The thermal oxidative decomposition of polyorganosiloxane fluids at metal surfaces

Abstract

The thermal oxidative decomposition of polyorganosiloxane fluids has been studied with particular reference to the formation of thin polysiloxane films on metal surfaces. Specific decomposition products were identified using infrared spectroscopic analysis, and showed that the thermal decomposition mechanism involved preferential oxidative attack at the substituent organic radical followed by condensation and cross-linking between the residual polysiloxane chains. However, in certain instances, metal oxides appear to catalyse the oxidative reaction so that cross-linked polysiloxane films were formed on metal surfaces at much lower temperatures than those at which bulk polymer decomposition is known to occur. The results of this work suggest that the reaction is initiated by highly reactive oxygen atoms or ions at the metal or metal oxide interface. The rate of growth of the resulting polysiloxane film at these low temperatures is determined by the rate of diffusion of these species through the film to the film-fluid interface, where further reaction can occur. At temperatures of the order of 100°C in air, these films are extremely thin, a limiting film thickness of 1000 Å or so being attained only after prolonged thermal treatment for 5–6 days. Thicker films are formed at higher temperatures.