The Vat Dyeing Process

Using anthraquinone as an example shows that the dye molecule is converted to the sodium leuco form in alkaline solution by the gain of two electrons [Eq. (4)]. 

For a long time, sodium dithionite (hydrosulfite) has been the most important of the reducing agents [5, p. 288-289] for vat dyeing. It is inexpensive and, when produced by modem methods, virtually free of heavy metals (Zn, Hg). Its reduction potential (-970 mV) is high enough to vat all vat dyes quickly and completely- 

The rate of vatting depends not only on the concentration of dye and reducing agent but also on the crystal form, surface, and dispersion of the pigment (i.e., on its finish quality [51]). Leuco compounds are soluble in alkali. In the case of anthraquinoid vat dyes, the pH of the vat is about 13. At lower values the risk of vat acid sediments exists. Reduction is usually performed at 50-60°C. At higher temperature, over-reduction of certain dyes can occur (i.e., reductive destruction of the dye molecule). 

The oxidation products of sulfur-containing reducing agents are sulfite and sulfate. In most countries, the concentration of these substances in wastewater must not exceed maximum permissible values stipulated by law. 

Thiourea dioxide has a stronger reducing effect (-1100 mV) than hydrosulfite. Therefore, with sensitive dyes, a risk of over-reduction exists. In addition, the oxidation products of thiourea dioxide contribute to the nitrogen and sulfur contamination of wastewater. 

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ZnS204. Made SO2 on aqueous suspension of Zn dust. Used in bleaches and the vat dyeing process. 

In the batchwise process the temperature can be raised to 80°C to promote levelness providing dyes not sensitive to reductive breakdown are used. In the continuous appHcation method the vat dye is padded onto fabric and dried under conditions that avoid migration, passed through a solution of sodium hydrosulfite and caustic, through a pad mangle, and then steamed in saturated steam for up to 60 s. 

Redox behavior of anthraquinone is shown in Scheme 4. The quinone moiety may be reduced to the hydroquinone form and converted to a leuco salt under alkali conditions. In general, the leuco salt has a strong affinity for cellulose and is soluble in water. The hydroquinone form is insoluble in water and has low affinity to cellulose. The preferred dyeing procedure depends on the structure and properties of the vat dye. The variables that are used to control the process include, e.g., strength and amount of alkali, reduction temperature, and the presence of salts. During the process of reduction, some side reactions, such as overreduction, hydrolysis,... 

Vat Dyes. Vat dyes are insoluble in water. Indigo, for example, an ancient blue dye, is probably the best-known example of an ancient vat dye others include woad and Tyrian purple. Since the process of dyeing requires that the dye be in solution, dyeing with a vat dye (or vat dyeing, as the process is known) is possible only after the vat dye has been made soluble by a relatively long and somewhat complicated chemical procedure. The terms vat dye and vat dyeing are probably derived from the large tanks or "vats", in which the process was carried out in ancient times. 

A vat dye is a water-insoluble colorant containing two or more keto groups. It can thus be brought into aqueous solution by a reduction process (vatting), which converts the vat dye into its alkali-soluble enolic (leuco) form. As the soluble sodium enolate the leuco vat dye has substantivity for cellulose. The application of vat dyes to cellulosic fibres (virtually the only fibre type on which their outstanding fastness properties can be exploited) thus proceeds in four stages ... 

These resemble the vat dyes in certain ways, although they are of indeterminate constitution and usually mixtures of different chemical species (section 6.4). The characteristic disulphide group (D-S-S-D in Scheme 1.2) is always present in the insoluble form of a sulphur dye, which is brought into aqueous solution by reduction to the alkali-soluble (leuco) form (D-S ). The soluble sodium thiolate form of the leuco sulphur dye has substantivity for cellulose. Thus the application of sulphur dyes to cellulosic fibres is a three-stage process (Scheme 1.2) broadly similar to that already outlined for vat dyes. 

In the vatting process the water-insoluble vat dye is reduced by sodium hydro-gensulfite (hydros) to the water-soluble phenate salt. This is less colored than the vat dye and has affinity for cotton. The cloth dyed with the phenate salt is placed in water, normally containing soap, and air is bubbled through the solution to regenerate the vat dye, which is trapped in the pores of the cotton (Scheme 2.4). The phenate salt may be stabilized by forming the sulfate ester. These stabilized, water-soluble salts of vat dyes, such as C.I. Solubilised Vat Blue 1 (5), are commercial products. 

In the dissolved form, the vatted dye is present as the sodium leuco compound, either as a single molecule or as a complex of a few dye molecules. In the exhaustion process, the higher the affinity, the more complete is the absorption by the fiber. The degree of absorption of dyes depends also on dye concentration, liquor ratio, temperature, and the electrolyte present in the dyebath. It is generally between 80 and 90 %. 

A greater measure of continuity is achieved in the Pad-steam process in which the cloth is first pigment-padded, dried, padded with sodium hydroxide and hydrosulphite, passed through a steam chamber to reduce and fix the vat dye, and finally finished off in a soaping and rinsing range. 

The vat-acid process has achieved a considerable measure of success and is based upon the use of a dispersion or suspension of the insoluble leuco compound instead of its sodium salt. The various reactions of an anthra-quinone vat dye are shown on page 503 (Speke, Hexagon digest, No. 7). The acid leuco compound is probably a mixture of the enol and keto/enol forms but the more the equilibrium favours vhe oxanthrone form the greater is the stability towards oxidation. The leuco derivatives are prepared by precipitation from a solution of the sodium salt on acidification. In order, however, that they shall be thrown out of solution in a state suitable for the formation of a stable suspension it is necessary that a dispersing agent should be present before addition of the acid. The vat dyes with the leuco compounds which disperse most easily are ... 

Tri-n-butyl phosphate is an effective anti-foaming agent which is used in paper making, printing ink manufacture, emulsion paints, and vat dyeing processes. It may be added to these systems as a 1-5 % solution in alcohol or acetone. Tri- -butyl phosphate can be used as a plasticiser or in low-temperature hydraulic fluids, but alternative tri-aryl derivatives are generally preferred (see above). The more important applications of orthophosphate esters are summarised in Figure 12.20. 

The first volume-yardage continuous process was the continuous pad-steam process for vat dyes on cotton. The vat dye dispersion was padded onto the cloth and dried this was followed by passage through a reducing bath, steaming for 30 seconds, an oxidizing bath... 

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). 

Polycyclic Aromatic Carbonyl Dyes. StmcturaHy, these dyes contain one or more carbonyl groups linked by a quinonoid system. They tend to be relatively large molecules built up from smaller units, typically anthraquinones. Since they are appHed to the substrate (usually cellulose) by a vatting process, the polycycHc aromatic carbonyl dyes are often called the anthraquinonoid vat dyes. 

The manufacturing process of anthraquinone vat dyes is more compHcated, and, in the extreme case of Cl Vat Blue 64 [15935-52-1] (12) (Cl 66730), requites 11 steps starting from phthaUc anhydride. 

Dihydroxyanthraquinone. This anthraquinone, also known as quinizarin [81-64-1] (29), is of great importance in manufacturing disperse, acid, and vat dyes. It is manufactured by condensation of phthalic anhydride (27) with 4-chlorophenol [106-48-9] (28) in oleum in the presence of boric acid or boron trifluoride (40,41). Improved processes for reducing waste acid have been reported (42), and yield is around 80% on the basis of 4-chlorophenol. 

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