New plastic closes the recycling loop

An astounding 91 percent of plastic is not recycled. Part of the reason for the world’s dismal recycling rate is that lack of collection and recycling infrastructure. Another reason is that recycled plastic is usually of a lower quality.

Researchers at the Lawrence Berkeley National Laboratory have now made a new plastic that can be broken down into its basic chemical building blocks and then rebuilt again and again without any loss in quality.

This means that the plastic, reported in Nature Chemistry, could be recycled in a closed loop. So less new material would have to be manufactured to create products. And by making plastic waste valuable, it would encourage recycling and divert plastic from landfills.

Making such a “circular plastic” is a grand challenge, staff scientist Brett Helms, who led the team, said in a press release. “We’re interested in the chemistry that redirects plastic lifecycles from linear to circular,” he said. “We see an opportunity to make a difference for where there are no recycling options,” such as phone cases, watch bands, shoes, and computer cables.

Plastics are polymers: very large molecules that are chains of smaller repeated building blocks called monomers. Manufacturers add chemicals to plastics to tweak their properties, like color or toughness. During recycling, plastics are mixed together and chopped into little pieces that are melted to make new material. The additives are difficult to remove, so it is hard to control the properties of the final reprocessed product. This is why recycled plastic is usually not used make a high-quality product, and cannot be recycled over and over.

The new polymer, which Helms and his colleagues call poly(diketoenamine)s (PDKs), is made of monomers that click together like Legos with chemical bonds that are reversible. Pulling apart monomers and separating the additives requires simply soaking the plastic in a sulfuric acid bath.

The mechanical properties of PDKs are comparable to the thermoplastics used in products such as cellphone cases and shoe soles. The team is now working on PDK plastics with different properties for various uses, and they are trying to incorporate renewable plant-based materials and other sustainable sources.

Source: Peter R. Christensen et al. Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds. Nature Chemistry, 2019.

Image: Dock and Bay/giphy

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