Quinacridone polymorphism

Figure 18-8 Two quinacridone polymorphs extended with Ti02.

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. 

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). 

Linear trans-quinacridones exhibit multiple crystal modifications. This phenomenon is reflected in the different reflection angles which are observed in the X-ray diffraction spectra of the pigment powders. This effect may be exemplified by describing the polymorphous phases of unsubstituted quinacridone. 

Quinacridone is the trivial name given to the five-ring heterocyclic system exemplified by the linear irani-quinacridone (2.69). The yellow-red to reddish-violet shade pigments based on this ring system show outstanding durability and are used in plastics, in industrial and automotive finishes and in exterior finishes. Like copper phthalocyanines the unsubstituted linear quinacridone exhibits polymorphism and two crystal forms, the reddish violet P-form and the red y-form, are commercially... 

Polymorphism in some specific groups of pigments 8.3.1 Quinacridones... 

The synthesis of quinacridone 8-1 was first claimed in a series of papers beginning in 1896 (Niementowski 1896, 1906 Ullman and Maag 1906 Baczynski and Niementowski 1919) all of which later proved to be an angular isomer of the linear compound first prepared in 1935 (Lieberman 1935 Liebermanefa/. 1935). Little was done with the compound until 1955 when Du Pont chemists recognized its polymorphic behaviour and associated favourable photochemical stability (Jaffe 1992). Jaffe has also noted that many acridone derivatives are polymorphic. Additional relevant references may be found in the recent review by Lincke (2000). 

At the lowest velocity 100 m/min inside fibres the crystalline structure consisting of both polymorphic modifications is formed. The K value equals 0.75. The content of p crystals is much higher than the content of a crystals. Nevertheless, the content of the p form is much lower in comparison to fibres coloured with quinacridone alone, but significantly greater in comparison to fibres coloured with phthalocyanine (Broda, 2003d). 

The red-violet compound attracted no particular attention until 1955, when DuPont chemists discovered two attractive syntheses, demonstrated its photochemical stability, and identified three polymorphic forms, each with distinct structural characteristics. In addition, methods were developed for the preparation of quinacridone in useful pigmentary forms. The first pigments were commercialized in 1958. 

Figure 18-3 Powder X-ray diffraction patterns of three polymorphic forms of quinacridone.

In 1958, production of both beta and gamma polymorphs of quinacridone began in the USA at Newport, Delaware, with an annual capacity of 115 tons. Today, approximately 40 years later, annual worldwide sales are estimated at 3400 tons, a 30-fold increase Major producers are Ciba SC, whose quinacridones are marketed as Mon-astral in North America and Cinquasia elsewhere. Clariant markets its products as Hostaperm , Sun Chemical as Sunfasf , Bayer as Quindo , and Dainippon as Fastogen . Ciba, Clariant and Sun account for about 80 % of all sales. The remainder is shared by Bayer, Dainippon and other smaller producers, including Sudar-shan in India. Estimated worldwide geographical distribution of quinacridone sales is as follows ... 

In contradistinction to the thermal process, the cycUzed product remains in solution in the hot PPA, presumably in the form of a solvated zwitterionic phosphoric acid salt such as 24 (Scheme 18.11, n= 1—4). The quinacridone product in its cmde state then is obtained by precipitation when the reaction solution is drowned into an appropriate protic solvent (the equivalent of an acid pasting process). As the properties of the solvents have profound effects on the subsequent polymorphic phase and quahty of the resulting pigments, a specified selection is required for each product code. 

Fairly recently a large particle size, opaque P-polytype has been commercialized [34]. Several other polymorphic forms of quinacridone have been claimed in the patent literature [35]. 

Quinacridones substituted in the peripheral rings are synthesized by the methods described earlier by including appropriately substituted aniUnes. Anilines that are p- and o-substituted afford 2,9- and 4,11-disubstituted quinacridones, respectively. By contrast, the inclusion of m-substituted aniUnes afford intermediate terephtha-lic acids that possess two distinct sites that compete for cyclization, subsequently resulting in the formation of the three possible isomers 3,10-, 1,8- and 1,10-disub-stituted quinacridones. The 3,10-isomer predominates in the mixture because it is formed by cyclization at the least stericaUy hindered sites of the intermediate product. In some instances, the three isomers can be separated. The individual compounds also can be distinguished by inspection of the NMR spectra of the mixtures [36]. Like the parent compound, many of the substituted derivatives, including 6,13-dihydroquinacridone [37], exhibit polymorphism For example,... 

Crystal structure In certain cases, chemically identical pigments existing in different crystal forms (also known as polymorphisms) may exhibit a different hue. Pigments that clearly show the effect of polymorphism on hue are quinacridone pigment (Pigment Violet 19), phthalocyanine pigment (Pigment Blue 15) and lead chromates. 

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