Pyranthrone pigments

Commercial attention focuses on the derivatives of the pyranthrone molecule at a varying level of halogenation. Most are orange but others exhibit a dull medium to bluish red shade. Due to their good weatherfastness pyranthrone pigments are used for high grade industrial finishes. 

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Pyranthrone pigments are related to the pyranthrone structure (99), which is formally derived from flavanthrone (Sec. 3.7.3.2) in that the nitrogen atoms are replaced by CH groups. 

Manufacture of the unsubstituted compound 99 (Pigment Orange 40, 59700) has recently been discontinued. Other commercially available pyranthrone pigments include bromo, chloro, or bromo/chloro derivatives of the parent structure. 

Commercially Available Pyranthrone Pigments and Their Application... 

P.R.216 lends color to all types of industrial paints, it is heat stable up to 200°C. Like other pyranthrone pigments, it is suitable for use in unsaturated polyester systems, in which it is resistant to peroxides. 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

Most pigments derived from vat dyes are structurally based on anthraquinone derivatives such as indanthrone, flavanthrone, pyranthrone, or dibromoan-thanthrone. There are other polycyclic pigments which may be used directly in the form in which they are manufactured. This includes derivatives of naphthalene and perylene tetracarboxylic acid, dioxazine (Carbazole Violet), and tetrachloro-thioindigo. Quinacridone pigments, which were first introduced in 1958, and recently DPP pigments have been added to the series. 

This class includes polycarbocyclic compounds which are at least formally derived from the anthraquinone structure. The products are considered members of the higher condensed carbocyclic quinone series, which even in the absence of additional substituents provide yellow to red shades. Halogenation is frequently found to afford cleaner shades and improved fastness properties. Heading the list of such derivatives are pyranthrone, anthanthrone, and isoviolanthrone pigments. 

Pyranthrone may be halogenated, for instance, in chlorosulfonic acid in the presence of small amounts of sulfur, iodine, or antimony as a catalyst. This procedure necessitates intermediate separation and purification of pyranthrone after manufacture, because the products, unless purified, fail to furnish the solvent fastness which is characteristic of a typical pigment. 

An opaque form of pyranthrone may also be prepared by treating the corresponding pigment in a polar organic solvent (such as isobutanol) at elevated temperature in the presence of some (0.5 to 10%) halogenated anthraquinone or another anthraquinone derivative. 

In addition to 6,14-dibromopyranthrone, there are some mixed halogenated pyranthrone derivatives which exhibit equally useful pigment properties. 

Until recently also some of the halogenated pyranthrone types had been offered to the market. Currently the producer has ceased their production and does not list them in his sales catalogues anymore. The pigments, however, are still available on the market, giving reason to furthermore characterize their application and fastness properties in this book. 

Brominated pyranthrone red, PR 216, Cl No. 59710 is a yellow-shade red, classed as a high-performance pigment because of its excellent fastness properties. Neither light nor dark shades will suffer on prolonged outdoor exposure. Again, the pigment finds use in plastics only when the economics can justify such a quality pigment. 

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