Polyethylene (PE) crosslinking technology is one of the important means to improve its material properties. The cross-linked modified PE can greatly improve its properties, which not only significantly improves the comprehensive properties of PE such as mechanical properties, environmental stress cracking resistance, chemical corrosion resistance, creep resistance, and electrical properties, but also significantly improves the temperature resistance level, which can increase the heat resistance temperature of PE from 70 °C to more than 100 °C, thereby greatly broadening the application field of PE.
Cross-linked polyethylene insulation is polyethylene under the action of high-energy rays (such as γ rays, α rays, electron rays, etc.) or cross-linking agents so that cross-linking between macromolecules can improve its heat resistance and other properties. The long-term working temperature of the cable using cross-linked polyethylene as insulation can be increased to 90 °C, and the instantaneous short-circuit temperature that can withstand can reach 170-250 °C.
Brief introduction
Polyethylene (PE) is one of the five general plastics, and its production and consumption rank first among various synthetic resins, in industry and agriculture and widely used in daily life. However, the high-temperature resistance of polyethylene is poor. Mechanical properties and chemical resistance sometimes do not meet the requirements of actual use. Therefore, the modification of polyethylene has always been the key to the development and application of polyethylene products, and polyethylene crosslinking technology is an important technology to improve its material properties. Cross-linked modified polyethylene can greatly improve its properties, which not only significantly improves the comprehensive properties of polyethylene such as mechanical properties, environmental stress cracking resistance, chemical corrosion resistance, creep resistance, and electrical properties. Moreover, the temperature resistance level is greatly improved, and the heat resistance temperature of polyethylene can be increased from 70 °C to more than 100 °C. As a result, the application range of polyethylene has been greatly broadened.
At present, cross-linked polyethylene (XLPE) has been widely used in pipes, films, cable materials, and foam products.
Performance and benefits
The molecules of polyethylene are composed of linear molecular chains. When the temperature increases, the binding force between the linear molecular chains (van der Waals force) is weakened, so that the entire molecular material is deformed, so the temperature resistance of polyethylene is poor. Cross-linked polyethylene (XLPE) A chemical chain bridge is erected between the molecules so that the molecules cannot be displaced, which overcomes the deficiency 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 mesh three-dimensional structure has excellent heat resistance. It will not decompose and carbonize below 200 °C, the long-term working temperature can reach 90 °C, and the thermal life can reach 40 years.
2. Insulation performance: XLPE maintains the original good insulation characteristics 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, stiffness, wear resistance, and impact resistance of XLPE are improved, thus making up for the shortcomings of PE being susceptible to environmental stress and cracking.
4. Chemical resistance: XLPE has strong acid and alkali resistance and oil resistance, and its combustion products are mainly water and carbon dioxide, which is less harmful to the environment and meets the requirements of modern fire safety.
Crosslinking principle
Polyethylene ([CH2-CH2]n, n-repeat unit number) is a polymer compound containing two elements of hydrocarbons and hydrogens, with linear or branched molecular structure macromolecular chains, solid form at room temperature, and crystal phase and amorphous phase coexistence form in solid form of polyethylene. The relative molecular weight of polyethylene is between 6,30 and <>,<>.
Polyethylene has excellent electrical insulation properties, but its poor heat resistance affects its use of raw materials for cable insulation. Due to the weak intermolecular interaction in the amorphous region, the melting temperature of most polyethylene is about 140 °C, and its mechanical strength decreases significantly when approaching the melting point of the polyethylene, and the cracking resistance also deteriorates.
When linear macromolecular chains are chemically or physically processed, the process of joining in the form of cross-linked bonds is called crosslinking or "vulcanization". Crosslinked polyethylene has the properties of mesh type and body structure, and its heat resistance will be enhanced with the increase of crosslinking, and the relative thermal elongation will decrease accordingly. Due to its significant improvement in mechanical properties and heat resistance, it has become a widely used power cable insulation material.
The method of crosslinking polyethylene by crosslinking to form cross-linked polyethylene is divided into two categories: chemical method and physical method, and the process methods realized in the industry mainly include the following five: high-energy irradiation crosslinking, silane crosslinking, peroxide crosslinking, ultraviolet crosslinking, and salt crosslinking. Among them, the peroxide crosslinking method (also known as chemical crosslinking) is a crosslinking method suitable for the production of medium and high-voltage level cables, and its principle is a series of free radical reactions triggered by the high-temperature decomposition of peroxide, and then PE is cross-linked. Peroxides are decomposed by heat to form free radicals, and the process of crosslinking reaction is as follows:
Crosslinking method
There are two types of crosslinking methods for polyethylene: physical crosslinking (radiation crosslinking) and chemical crosslinking. Chemical crosslinking is divided into silane crosslinking and peroxide crosslinking.
Physical crosslinking
Radiation crosslinking: polyethylene products, such as polyethylene sheaths, films, thin-walled tubes, and other products coated on the wire, are crosslinked with γ-rays and high-energy rays (causing polyethylene macromolecules to generate free radicals and form C-C crosslinked chains). The degree of crosslinking is affected by the radiation dose and temperature, and the crosslinking point increases with the increase of the radiation dose, so by controlling the radiation conditions, cross-linked polyethylene products with a certain degree of cross-linking can be obtained.
Cross-linked polyethylene produced by 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 the scrap rate is low; No additional free radical initiators (such as peroxides, etc.) are required during the crosslinking process, which maintains the cleanliness of the material and improves the electrical properties of the material; It is especially suitable for small-section, thin-walled insulated cables that are difficult to produce by chemical crosslinking. However, radiation crosslinking also has some disadvantages, such as the need to increase the accelerating voltage of the electron beam when crosslinking thick materials; For crosslinking of round objects such as wires and cables, it is necessary to rotate them or use several electron beams to make the irradiation uniform; One-time investment costs are considerable; The operation and maintenance technology is complex, and the safety protection problems in operation are also relatively harsh.
Chemical cross-linking
Chemical crosslinking is the use of chemical crosslinking agents to crosslink polymers, changing from a linear structure to a network structure.
The choice of crosslinking agent should depend on the polymer variety, processing technology, and product performance, the ideal crosslinking agent in addition to meeting some specific requirements should also have the following basic requirements: high crosslinking rate, stable crosslinking structure; large processing safety, easy to use, moderate validity period after adding resin, no premature or too late crosslinking drawbacks; does not affect the processing performance and use performance of the product; non-toxic, non-polluting, does not irritate the skin and eyes.
In chemical crosslinking, there are peroxide crosslinking, silane crosslinking, and azo crosslinking:
(1) Peroxide crosslinking and crosslinking agentPeroxide crosslinking, generally using organic peroxide as a crosslinking agent, under the action of heat, decomposes to generate active free radicals, which make the polymer carbon chain generate active points and produces carbon-carbon crosslinking to form a network structure. This technology requires high-pressure extrusion equipment so that the crosslinking reaction is carried out in the barrel, and then the product is heated using a rapid heating method, resulting in a crosslinked product. Therefore, the use of the peroxide crosslinking method to produce polyethylene pipe is not easy to control, the product quality is unstable, and continuous operation is more difficult.
(2) Azo crosslinking
The method is to mix the azo compound into PE and extrude at a temperature lower than the azo compound decomposition, and the extrusion is decomposed by a high-temperature salt bath, and the azo compound is decomposed to form free radicals, initiating polyethylene crosslinking. It is generally used for cypress gum materials with low melting temperatures and has few practical applications for plastics.
(3) Silane crosslinking and crosslinking agent
In the sixties of the twentieth century, silane crosslinking technology was successfully developed. The technology uses vinyl silanes containing double bonds to react with molten polymers under the action of initiators to form silane-grafted polymers, which are hydrolyzed in water in the presence of a silanol condensation catalyst to form a networked oxane chain crosslinked structure. Silane crosslinking technology has greatly promoted the production and application of cross-linked polyethylene due to its simple equipment, easy-to-control process, less investment, high degree of crosslinking of finished products, and good quality. In addition to polyethylene and silane, catalysts, initiators, antioxidants, etc. are also used in cross-linking.
Compared with other methods, the polyethylene products obtained by silane crosslinking have the following advantages:
(1) Less equipment investment, high production efficiency, and low cost.
(2) The process is highly versatile, suitable for all-density polyethylene, and also suitable for most polyethylene with filler.
(3) Not limited by thickness.
(4) The amount of peroxide is small (only 10% when peroxide is crosslinked alone), so fewer micropores are generated in the polyethylene insulation layer, which is conducive to maintaining the high insulation of polyethylene.
Main applications
Due to its excellent properties, cross-linked polyethylene is used as high-voltage, high-frequency, heat-resistant insulation materials and wire and cable claddings required by rockets, missiles, motors, transformers, etc. Manufacture of heat-shrinkable tubes, heat-shrinkable films, various heat-resistant pipes, foam plastics, corrosion-resistant chemical equipment linings, components and containers, manufacture of flame retardant building materials, etc. At present, the largest areas of use are mainly wire and cable, pipe, and foam.
1. Cross-linked polyethylene cable material
The heat resistance of the cable with cross-linked polyethylene as insulation is higher than that of polyvinyl chloride, it can be used for a long time at 90 °C, and the heat resistance temperature in short circuit can reach up to 250 °C; 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 environmental stress cracking. Once cross-linked polyethylene is burned by cables, carbon dioxide and water are produced, while PVC cables produce hydrogen chloride harmful gases when burning; In addition, the density of cross-linked polyethylene is about 40% smaller than that of PVC, which can significantly reduce the quality of overhead lines.
2. Cross-linked polyethylene pipe
The pipe produced by cross-linked polyethylene has the advantages of high creep strength, corrosion resistance, lightweight, and good heat resistance. The aluminum-plastic composite pipe using cross-linked polyethylene has strong air tightness and high burst stress resistance. It has an antistatic and shielding effect.
Compared with PVC pipe and ordinary polyethylene pipe, cross-linked polyethylene pipe does not contain plasticizers, will not mildew and breed bacteria; Does not contain harmful ingredients, meets FDA standards, and can be used in drinking water pipes; Good heat resistance, ordinary polyvinyl chloride and polyethylene pipe heat resistance is 60-75 °C, while cross-linked polyethylene pipe is 90 °C, the maximum instantaneous temperature can reach 185 °C, can withstand -75 °C low temperature; Wide operating temperature range, can be used for a long time under -75-95°C conditions, and the service life is up to long-term durability according to standards. High cross-linking, high density, good pressure resistance; Chemical corrosion resistance is very good, and environmental stress cracking resistance is excellent, even at higher temperatures, it can be used to transport a variety of chemicals and stress material with accelerated pipe, cross-linked polyethylene pipe is light in weight, only about 1/8 of metal pipe; Good corrosion resistance and wear resistance. The wear rate is less than 1/4 of the steel pipe, and the service life is 2-6 times that of the steel pipe; The inner wall is smooth, the fluid flow resistance is small, and at the same pipe diameter, the conveying flow is larger than that of metal pipe, and the noise is much lower; The transmission performance is good, and the transmission amount of liquid is increased by 30%-40% compared with the steel pipe; The thermal conductivity is much lower than that of metal pipes, so its thermal insulation performance is excellent. When used in the heating system, heat preservation is not required, and heat loss is small; It can be bent arbitrarily and will not be brittle and cracked; Excellent electrical insulation performance, easy installation, and installation workload of less than half of metal pipe, low installation cost.
Due to the excellent material performance of cross-linked polyethylene pipe. With completely non-toxic hygiene, it has been regarded as a new generation of green pipes, mainly used in the following aspects:
(1) Cold and hot water supply systems and pipeline drinking water systems for buildings;
(2) Chilled water system for building air conditioning;
(3) Residential heating system;
(4) Ground heating system;
(5) Piping of domestic water heater system;
(6) Transportation pipelines for beverages, alcohol, milk, and other fluids in the food industry;
(7) Chemical and petroleum industry fluid transportation pipelines;
(8) Refrigeration system and water treatment system pipeline.
(5) Good aging resistance and long service life.
