Page 301 - Basic Principles of Textile Coloration

290 DYEING CELLULOSIC FIBRES WITH DIRECT DYES

(1) SDC Class A direct dyes. These are self-levelling dyes with good migration,
even in the presence of salt. They usually require considerable amounts of
salt for good exhaustion because of their lower substantivity. These dyes are
relatively low molecular weight mono- and bis-azo dyes with several anionic
sulphonate groups per molecule. They therefore have good water-solubility
and do not aggregate to a significant degree in solution. Dyeing is started at
50 °C in the presence of added salt, the bath heated to the boil over 30–
40 min, and dyeing continued at the boil for up to an hour. Several further
salt additions, of increasing size, are required to promote exhaustion, the total
amount of salt (5–20% owf NaCl) depending upon the depth of shade and
the liquor ratio. Although these dyes would give greater exhaustion by dyeing
at lower temperatures, dyeing at the boil allows good levelling and adequate
penetration of the dyes into the fibre.

(2) SDC Class B direct dyes. These are salt-sensitive or salt-controllable dyes,
with poor levelling characteristics. They are of higher molecular weight than
Class A dyes, often bis- and tris-azo dyes with just a few sulphonate groups
per molecule. They have low to moderate substantivity in the absence of salt
but give much increased exhaustion on addition of small amounts of salt to
the dyebath. The dyeing method is the same as for Class A dyes but the
initial salt is omitted. Gradual addition of dissolved salt, at the boil, controls
the exhaustion.

(3) SDC Class C direct dyes. These very salt-sensitive dyes exhibit poor
migration. Level dyeing depends on the gradual increase of the dyeing
temperature and subsequent additions of limited amounts of salt. Levelling
agents may be required. These dyes are temperature-controllable. Dyeing is
started at low temperature without added salt. The bath is slowly heated,
with particular care in the temperature region where exhaustion is most
rapid. Some salt may be added during further dyeing at the boil. These dyes
are often polyazo dyes with few sulphonate groups and of high substantivity
for cellulose. At lower dyeing temperatures, they are very prone to
aggregation in solution and sensitive to salt addition. Salt in the dyebath
impedes exhaustion at low temperatures because it promotes even more
aggregation. The higher the degree of aggregation of the dye, the lower the
concentration of individual dye molecules in the solution that can diffuse into
the fibre, and therefore the lower the rate of dyeing. Dye aggregates are too
large to penetrate into the pores of cellulosic fibres.