Chemical recycling of plastic: Better living through chemistry


Chemical recycling plastic

Chemical recycling of plastic can deliver massive advantages, but it’s a technology in its infancy and needs scaling and standardising to be competitive. Ian Farrell looks at some of the technologies and processes emerging from the lab.

Plastic recycling is a mature, if somewhat crude, technology. Waste plastic is collected from various sources and sorted according to the polymer from which it’s made: PET, HDPE, polystyrene, etc.

It’s cleaned to remove any contaminants – usually by washing or shredding – before being melted into small pellets. These pellets are then used by plastic manufacturers to make plastic products, including food packaging, children’s toys, and even car parts.

Even though this mechanical recycling process has been around for a while, and has been continuously tweaked and improved, it’s not without its limitations.

Perhaps the most significant of these is the deterioration in quality that accompanies the process, meaning that high-grade plastic from, say, a drinks bottle may not be able to be used for this purpose again. Low-grade plastics may not be able to be recycled at all and must be incinerated or sent to landfill.

If you put a mixture of plastics into a chemical recycling process, you are going to get a mixture of monomers out the other end.

The problem is caused chiefly by the melting process, which causes some of the long polymer chains in the plastic to break down. This causes reduced strength and durability and can affect colour and translucency.

If only there was another way. Enter, stage left: chemical recycling. Instead of simply melting and reforming the sorted and cleaned plastic, some chemical recycling processes break down the material into its component monomers – the chemical building blocks that were joined together to make the long polymer chains that give plastic its unique physical properties.

Once these monomers are back in their original form, they are indistinguishable from virgin chemicals, which predominantly come from fossil fuels. This means they can be used in the same manufacturing processes that create brand-new plastic and give us a material that is just as good.

Chemical recycling can actually improve the quality of plastic, instead of degrading it. Take, for example, PET – also known as polyethylene terephthalate – which is used in everything from food packaging to clothing (where it’s simply called “polyester”).

plastic polymer granules
Chemical recycling can actually improve the quality of plastic, instead of degrading it, Ian Farrell, editor of Circular magazine, writes.

It’s made by reacting together two organic chemicals – terephthalic acid (TA) and ethylene glycol (EG) – to form very long polymer chains of alternating TA-EG groups. The chemical recycling of PET simply takes us back to the individual TA and EG molecules, which can then be polymerised again or used for something else entirely.

“I know purists would say that strictly speaking, recycling and circularity should mean returning a material in the same form in which it was collected, but this is the beauty and flexibility of chemical recycling – you have the choice,” says Dr Stuart Wagland, a researcher at Cranfield University, who has a special interest in recycling plastics. “The monomers could also be used as chemical building blocks in other industrial processes, such as pharmaceuticals.”

Wagland is keen to point out that chemical recycling is not a magical “black box” into which we can simply tip the contents of a kerbside wheelie bin. Plastics still need to be sorted and cleaned, and the conditions of the recycling process carefully monitored to ensure the right products.

“If you put a mixture of plastics into a chemical recycling process, you are going to get a mixture of monomers out the other end,” he warns.

“You have to sort somewhere – either at the start of the process or the end. Depending on the ingredients, it may well be easier to separate waste according to plastic type, just as you do for mechanical recycling.”

There is more than one way to break down a polymer into its constituent monomers. One of the best-known is pyrolysis, sometimes also called thermal cracking. It’s different from combustion because it takes place in an inert atmosphere without oxygen.

Temperatures between 250°C and 700°C are used to break apart the polymer, producing a mixed organic oil that must be purified to get the pure polymers we need, because pyrolysis is not a selective process; any organic material present is decomposed.

However, this lack of selectivity means that pyrolysis can be used on mixed waste streams, where plastic types cannot be separated beforehand – multilayer films, for example.

Much research effort is currently being spent looking at ways to use catalysts in pyrolysis processes to reduce the temperatures needed to break down the polymer chains – increasing the yields of valuable products while also requiring less energy.

“If a waste stream can be separated and purified before the recycling process, the thermal decomposition can be more controlled.”

If a waste stream can be separated and purified before the recycling process, the thermal decomposition can be more controlled.

For very pure waste streams the critical temperature at which certain polymers decompose into their monomer can be found. And because this polymer is the only material present, we don’t end up with as many of the impurities found in basic pyrolysis of a mixed feedstock.

Hydropyrolysis (sometimes called hydrocracking) does away with the inert atmosphere of other pyrolysis processes and uses steam at 350-500°C instead. The addition of hydrogen, from the water, improves the quality of the end products.

All of the above methods require solid waste plastic, but that’s not the only way of getting to the polymer chains. If we can dissolve the polymer chains in a solvent, creating a solution, it’s easier to break them apart selectively, giving us the monomers we want. These solvents can be anything from water (the process is then called “hydrolysis”), alcohol (“alcoholysis”) or amines (“aminolysis”).

Using high temperatures and pressures (and, consequently, a lot of energy) we can also use so-called supercritical water, which takes on special properties that make it an excellent solvent for this process.

Some researchers are even investigating ways to dissolve polymers in super acids – extremely strong acids that are then recovered and used again after the recycling process is complete. These techniques can also be used with naturally occurring polymers, such as lignin and cellulose, opening up new ways of recycling wood from waste streams.

The potential of this type of chemistry is undeniably huge, but it is a new technology compared with more traditional mechanical recycling. More research into how to scale chemical recycling is now needed before the waste and resources industry can reap the benefits.

Many of the processes are energy-intensive and could produce harmful by-products. Life-cycle analysis could help us understand these limitations and make a more meaningful comparison with mechanical recycling.

As well as scaling, future research could focus on the efficiency of processes to recover monomers from the products of depolymerisation and catalyst technologies, which reduce the amount of energy required.

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