Sulphur dyes

The first group consists of amorphous powders, insoluble or partly soluble in water, which must be solubilized by heating with sodium sulfide and water according to the Equation (8)  

For the sake of simplicity, each dye molecule D is assumed to have only one mercapto group. In strongly caustic alkaline baths, the reaction mode of Equation (9) is also possible  

This formula requires only one-fourth of the reducing agent usually needed. In the case of poly sulfide melt dyes, the reducing agent can act not just on the disulfide groups but also on the quinoneimine group (as in vat dyes). 

These ready-to-dye liquid dyes are in reduced form and contain additional reducing agents for stabilization. Low-sulfide grades are prepared by reduction of the base dyes with caustic/glucose. 

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Prepared by reduction of 4-nitrophenol or 4-nitrosophenoi. Can be diazotized and used as a first component in azo-dyes. Chief outlet is for sulphur dyes in which it is fused with sodium polysulphides. L/sed as a photographic developer. 

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

Sulphur dyes in the insoluble disulphide form and the Cl Solubilised Sulphur brands are reduced by the dyer as part of the application procedure. In the case of the Cl Leuco Sulphur brands reduction has already been carried out by the manufacturer, so that they are substantially in a form suitable for immediate application (section 1.6.2) The chemistry of the reduction of sulphur dyes is complex, as is the chemistry of the dyes themselves it has been well described elsewhere [204]. It is possible to describe the state of a reduced sulphur dye in alkaline sulphide or polysulphide solution by the general formula 12.42, but there are certain complications. In many cases the chromogen is not itself reduced, but in others, notably reddish browns, blues and navy blues based on indophenols, the chromogenic... 

Sodium hydroxide is the alkali usually used in conjunction with dithionite. Sodium carbonate is a possible alternative when Cl Solubilised Sulphur dyes are used but is insufficiently alkaline for the Cl Sulphur brands, requiring careful control if over-reduction and the associated lower yields are to be avoided [30]. Typical concentrations are given in Table 12.24. The system of sodium carbonate and sodium dithionite used to reduce blue and black Cl Solubilised Sulphur dyes is particularly suitable for flame-retardant viscose fibres that are sensitive to strong alkalis, since it preserves a satisfactory level of flame retardancy [30]. It is also possible to use a mixture of dithionite with sodium sulphide in alkaline media. 

Table 12.27 Typical print paste formulation for the application of liquid jaggery as a reducing system for sulphur dyes [211]...

Table 12.28 Analysis of exhaust dyebaths after dyeing of cotton fabric with sulphur dyes using sodium sulphide or thiourea dioxide [213]...

R-S-S03 Cl Solubilised Sulphur Dye L Ligand (gluconic acid or triethanolamine)... 

Table 12.30 Suitability of various sulphur dye types for application with anthra-quinonoid or iron-complex redox systems [214]...

Sodium chlorite is the basis of several proprietary oxidising agents for sulphur dyes. It is used at pH 10 (sodium carbonate) and 90-95 °C careful control of conditions is important as the reaction is rather slow. The presence of additives is necessary for the successful use of chlorite. The proprietary formulations contain stabilisers, EDTA-type sequestering agents... 

Hydroxyacetone (12.48), mentioned in section 12.8.1 in connection with sulphur dyes, is sulphur-free and biodegradable. This compound was originally proposed for use with vat dyes and continues to generate some interest. This agent can be used for the pad-steam application of vat dyes in the presence of high concentrations of sodium hydroxide (about 3.5-4.5 g/1). Hydroxyacetone does not cause over-reduction of indanthrone vat dyes but does give different shades with carbazole dyes, compared with sodium dithionite [218]. 

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