Monoazo (Monohydrazone) pigments based on 2-naphthol and derivatives

Abstract

This chapter describes the range of industrial monoazo pigments based on the 2-naphthol (β-naphthol) ring system. This group includes some of the earliest organic pigments introduced commercially and is also numerically the largest group of products currently described in the Colour Index. Most of the pigments within this group are red (with a few oranges), thus complementing the azoacetoacetanilides, which are mostly yellows. Three groups of monoazonaphthol-based pigments may be identified categorized according to the chemical structure of the coupling components used in their synthesis. The first group contains products based on 2-naphthol itself, a second is based on amide derivatives of 3-hydroxy-2-naphthoic acid (naphtharylamides), and the third is a series of metal salt azo pigments. The historical development of these pigments, outlined in an early section of this chapter, originated in the late 19th century with pigments described as ‘lakes’, derived from water-soluble anionic dyes absorbed on to inert colorless substrates, which were the forerunners of products now referred to as metal salt pigments. The non-ionic 2-naphthol-based pigments were introduced soon after. In the early to mid-20th century, a series of monoazonaphtharylamide (Naphthol AS) pigments were developed and introduced commercially. The pigments of this type that are currently manufactured can be sub-divided into products containing a single amide group (group 1) and higher performance products containing more than one amide or sulfonamide groups. Several group 1 pigments have diminished in importance over the years, while some higher performing group 2 pigments have grown in importance. The molecular and crystal structures of the range of pigments are presented and discussed in relation to their performance characteristics. The manufacture of the pigments involves the reaction of a diazotized aromatic amine with the appropriate 2-naphthol-based coupling component, using synthesis conditions typical of phenolic coupling components, followed by conditioning aftertreatments that are typical for azo pigments in general. Finally, there is an extensive discussion of the wide-ranging applications in which the individual pigments are used. While the pigments are especially well-suited to printing ink applications, many products also find use in paints and a few in plastics.