Iranian Polymer Journal / Volume 7 Number 4 (1998) 1026-1265198 Partial Replacement of Silica by White Rice Husk Ash in Natural Rubber Compounds : The Effects of Bonding Agents Hanafi Ismail* and Few Ling Chung School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Received : 14 April 1998; accepted 24 June 1998 ABSTRACT The effects of bonding agents on the partial replacement of silica by white rice husk ash (WRHA) in natural rubber compounds were studied . Results showed that the optimum weight ratio of WRHAlsilica to obtain maximum enhancement of tensile and tear strengths was 20130 (phrlphr) . Increasing the WRHA in weight ratio of WRHAlsilica decreases the cure time, t 9 o, scorch time, t2, hardness and elongation at break but increases the resilience of the vulcanizates. For similar vulcanizates, the incorporation of bonding agents enhanced the tensile . strength, tear strength, hardness, rubber-filler inter- action and resilience, whereas, the elongation at break decreased . The incor- poration of bonding agents also increases the too and t2. Key Words : silica, white rice husk ash, bonding agents, mechanical properties, rubber-filler interaction INTRODUCTION Bonding agents are used in rubber vulcanizates filled with non-reinforcing fillers, which by their nature show weak filler-rubber interaction . The incorpor- ation of bonding agents was found to improve the interaction between the non-reinforcing fillers and rubbers . Kishore and Rajalingam [1] reported that the mechanical properties of rubber vulcanizates filled with non-reinforcing fillers increased with the incor- poration of bonding agents . We also found [2—4] that the mechanical properties of oil palm fibre reinforced natural rubber composites improved with the incor- poration of various bonding agents. In our previous works [5, 6], the potential of white rice husk ash (WRHA) as a filler for natural rubber (SMR-L) compounds has been studied . The optimum loading of WRHA to obtain maximum mechanical properties was achieved at 10 phr after which there was deterioration in properties . In the case of epoxidized natural rubber (ENR 50), the maximum mechanical properties was observed to occur at 20 phr of filler loading . It was reported that WRHA has about 96% silica content with mean particle size of 5 .4μm, surface area of 1 .4 m2/g and density of 2 .2 Wm' [7, 8]. In this study the effects of bonding agents (resorcinol formaldehyde and hexamethylene- tetramine) on the partial replacement of silica by WRHA in natural rubber compounds have been {*) To whom correspondence should be addressed . 255