Polyethylene Terephthalate, or PET for short, is a type of plastic that is in common usage in many products today and is easily recyclable. PET is predominantly produced for the textiles industry as ‘polyester’, but this article will focus on the more pure form used in packaging. Primarily used in plastic drinks bottles, it is recognisable by the ‘spot’ on the base of the bottle, composed of moulded plastic.
Chemically, PET is a thermoplastic polymer, and is made up of polymerised, repeating units of C10H8O4.
Properties and Applications of PET
PET has a wide range of applications and is extensively used in many products containing plastic.
As PET is a thermoplastic, within a certain temperature band the polymer becomes extremely pliable, making it easy to mould in to shapes. This occurs between the glass transition temperature and the melting point, which for PET are 75oC and 255oC respectively. Once cooled, PET will return to a solid form, making the plastic polymer extremely useful for drinks bottles. It is also almost completely impermeable.
Other typical uses of polyethylene terephthalate include:
- Food packaging (microwavable meals)
- Thin film
- Solar cells
- Thermal insulation
- Adhesive tape
- Magnetic tape
- Polyester and associated textile products
A typical fizzy drink bottle, primarily constructed from PET. Image Credits: cityofyukonok.gov
Recycling of PET
Thermoplastics, such as PET, are generally easy to recycle because the polymer chain breaks down at a relatively low temperature, and so there is no degradation of the polymer chain during the recycling process. This allows PET to be recycled a large number of times before it becomes unusable, though contamination can reduce the number of ‘closed loop’ cycles that PET can go through.
The exact recycling process will vary from plant to plant, but in general the following steps are taken:
- Bottles are sorted by hand and unwanted materials are removed so that only bottles are left. Alternatively, the bottles can be sorted using an automated system.
- The bottles are then cleaned inside and out to remove any residual liquid or dirt to prevent contamination.
- The bottles can then be sorted infrared radiation techniques, to determine which polymers are present.
- If bottles are also sorted by colour, traditionally blue, natural, green and mixed, it can add further value to the plastic.
- Next the bottles are shredded into flakes, which are then washed again. Alternatively, the shredding step can be skipped and the bottle is melted and reformed into a different shape.
- The shredded plastic is then melted to produce plastic granulates, or pellets.
Plastic drinks bottles are often a mix of polymers, typically a mix of PET and PP or PVC. There are several methods of separation that can be used. To separate PET from any PP in the bottle, ‘sink-float’ separation is used. The separation is achieved because PET is denser than PP, with respective specific densities of 1.43-1.45 g/cm3 and .93-.95g/cm3, so PET will sink in a tank of water and PP will float.
Unfortunately, if PET is mixed with PVC then it can be very hard to separate the two polymers, as they have similar specific gravities and also look similar to the human eye. Separation of the two polymers is imperative however, because even a small amount of PVC in a batch of PET can ruin a melt. Near Infrared Radiation (NIR) techniques can be used, but darkly coloured plastics will absorb the radiation and so a distinction cannot be made.
To ensure that plastics polymers are recycled separately, they are assigned a resin code to distinguish the polymer. This is a randomly assigned number and can usually be found on the bottom of the plastic product. The resin identification code for polyethylene terephthalate is ‘1’.
Almost all (92%) local authorities in the UK offer plastic bottle collection schemes, either directly from households or from designated local sites. Reverse vending machines are also becoming increasingly popular, making it easier for people to recycle.
PET is an extremely useful material and use has skyrocketed in the last few decades. The market for bottled water has increased the need for PET and in Britain around 15 million plastic bottles are used every day. 11% of household waste is plastic, and around 40% of this is plastic bottles.
The energy consumption per volume of a PET bottle is around 5.4 MJ/litre, which is a lot less than glass or aluminium, but higher than PE (commonly used for milk bottles).
Recycling of PET will help to reduce dependence on oil and gas, reduce landfill waste and reduce greenhouse gas emissions into the atmosphere.
PET is predominantly recycled back into:
- Fibres (cheap carpets, fleeces)
- Food containers and bottles
- Film sheets
Sources and Further Reading