Thermal Degradation of Fluorosilicone Rubber Up to 250 °C

ABSTRACT Fluorosilicone (FVMQ) rubber is commonly used in many military aircraft applications, such as seals and gaskets, due to its exceptional heat stability, low temperature flexibility, and resistance to fuels and oil. A peroxide-cured and silica-filled FVMQ rubber compound was prepared and subsequently submitted to accelerated heat aging for up to 50 wk at temperatures between 75 and 250 °C. Heat-aged samples were then tested for hardness, physical property characteristics, and crosslink density by solvent swell and double quantum–nuclear magnetic resonance (DQ-NMR). The Arrhenius methodology was applied to the shifted experimental data assuming the principle of time–temperature superposition. The tensile strength and elongation at break both decreased upon thermal treatment. A small stiffness increase was observed by both the hardness and the tensile stress at 10% elongation test data. The characterization of the crosslink density by solvent swell testing was inconclusive. By contrast, the DQ-NMR testing clearly showed that the crosslink density decreases, whereas the number of chain defects increases. The unaged crosslink distribution is heterogeneous in nature, displaying much more heterogeneity than peroxide-cured silicone. Nonlinear Arrhenius behavior was observed between 75 and 250 °C, with a mechanism change at 175 °C. The FVMQ crosslink distribution becomes more heterogeneous with aging. A significant loss of low-molecular-weight FVMQ due to depolymerization and backbiting reactions was observed at higher aging temperatures. Reactions with the silica surface also occurred with the creation of Si–O bonds. No direct evidence of thermo-oxidative and oxygenation reactions was observed. Reaction mechanisms have been proposed to explain the degradation of FVMQ due to heat aging.