New Procedure PMMA can be recycled using less energy 

ETH researchers have developed a new method for breaking down PMMA into its constituent parts in an energy-efficient and cost-effective manner. The recycled material is extremely pure, making it suitable for use in high-quality products. What’s exciting for the industry is that it could adopt the process immediately.

Polymethyl methacrylate (PMMA, also known as acrylic glass or Plexiglas) is used in large quantities worldwide for a wide variety of applications—from “glass panes” to car taillights to furniture. All too often, acrylic glass ends up in the trash. Yet it could easily be recycled.

Materials scientist and ETH professor Athina Anastasaki has been working for years on efficiently breaking down acrylic glass into its basic building blocks and reusing them. In earlier work, her team showed that so-called depolymerization is possible in principle, but that it required specific conditions: For example, the ETH researchers previously had to chemically modify the polymer chains to incorporate “predetermined breaking points.” Other approaches required large amounts of solvents, catalysts, or UV light to initiate or sustain the depolymerization process.

New Approach Saves Energy and Money

Now, the ETH professor and her team are presenting a new, much simpler approach to PMMA recycling. They published it in the journal Nature Chemistry.

Using the new method, the researchers are able to break down the polymer almost completely into its building blocks, the monomers: the monomer yield is as high as 98 percent. The recycled material is also extremely pure, even when the source material contains dyes or impurities. New polymers of the same quality as those produced during the initial processing can be manufactured from the recovered monomers.

The process also requires significantly less energy than other methods. Depolymerization begins at just over 200 degrees Celsius. Previously, a temperature of at least 400 degrees was required. Last but not least, the process requires no solvents at all. “What makes our new approach so exciting is its simplicity,” says Richard Whitfield, first author of the study and a former research associate in Anastasakis’s research group.

One substance is enough

At the heart of the new process is a single chemical compound from the class of substances known as N-hydroxyimides, a commercially available and inexpensive reagent. It attacks the acrylic polymer chains, breaks them apart, and releases the monomers. Once this substance is added, the reaction proceeds on its own.

The choice of reagent was no accident. “We searched systematically until we came across this substance,” explains Anastasaki. This single substance is now sufficient to initiate depolymerization. 

Patent Filed, Licensees Sought

Other methods for chemical recycling are based on a process known as pyrolysis. In this process, plastic polymers are broken down into their constituent building blocks at high temperatures, usually exceeding 400 degrees. Furthermore, the purity of the recovered monomers is often low—they contain up to three percent impurities. This can compromise 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 the field of chemical plastics recycling is fierce, especially in the U.S., where large, well-funded research groups are working in this area,” says Anastasaki. Thanks to solid core funding from ETH Zurich, however, the materials scientist is able to keep up and is even one step ahead of her American competitors this time around.

The industry could adopt this approach immediately

The chemical industry is showing interest in the new method. This is because a key advantage is that the industry could implement this method using existing infrastructure and standard processes. “Our earlier approaches had the drawback that companies would have had to change their processes and equipment to implement them. That’s too expensive for most,” explains Anastasaki.

In the next steps, she plans to scale up the method and gain a detailed understanding of the reaction mechanisms. Based on this, the ETH researchers hope to develop substances that will further accelerate polymer degradation and optimize the monomer yield.

There is one small catch, however: The process works only for acrylic glass. To break down other plastics into their basic building blocks, different approaches are needed. Nevertheless, the materials researcher remains optimistic: “We will try to apply this principle to polystyrene, one of the most widely used plastics in the world,” says Anastasaki.