Saminathan Ratnapandian

Steaming time
Indigo source
Synthetic	Natural
A (caustic soda + TUD)	30 seconds	110.3%	108.2%
	60 seconds	118.7%	109.7%
	90 seconds	109.3%	142.0%
B (soda ash + TUD)	2 minutes	83.4%	103.4%
	4 minutes	100.0%	132.1%
	6 minutes	95.1%	135.7%

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Relative colour strength (%)
120
100
80
40
0
a b c d e f

a, b, c – synthetic indigo; d, e, f – natural indigo and steamed for 30, 60, 90 s respectively

Figure 6.7 Effect of steaming duration on colour strength (reduced in Bath A)

Figure 6.8 Effect of steaming duration on colour strength (reduced in Bath B)
The combination of synthetic indigo and sodium hydroxide resulted in the darkest shade at a steaming time of 60 seconds, while it was 4 minutes with sodium carbonate. The first combination yielded a shade that was darker by 18.7%. This was in agreement with Etters [123] and Blackburn et al. [116], who postulated that a higher amount of indigo would be absorbed by cotton at the higher alkalinity resulting from caustic soda. Such an increased absorption was probably caused by the swelling of cotton in the
presence of sodium hydroxide. In both cases an increase in steaming time irreversibly reduced (over-reduced) the indigo, which resulted in a lighter shade. This was consistent with the publications of Chakraborty et al. [50, 117, 201].

Table 6.5 shows that natural indigo gave the darkest shades at steaming durations of 90 seconds and 6 minutes for combinations with sodium hydroxide (Bath A) and sodium carbonate (Bath B) respectively. Consistent with the results for synthetic indigo, dyeing in the presence of sodium hydroxide produced a slightly darker shade. The necessity for a longer steaming duration may be explained by the fact that natural indigo is not as pure as its synthetic counterpart [116]. The increased steaming time ensured that sufficient energy was available for the indigo isomers to be reduced.

The lower crystallinity of natural indigo [116] probably facilitated a higher degree of reaction with the reducing agent and resulted in the relatively darker shades observed. A solubility test was conducted to attest to the lower crystallinity (in turn a lower particle size) of natural indigo. One (1) gram of indigo (natural or synthetic) was dissolved in 100 ml of water. The solutions were vacuum-filtered and the solids remaining after filtration were dried and weighed accurately. This weight was compared to the initial weight (1 gram) of dye added. It was determined that 95% of synthetic indigo remained insoluble while it was only 80% for natural indigo. This confirmed that natural indigo is more soluble than synthetic indigo.

The reflectance curves for natural indigo reduced in the presence of either sodium hydroxide (Bath A) or sodium carbonate (Bath B) as depicted in Figure 6.9 indicate near-equivalent shades. This confirms that cotton can be dyed using indigo by the pad

 steam process with a combination of sodium carbonate and TUD. Etters [123] has reported similar shade equivalency for indigo dyed in the presence of a mixture of sodium hydroxide and sodium carbonate as alkali. The work in this thesis is the first instance where sodium hydroxide has been totally replaced, although with the requirement for a longer steaming time.
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