Abstract:

Objective The annual production of waste textiles is enormous, and the existing treatment routes such as chemical incineration and physical landfill are commonly employed to deal with waste textiles. These methods not only severely pollute the environment but also result in significant resource waste. Therefore, recycling waste textiles is crucial for conserving resources and reducing pollution. Color stripping is a prerequisite for recycling the colored waste textiles. However, conventional color stripping processes are characterized by high energy consumption, low efficiency, and operational complexity. This research aims to develop an environmentally friendly color stripping process that is both low in energy consumption and easy to operate as an alternative to the conventional color stripping methods.

Method A reduction-oxidation stripping system, employing thiourea dioxide (TD) as the reducing agent and sodium persulfate (Na₂S₂O₈) as the oxidizing agent, was used to strip the dyed cotton fabrics colored with Reactive Blue KN-R. The influences of stripping agent concentration, alkali concentration, stripping time, and temperature on the color stripping rate and the strength retention rate of the fabric were investigated. After stripping, the K/S values of the fabrics were measured using spectrophotometers to determine the color stripping rate. The strength retention rate was measured using an electronic fabric strength tester. All samples were measured three to five times to obtain an average value.

Results In the reductive color stripping process, the concentration of NaOH significantly affected the removal of dye molecules. When the concentration of NaOH was increased from 1 g/L to 6 g/L, the stripping rate increased from 73.09% to 86.95%. Excessive dosage of TD resulted in slight decreases in both color stripping rate and strength retention rate, indicating that excessive TD concentration is not conducive to colorant removal. The color stripping rate increased with time but showed minimal improvement beyond 20 min. Higher temperatures improved the color stripping rate but also caused a notable decrease in strength retention rate. When the temperature of the stripping system reached 70 ℃, after stripping for 20 minutes, the fabric strength retention rate dropped below 90%. In the oxidative stripping process, the stripping rate was higher than 90.00% with the dosage of NaOH being more than 1 g/L. A gradually increasing tendency was also obtained as the concentration of the oxidant Na₂S₂O₈ being increased from 1 g/L to 6 g/L. Similarly, the color stripping rate was increased within 20 minutes and stabilized afterward. Higher temperature also increased the color stripping rate but damaged the cotton fabrics, reducing the strength retention rate. After TD reduction stripping and Na₂S₂O₈ oxidation stripping, the fabric color changed from blue to purple and eventually to white. Additionally, the sequential reduction-oxidation stripping system effectively stripped fabrics dyed with other reactive dyes. For fabrics dyed with Reactive Red M-3BE, Reactive Red X-3B, Reactive Blue KN-G, and Reactive Blue M-BE, the initial reductive color stripping rates were 91%, 93%, 64%, and 78%, respectively. The subsequent oxidative stripping further improved the rates to 97%, 98%, 83%, and 96%, respectively.

Conclusion The sequential stripping system of TD reduction and Na₂S₂O₈ oxidation can serve as an alternative to conventional color stripping methods. For the cotton fabrics dyed with Reactive Blue KN-R, the stripping rate can reach 96% under low-temperature conditions, while the strength retention rate remains above 90%. The optimized TD reductive stripping process includes a thiourea dioxide concentration of 2 g/L, NaOH concentration of 6 g/L, stripping time of 20 minutes, and stripping temperature of 60 ℃. The optimized Na₂S₂O₈ oxidative stripping process includes a sodium persulfate concentration of 6 g/L, NaOH concentration of 4 g/L, stripping time of 20 minutes, and stripping temperature of 60 ℃. Compared to traditional color stripping processes, the new method causes less fabric damage, uses fewer stripping agents, requires shorter stripping times, and operates at lower temperatures. Moreover, the sequential reduction-oxidation stripping system is applicable to a wide range of reactive dyes.

Key words: cotton fabric, color stripping, strength retention rate, thiourea dioxide, sodium persulfate, reduction-oxidation stripping system, recycling