Question

The after treatment of certain direct dyes with 0.25-2% copper sulphate and 1% acetic acid for 20-30 mins at 60°C to improve \(\underline{\hspace{2cm}}\)

• A. Washing fastness
• B. Light fastness
• C. Light and washing fastness both
• D. Perspiration fastness

Hint:

Remember common after-treatments for direct dyes: - Cationic fixing agents: Improve wash fastness. - Copper Sulphate: Improves light fastness (for specific 'after-coppering' dyes).

Answer 1

The Correct Option is B

Step 1: Identify the process. This is an after-treatment for direct dyes using a metallic salt (copper sulphate). 
 

Step 2: Understand the limitations of direct dyes. Direct dyes generally have poor wash fastness because they are held to the fiber by weak forces. Many also have moderate to poor light fastness. 
 

Step 3: Analyze the effect of the after-treatment. Certain direct dyes are designed to be "after-coppering" dyes. The copper ions (\(Cu^{2+}\)) from the copper sulphate form a more stable complex with the dye molecule inside the fiber. This complex is much more resistant to fading upon exposure to light. While it can offer a slight improvement in wash fastness, its primary and most significant effect is the improvement of light fastness. 

Conclusion: After-treatment with copper sulphate is specifically done to improve the light fastness of certain direct dyes.