Mechanical recycling degrades plastic quality each cycle ('downcycling'). Enzymatic recycling offers a closed loop: engineered enzymes break PET (polyethylene terephthalate, the plastic of bottles and polyester) back into its building-block monomers, which can be repurified and re-polymerised into virgin-quality plastic — indefinitely.
Working principle
Specialised enzymes called PET hydrolases (e.g. engineered variants of leaf-branch compost cutinase, or IsPETase from a PET-digesting bacterium) catalyse hydrolysis of the ester bonds in the polymer chain. This cleaves PET into terephthalic acid (TPA) and ethylene glycol (EG). The reaction runs in water at mild temperatures (around the polymer's glass transition), consuming far less energy than chemical depolymerisation. Protein engineering boosts the enzyme's activity and thermostability.
| Route | Output quality | Conditions |
|---|---|---|
| Mechanical | Degrades each cycle | Melt, simple |
| Chemical (glycolysis) | Monomers | High temp / catalyst |
| Enzymatic | Virgin-grade monomers | Mild, aqueous, low energy |
Why it mattersEnzymatic recycling enables a true circular economy for PET — infinite recycling without quality loss. Enzyme cost, reaction rate and tolerance to mixed/coloured waste are the scale-up hurdles.
Applications
- Bottle-to-bottle and textile-to-textile closed-loop recycling
- Processing coloured and mixed PET that mechanical recycling rejects
- Recovering value from polyester textile waste
References & further reading
- Tournier et al., “An engineered PET depolymerase to break down and recycle plastic bottles,” Nature, 2020.
- Yoshida et al., “A bacterium that degrades and assimilates poly(ethylene terephthalate),” Science, 2016.
- Lu et al., “Machine learning-aided engineering of hydrolases for PET depolymerization,” Nature, 2022.