Relaxation of Stress and Electrical Resistivity in Carbon-Filled Vulcanizates at Minute Shear Strains

Abstract

Abstract Measurements have been made of the relaxation of stress and electrical resistivity of carbon black-loaded vulcanizates subjected to shear deformation varying between 0.5 and 20%. Samples were initially brought into an equilibrium state by the application of a sinusoidal shear strain for a period of time long enough to ensure equilibrium condition, established by constancy in the dynamic stress- and resistivity-time cycles at each selected strain amplitude. It could be shown that at the small strains employed all phenomena were completely reversible, indicating the absence of rupture of elastomer chains or crosslinks and of strong bonds between carbon and elastomer, as well as of chemical aging. The shear stress relaxation rates show a small initial decrease with deformation and are fairly constant over the rest of the deformation range. Carbon black loading increases the rate—the more, the smaller the particles and the higher, the higher the loading—but the rate is independent of the carbon structure. Actual initial stress values follow the expected stress-strain relations. Resistivity relaxation rates, on the other hand, show a very rapid increase with shear deformation, more pronounced at higher structures and for smaller particles. Initial resistivity values are in complete accord with the dynamic behavior of vulcanizates. An explanation of the shear stress relaxation rates of the loaded vulcanizates has been made on the basis of reestablishment of previously disrupted weak bonds between particles and elastomer chains. Resistivity relaxation rates could be explained by assuming rebuilding of disrupted transient structures, while the unusual behavior of Graphon, with a zero relaxation rate, appears to be caused by its lack of bonding to elastomer chains. Finally, there are indications that the equilibrium resistivity behavior of a carbon-loaded vulcanizate subjected to shear strain at increasing amplitudes under reversible conditions is closely parallel to the reinforcement characteristics of the carbon black filler. Measurement at higher temperature permits calculation of an activation energy which appears to correspond to physical interaction.