Effect of zinc oxide on the reaction of TESPT silane coupling agent with silica and rubber

The addition of a coupling agent to silica-rubber compounds enhances filler-matrix compatibility. The coupling agent under study, TESPT, can also act as a sulfur donor, which leads to premature scorch in the mixer. The effect of the presence of zinc oxide in the internal mixer at higher dump temperatures was demonstrated by dynamic mechanical testing. A lower tendency to scorch was seen when zinc oxide was omitted during the internal mixing stage and added only later together with the curing additives on a two roll mill. Zinc oxide primarily interferes with the reaction between the coupling agent and the silica surface, as shown by the reaction rate constants obtained with rheological experiments. Further confirmation was obtained by HPLC-experiments: the reaction rate constants of TESPT with silica for the samples with zinc oxide present, were lower at higher temperatures in comparison with samples without zinc oxide. XPS data confirmed that zinc oxide can indeed react with the silica surface. When zinc oxide is included in the internal mixer stage, shorter scorch times are obtained for curing when compared to compounds where no zinc oxide is included during mixing and only added later on. These shorter scorch times are the result of filler flocculation of the silica during the curing stage.