Scientists are closing the loop on the recycling of mixed plastics

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ORNL’s organocatalyst deconstructs mixed plastics at different temperatures, making it easier to recover their individual monomers individually in a reusable form. Credit: Jill Hemman/ORNL, US Department of Energy

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ORNL’s organocatalyst deconstructs mixed plastics at different temperatures, making it easier to recover their individual monomers individually in a reusable form. Credit: Jill Hemman/ORNL, US Department of Energy

A small fraction of the mixed consumer plastics thrown away or placed in recycling bins are actually recycled. Nearly 90% is buried in landfills or burned in commercial facilities, which generate greenhouse gases and toxins in the air. Neither outcome is ideal for the environment.

Why aren’t more mixed plastics recycled? It is usually easier and cheaper to make new plastic products than to recover, sort and recycle used ones. Conventional recycling of mixed plastics used to mean manually or mechanically separating the plastics according to their constituent polymers.

To address the problem, scientists at the Department of Energy’s Oak Ridge National Laboratory used carefully planned chemical design, neutron scattering and high-performance computing to help develop a new catalytic recycling process. The catalyst selectively and sequentially deconstructs multiple polymers in compound plastics into pure monomers—molecules that react with other monomer molecules to form a polymer. The process offers a promising strategy to combat global plastic waste, such as bottles, packaging, foam and carpets.

The researchers’ analysis, published in Materials Horizons, compared using the new multifunctional catalyst to using separate catalysts for each type of plastic. The new catalyst will generate up to 95% less greenhouse gases, require up to 94% less energy and result in up to a 96% reduction in fossil fuel consumption.

“Our approach involves a custom synthetic organocatalyst—a compound made up of small organic molecules that facilitate organic chemical transformations. The organocatalyst can convert batches of mixed plastic waste into valuable monomers for reuse in the production of commercial-grade plastics and other valuable materials.” said Tomonori Saito, an ORNL synthetic polymer chemist and corresponding author. “This highly efficient chemical process can help close the recycling loop for mixed plastics by replacing first-use monomers with recycled monomers.

“Today, nearly all plastics are produced from fossil fuels, using first-use monomers produced through energy-intensive processes. “Establishing this type of closed-loop recycling, if used globally, could reduce annual energy consumption by about 3.5 billion barrels of oil,” added Saito.

credit: Materials Horizons (2023). DOI: 10.1039/D3MH00801K

A solution for recycling over 30% of all plastics

The new organocatalyst has proven to efficiently and rapidly degrade multiple polymers – in about two hours. Such polymers include those used in materials such as safety glasses (polycarbonates), foams (polyurethanes), water bottles (polyethylene terephthalates), and ropes or fishing nets (polyamides), which together account for more than 30% of global plastic production. So far, no catalyst has been shown to be effective on all four of these polymers.

The process provides many environmental benefits by replacing aggressive chemicals to deconstruct polymers, as well as offering good selectivity, thermal stability, non-volatility and low flammability. Its performance against multiple polymers also makes it useful for deconstructing increasing amounts of multicomponent plastics, such as composites and multilayer packaging.

Small-angle neutron scattering at ORNL’s Spallation Neutron Source was used to confirm the formation of deconstructed monomers from waste plastics. The method scatters neutrons at small angles to characterize the structure at various levels of detail, from nanometers to fractions of a micrometer.

Converting mixed plastic polymers into true recycled plastics

The organocatalyst deconstructs the plastics at different temperatures, which facilitates the sequential recovery of the individual monomers separately, in a reusable form. Polycarbonates degrade at 266°F (130°C), polyurethanes at 320°F (160°C), polyethylene terephthalates at 356°F (180°C), and polyamides at 410°F (210°C). Other plastics, additives and related materials such as cotton and plastic bags remain intact due to differences in their reactivity and can subsequently be recovered.

“The deconstructed monomers and the organocatalyst are water-soluble, so we can transfer them to water, where any impurities such as pigments can be removed by filtration,” said Dr. Arifuzaman, lead author of the study and a former postdoctoral synthetic organic chemist at ORNL. He is now a Fellow at Innovation Crossroads and CEO and Founder of Re-Du Company.

“The nearly pure monomers are then extracted, leaving the catalyst that is almost completely recovered by evaporation of the water and can be directly reused for multiple deconstruction cycles.”

More info:
Md Arifuzzaman et al, Selective degradation of mixed plastics by a customized organocatalyst, Materials Horizons (2023). DOI: 10.1039/D3MH00801K

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Materials Horizons

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