Sulfur dye

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Sulfur dyes are the most commonly used dyes 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. [1] Red sulfur dyes are unknown, although a pink or lighter scarlet color is available.

Contents

Chemistry

Sulfur linkages are the integral part of chromophore in sulfur dyes. They are organosulfur compounds consisting of sulfide (–S–), disulfide (–S–S–) and polysulfide (–Sn–) links in heterocyclic rings. They feature thiazoles, thiazone, thianthrene, 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: [2]

Fe + H2S → FeS + H2

Production, past and present

The forerunner of sulfur dyes is attributed to "Cachou de Laval", which was discovered by Groissant and Bretonniere in 1873 and is prepared by treating products with lignin (like sawdust or straw) with sulfide sources (like sodium hydroxide or sulphide mixed with sulphur). Subsequently, Henri-Raymond Vidal invented so-called Vidal Blacks in 1893 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. [3]

The most important member of the class is Sulfur Black 1. It is produced by the reaction of 2,4-dinitrophenol 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 derivatives, which are thought to form indophenol-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 thianthrene and phenothiazine subunits. The so-called sulfur bake dyes are produced from 1,4-diaminobenzene and diaminotoluene derivatives. These dyes are proposed to consist of polymers with benzothiazole subunits. Members of the sulfur bake dyes class are Sulfur Orange 1, Sulfur Brown 21, and Sulfur Green 12. [1]

Partial chemical structure of proposed for Sulfur Black 1. SulfurBlack1.svg
Partial chemical structure of proposed for Sulfur Black 1.

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 or sodium hydrosulfide 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 or sodium bromate 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 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, sulfur dyes are being slowly phased out in the West but they are used on a large scale in China. [3] Recent advances in dyeing technologies have allowed the substitution of toxic sulfide reducing agents. Glucose 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.

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<span class="mw-page-title-main">Thiol</span> Any organic compound having a sulfanyl group (–SH)

In organic chemistry, a thiol, or thiol derivative, is any organosulfur compound of the form R−SH, where R represents an alkyl or other organic substituent. The −SH functional group itself is referred to as either a thiol group or a sulfhydryl group, or a sulfanyl group. Thiols are the sulfur analogue of alcohols, and the word is a blend of "thio-" with "alcohol".

<span class="mw-page-title-main">Sodium carbonate</span> Chemical compound

Sodium carbonate is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water. Historically, it was extracted from the ashes of plants grown in sodium-rich soils, and because the ashes of these sodium-rich plants were noticeably different from ashes of wood, sodium carbonate became known as "soda ash". It is produced in large quantities from sodium chloride and limestone by the Solvay process, as well as by carbonating sodium hydroxide which is made using the Chlor-alkali process.

<span class="mw-page-title-main">Tie-dye</span> Technique of resist dyeing

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<span class="mw-page-title-main">Anthraquinone</span> Chemical compound

Anthraquinone, also called anthracenedione or dioxoanthracene, is an aromatic organic compound with formula C
14H
8O
2. Isomers include various quinone derivatives. The term anthraquinone however refers to the isomer, 9,10-anthraquinone wherein the keto groups are located on the central ring. It is a building block of many dyes and is used in bleaching pulp for papermaking. It is a yellow, highly crystalline solid, poorly soluble in water but soluble in hot organic solvents. It is almost completely insoluble in ethanol near room temperature but 2.25 g will dissolve in 100 g of boiling ethanol. It is found in nature as the rare mineral hoelite.

<span class="mw-page-title-main">Kraft process</span> Process of converting wood into wood pulp

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes.

<span class="mw-page-title-main">Sodium sulfite</span> Chemical compound

Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. It is also suitable for the softening of lignin in the pulping and refining processes of wood and lignocellulosic materials. A heptahydrate is also known but it is less useful because of its greater susceptibility toward oxidation by air.

<span class="mw-page-title-main">Sodium thiosulfate</span> Chemical compound

Sodium thiosulfate is an inorganic compound with the formula Na2S2O3·(H2O)(x) .Typically it is available as the white or colorless pentahydrate, It is a white solid that dissolves well in water. The compound is a reducing agent and a ligand, and these properties underpin its applications.

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<span class="mw-page-title-main">Sulfonic acid</span> Organic compounds with the structure R−S(=O)2−OH

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Calcium sulfide is the chemical compound with the formula CaS. This white material crystallizes in cubes like rock salt. CaS has been studied as a component in a process that would recycle gypsum, a product of flue-gas desulfurization. Like many salts containing sulfide ions, CaS typically has an odour of H2S, which results from small amount of this gas formed by hydrolysis of the salt.

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<span class="mw-page-title-main">Thiophenol</span> Chemical compound

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<span class="mw-page-title-main">Diphenylamine</span> Chemical compound

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<span class="mw-page-title-main">Mercaptobenzothiazole</span> Chemical compound

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References

  1. 1 2 Nagl, Gert (2000). "Sulfur Dyes". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_613. ISBN   978-3527306732.
  2. ↑ Peters R. H, "Textile Chemistry", Vol - II, Elsevier Publishing Company, London (1967)
  3. 1 2 Parikshit Goswami, Montu Basak "Sulfur Dyes" in Kirk-Othmer Encyclopedia of Chemical Technology, 2001, John Wiley & Sons. doi : 10.1002/0471238961.1921120619051409.a01.pub2.
  4. ↑ Industrial Dyes: Chemistry, Properties, Applications" Klaus Hunger, Ed. 2007, Wiley-VCH, Weinheim. ISBN   3-527-30426-6
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