Effects of addition of MgO fillers with various sizes and co-addition of nano-sized SiO2 fillers on the dielectric properties of epoxy resin

Complex permittivity (ε r’ and ε r”)conductivity (σ), and glass transition temperature (Tg) were measured for composites of epoxy resin and MgO fillers with various sizes in a wide temperature and frequency range. The effect of co-addition of nano-sized SiO2 was also examined. It has become clean that Tg decreases by the addition of MgO fillers, and its decrement becomes more with a decrease in filler size. Moreover, Tg decreases if the silica nanofillers are co-added. However, all the three important parameters for electrical insulation ability of polymers, namely ε r’, ε r”, and σ, decrease with the decrease in size of MgO fillers or by the addition of silica nanofillers at high temperatures and low frequencies. This is a very astonishing result, since it seems reasonable that a polymer with a high Tg should have a better insulating ability. Probably, the above-mentioned results indicate that the charge transport becomes more difficult, presumably resulting from the suppression of molecular motion with the decrease in size of MgO fillers and by the co-addition of SiO2 nanofillers. This indicates that the suppression of molecular motion becomes very strong if the filler size is small, making the carrier transport difficult even in polymers with low Tg.