Page 349 - Basic Principles of Textile Coloration

338 REACTIVE DYES

water at the boil without risk of hydrolysis. Formation of the reactive vinyl
sulphone group (Figure 16.1, (b)) requires the addition of alkali.

Reactive dyes for printing are usually dyes of low reactivity so that the print
paste can be stored for some time at room temperature without deterioration from
hydrolysis of the reactive group. Reactive dyes of low reactivity and relatively high
substantivity are valuable for dyeing using long (high) liquor ratios, using a winch
machine. Exhaust dyeing with low reactivity dyes at the higher temperatures
required for fixation allows better penetration of the dyes into the cotton fibres.
For continuous dyeing with reactive dyes stabilised liquid forms are available.
Although these contain special pH buffers and stabilisers to minimise the
hydrolysis reaction, they only have a limited shelf life.

Many commercial reactive dyes are dusty powders but all physical forms must
be handled with care. These dyes react with the amino groups in proteins in the
skin and on mucous surfaces. Inhalation of the dust is dangerous and a dust mask
is obligatory during handling. Reactive dye powders and grains are sometimes
hygroscopic and drums must be carefully re-sealed. Most reactive dyes have a
limited storage period, after which some deterioration can be expected.
Standardisation and comparison of reactive dye powders or liquids cannot be done
by the usual spectrophotometric procedure involving absorbance measurements of
standard solutions (Section 24.2). Both the reactive dye and its hydrolysed form
are equally coloured, but only the former is capable of reaction with the cellulose
during dyeing. Therefore, dyeings must be prepared and their colours compared
with standard dyeings. Chromatographic techniques usually allow separation and
quantitative measurement of the relative amounts of a reactive dye and its
hydrolysis product in a given dye (Section 24.5).

16.2.4 Fastness properties of reactive dyes on cellulosic fibres

In general, reactive dyes on cellulosic fibres give dyeings with good to very good
fastness to washing and other wet processes. Apparent inferior fastness to washing
is usually because of incomplete removal of unreacted and hydrolysed dye from the
material by washing after dyeing. The presence of unfixed dye can easily be tested
for by hot pressing a wet sample of the dyeing sandwiched between two pieces of
dry white cotton. Colour transfer to the white material indicates the presence of
unfixed dye in the dyeing. It is more likely to be a problem for deep dyeings.

The use of reactive dyes is growing rapidly, faster than for any other dye
application class. This is because these dyes also give dyeings of moderate to good