Effect of the β-crystal formation in isotactic polypropylene used for eco-friendly insulating material

In this paper, the melt blending method was used to prepare isotactic polypropylene (iPP) samples. The nucleating agents N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide (TMB-5) was chosen to induce the β-crystal. The quenching, 130
^o
C isothermal crystallization, natural cooling molding processes were used to control α- and / or β-crystals in the iPP samples. The crystalline modifications and crystallinity were analyzed by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) analysis. The mechanical and thermal degradation properties of these samples were investigated by tensile test and thermogravimetric analysis (TGA). The breakdown tests and the volume resistivity tests were conducted to characterize the electrical property of the samples. The experimental results show that the three molding processes does affect the content and thickness of β-crystals in the iPP sample, and the natural cooling and quenching molding will produce more β-crystals. Compared to the microcrystallites of α-crystal, the growth of β-crystals effectively improved the thermal and electrical properties of iPP, such as the thermal stability and breakdown strength. On another hand, the thinner β-crystal can effectively improve the mechanical and provide better stability in volume resistivity as temperature increases. Based on the above results, the mechanism of crystalline modifications influence on the electrical property of iPP discussed.