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An investigation of the electrical ageing in low density polyethylene

The electrical ageing effects in low density polyethylene (LDPE) may be related to an accumulation of electronic and ionic space charges in the bulk. A study of thermally stimulated discharge currents (TSDC), in which the sample is short circuited through an electrometer as its temperature is increased at a steady rate from sub-zero temperature to a few degrees below its melting point, have shown two unexpected results [1,2], The first is that the largest currents were obtained with the virgin samples (uncharged) and the second, that all samples produced a TSDC peak at the same temperature irrespective of the type and field of charging. Now the total charge released in a conventional TSDC experiment may not correspond to the actual trapped space charges due to a cancellation of a part of the charges flowing in opposite electrodes and recombination of ionic species. Furthermore, the observed current is the algebraic sum of the flow of charges of both polarities towards both electrodes in the existing internal field and, therefore, can lead to false conclusions regarding the polarity and the spatial location of the majority carriers.