Indigosol colours are solubilised vat dyes marketed in the form of water soluble vat dyes. The print of cotton fabric is carried out in following three stages:
The printing paste is prepared by dissolving suitable concentration of Indigosol colour in warm water. The solution is then brought to printable viscosity with the addition of gum Arabic or guar gum paste. Small quantity of sodium carbonate is added to improve the storage stability of print paste. After printing the fabric is dried in shade.
In this step the soluble form of Indigosol colour is converted into water insoluble form at the printed portion. On development the Indigosol dye gets converted to water insoluble vat dye form. Therefore, it shows the best all round fastness properties which is the characteristic of vat dyes.
The process of development is carried out by treating the fabric with solution containing sulphuric acid and sodium nitrite.
After development it is important to wash the fabric thoroughly to remove acid. It can be achieved by washing as
- Cold wash 2 times
- Wash with 2 g/l soda ash to neutralize acid
- Cold wash 2 times.
- Check the pH of the washed fabric which should be neutral.
Precautions to be taken during Indigosol printing
- Indigosol dyes are sensitive to light therefore the print paste or printed fabric while printing is in progress should not be exposed to light. Exposure to light will cause premature development of colour resulting in prints of low fastness properties and loss in depth of shade. Ideally the sunlight should be prevented in the hall where the block printing operation is carried out.
- Indigosol print pastes are not stable. The print paste cannot be stored for long time. Therefore, print paste just sufficient to use within a day must be prepared.
- The print paste stability can be improved with the addition of sodium carbonate in the print paste
- Sulphuric acid is a strong acid therefore after colour development it must be completely neutralized by thorough washing several times. Sodium carbonate may be added in the wash water to make sure that the acid is neutralized. If the acid is not completely neutralized, the fabric would lose its strength on storage and the printer would receive complaints from the customer. The fabric pH must be checked after washing and it should be neutral
- Gases liberated in the process are poisonous and workers should not inhale them. Proper ventilation must be provided during the development step.
Advantages of Indigosol colours
- Indigosol colours produce soft prints compared to pigment prints
- The prints show extremely good all round fastness properties which is the characteristics of vat dyes
- Printing process is simple
- The dyes are expensive
- Dark shades are difficult to produce
- The Indigosol dyes do not produce bright shades like pigment or reactive dyes.
Khadi print paste is available in the market as a white paste ready for printing. The exact composition of khadi paste is a trade secret but it is essentially prepared by the incorporation of titanium dioxide as white pigment. The content of titanium dioxide could be as high as 20-25%. This paste is used for producing white prints on white fabric or on dyed fabric. Because of high concentration of titanium dioxide and binder the prints are harsh. Harshness of the prints can be compensated by using plasticized binders which produce soft film on polymerization.
If pigment colour is incorporated in the khadi paste colour print effect can be obtained against colour background imitating colour discharge effects.
These are produced by using print paste containing metallic powders. Aluminium, bronze and copper powders are usually used. The type of the print effect obtained depends on the metal powder used in the print paste. For example
- Aluminium powder: Silver print
- Copper powder: Copper print
- Bronze powder: Gold print
Drawbacks of khadi and metallic prints
Following drawbacks are associated with these print pastes
- Because of high loading of titanium dioxide or metallic powders, the print pastes tend to dry quickly resulting in uneven saturation of block which produces uneven prints.
- The feel of the fabric at the printed portion is harsh
- The fastness property particularly rub fastness is poor.
- Considerable practice is required for producing satisfactory prints using these pastes.