Dielectric properties and resistance to corona and ozone of epoxy compositions filled with micro- and nano-fillers

This paper describes activities aiming to evaluate and compare the resistance to a prolonged corona and ozone exposure for a range of epoxy based compositions filled with micro- and nano-fillers of silica at different proportions. It has been earlier shown for this system that substitution of 5 wt% of micro-filler by nano-filler slightly improved mechanical properties, yielding an increase in Young´s modulus, tensile strength and elongation at break. At the same time, the toughness of the systems decreased with the addition of nano-fillers indicating a poor particle-matrix interaction, which was in accordance with the prior art claiming that this interaction is weaker in case of SiO₂ nano-particles, as for example compared to Al₂O₃ nano-particles. For the exposure to corona and ozone a methodology employing a multiple-needle electrode system was used, as recommended by CIGRE working group WG D1.14. Measurements of surface resistivity, bulk resistivity as well as dielectric response (DR) at broad frequency range (10^-4-10³ Hz) were performed on new samples of the investigated compositions and after each of two sequences of the corona-ozone treatment, lasting 100 hours each. It was found that the long-term corona-ozone exposure had obvious effect on surface resistivity for all the compositions investigated and the contents of the micro- and nano-fillers played a significant role in the observed changes. On the other hand, bulk resisitivity, dielectric permittivity as well as dissipation factor, all being the properties of material bulk, did not exhibit high sensitivity to the exposure.