Reactive Dyes form a covalent bond between the dye and fiber. It contain a reactive group (often trichlorotriazine), either a haloheterocycle or an activated double bond, that, when applied to a fibre in an alkaline dye bath, forms a chemical bond with an hydroxyl group on the cellulosic fibre.

In a reactive dye a chromophore contains a substituent that is activated and allowed to directly react to the surface of the substrate.

Reactive dyeing is now the most important method for the coloration of cellulosic fibres. Reactive dyes can also be applied on wool and nylon; in the latter case they are applied under weakly acidic conditions. Reactive dyes have a low utilization degree compared to other types of dyestuff, since the functional group also bonds to water, creating hydrolysis.

Reactive dyes are categorized by functional group:

 

Advantages of the Reactive Dyes

• Show improved fastness properties
• Simplify dyeing procedure
• Easy Washability
• Permanency of the color
• Good chemical binding
• Allows for a wide variety of chromophores to be used

 

Dyeing cycle and Important factors/phases in Reactive Dyeing

• pH of the substrate prior to dyeing
• pH of the dye bath
• pretreatment of the substrate
• solubility of the dyestuff
• dyeing temperature
• Quality of water and salt
• electrolyte concentration
• dyeing time
• Washing off sequence
• Type of alkali

Bifunctional Dyes - Dyestuffs containing two groups are known as bifunctional dyestuffs. These reactive dyes are designed in such a manner to have the capacity to react with the fibre in more than a single way.

Vinylsulphone Dye (VS) - Vinylsulphone Dyes are moderately reactive. The dyeing temperature is generally 600C and pH is 11.5 that gets applied by utilising a mixture of soda ash and caustic soda. These dyes show excellent fixation properties under proper alkaline condition. A typical example is the Remazol Black B (CI Reactive Black 5)

Monochlorotriazine Dye (MCT) - Normally these dyes are less reactive than vinylsulphone dyes. Reaction can take place in more energetic reaction conditions. That is typically 800 degreeC and pH value of 10.5, are essential for a proper fixation on cellulosic fibres. A typical monochlorotriazine dye is shown here.