A team of researchers from Virginia Tech has achieved a groundbreaking milestone in plastic recycling by developing a novel method to transform plastics into high-value chemicals known as surfactants.
Surfactants play a vital role in creating products like soap and detergent. Despite the apparent dissimilarity between plastics and soaps in terms of texture and application, a molecular connection between them exists.
Polyethylene, one of the world’s most widely used plastics, shares a striking resemblance in chemical structure to fatty acids, which serve as a precursor to soap. Both materials consist of long carbon chains, with fatty acids possessing an additional group of atoms at the chain’s end.
Guoliang “Greg” Liu, associate professor of chemistry in the Virginia Tech College of Science, recognized this structural similarity and proposed the possibility of converting polyethylene into fatty acids. He envisioned a multi-step process that could lead to soap production.
The primary challenge was to efficiently break down long polyethylene chains into shorter chains while retaining their usefulness. Liu believed that this innovative upcycling method could convert low-value plastic waste into high-value commodities.
Liu’s inspiration struck while he was relaxing by a fireplace during a winter evening. Observing the smoke produced by burning wood, he speculated on whether burning polyethylene in a controlled laboratory environment could yield a similar outcome. He wondered if the incomplete combustion of polyethylene could result in a residue comparable to smoke from burning wood.
“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,” said Liu, holder of 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 Ph.D. chemistry students Zhen Xu and Eric Munyaneza, Liu developed a specialized reactor resembling an oven for temperature-gradient thermolysis. This process involved heating polyethylene in a controlled environment where higher temperatures at the bottom of the reactor would break polymer chains, while cooling at the top would prevent further breakdown. The resulting residue, similar to chimney soot, was found to contain “short-chain polyethylene” or waxes.
This breakthrough marked the first step towards upcycling plastics into soap. Following additional steps, including saponification, the team successfully produced the world’s first soap from plastics. The research was a collaborative effort involving experts in computational modeling, economic analysis, and various fields.
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 published today in the popular journal Science.
“Our research demonstrates a new route for plastic upcycling without using novel catalysts or complex procedures. In this work, we have shown the potential of a tandem strategy for plastic recycling,” said Xu, lead author on the paper. “This will enlighten people to develop more creative designs of upcycling procedures in the future.”
The researchers note that their method can also be applied to polypropylene, another common plastic. This upcycling technique eliminates the need to separate the two plastics before processing, which is a significant advantage over traditional recycling methods.
The process is notably straightforward, requiring only plastic and heat for the initial transformation. The subsequent steps involve minimal additional ingredients to convert wax molecules into fatty acids and soap. This simplicity contributes to the method’s cost-effectiveness and limited environmental impact.
One of the exciting features of Liu’s new upcycling method is that it can be used on both these plastics at once, meaning that it’s not necessary to separate the two from each other. This is a major advantage over some recycling methods used today, which require careful sorting of plastics to avoid contamination. That sorting process can be quite difficult, because of how similar the two plastics are to each other.
Notably, the upcycling technique demonstrates economic viability, as soaps and related products often command a higher price compared to plastics by weight. Currently, the average price of soap and detergent amounts to about $3,550 per metric ton, and that of polyethylene is about $1,150 per metric ton. Furthermore, the demand for soaps and related products is comparable to the demand for plastics.
This research lays the foundation for reducing plastic waste by converting it into useful materials, leading to sustainable soap products and reduced plastic landfill waste.
“It should be realized that plastic pollution is a global challenge rather than a problem of a few mainstream countries. Compared to a sophisticated process and complex catalyst or reagent, a simple process may be more accessible to many other countries worldwide,” Xu said. “I hope this can be a good start for the war fighting plastic pollution.”
Researchers from the Department of Chemical Engineering also were part of this project and the resulting research paper.
As the team continues its efforts, this innovative solution could potentially revolutionize the fight against plastic pollution on a global scale.
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