Soap from plastic – chemists make magic

Groundbreaking research that grew out of a chemistry professor watching a fire burning in his fireplace one winter evening could one day make it economically viable to turn two common kinds of low value plastic waste into a high-value, useful product – soap.

The upcycling method is simple enough that it could be a good start for addressing the global challenge of plastic pollution, says Virginia Tech chemistry professor Guoliang “Greg” Liu. He and his team have produced soap using the method, their latest discovery in working to expand the frontiers of plastic recycling.

This breakthrough came from realizing the striking similarities between the chemical structure of polyethylene, and a fatty acid which is used as a chemical precursor to soap. Liu had long felt it should be possible to convert polyethylene into fatty acids—and with a few additional steps– produce soap. 

One winter evening by a fireplace, watching the smoke rise from the fire, he was struck by inspiration. Thinking about how the smoke was made up of tiny particles produced as the wood burned, Liu began to wonder what would happen if polyethylene could be burned in a safe laboratory setting. 

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

“Firewood is mostly made of polymers such as cellulose,” he says. “The combustion of firewood breaks these polymers into short chains, and then into small gaseous molecules before full oxidation to carbon dioxide. 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 help of two Ph.D. chemistry students in his lab, Zhen Xu and Eric Munyaneza, Liu built a small, oven-like reactor to heat polyethylene in a process called temperature-gradient thermolysis. The residue, composed of wax molecules, was the first step. Adding a few more steps, including saponification, the team made the world’s first soap out of plastics.

The upcycling method works on both polyethylene and polypropylene, which make up much of the plastic we encounter daily, and it’s not necessary to separate them. Compared by weight, soaps can actually be worth double or triple the price of plastics. The average price of soap and detergent is about $3,550 per metric ton, while polyethylene is about $1,150 per metric ton. 

Over time, he hopes recycling facilities around the world will begin to implement this technique, so consumers one day will be able to buy revolutionary sustainable soap products that also lead to reduced plastic waste in landfills.

(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. Photo by Steven Mackay for Virginia Tech.

“I think it’s important for people to realize that big global challenges like plastic waste can have — and most likely demand — multiple solutions,” said Liu. “We at Virginia Tech can contribute a small piece to the big puzzle and offer solutions to positively impact the world.”

It is his team’s second big discovery in terms of recycling plastics. In 2022, the team published a new method for recycling polystyrene, a major component in Styrofoam, which creates a very useful product called diphenylmethane (DPM). 

DPM is used as a precursor in drug development, polymer manufacturing, and even as a fragrance in consumer products. Its market price is 10 times higher than other materials that can currently be made from recycled polystyrene. An analysis by business experts from Santa Clara University and Dongbei University of Finance and Economics showed that because DPM has such a high economic value, the costs of collecting and processing the polystyrene would be fully justified.


An unexpected way to upcycle: Plastic waste transforms into soap. Virginia Tech news, Aug. 10, 2023

Researchers develop method for upcycling plastic waste into soap., Aug. 10, 2023

Researchers find a new method for recycling polystyrene. K-MAG, Nov. 10, 2022

From plastic waste to sustainable soap! An up-cycling innovation for a cleaner planet. Optimist Daily, Aug. 16, 2023

Virginia Tech researcher finds a new method for recycling polystyrene. Virginia Tech News, Aug. 19, 2022

Cover image: Researchers sort through various objects made from polystyrene, a type of plastic that is rarely recycled. Photo by Reilly Henson for Virginia Tech.