Hats off for new, lower temperature route to unzip plexiglass for recycling

A new lower temperature method to depolymerise acrylic glass into its monomer could offer an efficient, more environmentally friendly approach to recycle the material. The technique, which also uses no solvents, was developed by Swiss researchers who demonstrated it could produce high yields of pristine monomers ready for making new acrylic.

Acrylic sheets are made from polymethyl methacrylate (PMMA), producing lightweight and shatterproof alternatives to glass such as plexiglass and Perspex. However, converting the polymer back into monomers traditionally requires pyrolysis above 400°C. Existing methods also require solvents, which can contaminate the end product and cause undesirable discolouration.

Now, Athina Anastasaki and her colleagues at ETH Zurich, Switzerland, have developed what they claim is the first low temperature chemical recycling of acrylic that takes place below 200°C. Importantly, it also requires no solvents or light irradiation, which boosts the possibility of commercial scale-up.

Previous research into lower temperature methods for chemically recycling PMMA has generally focused on non-commercial, PMMA model materials. These were designed with chain-end functionalised polymers, which act as weak links that make them easier to depolymerise.

‘Although these model materials increased our kinetic and thermodynamic understanding, they could not meaningfully contribute to the plastic crisis and the need of immediate recycling, as commercial polymers typically lack these well-defined chain-ends,’ explains Anastasaki. ‘We wanted to investigate whether we could develop a much simpler process that could be applied directly on commercially relevant PMMA, without redesigning the polymer and ideally without using a solvent.’

To do this, the team set its sights on the hydrogen atom transfer (Hat) reaction. The researchers investigated whether eliminating a hydrogen atom from the polymer’s backbone could split the polymer mid-chain with no reliance on chain-ends under mild conditions.

Screening for potential Hat agents, including several established systems, the researchers found that they didn’t work well enough at the temperatures they wanted to work at. However, control experiments with an N-hydroxyimide reagent, which other groups have reported triggers efficient depolymerisation when added as co-monomers, led to a surprising discovery. The N-hydroxyimide unexpectedly triggered the Hat reaction, enabling splitting of the polymer chain into two fragments, with one fragment rapidly unzipping into monomers.

Optimising the system with N-hydroxytetrachlorophthalimide, the researchers showed efficient depolymerisation at temperatures between 180 and 230°C. The highest yield was 96%. However, putting the remaining 4% through a second cycle recycled 99% of the acrylic.

‘Using N-hydroxyphthalimides as an external radical source, rather than incorporating them into the polymer backbone, is a major step forward for practical PMMA recycling,’ comments Jon Husband at the University of Bath. In January, Husband published work on a recycling method that uses UV light to break down PMMA .

‘This paper takes important steps towards a scalable process by removing the need for light irradiation,’ says Husband. He explains that photochemical processes tend to scale poorly because light is absorbed and scattered as it passes through reaction mixtures.

‘In addition, the new method can be performed in bulk, which may offer advantages over solvent-based approaches,’ Husband adds. ‘However, this comes at the cost of higher temperatures than previous solution-phase methods.’ He also says the expense of tetrachloro-N-hydroxyphthalimide could be a significant hurdle.

Anastasaki says the leap from lab to industry is still a ‘substantial distance’ but the group has filed a patent. ‘We are currently exploring the possibility to scale-up our approach and also to investigate even less expensive N-hydroxyimide alternatives, thus increasing the viability of our methodology,’ says Anastasaki.

‘Given the remarkable progress being made by Athina’s group, alongside the work we are continuing at Bath, a commercially viable solution [to acrylic recycling] may not be far off,’ adds Husband.