A novel technique has been devised by researchers to transform plastics, including those from milk cartons, food containers, and plastic bags, into soap. The method involves heating the long carbon chains in plastics and rapidly cooling them. Scientists from Virginia Tech have pioneered this groundbreaking method for upcycling plastics into valuable chemicals known as surfactants, which are essential for soap and detergent production.
At first glance, plastics and soaps may seem worlds apart in terms of their characteristics and uses. However, on a molecular level, there exists a surprising similarity between them. Polyethylene, one of the most widely used plastics globally, shares a striking chemical structure with fatty acids, which serve as a precursor to soap. Both materials consist of long carbon chains, albeit with fatty acids having an additional group of atoms at the end of the chain.
Guoliang “Greg” Liu, an associate professor of chemistry at Virginia Tech, recognized this molecular resemblance and believed it could be possible to convert polyethylene into fatty acids, and subsequently, into soap with some additional steps. The challenge lay in breaking down the long polyethylene chains into shorter, but not excessively short, chains efficiently. Liu envisioned the potential for a new upcycling method that could transform low-value plastic waste into a high-value, useful commodity.
After pondering this question for some time, Liu was struck by inspiration one winter evening by a fireplace. He observed how smoke arose from the fire, created from tiny particles during the combustion of wood. Although it’s vital to note that plastics should never be burned in a fireplace for safety and environmental reasons, Liu wondered what might occur if polyethylene could be safely combusted in a laboratory setting. Would this incomplete combustion produce “smoke” akin to burning wood? And if so, what would this smoke consist of?
Liu hypothesized that if they broke down synthetic polyethylene molecules similarly to how firewood polymers like cellulose were broken down into short chains during combustion, they might obtain short-chain polyethylene-like molecules. With the assistance of two Ph.D. chemistry students, Zhen Xu and Eric Munyaneza, Liu constructed a small, oven-like reactor for this purpose. This reactor utilized a process called temperature-gradient thermolysis, where the bottom maintained a high enough temperature to break polymer chains, while the top cooled to prevent further breakdown. After the thermolysis, they collected the residue, resembling cleaning soot from a chimney, and discovered that it consisted of “short-chain polyethylene” or, more precisely, waxes. This marked the first step towards developing a method for converting plastics into soap.
By incorporating a few additional steps, including saponification, the research team successfully produced the world’s first soap from plastics. To further refine the process, experts in computational modeling and economic analysis were brought on board, some through connections with the Macromolecules Innovation Institute at Virginia Tech. Together, they documented and improved the upcycling process until it was ready for dissemination within the scientific community, resulting in a publication in the journal Science.
What sets this upcycling method apart is its ability to work on both polyethylene and polypropylene plastics, which constitute a significant portion of everyday plastic products, without the need for separating them. This simplicity, coupled with the method’s minimal requirements of plastic and heat, enhances its cost-effectiveness and reduces its environmental footprint.
For large-scale upcycling to be practical, the final product must have sufficient value to cover the process costs and surpass alternative recycling options. While soap may not initially appear to be a high-value product, it can be worth significantly more than plastics when measured by weight. Currently, the average price of soap and detergent stands at approximately $3,550 per metric ton, compared to polyethylene’s $1,150 per metric ton. Moreover, the demand for soap and related products rivals that of plastics.
This research lays the foundation for a new approach to waste reduction by redirecting used plastics into the production of other valuable materials. Over time, it is hoped that recycling facilities worldwide will adopt this technique, ultimately leading to the availability of innovative, sustainable soap products and a reduction in plastic waste in landfills.
In light of these findings, the economic viability of converting plastics into soaps becomes evident. Plastic pollution is a global challenge, and the simplicity of this process could make it accessible to many countries worldwide, contributing to the fight against plastic pollution.
Reference: “Chemical upcycling of polyethylene, polypropylene, and mixtures to high-value surfactants” by Zhen Xu, Nuwayo Eric Munyaneza, Qikun Zhang, Mengqi Sun, Carlos Posada, Paul Venturo, Nicholas A. Rorrer, Joel Miscall, Bobby G. Sumpter and Guoliang Liu, 10 August 2023, Science. DOI: 10.1126/science.adh0993.