Saminathan Ratnapandian

Chemical	Concentration (OWM)
Function
Indigo	1.5%	Dye
Sodium dithionite	3.0%	Reducing agent
Dextrose	2.0%	Stabiliser
Sodium hydroxide	3.0%	Alkali

The fabric sample was immersed in the clear dye solution for 30 minutes. During that period, the dye bath was stirred continuously. Care was taken to ensure that the fabric remained completely immersed, to prevent premature oxidation. After 30 minutes, the fabric sample was taken out and squeezed to remove most of the dye liquor. This sample was allowed to oxidise in atmospheric air at ambient temperature for 5 minutes. Oxidation was recognised by a return of blue colour to the fabric. The blue fabric was re-entered into the dye bath and the above dyeing sequence repeated for a further 30 minutes. After a final oxidation of 10 minutes, the dyed fabric was treated in 2 grams per litre (g/l) solution of acetic acid to neutralise residual alkali. This was followed by soaping off, at boil for 20 minutes, in a 1 g/l standard soap solution. The dyed fabric

sample was then rinsed sequentially in hot water and cold water. Final drying was carried out at 65^○C.


Vat dyeing method
Dyeing was achieved by adapting the padding process usually employed for vat dyes, that is, pad (dye) → dry → pad (reducing chemicals) → steam → oxidise → (sour) → soaping off. The sequence is schematically depicted in Figure 6.2. Sodium dithionite was not suitable for this process because of its thermal instability. As mentioned earlier, TUD attains maximum reduction capability at 100^○C and it has a slightly higher reduction potential than sodium dithionite [196, 199]. Hence TUD was selected as the alternate reducing agent.

Figure 6.2 Modified vat pad-dyeing process

Dye liquor was prepared according to the recipe given in Table 6.2. A fresh solution of sodium alginate and Delinol VB/LU was made up and allowed to settle for three hours. This was necessary to ensure that the sodium alginate swelled and became completely effective. Indigo (natural or synthetic) at 16 g/l, approximating a nominal 1.5% shade, was made into a paste with warm water and dispersed in this solution. This ensured even dispersion of the dye. Fabric samples of 10 g each were padded with the above dispersion using the 3 dip–3 nip technique with 80% wet pick-up at room temperature. The padded fabric was dried at 65^○C for approximately 2 hours and stored under clean conditions to avoid contamination. Drying facilitated pick-up during the subsequent chemical padding step.
Table 6.2 Pad liquor recipe