Large quantities of acrylic glass are used worldwide in a wide range of applications, from window panes and car tail lights to furniture. The polymer is also known by the brand name Plexiglas. All too often, acrylic glass still ends up as waste, even though it is well suited to recycling.
ETH Professor Athina Anastasaki, a materials scientist, has spent years working on ways to efficiently break down acrylic glass into its constituent building blocks for reuse. In previous studies, her team showed that this process, known as depolymerisation, was possible in principle, but only under specific conditions. For example, ETH researchers previously had to chemically modify the polymer chains to introduce "weak links". Other approaches required large quantities of solvents or catalysts, or UV light, to initiate or sustain depolymerisation.
New approach saves energy and money
The ETH professor and her team have now developed a new, much simpler approach to recycling acrylic glass polymers. They have published their findings in the journal Nature Chemistry .
Using the new method, the researchers can break the polymer down almost entirely into its constituent building blocks, known as monomers. The monomer yield can be as high as 98%. The recovered material is also extremely pure, even when the starting material contains dyes or impurities. The recovered monomers can be used to produce new polymers of the same quality as those produced during initial processing.
The process also requires significantly less energy than other methods. Depolymerisation begins at just over 200 degrees Celsius, compared with the minimum of 400 degrees previously required. Moreover, the process uses no solvents whatsoever. "What makes our new approach so exciting is its simplicity," says Richard Whitfield, lead author of the study and a former research associate in Anastasaki's group.
A single substance is all it takes
At the heart of the new method is a single chemical compound belonging to a class of substances known as N-hydroxyimides. This commercially available, inexpensive reagent attacks the acrylic glass polymer chains, breaking them apart and releasing the monomers. Once the reagent is added, the reaction proceeds on its own.
The choice of reagent was no coincidence. "We searched systematically until we found this substance." That single reagent is enough to initiate depolymerisation. "The only thing that surprised us was that a single chemical really was sufficient for the reaction," explains Anastasaki.
In their search for a suitable reagent, her group collaborated with ETH chemistry professors Tae-Lim Choi and Gunnar Jeschke. "Without the excellent collaboration with these two colleagues, many questions would have remained unanswered," Anastasaki emphasises.
Patent filed, licensees sought
Other methods of chemical plastics recycling rely on a process known as pyrolysis, in which plastic polymers are broken down into their constituent building blocks at high temperatures, usually above 400 degrees Celsius. Furthermore, the recovered monomers are often relatively impure, with impurity levels of up to 3%. This can adversely affect the quality of the plastic produced from them.
ETH Zurich has filed a patent application for the new method and is now seeking licensees. "Competition in chemical plastics recycling is fierce," says Anastasaki, "particularly in the USA, where large, well-funded research groups are working in this field." However, thanks to the solid core funding provided by ETH Zurich, Anastasaki can hold her own against these groups – and this time she is even one step ahead of her American competitors.
Industry could adopt the method immediately
The chemical industry has shown interest in the new method. One key advantage is that it could be implemented using existing infrastructure and standard processes. "The drawback of our previous approaches was that companies would have had to modify their processes and equipment to implement them. For most companies, that would be too expensive," explains Anastasaki.
Her next steps will be to scale up the method and gain a detailed understanding of the reaction mechanisms. Building on this knowledge, the ETH researchers aim to develop substances that will further accelerate polymer breakdown and improve the monomer yield.
There is, however, one small catch: the method only works with acrylic glass. Different approaches are needed to break down other plastics into their constituent building blocks. Nevertheless, the materials scientist remains optimistic: "We will try to apply the same principle to polystyrene, one of the most widely used plastics in the world," says Anastasaki.
