Saminathan Ratnapandian

Natural indigo
Synthetic indigo
Reducing bath		Bath A	Bath B	Bath A		Bath B
Steaming duration		90 seconds	6 minutes	60 seconds		4 minutes
Fading to light		4.5	5	5		5
Washing (colour change)		3.5	4.5	4.5		4
Washing (staining)	Nylon	3.5	3	3.5		3
	Acetate	3.5	3	3		3.5
Perspiration	Acid	5	5	5		5
	Alkaline	5	5	5		5
Rubbing	Dry	4.5	4	4.5		4
	Wet	3	4	3.5		4

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

2. Printing

   1. Shades obtained

Printing with indigo was achieved by a combined flash-age pad-steam process as described earlier. The printed indigo was solubilised by employing either a) an alternate reducing agent (TUD) in the presence of caustic soda as alkali; or b) TUD in combination with an alternate alkali followed by steaming. Similar to dyeing, all printed samples were pale-yellow in colour after steaming. This was proof that the indigo had been reduced. As can be observed from Figure 6.10, the samples acquired the typical blue shade of indigo after oxidation.

Figure 6.10 Printed indigo reduced using Bath A (TUD + caustic soda)
The reflectance curves depicted in Figure 6.11 brought out the 80% lighter shade produced by natural indigo as compared to synthetic indigo. Further, the close overlap of the curves for the two steaming durations (60 and 90 seconds) indicated that the longer duration did not significantly improve the final shade for both natural indigo and synthetic indigo reduced using Bath A.

Figure 6.11 Reflectance curves comparing print shades from synthetic and natural indigo reduced using Bath A

The evident contrast between the printing and dyeing results for indigo may be attributed to the higher degree of crystallinity of synthetic indigo [204] and the role of the thickener (Prisulon 1550S). The less crystalline natural indigo was probably bound intimately with the thickener as compared to synthetic indigo, and hence after being reduced could not readily migrate to the fabric. This resulted in lighter shades from natural indigo. In the additional 30 seconds of steaming time, the overall stability of the print paste may have prevented any additional quantity of the leuco-compound from being absorbed by the fabric. Further extension of steaming time caused dye degradation, which proved detrimental to the final shade.

2. Effect of alkali and steaming duration

The reflectance curves displayed in Figure 6.12 show that the shade obtained when caustic soda was replaced by sodium carbonate was lighter by approximately 15%. In addition, the colour strength relationship between natural and synthetic indigo was retained, that is, natural indigo produced only 80% of the shade of synthetic indigo. Although the duration of steaming affected the final shade, the trend observed disagreed with that of dyeing. The darkest shade for synthetic indigo reduced in the presence of sodium carbonate was obtained at a steaming time of 6 minutes, whereas 2 minutes were sufficient for natural indigo. During pad-dyeing, when sodium carbonate was employed as the alkali, natural indigo yielded the darkest shade at a steaming time of 6 minutes while it was 4 minutes for synthetic indigo. All other steaming durations consistently produced a lighter shade in the case of printing.

Figure 6.12 Effect of alkali on indigo printing

The results highlight the role of the thickener in deciding the final shade in these experiments. The thickener (Prisulon 1550 S) has an inherent pH of 9–11.5 [205]. Caustic soda would raise the alkalinity and facilitate increased uptake of the leuco- indigo. When steaming time was extended, the reduction of indigo became irreversible, and resulted in poor colour yield [50]. Sodium carbonate, a milder alkali, has a pH approximately equal to that of the thickener and so would maintain the integrity of the print paste hindering colour transfer into the substrate. The use of a lower thickener concentration would affect print paste rheology and thereby pattern definition [205]. On the other hand, steaming time of 6 minutes gave the darkest shades with synthetic indigo reduced in the presence of soda ash, probably because the available extra energy was able to promote additional dye migration.

3. Evaluation of fastness

The typical fastness properties of indigo were evident for all samples, as shown in Table 6.7. The darker samples, resulting from synthetic indigo, showed greater colour change (fading) on exposure to artificial sunlight or rubbing. This could be due to the fact that small changes in lighter shades are not readily apparent as compared to a similar change in darker shades. All samples performed well in the test for fastness to perspiration with a rating of 5, indicating no change in colour and no staining of adjacent multi-fibre fabric. Washing fastness yielded ratings of 3.5 to 4.5. The nylon and acetate components of the adjacent multi-fibre fabric were stained and rated at 3 to 3.5, similar to those for dyeing. The fastness ratings agree with the earlier dyeing results and it can be concluded that the alkali or reducing agent does not affect the fastness properties of indigo during printing.

Table 6.7 Fastness testing results (printing)