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| High Temperature Dielectric Properties of Polypropylene and Elastomer Blends for Cable Insulation |
| Jiang Yikai1, Xu Man1, Wang Ruofei1, Xu Jing2, Lu Wenxian2 |
1. State Key Laboratory of Electrical Insulation and Power Equipment Xi’an Jiaotong University Xi’an 710049 China;
2. Far East Cable Co. Ltd Wuxi 214000 China |
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Abstract Polypropylene (PP), which has excellent electrical properties, is a research hotspot of potential recyclable cable insulation materials at present. Nevertheless, PP is not capable as cable insulation material because of its stiffness and brittleness. Further modification should be studied for cable insulation applications. Elastomers are often used to improve the mechanical properties of PP. Because of the large difference in melting point between PP and elastomer, the enhanced relaxation of segments at high temperatures results in a significant deterioration of electrical properties. Therefore, β-nucleating agent was used to regulate the crystal morphology of PP in order to optimize the high temperature dielectric properties of polypropylene and elastomer blends.
Firstly, blends were prepared with and without nucleating agent in micro twin-screw extruder, for polyolefin elastomer (POE) contents of 20%, 40% and 60% in weight. In the case of blends nucleated with TMB-5, the PP was previously melt blended with the additive at 0.06 weight fraction of and subsequently blended with the elastomer. The changes in microstructure of specimens were tested through wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The morphologies of the crystals were observed by scanning electron microscope (SEM). The structure and molecular motion information of the specimen were reflected by the dynamic mechanical thermal analysis. The mechanical and electrical properties were all investigated, with emphasis on the dielectric properties at different temperatures.
The results show that, for samples incorporating β-nucleating agent, β-crystals appear and ethylene crystals disappear. The introduction of β-crystals significantly improves the tensile strength while retaining the toughening effect of POE. The tensile strength of POE40-β is 46.4% higher than POE40. Compared with the POE system, the high-temperature mechanical stability of the POE/nucleating agent system is improved, and the high-temperature AC breakdown strength also increases significantly. At 90℃, the AC breakdown strength of POE40-β is 11.4% higher than POE40. More importantly, the dielectric constant and the loss of the POE system significantly increase at high temperatures and low frequency. The more elastomer content, the more obvious the rise. For the POE/nucleating agent system, the addition of β-nucleating agent inhibits the occurrence of this phenomenon. The dielectric constant does not rise at high temperatures and low frequency, and the loss decreases by an order of magnitude from 10-2 to 10-3 at 90℃ and power frequency.
The following conclusions can be drawn from the experimental results: (1) The introduction of β-crystals enhances the flexibility and high-temperature mechanical stability of polypropylene and elastomer blends. (2) The specimen incorporating β-nucleating agent has higher AC breakdown strength. (3) The addition of β-nucleating agent inhibits the compatibility between polypropylene and elastomer and weakens the effect of interface polarization, which makes it show more excellent dielectric properties.
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Received: 07 October 2022
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2024, Vol. 39 
