access icon openaccess Ageing behaviour of a polyethylene blend: influence of chemical defects and morphology on charge transport

This is an open access article published by the IET and CEPRI under the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/)
Received 25/12/2019, Accepted 24/02/2020, Revised 12/02/2020, Published 26/02/2020

The properties of novel cable insulation systems will rely critically upon the morphology of the material. Here, a blend of high and low-density polyethylene (PE) was processed in order to generate three sets of samples with different morphologies. The influence of thermo-oxidative ageing at 120°C was then considered. The resulting chemical changes included the introduction of unsaturation and oxygen-containing groups and were determined by antioxidant consumption and oxygen permeability. Such chemical defects were found to be concentrated in the fraction of each system that was molten at 120°C and, consequently, served to inhibit recrystallisation following ageing. The resulting spatial distribution of charge trapping sites was therefore strongly dependent on morphology. The electrical conductivity of each system varied non-monotonically with ageing: short times reduced the conductivity; a rapid increase in conductivity over five orders of magnitude occurred beyond a critical ageing threshold. 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 influenced by chemical factors, crystallinity is relatively unimportant. This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous PE.

Inspec keywords: power cable insulation; oxidation; electrical conductivity; polyethylene insulation; ageing; recrystallisation

Other keywords: oxygen-containing groups; cable insulation systems; antioxidant consumption; polyethylene blend; charge transport; electrical conductivity; chemical defects; chemical morphology; low-density polyethylene; amorphous polyethylene; oxygen permeability; thermo-oxidative ageing

Subjects: Organic insulation; Surface treatment (semiconductor technology); Power cables

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