THERMALLY-STIMULATED CREEP SPECTROSCOPY IN THE STUDY OF PHYSICOCHEMICAL AGING OF LDPE

In polymeric power cables, the physicochemical ageing of the insulator is an important limiting factor of its lifetime. The present work rep orts a comparative study performed on additive-free low-density polyet hylene (LDPE) aged in different electrolytes under an electric field. The influence of the electric field, ageing temperature and ionic spec ies on LDPE was investigated by thermally stimulated creep (TSCr) spec troscopy. The TSCr spectrum of reference LDPE shows the existence of t hree mechanical retardation modes, labelled alpha, beta and gamma in o rder of decreasing temperature. The alpha-mode has been associated wit h mechanisms involving the rigid amorphous domains stressed by crystal lites, while the beta- and gamma-modes have been attributed to the amo rphous phase. The experimental data deduced from the analysis of TSCr spectra have been plotted on a compensation diagram: they define the ' 'structure'' of the disordered phases of reference LDPE. The comparati ve study of various aged LDPE samples shows that the para-crystalline phase or the rigid amorphous phase is affected first. In a second step , ageing mechanisms hinder the relaxational process of the amorphous p hases and the material becomes fragile and brittle at a rate that is a function of the ionic solution used.