**5.1 Solution blending or casting**

This involves the formation of the composites through dissolution of the polymer and fibers in a suitable solvent and subsequent drying of the solvent after the processing of the composites [62–64]. In solution casting, obtaining the right solvent that will dissolve the polymer and be removed with easy is a major task, although there are water-soluble polymers [65, 66]. Examples are polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), poly(ethylene oxide) (PEO) and poly(ethylene-co-vinyl acetate) (PEVA). Many others are dissolved using nonaqueous solvents such as chloroform, xylene, benzonitrile, tetrahydrofuran (THF), toluene, dimethylformanide (DMF) and acetone [67–69].

Solution casting helps to avoid thermo-mechanical degradation usually observed with the other methods of polymer processing which involve heating and vigorous mixing. It is used mostly for film formation. The amount of solvent used in this method makes solvent casting environmentally unsustainable. In addition, most of the organic solvents are hazardous to health and are temperature sensitive. Sur et al., in their work, prepared polysulfone nanocomposites using solution casting at elevated temperature [70]. To improve miscibility amongst the fibers and polymer matrix, the fibers were soaked using the solvent separately first before mixing with the dissolved polymer. In some cases, researchers coated the fibers with selected polymers to improve the fibers miscibility and interaction [71]. In general, solution casting is preferred for polymers with poor thermal stability and susceptible to thermo-mechanical degradation.

### **5.2 Melt blending**

For the past decade now, melt blending has become a method of choice in the processing or preparation of natural fiber composites. It involves the heating of the polymer matrix up to 10–30°C above its melting point and then introduced the fiber into the molten polymer with thoroughly mixing under shear [72]. Melt blending has gained much ground in terms of acceptability, because of the existent compatibility with existing processing technology (such as injection molding and

extrusion) [73–75]. These methods are environmentally friendly and do not involve the use of hazardous solvents. Many research works have reported the use of melt blending method [58, 76, 77]. One area of concern is the processing condition and the level of interaction between the fibers and the polymer matrix used. According to [61], to obtain good distribution of the fibers within the matrix, which is one of the conditions for improved mechanical and thermal properties, there should be a favorable enthalpy of interaction between the fibers and the polymer matrix. This results in good distribution of the fiber inside the matrix. In the absent of this favorable energy, the fibers will be poorly dispersed. The conditions for processing natural fiber composites will be discussed elaborately in subsequent sections.

### **5.3 In-situ polymerization**

This technique involves the polymerization of a monomer in the presence of another polymer, mostly in small quantities [78]. In-situ polymerization has been described as one of the important methods for compatibilizing polymer blends [79]. It allows the formation of covalent bonding between constituents which can result into graft or block copolymers that ultimately results in the development of a stable interface [72]. In-situ polymerization is noted to yield specific properties with conventional melt blending methods. Furthermore, it allows the preparation of composites with high fiber weight fraction because the homogeneity of the resultant composites is much greater than that obtainable from melt blending and solution casting [80]. Most thermoset NFPCs are prepared using in-situ polymerization methods [17, 78, 81]. According to Bounor-Legaré et al. [82], subject to the nature and reactiveness of the organic or inorganic precursors and the processing factors, different types of functionality can be fashioned.
