Patent Application: US-59738208-A

Abstract:
the present invention provides a method for improving the performance of polyimide membranes as used in solvent - resistant nanofiltration . more specifically the method of the present invention allows to improve the solvent stability of the polyimide membranes to solvents or solvent mixtures that would dissolve polyimide under the conditions applied during the filtration , such as dimethylforrnamide , n - methylpyrrolidinone , dimethylacetamide , tetrahydrofuran , y - butyrolacton , dimethylsulphoxide and chlorinated solvents .

Description:
fig1 : chemical structure of the commercial polyimide matrimid ® ( huntsman ) fig2 : generalized chemical structure of the commercially available polyimide under the trade name lenzing ® p84 in a first object the present invention provides a method for the modification of an ultra - or nanofiltration membrane comprising polyimide within its selective layer in order to increase the resistance of the membrane against organic solvents , while maintaining its permeability , said method comprising the cross linking of the polyimide using an amino - compound . preferably this cross linking step is followed by subjecting the cross - linked membrane to a solvent exchange procedure , where after the membrane is optionally dried . for cross linking the polyimide membrane it is preferred to use amino compounds selected out of the group consisting of cyclohexylamine , p - xylylene diamine , 1 , 2 - diaminoethane , 1 , 6 - hexane diamine , 3 - aminopropylmethyldiethoxysilane , tris ( 2 - aminoethyl ) amine , triethylenetetramine , pentaethylenehexamine , polyethylenimine , polyether diamines based predominantly on a polyethylene oxide backbone with a molecular weight of 50 to 20 , 000 , trimethoxysilylpropyl - substituted polyethyleneamine having a molecular weight of 200 to 200 , 000 , polyethyleneamine having a molecular weight of 1 , 000 to 200 , 000 , aqueous ammonium hyroxide , and isobutyl amine . preferably , the cross - linking of the polyimide membrane is obtained by completely or partially immersing the membrane in a solution comprising a suitable amine compound for an appropriate period of time . as one skilled in the art will appreciate , the rate of reaction between the amino - compound and the imide - containing material will vary strongly depending on their chemical identity and the process conditions . long reaction times or even curing at elevated temperatures may be necessary , depending on the chemical compositions of both the polyimide as the amino compound used for cross - linking . in an embodiment of the present invention the amine compound is dissolved in methanol , however , other solvents can be used , which solubilise the amine compounds without dissolving the uncrosslinked polyimide polymer or negatively affecting the cross linking reaction . examples of such alternative solvents are ethanol and isopropanol amongst others . upon termination of the reaction it is preferred that the cross linked polyimide membrane is thoroughly rinsed with methanol to remove all reacting compound from the membrane . thereafter , the membrane can be subjected to a solvent - exchange procedure in for instance glycerol - containing isopropanol before drying , to prevent pores from collapsing , which may have a negative effect on permeability in pressure - driven filtrations . in a particular embodiment the solvent exchange procedure comprises the immersion of the membrane in an isopropanol bath for at least two hours , followed by an immersion in a isopropanol - glycerol bath ( typically 60 - 40 ) for at least three days . preferably the method according to the present invention is used in the modification of membranes comprising more than 30 %, more preferably more than 60 %, most preferably more than 90 %, for instance more than 95 % of polyimide polymer in the selective layer as a fraction of the total polymer in the selective layer . in a preferred embodiment the method according to the present invention is used in the modification of membranes wherein polyimide is the only polymer in the selective layer of the membrane . in another preferred embodiment the polyimide comprised in the selective layer of the membrane does not comprise pendant carboxylic acid functions or siloxane . membranes suitable for modification according to the method of the present invention can be obtained by casting a polyimide - containing polymer solution . casting of the membrane may be performed by any number of casting procedures cited in the literature , for example u . s . pat . nos . 3 , 556 , 305 , 3 , 567 , 810 , 3 , 615 , 024 , 4 , 029 , 582 and 4 , 188 , 354 ; gb - a - 2 , 000 , 720 ; office of saline water r & amp ; d progress report no . 357 , october 1967 ; reverse osmosis and synthetic membranes , ed . sourirajan ; murari et al , j . membr . sci . 16 : 121 - 135 and 181 - 193 ( 1983 ). thus , the polymer or its derivatives may be dissolved in a suitable solvent or solvent mixture ( e . g . nmp and thf ), which may or may not contain cosolvents , partial solvents , nonsolvents , salts , surfactants or electrolytes , for altering or modifying the membrane morphology and its flux and rejection properties . the casting solution may be filtered by any of the known processes ( e . g . pressure filtration through microporous filters , or by centrifugation ), and cast on a support such as glass , metal , paper , plastic , etc ., from which it may then be removed . it is preferred , however , to cast onto a porous base support from which the membrane is not removed . such porous base supports may be non - woven , or woven , including cellulosics , polyethylene , polypropylene , nylon , vinyl chloride homo - and co - polymers , polystyrene , polyesters such as polyethylene terephthalate , polyvinylidene fluoride , polytetrafluoroethylene , polysulfones , polyether sulfones , poly - ether ketones , polyphenylene oxide , glass fibers , porous carbon , graphite , inorganic membranes based on alumina and / or silica ( possibly coated with zirconium and / or other oxides ). the membrane may otherwise be formed as a hollow fiber or tubelet , not requiring a support for practical use ; or the support may be of such shape , and the membrane is cast internally thereon . the concentration of polymer in the casting solution may vary as a function of its mw and additives , and may be for example , within the range of about 5 - 50 %, preferably about 10 - 50 %, most preferably about 15 - 30 %. the casting temperature may vary from about - 20 to about 100 ° c ., preferably about 0 to 60 ° c ., depending on the particular polymer , its molecular weight and the cosolvents and additives in the casting solution . the modification according to the method of the present invention results in ultra - and nanofiltration membranes with improved stability and high permeabilities in organic solvents and good rejection for low - molecular weight compounds between 200 - 2000 da , for instance between 200 and 700 da . the improved stability of the polyimide membranes obtained according to the present invention is particularly useful when the membranes are used in separation processes in organic solvents and more particularly in aprotic solvents such as dimethylformamide ( dmf ), n - methylpyrrolidinone ( nmp ), dimethylacetamide ( dmac ), tetrahydrofuran ( thf ), γ - butyrolacton ( gbl ), dimethylsulphoxide ( dmso ) and chlorinated solvents . the invention is further illustrated by way of understanding non - limiting examples . membranes have thus been created by treating matrimid 9725 ( huntman ) polyimide membranes with a solution of p - xylenediamine in methanol ( 10 % w / v ) for different times up until 2 hours . the membranes showed high solvent fluxes (˜ 3 l / m 2 bar h ) and retentions of bengal rose (˜ 98 %) and methylorange (˜ 80 %) in dmf , and were found to be stable in dmac , dmso and nmp . the results demonstrate the surprising efficacy of the chemical modification of a polyimidemembrane according to the method of the present invention . an 18 wt % solution of matrimid 9725 polyimide ( huntsman ) was made in nmp and thf ( ratio 2 : 1 ) by stirring overnight . the polymer solution was cast on a polypropylene non - woven support by an automated casting device set at a gap of 250 μm . the resulting film was immersed in a de - ionized water bath after 30 s of evaporation . parts of the resulting membrane were immersed in a solution of p - xylenediamine in methanol for cross - linking . after 5 , 60 and 120 minutes , the membrane slabs were removed and rinsed with methanol to remove all reactant . the membranes were then immersed in ipa until use for immersion experiments . parts of the cross - linked membranes were immersed in dmso for several days , after which they were again immersed in ipa until they were used for filtrations . filtrations were carried out in a stainless steel dead - end filtration cell , pressurized with nitrogen gas to 6 bars , with a solution of bengal rose in wa ( 35 μm ) on top of the cross - linked membranes , before as well as after their immersion in dmso . uncross - linked membrane slabs were immersed in dmso in which they dissolved after some hours . the immersion / filtration tests show that cross - linked membranes retain there excellent performance after immersion in dmso . membranes were prepared and cross - linked as in example 1 . cross - linked membrane slabs were immersed in nmp for several days . uncross - linked membrane slabs were immersed in nmp in which they dissolved after some hours . filtrations were carried out as in example 1 . the results show that 60 minutes or longer cross - linked membranes retain their excellent performance in wa after immersion in nmp . the first result proves the non - obvious success of a chemical modification to a membrane . membranes were prepared and cross - linked as in example 1 , but an nmp - exchanged clear solution ( nmp - cs ) was added as an extra component to the polymer casting solution . this nmp - cs emulsifies the polymer dope before casting , which was further treated as in example 1 . during this modified phase - inversion process called ‘ solidification of emulsified polymer dope by phase inversion ’ or ‘ seppi ’ ( gevers , 2006 ), a membrane is created with uniform spherical pores that is more resistant to compaction at high pressures . membranes were further treated as in example 1 . cross - linked membrane slabs were immersed in dmf . uncross - linked slabs that were immersed in dmf dissolved completely after some hours . filtrations were carried out as in example 1 . results show that a cross - linking treatment of 60 minutes or more is sufficient to create membranes stable in dmf , which retain their excellent performance in ipa after immersion in dmf . the first result proves the non - obvious success of a chemical modification to a membrane . membranes were prepared and cross - linked as in example 1 . cross - linked membrane slabs were immersed in dmac . uncross - linked membrane slabs that were immersed in dmac dissolved after some hours . filtrations were carried out as in example 1 . results show that the cross - linking treatment creates membranes that are stable in dmac , which retain their excellent performance in ipa after immersion in dmac . polyimide membranes of different compositions that were previously optimised for their use in ipa were prepared and cross - linked for 60 minutes as in example 1 . membranes with excellent fluxes and high rejections for bengal rose in dmf were obtained . polyacrylonitrile membranes may loose their mechanical stability under standard cross - linking conditions and they become fragile . cross - linking at temperatures close to the t glass (˜ 79 ° c .) of polyacrylonitrile or cross - linking for extended time or with highly - concentrated base solution but at lower temperatures can lead to a plastification of the membrane . a 19 % solution of lenzing p84 polyimide was made by dissolving the polymer powder ( evonik ) in nmp / thf mixtures ( ratio 5 : 1 ). the homogenised polymer solution was cast on a polypropylene / polyethylene non - woven backing by an automated casting device set at a gap of 250 μm . the resulting film was immersed in a de - ionized water bath after a 60 s evaporation time . the resulting membranes were immersed for 24 h in a solution of p - xylenediamine in methanol for chemical cross - linking , rinsed with methanol and posttreated involving immersion in 2 - propanol and a toluene / 2 - methyl - 4 - pentanone / mineral oil solution ( volume ratio 40 / 40 / 20 ). the membranes were used for the recycling of cu ( i ) catalysts from click - chemistry reactions performed in dmf and thf . reaction products should be retained , while pure solvent containing the cu ( i ) catalyst should permeate to be re - used in a next reaction . membranes were mounted in a dead - end , stainless steel filtration cell with an active membrane surface of 15 . 2 cm 2 , and sealed with kalrez ® o - rings . prior to filtration of a reaction mixture , membranes were pre - conditioned by a filtration with pure dmf . feed solutions ( generally 20 ml ) were stirred and pressurized to 10 bar with nitrogen gas . a 3 . 36 l / h . m 2 . bar permeability was measured with 86 . 8 % of the total cu - catalyst retrieved in the permeate . 1h nmr semi - quantitatively demonstrated the presence of some traces of the click reaction product in the permeates . aerts , s ., a . vanhulsel , et al . 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