ETH Zürich develops PMMA chemical recycling technique

A team of scientists at Eidgenössische Technische Hochschule Zürich (ETH Zürich) claims to have discovered a method to break down polymethyl methacrylate plastic (PMMA, commonly referred to as Plexiglas) into its monomer building blocks for potential chemical recycling.

The researchers from the ETH Zürich Laboratory of Polymeric Materials, led by Athina Anastasaki, say that by using additives the resulting building blocks “can be easily purified through distillation into virgin-grade starting products for the synthesis of new Plexiglas polymers.”

The university says the global annual production of PMMA is around 3.9 million metric tons, and that the polymer is gaining in popularity in the aerospace, automotive and construction industries and for the manufacturing of screens and monitors.

The process developed by the ETH researchers has been published in the journal Science.

“The presence of additives such as copolymers, plasticizers, dyes and most other plastics have minimal impact on chain scission," the researchers say. "Even when using multicolored Plexiglas, [the] yield remains between 94 and 98 percent.”

At least one existing PMMA “depolymerization” effort has been underway in Europe since late 2023, when Wayne, Pennsylvania-based Trinseo inaugurated a PMMA chemical recycling plant in Italy.

Last month, Trinseo announced it had been working with an Italian eyewear manufacturer to make recycled-content lenses with its chemically recycled PMMA.

“Our process is extremely simple. All we need is a chlorine-based solvent and to heat the dissolved recycling mixture to a temperature of between 90 and 150 degrees Celsius to start the depolymerization reaction with the aid of ultraviolet (UV) or visible light," Anastasaki says.

The existing method is based on pyrolysis, which involves the thermal decomposition of carbon chains at around 400 degrees Celsius.

“The large amount of energy required for this process, along with the costs associated with purifying the resulting mixture, severely limits the economic efficiency of pyrolysis,” ETH Zürich says.

The Swiss university says its solvent-based method was discovered by chance.

“We were actually looking for specific catalysts that would promote the targeted breakdown into monomers, but a control experiment led to the surprising revelation that the catalyst was not even necessary,” Anastasaki says. “The chlorinated solvent in which the crushed Plexiglas sample was dissolved was enough to virtually completely split the polymer with the help of UV light.”

Anastasaki credits help from other ETH researchers when following up on the discovery, including Tae-Lim Choi from the Laboratory of Polymer Chemistry and Gunnar Jeschke from the Institute of Molecular Physical Science.

The researcher says her preference for the future will be to dispense with the chlorinated solvent in the new recycling process.

“Chlorinated chemical compounds harm the environment,” Anastasaki says. “Our next goal is therefore to modify the reactions to enable them to work without the chlorinated solvent.”