Лако-красочные материалы — производство

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 sp2 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 of colour, about twice that of the anthraquinones for example (see Chapter 4), and reasonably bright colours. They are capable of providing reasonable to very good technical properties, for example fastness to light, heat, water and other solvents, although in this respect they are often inferior to other chemical classes, for example carbonyl and phthalocyanine colorants, especially in terms of lightfastness.

Perhaps the prime reason for the commercial importance of azo

colorants is that they are the most cost-effective of all the chemical classes of organic dyes and pigments. The reasons for this may be found in the nature of the processes used in their manufacture. The synthesis of azo colorants, which is discussed in some detail later in this chapter, brings together two organic components, a diazo component and a coupling component in a two-stage sequence of reactions known as diazotisation and azo coupling. The versatility of the chemistry involved in this syn­thetic sequence means that an immense number of azo colorants may be prepared and this accounts for the fact that they have been adapted structurally to meet the requirements of most colour applications. On an industrial scale, the processes are straightforward, making use of simple multi-purpose chemical plant. They are usually capable of production in high, often virtually quantitative, yields and the processes are carried out at or below ambient temperatures, thus presenting low energy require­ments. The syntheses involve low cost, readily available commodity organic starting materials such as aromatic amines and phenols. The solvent in which the reactions are carried out is water, which offers obvious economic and environmental advantages over all other solvents. In fact, it is likely that in the future azo dyes are likely to assume even greater importance as some of the other chemical types, notably anthra — quinones, become progressively less economic. This chapter contains a discussion of the fundamental structural chemistry of azo colorants, including a description of the types of isomerism that they can exhibit, and the principles of their synthesis. In the final section, the ability of azo yes to form metal complexes is discussed. Because of their prominence in most applications, numerous further examples of azo dyes and pigments will be encountered throughout this book.