The Effect Of Thermal Treatment On The Space Charge Behavior In EPDM Under 50 Hz AC Electric Field

Ethylene Propylene Diene Monomer(EPDM)is widely used as cable insulation due to the advantages of high temperature resistance,corona resistance and humidity resistance.It has been applied in many areas,such as coalmines,submarines,aerospace and other fields.Therefore,the investigation of properties of EPDM under operating condition is significant for the production and maintenance of corresponding cable.In this paper,the effect of thermal treatment on space charge behavior of EPDM under 50 Hz AC field was studied based on the variation of space charge response,and physicochemical properties inside EPDM.The thermal treatment temperature was in the range from 100℃ to 200℃.The space charge source and charge transport in EPDM under 50 Hz AC field are investigated based on the variation of experiment phenomenon.The space charge characteristics of EPDM under 50 Hz AC field were detected based on a modified PEA system with the AEPS principle.The electric field was in the range from 15 k V/mm to 46 k V/mm.The results show that the space charge accumulation is significant in the untreated sample.The distinct layered accumulation of heterocharge exists within EPDM near the semi-conductive electrode side.Homocharge accumulates within EPDM near the grounded aluminum electrode side.After thermal treatment at temperature from 100-150℃ for 18 hours,the space charge polarity inside the sample is the same as that of the untreated sample.Moreover,the amount of space charge significantly decreases with increasing temperature.After thermal treatment at 200℃ for 18 h,the space charge polarity near the grounded aluminum electrode side is the same as the untreated one.Homocharge accumulation appears near the semiconducting electrode side,which is different from that of untreated sample.The results of gas chromatography-mass spectrometry(GC-MS)showed that long-chain alkanes,α-methylstyrene,and benzyl alcohol exist in untreated EPDM rubber samples.After thermal treated at temperature from 100-150℃,the decrease is significant for the space charge accumulation and the content of three main types of impurities.Thus,it indicates that the ions associated with high-field dissociation of impurities is one of the main charge sources in untreated samples.According to the isodensity line of space charge and the migration direction of carriers under the electric field,the heterocharge accumulation near the semiconducting electrode derives from the ions associated with impurities dissociation,and the homocharge accumulation near the aluminum electrode derives from charge injection from electrode.Due to the higher content of impurity,the space charge formed by impurity ions significantly accumulated within untreated sample.It causes obvious electric field distortion in untreated sample.With the increase of polarization electric field from 15 k V/mm to 46 k V/mm,the distortion rate of the electric field first increases from 19.92%to 31.87%,and then decreases to 6.65%.The tendency of the electric field distortion rate may relate to the process of charge migration and trapping/detrapping,etc.The increase of conductivity and detrapping rate under high electric field may reduce the accumulation of space charge,leading to the suppression of the electric field distortion rate.After thermal treatment at 100-150℃ for 18 hours,no obvious oxidation and the decrease of AC breakdown field occurs in the sample.The electric field distortion rate decreases significantly and can reach to 6.88%,due to the decrease of impurity content.After thermal treatment at 200℃ for18 hours,significant oxidation occurs,and the AC breakdown field decreases by 11%.The homocharge accumulate within EPDM near the semi-conductive electrode and the ground aluminum electrode.According to the isodensity line of space charge and the migration direction of carriers under the electric field,it is determined that the homocharge is injected from electrodes.Due to the accumulation of the homocharge within EPDM near the interface,the electric field inside the sample obviously enhances,and the maximum electric field distortion rate rises to 21.37%.Considering the above analysis,the recommended temperature range of EPDM for thermal treatment is in from 100℃ to 150℃.It can not only avoid material damage,but also reduce space charge accumulation and suppress electric field distortion.