The Effects of Carbon Black on Rubber Hysteresis

Abstract The viscous modulus of oil-extended SBR loaded with carbon black relative to the gum stock increases with volume fraction of carbon black. This increase is much larger than predicted by the Guth-Gold equation and also greater than the increase in the elastic modulus. This behavior reflects the sum of at least three effects: geometrical (strain amplification), changes in the viscoelastic properties of the rubber, and formation of a three-dimensional network of carbon black and rubber. The geometrical effect increases with volume fraction of carbon black. Its per cent contribution to the viscous modulus decreases as the carbon black loading increases because of increased contribution of the viscoelastic effect at low loadings and of the network effect at high loading. The geometrical effect can be described reasonably well by the Guth equation with a shape factor and with the volume fraction increased by the additional immobilized rubber (bonded elastomer). The viscoelastic effect due to 60 phr black contributes at least 20 per cent to the viscous modulus and is related to structure, surface area, and volume fraction of the carbon black. It can be at least partially accounted for by rubber breakdown during mixing. At high volume fractions of carbon black (60 phr), network formation contributes about 25 to 45 per cent to the viscous modulus. It increases with decreasing interunit spacing which depends on the average volume of the carbon-black units and the amount of bonded elastomer. For a thermal black, N—990, the interpretation of the results is complicated by the distinct possibility that slippage contributes to hysteresis. We conclude, however, that at low loadings the increase of the viscous modulus is much greater than that of the elastic modulus and that network effects contribute to the viscous modulus at high loadings.