极性分子的结构异同性对接枝改性交联聚乙烯材料直流电性能的影响

doi:10.19595/j.cnki.1000-6753.tces.231423

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Abstract Cross-linked polyethylene (XLPE) materials are limited in their application as insulation materials for high-voltage direct current cables, due to issues of the temperature and field-dependent conductivity, as well as problems with space charge injection and accumulation. Generally, graft-modified XLPE can significantly improve the space charge distribution characteristics and reduce the DC conductivity. However, the difficulty lies in the micro-doping and low-gasification temperature of small molecule grafting monomers, which affects the difficulty of the melting grafting process. Additionally, the pre-grafted low-density polyethylene (LDPE) materials is severely affected by the initiation of free radicals during the preparation process, which affects the grafting rate and rheological properties of the materials. At the same time, the improvement of the DC electrical performance of pre-grafted and cross-linked XLPE materials still needs further exploration. Therefore, this paper proposes the preparation of graft-modified materials using a low-temperature high-pressure metering pump in conjunction with a parallel co-rotating twin-screw extruder, then evaluates the grafting rate, rheological properties, and crystallization characteristics of the materials. The pre-grafting materials is then diluted to the same grafting content to prepare graft-modified XLPE materials. Through tests on DC conductivity and space charge distribution, combined with first-principles calculations, the effect of pre-grafted XLPE materials on DC improvement is explored, as well as the influence of grafting monomer structural differences on the materials.
First, acrylate small molecule compounds (methyl acrylate (MA), butyl acrylate (BA), methyl methacrylate (MMA)) were prepared to graft LDPE materials. Based on the analysis of infrared spectroscopy and melt flow rate test, the characteristic absorption peak of carbonyl group (C=O) appeared at wave number 1 740 cm^-1, indicating the successful grafting of small molecules onto the LDPE macromolecular chains. The grafting masterbatch prepared with 0.2 phr peroxide initiator, dicumyl peroxide (DCP), showed a high grafting rate and acceptable melt flow rate. Compared to MMA monomer containing easily breakable bonds and methyl groups with steric hindrance, MA and BA grafting modified XLPE materials exhibited a higher grafting rate. The grafting masterbatch and LDPE material had similar crystallinity, with spherulite size of about 3.8~3.9 µm.
Further, the three different grafted materials were diluted in specific proportions and subjected to the cross-linking process. It was found in the temperature-dependent DC conductivity test that XLPE-g-MA exhibited the largest decrease in conductivity-temperature sensitivity. Under polarization voltage of 40 kV/mm at 80℃, XLPE-g-MA demonstrated excellent characteristics in the spatial charge distribution. Combined with the first-principles simulation calculation, it can be further obtained that the trap energy levels of the three grafted modified XLPE models are relatively consistent, and the improvement effect of the DC performance is significantly related to the electrostatic potential distribution of the grafted monomer. In summary, XLPE-g-MA exhibited excellent DC electrical performance and is highly promising as an insulation material for HVDC cables.

Key words： Cross-linked polyethylene grafting modification acrylate conductivity-temperature sensitivity electrostatic potential distribution

Received: 31 August 2023

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TM215.1

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	Wang Kai
	Zhao Xindong
	Zheng Haifeng
	Yang Xu
	Shao Manzhi
	Zhao Hong
	Wang Xuan

Cite this article:

Wang Kai,Zhao Xindong,Zheng Haifeng等. Influence of the Structural Similarities of Polar Molecules on DC Properties of Grafted Modified Cross-Linked Polyethylene Materials[J]. Transactions of China Electrotechnical Society, 2024, 39(1): 34-44.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.231423 OR https://dgjsxb.ces-transaction.com/EN/Y2024/V39/I1/34

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