Improving the dielectric properties of polypropylene for metallised film capacitors based on cyclic olefin copolymer blending

High conductivity loss and low breakdown strength of traditional polypropylene (PP) film in the high-temperature environment are the key factors limiting the application of metallised film capacitors (MFCs). The effect of cyclic olefin copolymers (COCs) with different glass transition temperatures (Tgs) on the dielectric performance of PP/COC composites at high temperature are studied. The results showed that the addition of COC enhanced the intermolecular interaction force, which led to the reduction of defects in the films. In addition, COC with high Tg inhibits the movement of molecular chains and enhances the thermal stability of the film, which limits the transport of carriers at high temperatures. The conductivity and breakdown strength of the modified films (with a Tg of 134°C for COC) at 125°C are 91.6% lower and 45.7% higher, respectively, than that of pure PP. This method shows great potential in improving the dielectric properties of PP at high temperatures.