Thioindigo pigment

These pigments also originated from the vat dye route. They have very little practical significance in the coating industry. Only chlorinated and methylated thioindigo derivatives have any noticeable commercial presence as pigments. 

They provide a range of colors from red-violet and maroon to brown shades based on the type and position of the substituents. 

As a class, these compounds are related to indigo, which is the oldest of all vat dyes. Indigo itself, during its comparatively short history as a commercial pigment, was used especially in rubber. Its structure was first described in 1883 by A.V. Bayer. Knowledge of this structure, together with the development of an industrial scale synthesis, facilitated the development of a series of indigo-based colorants. 

Thioindigo pigments have the following general structure  

In commercially important products, R typically represents a chlorine atom or a methyl group, and n stands for 2 or 3. The compounds assume the trans structure. 

The most important route involves cyclization of appropriately substituted phenylthioglycolic acids  

Suitable agents are chlorosulfonic acid, monohydrate, or concentrated sulfuric acid. Other methods include cyclization of phenylthioglycolic acid chlorides, which is afforded by Friedel-Crafts reaction with aluminum chloride, possibly in the presence of sulfuric acid/sodium chloride. The acid chloride is obtained from phenylthioglycolic acid with thionyl chloride or phosphorus trichloride. 

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Thioindigo Pigments. The thioindigos are red and violet pigments developed for textiles. Two red—violets. Pigment Red 88 [14295 3-3] and Pigment Red 198 [6371-39-9] are recommended for plastics because of their excellent fastness properties. 

Thiohydrazides, 13 575 colorant for plastics, 7 374t Thioindigo pigments, 19 444 Thioindoxyl... 

In this section a number of polycyclic pigments are discussed which have been used for a long time as vat dyes for textile fibers. Heading the list are perylene, perinone, and thioindigo pigments, as well as pigments derived from anthraquinone. 

Two important commercial yellows, one from phenylenediamine. Pigment Yellow 110 (XXrv), and the other from 2,6-diaminotoluene, Pigment Yellow 109 (XXV), have found use in automotive finishes. Prior to the research at Geigy, the class of isoindolinones had been looked at as potential colorants however, it was the Geigy chemists v/ho discovered that the chlorine substituents are vital to produce pigments with acceptable weatherfastness and insolubility for commercial use as was shov/n earlier to be the case for the thioindigo. Pigment Red 88. 

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