Electrothermal ageing of epoxy nanocomposites

Constant stress accelerated ageing experiments were conducted on unfilled epoxy and epoxy alumina nanocomposites with different filler loadings of 0.1, 1 and 5 wt%. Electrical (6 kV/mm), thermal (60oC) and combined electrothermal (6 kV/mm and 60°C) ageing experiments were performed for a duration of 250 h. The leakage current through the samples were continuously monitored and the variation in the tan δ values with ageing duration was also monitored. It was observed that the increase in the tan δ value with ageing duration was less for the epoxy alumina nanocomposites as compared to the unfilled epoxy. Dielectric spectroscopy measurements were performed on the samples before and after the ageing in the frequency range of 10^-2 to 10⁶ Hz. The permittivity and tan δ values were found to increase in the low frequency range. The volume resistivity of unfilled epoxy and epoxy alumina nanocomposites were also measured before and after the ageing. The volume resistivity improved marginally for the thermally aged samples, but reduced for the electrically aged and the electrothermally aged samples. The decrease in the value of volume resistivity was more for the multistress aged unfilled epoxy samples as compared to the multistress aged epoxy alumina nanocomposites. It was also observed that the unfilled epoxy samples having a higher value of tan δ failed first. The time to failure of the samples showed an increasing trend with an increase in the nano filler loading of epoxy alumina nanocomposites.