Comparison of stress-softenings in carbon-black filled natural rubber and styrene-butadiene rubber

The authors compared the mechanical behavior and, more precisely, the Mullins and the cyclic (post-Mullins) softenings of two filled rubbers. A crystallizing natural rubber and a noncrystallizing styrene-butadiene rubber of similar compositions resulting in similar cross-link densities and filled with 40 phr of N347 carbon-black fillers were tested in cyclic uniaxial tension at room temperature and at 85°C. Crystallization in filled rubbers is known to increase stress at high stretch, stretch at break, cycle hysteresis, and fatigue lifetime and to reduce crack propagation. In this study, it is shown that crystallization also seems to enhance the Mullins softening (softening at the first cycle) and to favor the apparent cyclic softening. Results reveal that natural rubber shows an amplitude dependence on the cyclic softening, whereas the styrene-butadiene rubber does not. Finally, results demonstrate that studying filled rubber softening cannot help predict lifetime.