Compared to carbon black, the use of silica as reinforcing filler for rubber results in lower hysteretic losses, for tyre applications leading to lower rolling resistance and consequently fuel savings. The compatibility of hydrophilic silica with a hydrophobic rubber polymer matrix is generally poor. Adding bi-functional coupling agents to the compounds, commonly bis(triethoxysilylpropyl) tetrasulphide (TESPT), enhances filler-matrix compatibility. The degree of hydrophobation of silica during rubber mixing then depends on many mutually interacting factors. Irreproducible conditions during mixing and vulcanisation are major causes of irreproducibility of silica-reinforced rubber compounds. Depending on the chemical composition of the coupling agent, the ultimate temperature obtained during the mixing process turns out to be the main factor governing the reactions of the coupling agent: the formation of a proper bond between the silica and the coupling agent, while avoiding a premature reaction with the rubber polymers, leading to premature scorch during mixing. Further, the mechanistic aspects of the reaction of various coupling agents, variants on TESPT, are covered, with silica as well as with the rubber. Of great importance are: the carbon and sulphur chain-lengths within the coupling agents, whether corrections are applied in the compound with elemental sulphur relative to the sulphur contained in the reference TESPT, and the moments the correcting amounts of sulphur are added to the compounds: during the first mixing stage, or together with the curing ingredients later-on in the process. The tensile properties of the vulcanised compounds are most prominently influenced. This is indicative of the dual role of sulphur: on the one hand as direct curative, on the other hand as part of the coupling agent becoming attached to the rubber polymers.
- Coupling agent