Vat Dyeing Process

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

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. 

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. 

Diaminoanthraquinone is an important intermediate for vat dyes and disperse dyes, and is prepared by oxidizing leuco-l,4-diaminoanthraquinone with nitrobenzene in the presence of piperidine. An improved process has been reported (45). 

Anthraquinone dyes have been produced for many decades and have covered a wide range of dye classes. In spite of the complexity of production and relatively high costs, they have played an important role in the areas where excellent properties ate requited, because they have excellent lightfastness and leveling properties with brUhant shades that ate not attainable with other chtomophotes. However, recent increases in environmental costs have become a serious problem, and future prospects for the anthraquinone dye industry ate not optimistic. Some traditional manufacturers have stopped the production of a certain dye class or dye intermediates that were especially burdened by environmental costs, eg, vat dyes and their intermediates derived from anthraquinone-l-sulfonic acid and 1,5-disulfonic acid. However, several manufacturers have succeeded in process improvement and continue production, even expanding their capacity. In the forthcoming century the woddwide framework of production will change drastically. 

Most vat dyes are based on the quinone stmcture and are solubilized by reduction with alkaline reducing agents such as sodium dithionite. Conversion back to the insoluble pigment is achieved by oxidation. The dyes are appHed by either exhaust or continuous dyeing techniques. In both cases the process is comprised of five stages preparation of the dispersion, reduction, dye exhaustion, oxidation, and soaping. 

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. 

Anthraquinone leuco dyes are widely known as vat dyes.10 Vat dyes possess extensively conjugated aromatic systems containing two or more carbonyl groups, e.g., anthraquinone, indigoid chromophores. The colored form of vat dyes are insoluble in water. The dyes are applied by a process whereby the dye is converted to the reduced form (leuco dye) which is soluble in water and can penetrate into a cellulosic fiber. On exposure to the atmosphere the leuco form is oxidized to the original quinoid form which then precipitates as an aggregate. Vat dyes generally have excellent chemical and photochemical stability. 

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,... 

The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water. Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. Disperse dyes for polyester fibres, for example, are only sparingly soluble in water and are applied as a fine aqueous dispersion. Vat dyes, an important application class of dyes for cellulosic fibres, are completely insoluble materials but they are converted by a chemical reduction process into a water-soluble form that may then be applied to the fibre. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic... 

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. 

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