Long-term performance of poly(vinyl chloride) cables : mechanical and electrical performances and the effect of plasticizer migration

Sammanfattning: Cables insulated with plasticized poly(vinyl chloride) were aged at temperatures between 80 and 180 °C in air and their conditions were assessed by indenter modulus measurements, tensile testing, infrared (IR) spectroscopy, differential scanning calorimetry (DSC) and liquid chromatography (HPLC). Electrical testing of oven-aged cable samples was performed in order to relate the electrical functionality during a high-energy line break accident to the mechanical properties and to establish a lifetime criterion. The mechanical data taken at room temperature after ageing could be superimposed with regard to ageing time and temperature. The ageing-temperature shift factor showed Arrhenius temperature dependence. The jacketing material showed an immediate increase in stiffness (indenter and Young’s modulus) and a decrease in the strain at break on ageing; these changes were dominated by loss of plasticizer by migration also confirmed by IR spectroscopy, DSC and HPLC. The core insulation showed smaller and also delayed changes in these mechanical parameters; the loss of plasticizer by migration was retarded by the closed environment and the changes in the mechanical parameters were due to chemical degradation (dehydrochlorination). Comparison with data obtained from this study and from other studies indicates that extrapolation of data for the jacketing insulation can be performed according to the Arrhenius equation even down to service temperatures (20-40 °C). Extraction of plasticizer of samples from cables that have been exposed to service for 25 years showed a minor decrease (within the margin of error) in plasticizer content with reference to that of unexposed cable samples. The low temperature deterioration of the jacketing is according to this scheme dominated by loss of plasticizer by migration.Numerical analysis were performed on desorption data obtained by liquid chromatography. The fitting of the data to Fick’s law showed a transition between 100 and 120 ºC. This was interpreted as a change from evaporation-control of migration at low temperatures to a diffusion-control of migration at the higher temperatures.