Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory in northern California have created a next-generation plastic that can be recycled again and again, into new materials of any color, shape, or form.
According to the EPA, as of 2015, Americans recycled less than 10 percent of the plastics they’ve used. This isn’t just down to laziness, many plastics simply cannot be effectively recycled. Even the most recyclable plastic, PET – or polyethylene terephthalate – is only recycled at a rate of 20-30%, with the rest typically going to incinerators or landfills, where the carbon-rich material takes centuries to decompose.
All plastics, from beverage bottles to automobile parts, are made up of large molecules called polymers, which are composed of repeating units of shorter carbon-containing compounds called monomers. Often, the chemicals added to plastics to give them specific characteristics — rigidity or flexibility, for example — bind so tightly to these monomers that they remain in the plastic even after it’s been processed at a recycling plant.
During processing, plastics with different chemical compositions – hard plastics, flexible plastics, clear plastics, candy-colored plastics – are mixed together and ground into bits. When that mishmash of chopped-up plastics is melted to make a new material, it’s difficult to predict what properties it will inherit from the original plastics.
Now a team of researchers has designed a recyclable plastic that 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. The creation of 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 Laboratory’s Molecular Foundry. “But we have discovered a new way to assemble plastics that takes recycling into consideration from a molecular perspective.”
“Circular plastics and plastics upcycling are grand challenges,” Christensen 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.”
The researchers want to divert plastics from landfills and the oceans by encouraging the recovery and reuse of plastics, which could be possible with polymers formed from PDKs.
“With PDKs, the immutable bonds of conventional plastics are replaced with reversible bonds that allow the plastic to be recycled more effectively,” said Brett Helms, a staff scientist in Berkeley Lab’s Molecular Foundry and leader of the team that made this discovery.
Unlike conventional plastics, the monomers of PDK plastic could be recovered and freed from any compounded additives simply by dunking the material in a highly acidic solution. The 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.”
A surprising number of everyday items including, phone cases, watch bands, shoes, computer cables and hard thermosets are created by molding hot plastic material and thus make recycling them extremely difficult, if not impossible.
“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.
The team behind the breakthrough. Left to right: Peter Christensen, Kathryn Loeffler, and Brett 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 researchers plan to develop PDK plastics with a wide range of thermal and mechanical properties for a diverse range of applications, including textiles, 3D printing, and foams. In addition, they are looking to expand the formulations by incorporating plant-based materials and other sustainable sources.