Page 430 - Basic Principles of Textile Coloration

DYEING COTTON/POLYESTER BLENDS 419

stable at 125 °C in a dispersion with a pH above 9. The overall dyeing time is just
over 5 h.

By using single-bath dyeing processes with combinations of reactive and
disperse dyes, it is possible to reduce dyeing times. The major limitation in this
respect is the long time required for removal of hydrolysed reactive dyes from the
cotton. The reactive dyes must also be stable to high temperature dyeing if they
are applied together with the disperse dyes. In addition, colour reproducibility of
the cotton fibres is more difficult when using reactive rather than simpler dyes
such as direct dyes.

20.3.4 Disperse/direct dye combinations

This combination allows relatively simple dyeing methods and the use of the less
expensive direct dyes. Because of the lower fastness to washing of direct dyes on
cotton, complete clearing of the disperse dyes staining this fibre is less essential.

There are two major methods for dyeing cotton/polyester with a combination of
direct and disperse dyes. In the two-bath method, the polyester and cotton are
dyed separately using the appropriate dyes. Disperse dyes are used for the polyester,
dyeing at 130 °C under pressure. For medium and heavy shades, an alkaline
reduction clear with hydros solution reduces and removes azo and anthraquinone
disperse dyes staining the cotton fibres. After rinsing and neutralising the fabric,
the direct dyes are applied to the cotton at 90 °C, with addition of salt if required.
This two-bath dyeing cycle takes about 6 h.

In the rapid one-bath dyeing method, dyeing takes place with a mixture of
disperse and direct dyes at 130 °C. This results in lower initial exhaustion of the
direct dye onto the cotton but good levelling. The bath is then cooled to 80–
90 °C and the direct dye further exhausted by addition of Glauber’s salt. The bath
is drained at 60–70 °C, and the fabric given two cool rinses. The total time
required is about 4 h. The direct dyes used must be stable at 130 °C and the
disperse dye should give minimal cross-staining of the cotton. Reduction clearing
of any disperse dye staining the cotton destroys the direct dyes. This process is
therefore less suitable for deep shades.

A major problem with using direct dyes on cotton is the poor washing fastness
of deep dyeings, which normally only satisfies the lowest requirements. The
washing fastness improves after the usual aftertreatments (Section 14.5), or after
resin finishing. The latter provides shrinkage control and dimensional stability for
knit goods and crease resistance for woven fabrics. The polyester avoids the usual