Page 435 - Basic Principles of Textile Coloration

424 UNION DYEING

damage polyamides because of hydrolysis. The risk of this is also higher for
extended dyeing times and at higher temperatures. Nylon fibres can be dyed with
acid, reactive or disperse dyes. The cellulosic component is usually cotton or
viscose, dyed with direct, vat, sulphur, reactive or azoic dyes. The high alkalinity in
dyeing cotton with vat, sulphur and azoic dyes excludes their use in the presence
of animal fibres. These dyes are, however, useful for dyeing the cotton in nylon/
cotton blends since the nylon is more resistant to alkaline solutions, provided the
dyeing temperature is limited. In the presence of wool, even reactive dyes for the
cellulose must be capable of fixation at lower temperature and pH values than are
normally used in cotton dyeing with these dyes.

Cotton/nylon fabrics can be dyed using disperse or acid dyes for the nylon, and
direct dyes for the cotton. Some direct dyes will give a solid shade dyeing both
cotton and nylon directly at pH 4–5. Alternatively, some direct dyes do not stain
nylon in the presence of a syntan, or do not stain the nylon at 80–90 °C in a
weakly alkaline bath. These can be used to correct the shade of the cotton. Better
washing fastness is obtained with a combination of fast acid dyes for the nylon,
and reactive dyes for the cotton.

If the percentage of cotton or nylon is below 20%, the production of a uniform
solid shade is not too difficult. For intimately blended fibres, it is even possible to
dye only the major component and leave the other fibre undyed. It is even easier if
one of the fibres predominates at the fabric surface as in a fabric with a cotton pile
on a nylon backing. Solid shades are more difficult to dye with 50/50 cotton/nylon
blends as both fibres will be visible.

Military cotton/nylon fabrics have a limited range of shades and orders of large
batches are common. For batch dyeing such fabrics, direct dyes of reasonable wet
fastness are used for the cotton in conjunction with a syntan to avoid their
staining of the nylon and, in a second bath, milling or metal-complex dyes are
applied to the nylon. The milling and metal-complex dyes do not cross-stain the
cotton and have good light and wet fastness.

Some interesting continuous dyeing methods are known using vat dyes for the
cotton component of cotton/nylon blends. This is possible because nylon is much
more resistant than wool to alkalis and reducing agents. For example, the cotton/
nylon fabric is padded with a mixture of vat and acid dyes, steamed to fix the acid
dye on the nylon, then padded wet-on-wet with hydros and caustic soda solution,
and steamed to reduce and fix the vat dye on the cotton. Rinsing, oxidising the
leuco vat dye, and scouring to remove superficial colour, complete the process.
This requires two steamers in the dyeing range. Since some acid dyes will