Azo pigments
As azo pigments are called organic pigments , one or more azo groups include -N = N-. Thus, azo pigments are azo colorants that are (practically) insoluble in the application medium . In the solid state, practically all commercial azo pigments are in the tautomeric hydrazone form. It should therefore correctly be called “hydrazone pigment” instead of “azo pigment”.
Classification and history
Azo pigments that contain exactly one azo group are called monoazo pigments ; if they contain exactly two azo groups, they are called disazo pigments . Azo pigments that contain more than two azo groups ( trisazo, tetraazo, ..., polyazo ) play no role as pigments in technology.
Historically, the azo pigments are children of the early 20th century, although their first representatives were discovered in the late 19th century ( Pararot , 1885).
Manufacturing
The azo pigments are produced in technology in two steps: by diazotization and subsequent coupling .
Insolubility is usually achieved in two ways:
- Synthesizing (soluble) azo dyes , the carboxylic acid - and / or sulfonic acid - substituents contain and sets it with metal salt (eg. Calcium - strontium - barium - or magnesium salts ) to form insoluble salts are ( laking ).
- One avoids the solubility-increasing substituents (z. B. sulfonic acid groups) and introduces the solubility-reducing substituents, z. B. carboxamide groups .
- A special case is e.g. B. with benzimidazolone pigments in which a polycyclic group serves as a substituent. Pigments in this group are therefore a kind of “hybrid” of azo pigment and polycyclic pigment.
properties
Most commercial azo pigments are practically insoluble, highly coloring, non-toxic powders. The difference between azo pigments and azo dyes is that azo pigments are usually not bioavailable due to their insolubility and therefore cannot be absorbed or decomposed by the body.
With azo pigments, theoretically all shades are accessible. In technology, yellow , orange , red and brown azo pigments are of the greatest importance; green and blue only play a subordinate role, as this area is now occupied by the cheaper and more stable copper phthalocyanine pigments .
Laked azo pigments, azo metal complex pigments and disazo condensation pigments
Azo pigments obtained by laking are called laked azo pigments . The azo metal complex pigments are to be distinguished from the laked azo pigments . This type of pigment is also obtained by reacting soluble azo pigments with metal salts (typically e.g. with cobalt or nickel salts ), but there is no salt formation, instead the metal ions are bound via coordinative bonds ( complex bonds ).
Disazo condensation pigments are a special type of disazo pigments. To synthesize these pigments, compounds containing one or two azo groups and bearing carboxylic acid groups are combined via a condensation reaction with substituted phenylenediamines or anilines .
literature
- W. Herbst, K. Hunger: Industrial Organic Pigments - Production, Properties, Applications. 3. Edition. Wiley-VCH, Weinheim 2004.
- H. Smith (Ed.): High Performance Pigments. Wiley-VCH, Weinheim 2002.