How reactive dyes work
Reactive dyes are water-soluble colourants that can form covalent chemical bonds with cellulose fibres, including cotton and viscose. This attachment helps produce good wash fastness—but only when unfixed dye is thoroughly removed after dyeing.
Different fibres need different dye chemistry. For background, see Textile Fibers and Materials.
Why alkali matters
In conventional exhaust dyeing of cotton, electrolyte helps promote dye uptake. Alkali then creates conditions that allow the dye’s reactive groups to bond with cellulose.
However, reactive dyes differ in their chemistry. Chlorotriazine and vinyl-sulfone systems, for example, have different application requirements. Temperature, alkali addition, and processing time must follow the dye supplier’s recommendations rather than one universal recipe.
Fixation competes with hydrolysis
The dye can react with water instead of the fibre. This competing reaction is called hydrolysis. Hydrolysed dye can no longer form the intended reactive bond with cellulose, although it may remain associated with the fabric.
Good process control therefore aims to achieve even uptake and efficient fixation while limiting unnecessary hydrolysis. Fabric preparation, circulation, temperature, and the salt-and-alkali addition sequence all matter.
Why wash-off is essential
After fixation, rinsing and suitable washing treatments remove residual chemicals, unfixed dye, and hydrolysed dye. Inadequate wash-off can leave colour that bleeds or stains adjacent materials during washing.
Wash-off conditions depend on the dye range, shade depth, substrate, and equipment. Supplier guidance should determine the appropriate sequence.
Troubleshooting priorities
- Poor wash fastness: Check fixation and wash-off before assuming the dye itself is unsuitable.
- Uneven colour: Review preparation, circulation, and chemical dosing.
- Unexpected shade variation: Check temperature, pH, water quality, and process consistency.
For polyester, the mechanism is different: read Disperse Dye.
Thanks for sharing valuable piece of information on reactive dyes. Being an employee with Alliance Organics it feels pleasure to announce that Alliance Organics has become one of the renowned dyestuffs manufacturer and major players in terms of market share along with Atul, Clariant India, Dystar, Ciba Specialities and IDI exporting high quality dyestuffs, Reactive Dyes, pigments and food colors across a variety of countries like USA, China, Indonesia, Russia and Brazil.
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