Scientists have discovered a surprising new recycling method making soap from plastic waste.

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Researchers have developed a new method to recycle plastic from milk cartons, food containers and plastic soap bags. Method: Heat long carbon chains in plastic and then cool quickly. Virginia Tech researchers have developed a new technology that converts plastic into valuable chemicals called surfactants, which are used in soaps, detergents and more. Plastic and soap have very little in common in texture, appearance and most aspects. What matters is how they are used. But at the molecular level, there is a surprising connection: polyethylene, one of the most widely used plastics in the world today, has a strikingly similar chemical structure to the fatty acids used as chemical precursors in soap. Both materials consist of long carbon chains, but fatty acids have an additional group of atoms at the end of the chain.

Guoliang “Greg” Liu, an associate professor of chemistry at Virginia Tech’s College of Science, has long argued that this similarity means it should be possible to convert polyethylene into fatty acids (and the process requires a few extra steps) to make soap. The challenge is how to split the long polyethylene chains into many short chains (but not too short) and how to do it efficiently. Liu believes there is potential for a new recycling method that can turn low-value plastic waste into valuable, useful raw materials. After thinking about the problem for some time, Liu was struck by an inspiration while enjoying a winter evening. Stove. He watched the smoke rise from the fire and thought about how the smoke was made up of tiny particles that form when wood is burned.

Guoliang “Greg” Liu holds a common water jug in his lab at Hahn Hall South. Credit: Photo by Steven Mackay for Virginia Tech.

Although plastic should never be burned in fireplaces for safety and environmental reasons, Liu began to wonder what would happen if polyethylene could be burned in a safe laboratory environment. Does incomplete burning of polyethylene produce “smoke” like burning wood? If someone caught this smoke, what would it be made of? “Firewood is primarily made of polymers such as cellulose. Burning wood breaks these polymers into short chains and then into small gaseous molecules, which are then completely oxidized to carbon dioxide,” said Liu, a Blackwood Life Sciences Junior recipient. Faculty scholarship in the Department of Chemistry. “If we break down synthetic polyethylene molecules in the same way, but stop the process before they completely break down into small gas molecules, then we should get short-chain polyethylene-like molecules.”

With the help of Xu Zheng and Eric Munyanez, two Ph.D. As a chemistry student in Liu’s lab, Liu built a small furnace-like reactor in which polyethylene could be heated using a process called temperature gradient pyrolysis. The bottom of the furnace is at a temperature high enough to break the polymer chains, while the top of the furnace cools down to a temperature low enough to stop any further degradation. After pyrolysis, they collected the residue (similar to cleaning soot from a chimney) and discovered that Liu’s hunch was correct: it consisted of “short-chain polyethylene” or, more precisely, wax.

A flask filled with waxes generated from waste polyethylene and polypropylene is heated in an oil bath, and the waxes are oxidized by a stream of airflow to produce fatty acids via catalytic oxidation. Credit: Photo by Steven Mackay for Virginia Tech.

Liu said this is the first step in developing a way to turn plastic into soap. After adding several steps, including saponification, the team created the world’s first plastic soap. To continue this process, the team enlisted the help of computational modeling, financial analysis and other experts. Some of these experts were brought to the team through contacts with Virginia Tech’s Institute for Macromolecular Innovation. Together, the group documented and refined the recycling process until it was ready to be shared with the scientific community. The work was recently published in the journal Science.

“Our research shows a new way to recycle plastics without using new catalysts or complicated procedures. In this work, we demonstrate the potential of a tandem strategy for plastic recycling,” said Xu, lead author of the paper. “This will inspire more creative recycling programs in the future.” Although polyethylene is the plastic that inspired the project. , the upcycling method also works with other plastics that called polypropylene. These two materials make up most of the plastics that consumers come into contact with every day, from product packaging to food containers to fabrics. An exciting feature of Liu’s new recycling method is that it can be used with both plastics, meaning they don’t have to be separated. This is a major advantage over some recycling methods used today, which require careful sorting of plastics to avoid contamination. Because the two types of plastic are so similar, this classification process can be difficult. Another advantage of recycling is that the requirements are very simple: plastic and heat. Although later steps in the process require some additional ingredients to convert the wax molecules into fatty acids and soap, the first conversion to plastic is a simple reaction. This contributes to the method’s cost-effectiveness and relatively low environmental impact.

(From left) Eric Munyaneza and Guoliang “Greg” Liu prepare plastic materials to upcycled into a fatty acid liquid in Liu’s lab at Hahn Hall South. Munyaneza is also an author on the Science journal study. Credit: Photo by Steven Mackay for Virginia Tech.

For recycling to be efficient at scale, the end product must be valuable enough to cover the cost of the process and make it more economically attractive than other recycling options. Although soap may not seem like a very expensive item at first, it can actually weigh two or three times the price of plastic. Currently, the average price for soaps and detergents is around $3,550 per ton and for polyethylene around $1,150 per ton. Moreover, the demand for soap and related products is comparable to that of plastic. Liu said the research lays the groundwork for new ways to reduce waste by converting plastic waste into other useful materials. In time, he hopes refineries around the world will begin to adopt the technology. If so, consumers will one day be able to purchase revolutionary sustainable soap products that will also reduce plastic waste in landfills.

Because of this, turning plastic into soap may prove to be economically feasible, added Liu, who also teaches the Nanoscience course in the School of Integrated Sciences and the Academy of Sciences’ Department of Materials Science and Technology. in Engineering from the Virginia Tech College of Engineering. “It must be recognized that plastic pollution is a global challenge, not a problem of a few ordinary countries. Compared with complex processes and complex catalysts or reagents, simple processes can be adopted much more easily by many other countries in the world,” Xu said. . “I hope this can be a good start in the fight against plastic pollution.”

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