Representation of Long-Time Creep in a Pure-Gum Rubber Vulcanizate

Creep may be expressed as ΔE/E ₁ = A log t + B(t-1) where ΔE is the increase of elongation above E ₁ the one-minute value, during the time t. The initial slope of a plot of ΔE/E ₁ against log t is A, a measure of physical creep. The limit of the final slope of a plot of ΔE/E ₁ against (t-1) is B, a measure of chemical creep. The validity of the equation is determined by the linearity of a plot of (ΔE/E ₁ - A log t) against (t-1) after A has been determined by the first plot. B is obtained as the slope. The equation is almost always valid and simultaneous equations can be used to determine A and B from only three observations, if desired. A, usually between 1 and 10 per cent/unit log t, is strongly dependent on cross-linking and nearly independent of temperature. B, ranging from 0.1 × 10^-5 to 10,000 × 10^-5 per cent/min is nearly independent of cross-linking and strongly dependent on temperature and specimen thickness. With an activation energy of 84-125 kJ (mol)^-1 (20-30 kcal (mol)^-1) it probably reflects oxidative degradation of the network, often initiated by ozone. The appearance time at which the creep is first observed to exceed A log t can be taken as equal to B ^-1. At high temperatures B is drastically increased with a corresponding strong reduction in appearance time. Creep in excess of that given by the equation is sometimes observed during a period immediately before rupture.