High-temperature Breakdown Performance Improvement of Polypropylene Films Based on Micromorphology Control

In this paper, a modification method of polypropylene (PP) films is proposed for capacitors by micro doping of carboxylated cellulose nanocrystals (C-CNCs). Results prove that crystallization process of films is promoted, due to the entanglement effect of acicular ultra-structures of C-CNCs on PP molecular chains. With a proper doping content (0.01 wt%), the thermal properties are elevated, and the DC conductivity decreases by raising crystallinity and limiting carrier movement. Modified films possess a restricted dielectric loss at low frequencies because of regular microstructures and decreased conduction loss. Based on DC experiments at different temperatures, the breakdown strength of C-CNC/PP films shows an increase by approximately 51% at 85 °C, basically equaled to the level of pure PP at 25 °C. The mechanism is attributed to deep trap sites, which are introduced by the large crystal region and result in short free path in carrier transport. This novel PP film shows a great potential for application under thermal-electrical compound field in power systems.