Estimation of Stress-Strain Curves of Aged Oil-Extended SBR Vulcanizates.

A contribution of the crosslinks changed in character by aging in oil-extended SBR vulcanizates to the stress-strain relation was examined with the following theoretical constitutive equation: f = {G - B(1 - 1/alpha) +C (alpha(2-)1) + D (alpha + 1/alpha(2-)2) +F (alpha(4-)1} (alpha- 1/alpha(2)) where f is nominal stress at an extension ratio ct, G corresponds to rigidity, B, C, D, and F are coefficients. The experimental stress-strain relations of oil-extended SBR vulcanizates aged for various periods of time at 100 degrees C coincided with the above relation in the extension range almost up to the break. The dependence of each coefficient on the crosslink densities was examined and found that a part of the crosslink densitiy in aged oil-extended SBR was effective on the stress. The rate of the effective crosslink densities for each coefficient was determined for each period of aging time. On the basis of the effective crosslink densities, a method estimating the stress-strain curves of aged oil-extended SBR vulcanizate was proposed.