Caustic soda plays several roles in textile dyeing. It provides the alkali that fixes reactive dyes on cotton, and it is essential for reducing vat dyes into a soluble form. Getting the amount right is the difference between a level, fast shade and a costly redye.
Reactive dyeing of cotton
Reactive dyes form a chemical bond with cotton fibre, but only in alkaline conditions. The usual sequence is:
- The dye is added with salt so it moves onto the fibre.
- Alkali is added to start fixation. Soda ash is the most common alkali; caustic soda is added for dyes and shades that need a higher pH, often in combination with soda ash.
- The goods are washed off and soaped to remove unfixed dye.
Too little alkali gives poor fixation and weak wash fastness. Too much, or adding it too quickly, can cause uneven dyeing and wastes dye through hydrolysis.
Vat dyeing
Vat dyes are insoluble in water. To apply them, they are reduced with a reducing agent such as sodium hydrosulphite in the presence of caustic soda, which forms the soluble leuco form that enters the fibre. After dyeing, the dye is oxidised back to its insoluble form inside the fibre, giving excellent fastness. Caustic concentration must be kept within the dye maker's range throughout, or the dye can come out of solution.
Controlling caustic in the dye house
- Dose by weight, using a known strength stock solution, rather than by guesswork.
- Check bath alkalinity by titration for critical shades.
- Use consistent quality caustic; low iron avoids dull shades on whites and pastels.
- Add alkali gradually, in portions or by dosing, for level dyeing.
Safety
Caustic solutions cause severe burns. Dye-house workers need goggles, gloves and aprons, and eyewash stations near dosing points. Always add caustic to water when preparing stock solutions.
Supply in Oman
For textile businesses with operations in Oman, a daily published price and ready stock of caustic soda flakes 99% are available at muscatchemical.com, with delivery within Oman.
