Saminathan Ratnapandian

Drawbacks of continuous dyeing
Since the entire process is in open width, care has to be taken to avoid stretching of the fabric. Once processing has started, any machinery breakdown would cause significant fabric damage due to excessive dwell time [12]. Colour bleeding during chemical padding can cause shade variation. This is avoided by using a high concentration of electrolytes in the padding liquor. Urea removed during washing-off is a source of pollution because it can be a nutrient source for algae [11, 14].

3. Printing

A large number of methods exist for applying dyes by the printing technique of which
direct printing is the most common. Direct printing involves applying the dye in the
form of a print paste consisting of dyes, thickeners and auxiliaries. The print paste is applied by means of engraved rollers (roller printing) or flat/rotary screens (screen printing). After dye application, the printed fabrics are dried and steamed, similar to the processes used for padding depending on the dye-fabric combination. Each colour requires its own screen, thus making the printing range significantly long. Pigment printing uses binders to secure the pigment to the fabric surface. This is generally followed by a simple drying-curing step. Transfer printing involves sublimation of dye from a patterned paper onto the fabric [11, 12].

Inkjet printing produces patterns directly on the fabric by spraying the dye solution and is gaining popularity. Production speeds at 20–30 m/min are slow as compared to rotary screen printing but this is compensated for by the inherent flexibility and quick change characteristics. Soluble dyes are ideally suited for inkjet printing. Insoluble dyes especially disperse dyes have to be finely dispersed to avoid clogging of the inkjet nozzles [11].

4. Unconventional dyeing

Trials to replace water by super-critical carbon dioxide have been successful for different classes of dyes and substrates [58, 59]. Shishoo [60] has collated the output from investigations into the use of plasma technology in textiles aimed at application areas such as a) elimination of water and chemical based pre-treatments; b) enhancing dye uptake; and c) imparting functional finishes. Ultrasonics [61], microwave and sonicator [62, 63] have been used to assist in the dyeing process. Hot-air drying has been supplemented by the use of radio frequency (RF) [64] and infra-red (IR) [65] techniques.

5. Improving textile colouration

Considering the variety of dyes and dyeing methods available, there is ample scope for improving textile colouration to increase productivity, reduce waste, optimize process parameters, etc. This may include:

1. 
   developing new dyes, dyeing machines and methods
   
2. 
   reuse of resources such as water and energy
   
3. 
   use of waste and by-products of textile and other industries; and
   
4. 
   applying unconventional technologies as discussed above.
   

The above may be summed up as developing processes for producing darker shades with given amounts of dye [11, 12, 14, 15].
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3.