PD resistance of thermally aged polyethylene and carbonyl-containing model polymers

Films of polyethylene (PE) with different degrees of crystallinity, including a crosslinked cable grade and a crosslinked grade containing 2-vinylnaphthalene, were aged in air at 373, 388 and 403 K for periods to 19000 h and subsequently exposed to external partial discharge (PD) in dry air at 298 K. Stress-strain measurements in a tensile testing machine, differential scanning calorimetry to assess the antioxidant concentration, crystallinity and crystal thickness, infrared spectroscopy to determine the concentrations of oxidation products, and size exclusion chromatography (sec) were used to characterize the samples before and after the oven aging. The thermal oxidation led to a combination of chain scission, as revealed by sec on the soluble fraction of aged samples, and crosslinking which was evident from the gel formation. Oxidation-induced chain scission increased the crystallinity of the crystal lamella stacks stable at the aging temperature, and lead to a pronounced brittleness of these materials. The fraction that was molten at the aging temperature and crystallized during the subsequent cooling phase dropped substantially in crystallinity after oxidation. Mere oxidation was not sufficient to cause a decrease in PD life, unless the carbonyl content was above a certain threshold value above which the PD life decreased rapidly with increasing concentration of carbonyl groups. It is suggested that the carbonyl groups enhance radical formation and thereby accelerate surface erosion, causing early PD breakdown. The PD life was not influenced by the degree of crystallinity, the average crystal thickness, the concentration of antioxidant or by the fracture toughness. Vinylnaphthalene acted in crosslinked PE as a stabilizer and prolonged the incubation period for oxidation.