Picking the Molecular Lock to Recycle Plastic
Sometimes yesterday’s innovation becomes today’s burden. That’s true, to a large extent, with plastic. The plastics industry came into its own during World War II, capitalizing on the material’s pliability to make synthetic goods, which helped conserve natural resources for the war effort. The use of plastics in consumer products surged after the conflict — culminating in the scene from the iconic 1967 film “The Graduate”, in which Dustin Hoffman receives employment advice from a friend’s parent at his college graduation party: “I want to say one word to you. Just one word. Plastics.”
The plastics boom might indeed have shaped the ’70s and subsequent decades, but Mr. McGuire — the corporate executive and advice-giver in the movie — was not farsighted enough to predict the heavy toll our reliance on plastics is taking on the environment. Imagine dumping a New York City garbage truck’s worth of plastic into the ocean every minute, every day, for a year. That’s how much plastic waste makes its way into our seas.
In the ViPER (Vilas Pol Energy Research) lab at Purdue, we’re working on a way to upcycle this ubiquitous material. The key lies in the molecular structure of the plastic itself. Polyethylene is one of the most prevalent plastics, used in grocery bags and a variety of consumer, industrial and medical products. One molecule is a long chain of carbon atoms, with two hydrogen atoms attached to each carbon atom. If you can upcycle the polyethylene back to carbon, you can close the loop on use and reuse. However, there’s a problem: the processes that achieve this are lengthy, inefficient and expensive.
We found an easier, more efficient way to get hold of the carbon. We immerse plastic bags in sulfuric acid, and then heat the mixture under pressure to just below polyethylene’s melting temperature. This sulfuric acid treatment, known as sulfonation, leads to a compound that can withstand the much higher temperatures required to hold the carbon.
We then heat the sulfonated polyethylene at 500–700° C in an inert atmosphere to extract pure carbon. We repurpose the freed carbon in our lab — which specializes in electrode materials preparation and battery fabrication — to manufacture anodes (the negative terminals) for lithium-ion batteries. These rechargeable batteries have applications ranging from remote-controlled devices to smartphones, electric vehicles and more.
There’s a plot twist. Many cell phone batteries require the mining of precious earth elements — materials that are ethically challenging to source. Turns out, the ability to upcycle plastic waste into usable battery components could make significant inroads into the sustainability pathway of more than just one consumable product.
We still face challenges. Sulfuric acid is not a very safe chemical, but we chose it because we needed a solvent that would react with plastic, which is very stable. Industry regularly works with sulfuric acid — to refine metals, manufacture fertilizers and produce other chemicals — so I’m confident we can find a solution that mitigates the risk.
We also need to address the issue of scale. We conducted our experiments in small laboratory reactors, and must migrate the process to larger industrial reactors, as well as justify the economics. The good news is, I don’t see too many hurdles in the path to larger-scale adoption.
For now, breaking up the menace that is polyethylene — and in the process generating something useful as a byproduct — is a significant step in the right direction. Mr. McGuire’s advice might not resonate the way it once did, and decades later, we’re still trying to find our way to a sustainable lifestyle. But every drop in the bucket helps, and we’re convinced this is a big one.
by Vilas Pol, PhD
Associate Professor, School of Chemical Engineering and School of Materials Engineering; Affiliate of Environmental and Ecological Engineering; and head of the ViPER (Vilas Pol Energy Research) Group and Laboratory, Purdue University
Related Content
Dr. Vilas Pol to receive 2019 AIChE Professional Achievement Award
American Chemical Society presents Salute to Excellence award to Dr. Vilas Pol
