A Comparison of the Properties of Rice Husk Ash, Silica, and Calcium Carbonate Filled 75 : 25 NR/EPDM Blends Wanvimon Arayapranee, 1 Garry L. Rempel 2 1 Department of Chemical and Material Engineering, Rangsit University, Phathum Thani 12000, Thailand 2 Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada Received 21 February 2008; accepted 20 May 2008 DOI 10.1002/app.28725 Published online 10 July 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The effects of incorporation of three differ- ent fillers, i.e., rice husk ash (RHA), silica, and calcium car- bonate (CaCO 3 ), over a loading range of 0–60 phr on the curing characteristics, processability, mechanical properties, and morphology of 75 : 25 natural rubber (NR)/ethylene- propylene-diene monomer (EPDM) blends were studied using a conventional vulcanization system. Filler loading and type influence the processability of the blends in which RHA and CaCO 3 offer better processing advantage over silica. The best improvement in the tensile and tear strength and abrasion resistance of the 75 : 25 NR/EPDM blends with additional fillers was achieved when filled with silica. However, RHA and CaCO 3 were better in resilience prop- erty compared to that of silica. The RHA filled blends showed higher failure properties and abrasion resistance but lower ozone resistance than that containing CaCO 3 . Scanning electron micrographs revealed that the morphol- ogy of the blend filled with silica is finer and more homog- enous compared to the blend filled with RHA and CaCO 3 . According to these observations, RHA can be used as a cheaper filler to replace CaCO 3 in rubber blends where improved mechanical properties are not so critical. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 1165–1174, 2008 Key words: natural rubber; EPDM; blend; rice husk ash; silica; calcium carbonate; filler INTRODUCTION Natural rubber (NR) is a natural biosynthesis poly- mer having an attractive range of properties, possess- ing excellent mechanical properties, and good processing characteristics. However, NR is highly sus- ceptible to degradation because of the presence of double bonds in the main chain. Degradation of NR is accelerated mainly by heat, humidity, light, ozone, radiation, etc. In general, improvement in the poor ozone resistance of NR can be achieved by blending it with low unsaturated rubbers such as ethylene– propylene–diene rubber (EPDM) obtained by poly- merizing ethylene and propylene with a small amount of a nonconjugated diene, which imparts usually good ageing characteristics, good resistance to weathering oxidation, and chemical resistance. 1 Blending with EPDM has been reported to be effec- tive in improving the ozone resistance of NR. 2 The influence of fillers is important because the elastomers are seldom used without fillers, and in some cases, blends of two or more elastomers are preferred to achieve the desired properties. When NR is blended with EPDM, it is often found neces- sary to modify it with fillers to improve the modu- lus, failure properties (tensile and tear strength), and abrasion resistance and reduce unit manufacturing costs while maintaining a high level of product qual- ity and performance. Many studies on the mechani- cal properties of various rubber compounds have been reported, including the effect of the type and the amount of rubber and filler, and the interaction between rubbers and fillers. 3–11 Filler dispersion also plays an important role in reinforcement. The effect of fillers on mechanical and other properties of the blends depends strongly on their shape, size and size distribution of the primary particles and their aggregates, surface characteristics, and degree of dis- persion and distribution. 12 A variety of black and nonblack fillers of the reinforcing and nonreinforcing types cater to this need. One of the most important fillers, which is added to rubber, is calcium carbon- ate (CaCO 3 ). Much work 13–15 has been done to study the use of CaCO 3 as a filler for polymeric materials. It not only lowers the cost but also improves me- chanical properties such as modulus, hardness, toughness, dimensional stability, thermal resistance, and flame retardancy. Among several fillers, silica is the most important reinforcing agent used in com- pounding recipes. In spite of being well known for its strong interaction with rubbers, this filler is rela- tively expensive when compared with natural fillers. Journal of Applied Polymer Science, Vol. 110, 1165–1174 (2008) V V C 2008 Wiley Periodicals, Inc. Correspondence to: W. Arayapranee (wanvimon@rsu.ac.th). Contract grant sponsor: Rangsit Research Institute, Rangsit University.