Monoazo Yellow Pigment Lakes

Monoazo yellow pigments lakes are basically synthesized by introducing acidic groups into the diazo or coupling component and by precipitating the product as an insoluble salt. Usually the precipitation is carried out with salt solutions of calcium, barium, strontium or manganese. 

A number of commercially available yellow monoazo pigment lakes are based on a pyrazolone sulfonic acid derivative as a coupling component. An example is the aluminum tartrazine lake, listed in the Colour Index as Pigment Yellow 100, 

Recent patents [5-7] also describe yellow pyrazolone monoazo pigment lakes based on the general structure 

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A more traditional approach consists of introducing sulfonic acid substituents into yellow monoazo compounds. These can simply be precipitated, i.e., converted to lakes by salt formation, especially with calcium, and thus afford the more solvent and migration resistant monoazo yellow pigment lakes. 

P.Y.169 is another calcium monoazo yellow pigment lake, which has been available for some years. Its hue is a reddish yellow. Its properties and application are similar to those of the much greener P.Y.168. 

Introducing acidic substituents into the basic structure of a typical monoazo yellow pigment makes it possible to convert the material into a lake by salt formation. This improves the application properties of a pigment compared to its non-laked counterpart. Such pigments exhibit particularly good migration resistance and heat stability, making them useful colorants for plastics. 

Tables 11 and 12 list the currently available non-laked monoazo yellow and orange pigments. Most of them carry a nitro substituent in the diazonium component, usually in ortho position relative to the azo bridge. The more migration-resistant Pigment Yellow 97 lacks a nitro group in its molecule. Seven monoazo yellow pigments are based on 2-nitro-4-chloroaniline or derivatives as a diazo component.

The currently available salt type monoazo yellow pigments are mostly based on 2-nitroaniline-4-sulfonic acid as a diazo component and are mostly sold as calcium lakes (Table 13). 

Monoazo Yellow and Monoazo Orange Pigment Lakes... 

Most non-laked monoazo yellow and orange pigments are derivatives of the following structure ... 

Table 11 Non-laked monoazo yellow and orange pigments.

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