Page 362 - Basic Principles of Textile Coloration

CONTINUOUS DYEING PROCESSES FOR COTTON 351

be carried out using a beam dyeing machine. Alternatively, other types of batch
dyeing machine or a continuous washing range can be used.

This semi-continuous pad–batch process offers many advantages and is now
widely used in place of jig dyeing. Provided that space is available for storage of the
fabric batches, the energy requirements for dyeing are minimal and the process is
more economic than batch dyeing for lots in the 1000–10 000 m range. The
fabric also often has a better handle and surface appearance because it is not
continuously circulating around in a winch or jet dyeing machine. The relatively
slow fixation process results in good dye penetration into the fibres.

16.5.3 Pad–heat dyeing
The pad–dry process is only suitable for reactive dyes with fairly high reactivity.
For dyes of lower reactivity, the dried fabric must be baked to promote further
fixation. In pad–dry dyeing, the fabric is first padded with a dye solution
containing sodium bicarbonate. During drying, bicarbonate is converted into
carbonate, which gives a higher pH and more effective fixation (Scheme 16.4).
The dye solution also contains a high concentration of urea (100 g l–1). This
helps the cotton fibres retain water during drying, possibly provides a fluid medium
for dye diffusion in the fibres at low water contents, and increases dye solubility.
Unfortunately, all this urea is removed on washing after fixation and poses a
pollution problem. Urea is a source of nitrogen nutrients for algal growth.

2NaHCO3 Na2CO3 + H2O + CO2

Scheme 16.4

In the pad–dry process, typically with the reactive DCT dyes, the drying rate is
fairly low and the cotton retains at least 18% water. If the fabric is fully dried, the
reaction ceases so the drying time must be longer than that for effective reaction
with the cotton. Typical conditions are 2–5 min at 105 °C in a hot flue or on
heated cylinders at 105 °C. The fabric temperature will not exceed 70 °C. The
initial drying cylinders may be much cooler to avoid excessively high initial
evaporation rates and migration.

The pad–dry–bake process is suitable for less reactive dyes such as MCT dyes.
Today, this type of process is used mainly in combination with the application of