A new process uses cheap catalysts to break down polyethylene and polypropylene into their building blocks, enabling common plastic objects to be turned into new products.
Plastic bottles and bags can be vaporised into chemical building blocks and turned into new plastics with all the properties of virgin material. There are hurdles still to overcome, but the new process is a big step towards a truly circular economy for plastic.
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| Polyethylene plastic bags are challenging to recycle Robert Sanders/UC Berkeley |
Around 5 billion tonnes of plastic has gone to landfill since the 1950s, and recycling efforts have only tackled 9 per cent of what we’ve produced. With current techniques, plastics degrade in each recycling round and end up in landfill after only a few cycles through this process.
John Hartwig at the University of California, Berkeley, and his colleagues had previously developed a process that breaks down waste plastic into its constituent parts, but it relied on expensive metal catalysts iridium, ruthenium and palladium, which were irrecoverably lost as part of the process. Hartwig says that the technique was “OK for an academic paper, for demonstration purposes, but nowhere near what you would need for something that could be conceived of ever becoming industrial.”
Now, his team has discovered an improved process that works on both polyethylene, from which most plastic bags are made, and polypropylene, which is used to make harder objects, and it relies only on catalysts considered so common that they are essentially “dirt”, says Hartwig.
Plastics consist of large molecules called polymers, which are made up of smaller units called monomers bonded together. The catalysts break the chemical bonds of polymers, turning them into gaseous monomers from which new plastics can be pieced together with all the properties of virgin material that has never been recycled.
In experiments, the team used two catalysts, sodium on aluminium oxide and tungsten oxide on silica, to turn a mixture of polyethylene and polypropylene into the monomers propylene and isobutylene with an efficiency of nearly 90 per cent.
Benjamin Ward at Cardiff University, UK, who wasn’t involved in the research, says recycling plastics is made harder by thousands of additives such as dyes, fire retardants and plasticisers, which can make up as much as a third of a finished product and contaminate the end product after recycling. “It defers the landfill. It defers the environmental problem. But it doesn’t prevent it altogether,” he says.
Ward believes this new process solves the additive problem, as stripping material down to its constituent gaseous monomers also removes the additives.
Hartwig warns that there are still many hurdles to overcome, and that the process has only been tested in the presence of a small number of common additives. “There will be additives that… will poison, will inhibit the catalyst,” he says. “We need to either find a way to separate those, which is maybe not optimal, or to find different catalyst structures or compositions that will be more resistant to some of those additives. That is absolutely a challenge.”
Cressida Bowyer at the University of Portsmouth, UK, says that even when we have a process that can split waste plastic into constituent parts and withstand additives, there are still additional concerns. “Toxicity and disposal of recycling end products [such as catalysts and additives] must be taken into account. These could outweigh any perceived benefits of recycling technologies,” she says. “Recycling should not be seen as any kind of solution or rationale to maintaining or increasing production of single-use and unnecessary plastics and continuing the current prevailing take-make-waste culture.”
Journal reference:
Science DOI: 10.1126/science.adq7316