Why is the temperature rating of biaxially oriented polypropylene film capacitors limited to 85°C?

• BOPP contains more rigid components (crystals + rigid amorphous fraction) than hot-pressed iPP. • BOPP exhibits a decrease in breakdown strength at the α c relaxation around 90°C. • Substantially decreased modulus due to the α c relaxation accounts for reduced breakdown strength/lifetime and thus low temperature rating of 85°C. Biaxially oriented polypropylene (BOPP) film capacitors have become the benchmark technology for DC-link and power electronic applications in electric vehicles due to their ultralow loss, high ripple current, high dielectric breakdown strength, and long operational lifetime. However, their maximum operating temperature is restricted to 85°C, a limitation that constrains performance in demanding power electronic environments (∼140°C). The origin of this temperature ceiling has remained insufficiently understood. In this work, we investigate the structure-property relationships that govern the breakdown strength and lifetime of BOPP films at elevated temperatures. Comprehensive analyses, including both structure and dielectric insulation characterizations, were performed to correlate microstructural transitions with dielectric performance. The results revealed that the α c relaxation of isotactic polypropylene around 90°C was the critical factor responsible for the deterioration of breakdown strength (and thus reduced lifetime). Because of 120° helical jumps at crystalline-amorphous interfaces, the α c relaxation lowered modulus, diminished the rigid fraction, and enhanced homocharge injection. Consequently, breakdown strength decreased sharply above 85°C. These findings provide a mechanistic explanation for the long-recognized 85°C rating of BOPP film capacitors. The insights gained not only clarify the intrinsic thermal limitation of BOPP but also point toward rational design strategies for high-temperature polymer dielectrics, such as advanced polyolefins, capable of extending the dielectric performance toward 150°C operation in next-generation electric vehicle power electronics.