Effect of Crystalline Characteristics on Space Charge and DC Breakdown Strength of Polypropylene

The crystalline morphology and crystallinity of PP under different forming conditions were observed by SEM, polarized light microscopy (PLM) and differential scanning calorimetry (DSC). The mechanical properties of different crystalline forms of PP were tested by dynamic mechanical analysis (DMA). The space charge characteristics of different crystal forms of PP were measured by electroacoustic pulse method (PEA), and DC breakdown strength tests were performed on different crystalline forms of PP. The results show that the crystalline morphology and crystallinity of PP are significantly different at different melting temperatures and crystallization temperatures. The higher melting temperature and crystallization temperature decrease the number of nuclei and promote the growth of spherulites. The results of DMA and PEA tests show that the small spherulites can reduce the storage modulus of PP. During the growth process of large spherulites, more structural defects are pushed out into the amorphous region, resulting in the increased β-relaxation loss and thus the space charge at room temperature increases significantly. The DC breakdown strength test results show that when PP forms large and sparse spherulites, the breakdown process can develop along a shorter path, and there are more impurity in the amorphous areas, so that the breakdown strength is significantly reduced. © 2019, Editorial Board of Polymer Materials Science & Engineering. All right reserved.