Up, up and away: WVU engineer uses microwaves to ‘upcycle’ a super-powered plastic

Polypropylene is a superhero that can do and be anything – or almost anything. It’s a plastic resin that keeps us healthy, transforming into medical sutures, prostheses, and surgical masks. It keeps us safe, contributing to the manufacture of impact-resistant car bumpers and hard hats. It keeps us entertained and energized. It’s a popular material for children’s toys, sports equipment like rackets and moisture-wicking performance apparel. And it keeps us comfortable. It’s in the rugs that cushion our footsteps – even in the insoles that support our aching arches.

Nontoxic and FDA approved, polypropylene also helps nourish us, forming the basis for everything from our yogurt tubs to takeaway boxes to reusable coffee cups. It contains no Bisphenol-A, a chemical other plastics can release into food and that has raised health concerns for its potential to alter hormonal function.

Cheap, lightweight and resistant to chemicals, fatigue, moisture, heat and freezing temperatures, polypropylene is all things to all people – almost.

What it’s not is recycled. Not often, that is.

According to WVU chemical engineer Yuxin Wang, assistant professor in the Benjamin M. Statler College of Engineering and Mineral Resources, 99% of the polypropylene in the world today will end up in a landfill. That’s true even though polypropylene has enormous potential for successful “upcycling.”

“What we think of as ‘recycling’ usually involves crushing a material into tiny pieces that can be used in a new product,” Wang explained. “We might recycle tires by breaking them down into small fragments that we can turn into flooring, for example.

“‘Upcycling’ goes further – it breaks a material down into its original chemical components. That gives much more flexibility in what the reclaimed materials can be used for.”

Recycled plastic just makes more plastic, but if Wang can recover the flammable gas propylene from post-consumer polypropylene, manufacturers can use it to produce a dizzying range of valuable products: adhesives, diapers, e-cigarettes, products that enhance the octane of the gas in our cars’ tanks, products that keep our baked goods moist and our skin and hair hydrated.

Up to now, polypropylene upcycling has been hindered by efficiency and cost considerations. Most attempts have relied on thermochemical approaches such as pyrolysis, in which the material is heated in an oxygen-free environment. But those techniques typically result in a propylene yield of less than 25%, meaning most of the original material is wasted.

Wang thinks he can help change that, thanks to a $1 million grant from the U.S. Department of Energy that will support his development of a new technology for polypropylene recovery that’s based on microwaves.

He sees microwave irradiation as the key to a practical, implementable, cost-competitive system for polypropylene upcycling – one that enables precise, selective control over the process, and one that allows propylene recovery to happen at much lower temperatures than in conventional thermal recovery processes: 300 degrees Celsius as compared to 600 or 700 degrees.

Professor Srinivas Palanki, chair of the WVU Department of Chemical and Biomedical Engineering, said this research needs to happen now.

“We make 150.3 million metric tons of propylene every year through energy-intensive processes like ethane steam cracking,” Palanki said. “These processes require very high temperatures. If research like Dr. Wang’s can lead to the recovery of usable propylene from polypropylene, we will have reduced energy and emissions throughout the lifecycle of these plastics, and we’ll be improving productivity and efficiency for U.S. manufacturers.”

And microwave-based upcycling isn’t just for polypropylene – it can work for all types of plastics. Wang has already begun focusing on applying his research to polyethylene and polystyrene, two other single-use plastics that are used in products like packaging and bottles and that have very low recycling rates.

His process uses microwaves to heat an intermediary “catalyst” material, which transfers the heat to the polypropylene waste in order to break it down. He and his graduate and undergraduate students are partnering with the Advanced Photon Source facility at Argonne National Laboratory in Illinois for access to cutting-edge technology like the synchrotron, a kind of particle accelerator.

“The students are learning about plastic upcycling mechanisms from Argonne engineers like Tao Li, from Northern Illinois University, who’s an expert in the catalytic reaction processes we’re interested in,” Wang said. “We’re building the relationship between our academic research program and a national lab, and it’s an opportunity for WVU students to be part of building a solution for the problem of America’s abundant plastic waste.”