New polymer can be recycled multiple times and upcycled without loss in performance

Introducing PDK, a new TLA you will want to add to your lexicon. It stands for poly(diketoenamine), a recyclable polymer developed by researchers at the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) that has the potential to square the circle in the circular economy. The material is described as recyclable plastic that, “like a Lego playset, can be disassembled into its constituent parts at the molecular level, and then reassembled into a different shape, texture, and color again and again without loss of performance or quality," according to a press release on the Berkeley Lab website. The research is profiled in the journal Nature Chemistry.

“Most plastics were never made to be recycled,” said lead author Peter Christensen, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry. “But we have discovered a new way to assemble plastics that takes recycling into consideration from a molecular perspective.”

poly(diketoenamine)
PDK monomers can be recovered and freed from any compounded additives simply by dunking the material in a highly acidic solution. Photo courtesy Peter Christensen et al./Berkeley Lab.

One of the obstacles to achieving a truly circular polymer—one that can be reused multiple times or upcycled into a new, high-quality product—are additives such as plasticizers that are tightly bound to monomers. When plastics with different chemical compositions are chopped up and melted to make a new material, it’s hard to predict the properties that the recycled material will inherit from the original plastics, explained the press release. “This inheritance of unknown and, therefore, unpredictable properties has prevented plastic from becoming what many consider the Holy Grail of recycling: a 'circular' material.” Uniquely, PDK contains reversible bonds that allow recovery of the constituent monomers from compounded additives simply by dunking the material in a highly acidic solution, explained the researchers.

As is often the case, this discovery was somewhat accidental. Christensen was applying various acids to glassware used to make PDK adhesives when he noticed that the adhesive’s composition had changed. Upon examination of the sample’s molecular structure, Christensen discovered that it was composed of the original monomers. Further research demonstrated that, in addition to breaking down PDK polymers into monomers, the acid bath also separated the monomers from entwined additives. The time-lapse video below, provided by Peter Christensen/Berkeley Lab, shows the degradation process.

The next step for the scientists was to prove that the material could be recycled into new plastic materials without inheriting the color or properties of the original materials. The press release gives the example of a broken black watchband, which otherwise would have been tossed in the trash, finding new life as a computer keyboard if it were made with PDK plastic or upcycled by adding additional features, such as flexibility.

The researchers are now looking at developing PDK plastics with a range of thermal and mechanical properties for applications as diverse as textiles, 3D printing and foams. In addition, they are looking to expand the formulations by incorporating plant-based materials and other sustainable sources.

“We’re at a critical point where we need to think about the infrastructure needed to modernize recycling facilities for future waste sorting and processing,” said Brett Helms, a scientist at Berkeley Lab’s Molecular Foundry who is leading the multidisciplinary team in this project. “If these facilities were designed to recycle or upcycle PDK and related plastics, then we would be able to more effectively divert plastic from landfills and the oceans. This is an exciting time to start thinking about how to design both materials and recycling facilities to enable circular plastics,” added Helms.

The technology is available for licensing and collaboration. Interested? Contact Berkeley Lab’s Intellectual Property Office.

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