Saminathan Ratnapandian

Chemical
Bath
A (g/l)	B (g/l)
Dextrose	10	20
Thiourea dioxide	25	25
Sodium hydroxide	12.5	--
Sodium carbonate	--	10

On emerging from the steamer, the pale green samples (the colour of leuco indigo) were allowed to oxidise at ambient temperature in an open atmosphere for 5 minutes. Samples padded with Bath A were then soured in a 2 g/l solution of acetic acid to neutralise residual alkali. Neutralisation was not required when Bath B was used because of the lower alkalinity of soda ash. The next step was washing off in a 1 g/l

solution of standard soap at the boil for 20 minutes. This was done to remove unfixed dye from the surface and promote dye aggregation inside the substrate. Dye aggregation improves fastness properties by enabling physical retention of the chemically non- reactive dye in cotton. Samples were then rinsed in hot and cold water, followed by oven drying at 65^○C.

2. Printing with indigo

A combined flash-age pad-steam process was adopted in this study. Nishkam and Kasturiya [200] detail the flash-age process as one of the industrial processes for printing with vat dyes, where fabric is printed with a paste containing dye (indigo), dried and developed by passing through a reducing bath, followed immediately by flash-aging with super-heated steam (125–140^○C and 100% RH) for 20–40 seconds. The pad-steam process is an alternate method where the fabric is printed using paste that contains metal salts such as sodium formaldehyde sulfoxylate in addition to dye, dried, padded in a reducing bath and immediately exposed to saturated steam for 5–10 minutes. It should be noted that in indigo printing the sodium or calcium or zinc salt of formaldehyde sulfoxylate is employed as the reducing agent due to the thermal instability of sodium dithionite [127]. Sodium hydroxide is the alkali generally employed. The major issues identified in regards to the above chemicals are the malodorous decomposition products especially of the zinc salt, and the care required in handling caustic soda.

In this research, a print paste containing indigo prepared as for the flash-age process and a reducing bath containing TUD were employed. The steaming for up to 10 minutes at 100^○C employed in the pad-steam process was utilised to ensure that the TUD attained its full reduction potential. Similar to the dyeing trials, the effect of substituting sodium hydroxide by sodium carbonate as alkali was evaluated.

The print paste constituents and their functions are listed in Table 6.4. An alkali-stable coagulating thickener (Prisulon 1550S) was necessary to ensure the integrity of the printed pattern during developing. The print paste was prepared according to the following procedure. First, the thickener was mixed with about 80% of the total water using a hand-mixer and allowed to swell for 2 hours. Simultaneously,the dye and

dispersing agent were made into a paste separately with the 20% balance of water. Next, the dye dispersion was added to the swollen thickener and thoroughly blended using the hand mixer to produce a homogenous print paste.

Table 6.4 Print paste recipe for indigo