Abstract
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, SiO2/LDPE nano-composites and MMT-SiO2/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, SiO2/LDPE and MMT-SiO2/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.
Highlights
It can be with morphology the increase ofof tensile the crystal cell spacing of sample microscope
It can be seen that with the of tensile ratio, the crystal cell spacing of sample becomes smaller and smaller, andincrease the crystal structure develops in a close direction
Becomes smaller and smaller, and the crystal structure develops in a close direction
Summary
The morphology of the insulating polymer used in cable is constantly changing during the use These inevitable mechanical stretching will change the local shape and structure in cables and the distribution of internal traps, which will further affect the electrical performance and safe operation of the cable. When subjected to tensile stress, the arrangement of molecular chains in both amorphous and crystalline regions will change [5,6] This will affect the crystalline structure of the polymer to a certain extent. A kind of micro/nano sheet structural material Cu2 O@OZrP was constructed When this sheet material was added to the matrix polyethylene terephthalate with the content of 0.2%, the composite showed good mechanical properties and antibacterial properties [14]. Considering the actual tensile stress of cables, the tensile ratio of 0%, 5%, 10% and 20% is selected to explore the effect of tensile stress on the crystalline structure and space charge characteristics of composite materials
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