OVERVIEW

Azo dyes and pigments constitute by far the most important chemical class of commercial organic colorant. They account for around 60 70% of the dyes used in traditional textile applications (see Chapters 7 and 8) and they occupy a similarly prominent position in the range of classical organic pigments (see Chapter 9). Azo colorants, as the name implies, contain as their common structural feature the azo ( N=N ) linkage which is attached at either side to two sp ² carbon atoms. Usually, although not exclusively, the azo group links two aromatic ring systems. The majority of the commercially important azo colorants contain a single azo group and are therefore referred to as monoazo dyes or pigments, but there are many which contain two (disazo), three (trisazo) or more such groups. In terms of their colour properties, azo colorants are capable of providing virtually a complete range of hues. There is no doubt though that they are significantly more important commercially in yellow, orange and red colours ( i.e. absorbing at shorter wavelengths), than in blues and greens. However, as a result of relatively recent research, the range of longer wavelength absorbing azo dyes has been extended, leading to the emergence of significant numbers of commercially important blue azo dyes and there are even a few specifically-designed azo compounds which absorb in the near-infrared region of the spectrum (see Chapter 2 for a discussion of colour and constitution relationships in azo dyes). Azo colorants are capable of providing high intensity...