Mercerization of cotton
Mercerization is a pre-treatment technique particularly used for cellulosic and cotton fibres. The process involves the treatment of fabrics with sodium hydroxide.
The treatment is normally applied either to yarn or to the fabric. Mercerization can be carried out in the slack state or under tension.
From a chemical point of view, mercerization of cotton fibers leads to a crystal structural transformation from cellulose I to II. A complete transition occurres at sodium hydroxide concentration levels of 15 and 20 wt%.
In particular, cellulose I swells immediately after contact with concentrated sodium hydroxide aqueous solution. The swelling breaks intermolecular hydrogen bonds and allows for re-organization of the cellulose chains.
Swelling most readily occurs at the amorphous interface between microfibrils; small lateral displacement of chains can then effect intermixing of molecules, producing a microstructure of antiparallel chains.
From a microstructural point of view, unmercerized cotton fiber has a ribbon-like structure with spiral twists longitudinally. Its surface is rough and non-uniform, the cross-section is irregular and kidney shaped with a lumen inside. As described in the figure below, when a cotton fiber is immersed in a sodium hydroxide concentrated solution, the cellulose begins to swell immediately and the cross-section turns into elliptical in a few seconds (Stage 4). On further swelling (Stage 5), the section rounds off with larger size and collapsed lumen. After washing with water, the cotton fiber shrinks (Stage 6). After drying, a further and final shrinkage occurs (Stage 7). Swelling is more significant when no tension is applied for mercerization. However, with no tension, fiber surfaces after mercerization still show residual creases and wrinkles which will influence the luster of the fabric.
Mercerized cotton has higher moisture regain and is more easily to wet out than unmercerized cotton fabrics. Swelling in caustic soda significantly breaks the hydrogen bonds in cotton fiber, increasing the number of available hydroxyl groups by 25%. In general, native cotton has moisture content of about 7%. Mercerized cotton with and without tension has moisture content of about 9% and 11% respectively. Moreover, mercerization imparts a silk-like luster to cotton and better mechanical properties.