Chemical Extraction
High-temperature treatment of raw cotton textiles in a sodium hydroxide bath removes non-cellulosic impurities like waxes, proteins, and pectins. This initial wet processing stage, known as alkaline scouring, prepares the grey fabric for subsequent bleaching and dyeing. The treatment hydrolyzes insoluble natural lipids into water-soluble compounds that can be washed away.
It stops being effective if the pH drops below eleven or if the temperature falls below ninety degrees Celsius.
Process Mechanism
The caustic soda reacts with the ester linkages in the cotton waxes to generate soap-like molecules that aid in further emulsification. Mechanical agitation in the jet dyeing machine or continuous scour range dislodges these loosened substances from the cellulose structure. Adding a chelating agent prevents insoluble metal soaps from redepositing on the fibers.
Without this step, subsequent dyeing would produce uneven shades and poor color fastness.
Quality Metric
Success is verified by measuring both the weight loss of the fabric and its water absorbency. Well-scoured fabric should absorb a drop of water within three seconds.
Industrial Precondition
The concentration of sodium hydroxide must be carefully controlled to avoid degrading the cotton cellulose itself. Excessive exposure can lead to chemical damage and reduce yarn tensile strength.
