New colour without stripping: supercritical CO₂ redistributes dyes in polyester
A University of Fukui team redistributes existing dyes between differently coloured polyester fabrics. It could simplify fibre-to-fibre recycling – but has so far only been tested in the lab.
Symbolic image · Photo: Daniel Capilla / Wikimedia Commons, CC BY-SA 4.0 (cropped and resized)
Coloured polyester is hard to recycle: removing the dyes takes large amounts of chemicals, water or energy. A team at the University of Fukui in Japan is taking a different route. Jianhua Ren, Kazumasa Hirogaki and Isao Tabata treated differently dyed polyester fabrics together with supercritical carbon dioxide at 120 degrees Celsius and 25 megapascals. The existing dyes migrate from higher to lower concentrations until the fabrics share a uniform new shade – without adding any new dye.
In one experiment, the researchers combined blue and yellow fabric at a 1:3 ratio. After 30 minutes the greens still differed, after 60 minutes they were nearly identical and after 90 minutes identical. Changing the proportions shifted the shade from yellowish green to blue green. A model that accounts for interactions between dyes predicted colour strength much more accurately than simple addition; microscopy showed the fibre structure was unchanged. The study has been online since May and will appear in The Journal of Supercritical Fluids on 1 November.
According to the university, more than 92 million tons of textile waste are generated worldwide each year, yet less than one percent of coloured textiles undergo closed-loop recycling. However, the researchers used fabrics with known dyes, not real used clothing. "The next step is to determine how the approach performs with real textile waste and more complex dye combinations," says Hirogaki.