Page 279 - Basic Principles of Textile Coloration

268 ACID, PRE-METALLISED AND MORDANT DYES

lactate, will gradually decrease the bath pH during dyeing (Scheme 13.4). To
obtain optimum wet fastness in medium to heavy shades on shrink-resist wool, the
dyeing may be aftertreated with a proprietary cationic agent. This presumably
complexes with the anionic dye, reducing its solubility and diffusion capability.

Loose wool and slubbing can be dyed continuously by padding with metal-
complex dyes, fixation usually being achieved by steaming. The pad bath contains
wetting agents, a thickener to prevent drainage in the fibre mass since the solution
pick-up is high (100–120%), as well as a variety of proprietary chemicals.

Coulombic interactions between dye anions and ammonium ion groups in the
wool are important for determining the rate of dyeing and the degree of migration
that is possible. As in the case of milling and super-milling acid dyes, the good
washing fastness of metal-complex dyes, however, is more likely a consequence of
hydrophobic interactions between the dye and the wool protein or between dye
molecules themselves leading to aggregation of the dyes in the fibre.

13.8.4 Optimised acid dye ranges
A number of commercial dye ranges for wool are available in which the member
dyes have a wide colour gamut and good compatibility. All the dyes have very
similar dyeing and fastness properties. They have a common dyeing method
despite being of several different types. The dyebath pH is around 4.5–5.0, close to
the isoelectric region for wool. This minimises damage to the wool. Their high
degree of exhaustion makes shade reproducibility much easier. Each range requires
the use of a proprietary levelling agent that ensures good coverage of root and tip
differences of the wool fibres. The dyes of the Sandolan MF (Clariant) range
behave like fast acid dyes. For dyeings with good levelness, an acid release agent
such as ethyl lactate is used in combination with a levelling agent. The Lanaset
range (Ciba-Geigy) contains milling, reactive and 2:1 dye–metal complex dyes,
whereas the Lanasan CF range (Clariant) has milling and 2:1 dye–metal complex
dyes. The last two ranges give very good washing fastness.

13.9 DYEING NYLON WITH ACID DYES

13.9.1 Introduction
Nylon molecules have amino groups at the end of the polyamide chains. Nylon,
like wool, can therefore be dyed with all types of acid dyes by an ion exchange
process. Nylon filaments are sensitive to mechanical, thermal and chemical