SULPHUR BLACK

Sulfur black dyes are high molecular weight polymeric compounds that are produced by the reaction of sulfur or sulfides with phenols and amines. They are commonly used in the dyeing industry owing to their cost effectiveness. Cotton-based textile companies are the key customers of sulfur black dyes. Various colorants that contain sulfur are available in the market; however, only those dyes that are soluble in water after reacting with sodium sulfides in alkaline conditions are considered actual sulfur black dyes. In alkaline conditions, sulfur black dye gets converted into leuco forms which are the soluble form and they show affinity to fibers. The commonly used reduced agents for the dyeing include sodium sulfhydrate, sodium polysulfide, glucose etc. Common salt when used with sulfur dyes acts as a catalyst. This facilitates the absorption process. Ease of application is the key benefit of the application of sulfur black dyes. These dyes can be used in cotton, nylon, and synthetics. Sulfur black dyes are highly complex. They consist of polysulfide and chromospheres side chains. The low water solubility is the basis of good wash –fastness of the dyed fabrics.

A technological development in the textile industry is considered a key factor driving the global sulfur black dyes market. Cost effectiveness is also an important factor that attract consumers toward sulfur black dyes. Growth in the cotton-based textile industry is expected to positively impact the global sulfur black dyes market. Increase in population, rise in disposable income, and overall economic developments are anticipated to boost the global sulfur black dyes market. Ease of application is another key factor driving the global sulfur black dyes market. Advancements in chemical and polymer sectors are anticipated to have significant influence on the sulfur black dyes market.

Sulfur dyes are most commonly used dyes manufactured for cotton in terms of volume. They are cheap, 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.

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 results 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.

Sulfur dyes are water-insoluble. In the presence of a reducing agent and at alkali pH’s at elevated temperature of around 80°C, the dye particles disintegrate, which then becomes water-soluble and hence can be absorbed by the fabric. Sodium sulfide or sodium hydrosulfides 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 affected 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 round 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.

The sulphur dyes have been in use as cotton dyes for over ninety years. As far as the application is concerned the sulphur dyes are akin to the vat dyes since they are water insoluble substances which first must be reduced to a soluble form by means of sodium sulphide, which is applied to cotton fibre and the dye being regenerated by oxidation (in air). This group of dyes is produced by heating relatively simple aromatic substance with sulphur or a sulphur compound where by aromatic rings are by disulfphide or disulphoxide bridges. These links are converted by the action of sodium sulphide into -SNa groups, smaller water soluble molecules being produced. On oxidation in air the molecules are linked together once more as sulphides.

The sulphur colour provides a full range of hues from yellow to black. None is bright and reds are almost absent apart from a few dull maroons and hordeaux shades.

The main representative among the sulphur colors is sulphur black T. The earlier sulphur blacks such as Vidal black FF from P-aminophenol and Immedial Black FF, extra from 2,4 -dinitro – 4- hydroxy diphenylamine, have been more or less completely displaced at the present time by the cheaper and faster black obtained by thionation of 2,4 – di-nitrphenol. This dye sulphur black T is made by all the leading dye manufacturers and sold in many different brands varying in the tone of the black and the physical form of the dye. Sulphur black is not merely the most important sulphur colour, but considering the quantities produced it is the most important single dye of any class.

The commercial importance of the dye is due to its case of preparation cheapness and dyeing and fastners properties full black shades are obtained on cotton by air oxidation after dyeing in the usual manner from a sodium sulphide bath and they have been started to be remarkably fast to light, acids, alkalies, scouring and milling; this is broadly true, the fastness to other agencies except chlorine being of the order of 4-5. The chloride fastness is poor as for other sulphur dyes. One disadvantage of some sulphur black is the tendering of the cotton fibre. Sulphur black shades which have been after treated with copper salts for imparting the brightness and shade on cotton wool unions which have been cross dyed with acid colours are particularly susceptible to the oxidation and tendering action. There are methods to avoid or minimize the action, but it would appear that it is possible to prepare sulphur blacks which are largely free from this effect. Tendering of sulphur black may also be partly prevented by complete oxidation of dye stuff in the interior of the fibre.

Although 2,4-dinitrophenol, sodium sulphide and sulphur are used in the manufacture of the various brands of the sulphur black T type of dye. The products are chemically by no means identical, the constitution (as yet undetermined) and the composition (including the sulphur content) are variable in accordance with the conditions employed for thionation. the probable mechanism of the reaction involves the formation of 4-nitro 2 aminophenol which may immediately condense with sulphur or undergo further reduction to the diamine, since the different products are obtained by starting with 4 Nitro 2 aminophenol or 2,4-diaminophenol. Dinitrophenol is prepared by the hydrolysis of 2,4 dinitrochlorobezene (free from water and acid, secting point above 470o) with boiling caustic soda solution, the product may be employed for thionation without isolating the phenol, the chloro compound may also be used directly by treatment with a mixture of aqueous caustic soda, sodium sulphide and sulphur.