Fukui Researchers Recolour Polyester With Supercritical CO2

University of Fukui researchers have recoloured polyester fabric in 90 minutes using supercritical CO2 (scCO2, carbon dioxide held above its critical point so it behaves as both gas and solvent), bypassing the dye-stripping step that makes coloured polyester costly to recycle.

The study, published in The Journal of Supercritical Fluids, treated polyester samples dyed blue, yellow and red at 120°C and 25 MPa. The dyes migrated between fibres until the fabrics reached uniform new shades. Microscopic analysis showed even dye distribution, and the fibre structure remained unchanged — a critical point for maintaining yarn quality in fibre-to-fibre recycling (turning old garments into new fibre rather than downcycling into insulation or wiping cloths).

What problem does this address?

Colour removal is one of the stubborn cost points in polyester recycling. Conventional stripping consumes large volumes of water, energy and chemicals, and it often degrades the polymer. The researchers cite a stark baseline: more than 92 million tonnes of textile waste are generated globally each year, yet less than 1% of coloured textiles go through closed-loop recycling.

"The textile industry currently faces a major challenge: large quantities of used textiles are discarded, while recycling coloured fibres can require dye removal, along with huge quantities of chemicals and water," senior assistant professor Jianhua Ren said. "Our research explores a way to change the colour of polyester fibres directly by redistributing the dyes already in them using scCO2."

How solid is the evidence?

The team measured colour strength before and after treatment using the Kubelka–Munk function (K/S), a standard colour-science calculation relating reflectance to dye concentration. By combining dyed fabrics in specific proportions, they achieved almost identical target colours across samples after 90 minutes.

They then built a predictive model that accounts for interactions between dye types, and the model significantly improved prediction accuracy for colour strength at multiple wavelengths. For recyclers, that framework matters more than the single experiment: it suggests a route to specifying output shades the way a dyehouse specifies a lab dip, rather than accepting whatever colour mixed waste happens to yield.

What the study does not show

Every claim here carries a caveat, and the researchers state the main one themselves. The experiments used fabrics with known dye types only. Mixed post-consumer waste — the actual feedstock of recycling operations — was not tested.

Professor Kazumasa Hirogaki of the University of Fukui said the work demonstrates a possible path to modify polyester colours without removing existing dyes. "The next step is to determine how the approach performs with real textile waste and more complex dye combinations," he said.

The team also acknowledged that industrial-scale validation remains outstanding. Lab conditions of 25 MPa pressure imply capital equipment that few sorting or recycling facilities currently operate, and the study reports no cost, throughput or energy-comparison data against conventional stripping. Sourcing teams should read this as a laboratory proof of concept plus a predictive framework, not a commercial process.

What comes next

The work signals an alternative strategy to current fibre-to-fibre routes: redistribute existing dyes rather than remove them. The researchers will now test the method against real textile waste, more complex dye combinations and larger volumes — the trials that will determine whether scCO2 recolouring can move from the lab to a recycling line.