Page 312 - Basic Principles of Textile Coloration

THE AFTERTREATMENT OF DYEINGS WITH DIRECT DYES 301

NH2

CH3 SO3 S
S N

N 7

N
N OH

CH3 SO3 S
S N

N

8

Figure 14.5 Primuline, CI Direct Yellow 59 (7); Primuline diazotised and developed with 2-
naphthol (8)

14.5.3 Aftertreatment with formaldehyde

This type of aftertreatment is applicable to a few mostly black dyes. Dye molecules
are linked by methylene groups, usually in ortho positions to hydroxyl or amino
groups. The dyeing is treated with acetic acid and formaldehyde in solution at 70–
80 °C. This aftertreatment may result in decreased light fastness.

14.5.4 Metal complex formation

In this type of aftertreatment, cupric or chromic ions convert the dye into a metal
complex. The most common ligand structure is a o,o¢-dihydroxyazo compound, or
its dimethyl ether. In the latter case, the methyl groups are displaced and the dye
forms the complex of a dihydroxyazo compound (9 and 10, in Figure 14.6).
Coppering involves treating the dyeing with acetic acid and copper sulphate
solution and heating to 70–80 °C. Complexation usually increases the light
fastness. The process results in a change in hue and may be reversible on repeated
washing due to de-metallisation. This results in a gradual decrease in washing
fastness. Both copper and chromium are environmentally undesirable. Copper
aftertreatment is still used for a few brown, navy and black shades, taking care that