Mechanism of the antioxidation effect of α-Fe2O3 on silicone rubbers at high temperature

Abstract

The high temperature resistance of silicone rubber (SR) could be significantly improved by adding hematite (α-Fe₂O₃). In this study, the variation of α-Fe₂O₃ and polysiloxanes was investigated by ultraviolet-visible absorption spectroscopy, X-ray diffraction, Mössbauer spectroscopy, and ²⁹Si NMR spectroscopy after SR aging for 12 h in air at 350 °C. The results indicate that Fe³⁺ is reduced to Fe²⁺ by organic free radicals (R˙) in the process of aging. Fe³⁺ captures R˙ to protect the polysiloxanes from being destroyed and is transformed into Fe₃O₄. Moreover, magnetic Fe₃O₄ is only found in the inner layer of SR; however, it is not detected in the outer layer. Therefore, a new mechanism of antioxidation is proposed: on the outer part of SR, the reduction–oxidation cycle of Fe³⁺ is formed because of sufficient oxygen; however, inside the SR, α-Fe₂O₃ is gradually reduced to Fe₃O₄.