Scientists from Japan said they may have discovered a process that supports sustainable polyester recycling. While real-world testing still needs to be done, the process, if it works in the field, would be groundbreaking.
The research was conducted by Jianhua Ren, senior assistant professor at 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.
In a research briefing report, the scientists said colored polyester is extremely difficult to recycle because the dyes “are hard to remove without using large quantities of chemicals, water or energy.” But they developed a “supercritical carbon dioxide treatment” that redistributes existing dyes between the polyester fibers, “creating uniform colors without removing the original dyes,” they said. By controlling fiber proportions, researchers 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,” the report noted.
“From the clothes we wear to our living spaces, colorful textiles add vibrancy to our everyday lives,” the researchers said. “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 percent of colored textiles undergo closed-loop recycling.”
“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,” Ren said. “Our research explores a way to change the color of polyester fibers directly by redistributing the dyes already in them using supercritical carbon dioxide.”
Researchers said they demonstrated the supercritical carbon dioxide treatment method at 120°C to mix and redistribute dyes—specifically blue BG, yellow G, and red 4G—across polyester fabrics. The research said by allowing dye molecules to migrate from higher to lower concentrations over time, varying fabric proportions and combinations successfully yielded uniform, blended shades.
To predict these outcomes accurately, the team said they quantified color strength using “Kubelka-Munk values” and developed a color-matching model incorporating “pairwise interaction coefficients determined” through a trust-region reflective algorithm. It’s heavy science, but it works.
“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.”
Hirogaki said 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 report noted that the findings provide a basis for further investigation into sustainable polyester recycling.