Charge packages in thermally aged polyethylene

A thermally aged (28 000 h at 373 K) and oxidized high-voltage polyethylene(PE) grade was tested in a high-resolution pulsed electro-acoustic (PEA) instrument in the mean electric fields of 20, 40 and 60 kV/mm to reveal the space charge distributions. Evaporated aluminum, gold, or sputtered gold/palladium were used as electrode materials. The specimens were exposed to different vacuum drying periods prior to the electrode application. DSC to assess crystallinity and crystal thickness and IR to determine the concentrations of oxidation products were used to characterize the sample. During voltage application charge packages were formed at the cathode. Only one charge package appeared upon voltage application. Vacuum treatment strongly reduced the magnitude of the package. The packages are suggested to be due to charge separation and faster migration of the negative charges than of the positive charges. Volatile species, either originating from additives or formed by the polymer degradation, are suggested to cause the charge package generation.