Scientists in California claim to have engineered a new “breakthrough” plastic that can be recycled over and over without sacrificing performance or quality.
Known as poly(diketoenamine)—or PDK, for short—the new material behaves almost like a set of Lego bricks, meaning it can be disassembled into its constituent parts at the molecular level and then reassembled into different shapes, textures and colors “again and again.”
But isn’t plastic already recyclable? Well, yes and no. Plastic, as the recycling codes at the bottom of water bottles and yogurt cups suggest, falls along a spectrum of suitability for the closed loop. Because of dyes, fillers or flame retardants, very few plastics can be recycled today without performance or aesthetic issues. Even PET, the most recyclable of them all, is only melted and remade into new products at a rate of between 20 percent and 30 percent.
“Most plastics were never made to be recycled,” said Peter Christensen, a postdoctoral researcher at the Lawrence Berkeley National Laboratory’s Molecular Foundry and lead author of the study, which was recently published in the journal Nature Chemistry. “But we have discovered a new way to assemble plastics that takes recycling into consideration from a molecular perspective.”
All plastic, Christensen noted, is made up of large molecules called polymers, which, in turn, consist of strings of shorter, carbon-containing units known as monomers.
Though additives—fillers for durability, for instance, or plasticizers for flexibility—make commercial plastic more useful, they’re also impossible for monomers to shake off. The problem is compounded when plastics of different compositions are mixed together, ground up and then melted to create new materials with hard-to-predict properties.
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It’s these unknowns, researchers said, that prevent plastic from becoming the “Holy Grail of recycling” and either continually recovered for reuse or upcycled into high-quality products.
“Circular plastics and plastics upcycling are grand challenges,” said Brett Helms, a staff scientist at the Molecular Foundry. “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.”
Employing polymers formed from PDK could change that. Not only does a dip in an acid bath break PDK polymers into monomers, but it also releases them from any lingering additives.
“With PDKs, the immutable bonds of conventional plastics are replaced with reversible bonds that allow the plastic to be recycled more effectively,” Helms said.
The scientists are framing their new recyclable plastic as a better alternative to many non-recyclable plastics in use today. Their next step is to develop PDK plastics with various thermal and mechanical properties for a host of applications, including textiles, 3-D printing and foams. Eventually, they plan to diversify formulations with plant-based 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,” Helms said. “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.”