Saminathan Ratnapandian

25



Reflectance (%)
20


15


10


5


0
400 450 500 550 600 650 700
Wavelength (λ) nm
Figure 6.9 Reflectance curves natural indigo (Bath A 90 s and Bath B 6 min)
By employing TUD, the release of sulfur dioxide and other toxic byproducts of oxidation of sodium hydrosulfite into the atmosphere can be avoided. This makes the process easy to control and friendly to the operators and the environment. These aspects are further enhanced by the use of sodium carbonate instead of caustic soda, which increases safety in handling and reduces disposal load.

3. Fastness properties

The fastness ratings of the indigo-dyed cotton fabrics are summarised in Table 6.6. The results reported are for those process combinations that yielded the darkest shades. All samples exhibited excellent fastness to light, with an average rating of 4–5 against blue wool standards after 48 hours of exposure to simulated sunlight. This is in agreement with the light fastness results of indigo reported by other researchers [202, 203].
Washing test results indicate a colour change of average to good with ratings ranging from 3.5 to 4.5. Staining of the nylon and acetate components of the adjacent multi- fibre fabric (rated at 3 to 3.5) indicates that the dye released into the wash liquor behaved like a disperse dye, as put forth by Etters [123]. Dry rubbing fastness was better than wet rubbing, being rated as good to very good (4 to 4.5) as against average to good (3 to 4). The dyed samples had a rating of 5 when tested for fastness to perspiration under both acid and alkaline conditions. There was no change of original colour or staining of the adjacent fabric. These results confirmed that the fastness
properties of indigo are not affected by the reducing agent or alkali employed during dyeing.
Table 6.6 Fastness testing results (dyeing)