Page 427 - Basic Principles of Textile Coloration

416 UNION DYEING

of the individual fibres after dyeing, they must be separated. This is not easy. For
simple evaluation of dyeing, small swatches of 100% polyester and 100% cotton
fabrics, with characteristics close to those of the fibres in the blend, can be sewn
onto the cotton/polyester material and their colours examined after dyeing. This
avoids having to separate the two kinds of fibres. Alternatively, a widely used
technique is to evaluate the colour of the dyed blend and then to dissolve out the
cotton with 70% aqueous sulphuric acid. The polyester is unaffected by this and a
polyester ‘skeleton’ remains, the colour of which can be examined. The colour of
the cotton is assessed from the difference between the colours of the original fabric
and the residual polyester skeleton.

Because a mixture of dyes is needed for dyeing this type of fibre blend, and
because there are a number of potential problems, consultation with a dye
manufacturer is recommended in selecting dyes and processes. Suppliers are able
to recommend disperse dyes that minimise staining of the cotton, suitable dyes for
the cotton, and point out inappropriate dye combinations.

20.3.2 Batch dyeing of cotton/polyester

Since the dyeing of polyester is invariably carried out at 120–130 °C under
pressure, enclosed beam and jet dyeing machines are widely used for dyeing
cotton/polyester blend fabrics. For lightweight materials in beam dyeing machines,
the multiple layers of the fabric on the beam must be permeable to the circulating
dye liquor. Knit goods do not have adequate dimensional stability for continuous
dyeing and are often dyed in jet machines. Good desizing, scouring and bleaching
are essential for bright or pale shades, and are usually carried out in the dyeing
machine before the actual dyeing. Heat setting at 180–200 °C for 30–40 s
reduces pilling and increases dimensional stability and crease recovery.

A dyeing carrier is not essential but will assist levelling of disperse dyes on the
polyester. To minimise staining of the cotton by the disperse dyes, it is common
practice to drain the dyeing machine while the liquor is still at high temperature.
Under these conditions, most of the unexhausted disperse dyes are still in solution.
Cooling the bath before draining tends to cause precipitation of dyes on the fabric.
Unfortunately, some disperse dyes do exhibit substantivity for cotton, but this is
low if the dyebath is above 100 °C. In the case of jet dyeing machines, blowing
out the exhausted dye liquor at high temperature may not be possible because of
the risk of setting creases into the fabric. In this case, the bath is cooled slowly and
then drained at the highest possible temperature that avoids creasing.