Polyethylene terephthalate
Polyethylene terephthalate is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods, and thermoforming for manufacturing, and in combination with glass fibre for engineering resins.
In 2020, annual global production of PET was 82 million tons. In the context of textile applications, PET is referred to by its common name, polyester, whereas the acronym PET is generally used in relation to packaging. PET used in non-fiber applications makes up about 6% of world polymer production by mass. Accounting for the >60% fraction of polyethylene terephthalate produced for use as polyester fibers, PET is the fourth-most-produced polymer after polyethylene, polypropylene, and polyvinyl chloride.
PET consists of repeating units. PET is commonly recycled, and has the digit 1 as its resin identification code. The National Association for PET Container Resources defines PET as: "Polyethylene terephthalate items referenced are derived from terephthalic acid and mono ethylene glycol, wherein the sum of terephthalic acid and mono ethylene glycol reacted constitutes at least 90 percent of the mass of monomer reacted to form the polymer, and must exhibit a melting peak temperature between 225 °C and 255 °C, as identified during the second thermal scan in procedure 10.1 in ASTM D3418, when heating the sample at a rate of 10 °C/minute."
Depending on its processing and thermal history, polyethylene terephthalate may exist both as an amorphous and as a semi-crystalline polymer. The semicrystalline material might appear transparent or opaque and white depending on its crystal structure and particle size.
One process for making PET uses bis terephthalate, which can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or by transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. It can also be obtained by recycling of PET itself. Polymerization is through a polycondensation reaction of the monomers with water as the byproduct.
Uses
Textiles
Polyester fibres are widely used in the textile industry. The invention of the polyester fibre is attributed to J. R. Whinfield. It was first commercialized in the 1940s by ICI, under the brand 'Terylene'. Subsequently E. I. DuPont launched the brand 'Dacron'. As of 2022, there are many brands around the world, mostly Asian.Polyester fibres are used in fashion apparel often blended with cotton, as heat insulation layers in thermal wear, sportswear and workwear and automotive upholstery.
Rigid packaging
Plastic bottles made from PET are widely used for soft drinks, both still and sparkling. For beverages that are degraded by oxygen, such as beer, a multilayer structure is used. PET sandwiches an additional polyvinyl alcohol or polyamide layer to further reduce its oxygen permeability.Non-oriented PET sheet can be thermoformed to make packaging trays and blister packs. Both amorphous PET and BoPET are transparent to the naked eye. Color-conferring dyes can easily be formulated into PET sheet.
PET is permeable to oxygen and carbon dioxide and this imposes shelf life limitations of contents packaged in PET.
In the early 2000s, the global PET packaging market grew at a compound annual growth rate of 9% to €17 billion in 2006.
Flexible packaging
film can be aluminized by evaporating a thin film of metal onto it to reduce its permeability, and to make it reflective and opaque. These properties are useful in many applications, including flexible food packaging and thermal insulation.Photovoltaic modules
BOPET is used in the backsheet of photovoltaic modules. Most backsheets consist of a layer of BOPET laminated to a fluoropolymer or a layer of UV stabilized BOPET.PET is also used as a substrate in thin film solar cells.
Thermoplastic resins
PET can be compounded with glass fibre and crystallization accelerators, to make thermoplastic resins. These can be injection moulded into parts such as housings, covers, electrical appliance components and elements of the ignition system.Other applications
- A waterproofing barrier in undersea cables.
- As a film base.
- As a fibre, spliced into bell rope tops to help prevent wear on the ropes as they pass through the ceiling.
- Since late 2014 as liner material in type IV composite high pressure gas cylinders. PET works as a much better barrier to oxygen than earlier used PE.
- As a 3D printing filament, as well as in the 3D printing plastic PETG. In 3D printing PETG has become a popular material - used for high-end applications like surgical fracture tables to automotive and aeronautical sectors, among other industrial applications. The surface properties can be modified to make PETG self-cleaning for applications like the fabrication of traffic signs for the manufacture of light-emitting diode LED spotlights.
- As one of three layers for the creation of glitter; acting as a plastic core coated with aluminum and topped with plastic to create a light reflecting surface, although as of 2021 many glitter manufacturing companies have begun to phase out the use of PET after calls from organizers of festivals to create bio-friendly glitter alternatives.
- Film for tape applications, such as the carrier for magnetic tape or backing for pressure-sensitive adhesive tapes. Digitalization has caused the virtual disappeance of the magnetic audio and videotape application.
- Water-resistant paper.
History
In the Soviet Union, PET was independently developed in the laboratories of the Institute of High-Molecular Compounds of the USSR Academy of Sciences in 1949, and its Russian name "Lavsan" is an acronym thereof.
The PET bottle was invented in 1973 by Nathaniel Wyeth and patented by DuPont.
Physical properties
PET in its most stable state is a colorless, semi-crystalline resin. However it is intrinsically slow to crystallize compared to other semicrystalline polymers. Depending on processing conditions it can be formed into either non-crystalline or crystalline articles. Its amenability to drawing in manufacturing makes PET useful in fibre and film applications. It is strong and impact-resistant. PET is hygroscopic and absorbs water.Transparent products can be produced by rapidly cooling molten polymer below the glass transition temperature to form a non-crystalline amorphous solid. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an orderly, crystalline fashion as the melt is cooled. While at room temperature the molecules are frozen in place, if enough heat energy is put back into them afterward by heating the material above Tg, they can begin to move again, allowing crystals to nucleate and grow. This procedure is known as cold crystallization. Amorphous PET also crystallizes and becomes opaque when exposed to solvents, such as chloroform or toluene.
A more crystalline product can be produced by allowing the molten polymer to cool slowly. Rather than forming one large single crystal, this material has a number of spherulites each containing many small crystallites. Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them, causing the resulting solid to be translucent. Orientation also renders polymers more transparent. This is why BOPET film and bottles are both crystalline, to a degree, and transparent.
Flavor absorption
PET has an affinity for hydrophobic flavors, and drinks sometimes need to be formulated with a higher flavor dosage, compared to those going into glass, to offset the flavor taken up by the container. While heavy gauge PET bottles returned for re-use, as in some EU countries, the propensity of PET to absorb flavors makes it necessary to conduct a "sniffer test" on returned bottles to avoid cross-contamination of flavors.Intrinsic viscosity
Different applications of PET require different degrees of polymerization, which can be obtained by modifying the process conditions. The molecular weight of PET is measured by solution viscosity. Viscosity is highly dependent on molecular parameters such as chain length and molecular weight. Due to the structural complexity of branched polymers, viscosity-based determination of molecular weight is best used with linear polymers. With dilute solutions, an empirical relationship can be derived between the viscosity and the hydrodynamic volume and molecular weight distribution. The preferred method to measure this viscosity is the intrinsic viscosity of the polymer. Intrinsic viscosity is a dimensionless measurement found by extrapolating the relative viscosity to zero concentration. Shown below are the IV ranges for common applications:| Application | IV |
| Textile fibers | 0.40–0.70 |
| Technical fibers | 0.72–0.98 |
| Biaxially oriented PET film | 0.60–0.70 |
| Sheet grade film for thermoforming | 0.70–1.00 |
| General purpose bottles | 0.70–0.78 |
| Carbonated drink bottles | 0.78–0.85 |
| Monofilaments and engineering plastics | 1.00–2.00 |