Diarylide Yellow Pigments

Diarylide yellow pigments, 74 317 79 433 Diaryliodonium salt photoinitiators, 74 270 Diaryliodonium salts, 79 108 photolysis of, 70 414... 

For decades, P.Y.l represented the standard source of yellow for all printing purposes. Today, however, P.Y.l has been replaced in this field by the stronger diarylide yellow pigments. 

P.Y.62 exhibits comparably low tinctorial strength. A 0.5% pigment concentration in HD PE is necessary to produce 1/3 SD colorations (containing 1 % Ti02), compared to the 0.17% pigment concentration required to afford equal depth of shade with the somewhat more reddish diarylide yellow pigment P.Y.13. 

Acetoacetarylides and l-aryl-pyrazolone-5 are the two most common coupling components. The former affords diarylide yellow pigments (Sec. 2.4.1), while the latter produces disazopyrazolone pigments (Sec. 2.4.3). 

The printing ink industry, currently the most important area of application for diarylide yellow pigments, first introduced these products around 1938 in the USA. 

The chemically most simple diarylide yellow pigment is made from bisdiazo-tized 3,3 -dichlorobenzidine and acetoacetanilide. This pigment exhibits high tinctorial strength, a property which is particularly useful for process printing inks, for which other diarylide yellow pigments were later developed. 

Commercially significant diarylide yellow pigments have the following chemical structure ... 

In order to manufacture a diarylide yellow pigment, the diamine, dissolved in hydrochloric acid or sulfuric acid, is bisdiazotized with an aqueous sodium nitrite solution. The resulting diazonium compound is subsequently coupled onto two equivalents of acetoacetarylide. Since a material with a very fine particle size is needed for a fast and complete coupling reaction, the coupling component is prepared by dissolution in an alkaline medium and reprecipitation with an acid, such as acetic acid or hydrochloric acid. Dichlorobenzidine cannot be diazotized stepwise. A lack of sodium nitrite does not produce the monodiazonium compound, but an excess of 3,3 -dichlorodiaminodiphenyl remains. 

Adsorption of e.g. rosin (abietic acid) at the pigment surface may - depending on the concentration of the rosin - reduce or accelerate the crystal growth. The presence of an excess amount of rosin during the production of diarylide yellow pigments of the Pigment Yellow 13 type affords an additional crystal modification, which can be identified by X-ray diffraction spectroscopy [4],... 

Diarylide yellow pigments range in shade from very greenish to extremely reddish yellow. The more greenish types almost exclusively contain 2,2, 5,5 -tetra-chlorobenzidine in place of the otherwise more common 3,3 -dichlorobenzidine. 

Most commercial varieties of diarylide yellow pigments are materials with comparatively fine pigment particles and specific surface areas between 50 and 90 m2/g. 

Monoazo yellow pigments, on the other hand, are products with larger particle sizes and with much smaller specific surface areas. P.Y.74 is the only species of which there are types with specific surface areas approaching those of diarylide yellow pigments (Sec. 2.3.4). 

The pigment industry produces primarily the more transparent diarylide yellow pigment types. This has its advantages, particularly in the field of printing inks, since yellow is printed as the last color in three or four color printing (Sec. 1.8.1.1). Highly transparent varieties are almost exclusively resinated and are often easy to disperse. 

A number of diarylide yellow pigments are also available in the form of very opaque products with small specific surface areas. These are used to an appreciable extent in Chrome Yellow free paints, in packaging printing inks, and other media. Fastness to light and weather are much improved in opaque varieties, compared to the corresponding transparent types their rheological characteristics are superior. Opaque pigments are not resinated. 

In practice, diarylide yellow pigments are frequently selected for their excellent fastness to a variety of organic solvents, in which they perform much better than monoazo yellow pigments. Properties such as migration or recrystallization... 

A variety of diarylide yellow pigments are very fast to light and weather - although monoazo yellow and orange pigments are superior in this respect. 

A number of diarylide yellow pigments meet the lightfastness requirements for wallpaper printing. A certain tendency to migrate precludes, for instance, the more lightfast monoazo yellow pigments from use in PVC wallpaper. 

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