Temperature Dependence of DC Dielectric Strength and Voltage Endurance of BOPP

Biaxially oriented polypropylene (BOPP) is the current gold standard for DC thin film capacitors. However, one of its major disadvantages is the poor high-temperature stability. It is the aim of this study to contribute to the understanding of the failure of BOPP at elevated temperatures. Therefore, capacitor-grade PP was extruded into a cast film using a twin-screw extruder and a water-cooled chill roll. BOPP films were produced using a laboratory stretching machine. The films were tested for their short-term dielectric strength and voltage endurance between 23 and 140°C. DSC, shrinkage and TSDC measurements were conducted to analyze the morphological and electrical changes in the material at elevated temperatures. An increase in temperature caused a reduction in dielectric strength and voltage endurance. The onset of space charge release by thermal trap activation coincided with a slow deterioration in both properties. The decrease was further enhanced as crystalline regions of low stability started to melt and orientation in amorphous regions started to be relaxed. This shows that trap states and morphological changes in the operating temperature range of thin film capacitors between 60 and 100°C should be the focus of future research.