Cross-linked polyethylene (PEX, XLPE) is a cross-linked modified product of polyethylene PE, which greatly improves the heat resistance and creep resistance of PE and expands the application range of PE.

The medium temperature of ordinary high-density polyethylene (PE-HD) pipes generally cannot exceed 40 ¡æ, while the service life of cross-linked polyethylene pipes in a hot water system of 70 ¡æ can reach 50 years. The disadvantage of cross-linked polyethylene is that the linear macromolecule PE, after cross-linking modification, its thermoplastic linear macromolecule becomes an insoluble and infusible mesh structure, or even a body structure, completely losing its thermoplasticity. The disadvantage is that the pipe cannot be hot-melt welded, and it is difficult to use it in the thermal pipeline system. In addition, its waste recycling materials cannot be hot-melt recycled, which is contrary to the low-carbon economy and is not conducive to environmental protection.

Cable manufacturers have begun to use XLPE cables to replace traditional oil-paper insulated cables because of their excellent electrical properties and the excellent characteristics of XLPE cables such as lightweight structure, easy winding and high transmission limit. With the increasing maturity of high-voltage technology and insulation technology, XLPE cables are now widely used in long-distance DC transmission systems.

Introduction

Polyethylene (PE) is one of the five general-purpose plastics. Its output and consumption rank first among various synthetic resins. It is widely used in industry, agriculture and daily life. However, polyethylene has poor high temperature resistance. Mechanical properties and chemical resistance sometimes cannot meet the requirements of actual use. Therefore, the modification of polyethylene has always been the key to the development and application of polyethylene products. Polyethylene cross-linking technology is an important technology to improve its material properties. Cross-linked polyethylene can greatly improve its performance, not only significantly improving the mechanical properties, environmental stress cracking resistance, chemical corrosion resistance, creep resistance and electrical properties of polyethylene. And the temperature resistance level is significantly improved, which can increase the heat resistance temperature of polyethylene from 70¡æ to above 100¡æ. Thereby greatly broadening the application range of polyethylene.

At present, cross-linked polyethylene (XLPE) has been widely used in pipes, films, cable materials and foam products. Definition

In order to improve the performance of polyethylene, many modification methods have been studied. Polyethylene is cross-linked, and a three-dimensional network structure is formed through the covalent bonds between polyethylene molecules, which rapidly improves the performance of polyethylene resin, such as: thermal deformation, wear resistance, chemical resistance, stress cracking resistance and other physical and chemical properties.

Properties

Polyethylene molecules are composed of linear molecular chains. When the temperature rises, the binding force (van der Waals force) between linear molecular chains weakens, causing the entire molecular material to deform, so the temperature resistance of polyethylene is poor. Cross-linked polyethylene (CLPE) builds a chemical chain bridge between molecules, so that the molecules cannot be displaced, overcoming the shortcomings of polyethylene. The performance comparison of cross-linked polyethylene and ordinary polyethylene is shown in Table 1.

Cross-linked polyethylene has the following advantages:

1. Heat resistance: XLPE with a three-dimensional network structure has excellent heat resistance. It will not decompose or carbonize below 300¡ãC, the long-term working temperature can reach 90¡ãC, and the thermal life can reach 40 years.

2. Insulation performance: XLPE maintains the original good insulation properties of PE, and the insulation resistance is further increased. Its dielectric loss tangent is very small and is not greatly affected by temperature.

3. Mechanical properties: Due to the establishment of new chemical bonds between macromolecules, the hardness, rigidity, wear resistance and impact resistance of XLPE are improved, thus making up for the shortcomings of PE being easily cracked by environmental stress.

4. Chemical resistance: XLPE has strong acid, alkali and oil resistance, and its combustion products are mainly water and carbon dioxide, which have less harm to the environment and meet the requirements of modern fire safety.

Crosslinking method

There are two crosslinking methods for polyethylene: physical crosslinking (radiation crosslinking) and chemical crosslinking. Chemical crosslinking is further divided into silane crosslinking and peroxide crosslinking.

Physical crosslinking

Radiation crosslinking: Polyethylene products, such as polyethylene sheaths, films, thin-walled tubes, etc., wrapped on wires, are irradiated with ¦Ã-rays and high-energy rays for crosslinking (initiating polyethylene macromolecules to produce free radicals to form CC crosslinking chains). The degree of crosslinking is affected by the radiation dose and temperature, and the crosslinking point increases with the increase of radiation dose. Therefore, by controlling the radiation conditions, crosslinked polyethylene products with a certain degree of crosslinking can be obtained.

The crosslinked polyethylene produced by the radiation crosslinking method has the following advantages: crosslinking and extrusion are carried out separately, product quality is easy to control, production efficiency is high, and scrap rate is low; no additional free radical initiator (such as peroxide, etc.) is required during the crosslinking process, which maintains the cleanliness of the material and improves the electrical properties of the material; it is particularly suitable for small-section, thin-walled insulated cables that are difficult to produce by chemical crosslinking methods. However, radiation cross-linking also has some disadvantages, such as the need to increase the acceleration voltage of the electron beam when cross-linking thick materials; for the cross-linking of round objects such as wires and cables, it is necessary to rotate them or use several electron beams to make the irradiation uniform; the one-time investment cost is considerable; the operation and maintenance technology is complex, and the safety protection issues during operation are also relatively harsh.

Chemical cross-linking

Chemical cross-linking uses chemical cross-linking agents to cross-link polymers, transforming the linear structure into a network structure.

The choice of cross-linking agent should depend on the type of polymer, processing technology and product performance. In addition to meeting some specific requirements, the ideal cross-linking agent should also have the following basic requirements: high cross-linking rate and stable cross-linking structure; high processing safety, easy to use, moderate shelf life after adding resin, no disadvantages of premature or late cross-linking; no effect on the processing performance and use performance of the product; non-toxic, non-polluting, and non-irritating to the skin and eyes.

Chemical crosslinking can be divided into peroxide crosslinking, silane crosslinking, and azo crosslinking:

(1) Peroxide crosslinking and crosslinking agents

Peroxide crosslinking generally uses organic peroxides as crosslinking agents. Under the action of heat, it decomposes and generates active free radicals. These free radicals generate active points on the carbon chain of the polymer and produce carbon-carbon crosslinks to form a network structure. This technology requires high-pressure extrusion equipment to allow the crosslinking reaction to proceed in the barrel, and then use a rapid heating method to heat the product to produce a crosslinked product. Therefore, the production of polyethylene pipes using peroxide crosslinking is difficult to control, the product quality is unstable, and continuous operation is difficult.

(2) Azo crosslinking

This method is to mix azo compounds into PE and extrude it at a temperature lower than the decomposition temperature of the azo compounds. The extrudate passes through a high-temperature salt bath, and the azo compounds decompose to form free radicals, which initiate polyethylene crosslinking. It is generally used for cypress rubber materials with low melting temperatures and has few practical applications for plastics.

(3) Silane crosslinking and crosslinking agents

Silane

Silane crosslinking technology was successfully developed in the 1960s. This technology uses double-chain vinyl silane to react with molten polymer under the action of an initiator to form a silane grafted polymer. In the presence of a silanol condensation catalyst, the polymer hydrolyzes when it comes into contact with water to form a network of oxyalkyl chain crosslinking structures. Silane crosslinking technology has greatly promoted the production and application of crosslinked polyethylene due to its simple equipment, easy process control, low investment, high crosslinking degree and good quality of the finished product. In addition to polyethylene and silane, catalysts, initiators, antioxidants, etc. are also required in crosslinking.

Compared with other methods, the polyethylene products obtained by silane crosslinking have the following advantages:

(1) Low equipment investment, high production efficiency and low cost.

(2) The process is highly versatile and is applicable to polyethylene of all densities and most polyethylene with fillers.

(3) Not limited by thickness.

(4) The amount of peroxide used is small (only 10% of that when cross-linked with peroxide alone), so fewer micropores are generated in the polyethylene insulation layer, which is conducive to maintaining the high insulation properties of polyethylene.

(5) Good aging resistance and long service life.

Main applications

Due to its excellent properties, cross-linked polyethylene is used as high-voltage, high-frequency, and heat-resistant insulation materials and wire and cable coverings required by rockets, missiles, motors, transformers, etc. It is used to manufacture heat shrinkable tubes, heat shrinkable films, various heat-resistant pipes, foam plastics, corrosion-resistant chemical equipment linings, components and containers, and flame-retardant building materials. At present, the largest areas of use are mainly wires and cables, pipes, and foam plastics.

1. Cross-linked polyethylene cable material

The heat resistance of cables with cross-linked polyethylene as insulation is higher than that of polyvinyl chloride. It can be used for a long time at 90¡æ, and the heat resistance temperature during short circuit can reach up to 250¡æ; the insulation resistance is high, the dielectric loss tangent is small, and it basically does not change with the change of temperature; it has good wear resistance and resistance to environmental stress cracking. Once the cross-linked polyethylene cable burns, it produces carbon dioxide and water, while the PVC cable produces harmful gas hydrogen chloride when it burns; in addition, the density of cross-linked polyethylene is about 40% smaller than that of PVC, which can significantly reduce the weight of overhead lines.

2. Cross-linked polyethylene pipe

The pipe produced with cross-linked polyethylene has the advantages of high creep strength, corrosion resistance, light weight, and good heat resistance. The aluminum-plastic composite pipe using cross-linked polyethylene has strong air tightness and high resistance to bursting stress. It has antistatic and shielding effects.

Compared with PVC pipes and ordinary polyethylene pipes, cross-linked polyethylene pipes do not contain plasticizers, will not mold or breed bacteria; do not contain harmful ingredients, meet FDA standards, and can be used for drinking water pipes; have good heat resistance, ordinary PVC and polyethylene pipes are 60-75¡æ, while cross-linked polyethylene pipes are 90¡æ, the highest instantaneous temperature can reach 185¡æ, and can withstand low temperatures of -75¡æ; wide operating temperature range, can be used for a long time under -75-95¡æ conditions, and the service life is up to 50 years. High degree of cross-linking, high density, good pressure resistance; good chemical corrosion resistance, excellent environmental stress cracking resistance, even at higher temperatures can be used to transport a variety of chemicals and accelerate pipe stress. Compared with materials, cross-linked polyethylene pipes are light in weight, only about 1/8 of metal pipes; good corrosion resistance and wear resistance. The wear rate is less than 1/4 of that of steel pipes, and the service life is 2-6 times that of steel pipes; the inner wall is smooth and the fluid flow resistance is small. At the same pipe diameter, the conveying flow rate is larger than that of metal pipes, and the noise is much lower; the transmission performance is good, and the liquid transmission volume is 30%-40% higher than that of steel pipes; the thermal conductivity coefficient is much lower than that of metal pipes, so it has excellent thermal insulation performance. When used in heating systems, no insulation is required, and the heat loss is small; it can be bent arbitrarily without brittle cracking; it has excellent electrical insulation performance, is easy and simple to install, and the installation workload is less than half of that of metal pipes, and the installation cost is low.

Due to the excellent performance of cross-linked polyethylene pipe materials. It is completely non-toxic and hygienic, so it has been regarded as a new generation of green pipes, and is mainly used in the following aspects:

(1) Cold and hot water supply systems and piped drinking water systems for buildings;

(2) Air conditioning cold water systems for buildings;

(3) Civilian residential heating systems;

(4) Floor heating systems;

(5) Household water heater system piping;

(6) Food industry beverage, alcohol, milk and other fluid delivery pipelines;

(7) Chemical and petroleum industry fluid delivery pipelines;

(8) Refrigeration system and water treatment system pipelines.

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