Scientists turn plastic waste into soap

The method could be effective in recycling plastics from milk cartons, food containers and plastic bags, say researchers at the Virginia Tech College of Science.

The chemical structure of polyethylene – one of the most commonly used plastics in the world today – is strikingly similar to that of fatty acids, which are 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. Due to this chemical similarities, the research team set out to develop a process to convert polyethylene into fatty acids, which could then be used to produce soap.

Guoliang 'Greg' Liu, associate professor of chemistry, came up with the idea while looking at a fireplace. He then began to wonder what would happen if polyethylene could be burned in a safe laboratory setting.

"Firewood is mostly made of polymers such as cellulose," Liu said. "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."

Liu's team built a small, oven-like reactor to heat polyethylene in a process called temperature-gradient thermolysis.

At the bottom, the oven is at a high enough temperature to break the polymer chains, and at the top, the oven is cooled to a low enough temperature to stop any further breakdown. After the thermolysis, they gathered the residue and found that it was composed of 'short-chain polyethylene' or, more precisely, waxes.

This was the first step in developing a method for upcycling plastics into soap, Liu said. Upon adding a few more steps, including saponification, the team made the world's first soap out of plastics.

"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 Zhen Xu, a lead author on the paper.

"This will enlighten people to develop more creative designs of upcycling procedures in the future."

Although polyethylene was the plastic that inspired this project, the upcycling method can also work on polypropylene. These two materials make up much of the plastic consumers encounter every daily.

One of the most promising 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.

"It should be realised 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."

The researchers' findings were published today in the journal Science.