CURE CHARACTERISTICS OF UNACCELERATED SULFUR VULCANIZATION OF EPOXIDIZED NATURAL-RUBBER

The curing characteristics of unaccelerated sulfur vulcanization of EN R 25 and ENR 50 were studied in the temperature range from 100-180 deg rees C. The range of sulfur loading was from 1.5 to 6.5 phr. The scorc h time was determined by Mooney Shearing Disk Viscometer whereas the i nitial cure rate, maximum torque, and reversion properties were obtain ed from the Moving Die Rheometer (MDR 2000). Results shows that ENR 25 gives a longer scorch time than ENR 50, an observation similar to tha t in an accelerated system reported earlier. For temperature < 120 deg rees C, scorch time depends exponentially on sulfur loading for both r ubbers. However, this dependence diminishes as temperature is increase d. This observation is attributed to the availability of activated sul fur molecules for vulcanization. The initial cure rate and maximum tor que increases with increasing sulfur loading. ENR 50, however, exhibit s higher value than ENR 25, suggesting faster cure in the former. For a fixed sulfur loading, reversion is a time and temperature-dependent phenomenon. It decreases with increasing sulfur loading because of the increase of cross-linking density for both rubbers studied. (C) 1996 John Wiley & Sons, Inc.