Creep of Pure-Gum Rubber Vulcanizates from Indentation-Time Measurements

The compliance J (limit of the ratio of strain to stress at zero deformation) has been determined from measurements of the indentation of a flat rubber surface by a rigid sphere, as a function of time t and temperature T. The results are subjected to two successive operations: (1) Jis multiplied by the absolute temperature T and (2) an empirically-determined number is added to the logarithm of the time at each temperature to make the values of JT agree as well as possible. For natural rubber from 25° to −40° C the shift required appears to correspond to a constant “activation energy” of 38kcal/mole; from −40° to − 60° C the shift is in quite good agreement with that predicted by the equation of Williams, Landel, and Ferry. Butyl rubber yields an “activation energy” of 20 kcal/mole while styrene-butadiene rubber gives a value of 22 kcal/mole. The resulting curve of JT against log t shows a sigmoid form with an increase of slope over 2 to 3 decades and a decrease at higher values. There is usually an extended region of nearly constant slope corresponding to the conditions of normal use of rubber products. For natural rubber this slope is 1 to 2% per decade; for the synthetics it is appreciably higher, reaching a value of 15% per decade for nitrile rubber. This behavior differs from that of a classical idealized polymer network, for which the compliance would approach an equilibrium value at long times.