Question

On mercerization, the strength and moisture absorption capacity of cotton fibre increases. The reason for increase is

• A. decrease in orientation and decrease in amorphous region
• B. increase in orientation and decrease in amorphous region
• C. increase in orientation and increase in amorphous region
• D. decrease in orientation and increase in amorphous region

Hint:

Mercerized cotton is preferred for bright colors because the increased amorphous region allows more dye to settle deep within the fiber.

Answer 1

The Correct Option is D

Concept: Mercerization is a fundamental chemical finish for cotton, discovered by John Mercer. It involves treating cotton yarn or fabric with a high concentration of Sodium Hydroxide (NaOH) at low temperatures, often under tension.

Step 1: Swelling and Structural Transformation.
When cotton is immersed in the caustic soda solution, the concentrated alkali penetrates the crystalline regions of the cellulose. This causes the fibers to swell significantly in the transverse direction. During this swelling, the natural "convolutions" (twists) of the cotton fiber disappear, and the crystal lattice changes from Cellulose I to Cellulose II.

Step 2: Relationship between Amorphous Content and Absorption.
The swelling process disrupts the highly ordered crystalline arrangement, leading to an increase in the amorphous region (the disorganized part of the fiber). Since moisture and dye molecules primarily enter through the amorphous regions, the fiber becomes much more absorbent. Simultaneously, the internal reorganization leads to a decrease in orientation, though the overall strength increases due to the removal of structural weak points and better stress distribution.