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%.