Cotton-polyester blends are popular fabrics, combining the comfort of cotton with the performance of polyester. But once you've combined those things—cotton fibers and fibers spun from PET—they're virtually impossible to separate, making recycling difficult.

However, a group of Chemistry students at the University of Copenhagen have figured out how to do it. The group had developed a method of breaking down PET bottles using hartshorn salt (a leavening agent used by bakers), an unspecified solvent (that they say is non-toxic), and high heat. They found that their process successfully depolymerized the PET bottles, breaking them down into their monomers.

Incredibly, when they tried their process on cotton-poly-blend clothing, they found that their salt/solvent mixture not only broke the polyester down, but left the cotton perfectly intact. "Due to the mild nature of the chemicals involved," the write, "the cotton fibers remain intact and in excellent condition."

Cotton fibers successfully left intact

The broken-down polyester

The group says their process is both practical and scalable.

"We can take a polyester dress, cut it up into small pieces and place it in a container. Then, add a bit of mild solvent, and thereafter hartshorn salt, which many people know as a leavening agent in baked goods. We then heat it all up to 160 degrees Celsius and leave it for 24 hours. The result is a liquid in which the plastic and cotton fibers settle into distinct layers. It's a simple and cost-effective process," explains Shriaya Sharma, a doctoral student of the Jiwoong Lee group at the Department of Chemistry and study co-author.

"The textile industry urgently requires a better solution to handle blended fabrics like polyester/cotton. Currently, there are very few practical methods capable of recycling both cotton and plastic—it's typically an either-or scenario. However, with our newly discovered technique, we can depolymerize polyester into its monomers while simultaneously recovering cotton on a scale of hundreds of grams, using an incredibly straightforward and environmentally friendly approach. This traceless catalytic methodology could be the game-changer," explains postdoc Yang Yang of the Jiwoong Lee group at the University of Copenhagen's Department of Chemistry, who serves as the lead author of the scientific research article.

"We're hoping to commercialize this technology that harbors such great potential. Keeping this knowledge behind the walls of the university would be a huge waste," concludes Yang Yang.