Study On The Process Of Electrochemical Reduction Of Vat Dyes

Vat dye is reduced by electrochemical reduction using clean energy instead of the traditional hydrosulfite.It is a development direction in the future due to the elimination of sulfate,sulfite waste liquor and sulfur dioxide pollution.Electrochemical reduction is divided into three types,including direct electrochemical reduction,indirect electrochemical reduction and electrocatalytic hydrogenation reduction.In this thesis,we studied the indirect electrochemical reduction because the highest current efficiency is only 80%and the reaction rate is limited by the low ion-radical concentration for the direct electrochemical reduction,as well as the cost is high,and the catalyst has shot life,low activity,conversion rate and current efficiency for the electrocatalytic hydrogenation reduction.The triethanolamine(TEA)and sodium gluconate(DGL)were selected as ligand for more detailed studies using dip dyeing process and pad dyeing process for the first time,because the low reducing ability and stability while the citric acid,tartaric acid,oxalic acid,hydroxy ethylenediamine triacetic acid and diethanolamine under alkaline condition during the indirect electrochemical reductionFirst of all,we used the Vat Yellow G as the dye,TEA as medium.The optimal formula was:NaOH 6 g/L,Fe2(SO4)3 4 g/L,TEA 9 g/L and U 3 V.The concentration of NaOH was reduced by 60%compared with the traditional sodium hydrosulfite dyeing process.However,the color depth,chroma and lightness were still the same,the washing fastness and rubbing fastness were 4-5 grades.Then,the color depth,chroma and lightness were still well when we operated electrochemical reduction for cotton fabric using Vat Blue RSN,Vat Red R and Vat Red FBB under the above condition.While the poor results were showed if we matched the dyes.In addition,we explored the pad dyeing process for the first time,the optimum condition was:NaOH 25 g/L,Fe2(SO4)3 6 g/L,TEA 9 g/L and U 3V,the results showed the excellent fastness and effect but relatively low color depth and chroma.In addition,we used the Vat Yellow GCN as the dye,DGL as medium.The optimal formula was:NaOH 24 g/L,Fe2(SO4)318 g/L,DGL 20g/L and U 6V.The color depth,chroma,lightness,washing and rubbing fastness were comparable with traditional method with the higher amount of chemical agents.It indicated the lower reducing capacity of DGL than that of TEA.Finally,we measured the content of ferric ions in dyeing fabric and found that it was hard to get rid of them by washing.We hypothesized that these iron ions may be associated with the cellulose molecules which could form stable complexes.The result showed that the lowest content was 81mg/Kg,the highest content was 218 mg/Kg under the condition of ferric sulfite was 0.015mol/L with different concentration of DGL;the lowest content of iron ions was 162mg/Kg,the highest content was 492mg/Kg when the concentration of ferric sulfite was 0.045mol/L at the same DGL concentration.The significance of this thesis were:firstly,the concentration of sodium hydroxide was reduced by 60%using Vat Yellow as the dye;we saved the cost and eliminated environmental pollution,accordingly.Secondly,the pad dyeing process of electrochemical reduction was studied for the first time,this could be a solid foundation for the continuous pad dyeing process.Finally,the first time for the content of iron ion in the fabric dyed during the process of electrochemical reduction was determined,it could help us clarify the fiber damage mechanism under the iron ion catalyzed hydrogen peroxide bleaching condition.