Ozone Pretreatment and Bleaching

Ozone (O₃) treatment uses powerful oxidizing gas for bleaching, decolorization, and impurity removal achieving results comparable to conventional bleaching with reduced water/energy consumption and no chemical residues. Ozone chemistry: O₃ molecule unstable, decomposing to O₂ + nascent oxygen [O] (extremely reactive, oxidation potential +2.07V vs. H₂O₂ +1.77V), direct oxidation and radical pathway (•OH, •O₂⁻ formation in water). Generation: corona discharge ozone generators producing 2-12% O₃ in oxygen (air-fed or oxygen-fed, latter giving higher concentration/efficiency), dosing 10-50 g O₃ per kg fabric typical. Ozone bleaching—replacing or supplementing peroxide bleaching, O₃ in gaseous or dissolved form oxidizing colorants (gossypol, lignin, melanin) achieving whiteness 70-80 (comparable to peroxide 75-82), processing in gaseous ozone chamber (fabric exposed to humid O₃ atmosphere, 40-60°C, 10-30 min) or aqueous ozone bath (O₃ dissolved in water via diffusers, concentration 10-40 ppm, pH 6-8). Advantages: ambient temperature (room temperature processing vs. 90-100°C peroxide, energy saving 80-90%), no auxiliaries (no stabilizers, activators, sequestering agents required), rapid (10-30 min vs. 60-120 min), minimal effluent (ozone decomposes to oxygen leaving no residues vs. peroxide needing extensive washing), and improved hand (no alkaline swelling, softer result). Limitations: ozone generation cost (3-5 kWh per kg O₃, equipment investment $100,000-500,000), incomplete whiteness (single ozone treatment typically 70-75 vs. peroxide 78-82, requiring combination treatment for highest whiteness), fabric strength retention concerns (ozone potentially more aggressive than peroxide if overdosed, 5-10% strength loss vs. 3-5% peroxide, requiring careful control), and safety (toxic gas, 0.1 ppm exposure limit requiring exhaust treatment, monitoring). Ozone applications beyond bleaching: Denim finishing—ozone fading creating vintage looks replacing stone washing (eliminating stones, reducing process time from 45-90 min to 10-20 min, water saving 80%, no pumice disposal), indigo decolorization precise and uniform. Decolorization—removing color from dyed fabrics for recycling, ozone breaking chromophores (decolorization 80-95% in 30-60 min enabling fiber-to-fiber recycling, demonstrating circularity potential). Desizing—ozone oxidizing starch, PVA, synthetic sizes (supplementing enzymatic desizing, 30-50% faster). Commercial systems: Jeanologia (ozone fading systems for denim, laser-ozone combinations), Tonello (ozone generators integrated in garment processing machines), Absolute Ozone (industrial ozone generators for textile applications). Future: ozone-peroxide synergy (sequential or simultaneous application achieving whiteness >85 with 50% reduced peroxide, 40% reduced temperature), ozone catalysis (transition metal catalysts enhancing •OH generation from O₃ decomposition improving efficiency 50-80%), and ozone-ultrasound combination (cavitation improving O₃ dissolution/distribution, accelerating reactions 40-60%).