A team of researchers at the US Department of Energy's Lawrence Berkeley National Laboratory has designed a recyclable plastic that can be disassembled into its principal parts at the molecular level, and then reassembled into a different shape, texture, and color again and again without loss of performance or quality. The new material, called polydiketoenamine, or PDK, was reported 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."
Christensen was part of a multidisciplinary team led by Brett Helms, a staff scientist in Berkeley Lab's Molecular Foundry. According to the researchers, the problem with many plastics is that the chemicals added to make them useful are tightly bound to the monomers and stay in the plastic even after it's been processed at a recycling plant.
"Circular plastics and plastics upcycling are grand challenges," he said. "We've already seen the impact of plastic waste leaking into our aquatic ecosystems, and this trend is likely to be exacerbated by the increasing amounts of plastics being manufactured and the downstream pressure it places on our municipal recycling infrastructure."
Unlike conventional plastics, the monomers of PDK plastic could be recovered and freed from any compounded additives simply by submerging the material in a highly acidic solution. The acid helps to break the bonds between the monomers and separate them from the chemical additives that give plastic its look and feel.
"We're interested in the chemistry that redirects plastic lifecycles from linear to circular," said Helms. "We see an opportunity to make a difference for where there are no recycling options." That includes adhesives, phone cases, watch bands, shoes, computer cables, and hard thermosets that are created by molding hot plastic material.
After testing various formulations at the Molecular Foundry, they demonstrated that not only does acid break down PDK polymers into monomers, but the process also allows the monomers to be separated from entangled additives. Next, they proved that the recovered PDK monomers can be remade into polymers, and those recycled polymers can form new plastic materials without getting the color or other features of the original material. The researchers believe that their new recyclable plastic could be a good alternative to many non-recyclable plastics in use today.
The researchers 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.
