Relaxation of Stress and Electrical Resistivity in Carbon-Filled Vulcanizates at Moderate and High Extensions

Abstract Measurements have been made of the relaxation of stress and electrical resistivity of stretched carbon black-loaded vulcanizates throughout the elongation range up to break. Data were obtained for vulcanizates with a number of carbon blacks, varying in particle size and in structure. The influence of the degree of loading as well as of the type of elastomer was investigated. In a gum vulcanizate there is initially a decline in rate of stress relaxation at increasing elongation as a result of the limited extensibility of the elastomer chains, followed by a constant rate at higher extensions. The level for a carbon-loaded vulcanizate is always higher than for the gum. At lower elongations this is due to an additional relaxation mechanism, the reformation of broken weak bonds between elastomer and filler particles upon standing. At medium and higher elongations, the stress relaxation rate for the carbon-filled vuleanizates increases sharply with increasing extension, due to persistent carbon chain alignment and, at still higher elongation, to rupture of carbon-elastomer bonds, both phenomena leading to additional modes of stress relaxation. Resistivity relaxation rates reflect changes in carbon black distribution. An initial increase at small extensions is due to rebuilding of destroyed transient structures. A decrease in relaxation rates for medium extension ranges is caused by persistent carbon chain alignment, less pronounced at increasing elongation. At the higher elongations carbon-elastomer bonds arc broken, leading to randomization of chains, resulting in negative relaxation rates with higher structure blacks at the highest elongations. All such effects depend to a large extent on the carbon structures. The influence of loading level, of particle size and of elastomer type is discussed.