Common Plastics For Wires And Cables

Polyethylene is a plastic with wide application and large consumption at present. According to the data in the table, polyethylene is a good electrical insulation material with small meson loss, high resistivity, high breakdown field strength, good weather resistance and processability. However, due to its low working temperature, it is mainly used as the insulation of communication cables. Medium density and high density polyethylene have high strength and hardness and low water permeability. They are mostly used as cable sheath. However, polyethylene has the disadvantage of large Z, that is, it is easy to burn and has strong black smoke. Therefore, its application has brought many hidden dangers to the environment.

Crosslinked polyethylene is an excellent thermosetting insulating material formed by adding crosslinking agent to low-density polyethylene. On the basis of inheriting many excellent properties of polyethylene, it improves the mechanical properties, weather resistance and allowable working temperature, so it has become a good insulating material for power cable Z at present.

Due to different crosslinking agents, different crosslinking processes are formed. At present, there are three kinds of chemical crosslinking, warm water crosslinking and irradiation crosslinking. Chemical crosslinking is mainly used for medium and high voltage cables (such as 10kV and above); Warm water crosslinking and irradiation crosslinking are mainly used for low-voltage cables (1kV and below).

The insulation performance of XLPE is closely related to its purity. The insulation of high-voltage and ultra-high-voltage cables above 35kV must adopt ultra-clean cross-linked polyethylene, which requires not only high purity of raw materials, but also high cleanliness of cross-linked process equipment and environment, and stable and reliable process.

It should be noted that the insulation properties of polyethylene and cross-linked polyethylene have a "quirk", that is, they are suitable for AC insulation rather than DC insulation, especially DC high voltage will reduce their insulation life. Therefore, the insulation of DC cables mostly adopts rubber insulation or oil paper insulation. Moreover, polyethylene and cross-linked polyethylene insulation have "fear of water", and their breakdown is often related to the existence of water, that is, the formation of "water branches" under high voltage, resulting in insulation damage. Therefore, when polyethylene and cross-linked polyethylene are used as the insulation of high-voltage and ultra-high-voltage cables, water shall be avoided in the process of processing, storage, transportation and insulation extrusion, and there shall be water blocking structures outside the cable insulation shield, such as metal sheath.

Compared with polyethylene, polyvinyl chloride and paper insulation, one Z great advantage of cross-linked polyethylene insulation is that the working temperature is increased by 20 ℃, which improves the safety of the cable and reduces the input cost of the cable. For example, when the line flow is the same (such as 300A), the cross-sectional area of copper conductor of polyethylene or polyvinyl chloride insulated cable (such as YV or VV) needs 120mm2, while the cross-linked polyethylene insulated tape men's basketball copper conductor needs only 70mm2. It can be seen how remarkable the advantages of XLPE insulated cable are.

PVC has good physical and mechanical properties and excellent process properties. It is not only the most used plastic in the 20th century, but also the main insulating material and sheath material of low-voltage wires and cables. However, in the 21st century, PVC will gradually shrink or even fade out in the cable market. There are two reasons. On the one hand, people's safety awareness has improved and hope to use halogen-free materials, so many halogen-free materials came into being. There is no doubt that it will become a new favorite of the cable industry in the 21st century and occupy the market. On the other hand, PVC has five weaknesses: first, it has high density, about 1.5 times that of XLPE, and high insulation cost; Second, the working temperature is low; Third, the dielectric loss is high, more than 100 times higher than that of cross-linked polyethylene; Fourth, poor cold resistance (- brittle at 15 ℃); Fifth, toxic gas (HCl) is released during combustion. The mechanical properties, electrothermal properties and insulation resistance of crosslinked PVC developed in recent years have been greatly improved. Some small section cables have entered the market by irradiation process and have been applied in equipment wires, high-voltage lead wires, automobile wires and building wiring, but their halogen shortcomings cannot be changed.

Fluoroplastics are widely used in wires and cables because of their high working temperature, small medium, good insulation, weather resistance, acid and alkali resistance, oil resistance and flame retardancy.