Advancing colour removal techniques for PES/CO textile recycling
Visual comparison between the original fibrous material of work shirts (35% cotton, 65% polyester) and this material treated with oxidative stripping.
Polyester (PES), a synthetic-based fibre, now represents over half of all textiles and blended fabrics, like polyester-cotton (PES/CO). Those materials are also among the most challenging to recycle, in particular because of the colourants and chemical finishings used in fabric treatment, which interfere with recycling processes, especially when the goal is to recover both polyester and cotton as high-quality raw materials.
This challenge is part of the PESCO-UP project, which focuses on developing sustainable solutions for blended textile waste. The project is working on processes to eliminate a wide range of impurities, particularly targeting the removal of colour from polyester (PES) and polyester-cotton (PES/CO) blends, while preserving the quality of the fibres or polymers as much as possible.
Why focusing on colour removal, and how can it be achieved?
PESCO-UP project partners Textile4ever, CuRe Technology, IOS, Valmet and, VTT has been testing various chemical and mechanical purification methods to make blended textiles suitable for recycling.
Removing dyes from used fabrics is more than just a cosmetic concern, as many dyes interfere with recycling by reacting poorly with chemicals, hindering fibre separation, or compromising the quality of the final product. PESCO-UP has identified colour removal as a critical step in preparing textiles for mechanical (“fibre-to-fibre") and chemical recycling, particularly in processes that aim to recover both polyester and cotton.
The partners explored several treatment options to remove dyes and other impurities:
Visual comparison between a fibrous material (50% cotton,
50% polyester) and this material treated with reductive stripping in strong alkaline media.
Reductive stripping in strong alkaline media: this chemical process uses substances like sodium hydroxide and sodium dithionite. While it proved to be one of the most effective methods for dye removal and is well suited for chemical recycling, it is important to note that strongly alkaline conditions can damage cotton and partially dissolve polyester fibres.
Oxidative stripping: another chemical approach, though less effective on more resistant dyes such as those commonly used in workwear.
Acid washing: primarily used to remove metal residues and enhance fibre brightness, often applied in combination with other stripping methods.
Sustainable solvent washing: Sustainable solvents worked very well in removing colorants from the polyester fibres dyed with sublimation inks while it showed limited applicability to remove colours from cotton.
In addition to the above treatments, the team also tested sustainable (“green”) solvents, and enzyme-based methods, particularly targeting the removal of elastane and specific fabric finishings. While some of these techniques showed also promising results on the removal of colours from pure polyester textiles, they were generally less effective on the removal of colourants from blends where both components had been dyed.
The application of sustainable solvents, need some optimization on PES/CO blends, where they are suitable for removal of certain types of dyes that are not chemically bonded to cellulose. In some cases, the polyester was completely dissolved, allowing the cotton to return to a near-original, undyed state. However, this approach is only practical when one fibre is intentionally sacrificed to recover the other, which is not ideal for achieving full circularity.
The most effective method identified for workwear colour removal so far is reductive stripping in a strongly alkaline environment. While it efficiently removes colour, it can also damage cotton fibres or partially dissolve polyester.
Workwear fabrics, often dyed with vat or reactive dyes, remain particularly challenging to decolorize. Reactive, reductive, and disperse dyes are especially resistant to removal, whereas sublimation inks used on polyester are much easier to strip. Therefore, for different feedstocks, it is essential to carefully select and adapt the decolouring method based on the specific fibre composition and the type of dyes involved. This tailored approach helps to maximize dye removal efficiency while minimizing damage to the fibres, thereby preserving the overall quality and integrity of the fabric.
Despite these trade-offs, the partners have achieved promising results, demonstrating that it is possible to remove most dyes and purify both polyester and cotton fibres to meet the standards required for further recycling steps.
What’s next?
The PESCO-UP partners are now working to scale up their methods and refine the process for industrial use. This includes optimising purification conditions such as temperature, pH, and chemical concentration to remove dyes with minimized damaging the fibres. They are also testing additional additives to boost the effectiveness of current methods, particularly for stubborn dyes. New fabric samples are being evaluated, including those designed with recyclability in mind, using dyes that are easier to remove. In parallel, the team is collaborating with clothing manufacturers to encourage the use of more recyclable materials and dyeing techniques from the outset.
The long-term vision is to establish a reliable process that fully purifies blended textiles, making it possible to recover and reuse both polyester and cotton fibers in high-quality products.
Recycling blended textiles like PES/CO is one of the biggest challenges in building a circular fashion economy. The work being done in colour removal by Textile4ever, CuRe Technology and IOS is a critical part of the process of purification of blend textiles.
While the progress made by PESCO-UP partners offers a good first step, there is no perfect solution yet. Through a combination of scientific innovation and industrial collaboration, we may soon be able to give our old clothes a truly sustainable second life.
