Effect of coupling agents on the vulcanizate structure of carbon black filled natural rubber

Energy loss in carbon black (CB) filled vulcanizates can be reduced by decreasing the CB volume fraction, reducing the localized deformation of rubber molecules between low-structure CB fillers, and increasing the crosslink density of vulcanizates. However, the decreased volume fraction and the use of low-structure CB degrade the mechanical properties of the vulcanizate. Furthermore, the increased crosslink density leads to drawbacks, such as the decrease of dynamic fatigue property. In this study, considering the performance trade-off from the energy loss reduction of CB-filled vulcanizate, the CB coupling agents (CCAs) system was introduced and its vulcanizate structure and mechanical properties were investigated. The coupling effect between CCAs and natural rubber molecules was confirmed by analyzing the vulcanizate structure using Flory–Rehner and Kraus equations. The vulcanizates with CCAs showed an increase in total crosslink density (TCD) compared to vulcanizates without CCAs. This increased TCD was attributed to the increase in crosslink density by the CB–rubber interaction (CRCD) and the increase in crosslink density by vulcanization reactions between vulcanization agents and matrix rubber (VRCD). The increase of TCD by CCAs improved the mechanical properties of vulcanizates. In particular, the energy loss characteristics were improved by increasing CRCD.