塑件内应力检测ESCR.docx

塑件内应力检测ESCR.docx

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塑件内应力检测ESCR

MATERIALSTESTING

EvaluatingEnvironmentalStressCrackingofMedicalPlastics

EricJ.MoskalaandMelanieJones

Whenaplasticisexposedtoachemicalenvironment,thematerialmayundergonumerouschanges.Thesecanincludeweightgainiftheplasticabsorbsthechemical,weightlossiftheplasticisdegradedbythechemicalorifthechemicalextractslow-molecular-weightcomponentsoftheplastic,dissolutionifthechemicalisagoodsolvent,orotherchangessuchasvariationsinopacityorcolor.Iftheplasticisunderstress,itmayalsoexperienceenvironmentalstresscracking,whichcanbedefinedasthecrazingandcrackingthatmayoccurwhenaplasticunderatensilestressisexposedtoaggressivechemicals.Thepotentialforenvironmentalstresscrackingisofparamountconcernwhenplasticsareusedinmedicaldevicecomponentssuchasluersandstopcocks.Intheseapplications,chemicalssuchasisopropanolandlipidsolutionscaninitiatecrazes—microcracksbridgedbypolymerfibrils—intheplasticandseriouslycompromiseitsmechanicalintegrity.

Productslikemedicalluerscanbesusceptibletocrazingwhenunderstressandexposedtoaggressivechemicals.Photo:

EastmanChemicalCo.

Theproperselectionofamedicalplasticrequiresathoroughanalysisandinterpretationofthephenomenonofenvironmentalstresscracking.Thegoalofthisarticleistoprovideaframeworkforevaluatingthesuitabilityofaplasticformedicalusesintermsofitsstress-crackbehavior.Ourstrategywillbetoexamine,insomedetail,therolesofthethreecriticalcomponentsofenvironmentalstresscracking:

thechemicalenvironment,theplastic,andthetensilestress.

THECHEMICALENVIRONMENT

Chemicalsthatcauseenvironmentalstresscrackingcanbedividedintothosethatswellorwetthepolymerandthosethatchemicallyreactwiththepolymer.Anexampleofthelatterwouldbethecausticoraqueoussodiumhydroxidethatcanhydrolyzepoly（ethyleneterephthalate）（PET）.1Thereductioninpolymermolecularweightfromthehydrolysiscanleadtocrazingandeventualcatastrophicfailure—amechanismthathasbeenidentifiedastheprobablecauseforstress-crackfailuresinone-piececarbonatedsoft-drink

Thisarticle,however,willemphasizechemicalsthatcausestresscrackingsimplybyswellingorwettingthepolymer.（Lipidsolutionsandisopropanolfallintothiscategory.）Itisthegeneralconsensusintheliteraturethatthemajorityofstress-crackfailuresexperiencedbyend-usersresultsfromthiscategoryof–5Numerousstudieshavelinkedtheabilityofasolventtoswellaplasticwithitsabilitytocrazetheplastic.Perhapsthebest-knownworkofthiskindisthatofKambouretal.,whodemonstrated—instudiesonpolycarbonate,6poly（phenyleneoxide）,7polysulfone,8andpolystyrene9—thattheabsorptionofthesolventandconcomitantreductioninthepolymer'sglass-transitiontemperaturecanbecorrelatedwithapropensityforstresscracking.Theyalsoshowedthatabsorptionoftheliquidbythepolymertendstobecorrelatedbythesolubilityparametersoftheliquidandpolymer.Thesolubilityparameter,

isdefinedas

whereCEDiscohesiveenergydensity,

Evapistheheatofvaporization,andVmismolarvolume.Hansenproposedthatthesolubilityparameterwascomposedofcontributionsfromthethreemajortypesofcohesiveforces,namelydispersive,polar,andhydrogenbonding,sothat

where

d,

p,and

harethedispersive,polar,andhydrogenbondingcomponentsofthetotalsolubilityValuesoftheseparametersforafewrepresentativechemicalsareshowninTableI.Ifthesolubilityparameterofthesolventisclosetothesolubilityparameterofthepolymer,thepolymerwillprobablyshowsomesolubilityinthesolventorundergosolvent-inducedcrystallization.Experiencehasshownthatabsorptionofaliquidbyapolymermaybebettercorrelatedbyusingthepartialsolubilityparameters.

Liquid

MolarVolume

（cm3mol-1）

d

（MPa½）

p

（MPa½）

h

（MPa½）

Total

（MPa½）

Isooctane

Heptane

Cyclohexane

Ethylbenzene

Dioctylphthalate

Toluene

Methylethylketone

Chloroform

Tetrahydrofuran

Cyclohexanone

Acetone

o-Dichlorobenzene

1-Pentanol

Nitrobenzene

i-Propanol

Ethanol

Dimethylsulfoxide

Methanol

Ethyleneglycol

Glycerol

Water

TableI.Solubilityparametersforselected

Becausethesolubilityparameterofapolymercannotbecalculateddirectlyfromtheheatofvaporization,indirectmethodssuchassolventswellingandgroup-contributionapproachesareused.Solventsthatswellordissolvethepolymermosteffectivelywillhavesolubilityparametersclosetothesolubilityparameterofthepolymer,inkeepingwiththeadagethat"likedissolveslike."Inthegroup-contributionapproach,thesolubilityparameterisdeterminedbyusingtheequation

whereFiisthemolarattractionconstantandViisthemolarvolumeforeachsubsegmentofthepolymerrepeatingunit（asdemonstratedforPETinTableII）.

TableII.Estimationofsolubilityparameterforpolyethyleneterephthalate（PET）usingthegroup-contributionPETmolecularstructureshownattop.

TheabilityofthesolubilityparameterapproachtocorrelatetheabsorptionbehaviorofplasticshasbeendemonstratedbytheauthorsusingPET,PCTG（acopolyester）,andpolycarbonate.Piecesof3-mil-thickamorphous,unoriented,extrudedfilmweresuspendedinsealedjarsaboveafewmillilitersofliquid.Thefilmswereremovedeverytwoweeksforweighinguntiltheyreachedanequilibriumweight.Theresults,listedinTableIII,indicatethatallthreeplasticsappeartoshowabroadpeakwithamaximuminliquidabsorptionatasolubilityparameterofapproximately20MPa½.ThesolubilityparametersforPET,PCTG,andpolycarbonate,asdeterminedbythegroup-contributionapproach,areMPa½,22MPa½,andMPa½,respectively,whichfallintotherangeofthebroadmaximuminliquidabsorption.（Thealcoholsareexceptionstothistrend,asseeninTableI,presumablybecauseoftheirstronghydrogenbondingcharacteristics.）Theseresultshighlightthedifficultyinusingthesolvent-swellingtechniquefordeterminingthesolubilityparameterofapolymerandinusingthetotalsolubilityparametertocorrelateapolymer'sabsorptionbehavior.

Liquid

Total

（MPa½）

PET

（%）

PCTG

（%）

Polycarbonate

（%）

Isooctane

0

0

0

Heptane

0

0

0

Cyclohexane

0

0

1

Ethylbenzene

6

17

27

Toluene

12

21

31

Methylethylketone

13

15

25

Chloroform

57

D

D

Tetrahydrofuran

18

33

D

Cyclohexanone

21

28

65

Acetone

12

13

24

o-Dichlorobenzene

25

50

60

1-Pentanol

0

1

0

Nitrobenzene

28

50

58

i-Propanol

0

0

1

Ethanol

1

4

7

Dimethylsulfoxide

16

14

40

Methanol

3

5

8

Ethyleneglycol

0

0

1

Glycerol

0

0

1

Water

0

0

2

TableIII.PercentageweightgainforPET,PCTGcopolyester,andpolycarbonatefilminvariousliquids.D=dissolved.Valuesinitalicsindicatethatthefilmbecameopaque.

THEPLASTIC

Allplasticscanbeclassifiedaseitheramorphousorcrystallinematerials.Inamorphousplasticssuchaspolystyreneandpoly（methylmethacrylate）,thepolymerchainsarerandomlyconfigured,displayingnosignificantorder.Incrystallineplasticssuchaspolyethyleneandnylon6/6,thepolymerchainsarealignedororderedintocrystallites.

Crystallineplastics,however,arenevercompletelycrystalline,butrathercontainregionsofamorphousmaterial.Afewplastics,amongthemPETandpolycarbonate,canbeentirelyamorphousorsemicrystalline,dependingonprocessingconditions.Atroomtemperature,thethermodynamicallyfavoredstatefortheseplasticsisthecrystallineform;however,iftheyarecooledrapidlyenoughfromthemelttobelowtheirglass-transitiontemperatures（Tg）,theywillremainintheiramorphousforms.Undernormalinjectionmoldingconditions,partsmadefromsuchplasticsareclear,indicatingtheabsenceofcrystallinity.IfthefinishedpartsareheatedtoaboveTgorareexposedtostrongsolvents,theywillcrystallize.Thelatterphenomenonisoftencalledsolvent-inducedcrystallization,andwasobservedduringtheabsorptionstudiesdiscussedabove.

ThenominallyamorphousPET,PCTG,andpolycarbonatefilmsturnedopaqueuponexposuretocertainsolvents（seeitalicizeddatainTableIII）,indicatingthatcrystallizationhadoccurred.AbsorptionoftheseliquidsdecreasedtheTg'softheplasticstoatleastambientconditions,givingthepolymerchainssufficientmobilitytoalignandcrystallize.ItwasnotedthatcrystallinitydevelopedmuchmorequicklyinPETandPCTGthaninpolycarbonate,primarilybecausethecopolyestershavemuchlowerTg's（approximately80°C）thandoespolycarbonate（approximately150°C）,andthereforeneededtoabsorblessliquidbeforeTgwasdepressedtoambienttemperature.Solvent-inducedcrystallizationmayhaveapronouncedeffectonstress-crackingbehavior,aswillbediscussedlater.

Althoughbothamorphousandcrystallineplasticsaresusceptibletoenvironmentalstresscracking,itisgenerallyrecognizedthatamorphousplasticstendtobemoreat–5Thecloselypackedcrystallinedomainsincrystallineplasticsactasbarrierstofluidpenetrationandthereforetendtoresistenvironmentalstresscracking.

THETENSILESTRESS

Plasticswillexhibitenvironmentalstresscrackingwhenexposedtoanaggressivechemicalenvironmentifandonlyifatensilestressispresent.Thetensilestressmaybeappli