Improvement of Dielectric and Energy Storage Performances in Surface Modified High-Crystallinity BOPP Capacitor Films

Abstract

High-crystallinity polypropylene (PP) has broad applications due to its excellent mechanical properties, thermal stability and processing properties. However, its capacitive energy storage performance deteriorates severely under high electric fields. In this paper, two surface modification methods, ultraviolet irradiation and surface fluorination, were adopted to modify the commercially biaxially stretched polypropylene (BOPP) film, and the effects of these methods on the surface structure, chemical composition, thermal properties, mechanical properties, dielectric properties and energy storage properties of the material were systematically studied. The results show that the introduction of oxygen-containing polar groups by ultraviolet irradiation and the formation of C-F bonds through fluorination treatment can effectively augment the electron injection barrier, suppress charge injection and migration and significantly improve breakdown strength, energy density and charge–discharge efficiency. The breakdown strength was significantly improved from 750 MV/m of pristine BOPP film to 790 MV/m of UV irradiated film and 831 MV/m of fluorinated sample, respectively. As a result, the energy density of the ultraviolet irradiated sample is 5.62 J/cm³ and the efficiency is 92%. The energy density of the fluorinated sample can reach 5.9 J/cm³ and the efficiency is 93%.