Surface structural changes of naturally aged silicone and EPDM composite insulators

In a long-term outdoor test with high direct and alternating voltages, silicone and ethylene propylene diene monomer (EPDM) rubber composite insulators have, at the beginning, shown a performance superior to that of glass and porcelain insulators. In the long-term test, however, the silicone rubber composite insulator has, in spite of the aging of both insulator types, kept its good performance, while the performance of the EPDM rubber composite insulator was drastically deteriorated. In order to get a better insight into the results obtained, the wettability and the surface structural changes of the insulators were studied by the drop deposition method using a goniometer and by advanced techniques such as scanning electron microscopy, electron spectroscopy for chemical analysis, Fourier-transform infrared spectroscopy, and secondary ion mass spectrometry. The results show that the differences in performance are related to the differences in the surface structural changes and in the dynamic ability of the surface to compensate the aging. Silicone rubber insulators have higher repellency than the EPDM insulators, especially when aged. The reason seems to be low silicone polymer diffusion from the bulk which covers the insulator surface, embeds the pollutants, and keeps the surface hydrophobic. Porcelain insulators with silicone elastomer coatings show lower water repellency than insulators with massive silicone rubber sheds. When aged, EPDM insulators do not, however, seen to possess the same dynamic recovery of their surface, thus becoming hydrophilic.<
>