Researchers have developed a new method of recycling plastics such as plastics from milk cartons, food containers and plastic bags into soap. Method: Heat long carbon chains in plastics and then cool them quickly.
Virginia Tech researchers have developed a new technique for turning plastics into valuable chemicals called surfactants, which are used to make soap, detergent and more.
Plastics and soaps tend to have little in common in terms of texture, appearance, and most importantly, how to use them. At the molecular level, however, there is a surprising connection between the two: the chemical structure of polyethylene – one of the most commonly used plastics in the world – is strikingly similar to that of fatty acid, which is used as a chemical precursor to soap. Both materials are made of long carbon chains, but fatty acids have an extra group of atoms at the end of the chain.
Guoliang “Greg” Liu, an associate professor of chemistry at the Virginia Tech College of Science, has long felt that this similarity implies that it should be possible to convert polyethylene into fatty acids—and with a few more steps to the process—to make soap. The challenge was how to split a long polyethylene chain into many short—but not too short—chains, and how to do it efficiently. Liu believed there was potential for a new upcycling method that could take low-value plastic waste and turn it into a high-value, useful commodity.
After pondering this question for some time, Liu was struck by inspiration while enjoying a winter evening by the fireplace. He watched the smoke rise from the fire and thought about how smoke is made up of tiny particles created when wood burns.
Although plastics should never be burned in a fireplace for safety and environmental reasons, Liu began to wonder what would happen if polyethylene could be burned in a safe laboratory environment. Would incomplete burning of polyethylene produce “smoke” just like burning wood? If someone caught that smoke, what would it be made of?
“Firewood is mostly made from polymers such as cellulose. Burning firewood breaks these polymers into short chains and then into small gaseous molecules before being fully oxidized to carbon dioxide,” said Liu, who holds the Blackwood Junior Faculty Fellowship of Life Sciences in the Department of Chemistry. “If we break down synthetic polyethylene molecules in a similar way, but stop the process before they break down into small gas molecules, then we should get short-chain polyethylene-like molecules.”
With the help of Zhen Xu and Eric Munyaneza, two Ph.D. chemistry students in Liu’s lab built Liu a small furnace-like reactor where they could heat polyethylene in a process called temperature gradient thermolysis. At the bottom, the furnace is at a high enough temperature to break the polymer chains, and at the top, the furnace is cooled to a low enough temperature to stop any further breakdown. After thermolysis, they collected the residue—much like cleaning soot from a chimney—and discovered that Liu’s hunch was correct: it consisted of “short-chain polyethylene,” waxes to be exact.
This was the first step in developing a method for turning plastics into soap, Liu said. After adding a few more steps, including saponification, the team produced the world’s first plastic soap. To continue the process, the team enlisted the help of experts in computational modeling, economic analysis, and more.
Some of these experts were introduced to the team through connections with the Macromolecules Innovation Institute at Virginia Tech. Together, the group documented and refined the upcycling process until it was ready to be shared with the scientific community. The work was recently published in a journal Science.
“Our research shows a new way to recycle plastics without the use of new catalysts or complex procedures. In this work, we have shown the potential of a tandem strategy for plastic recycling,” said Xu, lead author of the paper. “This will enlighten people to develop more creative designs for upcycling practices in the future.”
Although polyethylene was the plastic that inspired this project, the upcycling method can also work on another type of plastic known as polypropylene. These two materials make up most of the plastics that consumers encounter every day, from product packaging to food containers to fabrics. One of the exciting features of Liu’s new upcycling method is that it can be used on both of these plastics at once, meaning there is no need to separate them from each other. This is a big advantage over some recycling methods used today, which require careful sorting of plastics to prevent contamination. This sorting process can be quite difficult as both plastics are similar to each other.
Another advantage of the upcycling technique is that it has very simple requirements: plastic and heat. Although later steps in the process require some additional ingredients to convert the wax molecules into fatty acids and soap, the initial transformation of the plastic is a straightforward reaction. This contributes to the economy of the method and also to its relatively small impact on the environment.
For upcycling to be effective on a large scale, the end product must be valuable enough to cover the costs of the process and make it more economically attractive than alternative recycling options.
While soaps may not seem like a particularly expensive item at first, they can actually be double or triple the price of plastic by weight. Currently, the average price of soap and detergent is about $3,550 per ton, and the price of polyethylene is about $1,150 per ton. In addition, the demand for soaps and related products is comparable to the demand for plastics.
This research lays the groundwork for a new way to reduce waste by channeling used plastics into making other useful materials, Liu said. In time, he hopes that recycling facilities around the world will begin to implement the technique. If so, then consumers can expect to one day have the opportunity to buy revolutionary sustainable soap products that also reduce plastic waste in landfills.
Because of this, turning plastics into soaps can be shown to be economically viable, added Liu, who is also an associate faculty member in the nanoscience program, which is part of the College of Science’s Academy of Integrated Sciences as well as the Department of Materials Science and Engineering. Engineering at the Virginia Tech College of Engineering.
“It is important to realize that plastic pollution is a global challenge rather than a problem of a few major countries. Compared to a sophisticated process and a complex catalyst or reagent, a simple process can be more affordable for many other countries around the world,” Xu said. “I hope this can be a good start in the war against plastic pollution.”
Reference: “Chemical Upcycling of Polyethylene, Polypropylene and Blends into 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, August 10, 2023, Science.