The role of backbone chain rotation in the hydrophobicity recovery of polymeric materials for outdoor insulation

A series of experiments was conducted on polymeric insulators to examine the role of molecular bond motion in the recovery of hydrophobicity. The samples were also analyzed with characterization techniques which provided important information concerning surface topography, surface hydrophobicity level, and surface chemical composition. Silicone rubber and EPDM (ethylene propylene diene monomer) samples, in the form of cylindrical rods with 25 mm diameter and 150 mm in length, were subjected to dry band arcing in a fog chamber, to simulate outdoor service conditions. It is shown that the diffusion process is not the only mechanism responsible for the recovery of hydrophobicity in polymeric insulators. Heat treatment after hexane soaking showed evidence of recovery by molecular bond motion. Heat-induced changes occurring on the surface of polymeric insulators, as analyzed by ESCA (electron spectroscopy for chemical analysis) and COV (cross-over voltage) measurements, are comparable to those induced by dry band arcing. The formation of cyclic polymers, as well as their conformational changes during the thermal degradation process, helps explain the increase of oxygen level in the hydrophilic state, and the increase of carbon level in the hydrophobic state