Polyethylene terephthalate (PET) is a common type of plastic in the polyester family. It is used to make clothing fibers, containers for liquids and food, parts for manufacturing, and combined with glass fiber for engineering materials.
In 2020, the world produced 82 million tons of PET each year. In textiles, PET is often called polyester, while the abbreviation PET is used for packaging. PET used in packaging (not for fibers) makes up about 6% of all plastic production worldwide. Including the more than 60% of PET used for polyester fibers, PET is the fourth-most-produced plastic after polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC).
PET is made of repeating units of the chemical formula C₁₀H₈O₄. It is often recycled and has the number 1 (♳) as its resin identification code (RIC). According to the National Association for PET Container Resources (NAPCOR), PET is defined as a material made from terephthalic acid or dimethyl terephthalate and monoethylene glycol. At least 90% of the materials used must react to form the polymer, and the material must melt between 225°C and 255°C when tested using a specific method.
Depending on how it is made and heated, PET can be amorphous (clear and non-crystalline) or semi-crystalline (partially crystalline). Semi-crystalline PET may look clear (if particles are smaller than 500 nm) or white and cloudy (if particles are up to a few micrometers) based on its structure and particle size.
One way to make PET is by using bis(2-hydroxyethyl) terephthalate. This can be created through a reaction between terephthalic acid and ethylene glycol, with water as a byproduct (called a condensation reaction), or through a reaction between ethylene glycol and dimethyl terephthalate (DMT), with methanol as a byproduct (called a transesterification reaction). PET can also be made by recycling used PET. The final step in making PET is a polycondensation reaction of the monomers, which also produces water as a byproduct.
Uses
Polyester fibers are commonly used in the textile industry. The invention of polyester fiber is credited to J. R. Whinfield. It was first brought to market in the 1940s by ICI, under the brand name "Terylene." Later, E. I. DuPont introduced the brand "Dacron." As of 2022, many brands worldwide, mostly in Asia, produce polyester fiber.
Polyester fibers are often blended with cotton in clothing. They are also used as heat insulation layers in thermal wear, sportswear, workwear, and car upholstery.
Plastic bottles made from PET are widely used for soft drinks, including still and sparkling varieties. For beverages that are damaged by oxygen, such as beer, bottles use a multilayer structure. PET is combined with an extra layer of polyvinyl alcohol (PVOH) or polyamide (PA) to reduce oxygen passing through.
Non-oriented PET sheet can be heated and shaped to make packaging trays and blister packs. Both amorphous PET and biaxially oriented PET (BoPET) are clear to the naked eye. Dyes can be easily added to color PET sheet.
PET allows oxygen and carbon dioxide to pass through, which limits how long items in PET containers can stay fresh.
In the early 2000s, the global PET packaging market grew by 9% each year, reaching €17 billion in 2006.
Biaxially oriented PET (BOPET) film can be coated with metal to reduce its permeability and make it reflective and opaque (MPET). These properties are useful in flexible food packaging and thermal insulation, such as space blankets.
BOPET is used in the backsheet of solar panels. Most backsheets include a layer of BOPET joined to a fluoropolymer or a layer of UV-stabilized BOPET.
PET is also used as a base material in thin film solar cells.
PET can be mixed with glass fiber and materials that speed up crystallization to create thermoplastic resins. These resins can be molded into parts like housings, covers, electrical appliance components, and parts of ignition systems.
PET is used for:
– A waterproofing barrier in undersea cables.
– As a base for films.
– As a fiber, woven into bell rope ends to reduce rope wear.
– Since late 2014, as a liner in type IV composite high-pressure gas cylinders. PET blocks oxygen better than earlier materials like low-density polyethylene (LDPE).
– As a 3D printing filament, and in PETG, a type of PET used in 3D printing. PETG is popular for high-end uses like surgical tables and in automotive and aerospace industries. Its surface can be modified to make it self-cleaning for uses like traffic signs and LED lights.
– As one of three layers in glitter, acting as a plastic core coated with aluminum and topped with plastic to create a reflective surface. However, many glitter manufacturers have stopped using PET by 2021 due to requests for eco-friendly alternatives.
– As a film for tape applications, such as magnetic tape or adhesive tape backing. Digital technology has reduced the use of magnetic audio and video tapes.
– As water-resistant paper.
PET is used in:
– A mold for making bottles.
– A finished PET bottle.
– A PET bottle heated by a candle and changed to become opaque.
– PET clamshell packaging, used to sell fruit, hardware, and other items.
– Polyester yarn.
– Microfiber towels and cleaning cloths.
– Aluminized Mylar balloons filled with helium.
History
PET was patented in 1941 by John Rex Whinfield, James Tennant Dickson, and their employer, the Calico Printers' Association of Manchester, England. In 1950, E. I. DuPont de Nemours in Delaware, United States, first produced Dacron, which is a type of PET fiber. The company used the trademark name Mylar for boPET film in June 1951 and officially registered it in 1952. Mylar remains the most well-known name for polyester film. Today, the trademark is owned by DuPont Teijin Films.
In the Soviet Union, PET was developed independently in 1949 by scientists at the Institute of High-Molecular Compounds of the USSR Academy of Sciences. The Russian name for PET, "Lavsan," is an acronym formed from parts of the institute's full name in Russian.
The PET bottle was invented in 1973 by Nathaniel Wyeth and later patented by DuPont.
Physical properties
PET in its most stable form is a clear, semi-crystalline resin. However, it crystallizes more slowly than other similar materials. Depending on how it is processed, PET can be made into either non-crystalline (amorphous) or crystalline objects. Its ability to be shaped easily makes it useful for making fibers and films. PET is strong and resistant to breaking. It also absorbs moisture.
Clear products are made by quickly cooling melted polymer below a specific temperature (called the glass transition temperature, or Tg) to form a non-crystalline, amorphous solid. Amorphous PET is created by rapidly cooling the melted material. When heated above Tg, this glassy state begins to crystallize, a process called cold crystallization. Amorphous PET can also crystallize and become cloudy when exposed to certain solvents, such as chloroform or toluene.
A more crystalline product is made by cooling the melted polymer slowly. Instead of forming one large crystal, this material contains many small crystallized areas called spherulites, each with tiny crystallites (grains). Light scatters as it moves between these crystallites and the non-crystalline areas, making the solid translucent. Shaping the polymer can also increase its transparency. This is why BOPET film and bottles are both partially crystalline and transparent.
PET tends to absorb certain types of flavors, so drinks stored in PET containers sometimes need more flavor to compensate for what is absorbed by the container. In some European countries, when bottles are returned for reuse, a "sniffer test" is performed on some bottles to prevent flavor mixing from previous use.
PET used in different ways requires different levels of polymerization, which can be controlled by adjusting processing conditions. The molecular weight of PET is measured by solution viscosity. Viscosity depends on factors like chain length and molecular weight. Because branched polymers are complex, viscosity measurements are most accurate for linear polymers. In dilute solutions, an empirical relationship can be found between viscosity, hydrodynamic volume, and molecular weight distribution. The best way to measure this is through intrinsic viscosity (IV), a dimensionless value calculated by extending the relative viscosity (measured in dℓ/g) to zero concentration. Below are the IV ranges for common uses:
Copolymers
PET is often combined with other diols or diacids to improve its properties for specific uses. For example, cyclohexanedimethanol (CHDM) can be added to the polymer structure, replacing some of the ethylene glycol. Since CHDM is larger (with six more carbon atoms) than the ethylene glycol it replaces, it does not fit as well with the neighboring chains. This makes it harder for the polymer to form crystals and lowers its melting temperature. PET modified in this way is called PETG or PET-G (polyethylene terephthalate glycol-modified). It is a clear, amorphous thermoplastic that can be injection-molded, sheet-extruded, or made into filament for 3D printing. PETG can be colored during production. Replacing all ethylene glycol with CHDM creates a material called PCT.
Another common modifier is isophthalic acid, which replaces some of the 1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or 1,3-(meta-) linkage creates an angle in the chain, which also disrupts crystallinity.
These modified PETs are useful for certain molding processes, such as thermoforming, which is used to make tray or blister packaging from co-PET film, amorphous PET sheet (A-PET/PETA), or PETG sheet. However, crystallization is important in other applications where mechanical and dimensional stability are needed, such as seat belts. For PET bottles, small amounts of isophthalic acid, CHDM, diethylene glycol (DEG), or other comonomers can be helpful. If only small amounts of these are used, crystallization is slowed but not stopped completely. This allows bottles to be made using stretch blow molding ("SBM"), which results in clear and crystalline materials that form an effective barrier against aromas and gases, such as carbon dioxide in carbonated beverages.
Production
Polyethylene terephthalate (PET) is mostly made from purified terephthalic acid (PTA). It is also made, to a smaller degree, from mono-ethylene glycol (MEG) and dimethyl terephthalate (DMT). As of 2022, ethylene glycol is produced from ethene, which is found in natural gas. Terephthalic acid is made from p-xylene, which comes from crude oil. Usually, a catalyst made of antimony or titanium is used. A phosphite is added to help protect the material, and a bluing agent, like cobalt salt, is used to prevent yellowing.
In the DMT process, DMT and extra MEG are combined in a melted state at 150–200 °C with a basic catalyst. Methanol is removed through distillation to help the reaction proceed. Extra MEG is removed at higher temperatures using vacuum. A second step occurs at 270–280 °C, with continuous removal of MEG through distillation.
The reactions can be summarized as follows:
In the terephthalic acid process, MEG and PTA are combined directly at moderate pressure (2.7–5.5 bar) and high temperature (220–260 °C). Water is removed during the reaction and continuously removed by distillation.
Bio-PET is a version of PET made from plant-based materials. In Bio-PET, MEG is made from ethylene derived from sugar cane ethanol. A better method using the oxidation of ethanol has been proposed. It is also possible to make PTA from bio-based furfural.
There are two main ways to mold PET bottles: one-step and two-step. In the two-step method, two machines are used. The first machine creates a preform, which looks like a test tube, with the bottle-cap threads already shaped. The preform’s body is thicker because it will be stretched into its final shape in the second step using stretch blow molding.
In the second step, the preforms are quickly heated and then inflated inside a two-part mold to form the final bottle shape. Preforms are also used as containers for storing medical information in programs like the Vial of Life. The two-step process allows preforms to be produced at one location and finished at another, saving space and enabling "just-in-time" production. In the one-step method, the entire process—from raw material to finished container—happens in one machine. This is ideal for creating unusual shapes, such as jars or flasks, and reduces space, handling, and energy use compared to the two-step method.
PET can break down during processing. If there is too much moisture, a reaction called hydrolysis can reduce the material’s strength, making it brittle. If the material is exposed to high temperatures or stays in the machine too long, it can break down thermally or through oxidation, causing discoloration, lower strength, and the formation of acetaldehyde. This can also create "gel" or "fish-eye" spots. To reduce these issues, other chemicals like CHDM or isophthalic acid are added to lower the melting point. Stabilizers, such as phosphites, are also used.
Acetaldehyde can form when PET is improperly handled. It can cause a bad taste in bottled water. High temperatures, long processing times, high pressure, or fast machine speeds can increase acetaldehyde production. Exposure to light can also lead to its formation through a reaction called the Type II Norrish reaction.
When acetaldehyde forms, some of it stays in the container’s walls and can move into the product inside, changing its taste and smell. This is not a big problem for non-drink items like shampoo or fruit juice, which already contain acetaldehyde. However, for bottled water, keeping acetaldehyde levels low is important because even tiny amounts (10–20 parts per billion) can cause an off-taste.
Safety and environmental concerns
A report in Environmental Health Perspectives from April 2010 suggested that PET plastic might release hormone-like chemicals under typical use conditions and called for more research on this topic. Possible ways this could happen include the release of phthalates or antimony. A study in Journal of Environmental Monitoring from April 2012 found that antimony levels in deionized water stored in PET bottles stayed within EU safety limits even after brief storage at temperatures up to 60 °C (140 °F). However, water or soft drinks in PET bottles might sometimes exceed EU limits after less than a year of storage at room temperature.
Antimony (Sb) is a type of element used as a catalyst in PET production, often in forms like antimony trioxide (Sb₂O₃) or antimony triacetate. After manufacturing, some antimony remains on the surface of PET products and can be removed by washing. Antimony also stays inside the PET material and may leak into food or drinks. Exposing PET to boiling or microwaving can greatly increase antimony levels, possibly surpassing US EPA limits. The World Health Organization (WHO) sets a drinking water limit of 20 parts per billion, while the United States sets a limit of 6 parts per billion. Although antimony trioxide is not highly toxic when swallowed, its presence is still a concern. A Swiss study compared water in PET and glass bottles and found higher antimony levels in PET bottles, but these levels were still below safety limits. The study concluded that small amounts of antimony from PET bottles do not pose a significant health risk (1% of the WHO’s "tolerable daily intake"). A 2006 study also found similar antimony levels in PET-bottled water. The WHO has published a risk assessment for antimony in drinking water.
Fruit juice concentrates bottled in PET in the UK were found to contain up to 44.7 μg/L of antimony, which is above the EU’s 5 μg/L limit for tap water.
PET plastic used in clothing releases tiny fibers during use, washing, and drying. Plastic waste breaks down into small particles. These microplastics on river or seabed floors can be eaten by small marine life, entering the food chain. Because PET is denser than water, many PET particles may settle in sewage treatment plants. PET microfibers from clothing can become airborne, spread to fields, and be eaten by livestock or plants, eventually reaching humans through food. A study in Science of the Total Environment found PET made up 18% of microplastics in human lung tissue samples, with 0.69 ± 0.84 microplastics per gram of lung tissue. SAPEA stated that such particles "do not pose a widespread risk." PET breaks down when exposed to sunlight and oxygen. As of 2016, little information was available about how long synthetic polymers last in the environment.
Polyester recycling
Most thermoplastics can be recycled, but recycling PET bottles is more practical than other plastic uses because PET has high value and is mainly used for water and carbonated soft drink bottles. PET bottles are well-suited for recycling (see below). In many countries, a large percentage of PET bottles are recycled, such as about 75% in Switzerland. Recycled PET is often called rPET, R-PET, or post-consumer PET (POSTC-PET).
Recycled PET is commonly used to make polyester fiber, strapping, and non-food containers. Because PET is easy to recycle and many bottles are available as waste, it is also becoming popular as a material for carpets. PET can be thermally disposed of (burned) because it contains carbon, hydrogen, and oxygen, with only small amounts of other elements (no sulfur). PET can be recycled in three ways: chemically broken down into its original materials (PTA, DMT, and EG), mechanically reshaped without breaking the polymer structure, or processed using transesterification and other chemicals to create a new polyol. This polyol can be used in polyurethane foam or epoxy products, such as paints.
In 2023, a new process was introduced to use PET for making supercapacitors. PET, which contains carbon and water in a balanced chemical form, can be transformed into carbon sheets and nanospheres with a very large surface area. The process involves heating a mixture of PET, water, nitric acid, and ethanol under high temperature and pressure for eight hours, followed by centrifugation and drying.
In 2021 and 2022, major investments were announced to chemically recycle PET using glycolysis, methanolysis, and enzymatic methods to recover its basic building blocks. These methods will initially use bottles as raw materials but may later include recycling fibers.
PET is also valuable as a fuel in waste-to-energy plants because it has a high energy content, which helps reduce reliance on other energy sources.
PET breaks down naturally through ester hydrolysis, a process where enzymes like PETase and MHETase break it into smaller molecules: 2-hydroxyethyl terephthalic acid, ethylene glycol, and terephthalic acid. These enzymes, discovered in 2016, are being studied for recycling PET or managing waste. However, challenges remain, such as the enzymes' sensitivity to heat and their slow action on crystalline PET.