Page 433 - Basic Principles of Textile Coloration

422 UNION DYEING

cotton is not possible. In addition, a number of methods depend upon padding the
cotton/polyester fabric with a solution containing all the dyes for both fibres. The
colour develops on treatment with a solution of the appropriate chemicals in a
suitable dyeing machine. This might involve treatment with a carrier for dyeing
the polyester with the disperse dyes, followed by treatment with a solution of the
assistants for dyeing the cotton with the selected cotton dyes. The residual pad
liquor is usually added to the treatment bath. This procedure only requires one set
of weighings.

Cold pad–batch dyeing of cotton and cotton in cotton/polyester fabric has been
a big success and is widely practised. This process involves much less investment
in equipment than a continuous dyeing range and minimum energy requirements.

20.3.8 Continuous dyeing of cotton/polyester

The Thermosol process for dyeing polyester was discussed in Section 15.7.6. It is
used for dyeing woven cotton/polyester fabric. Vat, sulphur and reactive dyes for
the cotton component give good results in the pad–thermosol–pad–steam process.
This type of dyeing range (Figure 10.7) is so long that economic operation requires
large batches of woven fabric. Direct and azoic dyes may also be used for the
cotton component.

The dye pad contains a mixture of the dyes for the cotton and the polyester
along with an anti-migration agent. The dyes selected must therefore be
compatible at this stage and the dyes for the cotton must be stable to the
conditions of the subsequent Thermosol process.

The fabric is then carefully dried to avoid excessive dye migration, and
‘Thermosolled’. During this process, the disperse dyes, which are located mainly in
the more absorbent cotton fibres, vaporise and transfer to the polyester. After
cooling, the fabric passes into the chemical pad containing the appropriate
solution of cotton dyeing assistants that will allow dyeing of the cotton fibres
during subsequent steaming. The hydros for reduction of the vat dyes will clear
any disperse dye from the polyester surface. This cannot occur when using direct
or reactive dyes, so that effective sublimation of disperse dye from the cotton is
essential. The operation of a pad–thermosol–pad–steam dyeing range involves a
high degree of control to produce thousands of metres of uniformly dyed fabric of
high colour consistency. The method is useful for the dyeing of large lots of
material, usually well in excess of 10 000 m. Cotton/polyester fabrics can also be
dyed on a Thermosol range using solubilised vat dyes. These dyes colour both
fibres and in pale shades the colour difference between the two is often acceptable.