Supercritical CO2 polyester test advances dye recycling research

University of Fukui researchers in Japan treated blue, yellow and red polyester samples with supercritical carbon dioxide at 120°C and 25 MPa for 90 minutes, producing almost identical new colours. The team published the work in The Journal of Supercritical Fluids.

What did the experiment establish?

The researchers combined fabrics in specific proportions and observed dyes move between fibres until their colours became uniform. Microscopic analysis showed even dye distribution, while the team measured colour strength before and after treatment with the Kubelka–Munk function, K/S. The process did not change the fibre structure.

Those measurements supported a predictive model that accounts for interactions between dye types. The model significantly improved prediction accuracy for colour strength at multiple wavelengths.

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." He noted that large quantities of used textiles are discarded, while recycling coloured fibres can require dye removal, along with huge quantities of chemicals and water.

What barrier does the process target?

The researchers cited data showing more than 92 million tonnes of textile waste generated globally each year, while less than 1% of coloured textiles undergo closed-loop recycling. They identified colour removal from polyester as a significant barrier within the textile recycling chain.

Conventional dye-removal processes typically involve substantial energy or chemical use, according to the researchers. The method offers an alternative to current fibre-to-fibre recycling approaches: it combines laboratory evidence of direct dye redistribution with a predictive framework for producing new shades. It changes dye distribution within the fibre rather than removing the dye.

Professor Kazumasa Hirogaki of the University of Fukui said the work demonstrates a possible path to modify colours in polyester without removing existing dyes. He added: "The next step is to determine how the approach performs with real textile waste and more complex dye combinations." Dye removal remains the supply-chain barrier this research targets.

What remains unproven?

The experiments used only fabrics with known dye types and did not include mixed post-consumer waste. The team acknowledged that further investigation must validate the method with real-world textile blends and at industrial scales.

Trials with actual textile waste and greater volumes remain necessary before the approach can support commercial recycling operations. The researchers reported laboratory results only, with no factory capacity data, buyer-supplier commitments or production orders stated.

What does this mean for polyester sourcing?

For polyester recyclers and brands, the immediate sourcing decision is to treat supercritical CO2 dye redistribution as a development technology rather than a production-ready recycling route. The reported result covers laboratory samples, known dyes and a predictive colour-strength model, not mixed-waste throughput.

The paper describes the process as potentially more sustainable, but it reports no industrial-scale energy, chemical or water comparison with conventional polyester recycling. It also names no specific recycling standard or law against which recyclers can verify that claim.

The next sourcing gate is validation on mixed post-consumer streams and at greater volumes. Until those results and commercial commitments emerge, polyester buyers have no confirmed, industrially validated route to replace dye-removal stages in closed-loop recycling.