Sulfur dyes are the most commonly used

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

s manufactured for cotton in terms of volume. They are inexpensive, generally have good wash-fastness, and are easy to apply. Sulfur dyes are predominantly black, brown, and dark blue. Red sulfur dyes are unknown, although a pink or lighter scarlet color is available.

Chemistry

Sulfur linkages are the integral part of

chromophore A chromophore is the part of a molecule responsible for its color. The color that is seen by our eyes is the one not absorbed by the reflecting object within a certain wavelength spectrum of visible light. The chromophore is a region in the molec ...

in sulfur dyes. They are

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur ...

s consisting of

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...

(–S–),

disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

(–S–S–) and

polysulfide Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. Among the inorganic polysulfides, there are ones which contain anions, which have the general formu ...

(–S_n–) links in heterocyclic rings. They feature

thiazole Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular for ...

s, thiazone,

thianthrene Thianthrene is a sulfur-containing heterocyclic chemical compound. It is a derivative of the parent heterocycle called dithiin. It is notable for its ease of oxidation. Structure and synthesis Like other 1,4-dithiins but unlike its oxygen ana ...

, and phenothiazonethioanthrone subunits. Being nonionic, sulfur dyes are insoluble in water.

Process

Dyeing includes a few stages, viz. reduction, dyeing, washing, oxidation, soaping, and final washing. The anion is developed on reducing and solubilising at boil when it shows affinity for cellulose. Sodium sulfide (Na₂S), the reducing and solubilising agent, performs both reduction and solubilisation, producing thiols and then to sodium salt of thiols or thiolates, which are soluble in water and substantive towards cellulose. Higher rate of exhaustion occurs at 90-95 °C in presence of electrolyte. Dyed cellulosics exhibit a tendering effect on storage under humid atmosphere due to presence of excess free sulfur. Aftertreatment with sodium acetate is required to suppress that. H₂S liberated during dyeing forms corrosive metal sulfide. This restricts use of metal vessels except those made of stainless steel:Peters R. H, "Textile Chemistry", Vol - II, Elsevier Publishing Company, London (1967) :Fe + H₂S → FeS + H₂

Production, past and present

The forerunner of sulfur dyes is attributed to "Cachou de Laval", which is prepared by treating wood products with sulfide sources. Subsequently, the so-called Vidal Blacks were produced by reactions of various aniline derivatives with sulfur. These experiments demonstrated that deeply colored materials could be readily produced by combining aromatic compounds and sulfur sources.Parikshit Goswami, Montu Basak "Sulfur Dyes" in Kirk-Othmer Encyclopedia of Chemical Technology, 2001, John Wiley & Sons. . The most important member of the class is Sulfur Black 1. It is produced by the reaction of

2,4-dinitrophenol 2,4-Dinitrophenol (2,4-DNP or simply DNP) is an organic compound with the formula HOC6H3(NO2)2. It is a yellow, crystalline solid that has a sweet, musty odor. It sublimates, is volatile with steam, and is soluble in most organic solvents as well ...

and sodium sulfide in hot water. Like many sulfur dyes, details on the chemical reactions are poorly understood. It is accepted that the sulfide reduces the nitro groups to

aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile startin ...

derivatives, which are thought to form

indophenol Indophenol is an organic compound with the formula OC6H4NC6H4OH. It is deep blue dye that is the product of the Berthelot's reaction, a common test for ammonia. The indophenol group, with various substituents in place of O''H'' and various rin ...

-containing intermediates that are further crosslinked by reaction with sulfur. The result are insoluble, high molecular weight species. Sulfur Black 1 is imperfectly understood, and the material is probably heterogeneous. It is speculated to be a polymer consisting of

and

phenothiazine Phenothiazine, abbreviated PTZ, is an organic compound that has the formula S(C6H4)2NH and is related to the thiazine-class of heterocyclic compounds. Derivatives of phenothiazine are highly bioactive and have widespread use and rich history. T ...

subunits. The so-called sulfur bake dyes are produced from

1,4-diaminobenzene ''p''-Phenylenediamine (PPD) is an organic compound with the formula C6H4(NH2)2. This derivative of aniline is a white solid, but samples can darken due to air oxidation. It is mainly used as a component of engineering polymers and composites lik ...

and

diaminotoluene Toluenediamine may refer to these isomeric organic compounds with the formula C6H3(NH2)2(CH3): * 2,4-Diaminotoluene, precursor to toluene diisocyanate and azo dye Azo dyes are organic compounds bearing the functional group R−N=N−R� ...

derivatives. These dyes are proposed to consist of polymers with

benzothiazole Benzothiazole is an aromatic heterocyclic compound with the chemical formula . It is colorless, slightly viscous liquid. Although the parent compound, benzothiazole is not widely used, many of its derivatives are found in commercial products or ...

subunits. Members of the sulfur bake dyes class are Sulfur Orange 1, Sulfur Brown 21, and Sulfur Green 12. SulfurBlack1

Application method

Sulfur dyes are water-insoluble. In the presence of a reducing agent and at alkaline pH at elevated temperature of around 80 °C, the dye particles disintegrate, which then become water-soluble and hence can be absorbed by the fabric.

Sodium sulfide Sodium sulfide is a chemical compound with the formula Na2 S, or more commonly its hydrate Na2S·9 H2O. Both the anhydrous and the hydrated salts in pure crystalline form are colorless solids, although technical grades of sodium sulfide are gener ...

sodium hydrosulfide Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half-neutralization of hydrogen sulfide () with sodium hydroxide (NaOH). NaSH and sodium sulfide are used industrially, often for similar purpos ...

are suitable reducing agents. Common salt facilitates the absorption. After the fabric is removed from the dye solution, it is allowed to stand in air whereupon the dye is regenerated by oxidation. The regenerated parent dye is insoluble in water. Oxidation can also be effected in air or by

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%–6 ...

sodium bromate Sodium bromate, the inorganic compound with the chemical formula of NaBrO3, is the sodium salt of bromic acid. It is a strong oxidant. Uses Sodium bromate is mainly used in continuous or batch dyeing processes involving sulfur or vat dyes and a ...

in a mildly acidic solution. The low water solubility is the basis of the good wash-fastness of these dyed fabrics. These dyes have good all-around

colour fastness Colour fastness is a term—used in the dyeing of textile materials—that characterizes a material's colour's resistance to fading or running. Colour fastness is the property of dyes and it is directly proportional to the binding force between pho ...

except to chlorine bleaches. Because the dye is water-insoluble, it will not bleed when washed in water and will not stain other clothes. The dye, however, may have poor fastness to rubbing. The dyes are bleached by hypochlorite bleach.

Environmental issues

Due to the highly polluting nature of the dye-bath

effluent Effluent is wastewater from sewers or industrial outfalls that flows directly into surface waters either untreated or after being treated at a facility. The term has slightly different meanings in certain contexts, and may contain various polluta ...

, sulfur dyes are being slowly phased out in the West but they are used on a large scale in China. Recent advances in dyeing technologies have allowed the substitution of toxic sulfide reducing agents.

Glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

in basic solution is now used and both low-sulfide and zero-sulfide products are available. Future developments in the field of reducing dye levels by means of electrochemical processes are promising.

References

{{Dyeing Dyes Sulfur