Question

On mercerization, moisture regain of cotton fiber \(\underline{\hspace{2cm}}\)

• A. Decreases
• B. Remain unchanged
• C. Increases
• D. Indeterminate

Hint:

Mercerization of cotton improves several properties: - Lustre: Increases (due to smoother, rounder fiber surface). - Strength: Increases. - Dye Affinity: Increases (more accessible regions for dye). - Moisture Regain: Increases (more accessible amorphous regions).

Answer 1

The Correct Option is C

Step 1: Define Mercerization. Mercerization is a treatment for cotton fabric or yarn with a cold, concentrated solution of caustic soda (sodium hydroxide). 
 

Step 2: Understand the structural changes caused by mercerization. The strong alkali causes the cotton fibers to swell. This swelling changes the fiber's cross-section from a kidney-bean shape to a more circular shape. Crucially, it also changes the internal polymer structure. The treatment transforms the native crystal structure (Cellulose I) to a different, more accessible form (Cellulose II). This process increases the proportion of amorphous regions relative to crystalline regions and makes the hydroxyl groups in the amorphous regions more accessible. 
 

Step 3: Relate structural changes to moisture regain. Moisture regain is the weight of water in a material expressed as a percentage of its oven-dry weight. Water molecules are primarily absorbed in the amorphous regions of a fiber, where they can form hydrogen bonds with the polymer's hydroxyl groups. Since mercerization increases the accessibility of these amorphous regions, the fiber is able to absorb more water molecules. 

Conclusion: Mercerization increases the amorphousness and accessibility of hydroxyl groups in cotton, which leads to an increase in its moisture regain. The typical regain of cotton increases from about 7-8% to about 9-10%.