Discharge Style Printing Process on Cotton Fabric Step by Step

Discharge Printing Process:
Generally fabric is dyed with solid color before printing. Before going to discharge printing we should know what is discharge? Discharge means removal and discharging system means the process which can produce a white or colored effect on a previously dyed ground. Discharge printing is one style of printing. Discharge printing is also called as extract printing. Discharge design is a misnomer. It is a printing process that results in removing the original dye from the dyed fabric on which the design is printed. Hence the name of the design which suits that process. In this process a new color is fixed on the same spot from where the previous color is removed. So the double action of removing (discharging) of one color and fixing of another is done at the same time in a single process. These types of designs are prepared and printed on colored backgrounds, mostly medium or dark. With light or medium color in the background you can use additional darker colors along with discharge. Discharge style of printing is not widely used because production costs are high.

By this printing process, color is destroyed by one or multiple colors. Discharge printing can be done on cotton. Synthetics and blends are not suitable for this style of printing. The dyes used for printing must be suitable for discharging. The suitable recipe needs to be prepared. This discharging of color from previously dyed ground is carried out by a discharging agent which is actually a oxidizing and reducing agent capable of destroying color by oxidation and reduction.

In discharge printing the background color of the print is firstly applied to the fabric prior to printing. This background color can be applied by exhaust batch dyeing or by continuous dyeing processes. A print paste that, instead of containing dye, contains a reducing agent is then applied to the local areas of the fabric required by the design. It is essential to the printing style that the background color is produced from dyestuffs that can be ‘discharged’, that is, destroyed by chemical reduction in order for the pattern to be displayed after printing of the discharge paste. This process is illustrated in Figure 1.

The chemical reduction leaves a white image on the colored background. This technique allows for deep, rich background shades to be achieved. Common discharging chemicals are based on:

• Formaldehyde sulphoxylates such as sodium formaldehyde sulphoxylate (C.I. Reducing Agent 2)
• Thiourea dioxide (C.I. Reducing Agent 11)

It is possible to create an ‘illuminated discharge’ by incorporating a dye into the discharging paste. The dye or pigment in the paste must be resistant to the discharge chemical present also, so that it remains on the fabric when the illuminated discharge paste is printed. The illuminated discharge paste destroys the background color of the print, but the ‘illuminating’ color remains. This effect is illustrated in Figure 2.

Requirements for Discharge Printing:
Discharge styles have been important since the earliest days of textile printing. With any industrial process there must be sound technical and commercial reasons for its conception and continuation. In the case of discharge printing, the following considerations determine the usefulness of the process compared with other printing techniques.

1. Printed materials with large areas of ground color can be produced, the depth, levelness and penetration of which would be difficult, if not impossible, to obtain by a direct printing process.
2. Delicate colors and intricate patterns can be reproduced on grounds of any depth, with a clarity and sharpness that have become the hallmarks of this style. Intricate white patterns lose their crispness if left as unprinted areas in a direct, blotch print, because the print paste spreads unequally in different directions. In addition, a colored motif fitted into a blotch print either leaves unprinted white margins or forms a third color where fall-on occurs. In some cases such effects are acceptable, but they can be eliminated by using the discharge technique.
3. The extra processes required and the additional costs of discharge pastes mean that production costs are higher, but the aesthetically superior results give the product a higher value and enable profit margins to be maintained or even improved. The higher costs of discharge printing are often offset when applied to long-lasting designs used for scarves, ties, cravats and dressing gowns. As already indicated, in discharge styles the pattern is produced by the chemical destruction of the original dye in the printed areas. The discharging agents used can be oxidising or reducing agents, acids, alkalis and various salts. An early and, one might say, classical example is the discharge printing of cotton dyed with indigo, the characteristic color of which can be destroyed either by oxidation or reduction.

Sequence of Discharge Style Printing Process on Cotton Fabric:
In discharge printing, a predyed fabric is printed with a paste containing a reducing (discharge) agent and a dye (the illuminating color) that is resistant to reduction. The ground shade is simultaneously destroyed and replaced by the ‘illuminating’ color in the printed areas. Production of intricate patterns, usually strongly contrasting grounds, with great clarity, sharpness and fit, has become the hallmark of this style, which can be produced by direct printing methods only with great difficulty, if at all.

Sequence of discharge style printing process on cotton is done as the following way:

Fabric preparation

↓

Table preparation

↓

Fabric plaited on the table

↓

Apply printing paste with the help of screen

↓

Curing at 190ºC (belt speed 3m/min)

↓

Delivery

Advantages of Discharge Printing:

1. Large areas of ground color are possible.
2. Delicate colors and intricate patterns possible on deep ground color, excellent depth and clarity possible.
3. Very cool technique that removes the dye from the shirt.
4. Little to no feeling of the print on the shirt.
5. Vibrant colors.
6. The best way to print on dark shirts with non-traditional inks.
7. Something different from screen printing “norms”.
8. Still able to do a great amount of detail.
9. We offer discharge printing at no additional charge. In fact, it’s often cheaper than traditional ink.
10. Higher production cost but long lasting unique styles.

Disadvantages of Discharge Printing:
There are certain disadvantages to this style of printing. The choice of reducing agent depends upon the fiber being printed and the dyes used for the background. The background dyes need to be relatively easy to discharge, so they tend to be azo‐based colors. However, dyes with specific structural characteristics are more easily dischargeable than others. In general, monoazo disperse dyes based on azobenzene are most easily discharged.

Other disadvantages are:

1. It is an expensive process.
2. Two stage application involved in dyeing or padding and discharge printing.
3. Limited choice of ground and motif colors.
4. Requires rigid process care that any default will lead to damages.
5. Some sizes may discharge better than others.
6. Can be hard to use for photographic style or process printing.
7. Only works on natural fibers, like cotton, so 50/50 can’t be used.
8. Not all shirt colors will discharge (royal blue never works well, for example).
9. Process can be quirky.

What is Discharging Agents?
A stripping agent such as sodium hyposulfite which is used to remove dyes from fabric that has been vat-dyed or printed. Clearly, the most important methods of discharging are based on reduction. This general method can be varied and adapted to give discharges with most classes of dye in use and on most types of fiber. Indeed, to many printers the terms ‘reducing agent’ and ‘discharging agent’ are synonymous. The most widely used reducing agents are the formaldehyde sulphoxylates. The stability of these compounds is such that only limited losses of sulphoxylate occur during printing and prior to steaming. The use of sodium formaldehyde sulphoxylate (CI Reducing Agent 2, sold as Formosul or Rongalite C) was established as long ago as 1905, when it was recognized that methods based on this reducing agent offered many advantages.

References:

1. Textile Printing (Revised Second Edition) Edited by Leslie W C Miles
2. The Coloration of Wool and other Keratin Fibres Edited by David M. Lewis and John A. Rippon
3. Handbook of Value Addition Processes for Fabrics By B. Purushothama
4. An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
5. Fundamentals of Designing for Textile and Other End Uses by J.W. Parchure
6. Textile Engineering – An Introduction Edited by Yasir Nawab

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