Reactive Dyeing of Cellulosics

Reactive dyes contain reactive groups (monochlorotriazine MCT, dichlorotriazine DCT, vinyl sulfone VS, monofluorotriazine MFT) forming covalent bonds with cellulose hydroxyl groups via nucleophilic substitution in alkaline medium achieving excellent wash fastness. Reaction mechanism: Dye-Cl + Cell-OH + OH⁻ → Dye-O-Cell + Cl⁻ + H₂O (substitution) vs. Dye-Cl + OH⁻ → Dye-OH + Cl⁻ (hydrolysis—competing reaction wasting dye). Fixation efficiency 60-80% (20-40% hydrolyzes, removed in washing causing color loss and effluent pollution). Reactive dye types: Cold-brand (MCT, VS, application 30-40°C, 30-60 min, used for bright colors on delicate substrates), Hot-brand (DCT, application 60-80°C, 30-60 min, better exhaustion/fixation for dark shades, higher fastness), and Bifunctional (containing two reactive groups MCT + VS improving fixation 70-90%, reducing hydrolysis). Dyeing methods: Exhaust dyeing—batch process in dyeing machines (jet, winch, beam), procedure: (1) Dye dissolution and addition at 40-60°C with salt (NaCl/Na₂SO₄ 30-80 g/L promoting exhaustion—ionic strength compressing electrical double layer enabling dye approach to fiber), (2) Temperature programming (raising to 60-80°C over 20-40 min allowing gradual exhaustion), (3) Alkali addition (Na₂CO₃ 15-25 g/L or NaOH 3-8 g/L creating pH 10.5-11.5 activating cellulose for reaction), (4) Fixation time (30-60 min at temperature and pH), (5) Cooling and washing (hot rinse removing unfixed dye, soap wash removing hydrolyzed dye preventing bleeding, cold wash). Liquor ratio 1:5-1:15 (kg fabric to liters water) depending on machine. Continuous dyeing—pad-batch or pad-dry-steam for high production: (1) Padding (fabric passing through dye bath with salt, squeezing to 60-80% wet pickup uniformly impregnating fabric), (2) Batch storage (rolled fabric, dye migration, 4-8 hours room temperature fixation) or Steaming (saturated steam 102-105°C, 30-90 seconds, rapid fixation), (3) Washing (same as exhaust). Production rates 50-200 m/min vs. exhaust batch 20-50 kg/hour. Pad-batch advantages: low water (0.6-1.0 L/kg vs. exhaust 5-15 L/kg), low energy (room temperature vs. heated baths), high dye utilization (80-90% fixed vs. 60-80% exhaust). Color fastness: Wash fastness 4-5 (excellent—covalent bond), Light fastness 4-6 (good to excellent depending on chromophore—azo reactive typically 4-5, anthraquinone reactive 6-7), Perspiration, chlorine moderate (alkaline hydrolysis of bond in bleach, yellowing/fading after swimming pool exposure). Dye selection: Bright colors requiring cold-brand (temperature-sensitive substrates, minimizing shade change), Dark colors preferring hot-brand (better exhaustion, higher fixation efficiency reducing cost), and Critical fastness applications using bifunctional (maximum fixation, best wash fastness for childrenswear, sportswear with frequent washing).