Loss and Recovery of Hydrophobicity of Polydimethylsiloxane after Exposure to Electrical Discharges

Doktorarbeit / Dissertation aus dem Jahr 2001 im Fachbereich Chemie - Biochemie, KTH Royal Institute of Technology, Sprache: Deutsch, Abstract: Silicone rubber based on polydimethylsiloxane is used as high voltage outdoor insulation, due to its ability to preserve the hydrophobic surface properties during service, and even regain hydrophobicity after exposure to electrical discharges. The underlying processes for the hydrophobic recovery are diffusion of low molar mass siloxanes from the bulk to the surface and reorientation by conformational changes of molecules in the surface region. Only little is known of which factors are responsible for the long-term stability of this hydrophobic recovery. It is therefore important to increase the knowledge about the fundamental mechanisms for the loss and recovery of hydrophobicity of silicone rubbers, exposed to electrical discharges. Addition-cured polydimethylsiloxane networks, with known crosslink densities, were exposed to corona discharges and air/oxygen-plasma and the loss and recovery of hydrophobicity was characterised by contact angle measurements. The degree of surface oxidation increased with increasing exposure time with a limiting depth of 100- 150 nm, as assessed by neutron reflectivity measurements. The oxidation rate increased with increasing crosslink density of the polymer network, according to X-ray photoelectron spectroscopy. Within the oxidised layer, a brittle, silica-like top-layer was gradually developed with increasing exposure time. The hydrophobic recovery following the corona or air/oxygen- plasma exposures occurred at a slow pace by diffusion of cyclic oligomeric dimethylsiloxanes through the micro-porous but uncracked silica-like surface layer, or at a much higher pace by transport of the oligomers through cracks in the silica-like layer. The oligomers were present in the bulk, but additional amounts were formed during exposure to corona discharges. In addition high-temperature vulcanised silicone rubber specimens were aged in a coastal environment under high electrical stress levels (100 V/mm). The changes in surface structure and properties were compared to the data obtained from specimens exposed to corona discharges/plasma. The dominating degradation mechanism was thermal depolymerisation, initiated by hot discharges. This resulted in the formation of mobile siloxanes, of which the low molar mass fraction consisted of cyclic oligomeric dimethylsiloxanes. Oxidative crosslinking resulting in silica-like surface layers was not observed during these conditions.