Dischargeability testing
Dischargeability is the ability of a dyed ground shade to be removed cleanly by a discharging agent so that a printed pattern is left white or in a different colour. It is the property that makes discharge printing possible: the fabric is dyed first, then printed with a paste that destroys the colour where it touches, and the pattern that remains is the discharged area. A ground that discharges cleanly gives a sharp, high-contrast result. A ground that discharges only partly leaves a ghost of the original shade, and a ground that changes hue rather than disappearing produces an off-white that a buyer will reject.
What dischargeability measures
The property depends on the chemistry of the dye class rather than on the depth of the shade alone, which is why dischargeable ranges exist as separate products. Testing is done in the laboratory by printing the discharging paste onto dyed fabric and curing it, then comparing the discharged area against the original ground. Because the assessment is made against the shade the printer intends to produce, the result is read visually rather than on a grading scale.
The reference Nuchem follows
The discharging test is a Nuchem laboratory method, written for reactive-dyed fabric and run with a printing screen so the paste is applied the way a printer applies it. The paste is prepared to a fixed recipe before printing.
| Component | Quantity |
|---|---|
| Rongolite-C | 17 to 20 g |
| Titanium dioxide | 5 to 7 g |
| Sodium carbonate | 2.5 g |
| Plain sodium alginate 7 % paste | 50 to 70 g |
| Preparation | Dissolve the first three in 20 to 30 ml of hot water, then add the alginate paste |
How the test is run
The dyed fabric is laid flat and the paste is applied through a screen, as it would be in production. The specimen is then cured, which is the stage at which the discharging agent acts on the dye.
- The reactive-dyed fabric sample is plaited flat on the table.
- The discharging paste is applied to the fabric with a printing screen, so the pattern comes from the screen rather than from hand application.
- The printed specimen is cured at 150 to 170 °C, the stage at which the discharging agent destroys the ground colour.
- The discharged area is assessed visually against the original dyed ground.
How the result is read
The result is read visually. The discharged area is compared with the original dyed ground, and the assessment considers whether colour has been removed cleanly rather than only partly, whether the discharged area is white or carries a residual tint, and whether the surrounding printed ground is affected. Because discharge results depend on the paste, the curing temperature and the depth of the ground shade, a result belongs to a combination rather than to a dye alone.
Where a shade has to discharge, the COTFIX dischargeable range is the starting point, and the COTFIX dischargeable dyes page sets out that range. Fastness values for the range are published alongside the other grades on the fastness and solubility data page.
Frequently asked questions
A printing method in which the fabric is dyed first and a paste is then printed onto it to destroy the colour where the pattern falls. The pattern is produced by removing colour rather than by adding it.
Because the ability to be discharged depends on the dye chemistry, not only on the shade. A dye with the wrong chemistry will not discharge cleanly however light the shade.
The printed area retains a ghost of the original shade or an off-white tint, which is visible against the intended white and is normally rejected on inspection.
Because the question is whether the ground has been removed cleanly, judged against the shade the printer intends to produce. The grey scale measures change in a specimen, not the completeness of removal.
The dischargeable range is the correct starting point for a shade that has to discharge. A dye outside that range should be tested before it is specified for a discharge process.
Need a discharge trial on a specific ground shade?
Give the COTFIX grade and the intended discharge effect, and the laboratory will print and cure a trial so the result can be seen before it is specified on an order.