The Packaging Series: PET
As we move into a more diverse range of products including packaging, we wanted to take a closer look at the world of plastic. It is a product which has been a major talking point in recent years, and we have learned that we need to use it in a responsible way to ensure our planet’s health for future generations. However, if used in the right way and managed effectively plastic can be an incredible asset to the food and beverage industry. We are going to start by looking at PET, which is the most common plastic around.
What is PET?
PET (Polyethylene terephthalate) is the most common thermoplastic polymer resin of the polyester family and is used in a large variety of different objects such as clothing fibres, food and liquid containers, thermoforming in manufacturing and for engineering resins when used in combination with glass fibres. PET is the most widely recycled plastic across the globe. In the United States however, only 20% of PET material is actually recycled. This raises questions regarding the development of more thorough recycling processes and the development of recycling as a consumer habit.
Most of global PET production (over 60%) is for the purpose of synthetic fibres, whereas bottle production surprisingly accounts for only 30% of global demand. The abbreviation PET is largely used in reference to packaging but in the world of textile applications, its common name ‘polyester’ is used. Polyester is the fourth most commonly used and produced polymer, and makes up around 18% of world polymer production.
PET is commonly recycled and possesses the number 1 as its resin identification code. PET can be both transparent or opaque depending on its processing and thermal history. If PET is amorphous it will be transparent, if it is semi-crystalline it will appear either white or opaque depending on its crystal structure.
In 1940 England, PET was first prepared by John Rex Whinfield and James Tennant Dickson of the Calico Printers Association during a study of phthalic acid. Due to the wartime restrictions they were not able to patent PET until 1941. Overtime, PET eventually became one of the most widely produced synthetic fibres in the world and by the 1970s, improved stretch-moulding procedures had been developed which allowed PET to be rendered into durable transparent plastic bottles. The PET bottle was patented by Nathaniel Wyeth in 1973.
PET is produced by the polymerisation of ethylene glycol and terephthalic acid. They get heated together along with chemical catalysts to produce PET in the form of a molten and viscous mass which is then later spun into PET fibres or solidified to be later processed as a plastic.
PET is used in the production of plastic bottles which are widely used for soft drinks. Depending on the certain speciality of the bottle, PET can sandwich an additional (PVOH) polyvinyl alcohol layer to prevent any further oxygen permeability. This is done in the case of bottles designed for beer containment, for example.
PET is also used in the form ‘biaxially oriented PET’. This can be aluminised by evaporating a thin film of metal onto the film in order to reduce its permeability as well as giving it a reflective quality. These properties are of course useful in multiple spheres including flexible food packaging, thermal insulation and, because of its high mechanical strength, it can often be used in tape applications. PET film can be the carrier for magnetic tape or backing for pressure-sensitive adhesive tapes.
Another form, called ‘non-oriented PET’ can be thermoformed in order to produced packaging trays and blister packs. These packaging trays can be used for frozen dinners if crystallisable PET is used, as this has the property of being able to withstand both freezing and oven baking temperature changes. Other uses of PET include a substrate in thin film solar cells, waterproof barriers in undersea cables, bell rope tops to prevent wear and tear on ropes as they pass through a ceiling, liner material in composite high pressure gas cylinders, and as a 3D printing filament.
From the short description above, you can begin to see how diverse and useful PET can be. Due to its vast range of properties it has numerous outputs and uses it is very popular. It has resistance to water, a high strength to weight ratio, is practically shatterproof and has a wide availability as a recyclable plastic, making it a truly formidable commodity. It would not seem too far to say that PET is indeed the world’s most useful plastic.
Author: Betty Rook