A Review of Chemical Treatments on Natural Fibers-Based Hybrid Composites for Engineering Applications

Introduction

For the past few decades, the use of natural fiber reinforced composite materials has increased considerably due to its low price and biodegradable compared to synthetic fibers and thereby gained attention from both scientific and industrial communities. Natural fiber reinforced composites are primarily used in applications such as civil engineering and automobile sectors in seat backs, dashboards, door panels, package trays, headliners, and trunk liners etc., (Puglia et al., 2005). The classification of composites is given in Figure 1. Traditional synthetic fiber reinforced composites use different types of glass, carbon, aluminum oxide, aramid, Kevlar and many others as reinforcing materials because of high stiffness and strength properties (Rout et al., 2001). However, synthetic fibers have disadvantages such as processing cost, recyclability, biodegradability, energy consumption, health hazards, environmental safety etc., (Bledzki et al., 1999). Hence, natural fibers like bamboo, flax, hemp, sisal, kenaf, and jute have been well recognized amongst researchers as a potential reinforcement material for engineering fiber components. The mechanical properties of some natural and synthetic fibers are listed in Table 1. Natural fibers have advantages over their synthetic counterparts such as low cost, acceptable specific strength, lightweight, non-toxic, low density, high specific modulus, bio-degradable and easy processing (Abdelmouleh et al., 2007; Hapuarachchi et al., 2007). These advantages make natural fibers to widen its applications as well as it becomes potential replacement of synthetic fibers in composite sector materials. However, natural fibers possess certain drawbacks in their structural compositions such as cellulose, hemicelluloses, lignin, pectin and waxy substances which allow excess moisture absorption from the surrounding which tends to weak interface bonding between the fibers and matrix (Doan et al., 2006). The composition of some of the natural fibers are shown in Table 2. The hydrophilic nature of fibers and hydrophobic nature of matrix makes the couplings between the two phases are difficult and this tends to ineffective stress transfer at the interface of the composites which results in reduced physical and mechanical properties. Therefore, chemical treatments are certainly needed on the surface of natural fibers to improve interfacial adhesion. Chemical treatments may activate hydroxyl functional groups in their structure that can react with the fiber surface which changes their composition and can effectively interlock with the matrix as well as reduction of moisture absorption properties (Abdelmouleh et al., 2007). The chemically treated natural fiber reinforced composites are investigated by several researchers aiming to enhance physical, mechanical, and thermal properties through proper adhesion of fibers with the polymer matrix (Ray et al., 2001; Mishra et al., 2001; Weyenberg et al., 2003). The commonly available natural fibers reinforced composites are not the primary choice for advanced engineering applications due to its lack in specific mechanical and thermal properties compared to synthetic fibers reinforced composites. Therefore, hybridization of natural-natural or natural-synthetic fibers are well suited for advanced engineering components because of better load carrying capacity, impact energy absorption, high strength and stiffness compared to single fiber reinforced composites. Hybrid reinforced composites comprised only natural-natural fibers are potentially useful materials with respect to environmental concerns (Saw and Datta, 2009; Idicula et al., 2010). However, researchers are exploring the use of hybrid natural and synthetic fibers in composites to widen its applications and to improve good mechanical performance and reduce the processing cost (Hariharan and Khalil, 2005; Rosa et al., 2009).