Cross-linked polyethylene is to use chemical or physical methods to change the molecular structure of polyethylene from linear to three-dimensional network structure.
After cross-linking, polyethylene not only maintains the original excellent properties of polyethylene, but also overcomes and improves the mechanical, heat resistance, chemical resistance, creep resistance and environmental cracking resistance of polyethylene.
The physical cross-linking method is to irradiate polyethylene with high-energy particle rays (such as beta rays) generated by an electron accelerator, so that the polyethylene has a state of binding chains, and the polyethylene molecules with binding chains are combined with each other to form a cross-linking of a three-dimensional network structure. Therefore, physical crosslinking is also called radiation crosslinking.
The advantages of physical cross-linking are uniform cross-linking, less pinholes and bubbles, no scorch (local over-cross-linking), little energy consumption, and it belongs to cold cross-linking. Since physical cross-linking must have special additional equipment (electron accelerators and radiation-proof airtight sites, etc.), the radiation energy is proportional to the thickness of the insulating layer.
Therefore, the physical cross-linking method is mostly used to make films, and the insulating layer of cables (especially high-voltage cables) is relatively thick. Physical cross-linking methods are rarely used, and chemical cross-linking methods are mostly used.
The chemical cross-linking method is to mix a chemical cross-linking agent into the polyethylene material. During the cross-linking reaction, the cross-linking agent molecules break off and capture the hydrogen atoms in the polyethylene molecules to form polyethylene molecules with binding chains. become cross-linked polyethylene.
Commonly used crosslinking agents can be divided into two categories: organic peroxides and siloxanes.
When using organic peroxide as the crosslinking agent, the crosslinkable material needs to be heated and pressurized in a long pipeline after extrusion.
At present, there are two ways to heat and pressurize: one is to use high-pressure steam, and the other is to use electric heating and nitrogen pressure protection. The cross-linking method without water participation (the latter) is also called dry cross-linking, and the cross-linking method with water participation (the former) is called wet cross-linking. Due to the presence of a small amount of moisture inside the cross-linked polyethylene obtained by wet cross-linking, its electrical properties are poor, and it is not suitable for cables of commercial voltage level, so this method has been eliminated.
When organosiloxane is used as the crosslinking agent, the crosslinkable material needs to be hydrolyzed into unstable hydroxyl groups under the action of water or moisture at a certain temperature (70~90C) after extrusion. Under the action of polyethylene crosslinking. Therefore, it is also called silane crosslinking (referring to siloxane as a crosslinking agent) or warm water crosslinking (referring to the final completion of crosslinking in warm water).
The feature of silane crosslinking is that no special pipeline production line is needed, the energy consumption in production is low, and the crosslinking degree can reach 60%, but the silane crosslinking is completed in water, so the electrical properties are poor, and it is only suitable for medium pressure. The following wire and cable products.
At present, it is widely used in low-voltage power cables, overhead insulated cables of 10kV and below, various insulated wires of 450/750V and below, and the cores of control cables, etc., and has a tendency to replace polyvinyl chloride power cables.