Effect of Calcium Stearate Concentration on Tensile Properties of Epoxidized Natural Rubber

Abstract

The tensile strength and elongation at break of epoxidized natural rubber (ENR) was investigated by adding different concentration of calcium stearate. Two grades of epoxidized natural rubber (ENR), namely, ENR 25 and ENR 40 were used as model materials. Semi-efficient vulcanization (semi-EV) was used throughout the study. The tensile property of unaged and aged ENR vulcanizates was determined by using U-CAN electron tensile testing machine. Results indicate that tensile strength and elongation at break of unaged ENR 25 and ENR40 vulcanizates increase gradually with increasing concentration of calcium stearate up to about 1phr of calcium stearate and after which, the tensile strength and elongation at break drop with a further increase in calcium stearate loading. For a similar alcium stearate concentration, the tensile strength and elongation at break of ENR 40 vulcanizates exhibits a consistently higher value than that of ENR 25 vulcanizates, which is due to the higher crosslink density of ENR40 which contains more epoxide groups than ENR 25. For aged samples, the addition of calcium stearate could enhance the tensile strength and elongation at break of aged samples. Percentage retention of tensile strength and elongation at break also gradually increases with calcium stearate loading. That means that the incorporation of calcium stearate is found to improve the aging resistance of ENR vulcanizates. This is because calcium stearate neutralizes sulfur acids which can catalyze ring-opening of expoxide groups via ether crosslinks.