Naphthol pigments

lll is chemically related to the pyrazolone pigments P.O.34 and P.R.37. Its hue is somewhat bluer than that of its pyrazolone counterparts it equals that of Signal Red (RAL 3000). The lightfastness of P.R.lll is somewhere between that of P.O.34 and of P.R.37. P.R.lll performs similarly as far as other fastness properties are concerned. The pigment lends itself particularly to the coloration of rubber and PVC excellent dielectrical properties also render it suitable for cable insulations. Thermally, P.R.lll is not sufficiently stable to be used in polyolefins, styrene, ABS, and similar plastics. 

The same chemistry is involved as with monoazo yellow pigments, except that (3-naphthol pigments are obtained by coupling with 2-hydroxynaphthalene ((3-naph-thol) instead of acetoacetarylides. They have the general chemical structure  

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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. 

The (i-naphthol pigment Pigment Red 1 was the first red azo pigment to be submitted to three dimensional X-ray diffraction analysis [1], Today, three different crystal modifications are known, exhibiting divergent hues and crystal shapes [2] ... 

J-Naphthol pigments come in shades from yellowish orange to bluish red. They are tinctorially weak and in some applications considerably less strong even than monoazo yellow pigments. 

The technical applicability of (J-naphthol pigments is severely limited by poor fastness to organic solvents and migration. 

The more significant members of this family have not lost their impact in the field of paints, which is the main market for (3-naphthol pigments. 

Naphthol pigments are broad in scope in many fields. Special applications include cleaners and detergents, office articles, and artists colors, as well as match-head compositions and fertilizers. 

Only very few (3-naphthol pigments continue to play an important role in today s pigment industry. The list of important products includes Toluidine Red (P.R.3) and Dinitroaniline Orange (P.O.5). Other compounds, such as P.R.6, Parachlor Red, which is the positional isomer of P.R.4 P.O.2, Orthonitroaniline Orange, which is the positional isomer of the para toner P.R.l are only of regional importance. Table 16 lists the commercially available (3-naphthol pigments. The Colour Index numbers are listed along with the common names, since older products are frequently referred to by these names. 

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. 

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. 

Good general fastness properties make (J-naphthol pigment lakes suitable candidates primarily for the printing inks and plastics industry. Their primary area of application varies according to the type. (J-Naphthol pigment lakes are also employed in paints and in emulsion paints, but to a lesser degree. 

Table 19 lists the currently available (J-Naphthol pigment lakes. 

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. 

Table 19 Commercially available B-naphthol pigment lakes. General chemical structure ...

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. 

BONA pigment lakes are basically synthesized like (3-naphthol pigment lakes. 

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. 

The pigment is then laked according to the procedure described for [3-naphthol pigments (Sec. 2.7.1.1). Aluminum lakes are an exception. A soluble aluminum salt is first converted to aluminum oxide hydrate, which is washed to remove salt. The moist product is then combined with the dye solution, while a more soluble aluminum salt is added simultaneously. The insoluble pigment is finally washed salt-free and dried. 

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