polyethylene(PE) cross-linking technology is to improve itsMaterial propertiesOne of the important means of.The crosslinked PE can greatly improve its properties, not only its mechanical properties, environmental stress cracking resistance and chemical resistancecorrosiveEnergy, creep resistance, electrical properties and other comprehensive properties, and significantly improve the temperature resistance grade, which can increase the heat resistance temperature of PE from 70 ℃ to more than 100 ℃, thus greatly expanding the application field of PE.[1]
Cross linked polyethyleneinsulationIt means that polyethylene can cross link its macromolecules under the action of high-energy rays (such as γ ray, α ray, electron ray, etc.) or cross-linking agent, which can improve its heat resistance and other properties.The long-term working temperature of XLPE insulated cables can be raised to 90 ℃, and the transient short circuit temperature can be up to 170-250 ℃.[2]
Chinese name
Cross linked polyethylene[1]
Foreign name
Cross linked polyethylene[3]
English abbreviations
XLPE[4]
chemical formula
（C2H4）n[5]
Advantages
Improve mechanical, creep resistance and electrical properties[5]
polyethylene(PE) is one of the five generalPlasticOne, its output and consumption rank among varioussynthetic resinFirst, inIndustry、AgricultureAnd widely used in daily life.But the high temperature resistance of polyethylene is poor.Sometimes the mechanical properties and chemical resistance 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.The properties of crosslinked polyethylene can be greatly improved, which not only significantly improves the mechanical properties and resistance of polyethyleneEnvironmental stressCracking performance, chemical resistancecorrosiveCanCreep resistanceAnd electrical performance.Moreover, the temperature resistance grade is obviously improved, and the heat resistance temperature of polyethylene can be increased from 70 ℃ to more than 100 ℃.Therefore, the application range of polyethylene is greatly expanded.[4]
At present, cross-linked polyethylene (XLPE) has been widely used inPipe、film、Cable materialAnd foam products.[4]
Performance and advantages
Announce
edit
Table 1
The molecule of polyethylene is composed of linear molecular chains.When the temperature increases, the binding force (van der Waals force) between linear molecular chains weakens, causing deformation of the entire molecular material, so the temperature resistance of polyethylene is poor.Cross linked polyethylene (XLPE)[10]A chemical chain bridge is set up between the molecules, so that the molecules can not be displaced, which overcomes the shortcomings of polyethylene.See Table 1 for performance comparison of cross-linked polyethylene and ordinary polyethylene.[4]
Cross linked polyethylene has the following advantages:[5]
1、heat-resistingPerformance: XLPE with reticular three-dimensional structure has excellent heat resistance.Not under 200 ℃decomposeandcarbonizationThe long-term working temperature can reach 90 ℃, and the thermal life can reach 40 years.[5]
2、insulationPerformance: XLPE maintains the original good insulation characteristics of PE, and the insulation resistance further increases.The dielectric loss tangent is very small, and is not affected by temperature.[5]
4. Chemical resistance: XLPE has strong acid and alkali resistance and oil resistance. Its combustion products are mainly water and carbon dioxide, which is less harmful to the environment and meets the requirements of modern fire safety.[5]
Cross-linking principle
Announce
edit
Polyethylene ([CH2-CH2] n, n-number of repeat units) contains both hydrocarbonPolymer compound, with linear or branched chain molecular structure macromolecular chain, which is in solid form at room temperature, and coexistence of crystalline phase and amorphous phase in solid polyethylene.PolyethyleneRelative molecular weightBetween 60000 and 300000.[5]
Polyethylene has excellent electrical insulation performance, but its poor heat resistance affects the use of raw materials for cable insulation.Due to the weak intermolecular interaction in the amorphous zone, the melting temperature of most polyethylene is about 140 ℃. When approaching the melting point of polyethylene, its mechanical strength decreases significantly, and its anti cracking ability also decreases.[5]
When linear macromolecular chains are connected in the form of cross-linking bonds after chemical or physical treatment, the process is called cross-linking or "vulcanization".The crosslinked polyethylene has the network and body structure properties, and its heat resistance will increase with the increase of crosslinking degree, and the relative thermal elongation will also decrease accordingly.Due to the significant improvement of its mechanical properties and heat resistance, it has become a widely used power cable insulation material.[5]
The methods of cross-linking polyethylene to form cross-linked polyethylene can be divided into two categories: chemical methods and physical methods. There are five industrial processes: high-energy irradiation cross-linking, silane cross-linking, peroxide cross-linking, ultraviolet cross-linking and salt cross-linking.Peroxide crosslinking (also known as chemical crosslinking) is a crosslinking method suitable for production of medium and high voltage cables. Its principle is a series of crosslinking methods caused by the high-temperature decomposition of peroxidesFree radical reactionSo that PE can be crosslinked.Peroxide decomposes into free radicals when heated, and the process of crosslinking reaction is as follows:[5]
Cross linked polyethylene
Crosslinking method
Announce
edit
There are two cross-linking methods for polyethylene: physical cross-linking (radiation cross-linking) and chemical cross-linking.Chemical crosslinking can be divided into silane crosslinking and peroxide crosslinking.[6]
Physical crosslinking
Polyethylene sheath
Radiation crosslinking: willpolyethyleneArticles, such as polyethylene sheath wrapped on the conductorfilm、Thin-walled tubeγ for other products-radial, high-energy ray irradiation for cross-linking (initiating polyethylene macromolecule to produce free radicals, forming C-C cross-linked chain).The cross-linking degree is affected by radiation dose and temperature, and the cross-linking point increases with the increase of radiation dose. Therefore, cross-linked polyethylene products with a certain degree of cross-linking can be obtained by controlling radiation conditions.[7]
useRadiation crosslinkingThe crosslinked polyethylene produced by the method has the following advantages: cross-linking andExtrusion moldingSeparately, the product quality is easy to control, the production efficiency is high, and the scrap rate is low;No additionalFree radical initiator(such as peroxide) to maintain the cleanliness of the material and improve the electrical performance of the material;It is especially suitable for small section and thin wall insulated cables that are difficult to produce by chemical crosslinking method.However, radiation crosslinking also has some disadvantages, such as the need to improve the electron beamAccelerating voltage；For cross-linking of circular objects such as wires and cables, it is necessary to rotate them or use several electron beams to make irradiation uniform;The one-time investment cost is considerable;The operation and maintenance technology is complex, and the safety protection problem in operation is also relatively harsh.[8]
Chemical crosslinking
Chemical cross-linking is to use chemical cross-linking agent to make the polymer cross-linkedlinear structureChange into a network structure.[5]
The selection of cross-linking agent should depend on the polymer variety, 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, stable cross-linking structure;Great processing safety, easy to use, addresinThe later period of validity is moderate, without the disadvantage of early or late crosslinking;It does not affect the processing performance and service performance of the product;Non toxic, non polluting and non irritating to skin and eyes.[9]
In chemical crosslinking, there are peroxide crosslinkingsilaneClassification of cross-linking and azo cross-linking:[6]
(1) Peroxide crosslinking and crosslinking agent
Peroxide cross-linking generally uses organic peroxides as cross-linking agents. Under the action of heat, they decompose to generate active free radicals, which make the polymerCarbon chainThe active point is formed on the top of the, and the carbon carbon cross-linking is generated to form a network structure.This technology requires high-pressure extrusion equipment to make the crosslinking reaction take place in the barrel, and then use the rapid heating method to heat the products, so as to produce cross-linked products.Therefore, peroxide crosslinking method is adopted for productionPolyethylene pipeIt is difficult to control, the product quality is unstable, and continuous operation is difficult.[6]
(2) Azo crosslinking
This method is toAzo compoundIt is mixed into PE and extruded at a temperature lower than the decomposition temperature of azo compound. The extrudate passes a high temperaturesalt bath, azo compound decomposes to formFree radical, initiate polyethylene cross-linking.Generally used formeltLow temperature cypress gum materials have few practical applications for plastics.[6]
(3) Silane crosslinking and crosslinking agent
silane
In the 1960s, silane crosslinking technology was successfully developed.This technology uses thevinyl silane React with molten polymer under the action of initiator to formsilanegraftpolymerIn the presence of a silane alcohol condensation catalyst, the polymer is hydrolyzed in water to form a network of oxane chain crosslinks.Silane cross-linking technology has greatly promoted the production and application of cross-linked polyethylene due to its simple cross-linking equipment, easy process control, less investment, high cross-linking degree and good quality of finished products.Except polyethylenesilaneIn addition, it is also needed in cross-linkingcatalyzer、Initiator、AntioxidantEtc.[6]
Compared with other methods, polyethylene products obtained by silane crosslinking method have the following advantages:[1]
(1) Low equipment investment, high production efficiency and low cost.[1]
(2) The process has strong versatility and is suitable for polyethylene of all densities, as well as most polyethylene with fillers.[1]
(3) Not limited by thickness.[1]
（4）peroxideThe dosage is small (only 10% of that when peroxide is used alone for crosslinking)Insulation layerLess micropores are generated, which is conducive to maintaining the height of polyethyleneInsulation。[1]
(5) Good aging resistance and long service life.[1]
Main applications
Announce
edit
Due to its excellent performance, crosslinked polyethylene is used as a high-voltage, high-frequency, heat-resistant insulating material andWire and cableCladding - ManufacturingHeat shrinkable tube、Heat shrinkable filmVarious heat-resistant pipesFoam plastic, ResistancecorrosionChemical equipment oflining, components and containers, manufacturing flame retardant building materials, etc.At present, the fields with the largest consumption are mainly wires and cables, pipes and foamed plastics.[4]
1. Cross linked polyethylene cable material
XLPE cable compound
XLPE insulated cableheat-resistingsex ratiopolyvinyl chlorideHigh, it can be used for a long time at 90 ℃, and the maximum heat resistance temperature during short circuit can reach 250 ℃;insulation resistance High,mediumLoss angleThe tangent is small and basically does not change with the change of temperature;It has good wear resistance and environmental stress cracking resistance.Once the XLPE cable burnscarbon dioxideAnd water, while the burning of PVC cable produces hydrogen chloride harmful gas;In addition, the density of cross-linked polyethylene is about 40% less than that of PVC, which can significantly reduce the quality of overhead lines.[4]
2. Cross linked polyethylene pipe
The pipes produced with crosslinked polyethylene havecreepHigh strength, corrosion resistance, light weight, good heat resistance, etc.Aluminum plastic composite pipe with cross-linked polyethyleneAir tightnessStrong, resistant to blasting stress, and resistant tostatic electricityandshieldeffect.[4]
Cross linked polyethylene pipe
Compared with PVC pipes and ordinary polyethylene pipes, cross-linked polyethylene pipes do not containPlasticizer, will not mildew and breed bacteria;It does not contain harmful ingredients, meets FDA standards, and can be used for drinking water pipes;Good heat resistance, common PVC and polyethylenePipeThe heat resistance is 60-75 ℃, while the cross-linked polyethylene pipe is 90 ℃, the maximum instantaneous temperature can reach 185 ℃, and the low temperature of - 75 ℃ can be withstood;It has a wide temperature range and can be used for a long time at - 75-95 ℃, with a service life of up to 50 years.High crosslinking degree, high density, good pressure resistance;Good chemical corrosion resistance and environmental stress cracking resistance. It can also be used to transport a variety of chemicals even at high temperatures. Compared with materials with accelerated pipe stress, 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 the steel pipe, and the service life isSteel pipe2-6 times of;The inner wall is smooth and the fluid flow resistance is small. When the pipe diameter is the same, the transmission flow is larger than that of metal pipes, and the noise is much lower;transmission performance Good, the liquid transmission capacity is 30% - 40% more than that of steel pipe;thermal conductivityIt is far lower than metal pipeline, so its thermal insulation performance is excellent.When it is used in the heating system, it does not need thermal insulation, and the heat loss is small;It can be bent arbitrarily without brittle fracture;The electrical insulation performance is excellent, the installation is simple and easy, the installation workload is less than half of the metal tube, and the installation cost is low.[4]
Due to the excellent performance of cross-linked polyethylene pipe material.It is completely non-toxic and sanitary, so it has been regarded as a new generation of green pipes, mainly used in the following aspects:[4]
