Study On Ester-Activated And Esterase-Catalyzed System For Low-Temperature Bleaching Of Cotton Knitted Fabrics

Traditional hydrogen peroxide（H₂O₂）bleaching of cotton fabrics in the textile industry is usually carried out under nearly boiling temperature and strong alkaline condition,which not only has high energy consumption and low efficiency,but also produces strong alkaline effluent and causes serious damage to cotton fibers.To solve this problem,researchers found that the addition of activator to the H₂O₂ system can activate H₂O₂ to form peroxyacid with stronger oxidation activity in situ,which makes the bleaching temperature of cotton fabrics significantly lower,thus achieving the purpose of energy saving and emission reduction.Based on the activation of H₂O₂ low-temperature bleaching system,this thesis firstly incorporated glycerol triacetate（GT）as an activator into the H₂O₂system for low-temperature bleaching of cotton knitted fabrics,and the factors of p H,H₂O₂ dosage,GT dosage,bleaching temperature and time on the bleaching effect of cotton knitted fabric were discussed separately,and the bleaching properties of H₂O₂/GT system were compared and evaluated.Secondly,the H₂O₂/GT system was optimized by response surface methodology,and the bleaching mechanism of the H₂O₂/GT system was studied by the detection of hydroxyl radicals（HO·）.The single-factor test results showed that the H₂O₂/GT system could bleach cotton knitted fabrics at a lower temperature and obtain the best WI at p H 8-9,and the CIE whiteness index（WI）of treated cotton fabric could reach 72.29 and the bursting strength loss rate was only 1.16%,and the p H of the system was nearly neutral after bleaching.The addition of excessive hydrogen peroxide had no superposition effect on the H₂O₂/GT system.The addition of sodium pyrophosphate（TSPP）as a stabilizer or not has a large impact on WI of cotton knitted fabric.Based on the single-factor experiments,the H₂O₂/GT bleaching system was further optimized by central composite design of experiments（CCD）combined with response surface analysis method（RSM）using whiteness as the response value.The CCD experimental data were fitted to create a response surface quadratic model（RSQM）,and the ANOVA data of this quadratic model indicated that temperature was the most important factor affecting whiteness,followed by time and GT dosage,while TSPP dosage was an unimportant factor.The lack-of-fit test analysis showed that the quadratic model is suitable and could accurately respond and predict the whiteness.The optimized bleaching process was obtained by the analysis results from the RSQM:GT 40 mmol/L;TSPP 2 g/L;temperature 70℃;time 40 min.By using a fluorescent probe for HO·detection,it was found that HO·concentration increased nearly 4 times in H₂O₂/GT system compared to H₂O₂system without adding GT.By adding DMSO as a scavenger of HO·to the bleaching process of H₂O₂/GT system,it was demonstrated that the HO·concentration was closely related to the WI of cotton knitted fabric.On this basic,it was speculated that the bleaching mechanism of H₂O₂/GT system contained two bleaching paths:one was peroxyacetic acid（PAA）generated in situ by the reaction of H₂O₂/GT system bleached impurities directly to enhance WI,and the other was that HO·and other bleaching active substances generated from the decomposition of PAA and then remove impurities.In addition,the bleaching performance of H₂O₂/GT system was further confirmed by simulated decolorization experiments with mulberry pigment.Based on the H₂O₂/GT system,using the combined chemical-enzyme catalytic process of esterase-catalyzed in-situ synthesis of peroxyacid for oxidizing various substrates,a low-temperature catalytic bleaching system of H₂O₂/GT/Esterase was constructed and its bleaching performance was investigated.The experimental results showed that the addition of esterase had a certain promotion effect on the bleaching performance of H₂O₂/GT/Esterase system.When cotton knitted fabric was treated at 70℃for 60 min,its WI was as high as 72.43%with almost no loss of bursting strength.Sodium bicarbonate（Na HCO₃）was the most ideal alkali agent for H₂O₂/GT/Esterase system.The effect of type of carboxylic acid ester on the bleaching of cotton fabric mainly depends on the number of ester carbonyl groups and the nature of catalytic products.By measuring the H₂O₂ decomposition rate and PAA concentration at each stage of cotton knitted fabric bleaching process,it was hypothesized that PAA is the main component that plays a bleaching role in the H₂O₂/GT/Esterase system.Studies have shown that the H₂O₂/GT system can bleach cotton knitted fabric under mild environment.Compared with the hot alkaline peroxide bleaching system,the H₂O₂/GT system obtained an equivalent WI with almost no damage to the fabric,while producing no alkaline effluent.Therefore,H₂O₂/GT system may provide a more environmentally friendly bleaching approach of cotton as alternative to conventional alkaline high-temperature bleaching.H₂O₂/GT/Esterase system can be carried out under low temperature and nearly neutral conditions,and the combination of the system with enzymes for textiles will help to realize the pretreatment process of biological enzymatic desizing,scouring and bleaching of cotton fabrics in one bath.