Preparation of cotton fibers, yarns, and fabrics to undergo dyeing and finishing processes is critical because many problems that appear later can be traced back to inadequate preparation. Whatever the substrate—a fiber, a yarn, a fabric, or an entire garment—the first step of preparation is scouring to remove unwanted impurities. Scouring also improves absorbency and ensures that the cotton is uniformly clean.
Another major preparation process is bleaching, to produce a uniform white base, so that light or bright colors can be achieved. Various chemicals can produce the whiteness achieved by bleaching, but the most commonly used is hydrogen peroxide. Peroxide bleaching offers distinct advantages beyond good whiteness, as it is economical and flexible to use.
A third process often used with cotton fibers is mercerization, which changes the physical structure of the cotton fiber and thereby its physical properties and appearance. In mercerization, the substrate is exposed to a concentrated solution of caustic soda, usually under tension. Mercerization changes the cotton fiber’s cross-sectional shape and morphology.
The final step in the preparation of any substrate is rinsing and neutralizing any chemistry that has been used. To ensure that the fiber is clean, the solubilized matter created during scouring must be removed. The final pH of the substrate must be neutral, without any residual alkaline salts.