Physico-chemical study of thermally aged EPDM used in power cables insulation

Ethylene-propylene-diene monomer (EPDM) rubber used as power cables insulation was exposed to a thermal aging in oven for different temperatures ranging from 80 to 140 °C. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out to identify the main structure changes of the material before and after thermal aging. It is found that for pristine EPDM specimens and those subjected to thermal aging at 80 and 100 °C, there was no influence of these aging temperatures on the chemical structure of the material. On the other hand, an increase in oxidation with exposure time was observed for high temperatures (120 and 140 °C). Oxygenated species such as C-O- C, C=O and O-C=O groups were formed. The DSC analysis shows that the crystallinity of the EPDM remains very low. The thermal stability of EPDM, assessed by TGA, showed that although volatile gases migrated from the sample, the studied EPDM keeps high thermal stability. Note that thermal aging effect is not very significant on the material bulk while it is accentuated on its surface. Quantitatively, the carbonyl index was determined since the carbonyl groups are usually the most oxidation products on thermooxidative degradation of cross-linked EPDM.