Page 206 - Basic Principles of Textile Coloration

CONTINUOUS DYEING 195

vaporisation. This migrating water will carry with it any unfixed dyes and
chemicals in solution or fine dispersion. The extent of migration of unfixed
chemicals depends on the yarn and fabric construction and is smaller the higher
the absorbency of the fibres present, the lower the initial wet pick-up, the lower
the rate of drying, the larger the particle size of pigments, and the higher the
viscosity of the applied solution. It usually results in heavily dyed yarn surfaces,
with a deeper perceived shade, and sometimes reduced fastness to washing and
rubbing.

If the dye in the padded fabric is going to be fixed using heat, the fabric must
first be pre-dried with a minimum of migration, reducing the water content to a
value corresponding to the absence of significant amounts of water on the fibre
surfaces or in the fibre capillaries. For 50/50 cotton/polyester fabric, migration is
usually negligible at wet pick-up values below 20–25%, so the objective of pre-
drying is to initially dry the fabric to this level with a minimum of migration. For
fabrics of synthetic fibres that do not absorb water, the critical moisture content
below which migration is negligible is often much lower, around 15% water for
100% polyester fabrics. The padded fabric usually passes between gas fired infrared
heaters uniformly situated along each face of the fabric. The heating should be as
uniform as possible and the rate of drying not too high. The padding liquor may
contain migration inhibitors. These are usually polymeric materials that increase
the solution viscosity, or which cause flocculation of pigment particles to
mechanically hinder their movement. Once the wet pick-up has been reduced
below the critical value, the fabric is fully dried using steam-heated cylinders.

A Thermosol unit is used to rapidly heat polyester fabric to temperatures up to
200–220 °C for 30–60 s to promote fixation of disperse dyes padded onto the
polyester material (Section 12.6.2). It also serves for the fixation of reactive dyes
on cotton. This type of unit is now widely used for cotton/polyester fabrics (Figure
10.6).

Various types of steamers allow the continuous fixation of direct, reactive, vat
and sulphur dyes applied to cotton (Section 12.6.3). Fixation is achieved by
heating the padded fabric in saturated steam for 20–60 s. For dyes sensitive to air-
oxidation, such as reduced vat dyes, the steam must be air-free.

For the continuous dyeing of the important cotton/polyester fabrics, the
different dyes required for coloration of the two different fibres necessitate heat
fixation of the disperse dye in the polyester, in the Thermosol unit, and fixation of
the cotton dyes in the steamer (Figure 10.6). The most impressive continuous
dyeing ranges are therefore those for cotton/polyester dyeing. They include a dye