The ageing and charge transport dynamics in polyethylene system

Abstract

Polyethylene is widely used as the basis for the insulation in high voltage cables, commonly, in the form of cross-linked low-density polyethylene (XLPE). Under environmental and loading conditions, the physicochemical properties of the pristine insulation are gradually changed by a range of ageing mechanisms. The aim of this project was to investigate the thermal ageing characteristics of a polyethylene blend (HDPE/LDPE) in terms of chemical and physical changes and their effects on electrical parameters, such as dielectric response, space charge accumulation, charge transport dynamics and DC breakdown. The ageing conditions chosen included both a high temperature for short times (160 ºC from 1-6 h) and a low temperature for long times (120 ºC from 120-720 h), which were used to generate spatially different distributions of ageing products, termed, non-uniform and uniform distribution respectively. The process of ageing is associated with the diffusion of oxygen into specimens. In addition, on ageing at low temperature, the effects of morphology on thermo-oxidative ageing and electrical performance were investigated, using three distinctly different lamellar textures. Ageing characteristics under both ageing regimes were characterised using Fourier transform infrared-attenuated total reflectance (FTIR-ATR) and Raman spectroscopies to reveal the chemistry of ageing; these are represented, primarily, by the carbonyl concentration, which varied on the sample surfaces and throughout the sample thickness. Physical changes, such as crystallinity and morphology, were investigated by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Dielectric spectroscopy was used to monitor polarizability and dielectric loss. The pulsed electroacoustic (PEA) technique and DC conductivity were employed to investigate space charge and charge transport dynamics respectively.
Ageing at high temperature for short times formed a macroscopically non-uniform distribution of chemical defects (i.e. carbonyl groups) throughout the specimens, as a result of the kinetically controlled rate of oxygen consumption. The resulting specimens could, therefore, be considered as consisting of two differently oxidised layers. In contrast, ageing at low temperature for long times led to a macroscopically uniform distribution of chemical defects; in particular, chemical defects were found to be concentrated in the fraction of each system that was molten at 120 ºC and, consequently, served to inhibit recrystallisation following ageing. As expected, both ageing characteristics led to increases in polarizability and dielectric loss, corresponding to increasing the number of dipolar species present in the system. Specifically, dielectric spectra revealed a broad loss peak at frequencies above 1 Hz that increased in strength with increasing ageing time and was related to the motion of polar chain segments within the polymer. Concerning ageing at high temperature, space charge (SC) accumulation and charge transport dynamics were observed to exhibit non-monotonic forms of behaviour as a function of ageing time. Below a critical density of deep traps – correlated with the carbonyl content – SC accumulation and conductivity were suppressed, while above a critical trap density, both were enhanced. Concerning ageing at low temperature, SC accumulation and electrical conductivity of each system, again, varied non-monotonically with ageing time. Beyond the critical ageing threshold, a transformation from homocharge to heterocharge was observed, which also corresponded to a progressive increase in the accumulated charge. While the conductivity was found to be reduced after short ageing times, more prolonged ageing resulted in a rapid increase in conductivity over five orders of magnitude. Despite the pronounced structural differences between the morphologically distinct sets of samples, all exhibited comparable conductivity values beyond this threshold, implying that while charge transport is strongly influence by chemical factors, crystallinity is relatively unimportant. This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous polyethylene. In summary, the work described in this thesis has revealed new correlations between physicochemical changes, the distribution of ageing-induced defects, SC accumulation and charge transport dynamics, which provide increased understanding of the effects of early stages of ageing in polyethylene. This may lead to the identification of key ageing markers for assessment of insulation life expectancy or to facilitate the active design of novel polymer insulation systems that are better able to withstand the rigours of service.

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Identifiers

ORCID for Somyot Tantipattarakul: orcid.org/0000-0001-5814-1277

ORCID for Alun Vaughan: orcid.org/0000-0002-0535-513X

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

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