Page 361 - Basic Principles of Textile Coloration

350 REACTIVE DYES

effective than washing in a dyeing machine where longer washing times and
fresh baths are more easily arranged. The usual eight box washing range
allows three dilution rinses, 2–3 soapings and 2–3 final rinses. For initial
rinsing, static baths, into which no water flows, are more economical. The
concentration of salts accumulating in the static bath must not become so
high that soaping efficiency falls because the unfixed dye is too substantive. It
is common to have a counter-current flow of water through each short series
of boxes before the last bath overflows to the drain. The flow rate of water
must provide the required degree of dilution and of unfixed dye removal so
that the final fabric has acceptable fastness properties.

16.5.2 Pad–batch dyeing

Pad–batch dyeing involves padding the fabric with a pre-mixed alkaline solution of
the reactive dyes and then winding up the impregnated fabric on a suitable roller.
Fixation occurs during storage of the batched fabric at ambient temperature. The
dye and alkali solutions are usually mixed just before padding using metering
pumps to maintain the correct ratio. A 4:1 dye-to-alkali solution volume ratio is
very common. During storage, the roller may be continuously rotated at low speed
to avoid drainage of the internal liquid within the batch.

Obviously, the dyes must have adequate reactivity under the batching
conditions to give good fixation within about 24 h. The more reactive dyes give
effective fixation within 2–6 h. To avoid evaporation from the exposed surfaces
and edges of the roll, the fabric is stitched to a somewhat wider end-cloth that is
padded and finally wrapped around the entire wet roll before it is covered with a
plastic film. When padding on solutions with relatively high pH values, which are
necessary to promote fixation of less reactive dyes, there is always a risk of carbon
dioxide absorption on the selvages of the material even when these have been
accurately wound edge-on-edge. This decreases the solution pH since hydroxide
ion is converted into carbonate ion. The rate of fixation is then lowered. The
effect of this only becomes apparent after the final washing when the selvages
appear paler than the rest of the material. In pad–batch dyeing with Remazol dyes
(DyStar), the buffering action of sodium metasilicate added to the NaOH solution
is claimed to virtually eliminate the problem of paler selvages from carbon dioxide
absorption.

After storing, the batch of material must be thoroughly washed to remove
unfixed dye and chemicals. If the fabric is wound on a perforated beam, this can