Pretreatment of Fibers and Its Role in Composite Performance

In the development of a composite system, wettability, adhesion and more uniform distribution of cellulosic material within the matrix play a major role. Compatibility of nanofibrills in polymer matrix can be improved by decreasing the hydrophilicity of fibers through surface modification or by using suitable coupling agents. The availability 

The incorporation of surfactants is also reported to improve the compatibility. The hydrophilic head of surfactant adsorbs on the cellulose surface, whereas its hydrophobic tail finds proper solvency conditions in the matrix, thus deterring aggregation of the cellular inclusions via steric stabilization. 

The formulation plays a very important role in composite properties. Percolation theory predicts a maximum enhancement in composite properties when there are just enough nanoparticles properly dispersed in the matrix material to form a continuous structure. However, beyond the optimum filler loading, the properties generally decrease [84]. Uniform dispersion of fillers in the matrix is a major concern when it comes to the composite properties [85, 86]. Inferior mechanical property due to poor distribution of cellulosic nano elements has been reported by many researchers [87]. The relationship between quality of dispersion of the nano element and the resulting effect on the mechanical properties is well described in the review by Schaefer and Justice [88]. They came to the conclusion that almost all so-called nanocomposites have fallen far short of the most optimistic expectations, and the reason for their relatively poor performance can be attributed to large-scale agglomeration of filler loading. 

The dispersion of fibers in polymer latex to prepare composite has been reported for poly(6-hydroxyoctanoate) (PHO) [101, 102], polyvinylchloride (PVC) [103], waterborne epoxy [104] and polyvinyl acetate (PVAc) [94]. Most of the works focus on the use of non-polar, non-water-sensitive polymers, while keeping an aqueous media for the processing of the films to preserve the dispersion of the nanoparticles. In their pioneering work, Favier et al [94] adopted the technique of solvent casting using a synthetic latex obtained by the copolymerization between styrene (35 wt%) and butyl-acrylate (65 wt%) (poly(S-co-BuA)). Nanowhiskers were dispersed in the latex and evaporated. The nanocomposite films were obtained by water evaporation and particle coalescence at room temperature, that is at a temperature higher than Tg of poly(S-co-BuA), around 0 C. 

For a fiber-reinforced composite, the final performance depends on fiber aspect ratio, surface area and its uniform dispersion in the polymer matrix. Ihe fact that the cel-lulosic nano/microcomposites show superior performance even at very low loading makes them a desirable candidate in composite industry. However, retaining the nanodimension and uniform dispersability in polar polymer matrix, especially in nonpolar matrix, is a huge challenge. Many methods have been adopted to overcome these issues. Extensive research in this field in the last few years has given a clear idea on the techniques to be adopted to attain stable nanofibers. Pretreatment of fibers, such as different physical and chemical techniques, are found to improve fiber-matrix interaction. These methods combined with proper processing techniques can result in the development of cellulose nano/microcomposites with excellent properties. 

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