Effect of Temperature and Oxygen on the Strength of Elastomers

Abstract

Abstract 1. A linear relationship has been found to hold for SBR vulcanizates between the logarithm of the rate of steady tearing and the logarithm of air pressure, over the range 10 Pa to 100 kPa. The slope varied from 0.08 for a C-C crosslinked material to 0.18 for a polysulfide-crosslinked material. Similar effects of air pressure were found in the rate of crack propagation under intermittent loading. They are attributed to attack of oxygen (in air) on highly stressed bonds, either sulfidic or hydrocarbon. 2. Approximately linear relationships were found to hold between the logarithm of the rate of steady tearing and the reciprocal of test temperature for temperatures between 25°C and 130°C. Apparent activation energies were deduced from the slopes, of 135 kJ/mole for a polysulfide-crosslinked SBR compound, 105 kJ/mole for a C-C crosslinked SBR compound, and 85 kJ/mole for a sulfur vulcanizate of BR. For SBR, these values were independent of air pressure. 3. These activation energies are far too large to be accounted for by changes in segmental mobility (and hence in internal energy dissipation) with temperature, for simple viscoelastic materials at temperatures far above Tg. The discrepancies are attributed to a second temperature-dependent factor. It is suggested that the threshold tear strength T0, i.e., the strength under nondissipative conditions, is itself temperature dependent. 4. Estimates of the changes in T0 with temperature have been obtained from the observed discrepancies in temperature dependence of tear strength. They correspond to a decrease from about 100–250 J/m2 at 25°C to about 50 J/m2 at 130°C. These changes in T0 may reflect the stress-activated rupture of thermally-sensitive bonds.