In this paper, an improved method for the electric performance of polypropylene (PP) film was proposed to promote the safety and stability of power capacitors. Modified PP films containing three different polycyclic compounds were prepared, which showed good thermal properties and decreased DC conductivity. The DC breakdown strength of the modified PP films under both positive and negative voltage is increased compared with that of the original film. The deep traps introduced by polycyclic compounds and the decreased carrier mobility give an explanation of the decreased DC conductivity. A quantum chemistry calculation was further performed to clarify the mechanism for improving electrical performance, presenting that polycyclic compounds with a high electron affinity and low ionization energy can capture high-energy electrons, protecting the PP molecular chain from attack, and then increase the breakdown strength. It is concluded that the modified PP films by polycyclic compounds have great potential in improving the insulating properties of power capacitors.

The breakdown strength of polypropylene (PP) for film capacitors drops sharply in a high-temperature environment, which would lead to capacitor insulation failure frequently. This paper proposes a method to improve the breakdown strength of PP at high temperatures by introducing long-chain branches (LCBs). Different from traditional linear PP, the results show that LCBs can increase the number of spherulites and refine the grains due to heterogeneous nucleation, resulting in a reduction in weak areas in the films and an increase in trap densities and energy levels in long-chain branched polypropylene (LCBPP). In addition, the higher melt strength of LCBPP indicates that LCBs promote entanglement between molecular chains and hinder the sliding of chains, thereby reducing the influence of temperature on the free volume. Therefore, compared with PP, the conductivity of LCBPP is lower at high temperatures, and the breakdown strength increases by 16.2% at 105 °C. This method provides an idea for improving the breakdown strength of PP films at elevated temperatures from the perspective of regulating the microstructure.

The degradation of dielectric properties of polypropylene (PP) at high temperature affects the operational stability of metallised film capacitors. This study presents a method to improve the high‐temperature dielectric performance of PP through long‐chain branching modification and regulating the crystalline properties. Different amounts of nucleating agents were added to long‐chain branched polypropylene (LCBPP) to study the effect of crystallisation on electrical properties. The results show that the long‐chain branches improve thermal stability by enhancing the chain entanglement, and also increase the number of crystals due to heterogeneous nucleation. The addition of nucleating agents promotes the crystallisation of LCBPP, which increases the nucleation density and improves the crystallinity. The regulation of the microstructure of PP suppresses the relaxation of the chains and limits the charge transport. The dielectric constant of the modified films with long‐chain branching is slightly increased, and the doping of nucleating agents has little effect on the dielectric constant and loss of LCBPP. At 125°C, the conductivity of the modified LCBPP films with the addition of 0.05 wt% nucleating agent decreased by 2 orders of magnitude and the breakdown strength increased by 26.3%. This study has implications for improving the dielectric properties of PP films.

At high temperatures, the insulation performance of polypropylene (PP) decreases, making it challenging to meet the application requirements of metallized film capacitors. In this paper, the dielectric performance of PP is improved by long-chain branching modification and adding different kinds of nucleating agents. The added nucleating agents are organic phosphate nucleating agent (NA-21), sorbitol nucleating agent (DMDBS), rare earth nucleating agent (WBG-Ⅱ) and acylamino nucleating agent (TMB-5). The results show that the long-chain branches promote heterogeneous nucleation and inhibit the motion of molecular chains, thereby enhancing the dielectric properties at high temperatures. Nucleating agents modulate the crystalline morphology of long-chain branched polypropylene (LCBPP), which leads to a decrease in the mean free path of carriers and an increase in trap energy level and trap density. Therefore, the conductivity is reduced and the breakdown strength is improved. Among the added nucleating agents, NA-21 showed a significant improvement in the electrical properties of LCBPP films. At 125 °C, compared with PP, the breakdown strength of the modified film is increased by 26.3%, and the energy density is increased by 66.1%. This method provides a reference for improving the dielectric properties of PP.

The metal ash in polypropylene (PP) significantly weakens the breakdown performance, and the deashing process can reduce the ash content. In this paper, a novel deashing solution of metal deactivator (MD)/ethanol (ET) is prepared in an attempt to reduce the ash content and improve the breakdown performance. The film sample is prepared for the breakdown and conductivity performance tests, and the results reveal that the breakdown strength of the deashed sample is increased by 34%, the conductivity is greatly suppressed by 77% and the discharge energy density is significantly enhanced by 80 %. In addition, the total metal ash content of the deashed sample decreased by 65%, and the defect coefficient after crystallization is reduced by 70%. A stable ring structure is formed between MD and metal ashes, which plays a great role in deashing. Therefore, the metal ash ionization and the catalytic effect on the thermal-oxidative degradation of PP are significantly inhibited, resulting in a great improvement in the breakdown strength. This deashing method provides a feasible reference for the study of high-reliability capacitor films with excellent insulation performance.

A modification method of polypropylene (PP) films is proposed in this article for capacitors, based on the physical blending of the linear PP and long-chain branched PP (LCBPP). In order to evaluate the comprehensive properties of the samples with different mixing ratios, the micro morphologies, thermal, and electrical characteristics are studied. LCBPP is proven to possess the ability of heterogeneous nucleation. On the one hand, the addition of long-chain branched structures strengthens the intermolecular interaction and promotes the overlap of microcrystalline, which helps to reduce the amorphous region and improve the dimensional stability of the film. On the other hand, impurities and interfaces are introduced, resulting in the increased polarization. Therefore, the conductivity of the PP/LCBPP film reduces, the loss decreases, and the permittivity is basically unchanged. At 20 °C, a low proportion of the LCBPP is beneficial to the increase of the breakdown strength. While at 70 °C or 120 °C, the dense microstructures, promoted by a high proportion of the LCBPP, play a decisive role in suppressing the local field distortion and improving the breakdown properties. This method provides a feasible idea to improve the high-temperature resistance of the film from the perspective of molecular and aggregate structure design.