Interactions Between Silica and Epoxidized Natural Rubber with and without Silane

Unmodified squalene (Sq) and epoxidized squalene (ESq), as models for natural rubber and epoxidized natural rubber, were mixed with silica in the presence or absence of TESPT silane coupling agent. The results indicate that Sq and ESq with the help of TESPT can chemically link to the silica surface. The epoxidized squalene also produces chemical interaction with silica in the absence of TESPT, but to a lesser extent compared to the one with TESPT. On the other side, the silica in the Sq mixture without TESPT shows no traces of hydrocarbon, indicating that there is no noticeable chemical interaction involved. Further studies involving various levels of epoxidation on natural rubber (ENR) in the absence of TESPT coupling agent, as well as a combination of ENRs with different loadings of TESPT, provide a better understanding of the various factors which influence the properties of silica-filled ENR compounds.  Based on the overall properties, the best possible combination to optimize processibility, to reduce filler-filler interaction, and improve vulcanization rate as well as vulcanizate properties, is to utilize ENR with an epoxide content in the range of 20 – 30 mol%, together with a small portion of TESPT, i.e. 2 – 4 wt% relative to the silica content.  This leads to a reduction of approximately 60 – 80 % of TESPT when compared to the conventional NR compounds, where the optimal loading of TESPT was 9.0 wt% relative to the silica content.