Influence of high electrical fields on ageing and polarization properties of polyethylene

Abstract

The influence of high electrical fields (above 20 kV mm−1) on ageing, polarization and morphology of polyethylene (PE) is discussed. Infrared and positron annihilation spectroscopy measurements together with capacitance measurements tend to indicate that polymer morphology is modified by high fields. We show that accelerated electrical ageing characteristics of PE are directly linked to morphology changes induced by the field. Below a so‐called critical field, the activation volume of the ageing process is dependent on field‐induced strain. Above the critical field, the amorphous phase is significantly deformed and weak van der Waals bonds are broken, leading to another faster ageing regime. There is excellent agreement between the proposed model and experimental data obtained with PE‐insulated high voltage cables. The possible relation between the submicrocavity formation proposed in our ageing model and various polarization measurements is discussed. It is our contention that strong charge injection occurs only after submicrocavity formation, ie after weak bonds are broken. As is well known, polarization currents obtained under high fields are controlled by space charges. It seems that wave packets and the negative resistance observed above 100 kV mm−1 in PE are associated with a steady state in field‐induced defect formation. This suggests that space charges are related to the formation of submicrocavities and, therefore, are a consequence and not a cause of high field ageing. © 2002 Society of Chemical Industry

On the influence of chemical defects and structural factors on charge transport and failure in polyethylene

“…While macroscopic electrical degradation features, such as electrical trees, are reasonably wellunderstood, this is not the case for precursor phenomena, where a number of different processes have been suggested to lead to the modification of the material at the nanoscopic level. These include: chain scission at electric fields >20 kV/mm and consequent formation of sub-microcavities causing local accumulation of space charge [10]; charge recombination and consequent electroluminescence leading to local degradation through photo-oxidation [11]; field-induced mechanical stress and consequent propagating cracking associated with breaking of inter-lamellar tie chains [12].…”

On the influence of morphology and chemical defects on charge transport dynamics in polyethylene: thermal ageing and concentration gradient

“…In general, initiation is believed to involve the chemical and physical modification of the pristine polymer through the formation of defective regions at the nano-and micro-scales [17]. Crine [18] suggested that the initial stage of degradation under a high electric field (>20 kV mm −1 ) involves chain scission, the formation of free radicals and the generation of submicrocavities. Local accumulation of space charge (SC) then occurs, which increases the local electric field such that ageing is accelerated to failure.…”

Ageing behaviour of a polyethylene blend: influence of chemical defects and morphology on charge transport

“…Numerous studies of polymer ageing and its effect on the electrical behaviour of polymers have been reported. For example, it has been suggested that ageing is initiated at applied electric fields in excess of ∼20 kV/mm through chain scission processes, free radical generation, the formation of submicro‐cavities and the consequent accumulation of space charge [10]. This association of free volume and charge trapping is supported by a complementary theoretical study [11, 12].…”