The Impact of Age Stiffening on Fatigue Crack Growth in Natural Rubber and Polybutadiene

High-diene elastomers such as natural rubber and polybutadiene continue to stiffen over time during storage at ambient conditions. These elastomers also stiffen when heat aged at moderately elevated temperatures. Investigations were undertaken to determine what effect shelf age-stiffening and heat aging have on the fatigue performance of these elastomers. Fracture mechanics is used to study the propagation of cracks and characterize the rate at which strain energy is released as a crack grows.   This strain energy release rate is termed “tearing energy” and it can be used to describe the behavior of different materials intended for use in dynamic applications in the rubber industry. Fatigue Crack Propagation (FCP) testing was used to determine how crack growth behavior of rubber compounds changes with these two types of aging.  A NR compound, a BR compound and a NR/BR blend were used in the study.  These compounds were sulfur-cured, carbon black reinforced and contain a typical antidegradant package.  Classic heat aging for twelve weeks at 70 °C caused the crack growth rate to increase measurably for all three compounds.  Shelf age-stiffening both for one year at 21 °C and for six months at 40 °C had virtually no impact on the crack growth rate for the NR compound, but shelf age-stiffening increased the crack growth rate for both the BR compound and the NR/BR blend.