Compensating extra sulfur to TESPD-silica-filled NR compounds

With regard to the scorch safety of silica-filled compounds containing different levels of sulfur moiety in the silane structures, TESPT which contains an average sulfur chain length of four atoms is studied in comparison with TESPD containing only two sulfur atoms. The difference in sulfur moiety may cause inferior properties for TESPD-silica-filled compounds when compared to TESPT. This study presents an investigation of the efficiency of a sulfur compensation for compounds containing TESPD in comparison with one with TESPT. The overall results demonstrate that the addition of extra sulfur in the internal mixer during the primary non-productive step gives a positive contribution towards improved silica dispersion. This can be attributed to a more pronounced network formation that can suppress the mobility of silica aggregates, leading to a better degree of dispersion, as demonstrated by the lower Payne effect, higher amount of chemically bound rubber and a lower reinforcement parameter. The tensile properties of silica/TESPD vulcanizates are only slightly enhanced by adding free sulfur in the internal mixer compared to that of the TESPD compound without extra sulfur. Addition of extra sulfur during the productive step on the two-roll mill has practically no effect, as all benefits gained in the non-productive internal mixer step are by-passed; it then merely acts as some extra curative in an overall worse dispersed compound. Based on this investigation, extra sulfur at approximately 0.15 – 0.20 phr, or 3.4 – 5.1 wt% relative to the TESPD content, added during mixing of TESPD-silica-NR in an internal mixer is an optimal amount to achieve better scorch safety of the compounds, and comparable mechanical properties of the vulcanizates, compared to those based on TESPT without extra sulfur.