CONCENTRATION-EFFECT OF STEARIC-ACID ON SCORCH BEHAVIOR OF EPOXIDIZEDNATURAL-RUBBER

The effect of stearic acid on Mooney scorch time of epoxidized natural rubber (ENR 25 and ENR 50) and one grade of unmodified natural rubber (SMR L) was investigated in the concentration range of 0.5 to 14.5 ph r. Other parameters, namely accelerator systems, temperature, and fill ers (carbon black and silica), on the scorch property of ENR 25 in the presence of excess loading of stearic acid were also studied. Results indicate that scorch time increases with stearic acid loading for all the rubbers investigated, the rate of increase being fastest in ENR 5 0, followed by ENR 25 and SMR L. Mooney scorch time of ENR shows stron g dependence on stearic acid loading for delay-action accelerators and at lower temperatures of vulcanization. For a fixed filler loading, t he dependence of scorch time on stearic acid concentration is similar to that of gum stock. The retardation effect exhibited by excess stear ic acid on the vulcanization of ENR may be attributed to complex forma tion of chelates and the reduction in activation of adjacent double bo nds in ENR resulting from interaction between stearic acid and the epo xide group of ENR. (C) 1995 John Wiley and Sons,Inc.