Investigation of hydrophilic pollution layers on silicone rubber outdoor insulation

Polluted silicone rubber test plates were investigated before and after one year ageing at the Koeberg Insulator pollution Test Station (KIPTS), South Africa. The hydrophobicity of the rubbers was lost during the first two weeks after installation. The plates showed no sign of degradation after testing: no significant surface hardening or change of amount and molar mass distribution of the extractable silicones was found. The salinity and chemical composition of the hydrophilic pollution layers were similar a hydrophobic pollution layer from another geographical location. Additionally, it was concluded that the loss of hydrophobicity was not caused by biological growth. If the hydrophilic pollution layers were removed, hydrophobicity of the cleaned rubber surfaces recovered within a day. Optical microscope and scanning electron microscopy showed that the hydrophilic pollution from KIPTS formed a continuous, porous layer with a thickness of ~20 μm. The layer also contained high-aspect ratio structures. In comparison, the hydrophobic pollution layer was thinner, <;10 μm, and not homogenously distributed. It is suggested that the loss of hydrophobicity at KIPTS was caused primarily due to the structure of the pollution layer. In addition it was suggested this kind of structured pollution layer was a result of a fast deposition of moist salt in combination with other pollutants, both in form of particles and fibers.