A new type of plastic that can be recycled over and over again has the potential to help reduce the staggering amount of plastic waste that is crammed into landfills and pollutes the environment, but researchers recommend tweaking production processes to make the material cheaper and more environmentally friendly.
Unlike most plastics, polydiketoenamines, or PDKs, can be repeatedly recycled without diminishing their quality. Although recycling PDKs is relatively cheap, the initial cost to produce these novel plastics is about 30 times higher than it is for their conventional equivalents, according to a study published April 9 in Science Advances. That means there are potential hurdles to commercializing these plastics.
"There's a plastic waste crisis," said Corinne Scown, the study's senior author and a staff scientist at Lawrence Berkeley National Laboratory. "The plastics we use now were not designed with recycling in mind, and a lot of the things that make them so useful for a variety of applications, like their stability, make them really, really hard to deal with at their end of life. If we can design polymers that are easier to recycle, we can also design novel recycling processes that give us material that is closer to or indistinguishable from virgin quality resins."
Conventional plastics are usually recycled mechanically, which involves melting down the material but preserving its molecular structure. Because mechanical recycling degrades the plastic, the resulting products are "downcycled" into inferior forms with limited applicability, and the process cannot be repeated more than a few times.
"With mechanical recycling, what we are doing is just delaying the waste ending up in landfill. If it doesn't end up in landfill today, it is going to end up in landfill five to six years down the line," said lead author Nemi Vora, a postdoctoral researcher at Lawrence Berkeley National Laboratory.
Instead, chemical recycling involves breaking down a material's molecular structure into its individual units so it can be reformed back into pristine plastic.
"With chemical recycling, the product is either upcycled or it's used in the same exact application that you were recycling from," Vora said. "And the idea is that you keep it in the economy. So you completely divert that waste out of the landfill."
Although it's possible to recycle conventional plastics in such a way, huge amounts of energy are required to break apart their chemical bonds. To overcome this challenge, study co-authors Bret Helms and Peter Christensen, also from the Berkeley laboratory, recently developed the new material called polydiketoenamines. These plastics have special bonds that can be broken down with strong acid at room temperature, slashing the energy demands for chemical recycling.
With their potential for infinite recycling, PDKs could help reduce plastic waste, according to the researchers. But developing a thriving recycling industry for it will require that the novel material can compete with conventional plastics in terms of production costs and environmental impacts. To evaluate the best way of rolling out polydiketoenamines and identify ways to improve this potential industry, the researchers estimated costs and greenhouse gas emissions for PDK production and recycling.
The estimated cost to a facility of recycling them would be around $1.50 per kilogram, close to the cost of producing three commonly used plastics. But the initial cost of producing virgin PDK resin was estimated to be about 30 times that, at $45 per kilogram.
Although a price gap between recycling and virgin production costs can incentivize recycling, right now, "That gap is too large," Scown said, because high initial costs could dissuade manufacturers from investing in polydiketoenamines.
Through their analysis, the researchers suggest that almost 50% of production costs were due to a single compound called N,N′-dicyclohexylcarbodiimide, or DCC. According to Vora, the team that developed the novel plastic is now working on ways to reduce the use of this compound and bring down production costs.
Because recycling PDKs is much cheaper than producing them, Scown and Vora say the material should be used in products from which it can be easily recovered.
"You want to find products that have this sweet spot of, they don't stay in circulation for too long, but you can also get a lot of them back," Scown explained. Such items could include electronics, shoes and sunglasses, the recycling of which could be encouraged through take-back programs, initiatives by companies to recover products for recycling.
The team also calculated greenhouse gas emissions associated with processing polydiketoenamines. The amount of carbon dioxide emitted per kilogram of PDK was 86 kilograms for virgin resin and just 1.6 kilograms for recycled material, which is up to half that of greenhouse gas emissions associated with producing virgin conventional plastics.
Almost half of greenhouse gases from virgin PDK was related to chemicals involved in producing DCC, indicating that reducing use of this compound could make production more environmentally friendly, as well as cheaper.
According to Scown, PDKs are likely to be just one part of the solution to the problem of plastic waste.
"PDK plastics will be really great for some types of products. And I think in other cases, the existing polymers that we have offer properties that are really important. We need to invest in novel ways of recycling the plastics that we have because they're already so integral to our society," she explained. "In other cases, maybe you want to go the route of increasing the use of compostable plastics."
The study, "Leveling the cost and carbon footprint of circular polymers that are chemically recycled to monomer," published April 9 in Science Advances, was authored by Nemi Vora, Lawrence Berkeley National Laboratory and International Institute for Applied Systems Analysis; Peter R. Christensen, Jérémy Demarteau and Brett A. Helms, Lawrence Berkeley National Laboratory; Nawa Raj Baral, Lawrence Berkeley National Laboratory and Joint BioEnergy Institute; Jay D. Keasling, Lawrence Berkeley National Laboratory, University of California, Berkeley, Technical University of Denmark, Shenzhen Institutes for Advanced Technologies and Joint BioEnergy Institute; and Corinne D. Scown, Lawrence Berkeley National Laboratory, Joint BioEnergy Institute and University of California, Berkeley.