Preparation And Properties Of Polyvinylidene Fluoride Cation Exchange Membrane

The ion exchange membrane is a polymer membrane containing an ionic group and having a selective permeability to ions in the solution.As the core component of electrodialysis,electrodeionization,diffusion dialysis and other processes,ion exchange membranes are widely used in drinking water production,chlor-alkali production,wastewater treatment and other industries.However,at present,there are few kinds of ion exchange membranes produced in our country,and the majority of them in the market are still heterogeneous ion exchange membranes.Although the price of heterogeneous ion exchange membranes is cheap,the use range of heterogeneous ion exchange membranes is narrow,and the service life of heterogeneous ion exchange membranes is short.Homogeneous membrane with excellent properties are still dependent on imports.Polyvinylidene fluoride(PVDF)is a copolymer containing fluorine vinyl monomers.Compared with other commercial polymer membrane materials,it has excellent weather resistance,thermal stability,chemical stability and excellent mechanical properties.The preparation of polyvinylidene fluoride grafted styrene sulfonic acid membrane(PVDF-g-PSSA)is mainly composed of three steps: Alkalization,grafting and sulfonation.The process is simple,the cost is low,the reaction temperature is low,and the synthesis is fast.The oxidation resistance and selective permeability of polyvinylidene fluoride sulfonic acid membrane were studied.In terms of oxidation resistance,the three steps: alkalization,grafting and sulfonation of oxidation resistance was tested respectively,and the effect of sulfonation stage on the oxidation resistance of the membrane was determined.It is concluded that the position and quantity of sulfonic acid can not be controlled accurately,which leads to the decrease of crystallinity and loose structure of polyvinylidene fluoride sulfonic acid membrane,which is the essential reason for the decrease of oxidation resistance of polyvinylidene fluoride sulfonic acid membrane.Therefore,in order to improve the oxidation resistance of polyvinylidene fluoride sulfonic acid membrane,the sulfonation degree can be reduced properly when other properties of the membrane reach the standard.Subsequently,the effect of the degree of sulfonation on the selective permeability of the polyvinylidene fluoride sulfonic acid membrane was investigated.The selective permeability of the membrane was mainly affected by the density of the active group on the membrane and the pore size of the membrane.And the active group density of the membrane gradually increased at the initial stage of sulfonation,however,the introduction of sulfonic acid group will lead to loose structure and larger pore size of the membrane,which will lead to the decrease of the selective permeability.The various properties of the membrane affect each other,and it is impossible toachieve the best at the same time.Generally speaking,the larger the exchange capacity of the membrane,the greater the charge density in the pore,and the better of selective permeability,but the worse degree of association,leading to the loose structure and the poorer oxidation resistance of the membrane.Considering the sulfonation time as 5 hours,the ion exchange capacity at this time was 2.05 mmol/g,the chloride ion leakage rate was 11%,and the selective permeability was 89%.In terms of permeability,the self-made polyvinylidene fluoride sulfonic acid membrane has a permeability of 89% and a chloride ion leakage rate of 11%.Considering the timeliness of plasma treatment,the test uses low temperature plasma pretreatment for graft polymerization of styrene,it makes the surface of the membrane flat and uniform,and reduces the pores of the membrane to improve the selective permeability.The main influencing factors of plasma modification are plasma treatment time,graft monomer concentration,glow discharge power and gas atmosphere type.The results show that when the plasma treatment time is 3 min,the concentration of styrene monomer is 60%,the discharge power is 60 w,the effect of modification is the best,and the leakage rate of chloride ion is reduced from 11% to3%.Under the same conditions,the effect gas atmosphere type on the modification is further explored,and after the modification of the atmosphere of air,nitrogen and argon,the leakage rate of the chloride ions is 3%,2.8% and 2.5%,respectively,and the plasma modification effect in the argon atmosphere is best,however,the actual application of the modified atmosphere will still be air,considering the cost.The electron microscopy scan image before and after modification by polyvinylidene fluoride sulfonic acid membrane shows that the membrane before modification has many tiny pores under the condition of 1000 magnification,and the surface of the modified membrane is more compact and flat.This is due to chain free radicals formed on the surface of the membrane after low temperature plasma pretreat,and the interaction between chains and styrene monomers,which leads to cross-linking reaction and the formation of dense cross-linking layer on the surface of the membrane.This can effectively improve the chloride ion resistance of polyvinylidene fluoride sulfonic acid membrane.In order to test the practical application performance of polyvinylidene fluoride sulfonic acid membrane in membrane cobalt deposition,the laboratory simulated the field operation of the project,the four membranes:commercial heterogeneous membranes,commercial alloy membranes and PVDF-g-PSSA membranes modification before and after,which performance was studied in the experimental testAfter 48 h electrolysis,the modified PVDF-g-PSSA membrane has the best permeability.In terms of current efficiency,the modified PVDF-g-PSSA membrane need the highest tank voltage,but the current efficiency is also the largest.The current efficiency of other membranes are also over 90%,indicating the feasibility of the membrane cobalt deposition process.