Assembling plasma and ultraviolet radiation with cold-batch process towards a water and energy conservative cotton bleaching technology

Abstract

Cold pad-batch bleaching is an industrialized low-temperature cotton decolorization process, but the issues of low productivity, abundant chemical usage, and wastewater discharge limits its large-scale application. Therefore, an innovative process integrating plasma and UVA irradiations is introduced to cold pad-batch towards a conservation-oriented and highly-efficient bleaching target. Noteworthy, plasma and UVA irradiations play their respective role in bleaching, i.e., plasma induces an easily-wetted fiber surface for better contact with bleaching agents, while UVA boosts bleaching efficiency through catalyzing the decomposition of H₂O₂. Findings indicate that the combination of plasma and UVA radiations reduces the required H₂O₂ from 45 to 30 g/L, NaOH from 15 to 10 g/L, batch duration from 24 h to 1 h, and gets rid of penetrant. The polymerization degree of cotton is maintained due to the reduced chemical usage and shortened batch time. Single-factor experiment and central composite design prove that the UVA exposure time exhibits the highest impact on whiteness, and then comes the H₂O₂ and NaOH dosages, and plasma treatment duration. Optimal conditions (27.5 g/L H₂O₂, 12 g/L NaOH, plasma treatment for 180 s, UVA radiation for 85 min) results in a theoretical maximum whiteness of 80.5, making a 100 % improvement over the greige fabric. The mechanism study of plasma-assisted bleaching validates an increase in the number of oxygen-containing groups and a physically etched surface on cotton, which facilitates the infiltration of bleaching oxidants to completely degrade the color impurities. Demonstrated by life cycle assessment, the proposed bleaching technique exhibits a reduced environmental impact by saving chemical, energy, and water.