Epoxy resins crosslinked with acid anhydride hardeners are widely used in high voltage insulation. They have excellent dielectric, electrical and mechanical properties up to relatively high temperatures. However, neat epoxy systems exhibit low thermal conductivity and unsuitable coefficient of thermal expansion (CTE) when used with metallic materials such as high voltage conductors. A common solution to improve the thermal properties of the polymer is the addition of high loadings of micron-sized fillers with higher thermal conductivity, such as alumina. However, the high filler content leads to an increase in dielectric constant and to a large amount of interfaces in the bulk of the material, which are the weakest part of the composite. As a result, the addition of high proportions of inorganic fillers often causes a deterioration of breakdown strength and an electrical field enhancement at the interface with other media having lower permittivity 1,2.

In the present work, the influence of the shape and the size distribution of alumina micro fillers upon dielectric, electrical (breakdown strength), thermal (thermal conductivity and CTE) and dynamic mechanical properties of epoxy composites has been studied.

A DGEBA prepolymer has been fully crosslinked with an anhydride hardener. Two types of alumina have been added up to 66 wt% to the epoxy matrix. They differ by their shape and size distribution: the first alumina was monodisperse (3 µm) and nearly spherical (FIG. 2) and the second was polydisperse (1-20 µm) and irregular (FIG. 1).

The shape and size distribution of micro alumina have a limited influence upon the measured properties. This is a good indication for future modeling works.