There are very few plastics that can be recycled into reusable products due to various additives such as dyes and flame retardants. Polyethylene terephthalate (PET) is the most recyclable plastic, yet is only recycled at a rate of 20 to 30 percent.
In a groundbreaking new study, researchers at the U.S. Department of Energy’s Berkeley Lab have designed a recyclable plastic that can be disassembled into its components at the molecular level. As a result, the plastic can be reconstructed into a different shape and color repeatedly, like a Lego set. The new material, polydiketoenamine (PDK), can be broken down and reassembled without the loss of performance or quality.
“Most plastics were never made to be recycled,” said study lead author Peter Christensen. “But we have discovered a new way to assemble plastics that takes recycling into consideration from a molecular perspective.”
According to the researchers, the problem with many plastics is that the chemicals added to make them useful, such as the fillers that make plastic tough, are tightly bound to the monomers and stay in the plastic even after it has been processed at a recycling plant.
When plastics of different chemical compositions are mixed together during processing at plants, it is hard to predict what properties the recycled plastics will inherit. This prevents the new products from being “circular,” which is ideal because it means they can be reused and upcycled to make higher quality products.
“Circular plastics and plastics upcycling are grand challenges,” said Berkeley scientist Brett Helms. “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.”
“With PDKs, the immutable bonds of conventional plastics are replaced with reversible bonds that allow the plastic to be recycled more effectively,” explained Helms.
Unlike conventional plastics, the monomers of PDK plastic can be freed up from any compounded additives by simply exposing the material to a highly acidic solution. The acid breaks the bonds between the monomers and separates them from the chemical additives.
“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.”
The experts demonstrated that the recovered PDK monomers can be remade into polymers, and those recycled polymers can form new plastic materials which do not inherit the features of the original plastic. The researchers believe that PDKs will be a good alternative to many non-recyclable plastics used today.
“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 Helms. “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.”
The research is published in the journal Nature Chemistry.