Diarylide pigment

Diarylide pigments are organic compounds that are used as pigments in inks and related materials. They often are yellow or yellow-green. To some extent, these organic compounds have displaced cadmium sulfide from the market. Being pigments, these compounds exist as (yellow) powders of low solubility in water. They are similar to the simpler monoazo pigments called arylide yellows.^[1]

Production and properties

Synthesis of C.I. Pigment Yellow 13, a diarylide pigment.

The formation of diarylide pigments involves the reaction of doubly diazotized aromatic diamines (derivatives of benzidine) with acetoacetanilide. By varying both of these components, several useful pigments have been produced.^[2] A related family of organic pigments are the simpler arylides, which arise from the coupling of mono-diazonium salts with the same coupling partners.

The pigments' colors can range from yellow to yellow-green. One common diarylide yellow pigment is Pigment yellow 12. From the selection of diarylide yellow pigments shown below one can appreciate the subtle changes in the substituents. Worldwide production of organic pigments was estimated to be about 250,000 metric tons (t) in 2006, with about 25%, or 62,500 t, being diarylide yellows.^[3]

Being pigments, these compounds have very low solubility, especially in water. Single crystals for X-ray crystallography can however be grown from hot solutions in organic solvents. For example, crystals of Pigment Yellow 13, 14, and 63 were grown by slow cooling 200 °C solutions of the pigments in 1,2,4-trichlorobenzene. These vigorous conditions also indicate the considerable thermal stability enjoyed by these compounds.^[4] X-ray crystallography, spectroscopy]], and electronic structure calculations consistently show that these compounds exist as the (bis) keto-hydrazone tautomers, even though these structures are often depicted as diazo compounds.^[5]

• Selected diarylide pigments
• 
  Pigment yellow 83
• 
  Pigment Yellow 81
• 
  Pigment Yellow 17
• 
  Pigment Yellow 14
• 
  Pigment Yellow 16
• 
  Pigment Yellow 63

Uses

The diarylide yellows are the most common yellow pigments used in printing as well as a wide variety of other applications. Due to their stability, diarylide yellows are used in inks, coatings, and as plastic colorants. The pigment is insoluble. It is a standard pigment used in printing ink and packaging industry. The diarylide yellow pigment Yellow 12 is one of the three main colored pigments used in the four color process of color printing. As such, its use is ubiquitous in printing both in commercial applications and in home color printers, as well as in textile printing.

Toxicity and hazards

Diarylide yellow pigments are considered to be non-toxic. There is, however, evidence that these pigments degrade when exposed to temperatures above 200 °C to release 3,3'-dichlorobenzidine, a carcinogen that is listed in the U.S. EPA's Toxics Release Inventory. 3,3’-Dichlorobenzidine is structurally similar to one of the polychlorinated biphenyl congeners, PCB 11 (3,3’-dichlorobiphenyl), and some evidence indicates that the use of diarylide yellow pigments introduces PCB 11 to the environment.^[6][7]

See also

• List of colours

References

1. K. Hunger. W. Herbst "Pigments, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012. doi:10.1002/14356007.a20_371
2. Lewis, D. M., Modern Colorants Synthesis and Structure. Advances in Color Chemistry Series, Vol. 3. Edited by A. T. Peters & H. S. Freeman, Blackie Academic & Professional, Glasgow, 1995, xii + 245 pp. ISBN 0 7514 0209 5. Journal of Chemical Technology & Biotechnology 1996, 65, (2), 207-208.
3. Savastano, D., "The pigment report: although 2006 was a year of improvement, pigment manufacturers are coping with a wide variety of challenges, including raw material pricing and supply issues and overcapacity". Ink World 2007, March.
4. Barrow, M. (2002). "The Crystal and Molecular Structures of Three Diarylide Yellow Pigments, C. I. Pigments Yellow 13, 14 and 63". Dyes and Pigments. 55 (2–3): 79–89. doi:10.1016/S0143-7208(02)00068-2.
5. Christie, R. M.; Standring, P. N. (1989). "Colour and Constitution Relationships in Organic Pigments. Part 2 Disazoacetoacetanilides". Dyes and Pigments. 11 (2): 109–121. doi:10.1016/0143-7208(89)85031-4.
6. Rodenburg, L. A.; Guo, J.; Du, S.; Cavallo, G. J., "Evidence for Unique and Ubiquitous Environmental Sources of 3,3'-Dichlorobiphenyl (PCB 11)". Environmental Science & Technology 2009, 44, (8), 2816-2821.
7. Elizabeth Grossman (2013). "Nonlegacy PCBs: Pigment Manufacturing By-Products Get a Second Look". Environmental Health Perspectives. 121 (3): a86 – a93. doi:10.1289/ehp.121-a86. PMC 3621189. PMID 23454657. Archived from the original on 2014-02-21. Retrieved 2014-02-08.