Experimental model of a silica/epoxy interface submitted to a hygrothermal aging: a dielectric characterization

In order to investigate the influence of a hygrothermal aging on the organic/mineral interfaces of a silica filled epoxy resin, a macroscopic experimental model of a silica/epoxy interface was designed and realized. It consisted in coplanar interdigitated electrodes deposited on a quartz substrate moulded in epoxy resin. The dielectric properties have been followed at 80°C and 80% relative humidity. These properties have been compared with the evolutions of sheets and mouldings of unfilled epoxy resin submitted to the same conditioning. Measurements evidenced the formation of an interphase at the quartz/resin interface which appeared precisely when the water reached the quartz after its diffusion through the resin. The water bound to the resin during the conditioning of the unfilled samples induced a drop of resistance of nearly 3 decades. In the same time, due to the interphase which has grown at the organic/mineral interface, the resistance between the interdigitated electrodes fell of five decades, i.e. two more decades. This interphase is characterized by a surface resistivity of 80.10⁶ Ω. This data may be used to evaluate prebreakdown phenomena resulting from a hygrothermal aging of silica/epoxy insulations.