Polyethylene (PE for short) is a kind of ethylene monomer prepared by polymerizationThermoplastic resin。In industry, it also includes the copolymer of ethylene and a small amount of a-olefin.Polyethylene is odorless, non-toxic, wax like in hand, and has excellent low temperature resistance (the minimum use temperature can reach - 100~- 70 ° C).Good chemical stability, because the polymer molecule is connected by carbon carbon single bond, it can withstand the erosion of most acids and bases (not resistant to oxidizing acids).Insoluble in general solvents at room temperature, small water absorption, excellent electrical insulation.[1]
Polyethylene is very sensitive to environmental stress (chemical and mechanical effects), and can be processed by general thermoplastic molding methods.Polyethylene is widely used, mainly for making films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulation materials for television, radar, etc.With the development of petrochemical industry, polyethylene production has developed rapidly, accounting for about 1/4 of the total plastic output.By 2021, the world's total capacity will be 133 million tons, and it is estimated that the world's capacity will reach 157.02 million tons by the end of 2023.[2]
On October 27, 2017, the list of carcinogens published by the International Agency for Research on Cancer of the World Health Organization (WHO) was preliminarily sorted out for reference, and polyethylene was included in the list of three types of carcinogens.[3]
Chinese name
polyethylene
Foreign name
polyethylene
chemical formula
(CtwoHfour)n
CAS login number
9002-88-4
EINECS login number
618-339-3
Melting point
85 to 136 ℃
Water solubility
Insoluble
Density
0.91 to 0.96 g/cm ³
Appearance
Low molecular weight is colorless liquid, and high molecular weight is colorless milky white waxy particles or powder
Polyethylene was first synthesized by British ICI Company in 1922.In 1933, the British Burnemen Chemical Industry Company found that ethylene could polymerize under high pressure to produce polyethylene.This method was industrialized in 1939 and is generally called the high-pressure method.In 1953, K. Ziegler of the Federal Republic of Germany discovered that TiClfour-Al(CtwoHfive)threeAs a catalyst, ethylene can also be polymerized under lower pressure.This method was put into industrial production by Hearst Company of the Federal Republic of Germany in 1955, and is generally called low pressure polyethylene.In the early 1950s, Phillips Oil Company and Mobil Oil Company respectively used chromium oxide and molybdenum oxide catalysts to produce high-density polyethylene at relatively low temperature and pressure, and achieved industrial production in 1957.In the 1960s, DuPont Company in Canada began to prepare low-density polyethylene with ethylene and a-olefin by solution method.In 1977, Union Carbide and Dow Chemical successively adopted the low-pressure method to produce low-density polyethylene, called linear low-density polyethylene, of which the vapor phase method of Union Carbide is the most important.Linear low-density polyethylene (LLDPE) has similar properties to low-density polyethylene (LDPE), but also has some characteristics of high-density polyethylene (HDPE). In addition, it has low energy consumption in production, so it develops very rapidly and becomes one of the most eye-catching new synthetic resins.[4]
The core technology of low-pressure method is catalyst.TiCl invented by Ziegler in Germanyfour-Al(CtwoHfive)threeIt is the first generation catalyst for polyolefin, with low catalytic efficiency. Every gram of titanium is about several kilograms of polyethylene.In 1963, Solvay Company of Belgium pioneered the second generation catalyst with magnesium compound as the carrier, and the catalytic efficiency reached tens of thousands to hundreds of thousands of grams of polyethylene per gram of titanium.The use of the second generation catalyst can also eliminate the post-treatment process of removing the catalyst residue.Later, gas phase high efficiency catalyst was developed.In 1975, Italian MonteEdison Group Company developed a catalyst that can directly produce spherical polyethylene without granulation. It is called the third generation catalyst, which is another revolution in high-density polyethylene production.
classification
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Polyethylene can be divided into high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) and ultra-high molecular weight polyethylene (UHMWPE) according to different polymerization methods, molecular weights and chain structures.[5]
LDPE
Property: tasteless, odorless, non-toxic, lusterless, milky white waxy particles, density about 0.920 g/cmthree, melting point 108 ℃~126 ℃.Insoluble in water, slightly soluble in hydrocarbons, etc.It can resist the corrosion of most acids and alkalis, has low water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.[1]
Production process: there are two main methods: high-pressure tube method and kettle method.In order to reduce the reaction temperature and pressure, the tubular process generally adopts a low-temperature and highly active initiator initiated polymerization system, which takes high-purity ethylene as the main raw material, propylene, propane, etc. as the density regulator, and uses highly active initiator to conduct polymerization at about 200 ℃~330 ℃, 150~300MPa.Ethylene and low-pressure circulating gas are compressed to 25-30 MPa in the front turbine compressor, and then compressed to the reaction pressure (250-320 MPa) by the rear reciprocating ultrahigh pressure compressor, and preheated to 150-200 ℃ before being sent to the tubular reactor.In the tubular reactor, the polymerization reaction is carried out with air, oxygen or organic peroxide as the initiator, the reaction products are extracted after cooling, and the polymer and unreacted ethylene are separated in the high-pressure separator.[1]
Use: Mainly used as agricultural film, industrial packaging film, pharmaceutical and food packaging film, machine parts, daily necessities, building materials, wires, cable insulation, coating and synthetic paper.[1]
LLDPE
Properties: LLDPE and LDPE have different molecular structures and properties.Compared with LDPE, LLDPE has excellent environmental stress cracking resistance, electrical insulation, high heat resistance, impact resistance and puncture resistance.Production process: LLDPE resin is mainly produced by full density polyethylene plant, and the representative production processes are Innovene process and UCC Unipol process.[1]
Application: It can be used to produce films, daily necessities, pipes, wires and cables, etc.[1]
HDPE
Property: natural color, cylindrical or oblate particles, with smooth particles, the size of particles in any direction should be 2-5 mm, free of mechanical impurities, and thermoplastic.The powder is white, and qualified products are allowed to have yellowish color.It is insoluble in general solvents at room temperature, but it can swell when contacted for a long time in aliphatic hydrocarbon, aromatic hydrocarbon and halohydrocarbon, and slightly soluble in toluene and amyl acetate when above 70 ℃.Oxidation occurs when heated in air and affected by sunlight.It is resistant to most acids and alkalis.It has low water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.[1]
Production process: gas phase method and slurry method are adopted.[1]
Usage: It can be used to produce hollow containers of various sizes, pipes, calendering tapes and binding tapes for packaging, ropes, fishing nets, fibers for weaving, wires and cables, etc. for film products, daily necessities and industrial use.[1]
UHMWPE
Ultra high molecular weight polyethylene (UHMWPE) is a general term for polyethylene with a molecular weight of more than 1 million, which is mainly used in high-end fields such as plastic modification, pipes, high-strength boards, fibers, etc.UHMWPE is polymerized from ethylene. The production process is similar to that of ordinary slurry high-density polyethylene. Ziegler catalyst is used to polymerize ethylene under certain conditions, that is, ethylene polymerization, separation and drying.[5]
performance
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General characteristics
Polyethylene resin is non-toxic and tasteless white powder or particle, with milky white appearance and wax like feel, and low water absorption, less than 0.01%.The polyethylene film is transparent and decreases with the increase of crystallinity.The polyethylene film has low water permeability but high air permeability, which is not suitable for fresh-keeping packaging but for moisture-proof packaging.Flammability, oxygen index 17.4, low smoke during combustion, a small amount of melting drops, yellow on the flame and blue on the flame, with paraffin smell.Polyethylene has good water resistance.The surface of the product is non-polar, difficult to bond and print, and has been improved by surface treatment.Many branched chains make it have poor resistance to light degradation and oxidation.[6]
Polyethylene with molecular weight in the range of 10000~100000, and ultrahigh molecular weight polyethylene with molecular weight over 100000.The higher the molecular weight, the better its physical and mechanical properties, and the closer to the requirements of engineering materials.However, the higher the molecular weight, the more difficult the processing will be.The melting point of polyethylene is 100~130 ℃, and its low temperature resistance is excellent.Good mechanical properties can still be maintained at - 60 ℃, and the service temperature is 80~110 ℃.[7]
It is insoluble in any known solvent at room temperature, and can be slightly dissolved in toluene, amyl acetate, trichloroethylene and other solvents above 70 ℃.[6]
chemical properties
Polyethylene has good chemical stability and is resistant to dilute nitric acid, dilute sulfuric acid and any concentration of hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid, ammonia, amine, hydrogen peroxide, sodium hydroxide, potassium hydroxide and other solutions at room temperature.But it is not resistant to the corrosion of strong oxidizing acids, such as fuming sulfuric acid, concentrated nitric acid, chromic acid and sulfuric acid mixture, which will slowly erode polyethylene at room temperature.At 90~100 ℃, concentrated sulfuric acid and concentrated nitric acid will quickly erode polyethylene to destroy or decompose it.Polyethylene is prone to photo oxidation, thermal oxidation, ozone decomposition, and degradation under the action of ultraviolet light. Carbon black has excellent light shielding effect on polyethylene.After irradiation, crosslinking, chain breaking, formation of unsaturated groups and other reactions can also occur.[6]
Mechanical properties
Polyethylene has general mechanical properties, low tensile strength, poor creep resistance and good impact resistance.Impact strength LDPE>LLDPE>HDPE, other mechanical properties LDPE<LLDPE<HDPE.It is mainly affected by density, crystallinity and relative molecular weight. With the improvement of these indicators, its mechanical properties increase.The environmental stress cracking resistance is not good, but it is improved when the relative molecular weight increases.Good puncture resistance, among which LLDPE is the best.[6]
thermal properties
The heat resistance of polyethylene is not high, which is improved with the increase of relative molecular weight and crystallinity.Good low temperature resistance, brittleness temperature can generally reach below - 50 ℃;With the increase of relative molecular weight, the lowest temperature can reach - 140 ℃.The linear expansion coefficient of polyethylene is large, up to (20~24) × 10-5/K。High thermal conductivity.[6]
electrical properties
Because polyethylene is non-polar, it has excellent electrical properties with low dielectric loss and high dielectric strength. It can be used as frequency modulation insulation material, corona resistant plastic, and high-voltage insulation material.[6]
Environmental characteristics
Polyethylene is an alkane inert polymer with good chemical stability.It is resistant to corrosion of acid, alkali and salt aqueous solutions at normal temperature, but not to strong oxidants such as fuming sulfuric acid, concentrated nitric acid and chromic acid.Polyethylene is insoluble in general solvents below 60 ℃, but will swell or crack after long-term contact with aliphatic hydrocarbon, aromatic hydrocarbon, halohydrocarbon, etc.When the temperature exceeds 70 ℃, it can be slightly dissolved in toluene, amyl acetate, trichloroethylene, turpentine, mineral oil and paraffin.[6]
Due to the small amount of double bond and ether bond in polyethylene molecule, aging will be caused by sun and rain, so antioxidant and light stabilizer should be added to improve it.[6]
Machining characteristics
LDPE and HDPE have good fluidity, low processing temperature, moderate viscosity, low decomposition temperature, and do not decompose at high temperature of 300 ℃ in inert gas, so they are plastics with good processing performance.However, the viscosity of LLDPE is slightly higher, and the motor power needs to be increased by 20-30%;Melt fracture is easy to occur, so it is necessary to increase die clearance and add processing aids;The processing temperature is slightly higher, up to 200~215 ℃.Polyethylene has low water absorption and does not need drying treatment before processing.[6]
Polyethylene melt belongs to non Newtonian fluid, and its viscosity fluctuates slightly with temperature, but decreases rapidly with the increase of shear rate and presents a linear relationship, among which LLDPE decreases the slowest.
Polyethylene products are easy to crystallize in the cooling process, so mold temperature should be paid attention to during processing.To control the crystallinity of the product and make it have different properties.The molding shrinkage of polyethylene is large, so it must be considered when designing the mold.[6]
Chlorinated polyethylene: Random chloride obtained by partially replacing the hydrogen atom in polyethylene with chlorine.Chlorination is initiated by light or peroxide, and is mainly produced by aqueous suspension method in industry.Due to the difference of molecular weight and its distribution, branching degree, chlorination degree after chlorination, chlorine atom distribution and residual crystallinity of raw polyethylene, chlorinated polyethylene in the form of rubber to hard plastic can be obtained.It is mainly used as modifier of PVC to improve impact resistance of PVC.CPE itself can also be used as electrical insulation material and ground material.[7]
Chlorosulfonated polyethylene: When polyethylene reacts with chlorine containing sulfur dioxide, some hydrogen atoms in the molecule are replaced by chlorine and a small amount of sulfonyl chloride groups to obtain chlorosulfonated polyethylene.The main industrial method is suspension method.Chlorosulfonated polyethylene has good ozone resistance, chemical corrosion resistance, oil resistance, heat resistance, light resistance, abrasion resistance and tensile strength. It is an elastomer with good comprehensive performance and can be used to make equipment parts contacting food.[7]
Cross linked polyethylene: Cross linked polyethylene that uses radiation (X-ray, electron ray or ultraviolet radiation, etc.) or chemical method (peroxide or silicone crosslinking) to make linear polyethylene into a network or shape.The silicone cross-linking method is simple in process, low in operation cost, and the molding and cross-linking can be carried out step by step. Blow molding and injection molding should be adopted.The heat resistance, environmental stress cracking resistance and mechanical properties of crosslinked polyethylene are greatly improved than that of polyethylene, and it is suitable for large pipes, cables and wires, as well as roll molded products.[7]
Blending modification of polyethylene: after blending linear low-density polyethylene and low-density polyethylene, they can be used to process films and other products, and the product performance is better than low-density polyethylene.Polyethylene and ethylene propylene rubber can be blended to produce a wide range of thermoplastic elastomers.[7]
Production process
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Polyethylene can be divided into high pressure method, medium pressure method and low pressure method according to polymerization pressure.[8]
High pressure method is used to produce low-density polyethylene. This method was developed early, and the polyethylene produced by this method accounts for about 2/3 of the total polyethylene output so far. However, with the development of production technology and catalyst, its growth rate has lagged behind that of low-pressure method.Low pressure method includes slurry method, solution method and gas phase method.Slurry method is mainly used to produce high-density polyethylene, while solution method and vapor phase method can not only produce high-density polyethylene, but also produce medium and low-density polyethylene, also known as linear low-density polyethylene, by adding comonomer.Various low-pressure processes have developed rapidly.[8]
High pressure method
The method of polymerizing ethylene into low-density polyethylene using oxygen or peroxide as initiator.Ethylene enters the reactor after secondary compression, and is polymerized into polyethylene under the pressure of 100~300 MPa, temperature of 200~300 ℃ and initiator. The reactants are separated by decompression to recycle the unreacted ethylene. Molten polyethylene is extruded and granulated after adding plastic additives.[8]
The polymerization reactors used include tubular reactor (tube length up to 2000 m) and kettle reactor.The one-way conversion rate of tubular process is 20~34%, and the annual production capacity of single line is 100 kt.The single pass conversion rate of kettle process is 20~25%, and the annual production capacity of single line is 180 kt.[8]
Low pressure method
branchslurry 、Solution methodAnd gas phase method, except for the solution method, the polymerization pressure is below 2 MPa.The general steps include catalyst preparation, ethylene polymerization, polymer separation and granulation.[8]
① Slurry method: The polyethylene produced is insoluble in solvent and looks like slurry.Slurry polymerization has mild conditions and is easy to operate. Alkyl aluminum is often used as activator, hydrogen is used as molecular weight regulator, and kettle reactor is often used.The polymer slurry from the polymerization kettle passes through the flash kettle, gas-liquid separator to the powder dryer, and then goes to granulation.The production process also includes solvent recovery, solvent refining and other steps.Products with different molecular weight distributions can be obtained by using different polymerization kettles in series or in parallel.[8]
② Solution method: polymerization is carried out in solvent, but both ethylene and polyethylene are soluble in solvent, and the reaction system is homogeneous solution.The reaction temperature (≥ 140 ℃) and pressure (4-5MPa) are high.It is characterized by short polymerization time, high production intensity, and can simultaneously produce high, medium and low density polyethylene, which can better control the product properties;However, the polymer obtained by solution method has low molecular weight, narrow molecular weight distribution and low solid content.[8]
③ Gas phase method: ethylene is polymerized in gaseous state, usually in fluidized bed reactor.There are two kinds of catalysts, chromium system and titanium system, which are added into the bed quantitatively from the storage tank. High speed ethylene circulation is used to maintain the fluidization of the bed and eliminate the heat of polymerization reaction.The generated polyethylene is discharged from the bottom of the reactor.The pressure of the reactor is about 2 MPa, and the temperature is 85~100 ℃.The gas phase method is the most important method to produce linear low density polyethylene. The gas phase method saves solvent recovery, polymer drying and other processes, and saves 15% of investment and 10% of operation cost compared with the solution method.It is 30% of the investment of the traditional high-pressure method and 1/6 of the operation cost.Therefore, it has developed rapidly.However, the gas phase method needs further improvement in product quality and variety.[8]
Medium pressure method
Using chromium catalyst supported on silica gel, ethylene is polymerized under medium pressure in a loop reactor to produce high-density polyethylene.[8]
Processing and application: It can be processed by blow molding, extrusion, injection molding and other methods, and is widely used to manufacture films, hollow products, fibers, daily necessities, etc.In actual production, in order to improve the stability of polyethylene to ultraviolet radiation and oxidation, and improve the processing and use performance, a small amount of plastic additives need to be added.The commonly used UV absorbers are o-hydroxybenzophenone or its alkoxy derivatives, etc. Carbon black is an excellent UV shielding agent.In addition, antioxidants, lubricants, colorants, etc. are added to expand the application scope of polyethylene.[8]
Metallocene polyethylene technology
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Metallocene polyethyleneThe technology is to use the current polyethylene process to produce polyethylene products with narrow molecular weight distribution using metallocene catalysts or non metallocene catalysts. Foreign manufacturers include Dow, Exxon, LG and Mitsui. Domestic units include Qilu, Daqing and Dushanzi. At present, Guangzhou Petrochemical, Yangzi Petrochemical and Maoming Petrochemical are also actively developing metallocene products.
Metallocene polyethylene products have excellent optical properties and high permeability;The rigidity/toughness balance is conducive to thinning and resin simplification, excellent puncture resistance and tensile strength, and has great advantages in low temperature and shrink film and pipe materials.[9]
Determination of molecular weight of polyethylene
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The molecular weight and molecular weight distribution of high density polyethylene (HDPE) and low density polyethylene (LDPE) are mainly determined byGel chromatography(GPC).For ultra high molecular weight polyethylene (UHMWPE), the common GPC test method has certain difficulties, such as inappropriate chromatographic column, limited molecular weight of standard sample and difficulty in sample dissolution, which lead to the test accuracy and repeatability can not meet the requirements.At present, the molecular weight of UHMWPE samples is mainly measured by viscosity method.[10]
Characterization of Polyethylene with Different Molecular Weight
application
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High pressure polyethylene: more than half is used for film products, followed by pipes, injection molding products, wire wrapping, etc.[9]
Medium and low pressure polyethylene: mainly injection molding products and hollow products.[9]
Ultra high pressure polyethylene: due to the excellent comprehensive properties of ultra high molecular polyethylene, it can be used as engineering plastics.[9]
