Saminathan Ratnapandian

Results and discussion

1. Dyeing

   1. Shades obtained

The vat dyeing process sequence of pad (dye)  dry  pad (reducing chemicals)  steam  oxidise  soap  rinse  dry was found to be applicable for dyeing cotton fabric using both natural and synthetic indigo. Comparative reflectance curves of exhaust dyed and pad-dyed indigo (Figure 6.4 and Figure 6.5) identified a typical blue shade with maximum absorption wavelength (λ_max) of 660 nm in all cases. It should be noted that sodium dithionite was the reducing agent employed in exhaust dyeing, while it was thiourea dioxide during padding. Sodium dithionite could not be used in padding because of its thermal instability, mentioned earlier. For the padding results shown, both natural and synthetic indigo were reduced in the presence of sodium hydroxide (Bath A) and steamed for 60 s. The overlap of the curves indicates that the shades were of near-equal depth; in other words, TUD and sodium dithionite were equivalent reducing agents for indigo. A less than 5% variation among the triplicate trials confirmed the repeatability of the process.




Figure 6.4 Comparison between exhaust dyeing and padding – natural indigo



Figure 6.5 Comparison between exhaust dyeing and padding – synthetic indigo
Figure 6.6 compares reflectance curves for synthetic and natural indigo, both reduced using Bath A and steamed for 60 seconds. Once again the overlap of the curves represents an equivalence of shade. The marginal increase in absorption by synthetic indigo in the range of 570–660 nm indicates a slightly darker shade. This may be attributed to a higher degree of purity of the synthetic indigo [13, 112]. The results confirm that TUD functions as a reduction agent for both synthetic and natural indigo.
35


30