APPLICATION OF ADVANCED OXIDATION PROCESS FOR BLEACHING TEXTILE MATERIALS

Abstract

This study discusses the application of ozone to bleach raw cotton fabric using both laboratory scale set up and pilot plant. The quality of ozone bleached fabric was determined in terms of CIE whiteness, absorbency, degree of polymerisation (DP), copper number (CN) and tendering factor (TF) with regard to the conventionally bleached fabric (reference material). The application of ozonation process was also extended for color stripping of dyed cotton fabric at laboratory scale. The efficiency of ozone stripping process was measured in terms of lightness (L*), lightness difference (∆L*) and total color difference (∆E*) with respect to reference (conventionally stripped fabric samples). In case of laboratory scale application of ozone bleaching process, the maximum CIE whiteness (60-62) and best quality of bleached fabric was obtained at an ozone dose of 6 g/h, pH 5 and exposure time of 45 min. Strong acidic environment (pH 2-3) and exposure time longer than 45 min though marginally improved the degree of whiteness but deteriorated the quality of the fabric as indicated by higher copper number (0.19) and TF (0.94). Statistical analysis of experimental data confirmed that process parameters (ozone dose, pH and treatment time) significantly affected the efficiency of ozone bleaching process and pH had the greatest effect on whiteness and DP of bleached fabric followed in turn by ozone dose and treatment time. The dyeing quality of ozone bleached fabric in terms of colour difference (∆L*, ∆a*, ∆b* and ∆E*) and wash fastness properties was almost similar to the reference. The ozone bleaching waterbath was also reused several times to bleach multiple lots of raw cotton fabric and results demonstrated that CIE whiteness (60) remained stable even at 20 th reuse of same waterbath. This approach reduced the pollution load of effluent and saved water and chemicals. Since the efficiency of ozone bleaching process at pilot scale was not in parallel with its laboratory scale performance, various additives were used to enhance process efficiency. The best results in terms of whiteness (63.79), absorbency (10 sec) and strength of bleached fabric were achieved with an addition of 2 g/l of surfactant at optimal process conditions (ozone dose of 50 g/h, pH 5 and ozone treatment time of 45 min). However, other additives (peracetic acid and H2O2) did not show promising results. Dyed ozone bleached fabric samples were a bit lighter (ΔL* > 0.4) than reference. The values of ΔL*, Δa* and Δb* showed minor color difference between ozone bleached fabric and reference. However, total color difference values (ΔE* = 0.13 – 1.06) were within acceptable limit (ΔE*<1). Wash fastness properties of ozone bleached fabric were identical to the reference. The whiteness of ozone bleached fabric, however, did not stand firm with increasing storage time due to the presence of aldehyde groups and ozone residues. Among various treatments performed (hot washing at 50 ◦C, rinsing at 80 ◦C, reductive treatment) to avoid the decrease in degree of whiteness of ozone bleached fabric, washing with sodium borohydride was proved as one of the best treatments for color stability. Ozone color stripping efficiency was a function of pH, ozone dose and treatment time and maximum color stripping was achieved at an ozone dose of 10 g/h, pH 5 and treatment time of 45 min. The stripping efficiency decreased as the % owf of dyed fabric increased from 2% to 4% which implied that deceolorization of dyed fabric with higher initial dye concentration required higher ozone dose and/or longer exposure time. The color stripping efficiency of ozone for fabric samples dyed at different shades was almost similar to the reference (conventional stripping method). The ozone bleaching process was proved to be environmentally friendly because it decreases water consumption (no rinsing and neutralizing treatment), it is less energy intensive (applicable at ambient temperature), and reduces effluent load (involves no chemicals).