Effects of time and temperature on the reaction of TESPT silane coupling agent during mixing with silica filler and tire rubber

The addition of a coupling agent to silica-rubber compounds enhances the filler-matrix compatibility. Under certain mixing conditions the surface of the filler may be only partly activated, which may have an adverse effect on the properties in the final product. Some coupling agents may also act as sulfur donor. The dump temperature employed during mixing and the length of time the compound is exposed to that temperature govern the reaction mechanisms of the coupling agent and determine whether the agent leads to the formation of a silica-rubber bond or acts as a curing agent. A temperature of at least 130 °C is necessary to ensure that the reaction between the coupling agent and the silica proceeds, whereas the coupling agent starts to react with the rubber or to donate sulfur, resulting in scorching, at temperatures above 160 °C. An increase in the 300% modulus and/or G' at 100% strain above 150 °C is an indication of scorching caused by the sulfur in the coupling agent. No scorching is observed when a coupling agent without sulfur is used. Another important parameter is the mixing time. It was observed that the coupling agent must be mixed with the silica for at least 10 minutes at 150 °C to obtain a sufficient degree of coupling.