Azo dyes are synthetic dyes and numerically the largest group. Characteristic of azo dyes are one or more azo bridges (-N = N- ) as the chromophore.
Middle of the 19th century, industrialization had spread from England throughout Western and Central Europe. With the mechanization of spinning and weaving the textile industry received a boost, which increased the demand for dyes. These new requirements did not the production, of course, colorants prepared by what promoted the color research and the rapid development of the chemical industry.
Justus von Liebig in 1844 ventured the forecast, you 'll soon discover methods to produce color and drugs synthetically from coal tar.
1856 Finally, the Englishman William Henry Perkin succeeded in experimenting with coal tar, the synthesis of the first artificial Teerfarbstoffs Mauvein, but this is no azo dye. But on the Great Exhibition in London he caused a sensation, well this has encouraged the further research. Chemists discovered anywhere in Europe production routes for a whole range of such coal-tar dyes, Hofmann - violet, fuchsin, aniline yellow ( 1861 ) to Bismarck brown, the latter two being important representative of the group of azo dyes. Starting material was the aniline, which was obtained from coal tar, so this group is also called tar dyes. Today aniline yellow is still starting material for further azo dyes. In the Colour Index, it is listed as CI Solvent Yellow 1, a fatty dye, which is for use in the ester solvent. Congo Red (CI Direct Red 28) was the first bis azo dye (also diazo dye ) made in 1884. C. I. Disperse Yellow 8 was developed in 1926 as a water-insoluble disperse dye and is still used for dyeing polyester fabrics.
Structure and properties
Azo dyes constitute numerically the largest class of dyes. They are characterized by the general formula R1 -N = N- R2. The two radicals (R1 and R2, usually aromatic) can be identical or different radicals stand for. Are two azo groups in the dye molecule contain, so it is a diazo dye. Accordingly, there is also tri-, tetra-and polyazo.
Typical azo dyes, the azo group -N = N with the coloring ( chromophoric ) nitrogen double bond. These synthetic dyes used as the basis amines, in the simplest case of the aniline. Azo dyes achieve their diversity by the simple substitution of the hydrogen atoms on the benzene or the rings then auxochrome affect the azo bond and allow precise adjustment of color nuances.
Azo dyes often have polar and non-polar substituents and can be tailored to meet the required medium. Appropriate attachments they can form next to van der Waals forces and hydrogen bonds.
Representative of the group are color- stable, light-fast and can have vibrant colors. With a suitable constitution, they can be washable, dry cleaning and fast to rubbing on textiles.
The variety of azo dyes derived from the comparatively simple method of preparation. By diazotization can be aromatic amines, in the simplest case of coupling the aniline.
The synthesis is carried out in two steps:
This dye group is represented on the basis of various coupling possibilities with dyes in the entire color range, also the color properties are very good. Azo dyes are used for dyeing textiles, fats and oils, waxes for coloring, straw, wood, and paper. Also for coating materials, such as CD -R, they are in use. Azo dyes which can release toxic or carcinogenic amines are banned in Germany for daily use and tattooing. They may be applied either to the fabric colouration for jewelry and not for cosmetics.
Selected Azo dyes are tested and approved for their suitability as food dyes. Moreover, such dyes have a number of sulfonic groups to enhance its solubility in water. The high water solubility instead of a fat solubility of the color solid prevents the risk of storage in the body by the substances are more easily excreted through the urine.
Also come azo dyes for leather dyes used, is being taken here rather on lipid solubility, but also allowed coupling products not be used with certain amines.
Health concerns, there were, among others, also with colored Easter eggs with azo dyes, but which remained unconfirmed because a too low dose of the dye passes through the shell into the egg.
In medicine, serve selected products for staining of cancer cells. In clinical chemistry, they are still important to certain metabolites, such as bilirubin prove. They arise by complex formation between different substances and the substance to be detected and subsequently measured photometrically.
The azo bridge may be dependent on the pH value in protonated or deprotonated form. Thus, a shift in the color depth is connected. Azo dyes are therefore used as an acid -base indicators. Examples are methyl red, methyl orange, Congo red and Alizarin. In addition, there are also among the redox azo dyes.
It has been demonstrated that the human body is capable of cleaving by reductive cleavage of the azo dyes of the azo bridge recorded again in the starting materials. This can be done by intestinal bacteria, by azoreductases the liver or extrahepatic tissues. Therefore, there is the suspicion that all azo dyes which contain a releasable carcinogenic aryl amine have a carcinogenic potential.
Azo dyes, which are composed of at least one of these carcinogenic amines are prohibited in use funds in Germany ( Bedarfsgegenständeverordnung ). Also according to EU Directive 2002/61/EC, the use is prohibited in use of resources. This prohibition applies to textiles and leather that come with the human skin or oral cavity, direct and prolonged contact. Currently there are 24 such amines banned, of which benzidine ( benzidine dyes) is the best known. Benzidine azo dyes are suspected after occupational exposure, the risk of bladder cancer to increase significantly. According to the EU Directive may azo dyes which can release the finished product by reductive cleavage of azo groups such amines with more than 30 ppm, unused and appropriate textile and leather products are not placed on the market. The analytical limit varies depending on the amine to 5 ppm. In the EU, these dyes are no longer used in the textile and leather industry for years. As well as imports of consumer products ( such textiles from Asia ) must not be stained with such hazardous dyes, random tests are carried out by the competent authorities.
In contrast to azo dyes, azo pigments are practically insoluble in the application medium. Because of the insolubility azo pigments are not bioavailable, and therefore non-toxic and non-carcinogenic. They are used for printing inks, plastics, paints, toners, food packaging.
The European Commission commissioned on 25 March 2010 EFSA (European Food Safety Authority) to make a renewed health assessment questionable azo dyes. Since July 20, 2010 Food, the azo dyes tartrazine (E102 ), sunset yellow have S ( E 110 ), Azorubine ( E122 ), Allura red (E 129) or Cochineal Red A (E 124) included in the European Union with the separate warning " Can adverse effect on activity and attention of children " are marked.