How can we recycle plastics?

Header photo by Catherine Sheila from Pexels

Every day, 20.5 million plastic bottles enter the recycling system in the UK.

Those plastic bottles, made of polyethylene terephthalate (PET), may one day be used over and over after being recycled back into its monomers thanks to French company Carbios’s PET-eating enzymes. With the EU’s intent for all plastic packaging to be recyclable by 2030, that technology is revolutionary and could produce plastic just as perfect as its virgin counterpart- unlike the systems currently in place.

Recycling doesn’t have to be a mystery

Recycling seems mystical, but it’s simple when you peer behind the curtain. For most households, waste is sorted using magnets and air jets into their vague categories when it reaches the recycling facility.

It’s easy to separate aluminium cans from newspapers purely by their weight, with paper being easily removed using air jets and aluminium being removed in response to a powerful magnetic field.  

Plastics are more complicated to sort, but the difference in density between polyethylene (PE) milk bottles and polyethylene terephthalate (PET) water bottles can be exploited: if their ground-up particles are placed in water, PET will sink, and PE will float.

How plastics get sorted in a recycling plant in Germany. Credit to VDI Zentrum Ressourceneffizienz.

From there, the plastic can be sorted by colour and melted again, ready to be reformed into a new plastic water bottle. The problem with this process? It’s super energy-inefficient and the process degrades the plastic, making the final product far less desirable.

PET require relatively high temperatures of around 260°C to melt completely; those high temperatures also tend to break the carbon chain, reducing the molecular weight of the polymer which so many of the plastics’ qualities depend on. This chain scission yellows the plastic and hugely increases its flexibility, making it flimsy and less desirable by product designers.

What if we broke plastic down into its monomers?

Recycled PET rarely meets product specifications under current mechanical processes, but chemical recycling may eliminate this degradation issue altogether.

Chemical recycling involves the breaking of the chain links holding the plastic together, cleaving the super-long molecules back into their component materials. This isn’t an entirely new process; glycolysis has been readily used in industry to break PET back into its component terephthalic acid (PTA) using its other monomer, ethylene glycol (MEG).

Glycolysis involves the insertion of the glycol molecule into the chain of PET, swapping itself with the rest of the chain to break the super-long molecules first into much shorter chains, then dimers comprised of two chain links, and finally monomers containing only one.

This process is super-efficient but slow, since the glycol needs to diffuse into the bulk of the plastic before it can begin to degrade. Zinc salts or ionic solutions can speed this up by hours, but this still isn’t an ideal process for post-consumer waste.

Plastics meant for the public often have dyes or copolymers added to them to adjust their colours and properties. Blue or green plastic bottles may attract our attention far more than their clear counterparts, but those dyes and other impurities interact with the glycol used to chemically recycle the plastic, making this process useless for waste that we put in a recycling bin. Instead, glycolysis is widely used for post-industrial waste with far less contamination than its used counterparts.

Enzymes: nature’s catalysts

If we are ever to chemically recycle our plastics, enzymes are the way forward. After an enzyme able to break down PET into monomers was found outside a Japanese plastic bottle recycling facility in 2015, many have been hopeful it could be engineered for use in industrial processes- including Professor John Ward of UCL’s Department of Biochemical Engineering.

“If the enzymic depolymerisation can be done to release pure monomer then for certain plastics e.g. polyesters, then the monomers can be made back into the same plastics and not downgraded,” he said.

French bioengineering firm Carbios have claimed to do just this, engineering PETase to break down PET (or any similar polyester) into recycled terephthalic acid (rPTA) which can be purified and reused as a monomer. In February this year they demonstrated that this rPTA could make PET bottles just as pure as its virgin twin made with crude oil.

This breakthrough is huge, since terephthalic acid is usually produced from ethylene, a waste product from crude oil cracking. The production of terephthalic acid contributes to at least 49,500 tonnes of carbon dioxide emissions each year; a closed-loop process would not only limit these emissions, but also unlink plastic bottles from dwindling crude oil production.

The process is not yet perfect

Terephthalic acid is not the only component of PET; ethylene glycol, the degradation agent used in glycolysis, is also an essential monomer. This process only claims to use recycled terephthalic acid, but this does not eliminate the need for ethylene glycol and, much like current terephthalic acid production, glycol is usually made from crude oil.

The biggest downside of this method: only condensation polymers with ester or amide links can be depolymerised like this. Polyethylene milk bottles and polypropylene lipstick tubes are purely made of carbon and hydrogen, so the pure polymers have no ‘handle’ for enzymes to hold onto and break them into their monomers.

“For other plastics… the released compounds may not be able to be depolymerised but could be feedstocks for chemical synthesis and not burned”, Ward said. 

The process is far more complex than meets the eye, and very precise conditions are needed for the catalytic protein to do its job. “Enzymes will need to be made very cheaply and/or be secreted by bacteria or microorganisms that also have enzymes and pathways that can modify the monomers and convert them to other useful compounds,” Ward adds.

“There could be an integrated system where the depolymerisation and building of new chemicals can happen in the same reactor.”

Enzymatic recycling has historically been unviable for smaller or medium processors due to the sheer costs of the process and the volume of processing needed to make a profit. The pilot plant under construction in Lyon, set to open as early as late 2020, will demonstrate if this process is viable at commercial levels. 

With the EU’s circular economy deadline in 2030 looming, this kind of recycling plant is welcome innovation to start the clean-up of our waste.


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