Preparation and characterization of rice husk ash filled natural rubber film: effect of cross-linking agents on mechanical and electrical properties

Abstract

Natural rubber (NR) is one of the most important polymers produced by plants and is widely utilized as raw material for many products due to its excellent flexibility. Rice husk ash (RHA) is one of the major agricultural residues generated from the rice milling plant. In order to add value to NR and RHA, this study aims to use different types of RHA (black rice husk ash (BRHA), white rice husk ash (WRHA) and BRHA treated by acid washing (BRHAT)) as filler in NR composites. By the addition of alginate as a thickening and dispersing agent, a maximum of 100 per hundred rubbers (phr) of RHA could be integrated in NR matrix without phase separation. In addition, this study investigated the effects of crosslinking NR composite by CaCl2 compared to the common crosslinking process by sulfur vulcanization on mechanical and electrical properties of NR composites. Mechanical properties of the composite films with RHAs in terms of tensile strength, Young’s modulus and elongation at break were considerably enhanced, compared to the neat NR film. The composite films reinforced with WRHA demonstrated relatively better mechanical properties than those reinforced with BRHA and BRHAT, respectively. The crosslinking by CaCl2 was achieved on NR–WRHA, in which tensile strength and chemical resistance of the composite films were improved close to properties of sulfur vulcanized NR films. The NR composites were biodegradable in soil, with weight loss of 7.6–18.3% of the initial dry weight after 3 months. Dielectric constant, dielectric loss factors and electrical conductivity of the composites were enhanced with RHAs loading. However, dielectric constant, dielectric loss factor and electrical conductivity of NR composites with RHAs had dropped significantly after crosslinking the composites by CaCl2 or by sulfur vulcanization. Moreover, bacterial cellulose (BC) reinforced with skim/fresh natural rubber latex (SNRL/FNRL) were future developed by combining the prominent mechanical properties of multilayer BC nanofibrous structural networks and the high elasticity of NR. As compared to FNRL, SNRL could easily diffuse through the pores of a BC network. Many good benefits were obtained for the reinforcement with SNRL, including good mechanical properties, chemical resistance and noticeably improved dielectric properties. The SNRL-BC films are biodegradable and could be mostly or totally decomposed in soil within 6 weeks. The composite films developed in this study display potential for further application as semiconducting polymer films and flexible electronic devices for electronic applications.