What is polyethylene terephthalate (PET)?

Polyethylene terephthalate is a modern plastic used to make various types of packaging, technical components, fibres and household products. PET is used in many industries, but most actively in packaging. This thermoplastic polymer meets environmental requirements and is suitable for recycling and disposal. Among synthetic materials, it is considered the least harmful to the environment.

Key properties and advantages

Thermoplastic polyester is the result of a polycondensation reaction involving terephthalic acid and ethylene glycol. Simply put, polyethylene terephthalate is a modern polymer. It is currently one of the most popular types of plastic.

What does PET look like? It is a transparent, hard plastic with a smooth, shiny surface, sometimes with grey or yellowish specks. It turns white when crystallised, and its translucency allows you to see the contents of the packaging.

The unique properties and characteristics of PET distinguish it from many other plastics:

• resistance to deformation when heated;
• ability to melt and be recycled without losing important properties;
• retention of shape and dimensions in a temperature range from -50°C to +85°C;
• minimal water absorption;
• wear resistance of products, ability to withstand prolonged loads;
• immunity to the effects of chemicals and moisture;
• ease of pressing and moulding.

The melting point allows polyethylene terephthalate to be used for the manufacture of shaped products without destroying the structure of the material. It is easy to cut, drill, mill and weld. It is used for the manufacture of packaging, including vials, bottles, canisters and other containers of various volumes.

The characteristics of PET material determine its popularity in the packaging materials market:

• transparency, which makes thermoplastic polyester ideal for bottling mineral water;
• the ability to be coloured in any shade to meet consumer demands or protect the contents from sunlight;
• low weight, which allows PET containers to be packaged in polyethylene film without a pallet or with cardboard underneath;
• high impact strength of PET plastic, which is important for storage and transportation.

The use of polyethylene terephthalate in the production of containers for food, chemical and perfume products is also justified by its reasonable price, recyclability and the variety of production methods available, including melting, moulding, blow moulding and extrusion.

Unlike metal and glass containers, packaging made from thermoplastic polyester cannot cut you if it is damaged.

Application

Polyethylene terephthalate products are in demand in many industries, such as medicine, pharmaceuticals, cosmetology, trade, food, and chemical industries. PET is used to produce primary packaging and other products in the form of:

• flasks, bottles and jars for household chemicals, perfumes and cosmetics;
• lids and caps;
• various containers and receptacles;
• medical devices;
• dispensers, dosers and triggers compatible with plastic containers;
• preforms, etc.

PET has, to some extent, replaced glass and other types of plastic. If a 0.5 litre polymer bottle weighs approximately 28 grams, a glass container of the same volume will weigh approximately 350 grams.

Types of polyethylene terephthalate

In order to optimise the properties of the polymer, thermoplastic polyester is polymerised together with other chemical compounds in the form of modifying additives, as well as with the addition of flame retardants, heat and light stabilisers, and dyes. The result of melting is a high-molecular compound, or copolymer, with improved technical characteristics. Examples can be used to illustrate what polyethylene terephthalate with additives is.

PET-A

Amorphous polyethylene terephthalate is a transparent polymer characterised by impact strength, impact resistance, and cold and hot flexibility. Crystallisation begins when heated above 75°C - the polymer turns white and becomes brittle.

The material is used in advertising, retail, construction, and instrument making. Amorphous PET is used for UV and solvent printing. PET-A is used to make shop windows, light panels, stands, room partitions, and display screens.

PET-G

Polyethylene terephthalate with the addition of glycol has increased impact strength. Due to its amorphous structure, PET-G melts at lower temperatures while retaining its transparency. This property makes it a convenient material for moulding and pressing. PET glycol can be dyed, printed and finished with metal. It is used in the manufacture of signs, bottles and durable components for medical devices.

PET-HAG

A three-layer polymer combining two outer layers of PET glycol and an inner layer of amorphous PET-A polymer. This PET material combines the properties of both plastics. The material can be processed mechanically and bonds well. PET-GAG is suitable for printing, packaging, advertising products and commercial equipment.

Production technology

The raw materials used to produce PET are dimethyl ether or colourless crystals of terephthalic acid and ethylene glycol. There are two main stages in the production process: esterification and polycondensation.

In the first stage, a chemical polycondensation reaction occurs at a temperature of up to 2,700 °C and a pressure of up to 0.2 MPa. Long molecular chains of polyethylene terephthalate are formed, giving the material the necessary properties. When the material has finished melting, the mixture is squeezed out of the apparatus.

In the second stage, the resulting composition is cooled. The properties of polyethylene terephthalate depend on the cooling rate. If it is fast, the polyester will be amorphous and transparent. Slow cooling causes crystallisation and the appearance of a white colour. Since polyethylene terephthalate (PET) is mainly used for the production of containers and packaging, rapid cooling remains the most common method.

Controlling crystallinity during the cooling stage significantly expands the range of applications for the polymer. Amorphous plastic is used as a raw material for the production of films and containers, while crystalline plastic is used for the manufacture of structural products.

To improve its properties, the material is modified by:

• adding titanium oxide additives for a matt effect;
• using dyes and flame retardants;
• using photostabilisers to protect against ultraviolet rays;
• adding inert components, etc.

Popular technologies in the production of goods are moulding, extrusion, and forming. For example, pressure moulding is used to manufacture bottles, cans, and flasks of various shapes and volumes. Good barrier properties allow PET to remain a sought-after material in the production of containers.

Marking of containers and products

The basic requirements for PET are set out in GOST R 51695-2000. The origin and composition of polyethylene terephthalate products are marked with the abbreviation PET or PET in the international designation, as well as a triangle with three arrows and the number 1 inside:

• the number 1 is the value of a specific polymer;
• the triangle or ribbon is a recycling symbol.

The meaning of the symbols allows the properties of the polymer to be determined during use and subsequent recycling.

Other types of polymers

Common types of plastics are everywhere around us. Their chemical composition and technical characteristics determine their use in different industries. The most popular polymers include:

1. PVC (polyvinyl chloride). This thermoplastic vinyl chloride polymer is resistant to low temperatures and is often used to make window profiles and pipes. The properties of the material depend on the plasticiser and modifying additives in its composition. PVC is not environmentally friendly and is not suitable for recycling.
2. PP (polypropylene). This thermoplastic material is resistant to chemical exposure and heating. It is used to manufacture baking packaging, household appliances, cable insulation, water pipes, etc. It is easily recyclable.
3. PS (Polystyrene). An inexpensive and lightweight material. The polymer is used to make disposable tableware and food packaging.
4. PC (Polycarbonate). A transparent and durable plastic. It is used in the production of optical discs, car headlights, transport stops, and safety glasses.
5. FG (Fibreglass). A polymer with high strength and biological resistance. Suitable for the production of products requiring exceptional reliability.

A triangle symbol is used to indicate the type of polymer, with numbers inside indicating the specific type of plastic.