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附录Water-solublebinderwithhighflexuralmodulusforpowderinjectionmoldingMINSEOKSONG,MINSOOPARK,JINKONKIM∗NationalCreativeResearchInitiativeCenterforBlockCopolymerSelf-AssemblyandDepartmentofChemicalEngineering,ElectronicandComputerEngineeringDivisions,PohangUniversityofScienceandTechnology,Kyungbuk790-784,KoreaILBUMCHO,KYUNGHOKIM,HWANJINSUNG,SANGHOAHNResearchInstituteofScienceandTechnology,Pohang,Kyungbuk790-784,KoreaE-mail:jkkim@postech.ac.krAnewbindersystem,containingternarypolymerblendsofpoly(methylmethacrylate)(PMMA),celluloseacetatebutyrate(CAB)andpolyethyleneglycol(PEG),wasintroducedforthepowderinjectionmoldingofstainlesssteelpowders.TheweightfractionofPEGinthebinderwasmaintainedtobe0.65.WhentheweightfractionofPMMAinthebinderwas0.1–0.25(thustheweightfractionofCABwas0.1–0.25),thefeedstockswith56vol%ofsteelpowdersweresuccessfullyinjectionmoldedwithoutshowinganyjetting.Furthermore,thefeedstockshavemuchhigherflexuralmoduluscomparedwiththosepreparedbyabinderconsistingonly35/65(wt/wt)CAB/PEGblend.Threebinaryblends,PMMA/CAB,PMMA/PEG,andCAB/PEG,werecompletelymiscibleataninjectionmoldingtemperatureof130◦C.Theextractionprocessemployedinthisstudybecomesenvironmentallyfavorableduetothemajorcomponent(PEG)ofthebinderwhichiseasilyextractedbyasolventofwaterorethanol.Also,theshapemaintenanceduringthesolventextractionwasexcellent,andfinalsinteredpartshadexcellentdimensionalstability.Duetohighflexuralmodulusoffeedstocks,thisbindersystemcanbeemployedforpreparinglargesizedinjection-moldarticles._C2005SpringerScience+BusinessMedia,Inc.1.IntroductionPowderinjectionmolding(PIM)processingofmetalsorceramicshasbeenemployedtodeveloptinyandsophisticatedmetalorceramicparts,anditisaveryeconomicprocesscomparedwithacastingandamachining[1,2].AmongthemanystepsinthePIM,thesuitablechoiceofabinderbecomescrucialtodeterminethemechanicalpropertiesofthefinalproduct.Wax-basedbinderscontainingabackbonematerialofpolyethyleneorapoly(ethylene-ran-vinylacetate)havebeen\\widelyusedinthepowderinjectionmoldingindustries[3,4].However,organicsolventsuchashexaneandheptane,whichisnotanenvironmentallyfavorablesolvent,shouldbeusedtoextractthewax.Toavoidusinghexaneorheptaneduringtheextraction,effortshavebeenfocusedonfindingnewtypesofbindersthatwouldberemovedbyanenvironmentallyfavoredsolventsuchaswater[5–11].Daviesandcoworkers[7,8]introducedapoly(methylmethacrylate)/poly(ethyleneglycol)[PMMA/PEG]bindersystemforstainlesssteelpowders.Althoughaveryhighmolecularweight(∼1,000,000)ofPMMAwasused,thebinderwasmixedwithsteelpowderinthepresenceofwater,becausewaterisasolventoflowermolecularweight(<1500)PEGandanemulsionmediumofPMMA.However,withoutusingemulsioncontainingwaterforPMMAparticles,abinderconsistingofPMMAandPEGmightnotbemixedeffectivelywithsteelpowders.Previously,weintroducedawater-solublebinderconsistingofcelluloseacetatebutyrate(CAB)andvariousmolecularweightsofPEG.WefoundthatthecrystallizationofPEGdependinguponthemolecularweightaffectedprofoundlytheshapemaintenanceduringsolventextraction[12,13].Namely,whenthemolecularweightofPEGwaslargerthan3400,crackswerefoundingreenpartsafterthesolventextraction.AlthoughbindersofCAB/PEGwithmolecularweightlessthan2000couldbeinjectionmolded,jettingresultingfromlowviscosityofPEGwassometimesobserved.Also,mechanicalpropertiessuchasflexuralstrengthofgreenpartsarepoor,whichrestrictsonetomakinglargesizedinjection-moldedarticles.Inordertoovercomethesedisadvantages,weaddedPMMAtoCABasthebackbonematerial.TherationalbehindchoosingPMMAisthat(i)PMMAismisciblewithbothCABandPEGataprocessingtemperatureof130◦C；(ii)itshowsgoodadhesionwithmetalpowderduetohavingapolargroup,whichpreventsbinderseparationduringinjectionmolding；and(iii)itprovidesgoodmechanicalpropertiesofgreenparts.WecouldusehighermolecularweightofPEGupto8,000whenPMMAwasemployed.AbinderconsistingofPMMA/CAB/PEGternaryblendsexhibitedhigherflexuralmodulusandsuitablerheologicalpropertiesforinjectionmolding；thusalargesizedinjectionmoldedarticlewassuccessfullyprepared.Also,weobservedthattheshapemaintenanceduringtheextractionwasexcellent,andfinalsinteredpartshadexcellentdimensionalstability.2.Experimental2.1.MaterialsPMMAwasalsokindlysuppliedbyLGMMACo,andthemolecularweightandpolydispersityofPMMAweredeterminedbyusinggelpermeationchromatography(WatersCo.)withPMMAstandards.CABwith54mol%butyratesidegroupwaspurchasedfromAcrosCo.andPEGwithdifferentmolecularweightswaskindlysuppliedbyKoreaPolyolCo.(Korea).ThemolecularcharacteristicsofthesematerialsaregiveninTableI.CABandPMMAareamorphousmaterialswithaglasstransitiontemperatureof94◦Cand100◦C,respectively,measuredbyDSC(Perkin-ElmerDSC7series)ataheatingrateof20◦C/min.Themetallicpowderwasstainlesssteel(17-4PH,MitsubishiSteelMFGCo.)havingirregularshapeandamedianparticlesizeof8.86μm.2.2.SamplepreparationandrheologicalpropertiesThefeedstock,amixtureofthebinderandmetallicpowder,wasfirstpreparedbymixingat130◦Cfor1hbyusingtheKneadingBladeTypeMixer(IKAVISCCo.)；thenitwasshapedintoarectangularbar70.22×7.12×3.57mm(L×W×T)byusinga180tonprecisioninjectionmoldingmachine(SodickPlustechCo.；Japan)at130◦Candmoldtemperatureof25◦C.AnAdvancedRheometricsExpansionSystem(ARES)usingthedynamicoscillatorymodewithparallelplatefixture(25mmdiameter)wasusedtomeasuretheviscosityofthebinderitself.Thestrainamplitudewas5%,whichliesinlinearviscoelasticityrange.TABLEIMolecularcharacteristicsofblendcomponentsemployedinthisstudyMnPolydispersityindexTgorTm(◦C)PMMA63.0001.47Tg=100CAB11,0002.3Tg=94PEG-11,5001.15Tm=43PEG-22,0001.10Tm=50PEG-33,3501.14Tm=54PEG-88,0001.16Tm=562.3.TurbidityexperimentofPMMA/CABblendVariousblendcompositionsofPMMAandCABforturbiditytemperature(Tb)measurementwerepreparedbydissolvingapredeterminedamountofthemixtureintoluene(10wt%)andslowlyevaporatingthesolventover8hatroomtemperature.Thesamplethicknesswas10μm.Eachspecimenwasannealedinthehomogeneousstate(∼120◦C)for4h.TheTbofeachcompositionwasmeasuredfromthesuddenincreaseinlightscatteringintensityatascatteringangleof30◦uponheatingthemixturefrom120◦Catarateof0.1◦C/min.OncetheTbofaspecimenwasestimated,theexactTbwasdeterminedbythenakedeyeuponatemperatureincrementby0.5◦Cnearthetemperatureofinterest.Thespecimenwasheldateachtemperaturefor1h.ThemaximumerrorinthevaluesofTbdeterminedwas±0.5◦C.2.4.DebindingandsinteringTwostepsofdebindingofgreenpartswereconducted.ThefirststepwastoextractPEGoutofgreenpartsbydistilledwateratroomtemperaturefor24h.Thesecondstepwasreferredtoasthethermaldebinding,wherethebackbonescomposedofPMMAandCABwerecompletelyremovedandpre-sinteringofmetalpowdersoccurred.Finallythesinteringofmetalpowderswascarriedout.Thermaldebindingandsinteringwerecarriedoutinatube-typefurnace(LentonThermalDesignLtd.)underhydrogenatmospherebyusingthesamethermalhistoryasref.[12].3.Resultsanddiscussion3.1.MiscibilitybetweenbindercomponentsThebinderemployedinPIMhasamulti-componentsystem:(a)abackbonepolymerthatprovidesshapemaintenanceduringsolventextraction；(b)anextractablepolymerthatisremovedduringthefirststepofthedebindingprocess,and(c)asurfactanttobridgebetweenthebinderandpowder[1].Itisnecessarythatallcomponentsshouldbemiscibleatprocessingtemperatures.Otherwise,thebindercomponentsbecomemacrophase-separated.Previously,weshowedthatabinaryblendofCAB/PEGhavingamolecularweight(M)of35000showedlowercriticalsolutiontemperature(LCST)of168◦C[13].SinceMofPEGemployedinthisstudyislessthan8000,wefoundthattheCAB/PEGmixturedoesnotbecomephase-separatedupto250◦C.ThePMMA/PEGblendbecomescompletelymiscibleattemperaturesgreaterthanthemeltingtemperatureofPEG[14].Fig.1givesthephasebehaviorofthePMMA/CABblend,fromwhichtheLCSTwas142◦Cat20/80(w/w)PMMA/CABcomposition.Thus,weconcludethatMacrophaseseparationamongallthreebinaryblends(PMMA/PEG,CAB/PEG,andPMMA/CABblends)doesnotoccurataprocessingtemperatureof130◦C,whichisthemixingandinjectionmoldingtemperatures.Also,whentheamountofPMMAwaschangedfrom15to25wt%atafixedamountofPEG(65wt%),noturbiditywasdetectedupto200◦C.Figure1PhasediagramsofPMMA/CABblend.3.2.RheologicalpropertiesItisknownthatfeedstocksshouldhavealowenoughviscositytobeprocessedbyinjectionmolding.However,whentheviscositybecomestoosmall,jettingoccursduringtheinjectionmoldingprocess；thustheexactshapeofmoldedarticleisimpossible.Fig.2givesthecomplexviscosity(η*)asafunctionoffrequencyforthebinderscomposedofvariouscompositionsofPMMA/CAB/PEGswithtwodifferentmolecularweightsofPEGs.ItshowsthatasthecontentofPMMAandMofPEGincrease,theviscositybecomeslarger.WhenthebindersdonotcontainCAB,abinderconsistingof35/65(wt/wt)PMMA/PEGhaszero-shearviscosity(ηo)of100Pa·s,regardlessofmolecularweightsofPEG.Itisknownthatwhenηoofabinderislargerthan∼20Pa·s,thefeedstocksincludinghighvolumeofpowderscouldnotbemoldedbyinjectionmolding[1].Therefore,itisconsideredthatfeedstocksconsistingof15/20/65and25/10/65(wt/wt/wt)PMMA/CAB/PEG-8binderand56vol%ofsteelpowderscouldbeinjectionmolded.

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