Recycling plastic with enzymes

French biotechnology start-up Carbios has developed an enzymatic process for recycling polyethylene terephthalate (PET) back into its constituent monomers, from which new plastic can be produced with the same properties as virgin material.

‘Everybody believes PET is easily recyclable because bottles are easily recyclable in a mechanical way,’ says chief executive Emmanuel Ladent. ‘But food trays, for the most part, are not recyclable; most cosmetics packaging is not recyclable; the vast majority of polyester fibres, which are also PET, are not recyclable. So the vast majority of PET waste today is not easily recyclable.’

Rather than necessarily compete with mechanical recycling processes, such as those used for bottles, Carbios is targeting these more problematic waste streams. ‘Recycling polyester fibres is a huge market by itself. Two thirds of PET is actually polyester fibres,’ explains Ladent.

Chief scientific officer Alain Marty explains that the prospect of finding an enzyme that could break down the ester bonds within PET polymer chains was initially daunting. ‘I had a lot of colleagues working on cellulose. It’s a nightmare to degrade cellulose, and I was thinking that degrading PET with an enzyme would certainly be worse.’

The team started from an enzyme described by a Japanese research group in 2012, evolved to break down the waxy cutin later on plants’ leaves. This could break down a small amount of PET, but required large amounts of enzyme and took several weeks. Through a process of directed evolution, the team produced better enzymes that work faster, and could withstand higher temperatures. ‘We fully redesigned the active site,’ says Marty.

PET can be crystalline or amorphous, Marty explains, but the enzymatic degradation is much more effective on amorphous forms, so Carbios has also developed an extrusion pre-treatment process to ensure the material going into the enzymatic reactor is amorphous. ‘We end up with expanded pellets of amorphous PET with a high surface of exchange between the polymer and the enzyme,’ he says.

Boosting the enzyme’s thermal stability helps to speed up the reaction kinetics, but also to run the process close to the glass transition temperature of PET, which is about 70°C. ‘That allows us to take advantage of the polymer chains being more mobile,’ says Marty. However, PET can also recrystallise at these temperatures, so the process needs to be carefully controlled to minimise that competitive process.

At the end of the reaction, the liquid terephthalic acid and mono-ethylene glycol monomers can be separated, purified and sent directly for remanufacturing. ‘Almost all of the world production of PET uses these monomers,’ says Ladent. That means a recycling plant could very easily be coupled directly to existing PET production plants. And because the enzyme selectively breaks down PET, any small amounts of other polymers that are mixed in, such as polyamides or polyolefins, as well as any colours and additives, are left behind and can be easily filtered out. ‘That means we don’t need to have such an intensive sorting process at the beginning – the enzyme will do that itself,’ adds Marty.

Ladent describes how Carbios began scaling up its laboratory process in 2018. ‘In 2021, we constructed a demonstration plant in central France, built to be scalable.’ The company is now setting up a joint venture with chemicals producer Indorama Ventures to build a commercial-scale biorecycling plant adjacent to Indorama’s existing PET plant in Longlaville, close to the border with Luxembourg. The project will be funded by contributions from the two companies, as well as some support from the French government.

‘This first plant is going to be our own investment,’ says Ladent, but beyond that, Carbios intends to license its technology to partners worldwide. ‘If we want to minimise our carbon footprint and water consumption, it means processing waste locally, and producing the monomers which are used in the loop by the local industry,’ he adds. It’s also a question of speed: ‘If we want to deploy our technology with our own money, with our own fundraising, it will take decades. We don’t have that much time for the planet – we need to go fast!’

Regulations and demand from consumer brands are also pushing for greater recycling. The EU has set aggressive targets for incorporating 30% recycled material into all plastic packaging by 2030, and those targets are widely expected to increase relatively rapidly thereafter. ‘That kind of regulation is starting to be copied in some US states and in Asia,’ observes Ladent. ‘And more importantly, perhaps, many brands are making commitments that go beyond those regulations, in terms of incorporating more recycled content and creating more circular ways of managing waste. So the industry is pushing hard to go faster and recycle more products.’