Scientists close the loop on recycling mixed plastics

Scientists close the loop on recycling mixed plastics

ORNL’s organocatalyst deconstructs mixed plastics at different temperatures, facilitating the recovery of their individual monomers separately, in a reusable form. Credit: Jill Hemman/ORNL, US Department of Energy

Few of the mixed consumer plastics that are thrown away or placed in recycling bins end up being recycled. Nearly 90% is buried in landfills or incinerated in commercial facilities that generate greenhouse gases and airborne toxins. Neither outcome is ideal for the environment.

Why aren’t more mixed plastics recycled? It is generally easier and less expensive to make new the plastic products that recover, classify and recycle used ones. Conventional recycling of mixed plastics previously involved separating plastics manually or mechanically according to their constituent polymers.

To address the issue, scientists at the Department of Energy’s Oak Ridge National Laboratory used a 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 mixed plastics into pristine monomers: molecules that react with each other. monomer molecules to form a polymer. The process offers a promising strategy to combat global plastic waste such as bottles, packaging, foams and carpets.

The researchers’ analysis, published in Material horizons, compared the use of the new multipurpose catalyst with the use of individual catalysts for each type of plastic. The new catalyst would generate up to 95% less Greenhouse gasesThey require up to 94% less energy and result in up to a 96% reduction in fossil fuel consumption.

“Our approach involves a custom-made synthetic organocatalyst, a compound composed 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, ORNL synthetic polymer chemist and corresponding author. “This exceptionally efficient chemical process can help close the circle. to recycle mixed plastics by replacing first-use monomers with recycled monomers.

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

Credit: Material horizons (2023). DOI: 10.1039/D3MH00801K

A recycling solution for more than 30% of all plastics

The new organocatalyst has been shown to efficiently and rapidly deconstruct multiple polymers, in approximately 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 represent more than 30% of global plastic production. So far, no catalyst has been shown to be effective in these four polymers.

The process provides many environmental advantages by replacing harsh chemicals for deconstructing polymers, as well as offering good selectivity, thermal stability, non-volatility and low flammability. Its effectiveness against multiple polymers also makes it useful for deconstructing increasing amounts of multi-component plastics, such as composites and multi-layer packaging.

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

Conversion of mixed plastic polymers into true recycled plastics

The organocatalyst deconstructs plastics at different temperatures, making it easier to sequentially recover the individual monomers separately, in a reusable form. Polycarbonates deconstruct 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 associated materials, such as cotton and plastic bags, are left intact due to differences in their reactivity and can be recovered later.

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

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

More information:
Md Arifuzzaman et al, Selective deconstruction of mixed plastics using a customized organocatalyst, Material horizons (2023). DOI: 10.1039/D3MH00801K

Citation: Scientists close the loop on recycling mixed plastics (2023, October 22) retrieved October 22, 2023 from

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