Light yet sturdy, plastic is great – until you no longer need it. Because plastics contain various additives, like dyes, fillers, or flame retardants, very few plastics can be recycled without loss in performance or aesthetics. Even the most recyclable plastic, PET – or poly(ethylene 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.
Now a team of researchers at the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) has designed a recyclable plastic that, like a Lego playset, 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 new material, called poly(diketoenamine), 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. The other co-authors are Angelique Scheuermann (then of UC Berkeley) and Kathryn Loeffler (then of the University of Texas at Austin), who were undergraduate researchers at the time of the study.
All plastics, from water 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.
According to the researchers, the problem with many plastics is that the chemicals added to make them useful – such as fillers that make a plastic tough, or plasticizers that make a plastic flexible – are tightly bound to the monomers and stay in the plastic even after it’s been processed at a recycling plant.
During processing at such plants, plastics with different chemical compositions – hard plastics, stretchy plastics, clear plastics, candy-colored plastics – are mixed together and ground into bits. When that hodgepodge of chopped-up plastics is melted to make a new material, it’s hard to predict which properties it will inherit from the original plastics.
This inheritance of unknown and therefore unpredictable properties has prevented plastic from becoming what many consider the Holy Grail of recycling: a “circular” material whose original monomers can be recovered for reuse for as long as possible, or “upcycled” to make a new, higher quality product.
So, when a reusable shopping bag made with recycled plastic gets threadbare with wear and tear, it can’t be upcycled or even recycled to make a new product. And once the bag has reached its end of life, it’s either incinerated to make heat, electricity, or fuel, or ends up in a landfill, Helms said.
“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.”