Thermorheological Properties of Carbon Black Filled Elastomers

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Abstract A nonisothermal constitutive equation for a thermorheologically simple, linear viscoelastic materials has been developed. The influence of temperature on both the retardation times and the limiting compliance difference has been considered. This general nonisothermal equation can be reduced to previously published results for the creep T-jump experiment. Isothermal creep response was measured for the filled vulcanizates at various levels of strain. At relatively low strains, the true compliance was observed to increase with strain, the initial increase depending upon the carbon black volume fraction and structure. This increase in compliance was attributed to a breakdown of the carbon black network. A reversal of this trend at much larger levels of deformation was associated with orientation of the rubber chains. The creep rate at 300 seconds depends primarily on the level of strain, independent of the carbon black structure or loading. The apparent activation energy was determined by creep T-jump at various levels of stress for two series of filled compounds and an unfilled gum elastomer. For the filled materials, the activation energy decreased as the level of applied stress was increased. For example, the apparent activation energy of a SBR-1500 vulcanizate filled with 80 phr of N347 carbon black decreased from 167 to 71 kJ/mole as the applied stress was increased from 0.041 to 1.88 MPa. The activation energy at low stresses is a function of the carbon black volume fraction and structure. The apparent activation energy of the unfilled gumstock was 54 kJ/mole, independent of the level of deformation. The apparent activation energy of the filled vulcanizates approached that of the gumstock as the stress increased. The T-jump results and the isothermal data suggest the presence of two deformation mechanisms for the filled materials: (1) deformation of the carbon black network with an apparent activation energy dependent upon the loading and carbon black structure and (2) deformation of the rubber matrix surrounding the carbon black aggregates. At small deformations the first process is the rate controlling viscoelastic process and at large deformations the second process dominates. At intermediate levels of applied stress both mechanisms contribute to the deformation of the filled compounds.