Degradation of Polyethylene by Partial Discharge

To explore the fundamental concept concerning testing methods of corona resistance of insulating materials, some "roundrobin" tests with polyethylene have been performed by means of parallel-plate electrodes. The specimen films, degraded under the conditions of 6 kV ac impression and of static dry air, are examined with regard to weight, infrared spectrum, and other changes. From these results, information about degradation mechanisms was obtained. From these studies, the corona degradation behavior of polyethylene might be considered to include two kinds of chemical reactions. First, activated oxygen formed by discharge is able to oxidize perfectly the surface of the specimen exposed directly to discharge and to yield H2O, CO2, and other products, although the portion not directly exposed to the discharge is hardly affected owing to the extremely short life of these active species. At the same time, ozone with relatively long life diffuses into polyethylene and is able to form an ozonide by reacting with a terminal double bond, while it makes carboxylic acid and others by combining with the main chain in the polymer. Moreover, nitrogen oxide causes nitrate ester to be formed. Several conditions may control these two competitive reactions and the degree of competition is probably the reason for the fluctuation of results.