Thermally-stimulated creep spectroscopy in the study of physicochemical ageing of LDPE

Abstract

In polymeric power cables, the physicochemical ageing of the insulator is an important limiting factor of its lifetime. The present work reports a comparative study performed on additive-free low-density polyethylene (LDPE) aged in different electrolytes under an electric field. The influence of the electric field, ageing temperature and ionic species on LDPE was investigated by thermally stimulated creep (TSCr) spectroscopy. The TSCr spectrum of reference LDPE shows the existence of three mechanical retardation modes, labelled α, β and γ in order of decreasing temperature. The α-mode has been associated with mechanisms involving the rigid amorphous domains stressed by crystallites, while the β- and -γ-modes have been attributed to the amorphous 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 comparative 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 phases and the material becomes fragile and brittle at a rate that is a function of the ionic solution used.