Stress relaxation in rubbers containing reinforced fillers

Stress relaxation in rubber stocks containing various fillers was measured at room temperature, and an empirical equation found for relating decrease in tension, f t , over a period of time from 0.1 sec. to 6 months, namely:\documentclass{article}\pagestyle{empty}\begin{document}$ f_t = f_{1.0} t^{- n} $\end{document} where f 1.0 is force after 1 min. of relaxation, n is the relaxation rate of material (by definition) and t is time in minutes. Raw rubbers were examined under compression, and cured rubbers were tested under tension. In both cured and raw butadiene rubbers, stress relaxation was found to be a viscous‐controlled process. In raw rubbers, reinforcing carbon blacks decrease the rate of relaxation, while in cured rubbers the effect of carbon black is very small. However, in swollen (to V R = 0.25), cured rubbers, the rate of relaxation increases with increasing carbon black loading, indicating a slippage and/or breakage of some carbon black‐polymer attachments.