Page 258 - Basic Principles of Textile Coloration

THE APPLICATION OF ACID DYES IN DYEING WOOL 247

13.3.4 Super-milling acid dyes

The super-milling acid dyes have much the same properties as milling dyes but are
even more hydrophobic. They often have long alkyl chains in the dye molecule,
give very good washing fastness, and good to very good light fastness. Super-
milling acid dyes are widely used for yarn dyeing when the final fabric will be
scoured and milled.

Dyeing wool with both types of milling dye in the presence of sulphuric acid at
pH 3 results in very rapid absorption and unlevel dyeing, with no chance of
levelling by migration. Dyeing is therefore started at about pH 6.5–7.0. The rate of
dye absorption is controlled by the gradual temperature increase of the dyebath,
and later by a gradual decrease in dyebath pH to complete exhaustion, if required.
The initial pH of the dyebath must be even higher for dyeing pale shades to
mitigate the initial strike. The pH can be lower for dyeing loose fibre and tops
where levelness is less critical since the fibres will be blended together during
spinning. Dyeing is slow below 60 °C. It may, however, increase rapidly above
70 °C, once the dye aggregates in solution begin to break up with increasing
temperature. In the critical temperature range a decreased rate of temperature
increase avoids uneven dyeing.

A typical wool dyeing procedure using super-milling dyes follows the scheme
shown in Figure 13.2 except that the initial bath contains 2–3% owf of a weak acid
such as ammonium acetate. The use of ammonium acetate or sulphate as the
dyeing assistant gives a gradual decrease in the pH value as dyeing proceeds. It has
been proposed that heating such ammonium salts in solution liberates ammonia,
which escapes from the dyebath, thus giving a decrease in pH (Scheme 13.3). In
fact, the pH of ammonium acetate or sulphate solution decreases slightly as the
temperature rises even without ammonia loss. In some cases, ammonium
phosphate provides some neutralising and buffering action if there is any residual
alkali in the wool from scouring.

CH3CO2 + NH4 CH3CO2H + NH3

Scheme 13.3

Many milling and super-milling acid dyes give ‘skittery’ dyeings on wool despite
using a gradual temperature increase to control the dyeing rate and the colour
uniformity. Dyes penetrate more readily into the wool fibre in regions where