Advancements in Reactive Textile Dyes

Chemistry of Reactive Dyes

The dyeing principle based on the fibre reactivity involves the reaction of a functional group of the dyestuff with a site on the fibre to form a covalent link between the dye molecules and the substrate.²

The four structural feature of a typical reactive dyes molecule are:

• The chromophoric grouping, contributing the colour much of the substantively for cellulose.
• The reactive system, enabling them to dye to react with the hydroxyl group in cellulose.
• A bridging group that links the reactive system to the chromophore
• One or more solubilising group, usually sulphuric acid substituent attached to the chromophoric group for their colour, although the azo chromophore –N=N- is by far the most important

All reactive dye contain sodium sulphonate group for solubility and dissolve in water to give coloured sulphonate anions and sodium cations. Most reactive dyes have from one to four of these sulphonate group-reactive dye molecules. However, do have several specific structural features of their own.

The general form of the reactive dye is as follows.

S——R——B—-X

Where,

• S = Water solubility group
• R = Chromophore
• X = Reactive system
• B = Bond between reactive system and Chromophore

Chromophore (R):  It is the colour producing part. The Chromophore absorbs a certain wavelength of incident white light and reflects the rest, which incident on the eye retina giving colour vision.

Reactive system (X): Reactive system reacts with fibres forming XF dye covalent bond. It influences the fastness property.

Bridging group (B): Bridging group links R and X which determines stability and reactivity of the dye particle. In some, the reactive group is directly attached to the Chromophore and most reactive system contains a heterocyclic ring that contributes some substantivity for cellulose. The sulphatoethyl sulphone precursor of the vinyl sulphone reactive group contributes significantly to the aqueous solubility of reactive dyes.

Water solubility: This group imparts water to dye. Generally, it is sodium salt of sulphuric acid. One should always keep in mind that the easier is the application of the dye on the fibres the easier is the removal from fibres.