Resorcinol Azo Dyes

Browns are the most important shades for leather after blacks. Despite then-higher molecular weight and sole use on leather, these dyes are normally also classified as acid dyes. C.I. Acid Brown 123, 35030 [6473-04-7] (9), for example, although a tetrakisazo compound, is classified as an acid dye. The dyeing is more on the surface. 

The mono and disazo resorcinol dyes can act as indicators and change color very markedly, depending on pH. Monoazo and disazo varieties are often found as byproducts with higher molecular weight resorcinol dyes. This property makes them less obtrusive on leather. Because of their strong intermolecular forces of attraction, resorcinol dyes are very well fixed on leather, despite their low to medium molecular weight. Their indicator properties are scarcely noticeable, in contrast to the case with textiles. This explains why these dyes are not necessarily suitable for textiles, particularly polyamides. 

Although the major part of research on resorcinol dyes was actually completed in the 1930s, an increasing number of patent applications were filed into the 1980s. The more recent work was conducted to enhance the scope of available structures, to optimize synthesizing procedures, and to meet new fastness requirements by minimizing byproducts. 

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