Chemical Extraction
Alkaline preparation of cotton yarn or fabric utilises concentrated sodium hydroxide solutions to swell cellulose fibres and remove natural impurities. Cotton textiles undergo caustic soda pretreatment to eliminate natural waxes and seed coat fragments from the grey goods. The concentration of the bath determines whether the process acts as a simple scouring aid or induces full mercerisation.
Complete structural transformation occurs only when the concentration exceeds twelve percent by weight.
Structural Alteration
Swelling of the cotton cellulose begins at the sub-microscopic level when sodium ions penetrate the crystalline regions. This chemical action causes the flat, twisted cotton fibres to untwist and assume a round cross-section. The internal volume of the fibre increases, which provides more accessible hydroxyl groups for subsequent processing.
Consequently, the fabric develops a higher affinity for direct or reactive dyes.
Process Parameter
Temperature control remains a critical factor during the contact phase because lower temperatures favour deeper swelling of the cellulose. Industrial applications often run between fifteen and twenty-five degrees Celsius to achieve uniform results across the entire fabric width. High temperatures can cause uneven chemical penetration or premature degradation of auxiliary wetting agents in the bath.
The treated material requires thorough rinsing with hot water and subsequent acid neutralisation to prevent residual alkalinity from affecting the dyeing stage, as remaining alkali can cause premature fixation of reactive dyes.
Quality Verification
Evaluation of the treated substrate relies on measuring the barium activity number. Untreated cotton returns a value of one hundred, whereas fully treated fibres yield numbers above one hundred and fifty. This test ensures consistent dye yield across different production batches.
