Ultimate Tensile Properties of Elastomers. II. Comparison of Failure Envelopes for Unfilled Vulcanizates

Abstract

The tensile stress-at-break σb (based on the initial cross-sectional area) and the corresponding ultimate extension ratio λb of unfilled vulcanizates of silicone, hydrofluorocarbon (Viton B), butyl (both sulfur-cured and resin-cured), and natural rubber were determined at many strain rates and temperatures; the latter ranged from slightly above the glass transition temperature Tg, up to a temperature somewhat below that at which chemical degradation affected the results. For each vulcanizate except natural rubber, data obtained over an extended temperature range superposed to give a time- and temperature-independent failure envelope on a plot of log(σb273/T) vs log(λb−1), where T is the test temperature in °K; for natural rubber, data obtained between 90° and 120°C superposed, but those at lower temperatures did not because of strain-induced crystallization. For each vulcanizate, data at elevated temperatures gave, or tended toward, a line of unit slope on a plot of log (λbσb273/T) vs log(λb−1), where λbσb is the breaking stress based on the cross-sectional area at the moment of rupture. The position of each line corresponded to the equilibrium modulus Ee derived from stress—strain curves. Failure envelopes previously obtained for two styrene—butadiene vulcanizates, which had different crosslink densities, superposed to give a master failure envelope on a plot of log(λbσb273/T) vs logEe(λb−1). On this type of plot, failure envelopes for all the vulcanizates except silicone and natural rubber were found to be essentially identical. At a given value of λbσb, silicone had a smaller λb and natural rubber a somewhat larger λb than the vulcanizates of the three other rubbery polymers.