Adding a bi-functional coupling agent enhances filler-matrix compatibility. The most commonly used coupling agent is bis(3-triethoxysilylpropyl) tetrasulphide, known as TESPT, Si-69 or A1289. Its dual function encompasses a better dispersion of the filler during mixing and a chemical coupling between filler surface and polymer. The reactions that take place between the TESPT and the silica surface (primary reaction) and the rubber matrix (secondary) were studied.

Primary reaction The primary reaction was studied by using HPLC and dynamic mechanical measurements, from which kinetic parameters were calculated. There are indications that ZnO and/or stearic acid reacts as catalysts for the reaction. Mechanical testing revealed the mechanism of the secondary reaction of the coupling agent with the rubber matrix, silica-rubber bonding vs. the action as a curing agent.