To investigate the hygrothermal ageing of silica/epoxy interfaces in a filled epoxy resin, a macroscopic experimental model of such interfaces has been designed. It consisted of coplanar interdigitated electrodes deposited on a quartz substrate and moulded in epoxy resin. The dielectric properties were followed at 80°C and 80% relative humidity. These properties have been compared with the evolutions under the same conditioning of sheets and mouldings of the unfilled resin. Comparisons show that the resistance of the interdigitated sample was not due to the water-enhanced resin conductivity but to the conductivity of an interphase, which has been quantified. If the interphase is modeled as a uniform water layer at the quartz/resin interface, an estimation of the water thickness in the micrometer can be deduced from the dielectric measurements. This order of magnitude can be useful to understand prebreakdown phenomena resulting from a hygrothermal ageing of composite insulations.

Samples of epoxy filled with silica have been subjected to hygrothermal aging at 80°C and 80% relative humidity for several months. These samples were characterized by the measurements of resistances, partial discharges and breakdown voltages. Results for both silanized and non-silanized fillers were obtained. After ageing, large differences were observed in both breakdown voltage and resistance. The breakdown voltage was reduced by a factor of 10 after 74 days of ageing without silanization whereas the decrease was a factor of 2 with silanized fillers. The resistance was reduced by more than 4 decades in the former case and one decade in the second case. The breakdown was preceded by the occurrence of partial discharges. Partial Discharges Inception Voltage (PDIV) dropped by a factor of 10 between 32 days and 74 days of ageing. The PDIV was sensitive to the sample temperature: it was lower at larger temperatures. Partial Discharges (PD) patterns suggest that the discharges occurred in gaseous cavities. Based upon these measurements, a model of breakdown phenomenon based upon cavities filled of water vapour is proposed and discussed.

The hygrothermal conditioning of an epoxy resin at 80degC under 80% RH has been followed by weight measurements, therm ogravimetric analysis (TGA) and dynamical mechanical analysis (DMA). The samples are either filled with 60% by weight of silica flour or are not filled. Above an apparent saturation value of about 1.5% reached within a few days, a slight but significant mass uptake was observed in the filled resin, especially after 50 days. The TGA showed an evolution of the filled samples with conditioning after 50 days as well, which was not observed on unfilled samples. For the filled samples, the elastic modulus in the rubbery state decreased with conditioning. These evolutions have been attributed to the formation of a degraded inter-phase region due to hydrolysis occurring after the debonding of the filler-matrix interface caused by the absorbed water.