Mercerization - Caustic Treatment

Mercerization treats cellulosic textiles (primarily cotton) with concentrated sodium hydroxide (20-30% NaOH, 18-25°Be) under tension at 15-30°C causing irreversible structural changes improving luster, strength, dye affinity, and dimensional stability. Discovery: John Mercer 1844 (swelling effect), H.A. Lowe 1890s (tension application achieving luster). Mechanism: cellulose I crystal structure converting to cellulose II (native parallel chain arrangement transforming to antiparallel), fiber swelling (cross-section becoming rounder, more uniform from kidney-bean shape), molecular chain rearrangement, and surface smoothing. Process: (1) Impregnation—fabric saturated with cold NaOH (15-25°C, cold minimizing degradation while maximizing swelling), (2) Dwelling under tension—maintaining dimensional stability (preventing excessive shrinkage—untensioned cotton shrinks 20-25%), time 30-120 seconds, (3) Washing—removing alkali via counter-current washing (5-8 washing boxes reducing NaOH from 20% to <0.1%), and (4) Neutralization—dilute acid (acetic, formic) neutralizing residual alkali preventing yellowing. Effects: luster increase (light reflection improving 50-100% creating silk-like sheen), tensile strength increase (10-20% from better molecular orientation), elongation increase (15-25%), dye affinity increase (30-50% for reactive and direct dyes—more accessible hydroxyl groups), improved dimensional stability, and smooth hand. Process variations: Slack mercerization (no tension, used for stretch fabrics creating 15-20% permanent stretch via dimensional shrinkage and crystalline transformation), Chain mercerization (yarn mercerization before weaving for premium products), and Caustic washing (dilute NaOH 8-12%, similar effects but less pronounced used as economical pretreatment enhancing dye uptake 15-20%). Testing: baryta number (barium hydroxide absorption measuring crystalline transformation, mercerized >100 vs. unmercerized 60-80), cross-section examination (mercerized shows rounder profile), and swelling test (mercerized swells less in NaOH, structure already transformed). Applications: premium shirting, sateen fabrics, mercerized cotton thread (luster, strength), high-quality knitwear, and towels (improved absorbency). Economics: NaOH cost $400-600/tonne, recovery systems essential (multi-effect evaporation reconcentrating spent liquor from 5-8% to 20%, recovering >90% reducing cost and environmental load), justifying 10-20% premium for mercerized products.