres, crosslinking the fibre cell walls, or using coupling agents.19
Combining natural fibres with bio plastics to manufacture fully biodegradable composite materials has drawn attention for various multifunctional needs. Since, numerous polymers are biodegradable and retain antimicrobial and antioxidant stuff.20 Composites of polylactic acid (PLA) and natural plant fibre comprising <30 wt% of fibre have shown increased tensile modulus and reduced tensile strength. Adopting various chemical methods of transforming the surface of cellulosic fibres, such as cyanoethylation, esterification, and acetylation along with coupling agents/ compatibilizers, improves interface adhesion between the PLA matrix and the natural fibres. Natural fibres such as wood, bamboo and its flour, kenaf fibre, flax fibre, jute fibre, banana fibre, Grewia optia and nettle fibre, nanocellulose fibre, and jute- lyocell fibre have shown good compatibility with PLA.21-25
Polyhydroxyalkanoates (PHAs) belong to the family of linear polyesters made in nature by bacterial fermentation of sugar or lipids as intracellular carbon and energy storage granules.26 Their degrading temperature is 200-250°C. A composite made up of PHAs fibre along with natural plant fibre shows high glass transition temperature, higher thermal stability, and higher heat distortion temperature. Crystallinity also improves with fibril loading and the composite can be utilized for various structural applications.26-29 The chief copolymers or homopolymers of PHAs are poly (3-hydroxybutyrate) (PHB), poly (3-hydroxybutyrate-co-3-hy- droxyvalerate) (PHBV), poly (3-hydroxybutyrate-cohydroxyhexanoate), and poly (3-hydroxybutyrate-co-hydroxyoctanoate). Jute and lyocell fibres and bamboo micro-fabril fibres show good compatibility with a PHB matrix. Tea plant fibre, beer spent-grain fibre, and wood flour show good compatibility with PHA. Wood fibre, recycled cellulose fibre, and cellulose nanowhisker show good compatibility with PH В V.30
Starch can be converted to thermoplastic starch (TPS) in the existence of plasticizers (water, glycol, glycerine, sorbitol, etc.) under the pressure of high temperature and shear by forming hydrogen bonds with the starch.31 Pre-gelatinized cassava starch with Luffa fibre, thermoplastic corn starch with bleached E. urograndis pulp, cellulose derivatives/starch blends with sisal fibre, thermoplastic rice starch with cotton fibre, thermoplastic corn starch with sugar-cane and banana fibres, thermoplastic cassava starch with cassava bagasse cellulose nanofibrils, Thermoplastic maize starch with wheat straw cellulose nanofibrils, thermoplastic cassava starch with jute and kapok fibres all show good compatibility.31-35