Effects of bis (3-triethoxysilylpropyl) testrasulphane (TESPT) on properties of silica filled natural rubber compounds

Traditionally carbon black has been used as the reinforcing filler by the rubber industry. Although silica filler has been available for a long time its use has been restncted due to low reinforcement provided by silica. There are also processing problems peculiar to the use of silica, like long cure time and high viscosity. ThiS not only decreases the service life of the equipment due to increased wear and tear but also restricts commercial manufacture of silica-based products due to long cycle times. Blfunctional organosilanes have been used to improve reinforcement in rubber compounds. They react With the polymer on one hand and with the hydrophilic silica filler on the other. In this study the effect of an organosilane, bls(3-triethoxysilylpropyl) tetrasulphane (TESPT), was systematically studied on fifteen rubber compounds. The variables used were the amount of silane and the filler silica (60, 30, and 10 p.p.h r.). An attempt was made to find the amount of TESPT to be used to obtain optimum processing and reinforcing properties. The processing properties measured were cure time, scorch time, cure rate index and Mooney viscosity. The reinforcement was measured by calculating the difference between the maximum and minimum rheometer torques (A Torque) and by bound rubber measurements. Cross-link density measurements were also made on the compounds containing 30 p.p.h.r. silica. In 60 p.p h.r. mixes, 7 p p h.r. silane gave the optimum cure time and Mooney viscosity. For compounds containing 30 p.p h.r. silica cure time, Mooney ViSCOSity, cross-link density and bound rubber content all optimised at 4 p P h.r. silica. For 10 P p.h.r. silica no appreciable improvement in properties was observed. Interestingly when the results were looked at as a function of the silanel silica ratio the cure time, Mooney viscosity, cure rate index and A Torque all showed maximum improvement around 12 parts by weight, (pbw) TESPT per 100 pbw Silica. This corresponds to a monomolecular coverage of the silica surface by the TESPT. Thus there was a definite advantage in using TESPT,the optimum silane depending on the amount of silica present in the compound.