Supercritical CO₂ redistributes polyester dyes for fiber recycling

Dyed polyester fibers can acquire a new, uniform color by redistributing the dyes already present in them under supercritical carbon dioxide, according to an experimental study led by researchers at the University of Fukui in Japan. Differently colored polyester fabrics were treated together at 120 degrees Celsius and 25 megapascals, allowing their dyes to move and reach an even distribution without first removing the original colorants. By changing the proportions of the starting fabrics, the team produced several new shades, while a model incorporating interactions among dyes predicted the resulting color strength more accurately than a simple additive calculation.

The work demonstrates a laboratory route for modifying colored polyester that may support fiber-to-fiber recycling. Colored polyester presents a distinct challenge in textile recycling because removing dyes can require substantial quantities of chemicals, water or energy. Polyester is one of the most commonly used synthetic fibers in the textile industry, and the study examined a way to avoid the initial dye-removal step.

Instead of extracting colorants from the fiber, the researchers combined polyester fabrics carrying different colors and used supercritical carbon dioxide, abbreviated scCO₂, to promote movement of the dye molecules. Their objective was to retain the polyester fibers and their existing dyes while converting contrasting colors into a shared shade. The study was conducted by Senior Assistant Professor Jianhua Ren from the Headquarters for Carbon Neutral Initiatives at the University of Fukui, Professor Kazumasa Hirogaki and Researcher Isao Tabata from the university’s School of Engineering.

The experimental materials consisted of polyester fabrics dyed with Blue BG, Yellow G or Red 4 G. The researchers paired fabrics of different colors and varied the proportions in which those fabrics entered the treatment. All treatments were conducted at 120 degrees Celsius and 25 megapascals, allowing the researchers to examine dye migration and color development under a consistent set of high-pressure conditions. Under the treatment conditions, scCO₂ penetrated the polyester fibers and promoted dye movement.

Dye molecules migrated from regions of higher concentration toward regions of lower concentration, continuing until redistribution across the combined fabrics produced a more even color. Microscopy subsequently showed that the dyes were distributed evenly through the treated material, providing a physical observation consistent with the measured shift toward a uniform appearance. Time-dependent tests illustrated how two contrasting colors converged during treatment.

Blue BG-dyed fabric and Yellow G-dyed fabric were combined at a ratio of 1:3. After 30 minutes, the fabrics displayed different greenish shades. At 60 minutes, their colors had become nearly identical, and after 90 minutes both had reached the same green shade.

The progression from visibly different colors to a shared color supports the observation that dye migration continued over the course of the treatment. Changing the initial proportions shifted the final shade from yellowish green toward blue green, showing that the mixture of starting colors could be used to tune the outcome. The researchers expanded the experiments to three dye combinations, which produced a wider range of colors.

To describe these outcomes quantitatively, they measured color strength using the Kubelka–Munk function, expressed through K/S values. Their measurements were generally slightly lower than the theoretical sum of the individual contributions from the starting dyes. This difference meant that a straightforward calculation based on each dye’s separate contribution did not fully capture the observed color strength after redistribution.

The team therefore developed a color-matching model that included pairwise interactions between the dyes to account for the departures from additive behavior. Interaction coefficients in the model were determined from experimental data with a trust region reflective algorithm. Its performance improved substantially over the simple additive calculation at all three evaluated wavelengths.

At 410 nanometers, root mean square error decreased from 0.4388 to 0.1720. At 530 nanometers, it fell from 0.7758 to 0.2624, while at 680 nanometers it declined from 0.4017 to 0.0540. The model’s residuals were lowest at 680 nanometers in the reported comparison.

These measurements show that including interactions among dyes improved the agreement between predicted and experimentally observed color strength after supercritical-CO₂ treatment. Microscopy also showed that the polyester fiber microstructure remained unchanged after treatment. In combination with the observed dye redistribution, this result distinguishes the process from one in which the fibers themselves were structurally transformed: the reported modification was a change in the distribution of color within the existing polyester.

The researchers’ broader aim was to explore whether differently colored polyester fibers could be given a new uniform color while retaining both the synthetic material and its original colorants. The experiments further demonstrated that starting proportions, treatment duration and a predictive color model could be connected to a defined color outcome in the laboratory. The findings remain bounded by the materials and conditions tested.

The researchers used polyester fabrics with known dye compositions rather than real textile waste, and the experiments covered a limited set of dye combinations and starting proportions. The source study identifies testing with real waste streams, more complex dye combinations and larger-scale processing as necessary next steps. It also states that the method could help reuse colored polyester without first removing its dyes or adding new ones, but the present evidence is a laboratory demonstration rather than validation across the range of material encountered in discarded textiles.

The authors’ next phase of work specifically includes evaluating how the approach performs with actual textile waste and more complicated mixtures of dyes. “Our results show that differently colored polyester fibers can be given a new uniform color by allowing their existing dyes to redistribute,” Ren said. “By changing the proportions of the starting colors, we produced a range of shades and used the model to predict color strength after treatment.” The study, titled “Sustainable and predictable color change of dyed polyester fibers through supercritical carbon dioxide fluid treatment,” was made available online on May 29, 2026, and is scheduled for publication in Volume 237 of The Journal of Supercritical Fluids on November 1, 2026.

Taken together, the experiments show that scCO₂ can mediate dye movement in polyester, that the proportions of differently colored fibers influence the final shade, and that a model accounting for dye interactions provides more accurate estimates of color strength than simple addition.

Subject of Research: Experimental supercritical-CO₂ recoloring of dyed polyester for fiber-to-fiber recycling

Article Title: Recoloring polyester with supercritical CO₂ could simplify fiber-to-fiber recycling

Article References: Recoloring polyester with supercritical CO₂ could simplify fiber-to-fiber recycling. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: polyester recycling, supercritical carbon dioxide, dye redistribution, fiber-to-fiber recycling, textile dyeing, color matching, sustainable textiles, Kubelka–Munk function, polymer processing, Recoloring, polyester, supercritical