Softening of Vulcanized Carbon Black-Loaded Rubber during Extension

Abstract

Abstract The results of the observations recorded in this paper may be summarized by the following items. 1. A difference between the first and second load-extension curves is to be expected in any viscoelastic material, although the behavior of black-loaded rubber at large extensions differs in some fundamental respects from a simple viscoelastic material. 2. Within the experimental limits of about ±10 per cent, all of the energy used in softening the black-loaded rubbers studied appears as heat. 3. The ratio between the area of the hysteresis cycle obtained on first stretching a specimen of black-loaded rubber and that obtained during the second extension varies only slightly from compound to compound in the range of extensions studied. 4. Although the modulus of a black-loaded rubber decreases with increasing temperature, the ratio between the hysteresis area of the first extension and that of the second extension remains roughly constant over the temperature range studied. 5. There are electrical resistance changes that correspond closely to the mechanical softening effects experienced with black-loaded rubber. Result (2) suggests that if cross-linkage breaking or changes of state of rubber are specified as mechanisms for producing softening, then the potential energy changes required must be small, compared with the mechanical energy available. Result (5) shows that changes in arrangement of the carbon black particles are associated with the softening process, and therefore suggests that the softening is related to internal strains in the rubber. These results do not prove that the softening observed in rubber is similar in principle to the behavior of a simple viscoelastic model during the first and other extensions. However, these observations are consistent with this view, except for the difficulty connected with the relationship between the softening and the permanent set in the simple viscoelastic material, and the difficulty connected with the persistence of softening. A simple linear model is obviously bound to have considerable limitations in describing the properties of a black-loaded rubber at high extensions. The limitations may arise from the nonlinearity of rubber elasticity and the non-Newtonian nature of rubber flow; alternatively, it may be necessary to postulate some entirely different process, such as rubber heterogeneity or mechanical bond breaking.