Thermal Expansion Phenomena in Filled and Unfilled Natural Rubber Vulcanizates
Abstract As with polybutadiene and SBR compounds, filled and unfilled vulcanizates of natural rubber exhibit long transients in their linear thermal expansion response following sudden changes in temperature. This is particularly noticeable when the materials are prestretched well beyond the point of thermoelastic inversion. Incorporation of 50 phr of carbon black significantly reduces the magnitude of the linear thermal expansion coefficient for natural rubber compounds; this reduction is greater than expected on the basis of comparisons with previous data on filled SBR vulcanizates. Changes in the surface chemistry of the filler have little effect on the thermal expansion behavior. For the unfilled vulcanizate, an equation based on rubber elasticity theory was successful in predicting the strain dependence of the linear thermal expansion coefficient up to moderate levels of strain. At higher strains the large deviations from theory were observed.