Naphthol AS Pigment Lakes

The organic acid group of Naphthol AS pigment lakes is part of the diazo component a second site for salt formation can be provided by the coupling component. The plastics industry is the main user of such lakes. 

Commercially Available Naphthol AS Pigment Lakes General... 

There is no common structural principle to these pigments beyond the basic Naphthol AS pigment skeleton. Table 21 lists the commercially available Naphthol AS pigment lakes. 

Table 21 Commercially available Naphthol AS pigment lakes.

As a result of their salt character, (3-naphthol pigment lakes are faster to solvents and more resistant to migration than (3-naphthol pigments, but also less light-fast. They are only moderately fast to alkaline agents. The polar character of these pigments is responsible for their good heat stability. 

Pigment Red 53 has for some time dominated the (3-naphthol pigment lake market in Europe and Japan. Pigment Red 49, employed especially as the barium and less frequently as the calcium salt, is mainly used in the USA. Other pigments of this type are less important. 

The history of BONA pigment lakes parallels that of (J-naphthol pigment lakes. Literary evidence of the use of 2-hydroxy-3-naphthoic acid, which was first synthesized in 1887 by Schmitt and Burkard as a coupling component, dates from 1893 (Kostanecki). However, it was not until 1902 that the compound began to be employed in dye synthesis by AGFA (aniline - BONA). 

Technically important pigments in this group - similar to (J-naphthol pigment lakes - contain aminosulfonic acids as diazonium components. Therefore, these pigments always contain two acidic groups for salt formation. 

As with p-naphthol pigment lakes, there is only a limited number of BONA pigment lakes that are marketed in large volume. Two of these pigments, however, maintain an important position within the pigments industry. Table 20 lists the currently available BONA pigment lakes. 

Pigment synthesis follows the typical route to azo pigment lakes the aniline derivative or the aniline sulfonic acid is diazotized with sodium nitrite in an acidic medium (hydrochloric acid), followed by coupling on the Naphthol AS derivative, which is initially dissolved in an alkaline solution and then precipitated by adding inorganic or acetic acid. If a Naphthol AS sulfonic acid is used as a coupling component, it must be neutralized with alkali for dissolution and coupled directly. 

Lithol red C20H13N2O4SM (M= = Na, Ba, or Ca). Pigment Red 49 (15630). Pigment made by combining the intermediates, to-bias acid and beta-naphthol. This type of red is available as sodium, barium, and calcium toners, also lakes the sodium is the lightest shade, the barium is what may be termed a medium shade, and the calcium lithols are deep reds and maroons. 

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