Effect of Fluorination and Isothermal Crystallization on Polypropylene Electret Fiber Films for Transformer-oil Filtration

Nonwoven Polypropylene(PP) electret film is widely used for filtration due to its excellent filtration performance resulting from the electrostatic attraction between electret fibers and impurities. In this work, nonwoven PP films were fluorinated in F₂/N₂mixture gas and isothermally crystallized in an oven with elevated temperature. A point-grid-plate electrode system was utilized to prepare the electret PP filtration films by corona charging. The results from Fourier-Transform Infrared (FTIR) spectrum and Scanning Electron Microscopy (SEM) evidenced that hydrogen atoms had been replaced by fluorine atoms in the surface of the PP fibers during the fluorination reaction. It was also found that the fluorination lead to increased roughness of the PP fibers. The initial surface potential of the fluorinated PP film after corona charging was slightly higher than that of the nonfluorinated film. However, the former one showed higher decay rate than the latter at 90°C. The isothermal crystallization on the fluorinated PP films significantly improved the charge stability. This should be attributed to the increase of the crystallinity and the density of deep traps introduced during isothermal crystallization. After filtration using the fluorinated and isothermally crystallized PP electret films, the relative permittivity $\varepsilon_{r}$ and the loss factor tan$\delta$ of the aged transformer oil decreased to a low level that was almost identical to the new transformer oil.