A team led by researchers from Virginia Tech has unveiled a groundbreaking method for converting plastics into valuable chemicals called surfactants, integral components in the production of soap and detergent. This innovative approach, poised to revolutionize plastic recycling, was recently featured in the esteemed journal Science.
Though plastics and soaps may seem worlds apart in terms of their physical properties and applications, an unexpected molecular similarity connects them. Polyethylene, one of the most ubiquitous plastics globally, bears a striking resemblance to fatty acids, essential in soap manufacturing. Both substances comprise lengthy carbon chains, with fatty acids featuring an extra group of atoms at their chain’s end.
Guoliang “Greg” Liu, an associate professor of chemistry at the Virginia Tech College of Science, had long contemplated this likeness, pondering the possibility of transforming polyethylene into fatty acids, and ultimately, soap. The challenge lay in breaking the lengthy polyethylene chains into shorter ones efficiently.
Looking for inspiration
Inspiration struck Liu one winter evening while relaxing by a fireplace. Watching the smoke rise from the fire, he considered the microscopic particles produced during wood combustion. Although burning plastics is strictly discouraged due to safety and environmental concerns, Liu wondered what would happen if polyethylene underwent controlled combustion in a laboratory setting.
Could the incomplete combustion of polyethylene generate “smoke” akin to wood combustion? What would make up this smoke?
“Firewood is mostly made of polymers such as cellulose. The combustion of firewood breaks these polymers into short chains, and then into small gaseous molecules before full oxidation to carbon dioxide,” explained Liu, who holds the Blackwood Junior Faculty Fellowship of Life Sciences in the Department of Chemistry. “If we similarly break down the synthetic polyethylene molecules but stop the process before they break all the way down to small gaseous molecules, then we should obtain short-chain, polyethylene-like molecules.”
With the assistance of two Ph.D. chemistry students, Zhen Xu and Eric Munyaneza, Liu constructed a compact, oven-like reactor designed for temperature-gradient thermolysis. In this process, the bottom of the oven reaches a temperature high enough to break down polymer chains, while the top remains cool enough to halt further breakdown. After thermolysis, the remaining residue could be scraped like soot from a chimney and tested. What was found confirmed Liu’s hypothesis: it comprised “short-chain polyethylene,” or more precisely, waxes.
This marked the initial step in the development of a method to convert plastics into soap. With a series of additional steps, including saponification, the team achieved a groundbreaking feat: creating the world’s first soap from plastics. To further refine the process, the team collaborated with experts in computational modelling, economic analysis, and other relevant fields.
Improving the process
Several of these experts were introduced to the team through connections with the Macromolecules Innovation Institute at Virginia Tech. Together, they meticulously documented and honed the upcycling process, preparing it for sharing with the broader scientific community.
While polyethylene served as the initial inspiration for this project, the upcycling method is equally effective with another prevalent plastic-type, polypropylene. These two materials constitute a substantial portion of daily consumer plastics, from packaging to food containers to fabrics. A remarkable aspect of Liu’s new upcycling method is its ability to process both plastics simultaneously, eliminating the need for intricate sorting. This contrasts with some existing recycling methods, which demand meticulous plastic separation to prevent contamination, a notoriously challenging task given the striking similarity between polyethylene and polypropylene.
Furthermore, the upcycling technique boasts minimal prerequisites: plastic and heat. Although later stages require additional ingredients to convert wax molecules into fatty acids and soap, the initial plastic transformation is a straightforward reaction. This simplicity contributes to the method’s cost-effectiveness and its relatively modest environmental footprint.
