Polyethylene, also called polythene (PE), is the most widely made plastic. It is a type of polymer, mainly used for packaging, such as plastic bags, films, containers, bottles, cups, jars, and folders. As of 2017, more than 100 million tons of polyethylene resins are produced each year, making up 34% of all plastic production.

There are many types of polyethylene, most with the chemical formula (C₂H₄)ₙ. Polyethylene is usually a mix of similar polymers made from ethylene, with different values of "n." It can be low-density or high-density, with many other variations. Its properties can be changed through processes like crosslinking or copolymerization. All forms of polyethylene are non-toxic and resistant to chemicals, which makes it useful for many purposes. However, its chemical resistance also means it does not break down easily, causing long-term pollution if not disposed of properly. As a hydrocarbon, polyethylene is clear to not transparent (without added colors) and can catch fire.

History

In 1898, a German chemist named Hans von Pechmann accidentally created polyethylene while studying a chemical called diazomethane. His colleagues, Eugen Bamberger and Friedrich Tschirner, examined the white, waxy substance he made. They found it had long chains of −CH₂− and named it polymethylene.

In 1933, scientists Eric Fawcett and Reginald Gibson at Imperial Chemical Industries (ICI) in England accidentally made polyethylene again. They applied extremely high pressure to a mixture of ethylene and benzaldehyde and produced a white, waxy material. The reaction was caused by small amounts of oxygen in their equipment, making it hard to repeat at first. In 1935, another ICI chemist, Michael Perrin, developed a reliable method to make polyethylene under high pressure. This process became the foundation for industrial production of low-density polyethylene (LDPE) starting in 1939. During World War II, polyethylene was used to make insulation for radio cables because it worked well with high-frequency waves. Production in Britain stopped during the war, and the process remained secret until after the conflict ended.

In 1944, companies like DuPont and Union Carbide began large-scale production of polyethylene using technology licensed from ICI. A major advancement came with the discovery of catalysts that allowed polyethylene to be made at lower temperatures and pressures. In 1951, scientists Robert Banks and J. Paul Hogan at Phillips Petroleum developed a catalyst based on chromium trioxide. In 1953, Karl Ziegler in Germany created a system using titanium halides and aluminum compounds that worked even more gently. The Phillips catalyst was cheaper and easier to use, so both methods were widely used in industry. By the late 1950s, these catalysts were used to make high-density polyethylene (HDPE). In the 1970s, the Ziegler system was improved by adding magnesium chloride. In 1976, Walter Kaminsky and Hansjörg Sinn discovered catalysts called metallocenes. These catalysts, along with Ziegler-based ones, allowed polyethylene to be combined with other chemicals, leading to many types of polyethylene today, such as very-low-density and linear low-density polyethylene. Some of these materials, like ultra-high-molecular-weight polyethylene fibers, began replacing other strong materials in applications by 2005.

Properties

The properties of polyethylene depend on its type. Factors such as molecular weight, how the molecules are connected, and the presence of other chemical groups strongly influence its characteristics. A lot of research has been done to develop different kinds of polyethylene. Low-density polyethylene (LDPE) is softer and more transparent than high-density polyethylene (HDPE). Medium- and high-density polyethylene usually melt at temperatures between 120 to 130 °C (248 to 266 °F). Low-density polyethylene typically melts at 105 to 115 °C (221 to 239 °F). These melting temperatures vary depending on the type of polyethylene, but the highest possible melting temperature is reported to be 144 to 146 °C (291 to 295 °F). Polyethylene usually burns at temperatures above 349 °C (660 °F).

Most grades of LDPE, MDPE, and HDPE resist strong acids, strong bases, and mild oxidizing or reducing agents. Crystalline samples of polyethylene do not dissolve at room temperature. Polyethylene, except for cross-linked types, can be dissolved in certain solvents at high temperatures, such as aromatic hydrocarbons like toluene or xylene, or chlorinated solvents like trichloroethane or trichlorobenzene.

Polyethylene absorbs very little water. It has low permeability to water vapor and polar gases compared to most plastics. However, non-polar gases like oxygen, carbon dioxide, and flavorings can pass through it easily.

When burned, polyethylene produces a slow, blue flame with a yellow tip and releases a smell similar to paraffin (like a candle flame). It continues burning after the flame is removed and produces drips.

Polyethylene cannot be printed or bonded with adhesives without special preparation. Strong connections can be made using plastic welding.

Polyethylene is a good electrical insulator. It resists electrical breakdown, but it can become easily charged with static electricity. This can be reduced by adding materials like graphite, carbon black, or antistatic agents. When pure, its dielectric constant ranges from 2.2 to 2.4, depending on density, and its loss tangent is very low, making it suitable for use in capacitors. For the same reason, it is often used as insulation in high-frequency coaxial and twisted pair cables.

The transparency of polyethylene depends on its thermal history and film thickness. It can range from almost clear (transparent) to milky-opaque (translucent) or completely opaque. LDPE has the highest transparency, LLDPE has slightly less, and HDPE has the least. Transparency decreases when crystalline structures are larger than the wavelength of visible light.

Manufacturing process

Ethylene, also known as ethene (IUPAC name), is a gas made of carbon and hydrogen with the chemical formula C₂H₄. It can be thought of as two methylene groups (−CH₂−) connected together. Polyethylene must be very pure. It should have less than 5 parts per million of water, oxygen, and other alkenes. Acceptable impurities include nitrogen (N₂), ethane (a common material used to make ethylene), and methane. Ethylene is usually made from petroleum sources but can also be created by removing water from ethanol.

When ethylene is turned into polyethylene, the process follows this chemical equation:
Although ethylene is stable, it easily combines into long chains when exposed to catalysts. The most common method is coordination polymerization. Common catalysts include modified titanium(III) chloride, known as Ziegler–Natta catalysts. Another catalyst is the Phillips catalyst, which is made by placing chromium(VI) oxide on silica. Polyethylene can also be made through radical polymerization, but this method is rarely used and requires high-pressure equipment.

Joining

Common methods for joining polyethylene parts include:

• Welding: Hot gas welding, infrared welding, laser welding, ultrasonic welding, heat sealing, and heat fusion
• Fastening
• Adhesives: Pressure-sensitive adhesives (PSAs), solvent-type PSAs, polyurethane contact adhesives, two-part polyurethane, epoxy adhesives, and hot-melt adhesives

Solvent bonding is rarely used because polyethylene is nonpolar and has high resistance to solvents.

Pressure-sensitive adhesives (PSAs) work well if the surface of the polyethylene is treated with plasma activation, flame treatment, or corona treatment.

Classification

Polyethylene is grouped based on its density and branching. Its physical characteristics depend on factors like the amount and type of branching, crystal structure, and molecular weight. There are several types of polyethylene:

• Ultra-high-molecular-weight polyethylene (UHMWPE)
• Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX)
• High-molecular-weight polyethylene (HMWPE)
• High-density polyethylene (HDPE)
• High-density cross-linked polyethylene (HDXLPE)
• Cross-linked polyethylene (PEX or XLPE)
• Medium-density polyethylene (MDPE)
• Linear low-density polyethylene (LLDPE)
• Low-density polyethylene (LDPE)
• Very-low-density polyethylene (VLDPE)
• Chlorinated polyethylene (CPE)

In terms of production volume, the most widely used types are HDPE, LLDPE, and LDPE.

UHMWPE has a molecular weight in the millions, typically between 3.5 and 7.5 million amu. Its high molecular weight makes it very strong, but the chains do not pack tightly into crystals, resulting in a lower density than HDPE (e.g., 0.930–0.935 g/cm³). UHMWPE can be made using various catalysts, with Ziegler catalysts being most common. It is used in applications such as machine parts for handling cans and bottles, bearings, gears, artificial joints, ice rink edge protection, ship cable replacements, and chopping boards. It is also used in hip and knee implant parts. As a fiber, it is used in bulletproof vests.

HDPE has a density of 0.941 g/cm³ or higher. Its mostly linear molecules pack tightly, creating stronger intermolecular forces than highly branched polymers. HDPE can be made using chromium/silica catalysts, Ziegler–Natta catalysts, or metallocene catalysts. The degree of branching is controlled during production. HDPE has high tensile strength and is used in products like milk jugs, detergent bottles, butter tubs, garbage containers, and water pipes.

PEX is a medium- to high-density polyethylene with cross-link bonds added to its structure. This changes the material from a thermoplastic to a thermoset, improving its high-temperature performance, reducing flow, and increasing chemical resistance. PEX is used in some water plumbing systems because it can expand to fit over metal parts and then return to its original shape, creating a secure connection.

MDPE has a density range of 0.926–0.940 g/cm³. It can be made using chromium/silica catalysts, Ziegler–Natta catalysts, or metallocene catalysts. MDPE has good shock and drop resistance and is less likely to crack under stress than HDPE. It is used in gas pipes, sacks, shrink film, packaging film, carrier bags, and screw closures.

LLDPE has a density range of 0.915–0.925 g/cm³. It is a mostly linear polymer with many short branches, made by combining ethylene with short-chain alpha-olefins like 1-butene, 1-hexene, and 1-octene. LLDPE has higher tensile strength and impact resistance than LDPE. Thinner films can be made compared to LDPE, but they are harder to process. LLDPE is used in packaging films, cable coverings, toys, lids, buckets, containers, and pipes. It is most commonly used in film applications due to its toughness and flexibility.

LDPE has a density range of 0.910–0.940 g/cm³. It has many short- and long-chain branches, which prevent its chains from packing tightly into crystals. This results in weaker intermolecular forces and lower tensile strength but greater flexibility. LDPE is made using free-radical polymerization, which gives it unique flow properties. It is used in rigid containers and plastic films like plastic bags and wrap.

LDPE is made using a process that does not use a catalyst to control radical sites on polymer chains. In HDPE production, radicals form at the ends of chains because catalysts stabilize them. Secondary radicals (in the middle of a chain) and tertiary radicals (at branch points) are more stable than primary radicals (at chain ends). Adding ethylene monomers creates primary radicals, which often rearrange to form more stable secondary or tertiary radicals, leading to branching.

VLDPE has a density range of 0.880–0.915 g/cm³. It is a mostly linear polymer with many short-chain branches, made by combining ethylene with short-chain alpha-olefins. It is most commonly produced using metallocene catalysts, which allow better incorporation of co-monomers. VLDPE is used in hoses, tubing, ice and frozen food bags, food packaging, stretch wrap, and as an impact modifier when blended with other polymers.

Research has focused on the number and distribution of long-chain branches in polyethylene. In HDPE, even a small number of these branches (e.g., one in 100 or 1,000 per backbone carbon) can significantly affect the polymer’s flow properties.

The physical properties of polyethylene depend on its molecular structure. Molecular weight and crystallinity are the most important factors. Crystallinity depends on molecular weight and branching. The less branched the chains and the lower the molecular weight, the higher the crystallinity. Crystallinity ranges from 35% (LDPE/LLDPE) to 80% (HDPE). Polyethylene has a density of 1.0 g/cm³ in crystalline regions and 0.86 g/cm³ in amorphous regions. Density and crystallinity are closely related.

The branching in different types of polyethylene can be represented as follows:

The figure shows polyethylene backbones, short-chain branches, and side-chain branches. The polymer chains are shown as straight lines.

The properties of polyethylene depend on the type and number of chain branches. The branching depends on the production process: high-pressure (only LDPE) or low-pressure (all other types). LDPE is made using high-pressure radical polymerization, which creates many short- and long-chain branches. Short-chain branches form through intramolecular chain transfer reactions, which always produce butyl or ethyl branches due to the reaction mechanism.

Copolymers

In the low pressure process, α-olefins (such as 1-butene or 1-hexene) can be added to the polymer chain. These copolymers create short side chains, which lower the polymer’s crystallinity and density. This changes the material’s mechanical and thermal properties. For example, PE-LLD is made this way. Metallocene polyethylene (PE-M) is produced using metallocene catalysts and often includes 1-hexene as a copolymer. PE-M has a narrow molecular weight distribution, high toughness, excellent optical properties, and consistent comonomer content. Because of its narrow molecular weight range, PE-M flows less easily under strong shear forces. It has few low molecular weight components and low welding and sealing temperatures, making it ideal for food packaging.

Cyclic olefin copolymers are made by combining ethene with cycloolefins (like norbornene) using metallocene catalysts. These polymers are amorphous, highly transparent, and heat resistant.

Polyethylene with a multimodal molecular weight distribution contains several polymer fractions mixed together. These materials have very high stiffness, toughness, strength, stress crack resistance, and crack propagation resistance. They include equal parts of high and low molecular weight polymer fractions. The low molecular weight parts crystallize quickly and relax faster, while the high molecular weight parts connect crystallites, improving toughness and stress crack resistance. This type of polyethylene can be made using two-stage reactors, catalysts with two active centers on a carrier, or by blending in extruders.

Ethylene/vinyl alcohol copolymer (EVOH) is formally a copolymer of polyethylene and vinyl alcohol. It is made by partially hydrolyzing ethylene-vinyl acetate copolymer. EVOH typically contains more comonomer than usual. It is used as a barrier layer in multilayer films for packaging. Because EVOH absorbs water from the environment, it loses its barrier properties. To prevent this, it is placed as a core layer surrounded by other plastics like LDPE, PP, PA, or PET. EVOH is also used as an anticorrosion coating on street lights, traffic light poles, and noise barriers.

Copolymers of ethylene and unsaturated carboxylic acids (like acrylic acid) stick well to many materials. They resist stress cracking and remain flexible. When ethylene-acrylic acid copolymers lose protons, they form salts called ionomers. These materials bond well to metals, resist wear, and absorb water easily.

Ethylene-vinyl acetate copolymers are made similarly to LD-PE using high pressure polymerization. The amount of comonomer in the polymer greatly affects its properties. When the comonomer content is very high (about 50%), the material becomes a rubbery thermoplastic (a thermoplastic elastomer). Ethylene-ethyl acrylate copolymers behave similarly to ethylene-vinyl acetate copolymers.

Reactions of polyethylene

A basic difference exists between four types of crosslinking methods used for polyethylene: peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc), and azo crosslinking (PE-Xd).

Each method involves creating a special type of molecule, called a radical, in the polyethylene chain. This can happen through radiation (like light or energy from an electron beam) or through peroxides (chemicals like dicumyl or di-tert-butyl peroxide). Once radicals are formed, they can directly link together to create a crosslinked network or indirectly link through silane compounds.

Peroxide crosslinking (PE-Xa): This method uses peroxides to link polyethylene molecules. In the Engel process, a mixture of high-density polyethylene (HDPE) and 2% peroxide is heated in an extruder to high temperatures (200–250 °C). The peroxide breaks down into radicals, which remove hydrogen atoms from the polymer chain. These radicals then combine to form a crosslinked network. The resulting material is uniform, has low tension, and is more flexible and tougher than irradiated PE-Xc.

Silane crosslinking (PE-Xb): Silane compounds (like trimethoxyvinylsilane) help link polyethylene. First, the polyethylene is treated with silane using radiation or a small amount of peroxide. Later, in a water bath, silane groups form Si-OH molecules through a process called hydrolysis. These molecules then join together to form Si-O-Si bridges, linking the polyethylene. Catalysts like dibutyltin dilaurate can speed up this process.

Electron beam crosslinking (PE-Xc): Polyethylene can also be crosslinked using radiation from an electron accelerator or isotopic source. This process happens below the melting point of the polymer by removing hydrogen atoms. Radiation can penetrate up to 10 mm (β-radiation) or 100 mm (γ-radiation), allowing specific areas to avoid crosslinking. However, this method is less common than peroxide crosslinking due to high costs. Unlike peroxide crosslinking, this process occurs in solid polyethylene, linking molecules mainly in the amorphous regions while keeping the crystalline structure intact.

Azo crosslinking (PE-Xd): In the Lubonyl process, azo compounds are added to polyethylene after extrusion. The material is then heated in a salt bath to create crosslinks.

Chlorinated Polyethylene (PE-C): This material contains 34–44% chlorine and is used in blends with PVC. It improves impact resistance and weather resistance in PVC. It also softens PVC foils without causing plasticizers to move. PE-C can be crosslinked with peroxides to make elastomers used in cables and rubber. Adding PE-C to other polyolefins reduces flammability.

Chlorosulfonated Polyethylene (CSM): This material is used to make ozone-resistant synthetic rubber.

Braskem and Toyota Tsusho Corporation: These companies are working together to produce polyethylene from sugarcane. Braskem plans to build a new facility in Triunfo, Brazil, to make 200,000 short tons (180,000,000 kg) of high-density and low-density polyethylene annually from bioethanol made from sugarcane.

Environmental issues

The common use of polyethylene creates challenges for managing waste because it does not break down easily. Since 2008, Japan has improved its plastic recycling efforts, but still has a large amount of plastic wrapping that ends up as waste. Plastic recycling in Japan could become a $90 billion industry.

It is possible to quickly change polyethylene into hydrogen and graphene by heating. This process requires less energy than making hydrogen through electrolysis.

Scientists have tested methods to find an enzyme or organism that can break down polyethylene. Some plastics, like polyesters, polycarbonates, and polyamides, break down through water or air exposure. In some cases, bacteria or enzyme mixtures speed up this process. However, plastics with only carbon-carbon bonds in their structure, such as polyethylene, polypropylene, polystyrene, and acrylates, break down very slowly. Reports of early progress in this area often receive attention in the media.

Indian mealmoth larvae are said to break down polyethylene. Their digestive systems reduce the strength of polyethylene by 50%, its weight by 10%, and its molecular size by 13%.

The caterpillar of Galleria mellonella is also reported to consume polyethylene. This caterpillar digests polyethylene with the help of bacteria in its gut and enzymes in its saliva that break the plastic apart.

Nomenclature and general description of the process

The name polyethylene comes from the starting material, not the final compound, which has no double bonds. The scientific name polyethene is based on the name of the monomer used. During polymerization, the alkene monomer changes into a long chain alkane. In some cases, a naming system based on structure is used, and IUPAC suggests the name poly(methylene) (poly(methanediyl) is not preferred). The difference in names happens because the double bond in the monomer opens up during polymerization. The name is shortened to PE. Similarly, polypropylene and polystyrene are shortened to PP and PS. In the United Kingdom and India, the polymer is often called polythene, based on the ICI trade name, though this is not scientifically accepted.