Scientists have designed a recyclable plastic called poly(diketoenamine)s, or PDKs. Like a Lego playset, manufacturers can disassemble it into its fundamental parts at the molecular level. All plastics are made up of molecules called polymers, which are composed of repeating units of shorter compounds called monomers. But the chemicals in many plastics that make them useful are tightly bound to the monomers. They stay in the plastic even after it’s been recycled. In contrast, scientists can recover and free the monomers of PDK plastic from each other and additives simply by dunking it in a highly acidic solution. Manufacturers can then reassemble the plastic into a different shape, texture, and color again and again without loss of performance or quality.
The PDK plastics can be recycled many times without a loss in performance or aesthetics. Even the most recyclable plastic is only recycled at a rate of 20-30%. During processing, plastics with different chemical compositions are mixed together and ground into bits. When that hodgepodge is melted to make a new material, it’s hard to predict which properties it will inherit from the original plastics. As a result, plastics can’t be reused continually or “upcycled” to make a new, higher-quality product. The new PDKs are easy to make and can be re-manufactured without loss of performance. PDKs could turn plastic into a “circular” material that is infinitely recyclable. Fully recyclable plastic provides a path for reuse as an alternative to incinerating or placing in landfills, where it takes centuries to decompose.
The researchers first discovered the exciting circular property of PDK-based plastics when applying various acids to glassware used to make PDK adhesives and noticed that the adhesive’s composition had changed. Curious as to how the adhesive might have been transformed, they analyzed the sample’s molecular structure and found they were the original monomers. After testing various formulations at the Molecular Foundry (a DOE Office of Science user facility), they demonstrated that not only does acid break down PDK polymers into monomers, but the process also allows the monomers to be separated from entwined additives.
They also proved that the recovered PDK monomers can be remade into polymers, and those recycled polymers can form new plastic materials without inheriting the color or other features of the original material – so that broken watchband you tossed in the trash could find new life as a computer keyboard if it’s made with PDK plastic. They could also upcycle the plastic by adding additional features, such as flexibility.
The researchers next plan to develop PDK plastics with a wide 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.
Molecular Foundry, Berkeley Lab
The technical scope of this work was supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under US Department of Energy. Portions of this work, including organic and polymer synthesis and characterization, were carried out as a User Project at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy.
P.R. Christensen, A.M. Scheuermann, K.E. Loeffler, B.A. Helms. “Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds“ Nat. Chem. 11, 442-448, (2019). [DOI: 10.1038/s41557-019-0249-2]
Plastic Gets a Do-Over: Breakthrough Discovery Recycles Plastic from the Inside Out, Lawrence Berkeley National Laboratory press release