Observations on Structural Changes under Thermal Ageing of Cross-linked Polyethylene Used As Power Cables Insulation

The widespread use of cross-linked polyethylene (XLPE) as insulation in the manufacturing of medium and high voltage cables may be attributed to its outstanding physical, chemical, mechanical, and electrical properties. However, it is well known that degradation under service conditions is the major problem in the use of XLPE as cables insulation. Especially, thermal oxidative degradation produces low molecular weight and oxygenated products. Several diagnostic measurements have been used in this study in order to characterize XLPE behaviour under thermal ageing. Firstly FTIR has been conducted on fresh and aged samples in order to reveal the effect of thermal ageing on the chemical changes which take place in the XLPE structure. The main changes are the formation of carbonyl groups (aldehyde, ketone, etc.). Use of DSC analysis shows the effect of ageing on the melting peak temperature Tmmelting enthalpy L .Hm and lamellar thickness. In the last part we have applied X-ray diffraction to reveal the variations in percentage of XLPE crystallinity or addition of any new crystalline phase under thermal ageing. The obtained results showed that thermal ageing at temperatures above the melting temperature of XLPE has a great effect on the material structure.