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AZO DYES
The 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 to 70%
of all dyes. As one 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 colors. Research
is always continuing, though, so that now there are some viable
blue azo dyes on the market. The relationship between the
color of an azo dye has been more fully discussed in The Basis
of Color.
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, a vide range of possible
dyes are available, especially as the starting molecules are
readily available and cheap. Further, 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.
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