Page 431 - Basic Principles of Textile Coloration

420 UNION DYEING

losses of tear strength and abrasion resistance of the cotton with these resin
finishes. Cationic fixatives (to improve the washing fastness of the direct dyes on
the cotton) and softeners, can alter the shade of the dyeing and decrease the light
fastness. The possibility of thermal migration of the disperse dyes from the
polyester fibres into any hydrophobic softener on the fibre surface during final
drying and curing must also be considered. Any disperse dye transferred into the
surface layer of softener is easily removed during use.

So, when considering a combination of disperse and direct dyes for dyeing a
cotton/polyester blend, the fastness properties of the final dyeing are often
decisive. Direct dyes are more likely to be used in combination with disperse dyes
for colouring polyester/viscose fabrics since the washing fastness of direct dyes on
viscose is somewhat better than on cotton. Dyeing in a jet machine preserves the
characteristic soft handle of this type of fabric.

20.3.5 Disperse/vat dye combinations

Cotton/polyester can be dyed with a mixture of vat and disperse dyes using a
single-bath dyeing process. Both types of dyes are added at the beginning of the
dyeing cycle and, after the initial temperature increase, dyeing of the polyester is
completed at 130 °C. Some dye producers sell ready-mixed vat and disperse dyes
that reduce the number of required weighings. Shade control may be more difficult
with these. Additional dispersant maintains the dispersion of both types of
insoluble dye. Some vat dyes, of lower molecular weight, significantly stain the
polyester component, and should be avoided.

After dyeing at 125–130 °C, the cotton is already pre-pigmented with the vat
dye. The bath is then cooled and reduction of the vat dye carried out at around
60–70 °C by addition of caustic soda and hydros. The chemicals for vatting do
not affect disperse dyes since ionic species do not penetrate into the hydrophobic
polyester fibres. Reduction of the vat dye to the substantive leuco compound for
dyeing of the cotton and reductive clearing of superficial vat and disperse dyes
from the fibre surfaces occur simultaneously. The dyeing is then rinsed and the
leuco vat dye oxidised. The usual soaping follows. It is best to use closed, fully
flooded dyeing machines to minimise oxidation of the leuco vat dye. The process
requires about 5 h. Some vat dyes give better results when added after the
pressure dyeing of the polyester, but this takes longer because pre-pigmentation of
the cotton fibres takes more time. This latter process is useful if staining of the