低密度聚乙烯基料链结构对黏弹特性的影响

doi:10.19595/j.cnki.1000-6753.tces.230711

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摘要

高压电缆向高电压等级和大长度的发展,对交联聚乙烯绝缘料的加工性能提出了更高的要求。绝缘料的加工性能和电缆绝缘成型质量主要受低密度聚乙烯（LDPE）基料黏弹特性影响,基料的黏弹特性取决于其链结构。然而,由于LDPE基料分子链支化结构复杂,难以建立单一的链结构参数与黏弹特性的关联,因此精准地获得LDPE链结构特征对黏弹特性的影响对电缆绝缘优化升级至关重要。该文采用制备型升温淋洗分级工艺,对管式法LDPE和釜式法LDPE进行分级得到具有不同平均分子量的级份,采用凝胶渗透色谱法和旋转流变仪表征原料和级份的链结构参数及黏弹特性。结果表明,长链支化结构相似时,重均分子量越大,分子链间越容易产生缠结,黏度和模量越高,剪切变稀现象发生频率越低;重均分子量接近时,零切黏度与长链支化度具有非单调关系,过高的长链支化度会削弱分子链间的物理缠结,导致零切黏度降低。基于LDPE黏弹特性构效关系,结合电缆挤出成型工艺,设计更为适宜的基料黏弹特性参数,以此优化调控LDPE的链结构,是未来高端电缆绝缘材料研发的方向。

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关键词 ：高压电缆, 低密度聚乙烯, 黏弹特性, 分子链结构

Abstract：

As high voltage cable transmission systems develop to higher voltage levels and longer distances, the thickness and length of cable insulation layers are increasing, which places higher demands on the processing properties of crosslinked polyethylene insulating materials. The viscoelasticity of low-density polyethylene (LDPE) directly affects the processing performance of the insulating materials and ultimately determine the molding quality of cable insulation. While the molecular structure of LDPE is the key to determine its viscoelasticity. However, due to the complex branching architectures and broadly distributed chain structure of LDPE, it is challenging to establish the relationship between a single molecular chain structural parameter and viscoelasticity. Therefore, accurate obtainment of the influence of LDPE molecular chain structural characteristics on the viscoelasticity is significant for cable insulation optimization and upgrading.
In this paper, preparative temperature-rising elution fractionation (P-TREF) was carried out to separate a tubular LDPE and an autoclave LDPE into fractions with different average molecular weight. For each LDPE sample, two fractions with high content and large molecular weight differences were selected for subsequent experiments. Molecular structures of the original LDPE samples and their fractions were characterized by gel permeation chromatography and their viscoelastic parameters such as complex viscosity, storage modulus and loss modulus, were measured using an electromagnetic rheometer.
The results show that the LDPE resins prepared by different process exhibit significantly different molecular chain structure characteristics. The shape of molecular weight distribution for the autoclave LDPE is bimodal, with more molecules of higher molecular weight and a wider molecular weight distribution. It has higher long-chain branching degree and its molecular chain architecture is closer to a sphere. It is closer to star branched polymer. While the tubular LDPE features a unimodal molecular weight distribution curve with a smaller average molecular weight and a narrower molecular weight distribution. It has fewer long chain branching per molecule and its molecular chain architecture is closer to a rod. At the same weight average molecular weight, the zero-shear viscosity of autoclave LDPE is comparatively lower than that of tubular LDPE. For the two LDPE resins, the relationship between zero-shear viscosity and weight average molecular weight is as follows: for the autoclave LDPE, ${{\eta }_{0}}\propto M_{\text{w}}^{\text{3}.\text{4}}$, while for the tubular LDPE, ${{\eta }_{0}}\propto M_{\text{w}}^{\text{6}.\text{7}}$.The research in this paper also found that when the long chain branching topology is similar, the larger the weight average molecular weight, the more probable to create entanglement between molecular chains. Thus, the viscosity and moduli are higher and the shear thinning phenomenon appears at lower frequency. When the weight average molecular weight is close, the zero-shear viscosity has a non-monotonic relationship with the degree of long-chain branching. Excessive long-chain branches will weaken the physical entanglement between molecular chains, resulting in a decrease in the zero-shear viscosity.
Based on the structure-activity relationship of viscoelasticity of LDPE, and combined it with cable extrusion molding process, it is feasible to design more suitable viscoelasticity parameters of LDPE insulating materials, and optimize the chain structure of LDPE according to this. This is the future direction of the research and development of advanced cable insulating materials.

Key words： High voltage cable low-density polyethylene viscoelasticity molecular chain structure

收稿日期: 2023-05-22

PACS:

TM215

基金资助:

国家电网有限公司总部管理科技项目（SGSHDL00YJJS2000354）和国家自然科学基金智能电网联合基金项目（U2066204）资助

通讯作者: 李盛涛男,1963年生,教授,博士生导师,研究方向为电介质理论及其应用、电气功能材料及器件、极端条件下的绝缘材料和绝缘技术等。E-mail：sli@mail.xjtu.edu.cn

作者简介: 吴一帆女,1998年生,博士研究生,研究方向为高压电缆交联聚乙烯绝缘材料。E-mail：wyf980923@stu.xjtu.edu.cn。

引用本文:

吴一帆, 王诗航, 李盛涛, 摆音娜, 景政红, 徐治, 楼铁城. 低密度聚乙烯基料链结构对黏弹特性的影响[J]. 电工技术学报, 2024, 39(1): 3-12. Wu Yifan, Wang Shihang, Li Shengtao, Bai Yinna, Jing Zhenghong, Xu Zhi, Lou Tiecheng. Effect of Molecular Chain Structure on Viscoelasticity of Low-Density Polyethylene. Transactions of China Electrotechnical Society, 2024, 39(1): 3-12.

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https://dgjsxb.ces-transaction.com/CN/10.19595/j.cnki.1000-6753.tces.230711 https://dgjsxb.ces-transaction.com/CN/Y2024/V39/I1/3

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