Voltage stabilizers (VSs) with benzene derivatives have received great interest as high-performance additives for improving the electrical durability of polypropylene (PP)-based insulation materials for high voltage (HV) power cable applications. In this work, atactic polystyrene (aPS) was first used as a thermoplastic VS (TVS) to efficiently dope the benzene units into isotactic PP (iPP). The aPS TVS was incorporated in the iPP matrix through melt-mixing, and the domain size of immiscible aPS decreased with the decrease of aPS content. The smaller amount of aPS doping increased the direct current (DC) breakdown strength (BDS) of iPP, even at ultralow loading levels below 0.01 phr. The enhancement ratio of the BDS value of iPP with 0.001 phr of aPS was higher compared to the results obtained for other types of additives reported in the previous literature.
Various additives ranging from inorganic nanoparticles to organic additives have been suggested to improve the insulation performance of polymeric materials for high-voltage engineering applications. Herein, we present a simple method for doping fluorine into a polypropylene (PP) matrix by melt-blending of isotactic PP (iPP) with a small amount of polyvinylidene fluoride (PVDF) as a thermoplastic voltage stabilizer (TVS). During melt-mixing, the PVDF TVS, which is immiscible with PP, is gradually split into smaller domains within the iPP matrix and was finely distributed, especially at a low PVDF content. The well-distributed PVDF acted as a nucleating agent for the facile crystallization of PP molecules, thus increasing the crystallization temperature (Tc) and decreasing the spherulite size. We found that the direct current (DC) breakdown strength (BDS) values of the PVDF-doped iPP increased by 110% and 149% at 20 and 110°C, respectively, compared to those of the pristine PP. We hypothesize that the presence of fluorine sites as well as the increase in interfaces between spherulites with decreased size, without any significant degradation in the tensile strength and elongation at break below 1.0 phr of PVDF, were the reasons for our findings. Therefore, we anticipate that such PVDF-doped iPP is a potential candidate for high-voltage insulation systems.
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