A Quantitative Analysis of Influencing Parameters for Cure Torque in Silica-Filled SSBR Compounds

The maximum torque in the cure curve is commonly used as an indicator of crosslink density (CLD) in rubber compounds. However, in silica-filled systems, this relationship is affected by additional contributions to this torque, including filler-filler interactions, silane-mediated reactions (where silica-silane-polymer couplings contribute to the overall crosslink density), and interactions between silica and the accelerators. These contributions result in a higher complexity in the interpretation of cure curves in silica-filled compounds compared to carbon black-filled systems, while limiting the validity of maximum torque as a direct measure of crosslink density. This paper aims to subdivide the maximum torque of silica-filled styrene-butadiene rubber (SSBR) compounds into its constituent contributions arising from compound viscosity, filler-filler and filler-polymer interactions, and polymer crosslinking. Compounds based on a functionalized SSBR designed for chemical silica coupling were prepared at varying silica loadings, and the influence of each factor on the cure torque was investigated. At low silica loading, cure torque is governed primarily by intrinsic polymer characteristics, with a dominant contribution from polymer crosslinking. With increasing silica loading, the contribution of filler-filler and filler-polymer interactions increases significantly and progressively dominates the torque response, thereby reducing the apparent contribution of polymer crosslinking. Between 30 and 90 phr silica, while the estimated contribution of chemical crosslinking to the maximum torque decreased from about 37% to 28%, the contribution of filler-filler interactions increased from about 15% to 28%. Furthermore, without silane, the storage modulus at low strain was up to ten times higher than that of the corresponding compounds with silane, although the crosslink density remained close to that of the unfilled compound. Consequently, in highly filled silica compounds, cure torque cannot be directly correlated with crosslink density. These findings estimate the relative contributions of individual interactions to cure torque development in silica-filled rubber compounds, indicating that crosslink density alone cannot account for torque behavior at high filler loading.