What is polyethylene?

Polyethylene (PE) is a thermoplastic resin obtained by polymerizing ethylene. In industry, copolymers of ethylene with small amounts of alpha-olefins are also included. Polyethylene is odorless, non-toxic, feels like wax, has excellent low temperature resistance (minimum operating temperature can reach -100~-70°C), good chemical stability, and can resist most acid and alkali erosion (not resistant to oxidation nature acid). It is insoluble in common solvents at room temperature, with low water absorption and excellent electrical insulation.

Polyethylene was synthesized by the British ICI Company in 1922. In 1933, the British Bonemen Chemical Industry Company found that ethylene could be polymerized under high pressure to form 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 found that with TiCl4-Al(C2H5)3 as a catalyst, ethylene could also be polymerized under lower pressure. This method was put into industrial production in 1955 by the Hearst Company of the Federal Republic of Germany, and is commonly known as low pressure polyethylene. In the early 1950s, the Philips Petroleum Company of the United States discovered that using chromium oxide-silica alumina as a catalyst, ethylene could be polymerized to form high-density polyethylene under medium pressure, and industrial production was realized in 1957. In the 1960s, the Canadian DuPont Company began to make low-density polyethylene with ethylene and α-olefin by solution method. In 1977, Union Carbide Company and Dow Chemical Company of the United States successively used low-pressure method to make low-density polyethylene, called linear low-density polyethylene, of which the gas-phase method of Union Carbide Company was the most important. The performance of linear low density polyethylene is similar to that of low density polyethylene, and it has some characteristics of high density polyethylene. In addition, the energy consumption in production is low, so it has developed extremely 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. The TiCl4-Al(C2H5)3 system invented by Ziegler in Germany is the first-generation catalyst for polyolefins. In 1963, the Belgian Solvay Company 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 save the post-treatment process for removing catalyst residues. Later, high-efficiency catalysts for the gas phase method were developed. In 1975, the Italian Monte Edison Group Corporation developed a catalyst that can directly produce spherical polyethylene without granulation. It is called the third-generation catalyst, which is another revolution in the production of high-density polyethylene.

Polyethylene is very sensitive to environmental stress (chemical and mechanical action) and is less resistant to thermal aging than the chemical structure and processing of polymers. Polyethylene can be processed by conventional thermoplastic molding methods. It has a wide range of uses, mainly used to manufacture films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulating materials for TVs, radars, etc. With the development of petrochemical industry, polyethylene production has developed rapidly, and the output accounts for about 1/4 of the total plastic output. In 1983, the world’s total polyethylene production capacity was 24.65 Mt, and the capacity of the units under construction was 3.16 Mt. According to the latest statistics in 2011, the global production capacity reached 96 Mt. The development trend of polyethylene production shows that production and consumption are gradually shifting to Asia, and China is increasingly becoming the most important consumer market.

Polyethylene is divided into high density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) according to the polymerization method, molecular weight and chain structure.

Properties: tasteless, odorless, non-toxic, dull surface, milky white waxy particles, density about 0.920 g/cm3, melting point 130℃~145℃. Insoluble in water, slightly soluble in hydrocarbons, etc. It can withstand the erosion of most acids and alkalis, has low water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.
Production process: There are mainly two kinds of 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 high-activity initiator to initiate the polymerization system, with high-purity ethylene as the main raw material, propylene, propane, etc. The polymerization was carried out under the conditions of 330°C and 150-300MPa. The molten polymer that initiates polymerization in the reactor must be cooled and separated at high pressure, medium pressure and low pressure. After separation, it is sent to the inlet of the high-pressure (30 MPa) compressor, while the low-pressure circulating gas is cooled and separated and sent to the low-pressure (0.5 MPa) compressor for recycling, while the molten polyethylene is sent to the granulator after high-pressure and low-pressure separation. For granulation in water, during granulation, enterprises can add appropriate additives according to different application fields, and the granules are packaged and shipped out.
Uses: Injection molding, extrusion molding, blow molding and other processing methods can be used. Mainly used as agricultural film, industrial packaging film, pharmaceutical and food packaging film, mechanical parts, daily necessities, building materials, wire, cable insulation, coating and synthetic paper, etc.

Properties: Because the molecular structures of LLDPE and LDPE are obviously different, the properties are also different. Compared with LDPE, LLDPE has excellent environmental stress crack resistance and electrical insulation, higher heat resistance, impact resistance and puncture resistance. Production process: LLDPE resin is mainly produced by full density polyethylene equipment, and the representative production process is Innovene process and UCC’s Unipol process.
Uses: Produce films, daily necessities, pipes, wires and cables, etc. by means of injection molding, extrusion, blow molding and other molding methods.

Properties: natural color, cylindrical or oblate particles, smooth particles, particle size should be 2 mm ~ 5 mm in any direction, no mechanical impurities, thermoplastic. The powder is white powder, and the qualified product is allowed to have a slight yellow color. It is insoluble in common solvents at room temperature, but can swell in aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons when exposed to it for a long time, and is slightly soluble in toluene and acetic acid at temperatures above 70°C. Oxidation occurs when heated in air and under the influence of sunlight. Resistant to most acid and alkali erosion. It has low water absorption, can still maintain softness at low temperature, and has high electrical insulation.
Production process: two production processes of gas phase method and slurry method are adopted.
Uses: Use injection molding, blow molding, extrusion molding, rotomolding and other molding methods to produce film products, daily necessities and various sizes of hollow containers, pipes, packaging, calendering belts and tie belts, ropes, fishing nets and weaving. Fiber, wire and cable, etc.

General characteristics
Polyethylene resin is a non-toxic, odorless white powder or granule, milky white in appearance, with a wax-like feel, and low water absorption, less than 0.01%. The polyethylene film is transparent and decreases with increasing crystallinity. The polyethylene film has low water permeability but high air permeability, which is not suitable for fresh-keeping packaging but suitable for moisture-proof packaging. Flammable, with an oxygen index of 17.4, low smoke when burning, with a small amount of molten droplets, the flame is yellow on top and blue on the bottom, with paraffin odor. Polyethylene has better water resistance. The surface of the product is non-polar, difficult to bond and print, and has been improved by surface treatment. More branched chains have poor resistance to photodegradation and oxidation.
Its molecular weight is in the range of 10,000 to 100,000. If the molecular weight exceeds 100,000, it is ultra-high molecular weight polyethylene. The higher the molecular weight, the better its physical and mechanical properties, and the closer to the required level of engineering materials. But the higher the molecular weight, the more difficult it is to process. Polyethylene has a melting point of 100-130°C and has excellent low temperature resistance. It can still maintain good mechanical properties at -60 °C, but the operating temperature is 80~110 °C.
It is insoluble in any known solvent at room temperature, and can be dissolved in a small amount in toluene, amyl acetate, trichloroethylene and other solvents above 70°C.

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, acetic acid, ammonia water, amines, hydrogen peroxide, sodium hydroxide, potassium hydroxide, etc. solution. But it is not resistant to strong oxidative corrosion, such as fuming sulfuric acid, concentrated nitric acid, chromic acid and sulfuric acid mixture. At room temperature, the above-mentioned solvents will slowly erode polyethylene, while at 90-100°C, concentrated sulfuric acid and concentrated nitric acid will rapidly erode polyethylene, causing it to be destroyed or decomposed. Polyethylene is easy to be photo-oxidized, thermally oxidized, decomposed by ozone, and easily degraded under the action of ultraviolet rays. Carbon black has excellent light shielding effect on polyethylene. Reactions such as cross-linking, chain scission, and formation of unsaturated groups can occur after irradiation.

Mechanical properties
The mechanical properties of polyethylene are general, the tensile strength is low, the creep resistance is not good, and the impact resistance is good. Impact strength LDPE>LLDPE>HDPE, other mechanical properties LDPE crystallinity and relative molecular weight, with the improvement of these indicators, its mechanical properties increase. Environmental stress cracking resistance is not good, but when the relative molecular weight increases, it improves. Good puncture resistance, among which LLDPE is the best.

Thermal properties
The heat resistance of polyethylene is not high, and it improves with the increase of relative molecular weight and crystallinity. Good low temperature resistance, the brittle temperature can generally reach below -50 ℃; and with the increase of relative molecular mass, the lowest can reach -140 ℃. The linear expansion coefficient of polyethylene is large, up to (20~24)×10-5/K. High thermal conductivity.

Electrical characteristics
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 insulating material, corona-resistant plastic, and high-voltage insulating material.

Environmental characteristics
Polyethylene is an alkane inert polymer with good chemical stability. It is resistant to corrosion by acid, alkali and salt aqueous solutions at room temperature, but not resistant to strong oxidants such as oleum, concentrated nitric acid and chromic acid. Polyethylene is insoluble in common solvents below 60°C, but will swell or crack in long-term contact with aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, etc. When the temperature exceeds 60℃, it can be dissolved in a small amount in toluene, amyl acetate, trichloroethylene, turpentine, mineral oil and paraffin; when the temperature is higher than 100℃, it can be dissolved in tetralin.
Since polyethylene molecules contain a small amount of double bonds and ether bonds, sun exposure and rain will cause aging, which needs to be improved by adding antioxidants and light stabilizers.

Processing characteristics
Because 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, 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% to 30%; it is prone to melt fracture, so it is necessary to increase the die gap and add processing aids; the processing temperature is slightly higher, up to 200 to 215 °C. Polyethylene has low water absorption and does not require drying before processing.
Polyethylene melt is a non-Newtonian fluid, and its viscosity fluctuates less with temperature, but decreases rapidly with the increase of shear rate and has a linear relationship, among which LLDPE has the slowest decrease.
Polyethylene products are easy to crystallize during the cooling process, therefore, attention should be paid to the mold temperature during processing. In order to control the crystallinity of the product, so that it has different properties. Polyethylene has a large molding shrinkage, which must be considered when designing the mold.

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 by aqueous suspension method in industry. Due to the difference in molecular weight and distribution, branching degree, chlorination degree after chlorination, chlorine atom distribution and residual crystallinity of raw polyethylene, chlorinated polyethylene from rubbery to rigid plastic can be obtained. The main use is as a modifier of polyvinyl chloride to improve the impact resistance of polyvinyl chloride. Chlorinated polyethylene itself can also be used as electrical insulating material and ground material.
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 the suspension method. Chlorosulfonated polyethylene is resistant to ozone, chemical corrosion, oil, heat, light, abrasion and tensile strength. It is an elastomer with good comprehensive properties and can be used to make equipment parts that contact food.
Cross-linked polyethylene: Use radiation (X-ray, electron beam or ultraviolet radiation, etc.) or chemical method (peroxide or silicone cross-linking) to make linear polyethylene into a network or body-shaped cross-linked polyethylene. Among them, the silicone cross-linking method has a simple process, low operating costs, and the molding and cross-linking can be carried out in steps, so blow molding and injection molding are suitable. The heat resistance, environmental stress cracking resistance and mechanical properties of cross-linked polyethylene are greatly improved compared with polyethylene, and it is suitable for large pipes, cables and wires, and rotomolding products.
Blending modification of polyethylene: After blending linear low density polyethylene and low density polyethylene, it 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 thermoplastic elastomers with a wide range of applications.

Production Process
Polyethylene can be divided into high pressure method, medium pressure method and low pressure method according to the polymerization pressure.
The high pressure method is used to produce low density polyethylene. This method was developed early. The polyethylene produced by this method accounts for about 2/3 of the total output of polyethylene, but with the development of production technology and catalysts, its growth rate has been significantly behind the low pressure method. As far as the low pressure method is concerned, there are 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. vinyl. 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. Ethylene enters the reactor after secondary compression, and is polymerized into polyethylene under the pressure of 100-300 MPa, temperature of 200-300 °C and the action of an initiator. The polyethylene in the form of plastic is extruded and pelletized after adding plastic additives.
The polymerization reactors used are tubular reactors (with a tube length of up to 2000 m) and tank reactors. The single-pass conversion rate of the tubular process is 20% to 34%, and the annual production capacity of a single line is 100 kt. The single-pass conversion rate of the kettle method process is 20% to 25%, and the single-line annual production capacity is 180 kt.

Low pressure method
There are three types of 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.
①Slurry method: The resulting polyethylene is insoluble in the solvent and is in the form of a slurry. Slurry polymerization conditions are mild and easy to operate. Alkyl aluminum is often used as an activator, and hydrogen is used as a molecular weight regulator, and a tank reactor is often used. The polymer slurry from the polymerization tank is passed through the flash tank, the gas-liquid separator to the powder dryer, and then granulated. The production process also includes steps such as solvent recovery and solvent refining. Different polymerization kettles can be combined in series or in parallel to obtain products with different molecular weight distributions.
②Solution method: The polymerization is carried out in a solvent, but both ethylene and polyethylene are dissolved in the solvent, and the reaction system is a homogeneous solution. The reaction temperature (≥140℃) and pressure (4~5MPa) are high. It is characterized by short polymerization time, high production intensity, and can produce polyethylene with high, medium and low densities, and can better control the properties of the product; however, the polymer obtained by the solution method has low molecular weight, narrow molecular weight distribution, and solid material. The content is lower.
③Gas phase method: ethylene is polymerized in gaseous state, generally using a fluidized bed reactor. There are two kinds of catalysts: chromium series and titanium series, which are quantitatively added to the bed from the storage tank, and the high-speed ethylene circulation is used to maintain the fluidization of the bed and eliminate the heat of polymerization. The resulting 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 the production of linear low-density polyethylene. The gas phase method eliminates the process of solvent recovery and polymer drying, and saves 15% of investment and 10% of operating cost compared with the solution method. It is 30% of the investment of the traditional high pressure method and 1/6 of the operating fee. So it has developed rapidly. However, the gas phase method needs to be further improved in terms of product quality and variety.

Medium pressure method
Using a chromium-based catalyst supported on silica gel, in a loop reactor, ethylene is polymerized under medium pressure to produce high-density polyethylene.
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 to ultraviolet rays and oxidation, improve processing and performance, a small amount of plastic additives need to be added. 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 range of polyethylene.

High pressure polyethylene: more than half is used for 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 the excellent comprehensive properties of ultra-high molecular polyethylene, it can be used as engineering plastics.