Pigment polymorph, color

However, there are metal complex pigments. Without doubt the most important metal complex pigment is copper phthalocyanine (4). The phthalocyanines were discovered by accident in 19286 and now represent the second most important class of colorants after the azo colorants. Copper phthalocyanine itself exists in several polymorphic forms and gives beautiful blue and cyan colors with outstanding fastness properties.5-7 Halogenated copper phthalocyanines provide green pigments (see Section 9.12.4.3). 

The synthesis of linear turns-quinacridone (2.38) was reported in 1935 by Liebermann [26] and was cursorily looked at as a red vat colorant but not developed commercially. It was more than twenty years later that the DuPont company introduced these compounds as pigments under the trade name of Cinquasia. Their chemical structures are based on Cl Pigment Violet 19 (2.38). As with the phthalocyanines, this compound can exist in several polymorphic forms in this case there are three, termed a-, p- and y- forms only the last two being useful as pigments. The first three pigments were called Cinquasia Red B (y-form, size above 1000 nm), Cinquasia Red Y (y-form, size below 1000 nm) and Cinquasia Violet R (P-form). 

Like some other Naphthol AS pigments, P.R.170 is polymorphous the shades of the known crystal modifications are all within the same range of colors. 

History. Braun and Tschemak [23] obtained phthalocyanine for the first time in 1907 as a byproduct of the preparation of o-cyanobenzamide from phthalimide and acetic anhydride. However, this discovery was of no special interest at the time. In 1927, de Diesbach and von der Weid prepared CuPc in 23 % yield by treating o-dibromobenzene with copper cyanide in pyridine [24], Instead of the colorless dinitriles, they obtained deep blue CuPc and observed the exceptional stability of their product to sulfuric acid, alkalis, and heat. The third observation of a phthalocyanine was made at Scottish Dyes, in 1929 [25], During the preparation of phthalimide from phthalic anhydride and ammonia in an enamel vessel, a greenish blue impurity appeared. Dunsworth and Drescher carried out a preliminary examination of the compound, which was analyzed as an iron complex. It was formed in a chipped region of the enamel with iron from the vessel. Further experiments yielded FePc, CuPc, and NiPc. It was soon realized that these products could be used as pigments or textile colorants. Linstead et al. at the University of London discovered the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines from 1929 to 1934 [1-11]. The important CuPc could not be protected by a patent, because it had been described earlier in the literature [23], Based on Linstead s work the structure of phthalocyanines was confirmed by several physicochemical measurements [26-32], Methods such as X-ray diffraction or electron microscopy verified the planarity of this macrocyclic system. Properties such as polymorphism, absorption spectra, magnetic and catalytic characteristics, oxidation and reduc-... 

As stated in Section 5.1, all solid-state properties of different polymorphs will be different. Which of these parameters is of special importance for a particular type of industry and a particular application can vary. While color is probably the most important parameter of a pigment, taste and feel are decisive for foods, density for explosives, solubility, processability and stability in the pharmaceutical industry, and so on. In any case, it is imperative for aH industries that all possible forms are known and characterized, since the unexpected appearance of a new form may have very serious consequences. An example for that is the well-known Norvir Case [30], where the thermodynamically stable form was found only long after the product was on the market. Subsequently, the original metastable form could not be produced anymore and a new formulation for the product had to be developed [31] causing very high costs. 

Figure 18-8 demonstrates Ti02 extensions (10 parts pigment and 90 parts Ti02) of gamma and beta quinacridone polymorphs. These forms, as well as the red alpha polymorph, are chemically identical but have different arrangements of the molecules in their crystal lattices. The red gamma phase has become an article of commerce, while the thermodynamically less stable alpha phase, with similar color attributes, has not. 

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