Page 248 - Basic Principles of Textile Coloration

CONTINUOUS DYEING EQUIPMENT 237

for 30–60 s, the dye sublimes. The dye molecules dissolve directly in the polyester
fibres from the vapour phase. At such high temperatures, there is quite rapid
diffusion of the dye molecules into the polyester.

This type of dyeing process is widely used for cotton/polyester blends, using a
disperse dye for the polyester fibres and another type such as a reactive, vat or
direct dye for the cotton. The fabric is initially padded with all the dyes required
for dyeing the cotton and polyester. There may be as many as 6–8 dyes in the
mixture. Because the disperse dyes will be fixed using dry heat, the padded fabric is
first dried. This is a two-stage process. Infrared heaters gently pre-dry the fabric to
a water content below the critical value for migration (Chapter 10.5.3). Once
migration is no longer possible, drying is completed on steam heated cylinders.

The required heating for the Thermosol process can be done using a tenter
frame but this is not ideal. The oven would have to be quite long to run at high
speeds and there are problems from over-heating of the metal pins and plates that
hold the selvages. Hot flue fixation is common in Europe. The hot flue is a
chamber with a set of driven upper rollers and free lower ones. The fabric loops up
and down through these as it passes through the heated chamber. Hot air, from
oil- or gas-fired burners, blows up between the loops of fabric held on the rollers. If
combustion gases such as nitrogen oxides are likely to cause dye fading, the air
may be heated indirectly. The fabric is running at speed under some tension and
may extend so the exit rollers must be driven slightly faster to account for this.
There is no width control and the rollers must have quite large diameters and be
accurately aligned to avoid creases. The fabric enters and leaves through narrow
slots to retain the hot air.

Contact heating gives a more rapid temperature increase than convective
heating. This is important because sublimation and diffusion of the disperse dyes
probably occur quite rapidly once the required temperature is reached. The total
dyeing time in a typical contact Thermosol process is reduced to around 20 s.
This type of heating is used mainly in the USA where 60 cm diameter cylinders in
banks are fired using natural gas. Perforated suction drums can also be used in
which heated air is sucked through the fabric in contact with the drum. Care is
required to ensure that the flow of air does not carry away any vapour of the
sublimed dye.

12.6.3 Roller steamers [3]

Steamers are used for the continuous fixation of vat, sulphur, reactive and direct