Effect of electron beam irradiation on trap distribution of thermally aged PET film

Polyethylene terephthalate (PET) has been used in a wide range of applications as insulating material in electrical equipment which possibly operate in radioactive environment such as nuclear power plant, spacecraft and scientific facilities. The PET used in such an environment is exposed to a great risk of high radiation induced ageing that gives rise to the deterioration of its insulation property. Therefore, from the viewpoint of safety, it is of great importance to investigate the effect of radiation on ageing phenomena of PET. In this paper, PET film with thickness of 50μm was employed as test sample. The sample was thermally aged at 150°C for 180, 360 and 540 hours, after which the sample was irradiated in air by electron beam of 7.5 MeV with beam density ranging from 0.35 to 0.45 mA/cm
²
. The total accumulated dose was varied from 100 kGy to 500 kGy. Surface potential on the PET sample was established by dc corona charging through needle to plate electrode system, and the potential decay behavior was measured by means of an electrostatic voltmeter. The trap distribution and the volume conductivity of the sample were extracted from the potential measurement, and the effect of radiation dose as well as thermal ageing on the trap distribution was estimated. Obtained results show that with the increase of the ageing time and radiation dose, the trap density tends to be stable while the trap center appears to be shallower, and the volume conductivity of the sample increases gradually. It is suggested that the chemical as well as physical structure of PET sample is changed by the electron beam irradiation and the thermal ageing, which in turn influences the trap distribution of the material.