PET (PolyEthylene Terephthalate) is a strong but lightweight form of clear polyester. It is used to make containers for soft drinks, juices, alcoholic drinks, water, edible oils, household cleaners and other food and non-food applications. Being a polymer, polyethylene terephthalate's molecules consist of long chains of repeating units only containing the carbon (C), oxygen (O) and hydrogen (H) organic elements.
PET was first developed for use in synthetic fibres by British Calico Printers in 1941. The patent rights were then sold to DuPont and ICI who in turn sold regional rights to many other companies*.
Although originally produced for fibres, PET began to be used for packaging films in the mid 1960s and then, in the early 1970s, the technique for blowing bi-axially oriented bottles was commercially developed.
Bottles now represent the most significant use of PET moulding resins. Making a PET bottle starts from the raw material: ethylene and paraxylene. These two substances' derivatives (ethylene glycol and terephthalic acid) are made to react in order to obtain the PET resin. The resin, in the shape of small cylinders called pellets, is melted and injected into a mould to make a preform. The preform - a sort of test tube, shorter than the final bottle but with thicker walls - is then blow-moulded. During the blow-moulding phase, high-pressure air is blown into the preform allowing it to take the exact shape of the mould it is set into. The final product is, thanks to the molecular structure of the material, a transparent, unbreakable, resealable and lightweight bottle.
It is the strength of the material that contributes to the success of PET. Indeed, carbonated soft drinks can generate pressure inside the bottle reaching up to 6 bar. Such high pressure however, thanks to the alignment of macromolecules (crystallisation) occurring both during the resin spinning process and the blow-moulding process, is not capable of deforming the bottle nor can it make the bottle explode. Another advantage of the material lies in its physical properties that allow for great freedom in design.
Throughout the years, the PET industry has increasingly taken on environmental concerns, significantly decreasing the quantity of raw material needed for the manufacture of bottles. Nowadays, a 1.5 litre PET container is manufactured with just 35 grams of raw material.
Another striking feature of PET on the environmental side is that it is fully recyclable. In 1977, the first PET bottle was recycled and turned into a bottle basecup. Soon however, the fibre industry discovered the "new" material source and started using it for making textiles, carpets and non-wovens. Today, even though the "bottle to bottle" recycling process is growing, the fibre market is still the major outlet for recovered PET.
* Neumann, E H. Thermoplastic polyesters in Encyclopaedia of Packaging Technology, ed Bakker M. John Wiley, New York 1986.