Sulfur Dyeing Process

The H2S sulfanes are the subject of several reviews (129,133). Except for hydrogen sulfide these have no practical utiUty. Sodium tetrasulfide [12034-39-8] is available commercially as a 40 wt % aqueous solution and is used to dehair hides in taimeries, as an ore flotation agent, in the preparation of sulfur dyes (qv), and for metal sulfide finishes (see Leather Mineral recovery and processing). 

From an appHcations point of view, the sulfur dyes are between vat, direct, and fiber-reactive dyes. They give good to moderate lightfastness and good wetfastness at low cost and rapid processing (see Dyes, application and evaluation). 

The manufacture of sulfur dyes involves sulfurisation processes, the chemistry of which remains rather mysterious and may arguably be considered still to be in the realms of alchemy The processes involve heating elemental sulfur or sodium polysulfide, or both, with aromatic amines, phenols or aminophenols. These reactions may be carried out either as a dry bake process at temperatures between 180 and 350 °C or in solvents such as water or aliphatic alcohols at reflux or at even higher temperatures under pressure. C. I. Sulphur Black 1, for example, is prepared by heating 2,4-dinitrophenol with sodium polysulfide. 

Like nitrate, sulfate is also a constituent of textile processing wastewaters. Sulfate is generally added to the dye baths for ionic strength adjustment or it may be formed by the oxidation of sulfur species used in dyeing processes, such as sulfide, hydrosulfide, and dithionite [37]. 

Aerobic biodegradation processes often show unsatisfying results because a number of azo dyes are resistant to aerobic microbiological attack. The main process for removal of dyes in the aerobic part of a CWWT is based on an adsorption of the dyes on the biomass. Further problems in the destruction of chromophores result during the treatment of phthalocyanine dyes, anthraquinoid dyes, and vat and sulfur dyes, which contain rather persistent chromophores. 

Even now, knowledge of the constitution of sulfur dyes is rather fragmentary. Two reasons are that the preparation process yields complicated mixtures of related compounds and that sulfur dyes could not be obtained in pure form because of their amorphous and colloidal structure and their insolubility in common solvents. 

Nothing is known of the structure of bake dyes made from nitrogen-free intermediates (i.e., from which thiazole rings cannot be formed). This group includes sulfur dyes prepared from decacyclene by sulfur baking (dry fusion process), such as C.I. Sulphur Brown 52, 53320 [1327-18-0] and also by the sulfurization of anthracene. The latter dye is thus exceptional because it is used only as a vat dye it is largely insoluble in sodium sulfide. 

The polysulfide melt process yields mainly reddish brown, violet, blue, green, and black sulfur dyes, depending on the intermediate. 

In this process, the reaction of the intermediates-essentially indophenols, chiefly in leuco form, or indophenol-like substances-with sulfur is effected by heating with alkali metal polysulfide in an aqueous or alcoholic medium under reflux. When monoethers of ethylene glycol or diethylene glycol (such as Carbitol) are used, the dye can be obtained in solution form after melting, without distillation and elimination of the solvent. Accordingly, these solvents are used chiefly in the preparation of ready-to-dye sulfur dye solutions. The addition of hydrotropic substances such as sodium xylenesulfonate improves the homogeneity of the melt and hinders sedimentation of the dye on storage. 

The term acid dye derives from the dyeing process, which is carried out in an acidic aqueous solution (pH 2-6). Protein fibers contain amino and carboxyl groups, which in the isoelectric range (ca. pH 5) are ionized mostly to NH3+ and COO. In the acid dyebath the carboxylate ions are converted to undissociated carboxyl groups owing to the addition of acid HX (sulfuric or formic acid), which causes the positively charged wool (H3N+-W-COOH) to take up an equivalent amount of acid anions X (hydrogensulfate, formate Scheme 3.7) [5] ... 

Recently, the electrochemical reduction of sulfur dyes was studied. For continuous processes a mediator is not necessary. In discontinuous application it is possible to reuse dyebaths [72],... 

In the dyeing process, the pH is adjusted with sulfuric acid to 1.9-2.2 (pH 2.5 in the presence of auxiliary agents). After addition of sodium sulfate, the dyebath is heated to 40-50°C then the dissolved dye is added. Dyeing is carried out at boiling temperature for 90 min. The material is then cooled and rinsed ammonia or sodium acetate can be added to the last rinsing bath. A lowering of the dyeing temperature to 80 °C is possible in the presence of an ethoxylated fatty amine (pH 1.9-2.2). 

Disperseand Sulfur Dyes (see also Section 4.7.3). The thermosol pad steam process is the primary method for application of disperse and sulfur dyes. The sulfur dyes that are used preferentially in the ready-for-dyeing soluble form, and are especially inexpensive for muted shades, must be applied after the thermosol step because of their incompatibility with disperse dyes. 

Initially, sulfur dyes were water-insoluble, macromolecular, colored compounds formed by treating aromatic amines and aminophenols with sulfur and/or sodium polysulfide. R. Vidal developed these dyes in 1893 but they only became attractive for leather with the introduction of water-solubilizing groups. Today, the sulfur dyes can be divided into three classes conventional water-insoluble, leuco, and solubilized sulfur dyes. Most sulfur dyes are synthesized by condensation of aromatic amines with sulfur or sodium polysulfide in the so-called bake process, or else in water or under pressure as a solvent-reflux reaction. 

Sulfur blue dyes are often made using an organic solvent such as n-butanol, in what is known as the solvent reflux process. Examples are C.I. Sulfur Blue 9 and Sulfur Blue 13. In this case, intermediate structures are indophenols (e.g., 55). See Fig. 13.133. Similarly, sulfur dyes containing benzothiazine groups can be made from... 

English, French, and German 19th-century dye texts (16-19) suggest a penchant for the use of sulfuric acid, not only in the dyeing process to maintain an acidic pH, but more importantly, as a final sour or clearing rinse. 

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