Ellen Phiddian
Cosmos science journalist
A team of US and Chinese researchers has found a simple way to make soap out of plastic, perhaps creating a solution to some plastic pollution.
The team was able to break the long, hard to degrade molecules in polyethylene and polypropylene plastic (called polymers) into shorter molecules, yielding a waxy substance that can be turned into fatty acids.
These fatty acids can then be turned into a variety of useful materials, including soap.
The technique, described in Science, revolves around the similarity of plastic and soap molecules.
Both polyethylene and fatty acid molecules have long carbon chains attached to them, but polyethylene chains are much longer.
These chains can be broken up with heat, but this typically makes them far too short: they break into individual carbon atoms and then become polluting carbon dioxide molecules.
Senior author Guoliang Liu, an associate professor of chemistry at the Virginia Tech college of science, says he first came up with a solution while watching a wood fire on a winter evening.
“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.
“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 and colleagues built an oven which heated polyethylene at its base, but then cooled it at the top to stop it from breaking up too much.
This technique is called temperature-graded thermolysis.
The residue from the oven yielded waxes, which could then be converted into fatty acids.
From there, the product could be turned into a wide range of things, including more plastic, but the researchers focussed on soap and detergents. Soap is quite high value compared to other fatty acid products, which makes the process economically attractive, and the global demand for soap is similar to the demand for plastics.
The researchers have shown that their technique also works on polypropylene.
“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,” says co-lead author Zhen Xu, a PhD student at Virginia Tech.
In a second study, also published in Science, a team of US researchers found another pyrolysis method can convert dirty waste plastics into a class of molecules called olefins, which they could then convert into molecules called aldehydes.
Typically made from fossil fuels in high energy reactions, these substances are a key feedstock for a huge range of products, including plastics, resins, fibres, and pharmaceuticals.
The researchers believe that things made from their former plastic waste would be 60% less carbon intensive than the same materials made from petroleum stocks.