Gentle, Cost-Efficient Decolouring of Reactive-Dyed Cotton

Context

CHT Group is a global specialty-chemistry group with a strong focus on sustainable chemistry and deep expertise in textile finishing. This challenge addresses cotton textiles dyed with reactive dyes — a dominant dye class for cotton. Reactive dyes form a chemical bond between fibre and dye, which makes them highly wash-fast but also difficult to remove again.

CHT's role: CHT is a specialty-chemistry and textile-auxiliaries supplier, not a textile manufacturer. CHT develops and formulates the decolouring product and supplies it to its textile customers — dyehouses (colour correction) and recyclers (decolouring cotton for recycling) — who run the decolouring process on-site. CHT itself validates, tests and scales the product in its in-house laboratory jet machines.

The challenge deliberately focuses on cotton and reactive dyes. The following are explicitly relevant:

• Fibre: cotton (100% cotton in scope)
• Dye system: reactive dyes
• Application context: textile finishing (colour correction) and textile recycling

Problem Definition

CHT is looking for a gentler alternative to the aggressive state-of-the-art decolouring process. The core issues are:

• Fibre damage: current bleaching and reduction chemistry (hydrosulphite, caustic soda, chlorine) reduces the tensile strength and durability of the cotton fibre, limiting its reuse and recyclability.
• Sustainability: the process relies on aggressive chemicals; a milder, more sustainable process (e.g. enzymatic, aqueous, lower-temperature) is desired.
• Cost sensitivity: the market will not pay a premium for sustainability alone — the process must be cost-competitive with today's very cheap standard auxiliaries.
• Process compatibility: the solution must fit existing dyehouse equipment and workflows, i.e. aqueous treatment with standard textile finishing equipment such as jet-machines, within a reasonable cycle time.

Knowledge Base

The challenge covers two use cases that share the same underlying need — a solution that works for one is expected to work for the other:

Use case 1 — Textile finishing (colour correction): a dyehouse that has produced a faulty dyeing (wrong shade, misdosing or stains) needs to strip the colour and re-dye the fabric. This is CHT's current operational business.

Use case 2 — Textile recycling: to obtain white cotton that can be dissolved and re-spun (e.g. into viscose or modal) or torn and re-carded into new yarn, mixed-colour cotton must first be decoloured. This is the strategic future field, driven by regulation and market demand.

Current process (status quo): decolouring is carried out in an aqueous bath in a standard jet finishing machine. Reactive dyeings are stripped reductively and/or oxidatively (for reactive dyestuffs with a metal-complex component a sequestering agent is added, like HEPTOL ESW), followed by hot/cold rinsing and neutralisation. The chemistry is well known and not secret. CHT-Group / documents three standard stripping methods, which serve as the benchmark a new solution should be measured against:

Method I (reductive): 10.0 ml/l caustic soda 32.5% (38 °Bé) + 5.0 g/l sodium dithionite + 2.0 g/l Retinol A; 30 min at 80 °C; hot and cold rinse; neutralise.

Method II (oxidative): 3.0 g/l active chlorine, adjusted to pH 10–11 with caustic soda 32.5%; 30 min at 25 °C; cold rinse; followed by 1.0 g/l sodium bisulfite; 10 min at 40 °C; warm and cold rinse.

Method III (combined): reductive step as in Method I, followed by an oxidative step with active chlorine (sodium hypochlorite) as in Method II. Partial lightening is also possible with 5–10 g/l soda; 30 min at 95 °C; warm and cold rinse.

These processes are technically established but aggressive: oxidative systems such as hypochlorite reduce the average degree of polymerisation of the cellulose, which directly lowers fibre strength. A new solution should match these methods on decolouration while clearly outperforming them on fibre preservation and sustainability.