Dielectric Properties of Cross-Linked Polypropylene by Gamma-Ray Irradiation for Film Capacitor

In this paper, a modification method based on gamma (γ)-ray irradiation and applied to polypropylene (PP) films is proposed to enhance the dielectric properties for high-voltage direct current (HVDC) capacitors. With the addition of 0.2 wt% trimethylolpropane triacrylate (TMPTA), PP films are irradiated at doses of 0, 2, 5 and 10 kGy. The addition of sensitizer contributed to improving the crosslinking density of PP under the appropriate irradiation dose and the gel content can reach 22.54%. Experimental results show that dielectric loss and conductivity are decreased significantly after modification. Additionally, the breakdown strength of the modified films shows an increase of 13.8% and the energy density increases by 28.7% at 115 °C. It is found that the crosslinking of molecular chains forms stable three-dimensional network structures and improves the intermolecular forces, restricting transportation of charge carriers, which improves dielectric properties and the thermal stability of PP at high temperatures.