waterborne polyurethanes.docx
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waterborne polyurethanes.docx
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waterbornepolyurethanes
Chainextensionstudiesofwater-bornepolyurethanesfrommethylethylketoxime/
-caprolactam-blockedaromaticisocyanates
S.Subramani,I.W.CheongandJ.H.Kim
NanosphereProcessandTechnologyLaboratory,DepartmentofChemicalEngineering,YonseiUniversity,134Shinchon-Dong,Sudaemoon-Ku,Seoul120-749,RepublicofKorea
Received27May2004;
accepted3August2004.
Availableonline17November2004.
Abstract
Aseriesofwater-basedblockedaromaticpolyurethanedispersions(BPUDs)werepreparedbyprepolymermixingprocess,usingtoluene2,4-diisocyanate(TDI),4,4′-di-p-phenylmethanediisocyanate(MDI),poly(oxytetramethylene)glycol(PTMG),dimethylolpropionicacid(DMPA),methylethylketoxime(MEKO)and
-caprolactam(CL).Particlesize,viscosity,pH,molecularweight,storagestability,andde-blockingtemperatureoftheBPUDsweremeasuredandcompared.ChainextensionofBPUDwascarriedoutwiththreetypesofchainextenders,hexamethylenediamine(HMDA),isophoronediamine(IPDA)andtetraethylenepentamine(TEPA),atvariousde-blockingtemperatures.TensileandthermalpropertiesofthechainextendedBPUDfilmswereinvestigatedandresultsconfirmedthatMEKO-BPUDsde-blockedatlowertemperaturethan
-caprolactam-BPUDs.TheTgvaluesofthechainextendedBPUDswerehigherthanthoseoftheBPUDsandpurePTMG.Improvementinthethermalstabilityconfirmedthede-blockingandchainextensionreactionofblockedPUprepolymers.
Keywords:
Water-borne;Blocked
isocyanate;
Polyurethane
;Dispersion;Chainextension;De-blocking
ArticleOutline
1.Introduction
2.Experimental
2.1.Materials
2.2.SynthesisofaqueousanionicblockedPUdispersions
2.3.Characterizations
3.Resultsanddiscussion
3.1.SynthesisofblockedaromaticPUdispersions(BPUDs)
3.2.Molecularweights
3.3.Physicalproperties
3.4.Tensileproperties
3.5.Thermalproperties
4.Conclusions
Acknowledgements
References
1.Introduction
Polyurethanes(PUs)arehigh-classengineeringmaterialshavingawiderangeofenduses.Theirversatilityarisesfromtherichchoiceofstartingmaterialsorbuildingblocksthatcanbecombinedbythediisocyanate-polyadditionprocess[1].Generally,theyarewaterincompatibleandarepreparedbysolvent-bornemethods.Theeffectsoftheorganicsolventsonenvironmentaredetrimentalandhaveledtothedevelopmentofalternativetechniques,whichprovideeco-friendlysystems.Watercannotbeuseddirectlyeitherasasolventoradiluent,sincePUsarewaterinsolubleandoneofthechiefrawmaterials,isocyanates,arehighlymoisturesensitive.However,PUcanbedispersedinwaterbyusingexternalemulsifier[2]orbyincorporatingemulsifyinggroupsintothePUbackbone[3]and[4].Thelatterispreferred,sinceitleadstofilmswithsuperiorproperties.Theemulsifyinggroupscanbenon-ionicpolyethersegmentsorionicgroups.PUsmodifiedwithionicgroupsgiverisetodispersionsofhighstabilityandbetterpropertycomparedtotheirnon-ionicanalogues.PUscontainingionicgroups(<15%)arecalledPUionomers.Dependingonthetypeofionicgroupsincorporated,PUsareclassifiedasanionic[5]and[6],cationic[7]and[8]andzwitterionic[9].ThesePUionomerscanbedispersedinwaterbythefollowingvariousavailabletechniques,suchasacetoneprocess,pre-polymer-mixingprocess,ketimine–ketazineprocessandmeltdispersionprocess.
Generally,thepre-polymermixingprocessiscommerciallypredominanttopreparePUDs.Generally,aliphaticdiisocyanates,viz.isophoronediisocyanate(IPDI),hexamethylenediisocyanate(HMDI)arepreferredbecauseoftheirlowerreactivitywithwaterthanthearomaticisocyanates,suchas4,4′-di-p-phenylmethanediisocyanate(MDI),toluenediisocyanate(TDI).Leeandco-workerspreparedaqueousPUdispersionsbyapre-polymerprocessusingIPDI,TDIandMDIandstudiedthestructure–propertyrelationship[10].However,whentheMDI-basedPUpre-polymerwasdispersedinwater,theNCOgroupsreactedrapidlywithwatertoproducemanyprecipitates.Thedifferentreactivityofdiisocyanatesresultedinadramaticeffectonthestabilityofaqueousdispersion.Theisocyanategroupshavetobeprotectedbeforedispersioninwatertopreventthereactionofisocyanategroupswithwater,whichcouldbeachievedbyblockingtheisocyanategroupsbyusingtypical,blockingagentslikephenol,
-caprolactam(CL),methylethylketoxime(MEKO),etc.
Blockedisocyanatesarederivativesofisocyanatesoradducts,formedbythereactionofanisocyanatewithanactivehydrogenatom
(1).Whenheated,theblockedisocyanatedissociatesintoisocyanateandtheblockingagentandtheisocyanatethenreactswithanactivehydroxylfunctionalresintoformthermallystablebonds
(2).Theoverallreactioncanbeseenas:
Full-sizeimage(1K)
ViewWithinArticle
Full-sizeimage(1K)
ViewWithinArticle
whereBHisblockingagentandR′-OHiscross-linker.
Thethermalsplittingorcuringtemperatureofblockedisocyanateisacruciallimitingfactorforindustrialapplications.Generally160 °Cwith30 mindurationorlowerispreferable.Thistemperatureisspecificforaparticularblockingagent.Fewreviews[11],[12],[13]and[14]havebeenpublishedintheareasofblockedpolyisocyanatesandwater-dispersibleblockedpolyisocyanates,whichreferalargenumberofpatentsdescribingtheapplicationsofblockedisocyanates.Anumberofpatentsandreportsdisclosethatheterocycliccompounds,suchastriazoles,imidazolinesandimidazolescanbeusedasblockingagentsforisocyanates[15],[16],[17],[18]and[19].Amineswerealsousedasblockingagentsforisocyanates[20].
Itishighlyrequiredtoimprovethepropertiesofwater-bornecoatings.Thesolventandwaterresistanceproperties,mechanicalpropertiesandthermalstabilityhavetobeimprovedtoachievehighperformancematerials.Thesepropertiescanbeachievedbychainextendingwithshortchaindiaminesordihydroxycompoundstogethighmolecularweightpolymer.ThechainextensionisdoneafterdispersingthePUprepolymerinwaterotherwisetheresultanthighviscosityprepolymerscannotbedispersed.Hence,PUswithterminalisocyanategroupsaredispersedinwaterandsubsequentlyreactwithdiaminesordihydroxycompounds.Inthismethod,thechainextensioncannotbedoneovercertainpercentageevenifitisrequiredbecausehighchainextensionwillleadtohighmolecularweightnetworksresultingininstabilityofdispersion,andmoreover,itrequiresmoreamountoffreeisocyanateintheaqueousphase,whichwillresultinmanysidereactions.Theabove-mentionedproblemscanbeeliminatedandtherequiredpropertiescanbeeffectivelyachievedbyusingblockedisocyanateprocess.Intheblockedisocyanateprocess,thechainextensionorcross-linkingoccursonlyafterthecoatingofthedispersion,thatis,afterfilmformation.Hence,thepercentagechainextensionorcross-linkingcanbeachieved(increased)toourrequirement.Thepresenceoffreeisocyanatesgroupsareavoided,thus,increasingthedispersionstabilityinwatermedium.Thisisaneffectivewaytoimprovethestoragestabilityofthedispersionandthepropertiesofthecuredfilm.MergingthetechnologiesofaqueousPUdispersionsandblockedpolyisocyanatecouldresultinhighquality,one-packageandlowVOCs,coatingswithlongpot-lifeandlowfilm-formationtemperature.Inthiswork,synthesisandcharacterizationofwater-dispersibleanionicblockedpolyisocyanatedispersionsbasedonaromaticdiisocyanates(TDIandMDI)andsomeblockingagents(MEKOandCL)arepresented.Chainextensionstudiesoftheprepareddispersionswithdifferentdiaminesareconductedanddiscussed.
2.Experimental
2.1.Materials
TherawmaterialsusedinthisstudyarelistedinTable1.Allrawmaterialsarelaboratorygradechemicalsandwereusedasreceived.Polyols(PTMG)weredriedinvacuumat100 °Cfor16 hbeforeuse.N-Methyl-2-pyrrolidone(NMP)andacetonewerestoredoverwell-driedmolecularsieve.
Table1.
Listofrawmaterialsusedinthepreparationofanionicblockedpolyurethaneprepolymerdispersions
Designation
Chemicaldescription
Suppliers
PTMG-1000
Poly(oxytetramethylene)glycol(OHfunctionality = 2.0,MW = 1000)
DuksungCo.Ltd.,Korea
PTMG-2000
Poly(oxytetramethylene)glycol(OHfunctionality = 2.0,MW = 2000)
DuksungCo.Ltd.,Korea
DMPA
Dimethylolpropionicacid
Aldrich
MDI
4,4′-Methylenebiphenylisocyanae
Aldrich
TDI
Toluene2,4-diisocyanate
JunseiChemicalCo.Ltd.,Japan
MEKO
Methylethylketoxime
Aldrich
Caprolactam
-Caprolactam
WakopurechemicalindustriesLtd.,Japan
HMDA
Hexamethylenediamine
Sigma
IPDA
Isophoronediamine
Aldrich
TEPA
Tetraethylenepentamine
Aldrich
TEA
Triethylamine
DuksanPharmaceuticalCo.,Korea
NMP
N-Methyl-2-pyrrolidone
Lancaster
DBTDL
Di-butyltindilaurate
Aldrich
Full-sizetable
ViewWithinArticle
2.2.SynthesisofaqueousanionicblockedPUdispersions
Reactionschemeoutlinesthesyntheticprocessusedtoprepareanionicallymodifiedwater-dispersibleblockedPUpre-polymer(Scheme1).Poly(oxytetramethylene)glycol(PTMG;
= 1000or2000 g mol−1)anddimethylolpropionicacid(DMPA)inNMPwerefirstchargedintothereactionkettleequippedwitharefluxcondenser.Thereactionmixturewasstirredat75 °Cundernitrogenatmosphere.Aftercompletemixing,diisocyanate(MDIorTDI)wasaddedtothereactionmixture.Duringthereaction,thechangeofisocyanate(NCO)contentwasdeterminedbyusingastandarddi-n-butylaminebacktitrationuntilthetheoreticalendpointwasreachedwhichisapproximatelyabout2–3 hofreaction[21].Thenthereac
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