Azo compound#As dyes and pigments

As organic pigments, azo pigments is known that contain one or more N- azo -N =. Thus, azo azo colorants that (practically ) are insoluble in the application medium. In the solid state, virtually all commercial azo pigments in the hydrazone tautomeric form. It should read " hydrazone pigments " instead of " azo pigments " so correctly.

Classification and history

Azo pigments, which contain exactly one azo group, hot monoazo; they contain exactly two azo groups, as they are called Disazo. Azo pigments, azo groups containing more than two ( trisazo, tetraazo, ..., polyazo ) play in the art as pigments not matter.

Historically, the azo children of the early 20th century, although its first representatives were discovered in the late 19th century ( Para Red, 1885).

Production

The preparation of azo pigments is carried out in the art in two steps: by diazotisation followed by coupling.

Insolubility is achieved typically in two ways:

Properties

Most commercial azo pigments are practically insoluble, strong staining, non-toxic powder. The difference between azo and azo dyes is that azo pigments are usually due to their insolubility not bioavailable, and therefore can not be absorbed or decomposed by the body.

With azo pigments are theoretically all colors available. In art have yellow, orange, red and brown azo pigments of the greatest importance; green and blue play only a minor role, as this area is now occupied by the cheaper and more stable copper phthalocyanine.

Laked azo pigments, azo pigments and Disazo condensation pigments

By laking obtained azo laked azo pigments hot. To be distinguished from the laked azo pigments are azo pigments. Also this kind of pigments it is replaced by reaction with metal salts of soluble azo pigments (typically, for example, with cobalt or nickel salts ), but there is no salt formation, but the metal ions are bonded by coordinate bonds (complex binding).

Disazo condensation pigments are a special type of common pigments. For the synthesis of these pigments is associated - one or two azo groups containing - compounds bearing carboxylic acid groups via a condensation reaction with substituted phenylenediamines or anilines.