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 Molecular Foundry 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.”
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.
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 acid helps to break the bonds between the monomers and separate them from the chemical additives that give plastic its look and feel.
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 entwined additives.
They also proved that the recovered PDK monomers can be remade into polymers, and those recycled polymers can form new plastic materials without inheriting the color or other features of the original material – so that broken black watchband you tossed in the trash could find new life as a computer keyboard if it’s made with PDK plastic. They could also upcycle the plastic by adding additional features, such as flexibility.