333 / 2018-10-13 01:30:02

Dielectric Properties of Isotactic Polypropylene/SEBS-g-MA Composites

isotactic polypropylene;,space chage,dielectric properties,SEBS,conduction current

High voltage DC (HVDC) power cable technology has the advantages of large transmission capacity, low loss, negligible electromagnetic environmental effect, and trackless corridor, etc., which make it capable to solve the significant problem of power transmission channels during the fast development of electric industry. Meanwhile, HVDC cable technology also contributes to eco-friendly, resource-saving, safe, and reliable goal of power transmission lines. Compared to common crosslinked polyethylene materials, polypropylene (PP) has the advantages of non-crosslinking, high-temperature-resistant, recyclable, low production cost, and free maintenance work, and has been a hot spot in the research field of high performance HVDC cable insulating materials. PP materials have low impact strength due to high regularity of molecular structure. Adding low-molecular elastomer materials into the PP matrix is an effective and common method to improve the impact strength at low temperatures. However, the binary blending system of PP/elastomer composites usually decreases the stiffness and high-temperature resistance in some extent. In addition, the interfacial problem occurring in the binary system has also been of critical significance to the electrical, mechanical, and thermal properties. In this study, we prepare a binary system of isotactic PP (iPP) and Styrene-ethylene-butadiene-styrene triblock copolymer grafted with maleic anhydride (SEBS-g-MA) composites. iPP pellets were purchased from Sigma Aldrich with the melt index of 4 g/10 min (230 ℃/2.16 kg). SEBS-g-MA, existing in the form of dusted pellets provided by Kraton Corp., was selected as the elastomer. The SEBS-g-MA elastomer was prepared via the melt-blending technique and could be used as a flexibilizer to improve the impact strength of polyolefin resins with the typical MA graft degree of 1.7%. Different mass fractions, i.e., 1wt.%, 3wt.%, 5wt.%, 10wt.%, and 15wt.%, of SEBS-MA fillers were added into the iPP matrix via the melt mixing method in order to study the relationship between the mechanical performance and charge behavior. The obtained films with a thickness of ~200 μm were used for space charge and dielectric measurements, and those with ~100 μm were used for the measurements of breakdown strength (BDS), DC conductivity, and thermally stimulated depolarization current (TSC). The results show that there is no new characteristic peak appears compared with MA-SEBS/iPP-0% after the introducing of SEBS-g-MA. SEBS-g-MA was found to accelerate the crystallization of iPP in the polarizing optical microscopy (POM) observation, and obvious agglomerate phenomenon appears when the percentage of SEBS-g-MA reach 10%. Moreover, POM observation also reveals that although blending with varying mass fractions of SEBS-g-MA, no apparent distortion on the spherulite shape of iPP samples. The Weibull distribution of negative DC breakdown strength of pure iPP and SEBS-g-MA/iPP composites show that the breakdown strength of these iPP samples increases first, and then decreases with the rise of SEBS-g-MA mass fraction. Space charge behaviors under -100 kV/mm at 70℃ were also presented for iPP/SEBS-g-MA composite films. Finally, trap properties from TSC measurement were adopted to explain the microscopic mechanism of interfacial parameters for the binary system.