Preparation of aqueous dispersion of thermoplastic sizing agent for carbon fiber by emulsion/solvent evaporation

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DOI:10.1016/j.apsusc.2012.11.098

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http://dx.doi.org/10.1016/j.apsusc.2012.11.098

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Eprints ID: 8782

To cite this version:

Giraud, Isabelle and Franceschi-Messant, Sophie and Perez, Emile and

Lacabanne, Colette and Dantras, Eric Preparation of aqueous dispersion of

thermoplastic sizing agent for carbon fiber by emulsion/solvent evaporation.

(2013) Applied Surface Science, vol. 266 . pp. 94-99. ISSN 0169-4332

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T

oulouse

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OATAO

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Preparation

of

aqueous

dispersion

of

thermoplastic

sizing

agent

for

carbon

fiber

by

emulsion/solvent

evaporation

Isabelle

Giraud

_,

_Sophie

_{Franceschi-Messant}

_,

_Emile

_Perez

,

Colette

Lacabanne

,

Eric

Dantras

a_{LaboratoiredesI.M.R.C.P.,UMR5623CNRS,UniversitéPaulSabatier,31062ToulouseCedex09,France}

b_{LaboratoiredePhysiquedesPolymères,CIRIMAT,InstitutCARNOT,UniversitéPaulSabatier,31062ToulouseCedex09,France}

Keywords: Sizing Carbonfibers Composites Thermoplasticpolymer Aqueousdispersion Emulsion/solventevaporation

a

b

s

t

r

a

c

t

Inthiswork,differentsizingagentaqueousdispersionsbasedonpolyetherimide(PEI)wereelaboratedin ordertoimprovetheinterfacebetweencarbonfibersandathermoplasticmatrix(PEEK).Thedispersions wereobtainedbytheemulsion/solventevaporationtechnique.Tooptimizethestabilityandthefilm formationonthefibers,twosurfactantsweretestedatdifferentconcentrations,withdifferent concen-trationsofPEI.Thedispersionsobtainedwerecharacterizedbydynamiclightscattering(DLS)andthe stabilityevaluatedbyanalyticalcentrifugation(LUMiFuge).Theselecteddispersionsweretestedforfilm formationabilitybyscanningelectronmicroscopy(SEM),andthesizingperformancewasassessedby observationofthefiber/matrixinterfacebySEM.TheresultsrevealedthatanaqueousdispersionofPEI, stabilizedbysodiumdodecylsulfateasthesurfactant,ledtoverystablesizingagentaqueousdispersion withidealfilmformationandbetterinterfaceadhesion.

1. Introduction

Carbonfiberiswidelyusedasareinforcingmaterialin

com-posites,especiallyinadvancedcomposites[1,2].Ascarbonfibers

arebrittle,manyproblems,suchasfilamentbreakageandfluffing,

ariseduetomechanicalfrictionduringthemanufacturingprocess

[3–5].Therefore,carbonfibersaregenerallysizedorcoatedbya

sizinglayeronthesurface,whichisusuallyobtainedfroma

solu-tionoremulsionconsistingofpolymericcomponents[6,7].Sizing

easesfiberhandlingandcanalsoprovideacouplingagentforthe

fiber/matrixbond [8–11]. Thenatureof thesizingis oftenkept

secretbymanufacturersofcarbonfibers.However,sizingischosen

accordingtothenatureofthematrixandisgenerallyapre-polymer

orpolymer.Mostofthecompositesaremadefromepoxyresin,and

sizingagentsareoftenofthesamenature[5,12–14].Thisisa

prob-lemwhenthematrixisahigh-temperaturethermoplasticpolymer

sincethedegradationtemperatureofthistypeofsizingisaround

250◦_{C [15]. For polyimides, PEEK and other high-temperature}

thermoplastic polymers,thefunctional groups provided bythe

traditionalepoxy-compatiblesizingdonotreactchemicallywith

thesepolymersandweakinterfacialshearstrengthsresult[16].

Moreover,forcompositesmoldedwithpolyimidesorPEEK,high

processingtemperaturesduringmanufactureandcontinuoususe

∗ Correspondingauthor.

E-mailaddress:perez@chimie.ups-tlse.fr(E.Perez).

inhigh-temperatureenvironmentsdegradetheepoxysizingand,

consequently,weakenthefiber/matrixinterface,producingvoids

anddelaminations[17–19].Alloftheseobservationsunderlinethe

importanceofhavingsizingthatissuitableforhigh-temperature

thermoplasticmatrices.Fromapracticalpointofview,thesizing

formulationshouldbeeasytouse,non-toxicandenvironmentally

friendly.Inthispaper,wereportthefirstexampleofpreparation

anaqueousdispersionofathermoplasticsizingagentforcarbon

fiberbyemulsion/solventevaporation.

2. Experimental

2.1. Materials

ThepolyetherimidePEI(Ultem1000)wasobtainedfromSabic®_.

Thesodiumdodecylsulfate(SDS)andthechloroformwere

pro-videdbySigma–Aldrich,thebenzalkoniumchloride(BC)wasfrom

Fluka (C12 60%, C14 40%). The AS4 carbon fiber tow, provided

byHexcel,wastreatedunsizedandcontained12,000fibers.The

polyetheretherketone(PEEK)providedbyVictrexwasa 100mm

thickfilm.TheremoldingagentwasCIREX041WBfromSICOMIN.

2.2. PreparationofPEIdispersionsbyemulsion/evaporation

In order to reduce thetoxicity and to respect the

environ-ment,organicsolventsmustbeavoidedinthefinalsizingagent

formulation.Forthesereasons,wedecidedtoelaborateaqueous

Fig.1.Schematicrepresentationofthepharmaceuticalemulsification/evaporationprocess.

dispersions.Thesecanbemadebyavarietyofmethods[20–23]

leading,attheend,tostablehydrophobicparticlesinwater.The

preparationprocessdescribedherewaslargelyinspiredby

emul-sion/solventevaporation,anencapsulationtechniqueusedinthe

pharmaceuticalindustrytoprepareaqueousdispersionsof

poly-mernanoparticlesormicrospheres.

Emulsion/solvent evaporation involves a two-step process

(Fig.1):theemulsificationofa polymersolutioncontainingthe

encapsulatedsubstance,followedbyparticlehardeningthrough

solventevaporationandpolymerprecipitation.Duringthewater

emulsification, the polymer in solution in the volatile,

water-immisciblesolventisbrokenintomicrodropletsbytheshearstress

producedbyeitherahomogenizerorasonicatorinthepresenceof

asurface-activeagentuntilthepolymerprecipitates[24–27].

Thismethodwasusedtoprepareastableaqueousdispersionof

PEIasthesizingagent.Weusedtwodifferentsurfactants,sodium

dodecylsulfate(SDS)andbenzalkoniumchloride(BC)atdifferent

concentrations(0.3%,0.5%and1wt%).Thefinalconcentrationsof

PEIwere0.1%,0.3%,0.5%and1wt%.ThePEIdispersionat0.5wt%ina

0.5wt%surfactantsolutionwaspreparedasfollows.Ina5-mLflask,

0.1005gofPEIwasdissolvedin2mLofchloroform.Thissolution

waspouredintoanotherflaskcontaining20mLofthesurfactant

solution.Themixturewasemulsifiedbyultrasoundshearing(Vibra

Cell,BioblockScientific600W,20Hz).Theshearinglasted5minat

power4.Awaterbathwasusedtomaintainthesolutionatroom

temperature.Then,magneticstirringoftheemulsionat1200rpm

for12hallowedtotalevaporationofthechloroform.

2.3. CharacterizationofPEIdispersions 2.3.1. Particlesizeanalysis

Dynamiclightscattering(DLS)wasperformedusingaMalvern

InstrumentsNanoZSwithaHe–Nelaser(633nm)atascattering

angleof173◦_andat25±1◦_{C.Thehydrodynamicmeandiameterof}

thenanoparticleswasdeterminedusingthesoftwareprovidedby

MalvernInstruments.TheContinmodelwasappliedtoobtainsize

data.Alltheauto-correlationfunctionfitswerecheckedandfound

tobeinaccordancewiththeexperimentaldata.Fivemeasurements

weremadeoneachsamplewithanaccuracyofabout2nm.

2.3.2. Evaluationofstabilityusinganalyticalcentrifugation

Aseparationanalyzer(LUMiFuge,L.U.M.Berlin,Germany)was

usedtodeterminetheseparationbehaviorofdispersionsunderthe

influenceofvariouscentrifugalforces(5–1000×g).Thisapparatus

isbasedona low-speedcentrifugecombinedwithan

optoelec-tronicmeasuringsystemthatrecordsthelighttransmissionover

theentire sample cuvette (Fig. 2). The cuvettes containingthe

suspensionare positioned in thehorizontal planeon therotor

ofthecentrifuge.Duringcentrifugationalightsourcepositioned

abovetherotoremitsradiation(near-infrared)ontothesample.

TransmittedlightisdetectedbyaCCDlinesensorbelowtherotor

planeandisanalyzedbyamicrocontroller,whichgeneratesa

light-transmissionprofileof thesampleareafor everymeasurement

step.

This technique is very appropriate for the study and

opti-mizationofverystableaqueousdispersions.Thecentrifugalforce

acceleratesthedestabilizationofthedispersionandrapidly

deter-minestheshelflifeofthedispersion[28].Moreover,thepossibility

ofstudying8samplesatthesametimeenablesdifferent

formu-lationstobecomparedimmediately[29,30].Thedispersionsare

naturallystableover6monthssowechosetosimulate3yearsof

aging.Thedataacquisitioncorrespondedto255profilesrecorded

everyeverysecondsat4000rpm.Thetemperaturewas20◦_C.

2.4. Sizingtreatmentofcarbonfiberandcompositepreparation 2.4.1. Sizingofcarbonfiber

Differentmethodscanbeusedtosizecarbonfibers,such as

electrodeposition[31,32]orelectropolymerization[33,34],butthe

mostcommonisbathcoating.Wetestedthesizingatlaboratory

scaleso,inthiscase,themostsuitabletechniquewastospraythe

dispersiondirectlyontothefibersurface.Anunsizedfibertowwas

strainedbyaweighttokeepitvertical,allowinguniformspraying

ofthesizingatthefibersurface(Fig.3).Aftersizing,thefiberswere

driedatroomtemperature.

Fig.3. Schematicrepresentationofthesizingprocess.

2.4.2. Compositepreparation

WealsopreparedPEEK/unidirectionalcarbonfibersamplesat

laboratoryscale.Thesampleswerepreparedbyhotpress

mold-ing.ThepressusedwasaCarver4128CEequippedwithheating

plates.Theprocessingtookplaceinseveralsteps.Thesamplewas

firstprepared,thenmoldedinthehotpressandfinallycooledand

remolded.Inordertokeepallthecarbonfibersinthesame

direc-tionduringthedifferentsteps,thestrandsofcarbonfiberwere

insertedinafoldedPEEKfilm(Fig.4).

The sample was then placed in an aluminum mold

previ-ouslycoatedwiththeremoldingagent.Thenthemoldwasplaced

betweenthetwoplates,previouslyheatedto400◦_C,andkeptin

contactfor15mintoallowthePEEKtomeltuniformly.6MPaof

pressurewasthenappliedfor30stoletthePEEKimpregnatethe

fibers.Finally,thesamplewasaircooledandremoldedatroom

temperature.Thefinalsamplecontained30wt%ofcarbonfibers.

2.5. CharacterizationofPEIfilmsandcomposite 2.5.1. Scanningelectronmicroscopy(SEM)analysis

Thedifferentsampleswereexaminedusingascanningelectron

microscope(JEOLJSM6700F)withanacceleratingvoltageof5kV.

Thefilmsobtainedafternaturaldryingatroomtemperaturewere

mountedonaluminumstubsand sputtercoatedwithgold.The

compositeswerefreezefracturedinordertoobservetherupture

faces.

3. Resultsanddiscussion

3.1. Sizingagentformulation 3.1.1. Stabilitystudy

We selected PEIas thesizing agent becauseit is a

thermo-plastic polymer with high heat resistance [35], miscible with

polyetheretherketone(PEEK)[36],andsolubleinchlorinated

sol-ventslikechloroform.Severalfactorsinfluencethestabilityofthe

dispersion,suchasthenatureandtheamountofsurfactant.

Usu-ally,thechoiceofsurfactantdependsonthenatureoftheparticles

and,in particular,theirsurface charge.SincePEIhasnospecial

charge,thesurfactantcanbeanionicorcationic.Itisalsoimportant

Fig.4. Schemeofthesamplepreparation.

Fig.5.Influenceofthenatureandconcentrationofthesurfactantonthemean particlediameter([PEI]=0.5wt%).

todeterminetherightquantityofsurfactanttomaintainastable

dispersion.PEIconcentrationisalsoanimportantparameter.The

emulsion/evaporationmethodisnotsuitableforthepreparation

ofconcentrateddispersionsbut,inthecaseofsizing,thisisnota

limitationbecausetheconcentrationofpolymerdoesnotexceed

1wt%[37,38].

First,westudiedtheinfluenceofthenatureandthe

concentra-tionofthesurfactant,andalsotheinfluenceofthePEIconcentration

onthecharacteristicsofthesizingdispersions.

Thefirstparametertobeconsideredwastheparticlesizeas

itiswellknownthatthesmallertheparticlesare,themore

sta-blethedispersionwillbe.Dynamiclightscatteringmeasurements

(DLS)wereperformedonallthedispersions.Theinfluenceofthe

surfactantonthemeandiameterofparticlescanbeseeninFig.5.

Themeandiametersoftheparticleswerelessthan100nmand

favored stabledispersions.The natureofthesurfactantdidnot

haveasignificanteffectontheparticlesizeeventhoughthe

par-ticlesseemedsmallerwiththeBCsurfactant.Ontheotherhand,

thediametersvariednoticeablywiththesurfactantconcentration.

Thehighertheconcentrationwas,thesmallerweretheparticles.

Atlowconcentration,therewasnotenoughsurfactantto

main-tainsmalldropletsofchloroformandthisdeterminedthefinalsize

oftheparticles.Althoughthesmallestparticleswereobtainedfor

1wt%,theconcentrationof0.5%waspreferredinordertominimize

theamountofsurfactantinthefinalformulation.

Thesamestudywasperformedtoobservetheinfluenceofthe

PEIconcentration(Fig.6).Theparticlesizeincreasedquitelinearly

withtheconcentrationforbothsurfactantsbutthemean

diam-eterremainedunder 100nm.This resultwasrelated toseveral

factors.Thefirstwastheratiobetweentheconcentrationof

surfac-tantandtheamountofchloroformphasecontainingthedissolved

PEI[23].Thesecondwastheviscosityoftheorganicphase[23].

Fig.7.EffectofthePEIconcentrationontheclarificationkineticsfordispersionsat0.3%BC.

IncreasingthePEIconcentrationinchloroformincreasedthe

vis-cosityofthesolution.Giventhattheshearforceswerealwaysthe

same,whentheconcentrationofPEIwastoohigh,therewasnot

sufficientenoughavailabletocreatesmalldropletsofchloroform.

Theparticlesizeforthelowestconcentrationswasverysmall,

lead-ing,inprinciple, tothemoststable dispersions.However,for a

sizingformulation,thedispersionsmusthaveaminimumof0.5%

or1wt%ofPEI.

An interesting stability analysis consisted in determining a

destabilization velocity by accelerating the gravitation by

cen-trifugation. This kind of analysis could be performed withthe

“LUMiFuge”apparatus.Thistechniqueissuitabletooptimizevery

stabledispersions(stableformorethan6months).

Fromtheprofiles,anintegraltransmissionwascalculatedasa

functionoftime.Forinstance,theinfluenceofthePEIconcentration

onthestabilitycanbehighlightedimmediately(Fig.7).

Fromthisgraph,aclarificationvelocity,correspondingtothe

slopeofthefirstlinearpartofthecurves,wascalculatedbythe

“SEPView”software.Thesteepertheslope,themoreunstablethe

dispersion.Theclarificationvelocitywascalculatedforthedifferent

dispersionsandcomparedsoastohighlighttheeffectofdifferent

parameters.

AsshowninFig.8,thenatureofthesurfactantdidnothavea

significantinfluenceonthevelocity,exceptfor0.1wt%PEIsolution,

whereBCwaslessefficient.Consideringthesurfactant

concentra-tion,itseemsthat,from0.5wt%,thestabilityreachesaplateau.This

resultindicatesthatitisnotnecessarytousemorethan0.5wt%

surfactantsolutionstoincreasethestability.

As expected, the PEI concentration had a major impact on

thedispersionstability (Fig.9).Theclarificationvelocitytripled

between0.5%and1wt%.Althoughtheshelflifecannotbe

deter-mineddirectly from theclarification velocity, the real stability

Fig.8. Influenceofthenatureandconcentrationofthesurfactantontheclarification velocity([PEI]=0.5wt%).

periodextrapolatedfromthedatawasestimatedtobearound6

monthsforthe1%PEIdispersion.

Consideringtheaboveresults,boththetestedsurfactantswere

usable.However,benzalkoniumchloridemightbemoreinteresting

becauseofitsantimicrobialandlowfoamingproperties.

Concern-ingthedifferentconcentrations,agoodcompromiseseemstobe

0.5wt%ofsurfactantand0.5wt%ofPEI.

3.1.2. Filmformation

Forsizing,thecoating,andconsequentlytheformationofafilm,

isaveryimportantproperty.Alltheaqueousdispersionsprepared

wereabletoformfilmsafterwaterevaporation.Toensurethe

qual-ityofthefilm,twochosendispersionswereobservedbySEM.One

wasmadewithSDSandtheotherwithBC,andbothcontained

0.5wt%ofsurfactantand0.5wt%ofPEI.

ThesurfaceaspectofthePEIfilmobtainedwithSDSwasvery

homogeneous(Fig.10).Thecrackswereduetouncontrolled

evapo-ration.Thisparameterwillneedtobetakenintoaccountforfurther

applications.Themagnificationofthisfilmshowspartiallyfused

PEIparticles(Fig.11).Thisobservationistypicaloflatexfilm

for-mation,andisidealforahomogeneouscoating.

ThePEIfilmformedbytheBCdispersionwasverydifferent.

Fig.12isanSEMobservationofthisfilmshowingaheterogeneous

surface.Themagnificationshowsthat,infact,theparticlesformed

agglomeratesbutdidnotfuse(Fig.13).Thedifferenceinfilm

forma-tioncouldbeexplainedbytheabilityofthesurfactanttobedrained

outoftheevaporatingfilm[39–41].Wehavetoconsiderthe

affin-ityofthesurfactantwiththesurfaceofthePEIparticlestoexplain

thisbehavior.Itseemsthat,comparedtoBC,SDShasalower

affin-itywiththesurfaceoftheparticlesandismainlydrainedoutofthe

film,leadingtothefusionoftheunprotectedparticlesandfinally

toahomogeneousfilm.

Fig.9. InfluenceofthePEIconcentrationontheclarificationvelocity ([surfac-tant]=0.5wt%).

Fig.10.SEMobservationofthefilmfromtheSDSdispersion.

Fig.11.MagnificationofFig.10.

Fig.12.SEMobservationofthefilmfromtheBCdispersion.

Fig.13.MagnificationofFig.12.

Consideringthesefilmformationresults,thebestdispersions

forasizingapplicationseemtobethoseobtainedwithSDSasthe

surfactant.Itisveryimportanttoobtainahomogeneouscoatingon

thecarbonfibers.

3.2. Sizingevaluation

Theaimofthisstudywastoelaborateastableaqueous

disper-sionusableasathermoplasticsizingformulationforcarbonfibers.

Thesizing hasvariousroles,suchasfacilitating thehandlingof

fibersandimprovingtheinteractionsbetweenthematrixandthe

fibers.

Toevaluatetheeffectofthisnewsizing,PEEK/carbonfiber

com-positesweremade;onewithunsizedcarbonfibersandanother

withPEIsizedcarbonfibers.Thechosensizingwastheaqueous

dispersionwith0.5wt%ofPEIand0.5wt%ofSDS.Thebestwayto

highlighttheinfluenceofthesizingwastoobservethefiber/matrix

interface.Forthatpurpose,thecompositeswerefreezefractured

transversallyandobservedbyscanningelectronmicroscopy.

Fig.14correspondstoanunsizedcarbonfibercompositeand,as

wecansee,therearevoidsandnointeractionsbetweenthePEEK

matrixandthecarbonfibers.Incontrast,theinterfacebetweenthe

Fig.15. PEIsizedcarbonfibercomposite.

compositeandthePEIsizedcarbonfibersiscontinuous(Fig.15).

Inthiscase,thereisarealbondbetweenthePEEKandthecarbon

fibers.Theseobservationsconfirmnotonlythatthesizingremains

duringthecompositeprocessingbutalsothatthematrixandthe

carbonfibersareconnectedbythesizingagent.

4. Conclusions

Theanalysesperformedonthedifferentaqueousdispersions

revealedthatthequantityofparticleswasasignificantfactorfor

stability.TheparticlesizeincreasedgreatlywiththePEI

concentra-tion,whichtendedtodecreasethestabilityofthedispersion.The

LUMiFugestudyconfirmedthisresult.ThebestPEIconcentration

obtainable bytheemulsion/evaporation technique was0.5wt%.

Thedispersions werestable1 yearat 0.5wt%,nevertheless the

dispersionat1wt%remainedstablefor3months.Concerningthe

natureofthesurfactant,benzalkoniumandSDSallowedstable

dis-persionstobeobtained.Nevertheless,thebenzalkoniumdispersion

didnotformahomogeneousfilmandsowasnotsuitablefor a

sizingapplication.Incontrast,withSDSdispersion,thefilmwas

reallyuniformandweobservedacoalescencephenomenon

typi-caloflatexfilmformation.Theconcentrationofthesurfactantalso

hadaninfluenceontheparticlesizeandstability,and0.3%wasnot

enoughtoobtainagooddispersion.Thegaininstabilityobtained

at1%didnotjustifytheuseofsuchaconcentrationconsidering

thattherewasonly0.5wt%ofPEI.Sothebestconcentrationof

surfactantwas0.5wt%.

Consideringalltheresults,thechosendispersionforsizingwas

0.5%PEIand0.5%SDS.Theefficiencyofthisnewsizingagent

aque-ousdispersionwasappreciatedthroughSEMobservations,which

showedacontinuousinterfacebetweenthecarbonfibersandthe

PEEKmatrix.

Acknowledgements

WethankMrJ.M.Bergerat,AIRBUSIndustry,ToulouseFrance,

for useful discussions and advice. The financial support of FUI

INMAT2andAIRBUSisgratefullyacknowledged.WealsothankMr

D.KemmishandMrA.WoodfromVictrexInc.,fortheirinsightful

commentsandfruitfuldiscussions.

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