The Chemistry of Corona Degradation of Organic Insulating Materials in High-Voltage Fields and under Mechanical Strain

Organic materials used as high-voltage electrical insulation are altered chemically and physically by corona discharges. The basic behavior of polyolefin and fluorocarbon resins were investigated under conditions of high electrical stress. The presence of corona subjects a material to ozone, acid, ultraviolet light, and bombardment by electrons and ions. Degradation under these conditions is usually confined to the surface rather than the bulk of the material. The surface may be the outer surface of the material or the inside face of voids within the material. The present study includes the identification of by-products such as, for example, oxalic acid, which is found on the surface of polyethylene subjected to corona in the presence of humidity. The kinds of failure, cracking or erosion, and the effects of a number of variables including mechanical strain and humidity are presented. Also included is a discussion of electrical failures between the points of sharp needles imbedded opposite each other in a solid. Dendritic channels initiated by corona bombardment at the interface between the tip of one of the needles and the solid polymer slowly progress across the gap. The rate of dendrite growth is related to the gas pressure generated within the dendrite.