Comparison of Mullins Effect Anisotropy of the Elastomers Reinforced by Carbon-Black and Silica Filler

Abstract

Anisotropy of the internal damage caused by imposed deformation, which is called stress-softening or Mullins effect, was investigated for a carbon-black (CB) filled styrene butadiene rubber (SBR/CB) by a sequence of two tensile measurements. A wide-width pristine sheet specimen satisfying pure shear (planar) stretching geometry was first subjected to a single loading-unloading cycle with a maximum stretch (λm). The rectangular subsamples were cut out from the unloaded and relaxed sheet specimen such that the long axis had an angle of θ = 0° or 90° relative to the pre-stretching axis. Subsequently, uniaxial stretching of the subsamples was conducted, and the internal damage (D0 or D90) at each angle was evaluated as a function of λm from the difference in the uniaxial stress-strain data between the subsamples and the pristine samples. Almost no anisotropy (f = D0/D90 ≈ 1) was observed for the internal damage in the moderate λm regime of λm < 3. This is in contrast to a large damage anisotropy (f ≈ 3) in the corresponding λm regime previously reported for a silica filled SBR with silane coupling agent (SBR/Silica). The f values for SBR/CB in the high λm regime of λm > 3 increased with λm and became comparable to those in SBR/Silica. The significantly large difference in the internal damage anisotropy between SBR/CB and SBR/Silica reflects the differences in the types of destructed molecular bonding for the filler networks and filler/rubber interfaces between the two elastomers.