Page 259 - Basic Principles of Textile Coloration

248 ACID, PRE-METALLISED AND MORDANT DYES

weathering of the fibre tips or mechanical stress during processing has damaged
the epicuticle (Section 7.2.2). If the dye is unable to migrate, the result is an
uneven skittery dyeing. Skitteriness is the perceived uneven colour effect from
pale and dark fibres in proximity. Goodall’s dyeing method overcomes this. Dyeing
is commenced at the boil, at relatively high pH in the presence of ammonia and its
acetate. Under these conditions, the dye molecules readily penetrate into the
swollen wool fibres. The pH gradually falls during dyeing as ammonia escapes and
the dyeing rate slowly increases giving more uniform dyeings. Alternatively,
proprietary levelling agents may be used (Section 13.5.3).

Colour stripping is difficult with higher molecular weight dyes of greater
substantivity. Hot ammonia solution will remove some dye. This simply decreases
the number of ammonium ion groups in the fibre so that anionic dye molecules
held by ionic attraction are released. This may be assisted by a cationic/non-ionic
auxiliary product that holds the anionic dye in the bath by complex formation and
prevents its re-absorption by the fibres (Section 13.5.3). The non-ionic
component keeps the dye–auxiliary complex in suspension and avoids its
precipitation. Complete stripping of azo dyes is possible with reducing agents such
as zinc sulphoxylate-formaldehyde, which split azo dyes into amines, thus
destroying the chromophore. The stripping conditions should be as mild as
possible because of the tendency of wool to felt. Long, vigorous stripping increases
the risk of fibre damage and the development of a harsh handle, especially under
alkaline conditions.

Under neutral conditions, wool is slightly anionic, with more carboxylate than
ammonium ion groups. Even so, super-milling dyes exhibit high substantivity. The
desorption of super-milling dyes from dyed wool is only slight in solutions with a
pH value of up to 9.5. This shows that the dye binds to the fibre by forces other
than the electrostatic attraction between ions of opposite charge. Desorption only
becomes significant above pH 9.5 as the wool fibres begin to swell more. A dyeing
mechanism based on simple ion exchange does not explain the high fastness to
washing of these dyes. Other intermolecular forces are obviously involved,
including dipole, dispersion and hydrophobic interactions (Section 13.4.2).

13.4 MECHANISM OF WOOL DYEING [2]

13.4.1 Absorption of acids by wool
Wool contains about 820 mmol kg–1 of amino groups. Under acidic conditions,