Particle-filled rubber compounds are stiffer but show pronounced softening on stretching (termed the Payne effect at low strains and the Mullins effect at high strains). These effects obscure the effects of crosslinking and thus preclude a comparison of the elastic properties with molecular theories of rubber elasticity. However, they disappear on swelling the materials with a compatible liquid. A comparison of equilibrium swelling ratios for carbon-black-filled and unfilled samples suggests that the degree of crosslinking is not changed by adding carbon black. But the filled compounds swell less and thus the apparent degree of crosslinking is greater. (For example it is about twice as large for compounds containing 50 phr of HAF carbon black.) This is confirmed by measurements of the elastic modulus of swollen samples. We conclude that both weak and strong bonds are formed between rubber molecules and the surface of carbon black particles. The weak bonds disappear at low degrees of swelling whereas the strong ones withstand high swelling stresses and temperatures of up to 120oC. Procedures are suggested for estimating the actual degree of crosslinking from equilibrium swelling or elastic modulus.
* Based on a paper by A. N. Gent, J. A. Hartwell, and Ginger Lee, presented at the ACS Rubber Division Meeting held in San Francisco, April 2003.
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