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 (λ m ). 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 (D 0 or D 90 ) at each angle was evaluated as a function of λ m from the difference in the uniaxial stress-strain data between the subsamples and the pristine samples. Almost no anisotropy ( f = D 0 /D 90 ≈ 1) was observed for the internal damage in the moderate λ m regime of λ m < 3. This is in contrast to a large damage anisotropy ( f ≈ 3) in the corresponding λ m regime previously reported for a silica filled SBR with silane coupling agent (SBR/Silica). The f values for SBR/CB in the high λ m regime of λ m > 3 increased with λ m 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.