7.3 Dyeing with water-soluble dyestuffs.
Polyamide. Water-soluble dyestuffs for dyeing polyamide such as nylon 6 and 6.6 from PCE emulsion are selected acid dyes, reactive dyes containing sulphogroups, and metal complex dyestuffs. Suitable emulsifiers are non-ionic, weakly basic or anionic. Mixtures of these can also be used.
Dyestuffs which are absorbed from an aqueous bath below the boil, will also show a favorable absorption performance in emulsion dyeing below the azeotropic boiling point of the emulsion.
The dyeing times required to exhaust the bath are below those of a corresponding aqueous dyeing. But there is an increased tendency to produce barry dyeings when dyeing from solvents, compared with aqueous dyeing. Comparative absorption curves of a number of reactive dyes are shown in fig. 7.1. The mechanism of this dyeing process largely corresponds to that of an aqueous dyeing. The dyestuff is taken up by the fiber from the aqueous emulsion droplets absorbed by the fiber surface, which means the dyestuff is taken up from a highly concentrated solution and extremely short liquor. The ion exchange with the fiber is influenced by the kind and concentration of the emulsifier used. Mixtures of anionic, weakly cationic and non-ionic emulsifiers have proved particularly successful. Anionic emulsifiers alone will promote blocking of the fiber, while strongly cationic emulsifiers act as retarders with high dyestuff affinity and shift the equilibrium in the direction of the liquor. Furthermore, cationic emulsifiers will cause the dyestuff anion to travel from the aqueous to the organic phase of the solvent.
Fig. 7.1. Absorption curves of Verofix dyestuffs on Nylon-Helanca, from PCE and water
Wool. Wool dyeing with water-soluble dyestuffs from chlorinated hydrocarbon emulsion is determined by two factors:
- is a polyelectrolyte and requires water to be present, or a solvent of similar swelling capacity and similar dielectric constant, to absorb a dyestuff which contains ionic groups;
-wool will felt under dyeing conditions in chlorinated hydrocarbon emulsions. Hence only such wool is suitable for dyeing from an organic solvent, which has been given a non-felting finish.
Dyestuffs suitable for dyeing wool from PCE are acid, metal complex, and reactive dyes. The lower the solubility of the dyestuff in water, the greater the water content of the emulsion at a given color depth. It has been found that suitable products correspond to those which can be used for dyeing polyamide from emulsions.
The necessary amount of emulsifier depends on the type of equipment used, as already stated, assuming given water content of the emulsion. The additional use of Avolan® RE has given particularly good results with reactives. In this case the leveling agent also acts as an emulsifier.
The temperatures required in wool dyeing are usually higher than those employed when dyeing polyamide. Hence there is only a limited choice of dyestuffs which will go on the fiber below the azeotropic boiling point. In any case, machines which are to be used in wool dyeing must be equipped for dyeing temperatures above the azeotropic boiling point. In the case of dyestuffs, e.g. the Verofix® dyes, which will go on the fiber at lower temperatures but require fixation at an elevated temperature, fixation can be effected with a low water content emulsion after exhaustion is completed.
The dyeing mechanism corresponds to that of an aqueous dyeing from extremely short liquor. Absorption is therefore generally much faster than from a 1 00% aqueous liquor. The absorption curves of selected Verofix dyestuffs suitable for dyeing from solvents are shown in Figs. 10-1 5. In these graphs, the absorption curves of chlorinated wool are compared when dyeing from water and from an emulsion, at the same temperature and under equal conditions of equipment and operation. It will be observed that although the absorption curve from emulsion has a steeper gradient than that from water in the first one-third, somewhat higher degrees of fixation are obtained for the 100% aqueous dyeing.