A dyeing technique in which an insoluble azo dye is produced directly onto or within the fibre. This is achieved by treating a fibre with a diazo component and a coupling component. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique in that the final colour is controlled by the choice of the diazo and coupling components.
The azo compound class accounts for 60-70% of all dyes. As you might expect, they all contain an azo group, -N=N-, which links two sp hybridised carbon atoms. Often, these carbons are part of aromatic systems, but this is not always the case. Most azo dyes contain only one azo group, but some contain two (disazo), three (trisazo) or more.
In theory, azo dyes can supply a complete rainbow of colours. However, commercially they tend to supply more yellows, oranges and reds than any other colours. Research is always continuing, though, so that now there are some viable blue azo dyes on the market2. The relationship between the colour of an azo dye has been more fully discussed in The Basis of Colour.
Azo dyes give bright, high intensity colours, much more so than the next most common dye class (anthraquinones). They have fair to good fastness properties, but not so good as the carbonyl and phthalocyanine classes. Their biggest advantage it their cost-effectiveness, which is due to the processes involved in manufacture.
The general formula for making an azo dye requires two organic compounds- a coupling component and a diazo component. Since these can be altered considerably, an enormous range of possible dyes are available, especially as the starting molecules are readily available and cheap. Furthermore, the simplicity of the reactions mean that the process can be scaled up or down very easily, which is always a key factor in the cost of chemicals. Energy requirements for the reaction are low, since most of the chemistry occurs at or below room temperature. The environmental impact is reduced by the fact that all reactions are carried out in water, which is easy and cheap to obtain, clean and dispose of. As other dye classes become less viable from either an environmental or economic reasons, azo dyes become ever more attractive options.