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Endlessly recyclable materials could fix our plastic waste crisis


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One thing chemists do superbly is make bonds between atoms. We are now wading through the consequences of that success: plastic waste that ends up burned, landfilled or floating in the oceans. Plastics are polymers, long chains of molecules linked by strong chemical bonds. This is why they can be hard to degrade or recycle. Snipping apart those chemical bonds, to return to the small molecular building blocks, is often a tricky chemical problem.

There has been varying success in dealing with the main plastics we use. The low-hanging fruit is polyethylene terephthalate (PET), which is used to make plastic bottles. It can simply be shredded and remoulded into fresh bottles. No chemists need apply.

It is a different story with most other important plastics. Take polyvinyl chloride (PVC), which is ubiquitous in double-glazed windows and plenty besides. “PVC’s an absolute nightmare,” says chemist Anthony Ryan at the University of Sheffield, UK. There is no known way to recycle it, and even if you did, you would end up with vinyl chloride, a toxic compound that can increase the risk of cancer.

One job for chemists, then, is to devise new reactions that can break plastics into molecules that can be reused. Susannah Scott at the University of California, Santa Barbara, has recently had success doing this with polyolefins, a class of plastic that includes polyethylene. She developed a technique that uses a catalyst to break down these plastics into smaller molecules without having to use bucketloads of heat. These smaller molecules could be used in detergents, paints or pharmaceuticals.

We also need to design new plastics and plan from the start what will happen to them after they come to the end of their life. Chemists are starting to invent plastics that can be recycled infinitely or that break down into materials that nourish the soil.

One example is the plastic devised by Ting Xu at the University of California, Berkeley. Xu added tiny enzyme-containing capsules to the plastic. The material can be processed, heated and stretched into useful objects. But when its life is over, all you need do is soak the stuff in lukewarm water for a week or so. This releases the enzymes, which digest the plastic into small molecules. We will need plenty of new materials like this if we truly want to eliminate the scourge of plastic waste.

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