Hydrophobic stability of silicone rubber after water immersion

The hydrophobic stability of a virgin HTV silicone rubber formulation and corresponding formulations after initial 'accelerated ageing' by water immersion during 410 h at 23 or 90degC has been compared. This is in order to investigate the influence of moisture/water ingress on the hydrophobic recovery after corona discharges and after deposition of artificial pollution layers. The 'aged' materials exhibited a slower hydrophobic recovery after exposure to corona discharges, but on the other hand they exhibited a faster hydrophobic transfer through artificial pollution layers. Scanning electron microscopy revealed a significant surface erosion, exposing individual filler particles; even though no significant surface oxidation was observed using X-ray photoelectron spectroscopy. Moreover, the extractable amounts of silicone oils were analyzed using chromatography. It was found that both the amount and the types of the extractable silicones were similar for both virgin and 'aged' materials. The observed differences in hydrophobic recovery rates were explained primarily by the difference in surface roughness between aged and virgin samples. Finally, it was concluded that cyclic silicone oligomers (D₄ and D₅) were responsible for the initial hydrophobic transfer effect through the artificial pollution layer.