A Study on Low-Temperature Pretreatment of Knitting Cotton Fabrics by Hydrogen Peroxide/Bleach Activator Systems and Bleaching Mechanism

Author:Liu Kai

Supervisor:yan ke lu


Degree Year:2019





Scouring and bleaching in one bath is a common pretreatments of cotton greige knitted fabrics,which can remove natural impurities of cotton including pectin,pigments and cotton wax,so that cotton fabric can gain satisfactory CIE whiteness index(WI)and wettability for the following procedures.Cotton bleaching via alkaline hydrogen peroxide is often carried out at nearly boiling temperature,which means large energy consumption.To save energy in cotton bleaching,many researchers have tried to add bleach activators to aqueous H2O2 solution,proving an effective way.Bleach activators are those chemical agents which can produce peracids in situ through reacting with HOOˉ.Peracids have better oxidative ability than that of hydrogen peroxide,which can lower bleaching temperature and thus reduce energy consumption in cotton bleaching.Now more attention has paid to the procedures and mechanism of some bleach activators including tetraacetylethylenediamine(TAED),nonanoyloxybenzene sulphonate(NOBS)and N-[4-(triethylammoniomethyl)-benzoyl]-caprolactam chloride(TBCC).But the poor biodegradability of these bleach actiators and insolubility of TAED have limited their wide applications.Hence,it is essential to exploit new,safe and biodegradable bleach activators.In addition,it is essebtial to investigate the bleaching species and mechanism of H2O2/bleach activator systems,which would provide theorical base for exploiting H2O2/bleach activator systems.Because of the issues aforementioned,this thesis has explored new H2O2/bleach activator systems and investigated bleaching mechanism of these systems.The thesis includes five following parts:The first part investigated the bleaching performance of pentaacetyl glucose(PAG)as a bleach activator as well as bleaching mechanism of H2O2/bleach activator systems.PAG is a sugar derivative with many environmental advantages.It has biocompatible,nontoxic,and renewable features and can be synthesized by green solvents and catalysts.The H2O2/PAG bleaching system can get good WI,H2O2 decomposition rate and bursting strength under low-temperature conditions.By using a fluorescent probe for HO·detection,it was found that HO·concentration increased by10 times in H2O2/PAG system in comparison to H2O2 system without PAG.Scavenging HO·with DMSO proved that HO·was one of the important bleaching species in H2O2/PAG system.On this basis,a bleaching mechanism of H2O2/PAG system was proposed.For the mechanism,it may include two routes.One route was that peracetic acid bleached impurities directly to enhance WI and another was that HO·generated from decomposition of peracetic acid and then removed impurities.In addition,we investigated the reason for higher concentration of HO·in H2O2/bleach activator systems by chemical calculation and computational analysis.The relative concentrations of hydrogen peroxide and peracetic acid as well as their ions under varied pH conditions were also calculated to investigate the generation of other bleaching species.At last,we further profectly explained bleaching species and bleching mechanism of H2O2/bleach activator bleaching systems.The second part investigated the bleaching performance acetylated starches as bleach activators.Starch is an abundant,cheap,nontoxic,biodegradable and natural renewable material.Besides,acetylated starch can be synthesized by enzymatic methods.The results proved successful synthesis of acetylated starches with different degree of substitution(DS).Meanwhile,the low-temperature pretreatment performances of H2O2/acetylated starche systems were evaluated.The result explained that higher DS acetylated starch can promote bleaching performance.To improve the dispersiblity,these acetylated starch samples have been treated by nanoprecipitation,and the diameter of acetylated starch is about 600 nm.Dispersions of acetylated starch are stable and can be used in the bleaching process.The third part investigated the bleaching performance of o-phthalic anhydride(PA)as a bleach activator.Most bleach activators focus on amide and ester types,but there is no report on anhydride one in the cotton bleaching.PA was developed as a low-temperature activator in the H2O2 bleaching system for cotton fabric.H2O2/PA bleaching system can improve WI of the bleached cotton fabric,H2O2 decomposition rate,wettability and bursting strength under low-temperatures.In addition,the generation of singlet oxygen and its effect on the whiteness were discussed in H2O2/activator bleaching systems.Although the H2O2/PA system can significantly promote 1O2 generation,the large amount of 1O2 did not obviously affect the bleaching performance.By HO·detection,it was found that the H2O2/PA system has two bleaching routes like H2O2/PAG system.The forth part synthesized tetraacetylhydrazine(TH)and investigated the bleaching performance of TH as a bleach activator.In the past 20 years,reserchers have focused on the bleaching performance and mechanism of TAED.However,the poor solubility of TAED has limited its wide use.Therefore,the development of new bleach activators with good solubility is meaningful for lowing bleaching temperature.TH has better solubility than that of TAED.During bleaching process,NaHCO3 was used as alkali agent instead of NaOH to reduce the pressure of waste water teatment.In addition,alkali catalyzed bimolecular decomposition of peracids was recognized as one of key reasons to cause the loss of bleaching performance,but this is an assumption and has not been testified directly in H2O2/activator bleaching systems.By using a fluorescent probe of singlet oxygen,bimolecular decomposition was investigated in H2O2/TH bleaching system,proving that it is not the main reason to cause the loss of bleaching performance.In fact,the hydrolysis of TH may be the main reason under strong alkali conditions.At the same time,1O2 concentration may be related to peracid concentration.The fifth part investigated the relationship between structures of bleach activators and bleaching performance.We have explained that the H2O2/bleach activator bleaching process is a two-step reaction for in cotton bleaching and the whiteness of the bleached cotton fabric is a comprehensive result of these two steps.The four parts above have investigated the second-step reaction about bleaching routes and species.However,the first-step also has many peoblems which should be solved in the future including the structure merits of amide-type and ester-type bleach activators,the relationship between structures of bleach activators and bleaching performance.If these problems can be solved,many new bleach activators with better properties could be designed.By investigating Hirshfeld charge and WI,compounds such as TAED,TBCC and TH,were studied as well as their bleaching products,proving that Hirshfeld charge can predict and explore new bleach activators.Activation energy,Gibbs free energy and HSP distance also can be used as tools to analyze bleaching effect of potential ester bleach activators.The results testified that the ability to generate peracids of ester bleach activators may be related to their hydrolysis stability.In this way,some unnecessary expenditures can be reduced in the process of molecular design of bleach activators.