Berkeley Lab reinvigorates circular charge with breakthrough 100 percent recyclable plastic

08 May 2019 --- Scientists from the US Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a “next-generation plastic” that can be disassembled into its constituent parts at the molecular level and then reassembled into a different shape, texture and color multiple times without loss of performance or quality. The researchers believe that the new material – called poly(diketoenamine) – can help drive the transition from linear plastic lifecycles to a circular economy.

Certain challenges still need to be overcome before the new material can be commercialized. This includes integrating barriers to air and moisture for use as food packaging as well as finding a way to compete with the relatively high cost-effectiveness of conventional 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.

Unpredictable properties = unachievable circularity
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 they have been processed at a recycling plant.

This time-lapse video shows a piece of PDK plastic in acid as it degrades to its molecular building blocks – monomers. The acid helps to break the bonds between the monomers and separate them from the chemical additives that give plastic its look and feel. (Credit: Peter Christensen/Berkeley Lab)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 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 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.

Even the most recyclable plastic – PET – is only recycled at a rate of 20-30 percent, the researchers note. The remainder typically goes to incinerators or landfills, where the carbon-rich material takes centuries to decompose.

“Circular plastics and plastics upcycling are grand challenges,” Brett Helms, Staff Scientist at Lawrence Berkeley National Lab, tells PackagingInsights. “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
Plastics were never originally designed to be recycled, but the Berkeley Lab research team has discovered a new way to assemble plastics that takes recycling into consideration from a molecular perspective.

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.

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. “We see an opportunity to make a difference for where there are no recycling options,” says Helms. “That includes adhesives, phone cases, watch bands, shoes, computer cables, and hard thermosets that are created by molding hot plastic material.”

The researchers first discovered the circular property of PDK-based plastics when lead author Peter Christensen, a postdoctoral researcher at Berkeley Lab's Molecular Foundry, 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 analyzed the sample's molecular structure with an NMR (nuclear magnetic resonance) spectroscopy instrument. “To our surprise, they were the original monomers,” Helms notes.

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, the scientists 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. For example, a broken black watchband found in the trash could find new life as a computer keyboard if it's made with PDK plastic. It is also possible to upcycle the plastic by adding additional features, such as flexibility.

Moving toward 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. However, particular challenges must still be overcome before the new material can be commercialized. Most notably, how to compete with the cost-effectiveness of conventional plastics and how to add performance requirements such as barriers to air and moisture to the new material.PDK plastics are 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. (Credit: Peter Christensen et al./Berkeley Lab)

“Packaging for food and beverage must deliver on certain performance requirements, such as a barrier to air and moisture, while also being manufacturable,” continues Helms. “Recyclable alternatives should also meet those performance requirements, however, it is not yet clear how to do so. We are still in the early days of understanding how to bring together all these attributes in the same material.”

“Competing against existing plastics is a tough battle,” Christensen tells PackagingInsights. “Plastics are cheap. One of the biggest fundamental issues with recycling, in general, is that the people that make money from selling plastic products aren't the same people that make money from collecting and recycling plastics. So in addition to material problems (plastics were never designed to be recycled), as a society, we also face enormous issues with the capabilities of material recovery facilities, which vary significantly from city to city.”

“Often it's more economical to just burn discarded plastics than it is to recycle them. So, the key to commercializing this, or any other new plastic, is to find the right niche market to look at initially,” he explains.

Christensen gives the example of Tesla. Tesla didn't start by making cheap, affordable electric cars, they started by making the Roadster, a high-end, high-performance vehicle that was produced in very low volumes.

“I think this is where sustainability initiatives have failed in the past. Not only does it make business sense to start by going after high margin products while developing a new technology; rare, high-end, luxury items have a better chance at generating a brand identity and shifting consumer awareness and adoption.”Left to right: Peter Christensen, Kathryn Loeffler, and Brett Helms. (Credit: Marilyn Chung/Berkeley Lab)

To commercialize this plastic, scientists have to look for the plastic equivalent of the Tesla Roadster before going after low-cost, high-volume plastic products like food packaging, Christensen concludes.

A key element of the New Plastic Economy and Global Plastics Initiative is the identification of new polymers which allow complex multi-material products to be deconstructed, the plastics contained therein to be depolymerized, and the additives in those plastics to be dissociated from the original monomers in a cost-effective and scalable process. Those features are embodied by the Berkeley Lab next-gen plastics project.

The new material, poly(diketoenamine), was originally reported in the journal Nature Chemistry.

By Joshua Poole