Page 188 - Basic Principles of Textile Coloration

DYEING METHODS 177

finally be removed after drying by extraction with ethanol, if the dye itself is not
too soluble. Ion exchange methods are sometimes useful, as are chromatographic
methods (see Chapter 24). The yield of purified dye is always very low and it is
often difficult to ascertain if the dye is in fact pure and free from contaminants [1].

10.1.3 Dye selection
There are numerous factors involved in the selection of dyes for colouring a fabric
in a particular shade. Some of these are:
(1) the types of fibres present;
(2) the form of the textile material and the degree of levelness required – level

dyeing is less critical for loose fibres, which are subsequently blended, than it
is for fabric;
(3) the fastness properties required for any subsequent manufacturing processes
and for the particular end-use;
(4) the dyeing method to be used, the overall cost, and the machinery available;
(5) the actual colour requested by the customer.

The last point is not trivial. Although many types of dyes are available, in a wide
range of colours, customers have definite ideas about exactly what colour their
goods should be and will often provide a sample. The dyer will usually only be able
to match this colour by dyeing the goods with a mixture of two to four dyes. The
basic principles are much like those used in painting, many colours being obtained
by mixing suitable amounts of red, yellow and blue colorants.

Obviously, the dyes selected to colour a high quality cotton material for shirts
must be quite different from those for a cellulose acetate fabric for coat linings. For
the former, high washing fastness and moderate light fastness are essential, but are
insignificant for the latter. The dye manufacturers can assist in making a successful
choice of dyes but there is no substitute for experience.

10.2 DYEING METHODS
The objective of dyeing is the uniform coloration of the mass of fibres constituting
the material, usually to match a pre-specified colour. Any significant difference in
colour from that requested by the customer, and any unlevelness of the colour of a
fabric, will be immediately apparent. Many factors can influence the final colour
appearance. These include fibre characteristics such as the lustre, denier, staple