Additives to the Dye Bath

Water-Soluble Sulfur Dyes (C.I. Solubilised Sulphur). Water-soluble sulfur dyes are available in the form of Bunte salts (.S -arylthiosulfuric acid salts) of sulfur dyes and can be dissolved in hot water. The addition of alkali and reducing agent gives them an affinity for the fiber. If NaSH or another reducing agent is used, alkali must be added to trap H+ ions resulting from reduction. 

The leuco form is absorbed by the substrate and then oxidized to the insoluble dye. Since several mercapto groups are present per dye molecule, oxidation causes not only dimerization but also cross-linking of dye monomers . For polysulfide melt dyes, the quinonimine group is also involved in the reaction. This is apparent in the change in shade that occurs during reduction. 

With respect to both application and fastness, a distinction is made between sulfur dyes and sulfur vat dyes. The latter include dyes of the type Hydron Blue C.I. Vat Blue 43, 53630 [1327-79-3]) and Indocarbon C.I. Sulphur Black 11, 53290 [1327-14-6]). 

Reducing Agents. The reducing agents generally used to bring the dyes into the soluble form with affinity for the fiber are sodium sulfide or sodium hydrogensul-fide  

In the dye liquor, the amount of concentrated Na2S should be greater than or equal to 2.5 g/L. In aqueous solution, the sulfide ion is almost completely hydrolyzed. If NaSH and soda are used istead of NaOH, the lower alkalinity offers  

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Additives such as wetting agents, salts, carriers, retarders, and others may be added to the dye bath along with the dye if required to facihtate the dyeing process. 

As a rule animal fibres are not capable of decomposing the salts of acid dyestuffs, and in dyeing the colour-acids have to be set at liberty by the addition of a stronger acid to the dye-bath. Certain colour-acids, for example the sulphonic acids of amidoazo-compounds, have a different colour to that of their alkali salts. In dyeing wool with such colour-acids, the shade produced is that of f the salts and not that of the colour-acid, so that in such cases wool evidently plays the role of a base. 

The soluble (sulphonated) indulines are dyed on cotton mordanted with tannin and tartar emetic a considerable amount of almn being added to the dye-bath. Wool may be dyed directly from an acid bath the induline generally being used in combination with other acid colours. Silk is dyed in a bath containing boiled-off liquor the shades being brightened by addition of sulphuric acid. 

In addition to the dyebath, several auxiliary baths are used in batch textile processing to clean dyed goods and/or to fix the dye to the fiber. Considerable energy, water and chemicals are also wasted when these auxiliary baths are drained to the sewer. Plant pollution problems are also worsened by the additional effluent. Virtually all of the chemicals used in the auxiliary baths (sodium hydrosulfite and hydroxide, surfactants, etc.) are colorless, and thus do not lend themselves to simple... 

In regeneration of the fixation bath, no dye was added for hydrolysis after the Initial cycle as 1n the pilot-scale research. The pH, based on laboratory trials, generally remained around 10.5-11.0, and therefore only a 10 addition of soda ash to the fixation bath for each reuse cycle was assumed. No salt or sequesterant was added after the initial fixation cycle. 

The different behaviour of various textile fibres in dyeing may be explained by an assumption of different solvent power thus, silk dyes more readily than other fibres because the fibroin has a greater solvent power. Keratine again, the principle of the wool fibre, possesses a greater solvent power than cellulose, which is only capable of attracting and holding in solution a few dyestuffs, such as the tetrazo-dyes of the benzidine series, and in certain cases in this class the solvent power of the water in the dye-bath has to be decreased by addition of salt. 

DuPont [63] claimed a process for stabilisation of dyed poly(trimethylene terephthalate) fibres by adding a speciality benzotriazole in the dye bath (Cibafast USM Ciba). The structure of this additive was not revealed, but it most probably features extra substituents in the form of sulfonic acid(s) or derivatives thereof, which would allow the additive to be dispersed or dissolved in the aqueous dye bath. 

The transport of dye liquor along the packed bed fibrous assembly is represented by an MIM model. Thus, the supply of dye from the dye bath to the surface of the fibres at any position in the package is considered to consist of additive effects of two types of transport phenomena convective flow and dispersive flow through porous media. 

The relatively low pX values seen for the benzoyl acetanilides, especiaHy as two-equivalent couplers, minimize concerns over slow ionization rates and contribute to the couplers overaH reactivity. But this same property often results in slow reprotonation in the acidic bleach, where developer carried over from the previous step can be oxidized and react with the stiH ionized coupler to produce unwanted dye in a nonimage related fashion. This problem can be eliminated by an acidic stop bath between the developer and the bleach steps or minimized by careful choice of coupling-off group, coupler solvent, or dispersion additives. 

NaN02, in addition to its use with nitrates in heat-transfer molten-salt baths, is much used in the production of azo dyes and other organo-nitrogen compounds, as a corrosion inhibitor and in curing meats. 

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