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

In this article, a modification method based on gamma ( $\gamma $ )-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 3-D 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.