Fernando Gomollón-Bel discovers the story of one of the most ubiquitous artificial compounds
Find a plastic bottle. There’s probably one not far from you. Now look at the bottom of it: do you see a little triangle with a number one inside? Well, that means the bottle is made from the compound polyethylene terephthalate. Yes, it’s a bit of a mouthful. That’s why lazy chemists usually call it PET.
PET is a polymer – a macromolecule made out of thousands of repeating monomer units. To produce PET, chemists use two different kinds of monomers: terephthalic acid and ethylene glycol. Both units undergo a reaction called esterification, where an organic acid and an alcohol combine to form an ester and water. But PET is not a normal ester – it’s a polymer, a poly-ester – and its properties are very different from other esters.
PET can be spun into fibres. Check the labels on your sport clothes – if they say ‘polyester’ then it’s probably PET (or maybe another close family member). But PET is a versatile plastic – it can be crystallised into a very light, resistant material, ideal for making bottles. It’s cheaper than glass, it’s flexible, less fragile, and it can also be reused thousands of times. It’s also much lighter than glass and costs less to transport. It makes sense that PET bottles are now everywhere.
But there’s a significant downside to PET’s properties: it’s very hard to break down, so it’s resistant to biodegradation. Luckily it’s also among the most easily recycled polymers, so as long as people recycle their bottles instead of throwing them away, PET can be reused many times.
The easiest way to recycle PET is to chop bottles (and any other stuff made of PET) into small flakes and pellets. This waste product is quite valuable, because it can be melted down and used to create new containers.
There’s also a chemical way to recycle PET. Chemists know how to break down the ester bonds that hold PET together and obtain the monomers again. This process involves a couple of steps. First comes partial glycolysis, where the polymer is treated with ethylene glycol. This breaks the long polymer chains down into shorter chains, called oligomers, which melt at lower temperatures and so can be filtered to remove any impurities. Then, once the material has been purified, it’s completely broken down into the monomers, which are then purified again by distillation. Once the monomers are obtained, they can be used to produce new, shiny PET.
The final way to recycle PET goes back to making fibres. Used bottles can be turned into fabrics. This recycled material has become popular for making carpets, and in 2010, Nike reportedly recycled 13 million plastic bottles to make the shirts of 10 national football teams, including England.
A new ecosystem
But not all PET is recycled – there are already vast amounts of it in landfills across the world. And although it’s not traditionally biodegradable, some bacteria living in landfill sites have evolved to eat PET and use it as their main carbon source. Perhaps it was inevitable – in an environment where many species compete to use the smelly, rotten organic matter, the mountains of PET provide a new ecological opportunity – an untouched buffet of bottles.
But the days of these ingenious bacteria may be numbered. In 2009, the Australian town of Bundanoon banned the sale of bottled water. Similar bans followed in Concord, Massachusetts, many areas of Canada and San Francisco. Very recently, France passed a ban on all disposable plastic cutlery, plates and cups. Although this will benefit ecosystems in the long term, it may prove that for those dump-dwelling bacteria, adapting to eat PET is an evolutionary dead end.