Recycling research demonstrates new possibilities for a circular plastics economy powered by renewable energy

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HFIP’s central role: the solvent component that dissolves POM. A. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP): An effective solvent for polyoxymethylene (POM), the clustering of HFIP helped lower the σ*OH energy. B. Images of an intact cell before (left) and after (right) electrolysis. c. Reaction profile of POM bulk electrolysis in CH3CN:HFIP (26:4) at 3.5 V (60 °C), 0.1 M LiClO4. debt: Natural communication (2023) DOI: 10.1038/s41467-023-39362-z

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HFIP’s central role: the solvent component that dissolves POM. A. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP): An effective solvent for polyoxymethylene (POM), the clustering of HFIP helped lower the σ*OH energy. B. Images of an intact cell before (left) and after (right) electrolysis. c. Reaction profile of POM bulk electrolysis in CH3CN:HFIP (26:4) at 3.5 V (60 °C), 0.1 M LiClO4. debt: Natural communication (2023) DOI: 10.1038/s41467-023-39362-z

Despite efforts worldwide to move towards a circular plastic economy, more than three-quarters of the 400 metric tons of plastic produced globally each year still end up as waste.

A team of researchers at the University of Illinois Urbana-Champaign has demonstrated a way to recycle a form of plastic that is growing in use, but more challenging to recycle than other popular forms of plastic, using a renewable energy source of electricity.

Their study was published recently Natural communicationThey share their innovative process, which demonstrates the potential of using renewable energy sources in the transition towards a circular plastics economy.

„We want to prove this concept of bringing together renewable energy and the circular plastic economy,” said postdoctoral assistant and co-author Yuding Zhou, who worked on this groundbreaking research with two professors of chemistry at Illinois. Electrochemist Joaquin Rodriguez-Lopez.

The project originated with Moore, who had experience working with poly(phthalaldehyde), a form of polyacetal. Polyoxymethylene (POM) is a high-performance acetal resin used in a variety of industries, including automobiles and electronics. A thermoplastic, it can be shaped and molded when heated and hardened when cooled to a high degree of strength and stiffness, making it an attractive lightweight alternative to metal in some applications, such as mechanical gears in automobiles. It is manufactured by various chemical companies with slightly different formulations and names, including Delrin by DuPont.

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POM’s highly crystalline properties make it difficult to break down during recycling. It can be remelted and shaped, but the original material properties of POM are lost, limiting the usefulness of the recycled material.

„When the polymer was in use as a product, it was not a pure polymer. It also contained other chemicals such as coloring additives and antioxidants. So, if you melt it down and reshape it, the material properties are always lost,” Joe explained. .

The Illinois research team’s method uses electricity that can be drawn from renewable sources and takes place at room temperature.

This electro-mediated process restructures the polymer, breaking it down into monomers—molecules linked to other similar molecules to form polymers.

Significant chemical recycling advances have been achieved in the past five years in commonly known synthetic plastics such as PE, PET and PS.

However, there has been limited experimentation with the more challenging POM, particularly as a driver of the electrical process.

In the study, the researchers demonstrate their methods using Delrin. They begin by dissolving small beads of polymer in a liquid to loosen the bonds in the polymer chain. In the next step of electrocatalysis, Delrin polymer chains are broken down into monomer form.

The key is an organic solvent that dissolves the plastic. He tested several organic solvents, starting with rubbing alcohol, but found only one organic, hexafluoroisopropanol, commonly abbreviated as HFIP.

„It wasn’t instantaneous. It had to sit for hours,” Zhou said, explaining that they discovered that HFIP not only dissolves the polymer, but also plays an important role in depolymerization during the electrolysis step. „It is capable of acting as a proton donor (catalyst).”

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Basically, HFIP forms acid during electrolysis, and the researchers believe that the acid breaks down the polymer into monomers.

„We assume that’s what’s going on,” he said. „There may be other possible depolymerization mechanisms. But with all the experimental evidence we’ve gathered, it leans more toward an electro-mediated acidic depolymerization process.”

After successfully demonstrating their process on small beads of pure POM, they wanted to try a commercial product made of Delrin to further test their method, so they used keg clip sheds, which are commonly used in chemistry labs and are made of Delrin. Again, the process worked.

„For me personally, this work is like an encouragement that you’re not crazy enough to use electricity to break plastic. It’s possible. It’s very challenging. There are limits. There are a lot of walls to hit. But it’s possible,” Joe said. „Electricity can be used to break down a real product.”

Zhou said he wants to further explore the power of electrocatalysis in plastics repurposing by adopting the method as a flow system, aiming to selectively optimize POM for formic acid by testing the most challenging types of plastics.

„We hope this work will inspire other engineers and synthetic chemists, and encourage the industry to explore and think about using electricity to reconstruct synthetic plastics,” said Joe.

The article „Heterogeneous Electromediated Depolymerization of Highly Crystalline Polyoxymethylene” was published. Natural communication.

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