Resistance to corona discharge of HTV silicone rubber surface layers fluorinated at different temperatures

Abstract

In order to investigate the effect of fluorination temperature on the resistance to corona discharge of HTV silicone rubber, HTV silicone rubber samples were fluorinated using a F2/N2 mixture at temperatures of 25, 55, and 85 °C for 30 min. The surface fluorinated and unfluorinated (virgin) samples were simultaneously exposed to corona discharge generated by a multi-needle-to-plate electrode system. ATR-IR analysis and SEM observation indicate that the virgin surface layer is severely oxidized and degraded by corona discharge. In contrast with this, the surface layer fluorinated at 85 °C does not show obvious changes in the chemistry, surface morphology and thickness after corona exposure. However, the corona exposure led to definite changes in both the chemistry and surface morphology of the surface layer fluorinated at 55 °C, and its corona-resistance property appears to be worse than the surface layer fluorinated at 25 °C. The abnormal effect of fluorination temperature on corona resistance is explained by unusual changes of surface physicochemical characteristics with fluorination temperature. Measurements of contact angle show a much higher water contact angle on the fluorinated surfaces than the virgin surface before corona exposure, an initial significant decrease in water contact angle for all the surfaces just after the end of the corona exposure, and the subsequent recovery of contact angle only for the exposed virgin surface. Measurements of surface potential reveal that the potential decay rate obviously increases for the virgin sample after corona exposure, while the decay rate decreases rather than increases for the fluorinated samples after the same exposure. There is no significant difference in surface properties between the fluorinated samples before or after corona exposure. This is considered to be due to the combined effect of compositional and structural changes and the fact that the surface properties are very sensitive to the top several molecular layers.