Page 187 - Basic Principles of Textile Coloration

176 AN INTRODUCTION TO DYES AND DYEING

allows the rapid preparation of large volumes of dye solutions. Liquid dyes should
be stable to heating and cooling, resistant to sedimentation and stratification, and
not contain unsafe co-solvents or additives.

Commercial dye powders also contain various other chemicals besides the
principal dyestuff. These include diluents such as salts or starch, wetting agents,
dispersants, impurities from manufacture such as residual intermediate chemicals,
anti-dusting agents (oils), buffers (sodium carbonate or phosphates), stabilisers,
and shading dyes. In the manufacture of successive batches of a given dye, it is
almost impossible to obtain identical products because of slight process variations.
Addition of NaCl to solutions of water-soluble sulphonated dyes precipitates the
sodium salt of the dye but the amount of salt solution adhering to the filtered
precipitate varies from batch to batch. This solution evaporates during drying and
the salt remains in the product. Batch to batch variations in the colour yield (the
depth of colour attainable with a given amount of dye) and in hue, although slight,
are the norm. Because of this, each batch of dye must not only have the correct
physical form, with the appropriate additives, but also be standardised so that it
will give dyeings of the same hue and colour yield as previous batches. This is the
major reason for the presence of inert diluents and small amounts of shading dyes.
The latter, of course, must have similar dyeing properties to the parent dye. Some
dyes may even be available in more than one strength, such as the acid dye
Sandolan Rhodamine E-B 300% (CI Acid Red 52). This dye will produce a dyeing
with a given depth of colour using a third of the weight of the standard product,
which is rated at 100% strength.

Frequently a dye powder does not consist of a single principal coloured
chemical. Some products are mixtures of different isomeric colorants, or contain
substantial amounts of other coloured by-products from the dye-forming reaction.
Others are deliberate mixtures of different pre-made dyes, while some dyes are
prepared from mixtures of intermediates. In some cases, as for the sulphur dyes,
the manufactured product may be of unknown chemical constitution and may be a
complex mixture of coloured chemicals.

Commercial products are used directly for dyeing, but occasionally in dyeing
research there is a need for purified dyes. Dye purification is not easy. Washing of
water-insoluble dyes with hot water removes some additives. The product can
then be dried and recrystallised from an appropriate solvent. Usually, the product
obtained must then be finely ground with a dispersing agent so that it will readily
disperse in water. The purification of water-soluble dyes is much more difficult.
Sulphonated dyes may be salted out several times with sodium acetate, which can