Chemical Processing
Chemical Processing covers preparation before coloration: desizing, scouring, bleaching and mercerizing. The exam tests chemical action, recipe calculation, fibre damage and water and energy trade-offs.
Core concepts
- Desizing removes starch or synthetic size; enzymatic desizing is selective, while oxidative desizing needs close control.
- Scouring removes waxes, pectins, oils and dirt with alkali and surfactant; cotton gains absorbency.
- Bleaching with hydrogen peroxide destroys natural colour; stabilizers and chelators prevent catalytic fibre damage.
- Mercerizing uses concentrated NaOH, tension and washing; it improves lustre, strength, dye uptake and dimensional stability.
- Process control uses wet pickup, liquor ratio, temperature, time and rinsing.
Worked example
A 1,000 kg cotton lot is bleached at 1:10 liquor ratio with 3% owf H2O2 and 4 g/L NaOH. Liquor volume = 10,000 L. H2O2 = 0.03 × 1,000 = 30 kg. NaOH = 4 g/L × 10,000 L = 40,000 g = 40 kg. If a pad-steam route is used, the calculation changes to wet pickup rather than liquor ratio.
Traps
- Mixing owf (on weight of fibre) with g/L.
- Ignoring liquor ratio when converting a recipe to chemical mass.
- Overstabilizing or understabilizing peroxide, causing fibre damage or poor whiteness.
- Treating mercerization as a simple wash; tension and caustic recovery matter.