Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory have designed a recyclable plastic – called poly(diketoenamine) or PDK – which can be disassembled into its constituent parts at the molecular level and then reassembled into a different shape, texture and colour over and over again, without loss of performance or quality.
Initially reported in the journal Nature Chemistry, the discovery could revolutionise the way plastic is dealt with. Currently, even the most recyclable plastic – PET – is only at a rate of 20-30 per cent, with the rest typically going to incinerators or landfill.
“Most plastics were never made to be recycled, but we have discovered a new way to assemble plastics that takes recycling into consideration from a molecular perspective,” said Peter Christensen, lead author and post-doctoral researcher at Berkeley Lab’s Molecular Foundry.
According to Berkeley Lab’s news release, Christensen was part of a multi-disciplinary team led by Brett Helms, staff scientist in Berkeley Lab’s Molecular Foundry. The other co-authors are Angelique Scheuermann (UC Berkeley) and Kathryn Loeffler (University of Texas), both of whom were undergraduate researchers at the time of the study.
The researcher said 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.
The report mentioned that during processing, plastics with different chemical compositions – hard plastics, stretchy plastics, clear plastics, candy-coloured plastics – are mixed together and ground into bits. When that mixture 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 unpredictability has prevented plastic from becoming a completely “circular” material whose original monomers can be recovered for reuse for as long as possible, or upcycled to make a new, higher-quality product.
“Circular plastics and plastics upcycling are grand challenges. We’ve already seen the impact of plastic waste leaking into our aquatic eco-systems, 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,” Christensen said.
By breaking the polymers down easily, the plastic’s core units can be separated from any additives over and over again in what is described as a “closed-loop cycle”.
The researchers then tested the recovery process by contaminating PDK and acid solutions with other substances, including fibreglass and flame-retardant chemicals, finding the additives had little impact on their diketoenamine harvest.
They found that the end result is a plastic ingredient that can shake off any colours or strengthening agents in several easy steps to be turned back into another product.
“We’re interested in the chemistry that redirects plastic lifecycles from linear to circular. We see an opportunity to make a difference for where there are no recycling options,” Helms said.
“Right now, most of our recycling efforts are abysmal. While some countries are doing all they can, countries like the US are barely managing to recycle a quarter of their PET waste. Any methods we can find to make recycling easy for the consumer and appealing to the manufacturer are worth investigating.
“With PDKs, the immutable bonds of conventional plastics are replaced with reversible bonds that allow the plastic to be recycled more effectively. 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.”
Researchers next plan to develop PDK plastics with a wide range of thermal and mechanical properties. These plastics could be used for many kinds of cloth, as well as things such as 3D-printed materials and foams. In addition, the report also mentioned that the team is looking to include plant-based materials in the process.