They used a chemical reaction called microwave-assisted glycolysis, which can break up large chains of molecules — polymers — into smaller units, with the help of heat and a catalyst. They used this to process fabrics with different compositions, including 100% polyester and 50/50 polycotton, which is made up of polyester and cotton.
For pure polyester fabric, the reaction converted 90% of the polyester into a molecule called BHET, which can be directly recycled to create more polyester textiles. The researchers found that the reaction didn’t affect cotton, so in polyester–cotton fabrics, it was possible to both break down the polyester and recover the cotton.
Neat, so they can quickly break down the polyester back into monomers, and do so selectively without breaking down the cotton.