Grape skins and stems can be turned into a greener plastic

Wine production doesn’t use every part of the fruit. It leaves behind a pile of skins, stems, and seeds called grape marc – but we may be able to use that detritus to help produce durable plastics. Grapes are full of a chemical compound called polyphenols, which often act as antioxidants, preventing chemical reactions in which a molecule loses its electrons and a material degrades. In the human body, these reactions can damage cells; they can also make plastics brittle when they’re exposed to light and air for a long time. Most plastics forestall these reactions through stabilisers that contain antioxidant compounds. Audrey Diouf-Lewis at the University of Clermont Auvergne in France, and her colleagues used a polyphenol cocktail extracted from grape marc to stabilise plastics and make them last longer.

Grape garbage

First, they placed the raw grape marc from Pinot noir grapes into a microwave for 20 minutes and freeze-dried the resulting liquid into a light brown powder full of polyphenols. Then they incorporated the powder into the molecular matrix of melted polypropylene, a plastic used widely in packaging and reusable containers. They pressed the treated plastic into a sheet and tested it by subjecting it to air and ultraviolet (UV) light in an “accelerated ageing chamber”, and placing it in a dark oven set to 80°C and then 90°C. In the ageing chamber, plastic without stabiliser began to crack after 25 hours. Plastic treated with the grape marc stabiliser lasted twice as long before it began to get brittle. In the 80°C oven, untreated plastic lasted 7 hours before its molecules started to oxidise, while the treated plastic lasted 94 hours. Oxidation happened a bit faster at higher temperatures. “It’s not yet as good as the commercial stabilisers currently in use,” says Diouf-Lewis. “But it is a green stabiliser, so if you add more it’s not dangerous like if you add more from a petrol source.”

A world of varieties

In this experiment, the plastic was 2 per cent stabiliser by mass. Diouf-Lewis says that it could be increased to about 15 per cent to up the protection from oxidation, and her team is testing higher concentrations now. The process might be hard to scale up, though. As with wine, the qualities of grape marc depend on the type of grape and the growing conditions. “You can have exactly the same grapes from the same region, exactly the same people who make your wine, and one year can be different from another because conditions are different,” says Dimitris Argyropoulos at North Carolina State University in Raleigh. “A cocktail that works like this today, we don’t know that it’s going to work like this tomorrow.”

New Scientist, 18 May 2018 ;

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