High Temperature Capacitor Films Based on Grafted Polypropylene with Varied Melt Indices

Polypropylene dielectric capacitors are critical energy storage devices in high-voltage direct current (HVDC) transmission systems. However, their energy density and charge-discharge efficiency rapidly decline at high temperatures, failing to meet the current high-temperature operational requirements. Graft modification is a method for developing high-temperature capacitor films with high energy density, thermal stability, and excellent electrical properties. The changes in the melt index caused by grafting are key factors affecting their processability and electrical performance. This study explored the relationship between the melt index of grafted polypropylene and its high-temperature electrical performance. As the melt index increased, the high-temperature breakdown field strength and energy storage characteristics initially decreased and then improved. Through mass production on a BOPP film production line, it was found that samples with a melt index above 4 could not be continuously produced. Considering both film processability and high-temperature electrical performance, adjusting the melt index of grafted polypropylene to 2.9 was deemed optimal. This study provides technical support for the production of high-performance grafted polypropylene capacitor films.