From old jeans to new textiles: BioSusTex research advances circular denim recycling

A newly published study supported by BioSusTex shows how the colour could be removed from waste denim while preserving the cotton fibres for a second life.

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Jeans are designed to last. Their durability is one reason denim has become one of the world’s most widely worn fabrics, but it also creates a challenge when those jeans reach the end of their useful life. A new paper published in RSC Advances presents a promising approach for separating the indigo dye from cotton denim without significantly damaging the underlying fibres. The study, titled ‘Development of a closed-loop solvent-based recycling process for dyed denim waste’, was published on 18 July 2026 and was carried out by researchers from Aalto University. Supported by the BioSusTex project, the work could help remove one of the barriers preventing old denim from being turned into high-quality new textiles.

Why is denim difficult to recycle?

When we think about recycling jeans, the cotton may appear to be the most valuable part. However, the blue colour that gives denim its familiar appearance is also one of the reasons it is difficult to recycle effectively. Indigo is designed to remain strongly attached to the cotton fibres throughout repeated wearing and washing. This is useful while a pair of jeans is being used, but it becomes a problem at the end of its life. The remaining colour can restrict how the material is processed and what new products can be made from it.

As a result, waste denim is often downcycled into lower-value applications such as insulation, construction materials, composites or automotive interiors. Turning it back into fibres suitable for new clothing is still far less common. Existing colour-removal methods may also require several processing stages, harsh chemicals or conditions that can weaken the cotton.

This matters because denim is produced on an enormous scale. Global production exceeds four billion pairs annually, while denim manufacturing requires substantial quantities of cotton, water and energy. When discarded jeans are landfilled or incinerated, both the material itself and the resources used to produce it are lost.

Removing the blue while keeping the cotton

The researchers investigated a process using dimethyl sulfoxide, commonly known as DMSO, to extract indigo from denim. In brief, the solvent penetrates the cotton structure and loosens the interactions that hold the indigo in place. Importantly, it removes the dye mainly through physical extraction rather than by chemically destroying either the indigo or the cellulose from which cotton is made. The process was designed as a closed loop. After the indigo was removed, the solvent was recovered by distillation and used again. The extracted indigo was also separated for potential future recovery and reuse. The research team examined different solvent compositions, temperatures, treatment times and material-to-liquid ratios to identify conditions that removed the colour effectively while limiting damage to the fabric.

Promising results

Under the tested laboratory conditions, the process achieved several important results:

• Approximately 98% of the indigo colour was removed.

• The treated denim became sufficiently light to offer potential as a feedstock for fibre-to-fibre recycling or re-dyeing.

• The cotton retained approximately 98% of its tensile strength after the first treatment cycle.

• The recovered solvent continued to perform effectively through at least five reuse cycles.

• The stripped denim could be successfully re-dyed using commercial dyes including vat, reactive and direct.

The solvent-recovery step was particularly encouraging. DMSO recovery reached 99.4%, while colour-removal performance remained above approximately 96–98% across five distillation and reuse cycles, with no noticeable decline in effectiveness. Tests also indicated that the treatment largely preserved the internal structure, thermal stability and dye-binding capacity of the cotton. The process did not simply produce a pale piece of damaged fabric: it produced material that retained many of the properties needed for further textile processing.

What could this mean for the clothes we wear?

For consumers, this research points towards a future in which an old pair of jeans could provide raw material for new garments rather than becoming waste or being converted into a lower-value product. Removing the original colour gives recyclers greater flexibility. The recovered cotton could potentially be re-dyed, blended with other fibres or dissolved and regenerated into new textile fibres. This may help manufacturers make better use of existing cotton and reduce dependence on newly produced raw materials.

The approach also illustrates what “circularity” can mean in practice. It is not enough to collect used clothing: recycling processes must preserve the quality and value of the materials inside it. A technology that removes the dye but destroys the cotton would solve only part of the problem. The strength of this work lies in addressing colour removal, fibre preservation and solvent recovery together. It could ultimately contribute to clothing that contains more recycled material without requiring consumers to accept poorer quality, limited colours or reduced durability.

An important step, although not yet an industrial process

The results are promising, but the researchers emphasise that further development is required before the process could be used commercially. The present study was conducted at laboratory scale. Future work will investigate continuous processing, heat integration and operation over more than five solvent-reuse cycles. The team also plans to carry out techno-economic and life-cycle assessments to determine how the process compares environmentally and financially with existing recycling methods. The recovered indigo also contains small amounts of solvent and cotton impurities. Additional purification will therefore be needed before the dye can be confidently reused in future colouring processes. These are essential next steps. A successful textile-recycling technology must not only work scientifically; it must also be safe, affordable, energy-efficient and suitable for operation at industrial scale.

BioSusTex research supporting more circular textiles

This publication represents an important contribution to the BioSusTex ambition of developing safer and more sustainable approaches for textile production and circularity. By demonstrating that persistent indigo colour can be removed while retaining the value of the cotton and repeatedly recovering the processing solvent, the study provides a foundation for further work towards genuinely circular denim.

The research was supported by the European Union’s Horizon Europe programme through BioSusTex “Towards absolute safe and sustainable bio-based textile” under Grant Agreement No. 101135372, together with support from Business Finland’s TexirC project.

The full open-access paper is available in RSC Advances: “Development of a closed-loop solvent-based recycling process for dyed denim waste,” DOI: 10.1039/D6RA04250C.