Page 417 - Basic Principles of Textile Coloration

406 DYES SYNTHESISED IN THE FIBRE

19.3.2 Continuous dyeing methods

A variety of procedures are used for the continuous dyeing of cotton fabrics with
azoic combinations. Some involve continuous padding of the fabric with a solution
of the coupling component followed by batch development and soaping in a
dyeing machine.

A pad–dry–pad process is fairly common. For padding with an alkaline solution
of the coupling component, good fabric absorbency and effective size removal from
the warp are essential. The water used should also be soft since calcium and
magnesium ions readily precipitate insoluble salts of the naphtholate ion. Even
though the naphtholate ions are usually of only low to medium substantivity, a
small pad trough is used and padding with a solution as hot as 80 °C is common.
This decreases the substantivity even further, minimises bath concentration
changes and tailing and ensures complete solubility of the naphtholate and its
good penetration into the cotton. Intermediate drying of fabric padded with a
solution of a low substantivity Naphtol minimises bleeding and azo pigment
formation in the development bath. Addition of salt to the Naphtol solution helps
to suppress migration of the coupling component during drying. The impregnated
fabric should not be over-dried, to ensure rapid re-wetting in the development pad
bath. This second pad bath, with the diazonium ion solution, may contain a non-
ionic surfactant to ensure rapid fabric re-wetting and dispersion of any azo
pigment formed in the bath. Coupling takes place in a bath at a pH usually
between 4.0 and 6.0 at 20 °C, depending on the electrophilic nature of the
diazonium ion. After padding with the diazonium component, the fabric will be
skyed for a minute or so to allow time for the coupling reaction. The
aftertreatment involves the usual combination of rinsing and good soaping.

The application of azoic combinations to cotton fabrics such as terry cloth by a
wet-on-wet pad method is also popular. The fabric is padded with a solution of the
coupling component at 80 °C and skyed for up to a minute. This allows the
coupling component to penetrate into the fibres, and cooling of the fabric to the
point that there will be no thermal decomposition of the diazonium ion solution in
the next stage. Padding with a solution of the diazonium ion immediately follows
before skying again to complete development. The use of the more substantive
Naphtols prevents bleeding into the second, diazonium ion bath. Soaping with a
solution of a non-ionic surfactant and sodium carbonate is carried out in either a
rope or open-width machine.