A Raman spectroscopic study of bulk and surface ageing phenomena in polyethylene

Low density polyethylene (LDPE) films were subjected to a range of electrical stresses. Analysis of samples aged in the bulk by Raman microprobe spectroscopy indicated that changes in the spectra are only observed when samples are subjected to high electrical stresses for long periods of time. Even then, no specific chemical effects were detected; degradation only manifested itself in the form of mild fluorescence. Corona-induced surface reactions were also examined. Although the surface texture was found to change radically, subsequent Raman analysis revealed few chemical changes in the polymer, implying that erosion involves fragmentation of the LDPE and the formation of volatile by-products. No residues were detected on the samples. In addition to the LDPE itself, the corona electrode was also analyzed. This revealed the presence of extensive deposits with the fingerprint of sp² hybridized carbon. The significance of these results are discussed with respect to high field ageing processes, particularly, the development of electrical trees.