Page 257 - Basic Principles of Textile Coloration

246 ACID, PRE-METALLISED AND MORDANT DYES

13.3.2 Fast acid dyes

These are usually monosulphonated acid dyes of somewhat higher molecular
weight than typical levelling dyes. They dye wool by essentially the same dyeing
method (Figure 13.2) using acetic acid (1–3% owf) and Glauber’s salt (5–10%
owf). They have reasonable migration at the boil. In some cases, a levelling agent
may be recommended. These dyes are used where level dyeing is necessary but
when the washing and perspiration fastness of levelling acid dyes are inadequate.

13.3.3 Milling acid dyes

These anionic dyes have higher molecular weights and greater substantivity for
wool than levelling or fast acid dyes. They usually only have one sulphonate group
per dye molecule and therefore have lower water solubility. They aggregate
strongly, possibly even in boiling water, and give colloidal solutions. Dyeing with
milling dyes is carried out in the pH range from 5 to 7, usually starting in the
presence of sodium acetate or ammonium sulphate. Acetic acid may be added later
in the process to promote exhaustion by increasing the number of ammonium ion
groups in the wool. At best, milling dyes have low rates of diffusion in wool and
poor levelling ability. Besides simple anion–cation attraction, other types of
intermolecular forces bind the dye molecules to the wool. Milling acid dyes give
dyeings with moderate to good washing fastness, better than for levelling acid
dyes. In particular, they give dyeings that are reasonably fast to alkaline milling and
with fair to good light fastness.

When dyeing with milling acid dyes, Glauber’s salt is never added. Under
typical dyeing conditions, at pH 6, it increases the exhaustion of such dyes and
hinders levelling. The high concentration of sodium ions from the Glauber’s salt
helps to suppress the natural negative surface charge of the wool so that the dye
anions are not repelled to the same extent and therefore have higher substantivity.
The fibre’s negative surface charge is much less evident in acidic solution when
the wool has an excess of cationic ammonium ion groups. This effect of Glauber’s
salt is exactly the same as that of added salts in cotton dyeing and is discussed
more fully in Section 14.4.1.

Because of the poor migration of milling acid dyes, level dyeing initially depends
upon gradually increasing the dyeing temperature, and then possibly decreasing the
dyebath pH by addition of a small amount of acetic acid. Some of these dyes have a
tendency to stain cotton. They often have poor dyeing compatibility in mixtures and
are preferred for self-shades (shades determined by the colour of a single dye).