Abstract:

Objective As a healthy and environmentally friendly cotton fiber product, cotton nonwoven fabrics have been widely applied in various fields. All natural cotton fiber contains pigment impurities and needs to be scoured and bleached. Hydrogen peroxide (H₂O₂) bleaching is usually carried out in high-temperature and strong-alkali environment. The H₂O₂/activator system can effectively reduce the bleaching temperature and alkalinity. However, partial activators possess ecotoxicity. This research aims to explore for cleaner and more effective scouring and bleaching methods.

Method Firstly, the cotton nonwoven fabric was pre-treated with a plasma cleaning machine for 5 min. Subsequently, the fabric was immediately immersed in H₂O₂/PAG bleaching solution for treatment at 70 ℃ for 30 min in a constant-temperature dyeing machine. After the treatment, the fabric was squeezed and laundered thoroughly before being air-dried. The solution mentioned above contained H₂O₂, PAG, NaHCO₃, and EDTA-2Na. The fabric's whiteness and hydrophilicity were tested to evaluate the bleaching and scouring performance, respectively. The influences of plasma treatment on the chemical structure of cotton fiber were analyzed using X-ray photoelectron spectrum, scanning electron microscopy, and X-ray diffraction.

Results For achieving comparable bleaching effect via the NaOH and H₂O₂ routes, the H₂O₂/PAG system was able to reduce the bleaching temperature by 20 ℃ and to maintain the pH value at 5-7. On this basis, the introduction of plasma was shown to further reduce the H₂O₂ concentration by 50% or shorten the bleaching time by half, and the degree of polymerization was decreased by 3.6%. Plasma treatment significantly improved the hydrophilicity of cotton nonwoven fabrics. Specifically, the contact angle of water droplets reduced from 106.7° to 38.1°, and the wicking height within 30 min increased from 0 to 5.7 cm. Analysis of the X-ray energy spectrum indicated that both the content of oxygen elements and the number of polar oxygen-containing groups on the fiber surface increased after plasma treatment. Some grooves on the plasma-etched fibers were observed by means of scanning electron microscope. The results of the X-ray diffraction test showed a reduction in the cotton fiber's crystallinity by 13.4% compared with the untreated original fiber. The results of single-factor experiments demonstrated the following. Prolonging the plasma treatment time had little effect on the whiteness and yellowness of the nonwoven fabrics, but it evidently enhanced their hydrophilicity. An increase of H₂O₂ and PAG concentration produced more peracetic acid, contributing to an increase in whiteness and a decrease in the pH of the solution. The pH medium created by 30 mmol/L NaHCO₃ was suitable for the complete perhydrolysis of PAG, resulting in a near-neutral environment and desired bleaching results. Temperature was a significant factor affecting the whiteness of bleached fabrics. Fabrics' yellowness increased instead when the bleaching temperature exceeded 70 ℃. The whiteness showed a rapid increase within the scouring and bleaching duration of 0 to 30 min, indicating the high efficiency of the plasma/H₂O₂/PAG system. Further extension of time did not result in significant whiteness improvement.

Conclusion Approximate whiteness enhancement is obtained from plasma/H₂O₂/PAG and H₂O₂/NaOH system. However, 50% H₂O₂ concentration, 20 ℃ temperature, or 50% bleaching duration can be saved when using the front system. Moreover, plasma/H₂O₂/PAG system can provide a near-neutral pH environment. Thanks to the mild bleaching environment, the polymerization of cotton fiber only decreases by 3.6%. Plasma treatment increases the number of oxidation groups on the surface of the cotton fiber, induces physical etching on its surface, and improves the proportion of the amorphous zone. Above changes in fiber's structure achieves an apparent improvement in hydrophilicity of cotton non-woven fabric. As a result, the penetration and oxidation efficiency of the bleaching agent towards pigment impurities are correspondingly improved. When the optimal process parameters are applied (with concentrations of H₂O₂, PAG, and NaHCO₃ at 30, 7.5, and 30 mmol/L respectively, and plasma treatment for 5 min, followed by 70 ℃ bleaching for 30 min), the whiteness of the fabric can reach 82.1%, which is 23.8% higher than untreated fabric.

Key words: plasma, low-temperature scouring and bleaching, cotton nonwoven fabric, hydrogen peroxide, functional textile