Metal salt azo pigments

Historically, metal salt azo pigments have evolved from products referred to as ‘lakes’, which were prepared from established water‐soluble textile dyes, rendered insoluble by precipitation on to colourless inorganic substrates. The first azo lake pigment, Lithol Red, was reported in 1899. The majority of the current range of industrial metal salt azo pigments no longer contains the insoluble substrate as an integral constituent. Structurally, they may be considered as being derived from sodium salts of anionic azo dyes containing sulphonate and/or carboxylate groups by replacement of the sodium cation, usually with a divalent cation of an alkaline earth (calcium, strontium or barium) or the transition metal manganese. This review provides an account of the historical development of metal salt azo pigments and an overview of the chemical structural types of the current range of commercially important products in relation to their properties and application performance. An overview of the patent literature associated with the development of the products is also given. Importantly, the structural investigations which have been reported on this group of pigments, using a variety of microscopic, spectroscopic and crystallographic techniques, are reviewed.