Page 262 - Basic Principles of Textile Coloration

PROBLEMS OF DYEING WOOL LEVEL 251

washing fastness. The coulombic attractions between wool ammonium ions and
dye anions, however, determine the rate of dyeing.

13.4.3 Kinetics of wool dyeing
Above about 40 °C, increasing temperature increases the dyeing rate with acid
dyes but decreases the overall degree of exhaustion, in agreement with Le
Chatelier’s principle for an exothermic dyeing process. The rate of dyeing
increases by decreasing the dyebath pH, even though the diffusion coefficients for
acid dyes in wool are independent of pH. A lower pH increases the dye
concentration on the available wool fibre surfaces by increasing the number of
ammonium ion sites. This increases the concentration gradient for diffusion of the
dye into the fibres. Diffusion into the fibre is usually considered to control the rate
of dyeing. Because of this, wool fibres may be ring dyed at low dyeing
temperatures. This implies rapid mass transfer of dye to the fibre surface and rapid
adsorption, but slow diffusion into the fibre. At higher temperatures, when
diffusion is faster, rate-controlling mass transfer of dye to the fibre surfaces can
occur at low dye concentrations and when the relative movement of the goods and
dye liquor is inadequate.

The kinetics of wool dyeing is quite complex. The penetration of dye molecules
into wool fibres was discussed in Section 7.2.2. What is important from the
practical viewpoint is to control the dyeing rate so that dye is absorbed neither too
rapidly in the early stages of dyeing, giving unlevel coloration, nor so slowly that
the process is uneconomic. The objective is to produce a level, well-penetrated
dyeing in as short a time as possible. Some of the means of achieving this are
discussed in the following section.

13.5 PROBLEMS OF DYEING WOOL LEVEL

13.5.1 Temperature and pH control
One of the major objectives of dyeing is to produce goods with an even colour.
This is usually more of a problem when using rapid dyeing dyes that have little
migrating capability. Unlevelness of a dyeing arises from:
(1) unequal access of the fibres to the dye solution, resulting from densely packed

fibres or yarns and from poor agitation of the dyebath;
(2) variation of the temperature throughout the dyebath and the goods;
(3) uneven pH in the bath and the material.