Phosphonic acid Dyes

As in the case of chlorotriazines and vinylsulphones, dyes containing phosphonic acid groups had been prepared long before their utility as reactive dyes for cellulose was recognised [4]. The realisation that phosphonic acid derivatives could, under certain circumstances, react with alcohols to give phosphonate mono-esters was made in 1973 at the Stanford Research Institute. Subsequent development work by Burlington Industries and later ICI led to the commercial launch of the Procion T (ICI, now Zeneca) range of dyes in 1977. These dyes were unusual in being applied to cellulosic and blend fabrics under mildly acidic conditions (pH 5–6). This made them particularly suitable for polyester/cotton dyeing by the pad–dry–thermosol process, since the alkaline conditions normally required to fix conventional reactive dyes could be avoided.

The structures of many of these dyes were based on the versatile intermediate 3-aminophenylphosphonic acid attached to typical monoazo chromogens in various ways. One example was the 1:2 cobalt complex Procion Rubine T- 6BD Figure. These dyes were sold as aqueous solutions of the ammonium salts, which facilitated thermal dissociation to generate the free acid form. This in turn reacted with a carbodiimide, e.g. dicyandiamide, by one of two possible mechanisms, either or both of which may be operative. One mechanism involves formation of an O-acylisourea derivative of the dye phosphonate and the carbodiimide, the other proceeds via a dye phosphonic anhydride derivative. Dye fixation can occur by reaction of either of these derivatives with the hydroxy groups of cellulose.

More recently, the role of the carbodiimide in the dyeing process has been examined and it was concluded that the loss of this auxiliary by thermal decomposition was the most important factor limiting dye fixation. Later it was shown that fixation yields as high as 95% could be achieved by minimizing the decomposition of the carbodiimide.

A major attraction of the phosphonic acid dyes was the exceptional stability of the dye–fibre bonds, more stable than those from chlorotriazine dyes in acidic media and those from vinylsulphone dyes under alkaline conditions. Technical drawbacks of these first acid-fixing reactive dyes, however, included dye migration during drying, especially on heavy fabrics such as corduroy, and strength loss of the cellulosic fibres during thermofixation under the acidic conditions necessary for fixation. Consequently, the Procion T (ICI, now Zeneca) dyes were withdrawn in 1987, essentially for economic reasons.