Jiang Hongtao, Gao Junguo, Zhang Xiaohong, Guo Ning
Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150080, China.
Polymers (Basel). 2021 Dec 13;13(24):4354. doi: 10.3390/polym13244354.
Low density polyethylene (LDPE) is a good insulating material which is widely used in cable materials due to its excellent insulation and processability. However, in the DC high voltage environment, pure polyethylene materials still face many problems, the most serious of which is space charge accumulation. The cable will inevitably be subjected to tensile stress during production, installation and operation. Therefore, it is of great significance to study the effect of stretching on the microstructure and space charge characteristics for polymers and their composites. In this paper, MMT/LDPE micro-composites, SiO/LDPE nano-composites and MMT-SiO/LDPE micro-nano-composites were prepared by melt blending. Mechanical stretching was carried out on pure LDPE materials and the above three kinds of composite materials. Each material was stretched according to four stretching ratios, which are 0%, 5%, 10% and 20%. The crystal morphology was observed by polarizing microscope (PLM), the crystallization perfection was tested by differential scanning calorimetry (DSC), and the space charge distribution inside each sample was measured by pulsed electro-acoustic (PEA) method. At the same time, the average charge density and apparent charge mobility for samples during depolarization were calculated and analyzed. The experimental results show that when the pure low density polyethylene sample is not stretched, its crystal structure is loose. Tensile stress can make the loose molecular chains align in LDPE and improve its crystalline structure, which is helpful to restrain the accumulation of space charge inside the sample. For MMT/LDPE, SiO/LDPE and MMT-SiO/LDPE composites, their internal crystal structure is compact. Stretching will destroy their original crystal structure at first, and then disorder molecular chains inside the three composite materials. With the increase of stretching ratio, the molecular chains begin to orient along the direction of force, the crystallization tends to be perfect gradually, and the space charge accumulation in samples also decreases. From the calculation results of apparent charge mobility for each sample, with the increase of stretching ratio, the trap depth and trap density inside samples firstly increased and then decreased.
低密度聚乙烯（LDPE）是一种优良的绝缘材料，因其优异的绝缘性能和加工性能而被广泛应用于电缆材料中。然而，在直流高压环境下，纯聚乙烯材料仍面临许多问题，其中最严重的是空间电荷积累。电缆在生产、安装和运行过程中不可避免地会受到拉伸应力。因此，研究拉伸对聚合物及其复合材料微观结构和空间电荷特性的影响具有重要意义。本文通过熔融共混制备了蒙脱土/低密度聚乙烯（MMT/LDPE）微复合材料、二氧化硅/低密度聚乙烯（SiO/LDPE）纳米复合材料和蒙脱土-二氧化硅/低密度聚乙烯（MMT-SiO/LDPE）微纳复合材料。对纯LDPE材料和上述三种复合材料进行了机械拉伸。每种材料按照0%、5%、10%和20%这四个拉伸比进行拉伸。通过偏光显微镜（PLM）观察晶体形态，用差示扫描量热法（DSC）测试结晶完善程度，并用脉冲电声（PEA）法测量每个样品内部的空间电荷分布。同时，计算并分析了样品去极化过程中的平均电荷密度和表观电荷迁移率。实验结果表明，当纯低密度聚乙烯样品未拉伸时，其晶体结构疏松。拉伸应力可使低密度聚乙烯中疏松的分子链取向，改善其晶体结构，这有助于抑制样品内部空间电荷的积累。对于MMT/LDPE、SiO/LDPE和MMT-SiO/LDPE复合材料，它们内部的晶体结构致密。拉伸首先会破坏它们原来的晶体结构，然后使三种复合材料内部的分子链无序化。随着拉伸比的增加，分子链开始沿受力方向取向，结晶逐渐趋于完善，样品中的空间电荷积累也随之减少。从每个样品的表观电荷迁移率计算结果来看，随着拉伸比的增加，样品内部的陷阱深度和陷阱密度先增大后减小。
