Page 343 - Basic Principles of Textile Coloration

332 REACTIVE DYES

CHAPTER 16

Reactive dyes

16.1 THE DEVELOPMENT OF REACTIVE DYES

Dyeings of cotton with direct dyes (Chapter 14) have rather poor washing fastness
because only weak polar and dispersion forces bind the dye molecules to the
cellulose polymer chains. Direct dye molecules can therefore easily diffuse out of
the cotton during washing. The best fastness to washing requires precipitating an
insoluble pigment and mechanically trapping it within the cotton fibres. This type
of dyeing process with vat and azoic dyes is, however, much more complicated
than direct dyeing (Chapters 17 and 19).

The idea of immobilising a dye molecule by covalent bond formation with
reactive groups in a fibre originated in the early 1900s. Various chemicals were
found that reacted with the hydroxyl groups of cellulose and eventually converted
into coloured cellulose derivatives. The rather forceful reaction conditions for this
led to the false conclusion that cellulose was a relatively unreactive polymer.
Possibly because of this, a number of dyes now known to be capable of covalent
bond formation with groups in wool and cotton were not initially considered as
fibre-reactive dyes, despite the good fastness to washing of their dyeings.

In 1955, Rattee and Stephen, working for ICI in England, developed a
procedure for dyeing cotton with fibre-reactive dyes containing dichlorotriazine
groups. They established that dyeing cotton with these dyes under mild alkaline
conditions resulted in a reactive chlorine atom on the triazine ring being
substituted by an oxygen atom from a cellulose hydroxyl group (Figure 1.5). This is
shown in Scheme 16.1 where Cell–OH is the cellulose with a reactive hydroxyl
group, Dye–Cl is the dye with its reactive chlorine atom, and Cell–O–Dye the dye
linked to the cellulose by a covalent bond. The role of the alkali is to cause acidic
dissociation of some of the hydroxyl groups in the cellulose, and it is the
cellulosate ion (Cell–O–) that reacts with the dye.

Cell OH + HO Cell O + H2O
Cell O + Dye Cl Cell O Dye + Cl
Scheme 16.1

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