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New plant-based plastic can be printed—and fully recycled

Could this be the chemists’ answer to the plastic problem chemistry helped create?
February 25, 2021

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About 12 billion metric tons of plastic waste will be sitting in landfills or in the natural environment by 2050. To stop that deluge, and its associated carbon emissions, we’ll need many solutions. But a key one would be plastics that are not only made from sustainable non-petroleum sources but are also easy to recycle back into high-quality plastic.

A new plastic made by researchers at the University of Konstanz in Germany could be just the answer. It is a type of polyethylene—the world’s most commonly used plastic—made from plant and microalgae oils, and that can be recycled with near-perfect efficiency. The recycling method, also reported in the journal Nature, requires relatively low temperatures, making it more energy-efficient, and also recovers 96 percent of the starting material.

Polyethylene is the stuff of shopping bags and water bottles. The world makes millions of tons of it every year but a small percentage is recycled. That’s partly due to lacking recycling infrastructure, but also because today’s mechanical recycling techniques which involve chopping up the plastic and then melting and reforming it give material of lower quality. So recycled polyethylene is often used to make products such as rugs instead of reformed into plastic water bottles.

But there’s an alternative recycling technique called chemical recycling. This technique involves breaking the bonds in plastics, which are long chains made of smaller, simpler molecules, to separate those smaller molecules that can be restrung into new high-quality plastic. The problem is this process requires temperatures over 600°C, making it energy-intensive. And it recovers a limited amount of the starting material, sometimes less than ten per cent.

Stefan Mecking and his colleagues made their new bio-based plastic from oils that come from plants or algae, which they can convert into the building block molecules of plastic. The polyethylene-like plastic contains a small number of predetermined chemical links along its chains. It retains the physical properties of polyethylene during regular use, and it can be dyed and strengthened with the addition of carbon fibers. And when needed it can be easily deconstructed by undoing those chemical links.

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Breaking down the plastic requires temperatures of only 120°C, the researchers report, and the process allows them to recover most of the starting material. Plus, the recycled polyethylene made with the reclaimed material are as good as the original plastic. What’s more, the recycled material can be used for 3D printing, which the research team demonstrated by 3D-printing a strong yet supple phone case.

In a related News & Views article, Oxford University chemists Charlotte Williams and Georgina Gregory caution that this is still early-stage research, and that several engineering challenges will have to be solved for real-world success. That includes finding ways to produce the bioplastic at large scales and developing industrial methods for making and recycling the plastic. And most importantly the plastic will have to match the low cost of its petroleum-derived cousin.

Nevertheless, they write that the work is “exciting and inspiring, because it is extremely challenging to come up with plastics that can be derived from renewable resources, have outstanding properties, are compatible with large-scale manufacturing and processing techniques, and are fully recyclable—few materials meet all these criteria.”

Source: Manuel Häußler et al. Closed-loop recycling of polyethylene-like materials. Nature, 2021.

Image: AG Mecking, University of Konstanz



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