Colored polyester is difficult to recycle because the dyes are hard to remove without using large quantities of chemicals, water, or energy. Researchers developed a supercritical carbon dioxide treatment that redistributes existing dyes between polyester fibers, creating uniform colors without removing the original dyes. By controlling fiber proportions, they tuned shades and developed a model that accurately predicted color strength. This approach could support sustainable polyester recycling, although testing with real textile waste is needed.
From the clothes we wear to our living spaces, colorful textiles add vibrancy to our everyday lives. The world of the textile industry is evolving rapidly, embracing cutting-edge techniques to drive innovation and explore sustainable approaches. However, more than 92 million tons of textile waste are generated each year, yet less than 1% of colored textiles undergo closed-loop recycling. Polyester is one of the most commonly used synthetic fiber in the textile industry. Removing dyes from polyester can require substantial amounts of toxic chemicals, water, or energy-intensive processing, thereby creating barriers to fiber-to-fiber recycling.
Against this backdrop, a new study explores a different approach for fiber-to-fiber recycling: researchers use supercritical carbon dioxide (scCO₂) to redistribute dyes already present in polyester fibers and create a new shade. The research was conducted by Senior Assistant Professor Jianhua Ren from the Headquarters for Carbon Neutral Initiatives, University of Fukui, along with Professor Kazumasa Hirogaki, and Researcher Isao Tabata from the School of Engineering, University of Fukui, Japan.
"The textile industry currently faces a major challenge: large quantities of used textiles are discarded, while recycling colored fibers can require dye removal, along with huge quantities of chemicals and water. Our research explores a way to change the color of polyester fibers directly by redistributing the dyes already in them using scCO₂," shares Dr. Ren.
The researchers dyed the polyester fabrics with Blue BG, Yellow G, and Red 4 G, then treated differently colored fabrics together using scCO₂. Treatments were performed at 120°C and 25 MPa, while they varied starting fabric proportions and measured color strength using the Kubelka–Munk function (K/S). The study was made available online on May 29, 2026, and will be published in Volume 237 of The Journal of Supercritical Fluids on November 01, 2026.
The method relies on scCO₂ penetrating polyester fibers and promoting dye movement. Dye molecules migrated from higher to lower concentrations until redistribution occurred. In one experiment, Blue BG-dyed fabric and Yellow G-dyed fabric were combined at a 1:3 ratio. After 30 minutes, the fabrics showed different greenish shades. After 60 minutes, their colors became nearly identical, and after 90 minutes, both reached an identical green shade. Changing proportions shifted the shade from yellowish green toward blue green. Three dye combinations produced a wider range of colors.
The researchers quantified the color strength using the K/S values and found that the measured values were generally slightly lower than the theoretical sum of the individual dye contributions. Based on this analysis, they developed a color matching model to account for these differences using pairwise interactions between dyes. Interaction coefficients were determined from experimental data with a trust region reflective algorithm. The model reduced 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 even dye distribution, while the fiber microstructure remained unchanged.
"Our results show that differently colored polyester fibers can be given a new uniform color by allowing their existing dyes to redistribute," says Dr. Ren. "By changing the proportions of the starting colors, we produced a range of shades and used the model to predict color strength after treatment."
Prof. Hirogaki says, "The study provides a way to explore color modification without first removing the dyes from polyester. The next step is to determine how the approach performs with real textile waste and more complex dye combinations."
The approach could provide a way to reuse colored polyester fibers without first removing their dyes or adding new dyes. However, the study used fabrics with known dye compositions rather than mixed textile waste. Further research is needed to test different waste streams, dye combinations, and larger scale processing.
Overall, the study demonstrates a laboratory approach for directly changing dyed polyester color through scCO₂ treatment. By combining controlled dye redistribution with predictive color matching, the researchers provide a framework for designing new shades while retaining the polyester fibers and their existing dyes. The findings provide a basis for further investigation of sustainable polyester recycling.
