ImprovingAdhesiveBondingofCompositesthroughSurface

Characterization

BrianFlinnandMarcStaigerUniversityofWashington

DepartmentofMaterialsScience&Engineering

AMTASautumn2017reviewmeeting,11/8/17UniversityofWashingtonCtr forUrbanHorticulture

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Participants• FederalAviationAdministration

– AhmetOztekin,CindyAshforth,DavidWestland,CurtisDavies• CenterofExcellenceinAdvanceMaterialsinTransportAircraftStructures

(AMTAS)

• TheBoeingCompany• EpicAircraft• Textron

• UniversityofWashingtonDepartmentofMaterialScience&Engineering– Dr.BrianFlinn,PrincipalInvestigator– MarcStaiger ,RyanToivola,ShawnBaker,AmyChiu,AlexGray(Blush)– RitaTaitano Johnson(iGC)

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TwoTasks

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1. SurfaceCharacterizationusingInverseGasChromatography(iGC)Methods

– CharacterizationofAdherend Surfaces• Effectofsurfacepreparationonbondquality

2. AmineBlushinEpoxyPasteAdhesives– CharacterizationofBondline

• Effectofbondingenvironmentonbondquality

Introduction

Questions:• Whataretheconditionsforblushformation?

• Whataretheeffectsonbondquality?

• Canblushbemitigated?

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AmineBlush

• Amineblushisasurfacephenomenoninamine-curedepoxyadhesives

• Blushlayerformsinterfacebetweenadhesives,leadingtolowerstrength,hard-to-detect‘kissbonds’

Adhesive

Adherend

Adhesive

Adherend

Adhesive

Adherend

Adhesive

Adherend

Blush

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AmineBlushFormation

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• Bloom– Mobileaminecomponentdiffusestothesurfacecreatinggreasy,tackylayer.

• Blush- Theaminerichlayerreactswiththeatmospheretopotentiallyformcarbamicacid,carbamates,carbonates,orbicarbonates.

• Thebloom/blushprocessdegradestheoverallqualityofthedesiredepoxy-aminecrosslinkreaction.

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*

**

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MitigationTechniques

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• EnvironmentalControls

• IncubationTime

• SurfaceCombing/Scraping

• ProtectiveFilm

• Limitaminemobility

− Bulkyaminemolecularstructure

− Highresinsystemviscosity

Surfacescraping

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PreviousWork

• DOEfocusedon4variables:– Exposuretimeaftermixing– Temperature– Humidity– CO2

• MechanicalStrengthMeasurement– T-Peel(ASTMD1876)

• SurfaceCharacterization– FTIR(ATR)

• Samplesplacedinaconditioningchamberatpredeterminedconditions.

• Pressedinahydraulicpress.

• Postcuredafter48hrs.

• Samplescutonawaterjet

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• Aluminum substrates (primed)

• Bond does not extend full length of substrates

• After bonding, non-bonded substrate ends bent to a 90º angle

• Ideal failure is mode 1 - cohesive

T-Peel Test – ASTM D1876

PreviousWork– MechanicalResults

• Exposuretime,humidityandtemperaturehadthemosteffectonT-peelstrength

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0

2

4

6

8

10

12

14

-1 0 1

lbf/in

TemperatureEffect

0

2

4

6

8

10

12

14

-1 1

lbf/in

HumidityEffect

Load(lbf)

PredictedLoad(lbf)

0

2

4

6

8

10

12

14

-1 1

lbf/in

MixedExposureEffect

High – 80 °FLow – 60 °F

High – 85 %RHLow – 40 %RH

High – 45 minLow – 15 min

HighStrengthBonds

• Unexposedbond

• Fracturetravelsthroughthebulkadhesive

LowStrengthBonds

• HighexposureConditions

• Fracturetravelsthroughthebondline interface

• Flowlinesfromsqueeze-outarevisible

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PreviousWork– FailureMode

FTIRSurfaceCharacterization

• AttenuatedTotalReflectance(ATR)FTIRsamplingusedtocharacterizesurfacechemistrychanges.

• Measuressurfaceabsorption/transmittanceofIRenergy− 1st 0.5to5microns.

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• Vibrationalfrequenciesofbondinggroupsaredeterminedthroughaspringandmassmodel.Wherek=bondingstiffnessandm=atomicmass.

FTIRSurfaceCharacterization

EpoxyNetworkIRVibrationalAbsorbance

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C-H,N-HandO-HStretching

SingleCarbonBondsandFunctionalGroupBending/VibrationalModes.(FingerPrintRegion)

DoubleandTripleCarbonBondBending

PreviousWork– FTIRAnalysis

• Identifiedtheimportantpeakscorrespondingtoaminebloom/blush/saltformation• ConfirmedwithFTIRmeasurementsof“traveler”coupons• Arrowsindicategrowthofblush/bloomrelatedpeaks

Epoxyw/noblush

Epoxyw/highblush

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CurrentResearchPlan

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• ExamineFTIRtravelercouponsmadeofuncured epoxybondlinesratherthanepoxiesaftercure.

• Examinebondswithlowerexposuretimesthanstudiedinpreviousresearch.

• T-peelspecimenswithvaryingtimesbetweenspreadingandclose-out

• Examineenvironmentalconditionsforpost-spreadingexposuresandcorrelatewithFTIR.

FTIRresults

• Characterizationofamineblush:Closeexaminationof1750-1250cm-1 spectralregion• Assign1508cm-1 peaktoaromaticringofepoxymonomer(referencepeakintensity)• 1465cm-1 peakassignedtoaminemonomer• Bothpeakspresentinfullycuredadhesive• Nothinginregion1510-1575cm-1 ineithermonomerorcuredadhesive

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

12501300135014001450150015501600165017001750

Absorban

ce(o

ffset)

Wavenumber(cm-1)

partA- epoxypartB- aminecured

1508

1465

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FTIRResults

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

12501300135014001450150015501600165017001750

Absorban

ce(o

ffset)

Wavenumber(cm-1)

0minutes 2.5minutes 5minutes10minutes 15minutes partA- epoxypartB- amine

1312

• ExposuretimeafterspreadingofadhesiveatRT,in7milthickness.• Obviousnewspeciesrepresentedby1563cm-1 peak,growingovertime

15631483

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FTIRResults

0

0.2

0.4

0.6

0.8

1

1.2

12501300135014001450150015501600165017001750

Absorban

ce(o

ffset)

Wavenumber(cm-1)

0minutes 2.5minutes5minutes 7.5minutes10minutes 12.5minutes15minutes partA- epoxypartB- amine ammoniumbicarbonateNISTammoniumcarbonateNIST

• ComparisonwithNISTstandardssuggestsnewspeciesisnotammoniumcarbonateorbicarbonate

• Carbamateislikelybasedonpreviousliterature• 1563,1483,and1312cm-1 FTIRsignatureconsistentwithIR-absorption

peaksofcarbamatereportedinliterature. 19

0

0.1

0.2

0.3

0.4

0.5

0.6

12501300135014001450150015501600165017001750

Absorban

ce(o

ffset)

Wavenumber(cm-1)

-COOVibrationalMode(Carbamate)

AromaticVibrationalMode(Epoxy)

-COOVibrationalMode(Carbamate)

-NCOOVibrationalMode(Carbamate)

FTIRResults

• Analysis:Usearomaticringfromepoxyasareference,sinceitdoesnotparticipateinamineblushorcure.

• Plotratioofsuspectedcarbamatepeak,andsuspectedaminepeak,tothisreferenceovertime 20

FTIRResults

• Usingaromaticepoxyring(1508cm-1)asreference,plotchangesvstime.

• Rapidincrease,peakingat15minexposuretime,thenstableorslightdecrease.

• Suggestsrapidconversionofamineuponreachingsurface.

0

0.5

1

1.5

0 10 20 30 40 50 60 70

Peakra

tio

Exposuretime(min)

Carbamate/Epoxy

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FTIRResults

• Behaviorunchangedby– stoichiometry(50%excessamine),10xthickness,lowertempto60°F

• Increasingexposuretemperatureto122°Fpreventedblushformation,probablybyconsumingaminesinbulkbeforesurfacereactionscouldoccur

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70

Peakra

tio

Exposuretime(min)

standardconditionsthicksamplesexcessamine60F122F

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T-PeelResults

• Studiedeffectofpost-spreadtimeonpeelstrength

• Adherends:.020”anodizedaluminum.Conditions:70°F,41%RH

• MadeT-peelswithvaryingexposures,closed,curedunder10psiat150F1hr,150F1hrrampedpostcure

• Spread.020”adhesiveperadherend,compressedto.010”feelergauges

• Resultingbondline thickness~.015”

.020”PAAAl

Adhesivebondline

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Bondstrengthtesting

0

5

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15

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25

30

35

0 1 2 3 4 5 6 7

Load

(lbf)

Extension(in)

0min15min30min45min

0

2

4

6

8

10

12

14

16

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0 20 40 60

Peelstrength(lb-f/

in)

Exposuretime(min)

• Bondstrengthisaffectedbypost-spreadexposure• ConsistentwithFTIRdataonblushformationvstime• Note:samplesdesignedtoformblush andresultsmaybeconservative

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Bondstrengthtesting

• Adherends andfracturesurfacesof0exposure(top)and45minexposure(bottom)• Instrongbond,failureatadhesive-primerinterface(oldanodizedsurface)• Inweakbond,failureatbondline (blushlayer)

0exposurestrongbond

45minexposureweakbond

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FutureDirections

Spreading Close-outIsamineblushpresent?

FTIRspectroscopy

Yes

No

Mitigation

Exposure

• Itmaynotbepossibletocloseoutbondsin15minutesorless− Blushwillforminmostenvironmentswiththisadhesive.

• Mitigationstrategiesshouldbepursuedto“undo”effectsofamineblush.− Surfacecombing/scraping.− Adhesivesqueeze-out.− Sacrificial/protectivefilms.

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Conclusions

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• FTIRonuncured‘traveler’couponscanmonitorblushformationforpost-spreadexposures

• Short-timeexposuresafterspreadingcanproducesignificantamineblushandcanaffectbondstrengths

• T-peeltestingcontinuestobeasensitivetestforbondstrength,butmaynotrepresentallbondingproceduresandgeometries

Deliverables• Amethodologyfordetection,quantification,andmonitoringofamineblush

formationinbondsbeforeclose-out

• AtestforcorrelationbetweenT-peelstrengthandamineblushformation

FutureWork• CorrelateATR-FTIRmeasurementsof‘traveler’sampleswithT-peelstrength

measurementsforshortdurationexposuresinvariousenvironments

• Developamethodologyforgeneratingamapoftheenvironmentalprocessspacebasedonamineblush/FTIRmeasurements

• DemonstratemethodologyonEA9360adhesivesystem

• Explorepossibilityofhand-heldATR-FTIR+chemometric analysisasapredictorofbondstrengthbasedonIRchanges

• Exploreamineblushformationinotherpasteadhesiveformulations

• Exploremitigationstrategies–• Highturbulencesqueeze-outto‘breakup’amineblushlayer• FEPprotectivefilms,or“peel-ply”typeremovablelayerstostripblush

awayimmediatelybeforeclose-out• Combingpastelayerimmediatelybeforeclose-out

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