Charge Recombination Induced Luminescence of Chemically Modified Cross-Linked Polyethylene Materials

Very little is known on the chemical nature of the deep trapping centres that play a role in charge storage in insulating polymeric materials. The analysis of the luminescence emitted during a process of charge recombination can be helpful to unravel the nature of such centres. The spectral features of the luminescence induced by charge recombination are analyzed for three different cross-linked polyethylene (XLPE) materials, chemically-modified or not. In two XLPE formulations, a controlled amount of chemical groups containing an aromatic ring has been grafted to the polymer chains. The other formulation is the same XLPE without chemical modification. Photoluminescence is investigated to settle a background to identify the species involved in charge recombination. It is excited on films at room and liquid nitrogen temperature providing fluorescence and phosphorescence spectra. The luminescence excited by the recombination of trapped electrical charges is subsequently analyzed at low temperature and compared to the photoluminescence emission. The results obtained on chemically-modified XLPE provide a basis to identify the species involved in charge recombination in the non modified material. It turns out that aromatic compounds are involved in photo- and charge recombination-induced luminescence in all the tested XLPE, and that the luminescence has the same spectral features for the two stimulations, contrary to what is observed for low density polyethylene.