Cure and mechanical behavior of a black-filled natural rubber compound containing ground vulcanizates and model spherical inclusions

Cure and mechanical behavior of a carbon-black filled natural rubber (NR) truck tread tire composition containing cryogenically-ground vulcanizates were studied. The effects of particle size, concentration, type of matrix and particle base rubber, and particle modulus differentiated by curative level on the vulcanization, tensile, trouser tear and wear behavior were determined.;Addition of ground rubber reduced maximum Oscillating Disk Rheometry (ODR) torque and scorch times. Sulfur migration was shown to occur from the matrix to the ground vulcanizate phase accounting for the maximum torque reduction. The level of migration did not depend on particle size in the range less than 1 mm as determined by ODR response, but did depend on sizes greater than 1 mm as determined by direct sulfur analysis and measurement of model sphere modulus. Evidence was provided to show migration of 2-mercaptobenzothiazole (MBT) from the ground rubber to the matrix results in decreased scorch time.;Tensile strength decreases with addition of ground vulcanizate, however, the level of decrease depends strongly on particle modulus. Low modulus particles cause a minimal decrease in strength with little dependence of particle size, whereas higher modulus particles cause greater reductions with a larger dependence on size. The phenomenon was attributed to crack initiation and/or propagation processes. Tensile strength also has a minor dependence on sulfur migration. If sulfur depletion from the matrix is compensated, the strength increases.;Observation of the fracture process of testpieces containing a single sphere ruled out the possibility that differences in crack initiation from one sphere alone causes the strong dependence of strength on particle modulus. Furthermore, in this low concentration regime of one sphere, the type of matrix rubber dictated the behavior. For natural rubber, a rubber with high tearing resistance due to its ability to crystallize, interfacial adhesion has less effect on strength. However, for styrene-butadiene rubber (SBR), a rubber with lower tearing resistance, adhesion has a larger effect.;The trouser tear strength increases with increasing ground vulcanizate concentration. The increase is due in large part to the change in matrix crosslink density resulting from sulfur migration. When the sulfur level was adjusted, tear strength reverted to its initial value. Particle size has a minor effect on tear strength, but causes large deviation of the crack path. It is thought that black-filled natural rubber, already having the ability to deviate a crack in the absence of the particles, does not gain great increases in tear resistance with additional deviation. Furthermore, there was little dependence of tear strength on particle modulus suggesting crack propagation alone does not cause the dependence of tensile strength on particle modulus.;Wear resistance, measured with a rotary drum abrader (DIN), was found to be heavily dependent on the matrix modulus as well as the type of particle. Wear resistance decreased upon addition of ground rubber, but improved if the matrix sulfur level was increased to compensate for its depletion. Furthermore, particles of a more wear resistant SBR compound imparted improved wear resistance versus those with NR particles. It is likely the interface does not affect the wear behavior and the particles are abrading as bulk materials.