Overview
Polyethylene (PE) is a versatile thermoplastic resin made by polymerizing ethylene, often with small amounts of alpha-olefins. It is odorless, non-toxic, wax-like, and excels in low-temperature resistance (-100 to -70°C), chemical stability, and electrical insulation. Its low water absorption and resistance to most acids and alkalis make it highly durable. The British ICI Company first synthesized polyethylene in 1922, and in 1933, the British Bonemen Chemical Industry Company developed a high-pressure polymerization method that led to mass production by 1939.
Production techniques continued to evolve in the mid-20th century. In 1953, K. Ziegler in Germany pioneered a low-pressure method using a TiCl4-Al(C2H5)3 catalyst, which moved to industrial production in 1955. The 1960s saw U.S. companies advancing both high-density and low-density polyethylene production through the solution method and low-pressure processes, combining efficiency with improved material properties. Catalyst innovation has been central, transitioning from Ziegler’s first-generation catalyst to more advanced systems that significantly boosted efficiency and product quality.
Polyethylene finds extensive use across many industries due to its adaptability and ease of processing with standard thermoplastic methods. It serves in the production of films, packaging, containers, pipes, cables, and insulation materials. While sensitive to environmental stress and moderately resistant to thermal aging, polyethylene’s properties make it suitable for a wide range of industrial applications. As the petrochemical industry has grown, polyethylene output has soared, now making up about a quarter of global plastic production. Between 1983 and 2011, production capacity expanded from 24.65 Mt to 96 Mt, with Asia, especially China, emerging as a dominant consumer market.
Classification
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.
LDPE
Properties: Polyethylene is tasteless, odorless, and non-toxic, with a dull, milky white, waxy appearance. It has a density of about 0.920 g/cm³ and melts between 130℃ and 145℃. LDPE does not dissolve in water and only slightly dissolves in hydrocarbons. It resists most acids and alkalis, absorbs minimal water, remains flexible at low temperatures, and provides excellent electrical insulation.
Production Process: The primary methods are the high-pressure tube and kettle techniques. In the tubular process, manufacturers use a low-temperature, high-activity initiator to lower reaction temperatures and pressures, with high-purity ethylene as the main ingredient. Polymerization occurs at 330°C and 150-300 MPa. The molten polymer cools and separates at various pressures before processing. High-pressure and low-pressure separation systems manage circulating gas recycling and transport molten polyethylene to the granulator. During granulation, producers can add specific additives for different applications, then package the granules for distribution.
Uses: Polyethylene adapts well to injection molding, extrusion, and blow molding. It finds extensive use in agricultural films, industrial and food packaging, mechanical parts, building materials, wire and cable insulation, coatings, and synthetic paper.
LLDPE
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.
HDPE
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.
Performance
General characteristics
Polyethylene resin is a non-toxic, odorless white powder or granule with a milky white, wax-like appearance. It absorbs minimal water, less than 0.01%. Polyethylene film is transparent, but transparency decreases as crystallinity increases. The film has low water permeability but allows high air permeability, making it ideal for moisture-proof packaging rather than fresh-keeping. It is flammable, with an oxygen index of 17.4, producing low smoke and a yellow-to-blue flame with a paraffin odor when burned. It offers good water resistance, though its non-polar surface makes bonding and printing difficult, often requiring surface treatment. Branched chain variations show lower resistance to photodegradation and oxidation.
The molecular weight of polyethylene ranges from 10,000 to 100,000, with ultra-high molecular weight polyethylene exceeding 100,000. Higher molecular weight improves physical and mechanical properties, bringing them closer to engineering standards, though processing becomes more challenging. It melts between 100°C and 130°C and maintains excellent low-temperature resistance, keeping its mechanical strength down to -60°C, with an optimal operating range of 80°C to 110°C.
Polyethylene is insoluble in typical solvents at room temperature but can dissolve slightly in solvents like toluene, amyl acetate, and trichloroethylene 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
LDPE and HDPE have excellent processing performance due to their good fluidity, low processing temperatures, moderate viscosity, and low decomposition temperatures. They can withstand high temperatures of 300°C in inert gas without decomposing. However, LLDPE has a slightly higher viscosity, requiring a 20% to 30% increase in motor power. It is prone to melt fracture, so increasing the die gap and adding processing aids is necessary. The processing temperature for LLDPE is also higher, ranging from 200 to 215°C.
Polyethylene has low water absorption, so it doesn’t need drying before processing. As a non-Newtonian fluid, its viscosity is less sensitive to temperature but decreases rapidly with increased shear rate. LLDPE’s viscosity decreases the slowest. During cooling, polyethylene easily crystallizes, making mold temperature control crucial to adjust crystallinity and influence product properties. Additionally, polyethylene has significant molding shrinkage, which requires careful consideration in mold design.
Modification
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 production relies on three main methods based on polymerization pressure: high-pressure, medium-pressure, and low-pressure.
The high-pressure method primarily produces low-density polyethylene (LDPE). This method, one of the earliest developed, still accounts for about two-thirds of global polyethylene output. However, advances in production technology and catalysts have slowed its growth compared to the low-pressure techniques.
In the low-pressure method, three processes are commonly used: slurry, solution, and gas phase. The slurry method specializes in producing high-density polyethylene (HDPE). In contrast, both the solution and gas phase methods can produce a range of polyethylene types, including high, medium, and low-density polyethylene. Adding comonomers enables the creation of linear low-density polyethylene (LLDPE). These low-pressure processes have evolved rapidly due to their efficiency and versatility.
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 remains insoluble in the solvent, forming a slurry. Slurry polymerization conditions are mild and straightforward to manage. Manufacturers often use alkyl aluminum as an activator and hydrogen as a molecular weight regulator, typically relying on a tank reactor. The polymer slurry moves from the polymerization tank through a flash tank, a gas-liquid separator, and into a powder dryer before granulation. The production process also involves solvent recovery and refining. To achieve varying molecular weight distributions, they can combine different polymerization tanks in series or parallel.
②Solution method:
Polymerization occurs in a solvent, where both ethylene and polyethylene dissolve, creating a homogeneous solution. This reaction takes place at high temperatures (≥140℃) and pressures (4-5 MPa). The process has a short polymerization time and high production efficiency, allowing for the production of polyethylene with high, medium, and low densities. It also provides better control over the product’s properties. However, polyethylene produced by this method tends to have a lower molecular weight, a narrow molecular weight distribution, and a lower content of solid material.
③Gas phase method:
In the gas phase method, ethylene undergoes polymerization in a gaseous state, typically using a fluidized bed reactor. The process relies on two types of catalysts—chromium and titanium series—added from storage tanks to the reactor bed. High-speed ethylene circulation keeps the bed fluidized and dissipates the heat generated during polymerization. Polyethylene forms and exits from the bottom of the reactor, operating under conditions of approximately 2 MPa pressure and 85-100°C temperature.
This method is the leading technique for producing linear low-density polyethylene (LLDPE). It streamlines production by eliminating the need for solvent recovery and polymer drying, cutting 15% of investment costs and 10% of operating costs compared to the solution method. The gas phase approach also requires just 30% of the investment and 1/6 of the operational fees of the traditional high-pressure method. These advantages have driven rapid adoption, though improvements are still needed to enhance product quality and variety.
Medium pressure method
Using a chromium-based catalyst on silica gel, the polymerization of ethylene occurs in a loop reactor under medium pressure, producing high-density polyethylene (HDPE).
Processing and Application: Manufacturers process HDPE through blow molding, extrusion, injection molding, and other techniques, making it ideal for films, hollow containers, fibers, and everyday items. To boost stability against UV rays and oxidation, they add small amounts of plastic additives during production. Common UV absorbers include o-hydroxybenzophenone and its alkoxy derivatives, while carbon black effectively shields against UV rays. They also add antioxidants, lubricants, and colorants to expand HDPE’s application range and enhance its overall performance.
Application
Manufacturers use more than half of high-pressure polyethylene for film products, with additional applications in pipes, injection-molded products, and wire wrapping.
Medium and low-pressure polyethylene primarily serves in the creation of injection-molded and hollow products.
Ultra-high molecular polyethylene, thanks to its excellent properties, finds use in engineering plastics.