Transfer of hydrophobicity of polymeric insulating materials for high voltage outdoor application

This paper analyses the chemical structure and the amount of migrating polymer components into pollution layers on silicone rubber housing materials used in high voltage outdoor insulation and their respective effectivity in rendering the pollution layer surface hydrophobic. With Fourier transform infrared spectroscopy (FTIR) it was possible to observe a significant increase in the overall polydimethylsiloxane (PDMS) content of pollution layers on silicone rubbers between transfer times of weeks and several months. Contrarily the gas-chromatographic (GC) investigations revealed no significant increase during that period. Therefore, the low molecular weight (LMW) PDMS content detectable with gas chromatography - mass spectrometry (GCMS), which is generally accepted to be decisive for the transfer of hydrophobicity is likely only of subordinate importance. The LMW-PDMS extracted pollution was investigated again with FTIR, revealing that the LMWs detectable by GC constitute only a minor share of the PDMS transferred into the pollution layer. High-performance liquid chromatography (HPLC) allowed to directly corroborate this conclusion. With mass spectroscopy attached, the HPLC investigations also showed distinct differences in the molecular weight distribution of the transferred PDMS species for the investigated insulating materials. Accordingly, material specific PDMS content with individual molecular weight distributions seem to explain the observed differences in transfer capability of the investigated insulating materials.