Page 205 - Basic Principles of Textile Coloration

194 AN INTRODUCTION TO DYES AND DYEING

substantivities for the fibre, some dyes may be absorbed more strongly than others
and tailing then also results in unacceptable variations in hue. It can be minimised
by padding under conditions of low substantivity. This involves using a trough of
low volume, and running under conditions that cause more rapid replacement of
the dye liquor, a higher fabric speed and a greater wet pick-up. For continuous
dyeing, manufacturers often provide substantivity factors for appropriate dyes.
These allow calculation of the dye initially retained by the material, the steady-
state condition, and allow prediction of the degree of initial tailing. The pad bath
can then be diluted to the predicted steady-state concentration before dyeing
commences to avoid, or at least minimise, tailing.

Control of the shade over great lengths of material is a constant preoccupation
of continuous dyers. It depends upon a number of factors. A major one is
uniformity of the fabric absorbency, which depends upon its uniform preparation.
Once initial tailing has been eliminated, the shade along the fabric should not vary
provided that the pad roller pressure gives a constant wet pick-up, the dye solution
fed to the pad trough always has the same concentration, and the conditions of
fixation remain steady. Apart from lengthways variations in shade, side-to-side
variations can also be a problem if the pressure across the nip is not uniform. A
number of devices are used to ensure uniform nip pressure (Section 12.6.1). Other
problems that the dyer must avoid are marks on the padded fabric from
undissolved dye particles or undispersed pigment, or those arising from foam
picked up from the surface of the dye liquor in the pad, which is squeezed onto the
fabric surface.

10.5.3 Fixation procedures

Fixation procedures may be fully continuous or simple batch operations. Fully
continuous fixation usually involves heating the material in saturated steam, in
hot air, or by contact with heated metal cylinders. Sometimes, when using two
different types of dyes, a second fixation step is preceded by application of
additional chemicals in a second pad unit. In semi-continuous dyeing processes,
fixation is usually a batch process. This can be as simple as storing a covered roll of
impregnated fabric overnight, or fixation with an appropriate solution of chemicals
in a conventional batch dyeing machine.

Before entering any unit in which the padded fabric will be heated, it must be
completely dry. During drying, water evaporates from the surfaces of the yarns and
fibres and more migrates from the capillaries in the interior to replace water lost by