TBMembranesProcessingGuide

TechnicalBulletinSpecialtypolymerSProcessingGuideforPolymerMembranesOverthelastthirtyyears,theuseofporouspolymermembraneshasachievedasignificantpositionasacost-effectivemeansofnon-destructivelyseparatingcomponentsfromfluidmixtures.Membranetechnologypossessesanumberofattractivefeaturesinthatitcanbecarriedoutcontinuouslyundermildoperatingconditionsandrequireslittlespace.Itisalsoenvironmentallyfriendlyinthatenergyconsumptionisgenerallyverylowandfew,ifany,additivesarerequired.Itisbothacleaningandcleantechnology.Solvayofferstheindustry’sbroadestselectionofhigh-performancepolymersformembranes,including:• Solef®polyvinylidenefluoride(PVDF)• Halar®ethylenechlorotrifluoroethylene(ECTFE)• Udel®polysulfone(PSU)• Veradel®polyethersulfone(PESU)• Radel®polyphenylsulfone(PPSU)Thesematerialsareusedextensivelytomanufactureisotropicandanisotropicporoushollowfiberandflatsheetmembranesfortheentirerangeofthefiltrationspectrumfrommicrofiltration(MF)toreverseosmosis(RO).Dense-filmgaspurificationmembranesarealsoreadilypreparedwiththesematerials.ExcellentchemicalresistanceoveralargepHrange,hydrolyticstability,highstrengthandbroadagencycertificationsmakethesepolymerswell-suitedformembranesusedindemandingend-useenvironments.Typicalapplicationareasincludewaterpurification,wastewatertreatment,pharmaceuticalproduction,andbloodpurificationalongwithavarietyofindustrialprocessseparations,suchasfoodandbeverageprocessing,electropaintrecoveryandgasseparation.Manyoftheprocessesusedtomakeseparationmembranesrequiredissolvingthepolymerinasolvent.BecauseSolef®PVDF,Udel®PSU,Veradel®PESUandRadel®PPSUaresolubleinconventionalsolvents,theycanbesolventcastbytheDiffusion-InducedPhaseSeparation(DIPS)process.ThisprocessisalsoreferredtoasaNon-solvent-InducedPhaseSeparation(NIPS).TheDIPSprocessishighlysensitivetospinningprocessvariablesanddopesolutionvariability,makingitessentialtocloselycontrolboththemembranecastinganddopepreparationprocesses.Thiscanonlybeaccomplishedbyusinghigh-qualitypolymerswithminimallot-to-lotvariability.ThermallyInducedPhaseSeparation(TIPS)technologyisalsobecominganattractivemanufacturingmethodforporousmembranes.Temperatureprocessingallowsthepreparationofadefect-freemembranewithexcellentmechanicalperformance.Semi-crystallinepolymers,suchasSolef®PVDFandHalar®ECTFE,canbeprocessedbyTIPSduetotheirsolubilityinarangeofsolventsathightemperature.Thisdocumentprovidesusefulinformationaboutpolymerpropertiesaswellasguidelinesforpreparingdopesolutionsinordertoimprovethestabilityandefficiencyofyourmembranepreparationprocess.Solef®pVDFFluoropolymersPVDFisasemi-crystallinefluoropolymerobtainedbythepolymerizationofvinylidenefluoride.SolvaySpecialtyPolymersoffersawiderangeofPVDFpolymersthataremarketedundertheSolef®tradename.Solef®PVDFhomopolymersareusedformakingdurable,long-lastingmembranes.Theirtoughness,chemicalresistanceandoxidativestabilityallowmembranestotolerateawidevarietyoffeedstreamsandcleaningmethods.Thehighpurityofthesematerialsiswidelyrecognizedinotherspecialtymarketsegments,suchasthesemiconductorindustry,wherereleasedcontaminantsarenottolerated.InadditiontoPVDFhomopolymers,whicharerecognizedworldwidefortheirexcellentperformanceinmembraneapplications,Solvayhasdevelopedawiderangeofvinylidenefluoride-hexafluoropropylene(VF2-HFP)copolymersandvinylidenefluoride-chlorotrifluoroethylene(VF2-CTFE)copolymerstorespondtospecificneedsofvariousmarketsegments.Becausecopolymersarecharacterizedbylowercrystallinitythanhomopolymers,2\ProcessingGuideforPolymerMembranestheyhavehigherflexibilitythanhomopolymersandaresolubleinawiderrangeofsolvents,suchasketonesandtetrahydrofuran.Solef®PVDFpolymersarereadilysolubleintypicalsolventsthatareusedtomanufacturemembranes,andtheyofferawiderangeofdesirableproperties.Inparticular,Solef®PVDFsuspensionhomopolymersarewidelyusedinmembranefiltrationapplicationsduetoanumberofdesirablefeatures:• Excellenttoughnessanddurability• OutstandingchlorineandUVresistance• StabilityatpHlevelsfrom1to11• High-puritygrades• Globalagencyapprovals• EasytoformMFandUFmembranesSolef®PVDFhomopolymersencompassabroadrangeofmolecularweights;selectgradesareshowninTable1.Eachgradehasanarrowmolecularweightdistribution,whichmakesitpossibletocontrolviscositiesandeasiertofine-tunedopesolutionsaswellasmaximizeprocessstability.SolubilitySolef®PVDFpolymersaresolubleinaproticpolarsolvents,suchasdimethylformamide(DMF),dimethylacetamide(DMAC),N-methyl-2-pyrrolidone(NMP),triethylphosphate(TEP)anddimethylsulfoxide(DMSO).Solventpolarity,temperatureandpolymermolecularweightaffectthesolubilityofPVDF.ExamplesofbinaryphasediagramsofSolef®PVDFinNMPandTEPobtainedaccordingtoASTMD6038arereportedinFigures1and2.Additionaldatashowingsolubilityinothersolventsareavailableuponrequest.SolutionViscositySolutionviscositydependsonseveralparameters,includingtemperature,polymerconcentration,polymermolecularweightandthenatureofthesolvent.Figures3through7showcurvesofSolef®PVDFsolutionsmeasuredwithRheometerRheometricScientificRSFIIIinsteadyratesweepmodeusingconcentriccylindersgeometry.Co