Factors Governing Reactive Dye Uptake

All conventional reactive dyes for cellulose, irrespective of whether they react by nucleophilic addition, substitution, or both mechanisms, rely on the reactivity of the cellulosate anion as the nucleophilic reagent and hence hydrolysis of the dye by reaction with hydroxide ions from water will always compete with the desired fixation reaction. Reaction between the dye and cellulose can occur only when the dye has been absorbed into the cellulose phase. Thus the kinetics of the dye–cellulose reaction is strongly influenced by the rate of absorption of dye. The ratio of the rate constants for reaction of the dye with the fibre and with water is a constant for a given dye over a wide range of alkaline pH values.

The efficiency of fixation is a function of:

1. The reactivity ratio, the ratio of rate constants for the fixation reaction and hydrolysis;
2. The substantivity ratio, the relative concentrations of dye absorbed into the substrate and remaining in the dyebath;
3.  The diffusion coefficient of the dye in the substrate;
4. The liquor ratio; and
5. The surface area of the substrate available for absorption of dye [1].

The lower the linear density of the fibre, i.e. the greater the surface area per unit weight, the more efficient is the dyeing. The substantivity ratio is the most influential of the factors governing fixation efficiency. Dyes of higher substantivity diffuse more slowly than less substantive dyes. Changes in dyebath conditions that increase substantivity tend to decrease the diffusion coefficient. Lowering the liquor ratio favours increases in the rate and efficiency of fixation. The full effects of this are never completely realised, however, because the higher dyebath concentration necessary at the lower liquor ratio implies a decrease in substantivity ratio, offsetting some of the expected gain.

Substantivity ratio remains approximately constant within the pH range 7–11 at a given electrolyte concentration, but above pH 11 there is a marked fall in substantivity, especially with highly sulphonated dyes. As the applied con-centration of dye is increased at constant electrolyte concentration, the substantivity ratio and hence the efficiency of fixation are lowered. Thus fulldepth dyeings require longer for completion of the reaction and the percentage fixation is usually inferior. In order to attain the maximum rate and efficiency of fixation, more electrolytes are needed, but this increases the risk of aggregation and possible precipitation with dyes of limited solubility.

An increase of dyeing temperature lowers the substantivity ratio and accelerates the rate of hydrolysis of the dye; both of these effects reduce the fixation efficiency. The rates of diffusion into and reaction with the fibre are also accelerated, however, and these factors both favour fixation of the dye. An increase in electrolyte concentration always enhances substantivity without impairing reactivity providing the dye remains completely dissolved. The beneficial effects of electrolyte addition are most evident with the more highly sulphonated dyes at relatively high pH and applied depth.

Informative studies of the relationships between dye structure and substantivity [2] and between dye structure and levelling properties [3] are available. The interaction between these dyeing properties and the controlling parameters in exhaust dyeing with reactive dyes, such as applied concentration, pH, temperature and electrolyte addition, is the key to achieving successful and reproducible dyeing.

References:

1. H H Sumner and C D Weston, Am. Dyestuff Rep., 52 (1963) 442.
2. M Haelters, Melliand Textilber., 61 (1980) 1016.
3. N Harada et al., J.S.D.C., 107 (1991) 363.