Page 359 - Basic Principles of Textile Coloration

348 REACTIVE DYES

are not part of the chromophoric system. This problem can be partly overcome, for
example by linking together two DCT dyes with a suitable diamine to give a dye
with two MCT groups, as in the Procion H-E dyes (4). This increase in molecular
size usually results in an increase in substantivity that is of value for exhaust
dyeing with higher liquor ratios but which can impede washing-off of unfixed dye
after dyeing.

Bifunctional dyes with two reactive groups of different reactivity towards the
cotton, which have different optimal fixation conditions, give a more uniform
degree of fixation over a wider range of dyeing temperature and fixation pH than
dyes containing two identical groups. Therefore, process control does not need to
be so stringent. These types of reactive dyes give quite high fixation yields and
thus less colour in the dyehouse effluent. Other important types of bifunctional
reactive dyes include the MFT-VS type (Cibacron C, Ciba) and the MCT-VS type
(5) used in the Sumifix Supra dyes (Sumito). The Kayacelon React range of dyes
(Nippon Kayaku) are also bifunctional reactive dyes, having two NT reactive
groups in each dye molecule.

16.5 CONTINUOUS DYEING PROCESSES FOR COTTON

16.5.1 Introduction
The first step in continuous dyeing involves padding dye solution onto full-width
woven fabric. This is a very low liquor ratio dyeing method that gives the expected
improved exhaustion and fixation, under appropriate conditions. The very rapid
fixation (< 60 s) at high temperatures that is possible in fully continuous dyeing
gives high productivity for long runs of a given colour. On the other hand, the
slow dye fixation achieved during storage of a roll of impregnated fabric at room
temperature (4–24 h) results in better dye diffusion and penetration into the
fibres (Section 16.5.2).

There are several problems that must be considered in continuous dyeing with
reactive dyes:
(1) dyes of low substantivity are desirable to avoid preferential dye absorption

during padding and the initial colour tailing that it causes (Section 10.5.2).
This is less of a problem with reactive dyes than with direct or sulphur dyes,
which have greater molecular size and therefore higher substantivity. Tailing
caused by dyes with different substantivities can result in an initial drift of the