Deterioration of internal interfaces between silicone and epoxy resin

Abstract

Interfacial defects, such as loss of adhesion together with environmental effects, such as moisture intrusion into the defect, can cause interfaces to become weak points in an insulation system. Silicone rubber moulded onto internal epoxy components is commonly used in high voltage outdoor composite insulation systems. For evaluating the interfacial properties of such a material combination, samples containing interfacial defects were prepared. In the samples a transparent silicone rubber was moulded onto a primer pre-treated epoxy substrate, while the spots to be characterized by a loss of adhesion were left unprimed. In this way air-filled defects between the two materials were created. The samples were thereafter exposed to an electric aging test with a needle-ring electrode arrangement attached to them. The electrode arrangement provided stress characterized by a high content of tangential field component along the interface and the field strength at the needle tip was kept at high enough level to maintain corona discharge activity. The ageing was performed under normal and humid conditions. The latter was performed to secure moisture ingress into the interface area through the silicone rubber. After the testing, the defect surfaces were analysed chemically to investigate reactions involved in the ageing processes. In addition, optical microscopy was used to evaluate a risk for void growth, which could potentially be caused by hydrolysis of the primer. The defect area was chemically evaluated by infrared spectroscopy