Question

A sample of cotton fabric is dyed with a vat dye at 60°C till equilibrium dye uptake is reached. Another sample of the same cotton fabric is dyed with the same dye at 90°C till equilibrium, keeping all other parameters same. Amongst the following, the correct statement(s) is/are:

• A. Dye exhaustion will be higher at 90°C as compared to that at 60°C
• B. Dye exhaustion will be lower at 90°C as compared to that at 60°C
• C. Levelness achieved at 90°C will be higher as compared to that at 60°C
• D. Levelness achieved at 90°C will be lower as compared to that at 60°C

Hint:

Distinguish {equilibrium} effects from {rate} effects. Higher temperature speeds up dyeing (kinetics) but often lowers final equilibrium uptake (thermodynamics), while improving levelness via enhanced migration.

Answer 1

The Correct Option is B, C

Step 1: Effect of temperature on vat-dye {equilibrium exhaustion}
- In vat dyeing, the dye is applied in its leuco (reduced) form which has affinity for cellulose.
- Increasing temperature generally decreases the {thermodynamic substantivity} (affinity) of the leuco dye for the fibre, even though it speeds up the kinetics.
- At true equilibrium (all else same), higher temperature tends to shift the balance toward the bath, giving {lower} exhaustion at 90°C} than at 60°C.
$\Rightarrow$ (B) is true; (A) is false.
Step 2: Effect of temperature on levelness
- Higher temperature increases diffusion and migration of dye within fibres and between more/less dyed regions.
- Better mobility helps correct shade differences, improving levelness.
$\Rightarrow$ (C) is true; (D) is false.
Final Answer: Dye exhaustion is lower at 90°C (B), and levelness is higher at 90°C (C).