Researchers at Berkeley Lab in California have developed a ‘next generation’ plastic which can be recycled indefinitely on a molecular level, allowing it to be formed back into new products of any shape or colour with no dergedationdegradation.
Light yet sturdy, plastics are great for many applications – until you no longer need itthem. Because plastics contain various additives, such as 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% in the US, according to the Environmental Protection Agency. The rest typically ends up in landfill, where the carbon-rich material takes centuries to decompose, or goes to incineration.
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 colour 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,” explains 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.
Recycling at a Molecular Level
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 plasticisers 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-coloured plastics – are mixed together and ground into bits. When that hodgepodge hotchpotch of chopped-up plastic 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.
Currently, when a reusable shopping bag made with recycled plastic gets threadbare from 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 says.
“Circular plastics and plastics upcycling are grand challenges,” he conctinues. “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.”
One Monomer at a Time
The researchers want to divert plastics from landfill, and for that matter the oceans, by incentivising 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,” says Helms.
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.
“We’re interested in the chemistry that redirects plastic lifecycles from linear to circular,” Helms adds. “We see an opportunity to make a difference for where there are no recycling options.” That includes adhesives, phone cases, watch bands, shoes, computer cables, and hard thermosets that are created by moulding hot plastic material.
The researchers first discovered the circular property of PDK-based plastics when Christensen was applying various acids to glassware used to make PDK adhesives, and noticed that the adhesive’s composition had changed. Curious as to how the adhesive might have been transformed, Christensen analysed the sample’s molecular structure with an NMR (nuclear magnetic resonance) spectroscopy instrument. “To our surprise, they were the original monomers,” Helms exclaims.
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.
Next, they proved that the recovered PDK monomers can be remade into polymers, and those recycled polymers can form new plastic materials without inheriting the colour or other features of the original material – so that broken black watch band 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.
Moving Towards a Circular Future for Plastics
The researchers believe that their new recyclable plastic could be a good alternative to many non-recyclable plastics in use today.
“We’re at a critical point where we need to think about the infrastructure needed to modernise recycling facilities for future waste sorting and processing,” explains 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 next plan to develop PDK plastics with a wide range of thermal and mechanical properties for applications as diverse as textiles, 3D printing, and foams. In addition, they are looking to expand the formulations by incorporating plant-based materials and other sustainable sources.