Bleaching - Whiteness Enhancement

Bleaching destroys natural color pigments (gossypol in cotton, lignin in bast fibers, melanin in wool) and residual impurities improving whiteness and color clarity for subsequent dyeing/finishing. Bleaching agents: (1) Hydrogen peroxide (H₂O₂, most common)—oxidizing colorants, concentration 5-30 g/L, pH 10-11 (stabilized with silicate, phosphate preventing decomposition), temperature 80-100°C, time 30-120 min. Chemistry: perhydroxyl anion (HOO⁻, active bleaching species) oxidizing chromophores breaking conjugated double bonds. Advantages: mild (minimal fiber damage <5% tensile loss), predictable, environmentally acceptable (decomposes to water + oxygen), suitable for all natural fibers. (2) Sodium hypochlorite (NaOCl)—powerful oxidizer, 2-5 g/L available chlorine, pH 10-11, 20-30°C, 30-60 min, faster than peroxide but risks fiber damage (>10% strength loss), yellowing if overdosed, and environmental concerns (AOX formation). Mainly developing countries, declining usage. (3) Sodium chlorite (NaClO₂)—acidic bleaching (pH 3.5-4.5, 70-80°C) generating chlorine dioxide, most powerful cellulosic bleach achieving highest whiteness (index 85-90 vs. peroxide 75-82) but toxic fumes (ClO₂), corrosive, environmental issues limiting to specialty applications (high-whiteness textiles, pulp bleaching). (4) Reductive bleaching—sodium dithionite (Na₂S₂O₄, hydrosulfite), mechanism different (reducing colorants rather than oxidizing), pH 5-7, 60-80°C, used for discharge printing, wool/silk whitening (oxidative bleaches causing excessive damage to proteins). Bleaching sequences: Continuous bleaching (pad-batch: fabric impregnated with peroxide, rolled and stored 4-16 hours at room temperature then washed; pad-steam: peroxide application, steaming 60-120 seconds, rapid for towels, knits), Semi-continuous (J-box steaming after padding), and Batch (winch, jet, beam dyeing machines). Fiber-specific: cotton achieves 75-85 whiteness index, wool/silk limited whitening (enzyme bleaching, mild peroxide 3-5 g/L, 40-50°C preventing damage), synthetics no bleaching needed (inherently white) or optical brightening. Post-bleaching: antichlor treatment if hypochlorite used (sodium bisulfite neutralizing residual chlorine preventing tendering), thorough washing removing all chemicals preventing yellowing/degradation in storage.