Page 434 - Basic Principles of Textile Coloration

DYEING COTTON/NYLON BLENDS 423

20.4 DYEING WOOL/POLYESTER BLENDS

Fabrics containing a blend of polyester and wool have superior abrasion and crease
resistance to those made from wool alone, yet they retain the valued warmth and
handle of wool fabrics. In many cases, where the highest quality and good fastness
properties are essential, the two fibres may be dyed separately before blending and
fabric construction.

The dyeing of wool/polyester mixtures, using acid dyes for the wool and disperse
dyes for the polyester, allows the production of solid shades and all types of cross-
dyeing. In the two-bath dyeing process, the polyester is first dyed with the disperse
dyes under pressure at up to 130 °C using a weakly acid dyebath at around pH 5.
Dyeing times should be short since the wool is more prone to damage at
temperatures above 105 °C. In dyeing wool/polyester unions, the disperse dyes
may stain the wool fibres in the blend and the long time required for their
migration to the polyester can cause a loss of wool quality. Any residual disperse
dyes in the wool must be removed under the mildest conditions possible by
soaping with a non-ionic detergent solution at 70 °C. Alternatively, reduction
clearing with a weakly alkaline ammonia solution of hydros or formaldehyde-
sulphoxylate avoids damaging the wool. Acid dyes then dye the wool under weakly
acidic conditions in a second bath.

In the one-bath process, fast acid and disperse dyes are applied simultaneously
from a weakly acidic dyebath, followed by soaping. In deep shades, staining of the
wool by disperse dyes is heavy and unless the soaping treatment is effective, the
washing fastness of the dyeings may be inferior. The one-bath dyeing method is
useful mainly for pale to medium depths of moderate fastness. Both the acid and
disperse dyes are applied simultaneously using a polyester dyeing carrier at the boil
at pH 4–5. The use of a carrier avoids the lengthy times for the transfer of disperse
dye held by the wool to the polyester. The wool must be cleared of disperse dye by
scouring with non-ionic detergent. Reduction clearing is not possible in this case,
since the wool is already dyed with dyes sensitive to reducing agents.
Alternatively, a one-bath process at 120 °C uses a wool protection agent and
there is little cross-staining of the wool by the disperse dye.

20.5 DYEING COTTON/NYLON BLENDS

In dyeing polyamide/cellulosic blends, there is a diversity of blends types and
dyeing methods. The polyamides include wool, silk, nylon and the polyurethanes,
although the latter are not strictly polyamides. Dyeing at pH values above 10 may