Polyethylene (PE) is a thermoplastic resin obtained by polymerization of ethylene monomers. In industry, it also includes copolymers of ethylene and a small amount of α-olefins. Polyethylene is odorless, non-toxic, feels like wax, and has excellent low-temperature resistance (the lowest operating temperature can reach -100~-70°C). It has good chemical stability because the polymer molecules are connected by carbon-carbon single bonds and can withstand corrosion from most acids and alkalis (not resistant to oxidizing acids). It is insoluble in general solvents at room temperature, has low water absorption, and has excellent electrical insulation.
Polyethylene is very sensitive to environmental stress (chemical and mechanical effects) and can be processed using general thermoplastic molding methods. Polyethylene has a wide range of uses, mainly used to make films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulation materials for televisions, radars, etc. With the development of petrochemicals, polyethylene production has developed rapidly, and its output accounts for about 1/4 of the total plastic production. As of 2021, the world’s total production capacity is 133 million tons, and it is expected that by the end of 2023, the world’s production capacity will reach 157.02 million tons.
On October 27, 2017, the World Health Organization’s International Agency for Research on Cancer published a preliminary list of carcinogens for reference, and polyethylene was included in the list of Class 3 carcinogens.
Research History
Polyethylene was first synthesized by ICI in the UK in 1922. In 1933, Brunner Mond Chemical Industries in the UK discovered that ethylene can be polymerized under high pressure to produce polyethylene. This method was industrialized in 1939 and is commonly known as the high-pressure method. In 1953, K. Ziegler of the Federal Republic of Germany discovered that ethylene can also be polymerized under relatively low pressure using TiCl4-Al(C2H5)3 as a catalyst. This method was put into industrial production by the Hoechst Company in the Federal Republic of Germany in 1955 and is commonly known as low-pressure polyethylene. In the early 1950s, Phillips Oil Company and Mobil Oil Company used chromium oxide and molybdenum oxide catalysts respectively to produce high-density polyethylene at relatively low temperatures and low pressures, and achieved industrial production in 1957. In the 1960s, DuPont of Canada began to use ethylene and a-olefins to produce low-density polyethylene using a solution method. In 1977, Union Carbide and Dow Chemical Company in the United States successively used low-pressure methods to produce low-density polyethylene, called linear low-density polyethylene. Among them, Union Carbide’s gas phase method was the most important. The performance of linear low-density polyethylene is similar to that of low-density polyethylene, but it also has some characteristics of high-density polyethylene. In addition, the energy consumption in production is low, so it has developed very rapidly and has become one of the most eye-catching new synthetic resins.
The core technology of the low-pressure method lies in the catalyst. TiCl4-Al(C2H5)3 invented by Ziegler of Germany is the first generation catalyst for polyolefins. Its catalytic efficiency is low, and about several kilograms of polyethylene can be obtained per gram of titanium. In 1963, the Belgian Solvay Company pioneered the second generation catalyst with magnesium compounds as carriers, and the catalytic efficiency reached tens to hundreds of thousands of grams of polyethylene per gram of titanium. The use of the second generation catalyst can also save the post-treatment process of removing catalyst residues. Later, a gas phase method high-efficiency catalyst was developed. In 1975, the Italian Monte Edison Group developed a catalyst that can save granulation and directly produce spherical polyethylene. It is called the third generation catalyst and is another revolution in the production of high-density polyethylene.
Classification
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) depending on the polymerization method, molecular weight and chain structure.
LDPE
Properties: Tasteless, odorless, non-toxic, matte surface, milky white waxy particles, density about 0.920 g/cm3, melting point 108℃～126℃. Insoluble in water, slightly soluble in hydrocarbons, etc. Resistant to corrosion by most acids and alkalis, low water absorption, able to maintain softness at low temperatures, and high electrical insulation.
Production process: There are mainly two types: high-pressure tubular process and kettle process. In order to reduce the reaction temperature and pressure, the tubular process generally adopts a low-temperature and high-activity initiator to initiate the polymerization system. It uses high-purity ethylene as the main raw material, propylene, propane, etc. as density adjusters, and uses high-activity initiators to carry out polymerization reactions at about 200℃~330℃ and 150~300MPa. Ethylene and low-pressure circulating gas are compressed to 25-30 MPa in the front-end turbo compressor, and then compressed to the reaction pressure (250-320 MPa) by the rear-end reciprocating ultra-high pressure compressor. They are preheated to 150-200℃ and then sent to the tubular reactor. In the tubular reactor, the polymerization reaction is carried out using air, oxygen or organic peroxide as the initiator. After cooling, the reaction product is extracted, and the polymer and unreacted ethylene are separated in the high-pressure separator.
Uses: Mainly used as agricultural film, industrial packaging film, pharmaceutical and food packaging film, mechanical parts, daily necessities, building materials, wire and cable insulation, coating and synthetic paper, etc.
LLDPE
Properties: Due to the significant difference in the molecular structures of LLDPE and LDPE, their properties are also different. Compared with LDPE, LLDPE has excellent resistance to environmental stress cracking and electrical insulation, higher heat resistance, impact resistance and puncture resistance, etc. Production process: LLDPE resin is mainly produced using full-density polyethylene equipment, and the representative production processes are the Innovene process and UCC’s Unipol process.
Uses: Can be used to produce films, daily necessities, pipes, wires and cables, etc.
HDPE
Properties: Natural color, cylindrical or oblate particles, smooth particles, the size of the particles in any direction should be 2~5 mm, free of mechanical impurities, and thermoplastic. The powder is white powder, and qualified products are allowed to have a slight yellow color. It is insoluble in general solvents at room temperature, but can swell in aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons after long-term contact, and is slightly soluble in toluene and amyl acetate above 70°C. It will oxidize when heated in air and affected by sunlight. It can withstand corrosion from most acids and alkalis. It has low water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.
Production process: Two production processes are used: gas phase method and slurry method.
Uses: It can be used to produce film products, various hollow containers of various sizes for daily necessities and industrial use, pipes, calendered tapes and ligatures for packaging, ropes, fishing nets and weaving fibers, wires and cables, etc.
UHMWPE
Ultra-high molecular weight polyethylene (UHMWPE) is a general term for polyethylene with a molecular weight of more than 1 million. It is mainly used in high-end fields such as plastic modification, pipes, high-strength plates, fibers, etc. Ultra-high molecular weight polyethylene is produced by the polymerization of ethylene. The production process is similar to that of ordinary slurry high-density polyethylene. Both use Ziegler catalysts to polymerize ethylene under certain conditions, that is, ethylene polymerization, separation, and drying.
Performance
General Features
Polyethylene resin is a non-toxic, odorless white powder or granule with a milky white appearance and a waxy feel. It has a low water absorption rate of less than 0.01%. Polyethylene film is transparent and decreases with increasing crystallinity. Polyethylene film has low water permeability but high air permeability. It is not suitable for fresh-keeping packaging but suitable for moisture-proof packaging. It is flammable and has an oxygen index of 17.4. It produces low smoke when burning, with a small amount of molten droplets. The flame is yellow on the top and blue on the bottom, and has a paraffin smell. Polyethylene has good water resistance. The surface of the product is non-polar and difficult to bond and print, which can be improved by surface treatment. It has many side chains, which makes it poorly resistant to light degradation and oxidation.
The molecular weight of polyethylene is in the range of 10,000 to 100,000. The molecular weight of polyethylene is over 100,000 and is called ultra-high molecular weight polyethylene. The higher the molecular weight, the better its physical and mechanical properties, and the closer it is to the requirements of engineering materials. However, the higher the molecular weight, the more difficult it is to process. The melting point of polyethylene is 100~130℃, and it has excellent low temperature resistance. It can still maintain good mechanical properties at -60℃, and the operating temperature is 80~110℃.
It is insoluble in any known solvent at room temperature, but can be dissolved in small amounts in toluene, amyl acetate, trichloroethylene, and other solvents above 70°C.
Chemical properties
Polyethylene has good chemical stability. At room temperature, it can withstand dilute nitric acid, dilute sulfuric acid, and any concentration of hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid, ammonia, amines, hydrogen peroxide, sodium hydroxide, potassium hydroxide and other solutions. However, it is not resistant to corrosion by strong oxidizing acids, such as fuming sulfuric acid, concentrated nitric acid, and a mixture of chromic acid and sulfuric acid. At room temperature, it will slowly corrode polyethylene. At 90-100°C, concentrated sulfuric acid and concentrated nitric acid will quickly corrode polyethylene, causing it to be destroyed or decomposed. Polyethylene is easily photo-oxidized, thermally oxidized, and ozone-decomposed. It is easily degraded under the action of ultraviolet rays. Carbon black has an excellent light-shielding effect on polyethylene. After being irradiated, it can also undergo reactions such as cross-linking, chain breaking, and the formation of unsaturated groups.
Mechanical properties
The mechanical properties of polyethylene are average, with low tensile strength, poor creep resistance and good impact resistance. Impact strength LDPE > LLDPE > HDPE, and other mechanical properties LDPE < LLDPE < HDPE. It is mainly affected by density, crystallinity and relative molecular weight. As these indicators increase, its mechanical properties increase. Environmental stress cracking resistance is poor, but it improves when the relative molecular weight increases. Puncture resistance is good, among which LLDPE is the best.
Thermal properties
The heat resistance of polyethylene is not high, but it improves with the increase of relative molecular weight and crystallinity. It has good low temperature resistance, and its brittle temperature can generally reach below -50°C; and as the relative molecular weight increases, it can reach a minimum of -140°C. The linear expansion coefficient of polyethylene is large, and can reach up to (20~24)×10-5/K. The thermal conductivity is relatively high.
Electrical properties
Because polyethylene is non-polar, it has excellent electrical properties such as low dielectric loss and high dielectric strength. It can be used as frequency modulation insulation material, corona-resistant plastic, and high-voltage insulation material.
Environmental characteristics
Polyethylene is an inert alkane polymer with good chemical stability. It is resistant to corrosion by acid, alkali, and salt aqueous solutions at room temperature, but is not resistant to strong oxidants such as fuming sulfuric acid, concentrated nitric acid, and chromic acid. Polyethylene is insoluble in general solvents below 60°C, but will swell or crack if in long-term contact with aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, etc. When the temperature exceeds 70°C, it can be dissolved in toluene, amyl acetate, trichloroethylene, turpentine, mineral oil, and paraffin in small amounts.
Since polyethylene molecules contain a small amount of double bonds and ether bonds, exposure to sunlight and rain will cause aging, and antioxidants and light stabilizers need to be added to improve this.
Processing characteristics
LDPE and HDPE have good fluidity, low processing temperature, moderate viscosity, low decomposition temperature, and do not decompose at high temperatures of 300°C in an inert gas, so they are plastics with good processing performance. However, LLDPE has a slightly higher viscosity, requiring an increase in motor power by 20-30%; it is prone to melt fracture, requiring an increase in die gap and the addition of processing aids; and a slightly higher processing temperature of 200-215°C. Polyethylene has a low water absorption rate and does not require drying before processing.
Polyethylene melt is a non-Newtonian fluid. Its viscosity fluctuates slightly with temperature, but decreases rapidly with increasing shear rate in a linear relationship. Among them, LLDPE decreases the slowest.
Polyethylene products tend to crystallize during the cooling process, so the mold temperature should be paid attention to during the processing to control the crystallinity of the product and give it different properties. Polyethylene has a large molding shrinkage rate, which must be taken into consideration when designing the mold.
Modified
The modified varieties of polyethylene mainly include chlorinated polyethylene, chlorosulfonated polyethylene, cross-linked polyethylene and blended modified varieties.
Chlorinated polyethylene: A random chloride obtained by partially replacing hydrogen atoms in polyethylene with chlorine. Chlorination is carried out under the initiation of light or peroxide, and is mainly produced in industry by the aqueous suspension method. Due to the differences in the molecular weight and distribution of the raw polyethylene, the degree of branching and the degree of chlorination after chlorination, the distribution of chlorine atoms and the residual crystallinity, chlorinated polyethylene ranging from rubber to hard plastic can be obtained. Its main use is as a modifier for polyvinyl chloride to improve the impact resistance of polyvinyl chloride. Chlorinated polyethylene itself can also be used as an electrical insulating material and ground material.
Chlorosulfonated polyethylene: When polyethylene reacts with chlorine containing sulfur dioxide, some of the hydrogen atoms in the molecule are replaced by chlorine and a small amount of sulfonyl chloride groups to obtain chlorosulfonated polyethylene. The main industrial production method is the suspension method. Chlorosulfonated polyethylene has good ozone resistance, chemical corrosion resistance, oil resistance, heat resistance, light resistance, wear resistance and tensile strength. It is an elastomer with good comprehensive properties and can be used to make equipment parts that come into contact with food.
Cross-linked polyethylene: Linear polyethylene is made into a mesh or body-shaped cross-linked polyethylene by radiation (X-ray, electron beam or ultraviolet irradiation, etc.) or chemical method (peroxide or silicone cross-linking). The silicone cross-linking method has a simple process, low operating cost, and molding and cross-linking can be carried out in steps. Blow molding and injection molding are suitable. The heat resistance, environmental stress cracking resistance and mechanical properties of cross-linked polyethylene are much better than those of polyethylene, and it is suitable for large pipes, cables and wires, and rotational molding products.
Blending modification of polyethylene: After linear low-density polyethylene and low-density polyethylene are blended, they can be used to process films and other products, and the product performance is better than that of low-density polyethylene. Blending polyethylene and ethylene-propylene rubber can produce a widely used thermoplastic elastomer.
Production process
Polyethylene can be divided into high pressure process, medium pressure process and low pressure process according to the polymerization pressure.
The 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 production. However, with the development of production technology and catalysts, its growth rate has lagged far behind the low-pressure method. In terms of its implementation methods, the low-pressure method includes slurry method, solution method and gas phase method. The slurry method is mainly used to produce high-density polyethylene, while the solution method and gas phase method can not only produce high-density polyethylene, but also produce medium and low-density polyethylene by adding comonomers, also known as linear low-density polyethylene. Various low-pressure processes are developing rapidly.
High pressure method
A method of polymerizing ethylene into low-density polyethylene using oxygen or peroxide as an initiator. After two-stage compression, ethylene enters the reactor and polymerizes into polyethylene under the action of an initiator at a pressure of 100-300 MPa and a temperature of 200-300°C. The reactants are separated by reduced pressure, and the unreacted ethylene is recovered and recycled. The molten polyethylene is extruded and granulated after adding plastic additives.
The polymerization reactors used are tubular reactors (tube length can reach 2000 m) and tank reactors. The tubular process has a single-pass conversion rate of 20~34% and a single-line annual production capacity of 100 kt. The tank process has a single-pass conversion rate of 20~25% and a single-line annual production capacity of 180 kt.
Low pressure method
There are three types of polymerization methods : slurry method, solution method and 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.
① Slurry method: The polyethylene produced is insoluble in the solvent and is in the form of a slurry. The polymerization conditions of the slurry method are mild and easy to operate. Alkyl aluminum is often used as an activator and hydrogen as a molecular weight regulator. A kettle reactor is often used. The polymer slurry coming out of the polymerization kettle passes through a flash kettle, a gas-liquid separator, to a powder dryer, and then to granulation. The production process also includes steps such as solvent recovery and solvent refining. By using different combinations of polymerization kettles in series or in parallel, products with different molecular weight distributions can be obtained.
② Solution method: The polymerization is carried out in a solvent, but ethylene and polyethylene are both dissolved in the solvent, and the reaction system is a homogeneous solution. The reaction temperature (≥140°C) and pressure (4-5MPa) are relatively high. The characteristics are short polymerization time, high production intensity, and the ability to produce high, medium, and low density polyethylene, and better control of product properties; however, the polymer obtained by the solution method has a lower molecular weight, a narrow molecular weight distribution, and a lower solid content.
③ Gas phase method: Ethylene is polymerized in the gaseous state, generally using a fluidized bed reactor. There are two types of catalysts, chromium and titanium, which are quantitatively added to the bed from a storage tank. High-speed ethylene circulation is used to maintain the fluidization of the bed and remove the heat of the 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°C. The gas phase method is the most important method for producing linear low-density polyethylene. The gas phase method eliminates the steps of solvent recovery and polymer drying, and saves 15% of investment and 10% of operating costs compared to the solution method. It is 30% of the investment of the traditional high-pressure method and 1/6 of the operating cost. Therefore, it has developed rapidly. However, the gas phase method needs to be further improved in terms of product quality and variety.
Medium pressure method
High density polyethylene is produced by polymerizing ethylene at medium pressure in a loop reactor using a chromium catalyst supported on silica gel.
Processing and Application: It can be processed by blow molding, extrusion, injection molding and other methods, and is widely used in the manufacture of films, hollow products, fibers and daily sundries. In actual production, in order to improve the stability of polyethylene against ultraviolet rays and oxidation, and improve processing and performance, a small amount of plastic additives needs to be added. Commonly used ultraviolet absorbers are o-hydroxybenzophenone or its alkoxy derivatives, and carbon black is an excellent ultraviolet shielding agent. In addition, antioxidants, lubricants, colorants, etc. are added to further expand the application range of polyethylene.
Metallocene Polyethylene Technology
Metallocene polyethylene technology utilizes current polyethylene processes to produce polyethylene products with narrow molecular weight distribution using either metallocene catalysts or non metallocene catalysts. Foreign production companies include Dow, ExxonMobil, LG, and Mitsui, while Chinese companies include Qilu, Daqing, and Dushanzi. Currently, Guangzhou Petrochemical, Yangtze Petrochemical, and Maoming Petrochemical are also actively developing metallocene products.
Metallocene polyethylene products have excellent optical properties and high transparency; they have a good balance of rigidity and toughness, are conducive to thinning and resin simplification, have excellent puncture resistance and tensile strength, and have great advantages in low temperature and shrink film and tube materials.
Determination of molecular weight of polyethylene
The molecular weight and molecular weight distribution of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) are mainly measured by gel permeation chromatography (GPC). However, for ultra-high molecular weight polyethylene (UHMWPE), the conventional GPC test method has certain difficulties, such as inappropriate chromatographic columns, limited molecular weight of standard samples, and difficulty in sample dissolution, resulting in the test accuracy and repeatability failing to meet the requirements. Currently, the molecular weight of UHMWPE samples is mainly measured by the viscosity method.
Applications
High-pressure polyethylene: More than half is used in film products, followed by pipes, injection-molded products, wire wrapping, etc.
Medium and low pressure polyethylene: mainly injection molding products and hollow products.
Ultra-high pressure polyethylene: Due to its excellent comprehensive properties, ultra-high molecular weight polyethylene can be used as an engineering plastic.
