Ellen Phiddian
Cosmos science journalist
Chemists have developed a simple way to turn a common type of plastic (PET) back into virgin material – using iron and alcohol.
Importantly the method is selective, meaning it could recycle the PET in mixtures of plastics and textiles.
The Japanese and Malaysian researchers have published their work in Industrial Chemistry and Materials.
Polyethylene terephthalate (PET) is a common polyester plastic, used widely in textiles and drink containers.
While PET water bottles can be recycled by melting and reforming, this can yield lower quality plastic, and requires careful separation of the PET from all other materials.
Mixed waste and polyester textiles can’t be easily recycled using this method. Fabrics made from mixtures of fibres (like poly-cotton blends) are particularly difficult to recycle, because individual polyester and other fibres must be unravelled first.
Instead, chemists have been working on ways to break up PET molecules. Like all plastics, PET is a polymer – made of long chains of repeating units called monomers.
Chemists have been investigating depolymerisation reactions that can turn the PET back into re-usable monomers, but most known methods require high industrial temperatures and corrosive acids or bases.
These researchers’ new method can be done at under 200°C (low by industrial standards), and uses just alcohol and a cheap iron-based catalyst.
The researchers found that mixing PET with ethanol and ferric chloride (FeCl3) or iron(III) bromide (FeBr3) for a couple of days, at 160-180ºC, yielded an almost-pure mixture of the monomers used to make PET.
These monomers – diethyl terephthalate and ethylene glycol – could be used to make more high-quality PET, or they could be used in other materials.
The ferric chloride catalyst worked particularly well, so the researchers trialled it on a waste textile that was 65% PET and 35% cotton.
They could separate out the monomers from the textile, yielding pure cotton and pure monomers in 16 hours.
They could also selectively remove PET from textile blends of different plastics.
In their paper, the researchers point out that ferric chloride is a cheap catalyst already widely used in industry.
“We believe that the method could provide a possibility of a clean chemical recycling process in the presence of a commercially available catalyst,” they write.
The team is now investigating whether they can achieve the same results in milder conditions.
