Saminathan Ratnapandian

Reducing bath	Bath A
Bath B
Indigo source	Synthetic	Natural	Synthetic	Natural
Steaming time	60 s	60 s	6 min	2 min
Fading to light	4	4.5	4.5	5
Washing	3.5	4.5	4	4.5

Perspiration	Acid	5	5	5	5
	Alkaline	5	5	5	5
Rubbing	Dry	4	4.5	4	4.5
	Wet	3	4	3.5	4

Bath A - Sodium hydroxide and TUD Bath B - Sodium carbonate and TUD

6. Conclusions

The results of this investigation prove that the pad/print (dye)  dry  pad (reduction chemicals)  steam  oxidise process can be employed to dye or print cotton with either natural or synthetic indigo. Thiourea dioxide is able to replace sodium dithionite (sodium hydrosulfite) as a reducing agent for indigo. Similarly sodium carbonate served as an alternate for sodium hydroxide although at the cost of added steaming time. This was the first instance where both reducing agent and alkali were replaced during indigo colouration. It should be noted that process parameters using the alternate chemicals have not been optimised, that is, the lowest amount of chemicals required to reduce a given concentration of indigo has not been determined.

In the dyeing process, the steaming time required to yield the darkest shade for synthetic indigo reduced in the presence of sodium hydroxide was found to be 60 seconds, while it was 90 seconds for natural indigo. When sodium hydroxide was replaced by sodium carbonate as the alkali, steaming time was increased to 4 and 6 minutes for synthetic and natural indigo respectively. In the above dyeing experiments, natural indigo gave about 20% darker shade than synthetic indigo. This may be attributed to the lower crystallinity of the former.

A combined flash-age pad-steam method was used for printing with either natural or synthetic indigo on cotton. Thiourea dioxide proved to be a viable reducing agent. In contrast to dyeing, natural indigo yielded a 20% lighter shade than the synthetic indigo. The use of sodium carbonate, although effective in replacing sodium hydroxide, was not able to produce an equivalent depth of shade. The steaming time required to yield the darkest shade for indigo reduced in the presence of sodium hydroxide was

determined to be 60 seconds. However, when sodium carbonate was used as the alkali, the steaming time had to be increased to 6 and 2 minutes for synthetic and natural indigo respectively. The shade obtained and optimum steaming time was affected by: a) the stability of thickener to alkali; and b) the interaction of dye with the thickener.
Alternate thickeners that address these issues may be investigated in the future.
The use of thiourea dioxide or the additional replacement by sodium carbonate did not affect the fastness ratings of the dyed or printed samples. The ratings for the common desirable properties of light fading, washing, rubbing and perspiration were acceptable for textiles coloured using indigo.

The impact of the extended steaming duration is an important factor to be considered prior to adopting the methods developed above on a commercial scale. On the whole, the use of thiourea dioxide and sodium carbonate for reducing indigo during colouration by padding may be considered as a sustainable advancement.

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Chapter 7