Space charge and electroluminescence characteristics of thermally aged LDPE films

Polymeric insulation aging is closely related to the change of its physical and chemical properties. An attempt is tried to correlate the space charge and electroluminescence (EL) behaviors of aged low-density polyethylene (LDPE) samples films. LDPE samples were aged in a thermal aging oven with five different time intervals (120, 240, 360, 480 and 600 h). After thermal aging, their molecular structures were analyzed via the Fourier transform infrared (FTIR) absorption spectra, and due to oxidation reaction, two new peaks appeared at ∼1725 and 3380 μm corresponding to carbonyl and hydroxyl groups, respectively. Then the pulsed electro-acoustic (PEA) method was employed to measure the space charge distribution for each sample, and aged samples showed a higher space charge distribution. EL phenomena from aged LDPE surface were investigated under stepped ac voltages, and aged samples showed a relatively lower EL onset voltage and intensity than that without aging. It is considered that EL occurs due to the radiative recombination of electrons and holes injected from both electrodes. New products due to the oxidation reaction can result in more charge traps, which will reduce the surface contact potential barrier and also enhance the charge accumulation inside, and hence lead to the space charge and EL phenomena observed.