Disperse Dyeing of Polyester

Disperse dyes are non-ionic, hydrophobic colorants (molecular weight 250-500, water solubility <1-50 mg/L) applied to hydrophobic synthetic fibers (polyester 80%, polyamide, acetate) where dyes diffuse into fiber's amorphous regions dissolving in polymer matrix. Mechanism: Dye particles disperse in water (via dispersing agents—anionic/nonionic surfactants preventing aggregation), dye molecules dissolve from particle surface, adsorb onto fiber surface, diffuse into fiber (solid solution, no chemical bonding, thermodynamic equilibrium between dye in bath and fiber). Disperse dye classification by heat stability and application temperature: Low energy (LE, 110-120°C, lower fastness, economical, pastel shades), Medium energy (ME, 120-130°C, medium fastness, general purpose), High energy (HE, 130-140°C, excellent fastness, deep shades, highest sublimation resistance). Dyeing methods: Carrier dyeing—atmospheric pressure, 98-100°C, carriers (organic compounds like benzoic acid, salicylic acid, aromatic esters—banned in many countries for toxicity) swelling polyester enabling dye diffusion at lower temperature, slow (60-120 min), largely obsolete except small-scale. High-temperature (HT) dyeing—pressurized sealed vessels (jet, beam dyeing machines), 120-135°C, 1.5-2.5 bar, procedure: (1) Dispersing agent addition (0.5-1.5 g/L), (2) Dye addition at 50-60°C with pH control (pH 4.5-5.5 using acetic acid preventing aggregation and dye hydrolysis), (3) Temperature ramping (1-2°C/min to 130-135°C over 30-60 min allowing gradual diffusion), (4) Holding time (20-45 min at temperature achieving equilibrium exhaustion 85-98%), (5) Cooling (gradual, 2-3°C/min to 80°C, rapid cooling causing dye precipitation on surface), (6) Reduction clearing (sodium hydrosulfite 1-2 g/L, NaOH 1-2 g/L, 70-80°C, 20 min removing surface dye improving fastness). Liquor ratio 1:6-1:12. Thermosol dyeing—continuous process for polyester, (1) Padding (dye dispersion with dispersing agent, thickener, 70-80% wet pickup), (2) Drying (120-150°C, complete water removal), (3) Thermosolling/heat-setting (180-220°C, 30-60 seconds in hot-flue, dye subliming and diffusing into fiber), (4) Reduction clearing. Production rates 30-60 m/min, suitable for light-medium shades (heavy shades require exhaust for economics/uniformity). Fastness: Wash 4-5 (excellent—dye trapped in fiber matrix), Light 3-6 (variable—azo disperse 3-4, anthraquinone 5-6, careful selection required for outdoor), Sublimation (heat migration)—critical concern, HE dyes essential for dark shades, heat-setting temperatures (ironing 150-180°C causing fading if inadequate sublimation fastness), exhaust fumes in factories (subliming dye depositing on machinery, ventilation critical). Auxiliaries: Dispersing agents (preventing aggregation, typical 0.5-2 g/L), pH buffers (maintaining pH 4.5-5.5 preventing hydrolysis and precipitation), carriers (rarely), antifoaming agents (preventing foam in jets), and sequestering agents (chelating Ca²⁺, Mg²⁺ preventing precipitation).