Know Your Plastics: Polyethylene Terephthalate (PET or PETE)

Different PET products

For every container used for soft drink and water, shampoo and liquid soap, cooking oil, food dressing and spread and;

For every oven food tray, roasting bag, curtain, upholstery, thread, tyre cord filament, industrial fibre, and industrial filtration fabric, you are more than likely looking at a plastic product made of Polyethylene terephthalate.

Still in doubt? Look for the inscription “bubble” or the recycling code “1” encased in a triangle of chasing arrows on the container or the item itself and, there you have it! You have identified a polyethylene terephthalate AKA PETE AKA PET!

Polyethylene Terephthalate recycling code and acronyms

Background

The very first preparation of PET was made during a 1940 study of phthalic acid by Rex J. Whinfield and James T. Dickson of the Calico Printers Association, England. However, the war at the time delayed the publication of its patent specification. This stalled the production of the Terylene fibre by Imperial Chemical until 1954. By 1945, DuPont had developed its PET process from terephthalic acid and production of, what is now known as Dacron fibre began in 1953. The technology for PET film was developed in the 1950s while the blow-stretch mould used to produce PET bottles was developed in the 70s. The PET bottle patent was issued in 1973.

Did you know that Dacron, Mylar, Recron, Lavsan and Terylene are all trade names for polyethylene terephthalate?

Description

A member of the polyester family of polymers, polyethylene terephthalate is a clear, strong, wrinkle-free, lightweight, thermoplastic resin, film or fibre that discolours when bent. Often referred to as polyester, PETE (or PET for short) is produced by heating ethylene glycol, a colourless liquid, and terephthalic acid, a semi-crystalline solid, in the presence of chemical catalysts. The result? A molten, viscous mass that is either directly made into fibres or solidified to be processed into the plastic later.

Polyethylene Terephthalate material

In fibre form, PET is spun into high strength textiles that are stiff and very resistant to deformation. These fibres are in wrinkle-resistant applications such as:

- In durable-press fabric blends that do not hold wrinkles like rayon, wool and even cotton bends and artificial silk. Such fabric is in clothing and furnishing such as curtains, bedsheets and spreads

- In fibre filling used in pillows, furniture, cushions, carpets and insulated clothing.

- In industrial products as automobile tire yarns; seat belts, drive belts and conveyor belts; disposable medical clothing (PPE) reinforcements for hoses i.e. garden and fire.

When processed into plastic, the high strength level of PET allows for shaping into PET films and transparent containers that are rigid and highly impermeable to gas and liquids. The most common application of PET plastic is the bottling of carbonated beverages and food jars. PET film is used in CDs and DVDs. NOTE: PET containers used for food are processed at low temperatures. This means that such containers should not hold hot food. The lowest temperature at which such PET will soften is 70^oC (160^oF).

Did you know that polyethylene terephthalate in textile applications is called polyester, while in food and beverage packaging; it is known as PET or PET-resin?

PET materials can be both transparent (amorphous) or opaque and white (semi-crystalline). This duality means you can find a group of thermoplastic PET materials that is stiff, hard and ductile, with good strength and yet another group that is less stiff and hard but with better ductility.

Characteristics of PET

As general-purpose thermoplastic, PET resins, films and fibres have some excellent properties:

- They are highly flexible

- They have significantly high strength levels i.e. they are very strong.

- They are easy and efficient to move because of their lightweight.

- They do not react to water or moisture and attacks by microorganisms.

- They are generally stable, dimensionally, thermally, chemically and mechanically. The stiffness in PET polymer chains gives excellent chemical, hydrolytic and solvent resistance.

- They have a thermal resistance of 150-175^oC

- They have a high resistance to creasing and abrasions

- PET can be rigid or semi-rigid; this is a factor of the processing parameters employed.

- PET is an excellent electrical insulator.

- Pet can be highly transparent, making it a fitting replacement for glass in some applications.

- When it fractures, it does not break. In other words, it has good fracture resistance and is virtually shatterproof.

- Gas permeability level for PET is low. This feature is perfect for preventing the escape of carbon-dioxide in carbonated drinks and limiting oxygen in food products which can cause spoilage.

- They are easily recycled.

Issues with PET

Although PET is the most widely produced thermoplastic, it contains antimony trioxide – a carcinogenic substance that can cause cancer in living tissues. The chance of releasing the antimony trioxide in PET is highest:

- In liquids left in PET containers for very long periods.

- In warm temperatures in enclosed spaces like cars, garages and poorly ventilated warehouses.

Oxidation can affect polyethylene terephthalate. This effect is why beverages like beer and wine are rarely not stored in PET containers. Such drinks are brewed with a long shelf life but, prolonged interaction with PET will lead to degradation in taste within a short time.

PET Recycling and Sustainability

Though PET materials come from crude oil and natural gas, they are relatively sustainable compared to other container materials like glass and aluminium.

The combination of high strength and lightweight makes PET one of the most energy-efficient materials available. The more PET product delivered in less packaging material and with no damage, the less the transportation fuel.

As the most produced thermoplastic, it is also the thermoplastic with the highest amount of waste and the most widely recycled plastic worldwide. Recycling is essentially a means of re-capturing and re-using the internal energy contained in materials. This energy, known as resource energy, is as high as 40% in PET. Conventional recycling methods involve the washing and melting of PET containers, spun into fibres for carpets, fibrefill in sleeping bags and winter jackets, industrial strapping, rope, clothing and protective packaging. If PET collected is in an acceptably pure condition, it may be re-made back into its original uses.

Recycling (degradation) of the PET is mainly by depolymerisation. Essentially, it is the process in which the PET chain is chemically broken down into its constituent monomers using different chemical agents. Monomers constituting PET are Dimethyl terephthalate, terephthalic acid (TPA), ethylene glycol (EG) and bis terephthalate. The chemical agent used determines the degradation technique implemented, whether it is hydrolysis, methanolysis or glycolysis. The most straightforward approach to recycle large quantities of PET commercially is glycolysis.

Due to the ease of production and the improper disposal of PET over the years, a global environmental waste problem has emanated that conventional recycling methods are struggling to manage. This problem necessitated the development of some unconventional applications that work towards the sustainable use of PET plastic. They include:

Nzambi Matee, founder of Genje Makers

PET in the Production of Concrete; Dioctyl terephthalate or DOTP, an oily substance gotten from waste PET, is compatible with the materials used in making concrete as a building material. The years of research put into improving the performance of PET-reinforced concrete has proved successful. Several innovators in various parts of the world now provide low-cost blocks and even shelter with this method. A practical example is the Gjenge Makers factory in Nairobi, Kenya that produces 1,500 bricks per day from plastic waste. Another example is the Rivesti mosaic tiles made by Plaslatina in Sao Paulo, Brazil.

PET in prostheses: In the health sector, Polyethylene terephthalate has various successful applications including, cardio-vascular autologous grafts for large diameter vascular arteries, heart valves and surgical meshes.

There you have it! A summary of the most important details of Polyethylene Terephthalate!

Did we miss anything? Would you like to know more about something specific?

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The next article will feature the material with the recycle code “2”. Can you guess?

Stay Green!

March 18^th 2021 was World Recycling day. As they say in Bollywood, It is “an auspicious day” for every sustainability-conscious individual and entity to create awareness and share the successes of recycling. What a wonderful coincidence that this global celebration falls within our series on the most mismanaged material on earth!