"Black Scorch" in EPDM Compounds
C. Daniel, J.G. Pillow and W.Steinhoff, DuPontDow Elastomers
While black scorch is known in the rubber industry, it is not well documented. The phenomenon of "black scorch" has serious consequences on the processing of EPDM compounds because it can result in a significant change in viscosity and a shortening of the scorch time even when no curative is present. However the effect is reversible and if the compound is put back through the mixer it may be restored back to its original viscosity, only for further processing to cause yet another increase. The change in viscosity is accompanied by a reduction in the Mooney scorch time of the compound. This effect is much less reversible than the viscosity increase and was therefore found to be an effective way to study black scorch. The variables for EPDM elastomer and carbon black were examined in turn to find out which are the most critical in shortening the scorch time. The Rubber Process Analyser (RPA) was used to show what initiated the network formation, which was not found in a standard rheometer. This paper describes how the effect can be seen, what influences it, and what can be done to inhibit and minimise it.