Next-gen plastic could revolutionise recycling

Researchers have designed a recyclable plastic that can be broken down to molecular constituents and then rebuilt, without compromising performance or quality. The new plastic marks a step towards a circular plastic future.

Despite our best efforts, very few plastics can be successfully recycled. In addition to many different types of plastic being used to make disposable materials and packaging, plastics contain additives such as dyes, fillers and flame retardants that make recycling challenging. Even the most recyclable plastic, PET (polyethylene terephthalate), is only recycled at a rate of 20–30%, with the rest typically going to incinerators or landfills.

However, a team of researchers at the US Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) has designed a next-generation plastic that, like Lego, can be broken down into molecules then reassembled into a different shape, texture and colour again and again, without compromising quality. The new material, called polydiketoenamine 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 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, plastics with different chemical compositions are mixed together and ground into bits. When this blend of plastics is melted to make a new material, it's hard to predict which properties it will inherit from the original plastics. This situation has prevented plastic from becoming a ‘circular’ material whose original monomers can be recovered for re-use for as long as possible or upcycled to make a new, higher quality product.

Brett Helms, team leader and staff scientist at Berkeley Lab's Molecular Foundry, explained that when a re-usable shopping bag made with recycled plastic gets threadbare, it can't be upcycled or even recycled to make a new product. 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.

"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."

Recycling plastic one monomer at a time

The researchers' aim is to divert plastics from landfills and the oceans by incentivising the recovery and re-use 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," Helms said.

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 life cycles from linear to circular," said Helms. "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 exciting 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 said.

After testing various formulations at the Molecular Foundry, the team 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, the researchers 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 watchband you threw away could find new life as a computer keyboard if it's made with PDK plastic. The plastic could also be upcycled by adding additional features, such as flexibility.

Moving towards a circular plastic future

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," said 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," he continued.

The researchers 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.

To read the journal article, click here.

Source: DOE/Lawrence Berkeley National Laboratory. Image credit: ©stock.adobe.com/au/Bits and Splits