Effects of Zinc Stearate Replacement and Zinc Oxide/Zinc Stearate Ratio on Rheological Properties and Curing Characteristics of Carbon Black-Filled Rubbers

This work aims to study the substitution of zinc stearate (ZS) as an activator and coactivator with stearic acid (SA), its mechanism of action and also the influence of ZnO/ZS on the rheological and curing characteristics of styrene-butadiene (SBR), butadiene (BR) and nitrile-butadiene (NBR) rubbers-filled carbon black. Despite accelerating the curing reaction rate of SBR, BR and NBR by ~119, 145 and 22%, the ZS chemical increased the maximum torque in the oscillating disc rheometer (ODR) by 13 and 17% for BR and NBR, respectively, with a slight change for SBR. The minimum torque in the ODR test also increased by ~25 and 11% for BR and NBR in the presence of ZS, while SBR experienced a ~8% reduction. The decrease in the ZnO/ZS ratio changed the rheological behavior of SBR to an insignificant overcuring reversion in the maturing zone. The curing time of SBR was decreased from 15.09 to 11.31 min by reducing the ZnO/ZS amount from 2/1 to 1/1, while the scorch time was almost similar in the BR and NBR-based compounds. Reducing the ZnO/ZS amount raised the maximum vulcanization rate of SBR by ~50% (0.687 to 1.03 1/min), while its trend was descending in the BR and NBR compounds by ~53% and 20%. Moreover, the decrease of ZnO in the recipe containing the ZS chemical decreased the crosslinking by ∼16% (from 8.065 to 6.768×105 mol/g) and 4% (from 0.278 to 0.268×105 mol/g) for the BR and NBR compounds, while it did not vary the SBR crosslink density.