Natural fibers matrix interactions, chemical

Natural fibers-reinforced polymer matrixes provide more alternatives in the materials market due to their unique advantages. Poor fiber-matrix interfacial adhesion may affect the physical and mechanical properties of the resulting composites due to the surface incompatibihty between hydrophilic natural fibers and non-polar polymers. The results presented in this chapter focus on the properties of palm and pineapple fibers in terms of their physical and chemical structure, mechanical properties and processing behavior. The final properties of these fibers with thermoplastics matrixes are also presented, paying particular attention to the use of physical and chemical treatments for the improvement of fiber-matrix interaction. 

The chemical and/or physical interaction between the silane-treated natural fibers and the polymer matrix is an important factor for improving the mechanical properties of biocomposites. Physical mixing of silane-treated natural fibers with thermoplastic resins can enhance the fiber-matrix interaction through in-termolecular entanglement or acid-base interaction [56]. In this case, limited improvement in the mechanical properties may be expected. The mechanical properties may be marginally increased by the increased wettability and the uniform dispersion of silane-treated natural fibers into the thermoplastic matrices [71]. The molecular chains may be interdiffused into the natural fiber surfaces, forming a (semi)interpenetrating polymer network [65, 72]. 

In thermoplastic/fiber systems, the fiber may act as a series of nucleating sites for the polymer, resulting in a transcrystaUine region around the fiber. In this chapter, a large number of factors have been demonstrated to contribute to the interactions across the interface. The effect of various conditions such as the chemical modification of natural fibers and nucleation ability of semicrystalUne matrix on the formation of transcrystallinity was investigated. 

Chemical or physical treatments can be applied to natural fibers to modify their surface (polar groups) and/or their composition (lignin, cellulose and hemicellulose content). Several treatments may enhance the interaction between the phenolic matrix and natural fibers, such as mercerization, succinic anhydride, ionized air, and others (Leao, 1997 Mu et al., 2009 Trindade et al., 2008). Barreto et al. (2010) prepared composites of a phenolic matrix. 

This chapter summarizes different surface modifications available to physically, chemically, or biologically modified natural fibers. Such a modification is required to improve the interaction between the fiber and matrix, which could attribute to better interlocking and in turn result in enhanced bulk mechanical properties of the resulting composites. 

Chemical Approaches to Promote Natural Fiber-Polymer Matrix Interactions... 

Composites of natural cellulosic fibers may have the best option to the array because the nanofibers can interact with other natural materials to form highly ordered structures. The cellulose microlibrils have hydroxyl groups (OH) on their surface, which can form covalent bonds with the matrix. In literature, three alternative routes for the preparation of composites of cellulose are known [40]. The first route refers to the incorporation of hydrophobic libers in matrices such as polyethylene, polypropylene and polystyrene. In this case, it is necessary to a chemical or physical treatment, so that the surfaces of the matrix and the libers... 

Another chemical modification studied was poly(vinyl alcohol) (PVA). The modification was conducted in water hyacinth fiber before mixing with low-density polyethylene and natural rubber to form a LDPE/NR/WHF composite. The tensile strength and modulus, melting temperature and water absorption resistance of the LDPE/NR/ WHF composites are reported to increase after PVA modification. However, the elongation at break decreased after the modification. Lower value of interparticle spacing was also found in PVA-modified WHF composites, enhancing the interparticle interaction between WHF and the matrix. 

To improve adhesion of binders to fibres, including carbon fibers, methods of surface treatment by cold plasma were developed. In the course of such treatment, the removal of a weak border layer of the fiber proceeds and the contact between the surface and a binder is improved. At the same time, the number of active centers capable of chemical interaction with a binder increases and the wetting becomes better. It may be expected that pol5mierization under plasma action may also serve as a tool adhesion improvement at the phase border. In spite of the existence of many ways of surface treatment of the reinforcement surface, no model of interaction was proposed which is effective in predicting the t5T)e of reinforcement by surface treatment of a given filler-matrix combination. According to Drzal, the major reason for this lack of theoretical developments is in the over-simplification of the composition and nature of the filler-matrix interface. 

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