A study of the temperature dependence of electrical insulating properties of films of PVC subjected to thermolysis modelling thermal aging in solution

Abstract

Experiments on measuring the conductive properties in a series of “thin” (10–12 µm) and “thick” (170–250 µm) film samples of unplasticized and plasticized PVC previously subjected to thermolysis (partial dehydrochlorination in solution) simulating the thermal aging of polymer PVC insulation in electrical cables and coatings have been carried out. In this case, PVC macromolecules contain chains of polyene conjugated bonds (PCBs), so that the samples subjected to the treatment are copolymers of vinyl and vinyl chloride. In all cases, the temperature dependence of electrical conductivity was studied in the range close to the performance (from 15 to 85°C) using a standard (GOST) measurement technique. At the same time as the temperature increases for the “thin” samples, abrupt changes in the conductivity by more than ten orders of magnitude were observed with the transition from low conductivity state (LCS) into a high conductivity state (HCS). This transition has a pronounced two-stage character, which is a quasi-stable intermediate semiconducting state (SCS, Rv = 104–105 Ω) was clearly recorded between the limit states of LCS (resistance of the samples Rv>1012 Ω) and HCS (Rv = 0.5 Ω). For a series of “thick” samples of PVC films, the temperature dependence has the shape with completely different, smooth character. It has been noted that the greatest influence on the degradation of electrical properties of the PVC film samples (simulating traditional types of flexible PVC cable insulation) exerts not only the decrease (volatilization) in concentration of traditional plasticizers (such as DOP), but the appearance of PCB formed because of their thermal aging, which eventually lead to the total degradation of their electrical insulating properties by 1.5–2 orders of the magnitude as compared to the initial value.