Determination of the aging mechanism of single core cables with PVC insulation

Two set of single core cables with PVC insulation with different decreases of electrical resistivity have been analyzed to determine the aging mechanism responsible for this decrease of electrical properties. Four different aging mechanisms can be considered: oxidation, effect of water ingress, loss or migration of plasticizers and dehydrochlorination. IR spectroscopy has not revealed any oxidation nor the presence or the effect of water. IR microspectroscopy has confirmed the uniform composition of the polymer throughout the sample. SEM and EDX analyses have highlighted clusters of lead chloride. The presence of this compound results from the reaction of the thermal stabilizer due to the mechanism of dehydrochlorination. However, these techniques did not distinguish between the two samples of different resistivity. UV spectroscopy revealed the presence of double bonds in the two samples and show that the concentration is higher in the cable with the worse electrical properties. The formation of double bonds is a consequence of the dehydrochlorination and cannot result from one of the three other aging mechanisms considered. Although the measurement of chloride ion concentrations by coulometry should have confirmed this aging mechanism, the surprisingly low measured levels of chloride ions are discussed with regards to the adequacy of the method to measure lead chloride in cluster forms.