Naphthol AS pigment

For Naphthol AS pigments (Naphthol Red pigments), the same chemistry is involved as with monoazo yellow pigments, except that coupling is carried out with arylides of 2-hydroxy-3-naphthoic acid. The reference compound, Naphtol AS is the anilide of 2-hydroxy-3-naphthoic acid  

Substitution in the anilide ring affords a series of Naphthol AS derivatives, although only a limited number are commercially recognized. 

In 1892, the chemist Schopf, in an attempt to prepare 2-phenylamino-3-naph-thoic acid, developed a synthetic route leading to the anilide of 2-hydroxy-3-naph-thoic acid. His method continues to be used today, if only in a slightly modified form. He added phosphorus trichloride to a molten reaction mixture containing aniline and 2-hydroxy-3-naphthoic acid (beta-oxynaphthoic acid, also known as BONA) and received Naphthol AS in good yield. Modern processes differ from this principle only in terms of reaction control the synthesis is now carried out in the presence of organic solvents, such as aromatic hydrocarbons. 

Naphthol AS was not rediscovered until 1909, when BASF in Germany claimed a patent for a diazotization dye which could be developed by diazotizing primu-line on the fiber and then coupling with Naphthol AS in an alkaline solution. 

Naphthol AS derivatives that have retained commercial importance are listed in Table 10, together with their name, constitution, and Colour Index number (Sec. 2.1.2, p. 195). 

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Another important pigment in this class is Pigment Red 170 [6985-95-17, which provides medium shades of red, and when particle-grown produces an opaque modification which shows improved migration resistance and lightfastness. Like some other Naphthol AS pigments it shows the phenomenon of polymorphism. 

Naphthol AS pigments, 9 425 Naphthol couplers, 19 252-253 in chromogenic chemistry, 19 251 a-Naphthol derivatives, 21 145 P-Naphthol derivatives, 21 145 Naphthol dyes, 9 182, 407 08 Naphthol reds/maroons, 19 437-438 Naphtholsulfonic acids, 9 356 N aphthopyrans... 

These pigments are obtained by coupling substituted aryl diazonium salts with ary-lides of 2-hydroxy-3-naphthoic acid (2-hydroxy-3-naphthoic acid anilide = Naph-tol AS). They provide a broad range of colors from yellowish and medium red to bordeaux, carmine, brown, and violet their solvent fastness and migration resistance are only marginal. Naphthol AS pigments are used mainly in printing inks and paints. 

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. 

Comparative three dimensional X-ray diffraction studies have been carried out on a red and a brown representative of the Naphthol AS pigment series, differing only by the presence or absence of one methoxy group in the anilide function of the coupling component (4). 

Both molecules favor the o-quinone-hydrazone form over the o-hydroxyazo form. Corresponding studies on a selection of monoazo pigments and several (i-naphthol and Naphthol AS pigments are described in the respective chapters. 

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... 

The application fastness of the pigment, however, is more satisfactory. P.O.44 may be dry cleaned and resists dry heat up to 210°C for 30 seconds while P.O.5 and also the somewhat redder Naphthol AS pigment P.R.10 is less fast. P.O.44 is also used in PVC coatings. 

Technically significant Naphthol AS pigments are therefore divided into two groups, according to whether their diazo component carries... 

Today, 80 years after their discovery, Naphthol AS pigments continue to play a major role among organic pigments. Although comparatively many derivatives are known, only few are produced in large volume. 

Naphthol AS pigments have the following general chemical structure ... 

For group I Naphthol AS pigments, further treatment is not necessary. The pigment suspension may be heated to 60 to 80°C for a short time before it is filtered. 

Commercially available Naphthol AS pigments afford a variety of shades ranging from yellowish to very bluish red including all possible intermediate shades, such as bordeaux, maroon, violet, and brown. Most are tinctorially very strong compared to other pigments that cover the same range of shades. 

A variety of Naphthol AS pigments are polymorphous they display at least two crystal modifications. The list includes P.R.9,12, and 187. A wide variety of dia-zonium compounds and coupling components with different substitution patterns have been developed in order to alter the properties of the resulting Naphthol AS pigments. This is particularly true for parameters such as fastness to solvents, in... 

Table 17 lists a number of commercially available pigments, along with their chemical structures, in order to illustrate the different structural types of Naphthol AS pigments. Fastness to solvents and migration resistance improve from top to bottom, i.e., with increasing number of CONH groups in the molecule. The first example, a simple (3-naphthol pigment, is the skeleton from which all other species are derived. 

The heat sensitivity of a Naphthol AS pigment is largely a function of its chemical constitution and may, for instance in print, range from under 120°C to 200°C. 

Almost all Naphthol AS pigments, with a few exceptions, are excluded from metal deco printing. However, a number are used in large volume by the textile printing industry. Poor migration and heat resistance make most members of this class inapplicable to plastics. 

Naphthol AS pigments are used to an appreciable extent in special areas, such as in office articles, artists colors, cleaning agents and detergents, including soaps. They are used to color paper, both mass colored paper and surface coated paper. 

The number of Naphthol AS pigments that have gained commercial recognition is comparatively large (Table 18). There is a dual classification system according to the substitution pattern (Sec. 2.6.2). 

Somewhat of an intermediate position is assumed by a compound which may be considered both a disazo and a Naphthol AS pigment. Pigment Blue 25 is an interesting example of a cross compound, derived from coupling bisdiazotized 3,3 -dimethoxy-4,4 -diaminodiphenyl with Naphthol AS. 

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