Structure and Properties of Isotactic Polypropylene and Ethylene-propylene Copolymer

Polypropylene (PP) is well-known for its excellent electrical and thermal properties, as well as good recycling performance, so it has the potential to replace the cross-linked polyethylene (XLPE) as the cable insulation material in the future. Two kinds of PP such as isotactic homo-propylene (iPP) and ethylene-propylene copolymer (EPC) are selected to study the microstructure, impact strength, and electrical properties. The results show that, there is only α spherulite in iPP and EPC, the spherulites of EPC are smaller and denser than iPP and spherulites of EPC have no distinct interface. The growth rate of the spherulites in EPC is slower than that of iPP, but the crystallization rate is faster. In the different crystallization process, the degree of crystallinity of EPC is lower but the melting point of EPC is higher than that of iPP. The ethylene copolymer molecular chain fragment of EPC can be formed into rubber (EPR) structure, which enhances the EPC impact strength, and temperature of the low temperature brittleness of EPC is -57.3℃, which is much lower than iPP -5℃. It is found that, at room temperature, the DC breakdown and volume resistivity of iPP are higher than those of EPC. However, with the increasing temperature, the breakdown strength and volume resistivity of EPC become more stable. At 90℃, the DC breakdown strengths of iPP and EPC drop by 27% and 21%, and the volume resistivities of iPP and EPC drop by 128.8×1015 Ω·cm and 52.5×1015 Ω·cm, respectively. The space charge density of iPP is about three times of EPC, so EPC is more suitable for extrusion insulation material of HVDC cables. The improvements of EPC properties are influenced by the phase structure and spherulite structure. © 2017, High Voltage Engineering Editorial Department of CEPRI. All right reserved.