Leuco dyes pigments

The chemistry of quinone dyes has been discussed in a series of books entitled The Chemistry of Synthetic Dyes, edited by Venkataraman.1 The general chemistry of quinoid compounds has been discussed by Patai.2 There have been many books that cover quinoid compounds as dyes and pigments but very few discuss the chemistry of the corresponding leuco dyes. Traditional vat dyes are applied to cellulosic fiber in the leuco form. The chemistry of the leuco form of vat dyes is rather simple. Some leuco quinones are quite stable in the solid state and can be stored for a year. Other leuco dyes are unstable in solution and gradually undergo aerial oxidation. 

The dyeing behavior of vat dyes, i.e., their absorptive behavior and leveling capacity, is determined basically by the substantivity and diffusion of the leuco compounds. The chemical stability and insolubility in water of the dye pigment that results from reoxidation in the fiber account for the generally unsurpassed light-, wet-, and weatherfastness of vat dyeings. 

After absorption by the fiber, the leuco dye is converted to the original pigment by oxidation and, in this way, fixed to the fiber. Oxidation can be achieved with atmospheric oxygen or by the addition of oxidants, such as hydrogen peroxide, perborate, or 3-nitrobenzene sulfonic acid, to the liquor. 

The leuco dye present in wastewater is also converted to water-insoluble pigment, which can easily be separated mechanically or adsorbed on clarification sludge. Thus, colorization of the wastewater by vat dyes is negligible. 

Flavanthrone. Flavanthrone [475-71-8] (177) (Cl 70600) has exceUent dyeiag properties, which ate due to the stabUity of the leuco form, but its fastness is not satisfactory. Only the unsubstituted flavanthrone is used as a vat dye, ie. Cl Vat YeUow 1 (177). It is mainly used as a pigment, ie. Cl Pigment YeUow 24. Synthetic routes ate iUustrated ia Figure 10. 

As a beginning toward understanding and preventing this phenomenon, the effect of selected amines and phenol stabilizers on the pigment-sensitized photooxidation of the leuco triphenylmethane dye I to the blue dye II was determined (Scheme 2). 

Vat dyes are water-insoluble, organic pigments that are used to dye cotton and other cellulose libers. The principle of vat dyeing is based on chemical reduction of these dyes to the leuco compounds, which are soluble in aqueous alkali and exhibit fiber affinity, followed by reoxidation within the fiber to the water-insoluble starting dye. 

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). 

As a result of their high price, low substantivity, and toxicological problems during production, the importance of the leuco esters of vat dyes is decreasing. They can be replaced easily by pigment coloring or by reactive or vat dyes. 

Many leuco compounds of vat dyes have a very strong affinity for cellulose and this can make it difficult to obtain uniform penetration in tightly constructed or packed goods, especially in yarn in package form. The insoluble pigment, however, has no affinity and if this can be distributed evenly and subsequently reduced and oxidized in situ the problem of penetration should be overcome. 

The Abbott-Cox process relies upon exhausting the suspended pigment. In many cases exhaustion is difficult to control and irregularity in the shade can be the result. Vat dyes in their insoluble oxidized state are available in a super finely divided form which may be added to the dyebath and circulated through the goods on which they will be uniformly deposited without the use of an electrolyte to bring about coagulation. After a suitable period sodium hydroxide is added, followed by sodium hydrosulphite when the leuco compound becomes adsorbed on the fibre and is... 

Klingsberg (jf.S.D.C., 1954, 70, 563) describes a method specially designed for the chromatographic examination of vat dyes. Owing to the insolubility in virtually all solvents of the pigment form, the separation must be carried out in the leuco state. The reduced vat dyes, however, have such a marked affinity for cellulose that paper chromatography must be ruled out, but strips of material made of hydrophobic synthetic fibres, of which polyesters are the best, may be substituted. Rao, Shah, and Venka-... 

---