Polydimethylsiloxane and alumina trihydrate system subjected to dry-band discharges or high temperature part I: chemical structure

This is the first of a two-part series on the chemical, morphological, and electrical changes of polydimethylsiloxane (PDMS) and alumina trihydrate (ATH) material system subjected to dry-band discharges produced on the material surface by sustained moisture and contamination buildup. This system possesses both stable hydrophobicity restricting water-film formation as a path of leakage current and high resistance to electrically conductive path (track) formation induced by dry-band discharges, contributing to the recent innovation of polymeric outdoor insulation technology. The PDMS/ATH system has been believed to be resistant to track formation (tracking). However, it was observed in the present study that the PDMS/ATH system occasionally allows track formation leading to a dielectric breakdown in an internationally standardized tracking test, with the carbon concentrations in the formed track being measured at a mere ∼1% wt. The unknown phenomenon of tracking of the PDMS/ATH system was studied here from the perspective of chemical structure and of electrical insulation. This first paper describes the chemical and morphological changes of the PDMS/ATH system subjected to dry-band discharges. A comparison with the thermal decomposition behavior of the PDMS/ATH system heated in the presence and absence of oxygen was carried out to better our understanding of the tracking mechanism. Amorphous SiO₂ and mullite 3Al₂O₃.2SiO₂ were primarily produced by the dry-band discharges, while cristobalite SiO₂ and mullite 3Al₂O₃.2SiO₂ were produced by thermal decomposition in either air or nitrogen gas. Cristobalite SiO₂ and mullite 3Al₂O₃.2SiO₂ are basic ingredients of ceramic insulators, which may contribute to recycling of the PDMS/ATH system. The structure of carbon deposited in the track was not absolute graphite, and its concentration was much higher than that of the heated system. Finally, we propose herein models of the chemical changes induced by dry-band discharges as well as heat treatments.