Diazo pigments

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Naphthol AS Coupling Components. Naphthol AS components are the aryhdes of either o-hydroxyarylcarboxycHc acids or acylacetic acids. They are free of sulfo and carboxyl groups, but form salts with bases these salts dissolve in water to give colloidal solutions, which couple with diazo components to form colored pigments. The whole class derives from the anilide of 3-hydroxy-2-naphthoic acid [92-70-6] Naphthol AS (85) (Cl Azoic Coupling Component 2). 

Insoluble sulfonated pigments are made from colorants that contain a sulfonic acid group that is easily converted into an insoluble metal salt. In most cases, the sulfonic acid group is ortho to the diazo further reducing the solubilizing characteristics of the sulfonic grouping. The shade of these products is affected by the metal incorporated into the molecule and the physical characteristics of the colorants. D C Red Nos. 7 (19b) and 34 (26) are insoluble sulfonated pigments. 

Coupled with the latter investigations, it was shown that a diazo suspension can be fed through a micro device without leading to plugging [55]. This broadens the scope of micro processing applicability towards azo pigment formation. 

OS 39] ]R 25] ]P 27] A 24 h run of a pilot-scale micro reactor for azo pigment production was performed using a diazo suspension [55], At the end of this period, the pressure loss of the micro reactor increased exponentially. Special means were developed to prevent clogging and unstable operation. By partial removal of the deposits, the pressure loss was brought back to normal. 

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. 

Naphthalenesulfonic acid lake pigments are based on naphthalenesulfonic acid as a coupling component introduction of an additional S03H function as part of the diazo component is possible. 

The basic hue of an azo pigment is primarily defined by the structure of the coupling component, since pigment manufacturers focus almost exclusively on substituted anilines as diazo components. Shades of yellow, for instance, are preferably produced by using acetoacetic arylides CH3COCH2CONH-Ar or heterocyclic coupling components based on the structure... 

The correlation between constitution (substitution pattern of the diazo component) and solubility of an azo compound has been studied on a selection of Naphthol AS pigments [17]. Substituents being identical, their position on the aromatic... 

Azo pigments are typically formed by a reaction sequence of diazotization and coupling, involving a primary aromatic amine, which is referred to as a diazo component, and a nucleophilic aromatic or aliphatic compound with active methylene groups as a coupling component [1-3]. 

Another family of technically important diazo components for pigment formation includes a series of aromatic diamino compounds, primarily 3,3 -dichlorobenzidine, and to a lesser extent 3,3 -dimethoxybenzidine (o-dianisidine), 3,3 -dimethylbenzidine (tolidine) and 2,2, 5,5 -tetrachlorobenzidine ... 

The synthetic route to monoazo yellow pigments involves the coupling of a diazo-tized substituted aniline with a coupling component containing an active methylene moiety in a linear structure. 

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. 

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). 

Structurally, there are two basic types of disazo pigments, depending on whether the bifunctional element is introduced through the diazonium compound or through the coupling component. A bifunctional diazo component results in products of the type... 

Structurally based on 3,3 -dimethoxybenzidine (o-dianisidine) as a diazo component, P.O.16 is also known as Dianisidine Orange. At present, the pigment only enjoys some importance in Europe, the USA and in Japan. 

P.O.44 has lost most of its commercial importance and is at present only applied to a limited extent. This is also true for other pigments whose synthetic route involves 3,3 -dimethoxybenzidine as a diazo component. P.O.44 provides a very reddish orange shade, which is much redder than the color of the (3-naphthol pigment P.O.5. Although P.O.44 is more resistant to solvents than P.O.5, the reverse is true for lightfastness. Standardized letterpress proof prints containing P.O.44, for instance, equal step 3 on the Blue Scale, while equally deeply shaded P.O.5 prints equal step 6 on the Blue Scale. 

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