Research On Ozone Bleaching Process And Mechanism Of Hemp Fabric

In the bleaching process of hemp fabrics,sodium hypochlorite and sodium chlorite are the most commonly used bleaching reagents,which can effectively remove lignin and other colored impurities from hemp fibers.However,this process consumes a lot of energy and water,and there are AOX problems in chlorine-containing bleaching agent,which will bring great harm to the environment and personnel.Therefore,it is imperative to develop a new green bleaching technology.In recent years,ozone has been successfully applied in the pulp bleaching process of paper industry,which provides a reference for the bleaching treatment of hemp fabrics.To research the effect of ozone bleaching on hemp fabrics,linen fabric was chosen as the research object and the effect of various process parameters such as bleaching in ozone(gas phase)or in ozonated solution(liquid phase),bleaching time,water pick-up value and bleaching pH value on ozone bleaching of linen fabric was studied.The results show that the optimum parameters of ozone gas-phase bleaching of linen fabrics are as follows: 20% water pick-up value,room temperature,pH 7 and 20 minutes;and the optimum parameters of liquid-phase bleaching are: room temperature,pH 7 and 30 minutes.Desizing and scouring pretreatment can significantly improve the ozone bleaching effect of linen fabrics,and the efficiency of ozone gas phase bleaching is better than ozone liquid phase bleaching.However,compared with traditional bleaching process,ozone bleaching effect still needs to be further improved.Because the bleaching effect of hemp fabrics was mainly attributed to the removal of lignin,the lignin in jute fiber is taken as the research object in this experiment to further reveal the mechanism of ozone bleaching.The effect of ozonation on the chemical structure of isolated lignin was studied by Fourier transform infrared spectroscopy,gel permeation chromatography,elemental analysis and nuclear magnetic resonance.The results show that the degradation of lignin by ozone may occur only on the surface of lignin particles.The aromatic ring structure of lignin will be destroyed forming small molecular aliphatic compounds which can be dissolved in water.Pyrolysis analysis showed that the relative content of guaiacyl/syringyl structure of lignin after bleaching changed from 37:61 to 41:58.The relationship between the changes of lignin structure and NMR chemical shifts at the qualitative and quantitative levels was discussed by nuclear magnetic analysis(1H,HSQC)technology.Based on the theoretical calculation method in quantum chemistry,the reaction between H,G,S lignin units and ozone was studied to explore the mechanism of ozone oxidation of linen lignin.The theoretical calculation includes the optimization of the spatial structure of lignin units,the analysis of reaction sites,the electronic energy and the change of molecular space structure on the reaction path and the effect of functional groups on the reaction energy barrier.The results show that in the aromatic ring region,ozone will react with the aromatic ring in perpendicular direction to the ? electron plane;in the side chain region,ozone will react with C=C in perpendicular direction to the plane formed with double bond and connected hydrogen atom.Energy analysis shows that the electrophilic addition reaction between lignin unit and ozone was exothermic,and the addition reaction barrier on side chain is lower than aromatic ring addition.The results of functional group analysis show that the substitution of methoxy and hydroxyl groups in aromatic rings will activate ortho-and paraaddition and inactivate the meta-addition.