Plastic bags and bottles can be recycled into soap

The plastics polyethylene and polypropylene, which are used in shopping bags, bottles and food packaging, can be turned into the ingredients for detergents

By Chen Ly

10 August 2023

Water bottles like this one, held by researcher Greg Liu, can be melted into useful ingredients

Steven Mackay for Virginia Tech

Two types of common plastic can be turned into the key molecules that are used to make soap.

“Plastic pollution is a major problem we face today,” says Greg Liu at Virginia Tech. So, he and his colleagues looked to see whether discarded polyethylene, a major contributor to pollution used in shopping bags and bottles, could be made into something more useful.

Polyethylene is made up of long carbon chains, which have a similar structure to the fatty acids used to make surfactants, an ingredient in soaps and detergents, says Liu.

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“We realised that if we can somehow break the polyethylene chain into smaller chunks and then add acid groups at the end of the chain, then we pretty much make a fatty acid,” he says.

By using an oven that was hotter at the bottom and cooler at the top, the team heated polyethylene to a high enough temperature to break the chains down into smaller pieces without it turning into gas.

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The resulting product was short-chain polyethylene, a type of wax. The researchers then added oxygen molecules to the end of these short chains of wax, effectively turning them into fatty acids.

A chemical reaction called saponification can then be used to turn the fatty acids into soap.

The plastic-derived fatty acids are chemically no different to fatty acids obtained from traditional sources, such as animals. This means it can be used to make a host of products, including bars of soap or laundry detergent.

The researchers found that this upcycling method also works on polypropylene, another pervasive type of plastic found in coffee cups and food packaging.

As well as helping to cut down on plastic pollution, Liu says that the process provides another stream of fatty acids for the detergent-making industry, reducing the dependence on animal fats.

The researchers hope to explore whether this upcycling process can be applied to other types of plastics.

Journal reference

Science DOI: 10.1126/science.adh0993

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