Phase transformation via optimal impregnation to enhance dielectric performance of polypropylene

The development of smart grids and new power systems has imposed higher requirements on the dielectric performance of polypropylene (PP) in oil-impregnated capacitors. Herein, the dielectric properties of PP films are successfully improved by adjusting their crystalline characteristics. The results demonstrate that under the optimal impregnation process, the capillary effect enhances impregnation efficiency, thereby inducing superior compatibility between the PP films and oil. This facilitates the phase transformation of β-crystals into α-crystals with a more perfect crystal structure during high-temperature impregnation. The resultant increase in the α-crystal content is accompanied by a corresponding enhancement in overall crystallinity. It consequently increases the dielectric constant of PP, while retaining an extremely low dielectric loss. Notably, the phase transformation also hinders the mobility of PP molecular chains, thereby weakening the charge energy accumulation process. These synergistic effects lead to a breakdown strength of up to 537 kV/mm, representing an 11.0% increase compared with the unimpregnated PP films. This study proposes a promising strategy for enhancing the dielectric performance of PP via phase regulation, which paves the way for its application in high-performance capacitors.