Chemical analysis of outdoor silicone materials after electrical and environmental testing

In this work the results of electrical and environmental testing of outdoor silicone rubber insulating materials are presented. Degradation paths in silicone insulation were occasionally observed in the artificially accelerated and field tests. Chemical analysis of material in the degradation path has shown that the original material had transformed into the orthorhombic mullite Al₆Si₂O₁₃. The formation of mullite in the degradation paths indicates that local surface temperatures during discharge activity on the surface may well be over 1200degC degrees. Alumina trihydrate (ATH) filler was observed to be homogeneously distributed in the studied material. The filler had platelet-like form and was hexagonal in shape. The filler particles were several micrometers in size with submicrometer thickness. In addition to the ATH filler, occasional impurities such as NaCl and KCl salt crystals were observed in the bulk material. Soxhlet extraction has shown that tested high temperature vulcanised rubbers (HTV) contained about 2%w of low molecular weight (LMW) silicone species while liquid silicone rubber (LSR) contained about 4%w of LMW silicone.