REACTIVE DYES: HISTORICAL BACKGROUND AND SHORT DISCUSSION

It was not until the early 1950s, following the marketing in 1952 by Hoechst of two Remalan (HOE) vinylsulphone dyes capable of reacting with wool, that ICI was successful in devising a reactive dyeing process that enabled cellulose to be dyed with a trichromatic mixture of dyes under practical conditions. Cotton fabric was pretreated with alkali and dried before immersion in a solution of the highly reactive dichlorotriazine dyes. Various refinements of the process were necessary (adding salt to enhance substantivity, lowering the pH and buffering the dyebath to minimise dye hydrolysis) before these novel Procion (ICI, now Zeneca) dyes could be marketed in 1956.

Exploitation of the dichlorotriazine reactive system soon led to parallel development of the much less reactive monochlorotriazine dyes, readily made by a substitution reaction between an arylamine and the dichlorotriazine precursor. More stable padding liquors could be prepared using the aminochlorotriazine types and the range of reactivities offered by these two classes of dyes in combination with various alkalis greatly extended the scope of novel continuous dyeing methods for them.

At this stage, however, the limitations of continuous dyeing requirements became a temporary constraint on the adoption of reactive dyeing and attention turned to the development of batchwise methods. It was quickly demonstrated that optimal temperatures of dyeing should be sought (40°C or lower for dichlorotriazines, 70°C or higher for aminochlorotriazine dyes).

The major breakthrough came when it was realised that a neutral exhaustion in salt solution to achieve moderate uptake should precede the alkali addition to promote further exhaustion at a controlled rate determined by the dye–fibre reaction that proceeds at an optimal alkaline pH and temperature.

Over the decades since the commercial introduction of reactive dyes, their use has grown steadily rather than spectacularly. When they first appeared it was predicted that reactive dyes would largely replace azoic combinations, direct dyes and sulphur dyes, displace vat dyes from outlets where fastness to bleaching was not essential and eventually dominate the dyeing of cellulosic materials. This did not occur and even in the relatively sophisticated markets they do not account for more than 30% of all dyes consumed on cellulose. World-wide, the traditional uses of direct and sulphur dyes on woven cotton fabrics remain dominant and reactive dyes only account for about 10% of total consumption on this basis of comparison. In the USA, however, where vat dyes have been used preferentially for fast-dyed cottons, there has been a more gradual trend in favour of reactive dyes. Greater demand for brilliant hues, a shift towards cotton and cotton-rich blends for apparel and a greater prevalence of short-run lots in the US textile industry have all contributed to this trend.

Although the dyeing cycles of direct and reactive dyes are broadly similar, a major difference becomes apparent when unfixed or hydrolysed reactive dye has to be washed off thoroughly in order to achieve the desired superior wet fastness of the reactive dyeing. As much as 50% of the total cost of a reactive dyeing process must be attributed to the washing-off stages and treatment of the resulting effluent. This aspect of the process should be recognised as a major limitation that prevents reactive dyes from achieving the degree of success that was predicted for them at the time of their discovery. Certain other deficiencies are associated with the limited stability of specific types of dye–fibre bond to various conditions of treatment of the dyed fibres.

Research into novel reactive dyeing systems and application methods has remained highly active and some notable developments have taken place during recent years. Aminofluorotriazine dyes have joined the vigorous competition between the major reactive systems, and bifunctional systems reacting by two distinctly different mechanisms have also appeared. Bis(aminonicotinotriazine) dyes that react with cellulose under neutral conditions and reactive dyes containing phosphonic acid groups capable of fixation under hot, dry, acidic conditions represent two quite different novel approaches to reactive dye fixation that have been commercialised.

Pretreatment of cellulosic materials with cationic agents of various kinds that enhance uptake of anionic dyes and facilitate the fixation of reactive dyes in the absence of either salt or alkali has also attracted considerable interest. Novel chromophores, cold-dissolving granules and liquid brands, improved automation of application methods and greater awareness of the environmental impact of reactive dye wastes have all received attention over this recent period.