Effect of metal oxides as activator for sulphur vulcanisation in various rubbers

Two widely different rubbers, viz. EPDM and s-SBR have been selected in the present study. The influence of the basicity, the crystal structure and the ability to form complexes of various metal oxides were studied. It was observed that neither CdO, PbO, BaO, CaO, MgO and BeO are proper substitutes for ZnO as activator in thiuram-accelerated vulcanisation of EPDM, nor do they show a synergistic effect with ZnO. In s-SBR compounds, however, it is demonstrated that CaO and MgO can function as activator of cure for sulphur vulcanisation, retaining the curing and physical properties of the rubber vulcanisates. Model Compound Vulcanisation has been used to elucidate the influence of the activators MgO and CaO on the vulcanisation mechanism. It can be concluded that depending on the exact requirements for a specific compound, certain metal oxides represent an alternative route to reduce the zinc level and therefore to minimise the environmental impact.