Elastic Modulus of Vulcanizates as Related to Carbon Black Structure

Abstract

Abstract The elastic modulus Gf′ of filled SBR vulcanizates has been measured as a function of strain amplitude A. At low loadings of carbon black (20 phr), where Gf′ is practically independent of A, the value of Gf′ (at 25° C, 0.25 Hz) can be predicted within 5 per eent from the gum modulus Gg′ by the Guth—Gold equation with due allowance for occluded volume, as calculated from the dibutyl phthalate absorption. At normal loadings (50 phr) the value of Gf′ can be predicted fairly well from this treatment if the measurements are made at high amplitude. At 25° C, 0.25 Hz, Gf′ is about 10 per cent higher than predicted due to the residual effect of the carbon black network. The dependence of the high-amplitude Gf′ on frequency and temperature is essentially the same for various reinforcing and semi-reinforcing blacks. Consequently, the value of Gf′ for one black can be calculated from the values found for other blacks. The dependence of Gf′ on A is primarily a function of surface area, but in two cases the dependence is unusually steep for different reasons. With graphitized blacks the low-amplitude Gf′ is higher than that of the ungraphitized blacks, but the high-amplitude Gf′ is low, probably due to molecular slippage. “New technology” blacks, including N339 and N351, give a high-amplitude Gf′ comparable to that of conventional blacks of the same dibutyl phthalate absorption, but give a higher Gf′ at low amplitude, indicating a more effective network structure.