Commonly used plastics for wires and cables include polyethylene, cross-linked polyethylene, polyvinyl chloride, polypropylene, polyolefin, fluoroplastics, nylon, etc.
Polyethylene is currently the most widely used and most used plastic. From the data in the table, it can be seen that polyethylene has low meson loss, high resistivity, high breakdown field strength, good weather resistance, and good manufacturability. It is currently the best electrical Insulation Materials. However, due to its low operating temperature, it is mainly used as insulation for communication cables. Medium-density and high-density polyethylene have high strength and hardness, and their water permeability is low, and they are mostly used as cable sheaths. However, polyethylene has the biggest disadvantage, that is, it is easy to burn and has strong black smoke, so its application has brought many hidden dangers to the environment.
Cross-linked polyethylene is an excellent thermosetting insulating material formed by adding a cross-linking agent to low-density polyethylene. On the basis of inheriting many excellent properties of polyethylene, it has improved mechanical properties, weather resistance and allowable working temperature, thus becoming the best insulating material for power cables at present.
Due to the different cross-linking agents added, different cross-linking processes are formed. At present, there are three kinds of chemical cross-linking, warm water cross-linking, and radiation cross-linking that are most used. Chemical crosslinking is mainly used for medium and high voltage cables (such as 10KV and above); warm water crosslinking and radiation crosslinking are mainly used for low voltage cables (1kV and below).
The XLPE insulated cable performance of cross-linked polyethylene is closely related to its purity. The insulation of high-voltage and ultra-high voltage cables above 35KV must be ultra-clean cross-linked polyethylene, which not only requires high purity of raw materials, but also requires high cleanliness of cross-linking process equipment and environment, and the process is stable and reliable.
It should be particularly pointed out that the insulation properties of polyethylene and cross-linked polyethylene have a "quirk", that is, they are suitable for AC insulation, not DC insulation, especially DC high voltage will reduce their insulation life. Therefore, the insulation of DC cables is mostly rubber insulation or oil-paper insulation. Furthermore, polyethylene and cross-linked polyethylene insulation have "waterphobia", and their breakdown is often related to the presence of water, that is, "water branches" are formed under high voltage, leading to insulation damage. Therefore, when polyethylene and cross-linked polyethylene are used for the insulation of high-voltage and ultra-high-voltage cables, they are especially "water-proof" during their processing, storage and transportation, and insulation extrusion. Moreover, there should be a water-blocking structure outside the cable insulation shield, such as metal jacket.
Compared with polyethylene, polyvinyl chloride and paper insulation, one of the biggest advantages of cross-linked polyethylene insulation is that the working temperature is increased by 20°C, 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 the copper conductor of the polyethylene or PVC insulated cable (such as the YV type or VV type) requires 120mm2, while the cross-linked polyethylene insulation tape male basketball copper conductor cross-sectional area only needs 70mm2 is sufficient. It can be seen how remarkable the advantages of XLPE insulated cables are.
Polyvinyl chloride has good physical and mechanical properties and excellent process performance. It is the most used plastic in the 20th century. It is also the main insulation material and sheath material for low-voltage wires and cables. But entering the 21st century, PVC will gradually shrink or even fade out in the cable market. There are two reasons for this. On the one hand, people’s safety awareness has increased and they hope to adopt halogen-free materials, so many halogen-free materials have emerged. There is no doubt that it will become the new favorite of the 21st century cable industry and squeeze the market. On the other hand, PVC has five weaknesses: one is its high density, which is about 1.5 times that of cross-linked polyethylene, and its insulation cost is high; the other is its low operating temperature; the third is its high dielectric loss, which is higher than that of cross-linked polyethylene. One hundred times higher; fourth, poor cold resistance (brittle at -15 degrees); fifth, toxic gas (HCL) is released during combustion. In recent years, the mechanical properties, electrical heating properties, and insulation resistance of the cross-linked polyvinyl chloride developed in recent years have been greatly improved. Some small cross-section cables have entered the market using the irradiation process. They have been used in equipment and installation wires, high-voltage lead wires, automotive wires and building wiring. Application, but the shortcomings of halogen cannot be changed.
Fluoroplastics are widely used in wires and cables due to their high working temperature, small medium, insulation, weather resistance, acid and alkali resistance, oil resistance, and good flame retardancy. Among them, polyperfluoroethylene propylene is particularly popular because of its good manufacturability. But its price is expensive, users have to think twice.
Low-smoke halogen-free polyolefin is a new type of cable sheathing material that has not been developed in the 20th century. Its biggest advantage is its flame retardancy, low smoke and non-toxic gas during combustion, and it is increasingly widely used in important public buildings.