Elucidation of the role of ZnO in sulfur cure in novel EPDM-CTS blends

Abstract

A new concept is introduced in this study, suggesting the role played by the ZnO crystal in sulfur crosslinking polymers, particularly, rubber blends of EPDM-CTS. The study is conducted in a polymer blend of ethylene propylene diene terpolymer (EPDM) and cyclic tetrasulfide (CTS). CTS is a reactive low molecular weight polymer, which can act as a sulfur donor. It is proposed that the sulfur crosslinking occurs at the surface of the ZnO crystals. A reaction site is created on the surface of the ZnO crystal by the reaction with stearic acid, which creates a “template” or active site on the surface as a catalytic site for the CTS reaction to take place. Various experiments are performed to substantiate the newly proposed role the crystalline ZnO structure plays in influencing the initiation of the sulfur crosslinking. The investigation is conducted in rubber compounds in the absence of fillers and other additives and additionally in chemical model studies. The analyses conducted in rubber compounds are performed on fully or partially cured rubber to study the evolution of the chemical process involved in the CTS reaction and the curing of the rubber blends. Step-cured compound allows analyses of the transformation of the ZnO crystal size during the sulfur crosslinking process, and the ability to gauge the step transformation that the ZnO crystal is undergoing during the crosslinking process. Additional chemical model studies are used in the study of the influence of the ZnO crystal on sulfur crosslinking, to confirm the reaction path and by-products of reaction. Lastly, quantum mechanical (QM) and molecular mechanics (MM) calculations are applied in support of the suggested mechanism of reaction.