Recolouring polyester without stripping out its old dyes could make fibre-to-fibre recycling simpler. Researchers at the University of Fukui in Japan have used supercritical carbon dioxide (scCO2) to move existing dyes between differently coloured polyester fabrics, giving them a new, uniform colour. They also built a model that predicts the resulting colour strength. The work, reported in The Journal of Supercritical Fluids, was described in a University of Fukui story carried by TechXplore on 22 October 2026.
Why coloured polyester is hard to recycle
According to the release, more than 92 million tons (83 million metric tonnes) of textile waste are generated each year, yet less than 1% of coloured textiles go through closed-loop recycling. Polyester is one of the most widely used synthetic fibres. Removing its dyes can require large amounts of toxic chemicals, water or energy-intensive processing, which creates barriers to turning old fibres into new ones.
The study was led by senior assistant professor Jianhua Ren of the Headquarters for Carbon Neutral Initiatives at the University of Fukui, with professor Kazumasa Hirogaki and researcher Isao Tabata of the university’s School of Engineering. “Our research explores a way to change the color of polyester fibers directly by redistributing the dyes already in them using scCO2,” says Ren.
How recolouring polyester with CO2 works
The team dyed polyester fabrics with three dyes, Blue BG, Yellow G and Red 4 G, then treated differently coloured fabrics together in scCO2 at 120°C and 25 MPa. The scCO2 penetrates the polyester fibres and helps the dye molecules move from areas of higher concentration to lower concentration until they are redistributed.
In one experiment, blue-dyed and yellow-dyed fabrics were combined at a 1:3 ratio. After 30 minutes, the two fabrics showed different greenish shades. After 60 minutes, their colours were nearly identical, and after 90 minutes both had reached the same green. Changing the starting proportions shifted the shade from yellowish green towards blue green, and combining all three dyes produced a wider range of colours.
A model that predicts the colour
The researchers measured colour strength using the Kubelka–Munk function, known as K/S. The measured values were generally slightly lower than the theoretical sum of each dye’s contribution, so they built a colour-matching model that accounts for pairwise interactions between dyes. The interaction coefficients were fitted from experimental data using a trust region reflective algorithm.
The model cut prediction error, measured as root mean square error, from 0.4388 to 0.1720 at 410 nm, from 0.7758 to 0.2624 at 530 nm and from 0.4017 to 0.0540 at 680 nm. Microscopy showed that the dye was evenly distributed and that the fibre microstructure was unchanged.
What recolouring polyester could mean for recycling
The approach could offer a way to reuse coloured polyester without first removing its dyes or adding new ones. “The study provides a way to explore color modification without first removing the dyes from polyester,” says Hirogaki. For another story on giving old materials a second life, see our report on WPI’s upcycled battery materials.
The case for and against
The case for this work is that it flips the usual recycling problem. Instead of fighting to get dyes out, it reuses them, and the predictive model means the final shade can be planned rather than guessed.
The case for caution is that the study used fabrics with known dyes, not mixed textile waste. The researchers themselves say the next step is to test real waste, more complex dye combinations and larger-scale processing. The release also does not give figures for the method’s energy or CO2 use.
My view on recolouring polyester
As a writer, I feel this is a clever idea for one of fashion’s messiest problems. Old clothes come in every colour, and a process that blends them into a predictable new shade sounds useful. Real waste bins are far messier than a lab, so I will be watching for those next tests.
FAQ: University of Fukui recolouring polyester study
How does recolouring polyester with supercritical CO2 work?
Differently dyed polyester fabrics are treated together in scCO2 at 120°C and 25 MPa, and their dyes redistribute into a new uniform colour.
Can the final colour be predicted?
Yes. The team built a colour-matching model that accounts for interactions between dyes and sharply reduced prediction error.
Has the method been tested on real textile waste?
Not yet. The study used fabrics with known dyes, and testing with real waste is the next step.