A new approach to the electrical ageing problem in insulating polymers

According to Kao´s model of electric breakdown in insulators, charge carrier injection from electrical contacts and subsequent dissociative trapping and recombination play a decisive role in the breaking of polymer chains and the creation of free radicals or low-weight molecules and hence traps. The authors believe that electrical ageing is due to this gradual degradation process. On the basis of this concept coupled with the chemical decomposition process, they have derived a thoretical formula for the evaluation of the lifetime of insulating polymers under various electrical stressing conditions. They have also carried out an experimental study on polypropylene by measuring the changes in material density, radical and trap concentration, breakdown strength, and other properties as functions of applied electric field and stressing time. It is found that destructive breakdown occurs when the accumulation of field-induced radicals or traps reaches a certain critical level. Thus, the lifetime of an insulating polymer can be predicted by the time required for this critical level to be reached