1、百度文庫-好好學(xué)習(xí)，天天向上百度文庫-好好學(xué)習(xí)，天天向上 MATERIALSTESTINGEvaluatingEnvironmentalStressCrackingofMedicalPlasticsEricJ.MoskalaandMelanieJonesWhenaplasticisexposedtoachemicalenvironment,thematerialmayundergonumerouschanges.Thesecanincludeweightgainiftheplasticabsorbsthechemical,weightlossiftheplasticisdegradedbyth

2、echemicalorifthechemicalextractslow-molecular-weightcomponentsoftheplastic,dissolutionifthechemicalisagoodsolvent,orotherchangessuchasvariationsinopacityorcolor.Iftheplasticisunderstress,itmayalsoexperienceenvironmentalstresscracking,whichcanbedefinedasthecrazingandcrackingthatmayoccurwhenaplasticun

3、deratensilestressisexposedtoaggressivechemicals.Thepotentialforenvironmentalstresscrackingisofparamountconcernwhenplasticsareusedinmedicaldevicecomponentssuchasluersandstopcocks.Intheseapplications,chemicalssuchasisopropanolandlipidsolutionscaninitiatecrazesmicrocracksbridgedbypolymerfibrilsinthepla

4、sticandseriouslycompromiseitsmechanicalintegrity.Productslikemedicalluerscanbesusceptibletocrazingwhenunderstressandexposedtoaggressivechemicals.Photo:EastmanChemicalCo.Theproperselectionofamedicalplasticrequiresathoroughanalysisandinterpretationofthephenomenonofenvironmentalstresscracking.Thegoalof

5、thisarticleistoprovideaframeworkforevaluatingthesuitabilityofaplasticformedicalusesintermsofitsstress-crackbehavior.Ourstrategywillbetoexamine,insomedetail,therolesofthethreecriticalcomponentsofenvironmentalstresscracking:thechemicalenvironment,theplastic,andthetensilestress.THECHEMICALENVIRONMENTCh

6、emicalsthatcauseenvironmentalstresscrackingcanbedividedintothosethatswellorwetthepolymerandthosethatchemicallyreactwiththepolymer.Anexampleofthelatterwouldbethecausticoraqueoussodiumhydroxidethatcanhydrolyzepoly(ethyleneterephthalate)(PET).1Thereductioninpolymermolecularweightfromthehydrolysiscanlea

7、dtocrazingandeventualcatastrophicfailureamechanismthathasbeenidentifiedastheprobablecauseforstress-crackfailuresinone-piececarbonatedsoft-drinkThisarticle,however,willemphasizechemicalsthatcausestresscrackingsimplybyswellingorwettingthepolymer.(Lipidsolutionsandisopropanolfallintothiscategory.)Itist

8、hegeneralconsensusintheliteraturethatthemajorityofstress-crackfailuresexperieneedbyend-usersresultsfromthiscategoryof-5Numerousstudieshavelinkedtheabilityofasolventtoswellaplasticwithitsabilitytocrazetheplastic.Perhapsthebest-knownworkofthiskindisthatofKambouretal.,whodemonstratedinstudiesonpolycarb

9、onate,6poly(phenyleneoxide),7polysulfone,8andpolystyrene9thattheabsorptionofthesolventandconcomitantreductioninthepolymersglass-transitiontemperaturecanbecorrelatedwithapropensityforstresscracking.Theyalsoshowedthatabsorptionoftheliquidbythepolymertendstobecorrelatedbythesolubilityparametersoftheliq

10、uidandpolymer.Thesolubilityparameter,6,isdefinedaswhereCEDiscohesiveenergydensity,istheheatofvaporization,andVmismolarvolume.Hansenproposedthatthesolubilityparameterwascomposedofcontributionsfromthethreemajortypesofcohesiveforces,namelydispersive,polar,andhydrogenbonding,sothatwhered,%,and&harethedi

11、spersive,polar,andhydrogenbondingcomponentsofthetotalsolubilityValuesoftheseparametersforafewrepresentativechemicalsareshowninTableI.Ifthesolubilityparameterofthesolventisclosetothesolubilityparameterofthepolymer,thepolymerwillprobablyshowsomesolubilityinthesolventorundergosolvent-inducedcrystalliza

12、tion.Experiencehasshownthatabsorptionofaliquidbyapolymermaybebettercorrelatedbyusingthepartialsolubilityparameters.百度文庫-好好學(xué)習(xí)，天天向上百度文庫-好好學(xué)習(xí)，天天向上 Becausethesolubilityparameterofapolymercannotbecalculateddirectlyfromtheheatofvaporization,indirectmethodssuchassolventswellingandgroup-contributionapproach

13、esareused.Solventsthatswellordissolvethepolymermosteffectivelywillhavesolubilityparametersclosetothesolubilityparameterofthepolymer,inkeepingwiththeadagethatlikedissolveslike.Inthegroup-contributionapproach,thesolubilityparameterisdeterminedbyusingtheequationwhereFiisthemolarattractionconstantandVii

14、sthemolarvolumeforeachsubsegmentofthepolymerrepeatingunit(asdemonstratedforPETinTableII).2Mcda-rWlume-fcVcirAnol-1TableII.Estimationofsolubilityparameterforpolyethyleneterephthalate(PET)usingthegroup-contributionPETmolecularstructureshownattop.百度文庫-好好學(xué)習(xí)，天天向上百度文庫-好好學(xué)習(xí)，天天向上百度文庫-好好學(xué)習(xí)，天天向上 Theabilityoft

15、hesolubilityparameterapproachtocorrelatetheabsorptionbehaviorofplasticshasbeendemonstratedbytheauthorsusingPET,PCTG(acopolyester),andpolycarbonate.Piecesof3-mil-thickamorphous,unoriented,extrudedfilmweresuspendedinsealedjarsaboveafewmillilitersofliquid.Thefilmswereremovedeverytwoweeksforweighingunti

16、ltheyreachedanequilibriumweight.Theresults,listedinTableIII,indicatethatallthreeplasticsappeartoshowabroadpeakwithamaximuminliquidabsorptionatasolubilityparameterofapproximately20MPa%.ThesolubilityparametersforPET,PCTG,andpolycarbonate,asdeterminedbythegroup-contributionapproach,areMPa%,22MPa%,andMP

17、a%,respectively,whichfallintotherangeofthebroadmaximuminliquidabsorption.(Thealcoholsareexceptionstothistrend,asseeninTableI,presumablybecauseoftheirstronghydrogenbondingcharacteristics.)Theseresultshighlightthedifficultyinusingthesolvent-swellingtechniquefordeterminingthesolubilityparameterofapolym

18、erandinusingthetotalsolubilityparametertocorrelateapolymersabsorptionbehavior.LiquidTotal6(MPa)PET(%)PCTG(%)Polycarbonat(%)Isooctane000Heptane000Cyclohexane001Ethylbenzene61727Toluene122131Methylethylketone131525Chloroform57DDTetrahydrofuran1833DCyclohexanone212865Acetone121324oDichlorobenzeine25506

19、01Pentanol010Nitrobenzene285058iPropanol001Ethanol147Dimethylsulfoxide161440Methanol358Ethyleneglycol001Glycerol001Water002TableIII.PercentageweightgainforPErcopolyester,andpolycarbonatefilminvariodissolved.Valuesinitalicsindicatethatopaque.,PCTGusliquids.D=hefilmbecameTHEPLASTICAllplasticscanbeclas

20、sifiedaseitheramorphousorcrystallinematerials.Inamorphousplasticssuchaspolystyreneandpoly(methylmethacrylate),thepolymerchainsarerandomlyconfigured,displayingnosignificantorder.Incrystallineplasticssuchaspolyethyleneandnylon6/6,thepolymerchainsarealignedororderedintocrystallites.Crystallineplastics,

21、however,arenevercompletelycrystalline,butrathercontainregionsofamorphousmaterial.Afewplastics,amongthemPETandpolycarbonate,canbeentirelyamorphousorsemicrystalline,dependingonprocessingconditions.Atroomtemperature,thethermodynamicallyfavoredstatefortheseplasticsisthecrystallineform;however,iftheyarec

22、ooledrapidlyenoughfromthemelttobelowtheirglass-transitiontemperatures(Tg),theywillremainintheiramorphousforms.Undernormalinjectionmoldingconditions,partsmadefromsuchplasticsareclear,indicatingtheabsenceofcrystallinity.IfthefinishedpartsareheatedtoabovegTorareexposedtostrongsolvents,theywillcrystalli

23、ze.Thelatterphenomenonisoftencalledsolvent-inducedcrystallization,andwasobservedduringtheabsorptionstudiesdiscussedabove.ThenominallyamorphousPET,PCTG,andpolycarbonatefilmsturnedopaqueuponexposuretocertainsolvents(seeitalicizeddatainTableIII),indicatingthatcrystallizationhadoccurred.Absorptionofthes

24、eliquidsdecreasedtheTgsoftheplasticstoatleastambientconditions,givingthepolymerchainssufficientmobilitytoalignandcrystallize.ItwasnotedthatcrystallinitydevelopedmuchmorequicklyinPETandPCTGthaninpolycarbonate,primarilybecausethecopolyestershavemuchlowerTgs(approximately80C)thandoespolycarbonate(appro

25、ximatelyg150C),andthereforeneededtoabsorblessliquidbeforeTgwasdepressedtogambienttemperature.Solvent-inducedcrystallizationmayhaveapronouncedeffectonstress-crackingbehavior,aswillbediscussedlater.Althoughbothamorphousandcrystallineplasticsaresusceptibletoenvironmentalstresscracking,itisgenerallyreco

26、gnizedthatamorphousplasticstendtobemoreat-5Thecloselypackedcrystallinedomainsincrystallineplasticsactasbarrierstofluidpenetrationandthereforetendtoresistenvironmentalstresscracking.THETENSILESTRESSPlasticswillexhibitenvironmentalstresscrackingwhenexposedtoanaggressivechemicalenvironmentifandonlyifat

27、ensilestressispresent.Thetensilestressmaybeappliedexternallyormaysimplybeaconsequenceofresidual,ormolded-in,stresses.ResidualstressescanbeminimizedthroughtheuseofproperdesignguidelinesandthecontrolofcriticalvariablesintheinjectionmoldingExternallyappliedstressescanresultfromsubassemblyprocesses,ship

28、pingandstorageconditions,orimproperpacking.Anexternallyappliedtensilestressmayalsobepartoftheintendedend-useofthedevice.Afemaleluer,forexample,maybesubjectedtoextremelyhighhoopstressesuponinsertionofthemaleObviously,themostreliablemethodforevaluatingthestress-crackresistanceofaplasticinagivenapplica

29、tionistoanalyzeitsperformanceundersimulatedend-useconditions.Alternatively,stress-crackresistancecanbedeterminedbysometypeofstandardtestingprocedurewhoseresultscanberelatedtothestressandstrainlevelsobservedinend-useconditions.Afewofthenumerousteststhathavebeendevelopedtoevaluateenvironmentalstress-c

30、rackresistancearelistedintheboxonpage41.Thetestsdifferprimarilyinthewaytheexternalstressisapplied.ASTMD1693describesatestforevaluatingthestress-crackresistanceofethyleneplasticsinenvironmentssuchassoaps,wettingagents,oils,ordetergents.Stripsofaplastic,eachcontainingacontrolleddefect,areplacedinabend

31、ingrigandexposedtoastress-crackingagent.Thenumberofspecimensthatcrackoveragiventimeisrecorded.ISO4600detailsaballorpinimpressionmethodfordeterminingstress-crackresistance.Inthisprocedure,aholeofspecifieddiameterisdrilledintheplastic.Anoversizedballorpinisinsertedintheholeandtheplasticisexposedtoastr

32、ess-crackingagent.Afterexposure,tensileorflexuraltestsmaybeperformedonthespecimen.Aconstanttensile-stressmethodisoutlinedinISO6252,inwhichatestspecimenisexposedtoaconstanttensileforcewhileimmersedinastress-crackingagentsoastodeterminetime-to-ruptureunderaspecifiedstress.Variationsofthistestincludeat

33、ensilecreeptestthatmonitorsstrain,andamonotoniccreeptestthatusesaconstantstressingrateinsteadofafixedAnotherbent-stripmethodforevaluatingstress-crackresistanceispresentedinISO4599.Inthistest,stripsofaplasticarepositionedinafixedflexuralstrainandexposedtoastress-crackingagentforapredeterminedperiod.A

34、fterexposure,thestripsareremovedfromthestrainingrig,examinedvisuallyforchangesinappearance,andthentestedforsomeindicativepropertysuchastensilestrength.CommonlyUsedTestsforEvaluatingStressCrackResistanceofPlasticsASTMD1693EnvironmentalStressCrackingofEthylenePlasticsISO460ResistancetoBall/PinImpressi

35、onMethodISO6252ResistancetoESConstant-TensileStressMethodISO4599ResistancetoBent-StripMethodCriticalStrainFractureMechanicsAvariationoftheISO4599testwasperformedbytheauthorsonmedical-gradeversionsofPCTG,polycarbonate,andanacrylicresin.Injection-moldedtensilebarswereplacedunderfixedstrainsof0%,%,%,an

36、d%andexposedtoalipidsolutionandisopropanolusingawet-patchtechnique.Aftera72-hourexposureperiod,thespecimenswereremovedfromthestrainrig,rinsedcleanofchemicalwithdistilledwater,allowedtoequilibrateatambientconditionsfor24hours,andthentestedforresidualtensilepropertiesaccordingtoASTMD638.Theresultsfors

37、pecimensexposedtoisopropanolandtolipidsolutionaredisplayedinTablesIVandV,respectively.Inisopropanol,polycarbonatecrazedatstrainlevelsof%andhigher,resultingindramaticlossesintensileproperties.PCTGalsocrazedatstrainlevelsof%andhigherbutmaintainedmostofitstensileproperties.Theacrylicresinfracturedonthe

38、strainrigshortlyaftertheappearanceofcrazing,atastrainlevelaslowas%.Inthelipidsolution,polycarbonatecrazedatastrainlevelaslowas%,andfracturedwhilestillonthestrainrigatastrainlevelof%.Theacrylicresincrazedonlyatthe%strainlevel,anddemonstratedadramaticlossoftensilepropertiesforthisonestrainlevel.PCTGcr

39、azedatastrainlevelaslowas%butretainedmostofitstensileproperties.OnecanspeculatethattheabilityofPCTGtoretainmostifitsmechanicalpropertiesdespitebeingheavilycrazedislinkedtoitsabilitytoundergorapidsolvent-inducedcrystallization.Thecriticalstraintestattemptstodeterminetheminimumstrainrequiredtoinitiate

40、crazinginthepresenceofastress-crackingagent.ThetestismostcommonlyperformedusingaBergenellipticalstrainrig:astripofplasticisplacedontherig,whichispatternedafteraquarterofanellipse,andexposedtoastress-crackingThestrainatanypointalongtheellipticalrig,e,isgivenbytheequationwhereaisthesemimajoraxis,bisth

41、esemiminoraxis,tisspecimenthickness,andXisthedistancealongthesemimajoraxistothepointofinterest.Withaspecimen,anellipticalrigwitha=10in.andb=5in.willgiveminimumandmaximumstrainsof%and%,respectively.ResultsofcriticalstraintestsonPET,PCTG,andpolycarbonatearelistedinTableVI.Injection-moldedflexuralbarsw

42、erestrappedtoaBergenellipticalstrainrigandexposedtotheliquidsusingawet-patchtechnique.Thewidthofthepatchwassmallerthanthewidthoftheflexuralbartoavoidexposingtheedgesoftheflexuralbartotheliquids.Polycarbonatedisplayedoutstandingcritical-strainvaluesinalcoholsandaliphatichydrocarbonsbutverylowcritical

43、-strainvalues(2HeptaneCyclohexaneEthylbenzeneTolueneMethylethylketone.031ChloroformTetrahydrofuran.076CyclohexanoneAcetoneBo-DichlorobenzeneX1PentanolNitrobenzeneD2百度文庫-好好學(xué)習(xí)，天天向上百度文庫-好好學(xué)習(xí)，天天向上 Dimethylsulfoxide2Ethyleneglycol222Glycerol222Water222TableVI.CriticalstrainforPET,PCTGcoppolycarbonateinva

44、riousliquids.X=nottesdissolvedsurfacewithnocrazing;B=broketosolvent.PCTGtensilebarsthathavebeeisopropanolareshownbefore(left)andaftfixedstrain.)lyester,andted;D=heavilyuponexposurenexposedtoer(right)a%Severallaboratorieshaveusedtheconceptsoffracturemechanicstoevaluatestresscrackingin-19Thebasicpremi

45、seoffracturemechanicsisthatthestrengthofamaterialisdeterminedbythepresenceofflaws.Infracture-mechanicstesting,awell-definedflaworcrackismachinedintoaplastic.Thespecimenisstressedandthegrowthoftheflawinthepresenceofastress-crackingagentismonitoreduntilfailure.Whereasfracturemechanicsisidealforstudyin

46、gtheeffectofapreexistingcrackontheresidualstrengthofaplastic,itprovidesnoinsightintothemechanismforinitiationofacrackorcrazeuponexposuretoastress-crackingagent.MaterialConditioiStrainanYield(%)itStressaBreak(%)itStressaYield(MPa)itStressaBreak(MPa)AppearanceControlA0%APCTG%A%C%CControlA0%APolycarbon

47、at,e%A%NoyieldingNoyieldingC%NoyieldingNoyieldingCControlA0%AAcrylic%FracturedFracturedFracturedFracturedC%FracturedFracturedFracturedFracturedC%FracturedFracturedFracturedFracturedCaA=nochangeinappearance;B=edgecrazing;C=fullwidthcraziTableIV.TensilepropertiesofPCTG,polycarbonate,andacryliisopropan

48、ol.ng.cafterexposuEachoftheaforementionedtestshasitsadvantagesanddisadvantages.Thechoiceofthebestmethodforevaluatingtheperformanceofamedicaldevicematerialwilldependonwhichtestmostcloselysimulatesend-useconditions.,constantstressversusconstantstrain,etc.)andonthefailurecriterionselectedbythedesigner.

49、Criticalstrainisanexcellentmethodforevaluatingstress-crackresistanceifthemereappearanceofcrazingconstitutesafailure,sincethetestdeterminestheminimumstrainrequiredtoinitiateacraze.However,itshouldbeemphasizedthattheappearanceofcrazingdoesnotnecessarilyindicatealossofmechanicalproperties.Theresultssho

50、wninTablesIVandVdemonstratethataplasticsuchasPCTGcanbeheavilycrazedbyachemicalbutundergonolossinmechanicalproperties.Thiswasnotthecaseforpolycarbonateandacrylic,inwhichtheappearanceofcrazingsignaledadeteriorationofmechanicalproperties.Ifthefailurecriterionisretentionofmechanicalpropertieswithoutrega

51、rdtoaestheticappearance,anyoftheISOtestsorvariationsthereofthatmeasureretentionofmechanicalpropertiesmaybetheappropriatechoice.MaterialConditioiStrainanYield(%)itStressaBreak(%)itStressaYield(MPa)itStressaBreak(MPa)AppearanceControlA0%APCTG%A%C%CControlA0%APolycarbonat,e%A%NoyieldingNoyieldingB%Frac

52、turedFracturedFracturedFracturedBControlA0%1AAcrylic%A%A%NoyieldingJNoyieldingBaA=nochangeinappearance;B=edgecrazing;C=fullwidthcraziTableV.Tensilepropertiesafterexposureto20%lipidsolution.CONCLUSIONTherolesofthechemicalagent,theplastic,andthestressareallimportantfactorsincausingenvironmentalstressc

53、racking.Chemicalsthattendtoswellorwetaplasticareoftenactivestress-crackingagents;however,thesolubility-parameterapproachisonlymodestlyusefulforcorrelatingstress-crackingbehaviorwiththeabilityofaliquidtoswellaplastic.Semicrystallineplasticstendtobemoreresistanttoenvironmentalstresscrackingthanamorpho

54、usones.Itwasdemonstratedthatamorphousbutcrystallizableplastics,suchasPETandPCTG,canbeheavilycrazedbyastress-crackingagentandyetretaintheirmechanicalpropertiesduetosolvent-inducedcrystallizationofthecrazingfibrilsinthepolymer.Finally,severaltestsforevaluatingstress-crackresistancewerereviewed.Foragiv

55、enmedicalapplication,theoptimaltestmethodwilldependonthefailurecriteriaestablishedbythedesignersofthecomponentordevice.REFERENCESZeronianSH,WangHZ,andAlgerKW,FurtherStudiesontheMoisture-RelatedPropertiesofHydrolyzedPoly(ethyleneterephthalate),JApplPolymSci,41:527,1990.MoskalaEJ,EnvironmentalStressCr

56、ackinginPETBeverageContainers,inProceedingsofBev-PakAmerica96,Ft.Lauderdale,FL,Directions21,Inc.,8,1996.百度文庫-好好學(xué)習(xí)，天天向上百度文庫-好好學(xué)習(xí)，天天向上 KramerEJ,EnvironmentalCrackingofPolymers,DevelopmentsinPolymerFracture,Vol1,chap3,AndrewsEH(ed),London,AppliedScience,1979.KambourRP,AReviewofCrazingandFractureinThermoplastics,MacromolRev,7:1,1973.WrightDC,EnvironmentalStressCrackingofPlastics,Shawbury,UK,Rapra,1996.KambourRP,GrunerCL,andRomagosaEE,Bisphenol-APolycarbonateImmersedinOrganicMedia.SwellingandResponsetoStress,Macromol,7:248,1974.BernierGA,andKambourRP,TheRoleofOrganicAgentsinth