Hydrophobicity loss and recovery of silicone HV insulation

Most of the silicone materials used for Hv outdoor insulation are high-consistency, heat cured polydimethylsiloxane (PDMS) elastomers. The unique properties of PDMS that make it suitable for HV applications are reviewed. The surface of these elastomers can be rendered hydrophilic by exposure to discharges. A time and temperature dependent hydrophobic recovery ensues when exposure ceases. A variety of surface characterization investigations have established that corona exposure forms a brittle, wettable, silica-like layer on the surface of most PDMS elastomers. This is consistent with similar effects from oxygen and inert gas plasma treatment. There is still considerable debate as to the relative importance of the two major mechanisms postulated to account for the hydrophobic recovery after corona discharge. The diffusion mechanism invokes migration of low molecular weight species from the interior to the surface, while the reorientation or overturn mechanism envisages a surface reorganization with polar entities such as silanol groups resulting from surface oxidation rotating away and being replaced by methyl groups in the outermost surface layers. In our view, the highly crosslinked silicalike layer cannot reorient readily between hydrophilic and hydrophobic states at the surface, suggesting that diffusion of low molecular weight PDMS components is the more important mechanism of hydrophobic recovery. Recent data obtained on PDMS samples free from lowmolecular- weight diffusible species show that hydrophobic recovey may be due to in-situ depolymerization.