Reactive dyes are dyes that form a covalent bond with the fibre during dyeing. They are used mainly on cellulosic fibres such as cotton, viscose, modal, lyocell and linen.
Because the dye becomes part of the fibre, wash fastness is usually good and colours are bright. That is why reactive dyes are the largest dye class for cotton knits, towels, shirting and home textiles.
How they work
Each reactive dye molecule has a coloured part and a reactive group. In an alkaline bath, usually with soda ash and salt, the reactive group bonds with the hydroxyl groups in cellulose. Some dye reacts with water instead and is lost. That hydrolysed dye must be washed off, or it will bleed later.
The share of dye that bonds to the fibre is called fixation. Higher fixation means less colour in the effluent and less washing, which matters to mills with strict discharge limits.
Main types you will see
- Vinyl sulphone dyes react at moderate temperatures and are widely used in exhaust dyeing.
- Monochlorotriazine dyes need higher temperatures and suit some printing and continuous processes.
- Bifunctional dyes carry two reactive groups, often one of each type, for higher fixation and better reproducibility.
- Cold-brand dyes react at low temperatures and suit cold pad-batch dyeing.
Fibres and limits
Most reactive dyes are made for cellulose. Some ranges are designed for wool, silk or nylon, where they bond with amino groups. They are not used on polyester, which needs disperse dyes.
Reactive dyes are sold under a Colour Index name such as C.I. Reactive Red 195, but two products with the same name can differ in strength and shade. Always buy against a standard, as set out in how to write a specification for a reactive dye.
What to check before you buy
Reactive dyes are sensitive to moisture and heat in storage, because the reactive group can slowly hydrolyse. A dye that tested well at the plant can lose strength if it sits in a hot warehouse for months. Ask for the manufacturing date on every drum or bag, and test retained samples on arrival.
- Ask for strength as a percentage against your current standard, measured by a dyeing test, not only by absorbance.
- Ask for fixation and exhaustion figures from a lab dyeing on your fabric type.
- Ask for the content of salt or diluent, since some products are cut with standardising agents to hit a strength.
- Ask for solubility at your dyeing temperature, which matters for continuous and pad processes.
- Ask for fastness to washing, light and rubbing on your substrate.
Next step
Gujarat is a large source of vinyl sulphone and bifunctional reactive dyes. We find manufacturers, compare their samples against your standard and have lots tested before loading. To start, book a free sourcing review.