Flow and mixing efficiency characterisation in a CO2-assisted single-screw extrusion process by residence time distribution using Raman spectroscopy

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Flow and mixing eﬀiciency characterisation in a

CO2-assisted single-screw extrusion process by residence

time distribution using Raman spectroscopy

Audrey Common, Élisabeth Rodier, Martial Sauceau, Jacques Fages

To cite this version:

Audrey Common, Élisabeth Rodier, Martial Sauceau, Jacques Fages. Flow and mixing eﬀiciency

characterisation in a CO2-assisted single-screw extrusion process by residence time distribution using

Raman spectroscopy. Chemical Engineering Research and Design, Elsevier, 2014, 92 (7), p. 1210-1218.

�10.1016/j.cherd.2013.10.013�. �hal-01611624�

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Flow

and

mixing

efficiency

characterisation

in

a

CO

2 -assisted

single-screw

extrusion

process

by

residence

time

distribution

using

Raman

spectroscopy

Audrey

Common

1

_,

_Elisabeth

_Rodier,

_Martial

_Sauceau,

_Jacques

_Fages

∗

UniversitédeToulouse,EcoledesMinesd’Albi,CNRS,RAPSODEECentre,81013Albi,France a b s t r ac t

Hot-meltextrusionofabio-sourcedpolyamidehasbeenimplementedinasingle-screwextruderwithsupercritical carbondioxideinjection.CO2actsasaplasticiserintheextruderbarrelandasaphysicalblowingagentatthedie.

ToinsureabettermixinganddissolutionoftheCO2intothepolymermelt,additionofastaticmixerbetweenthe

screwtipandthediewastested.TheeffectofboththestaticmixingelementandtheCO2injectiononthemelt

flowbehaviourhasbeenelucidated.Arecenttechniqueofin-lineRamanspectroscopywasimplementedtomake aresidencetimedistributionstudy,usingtitaniumdioxideasatracer.Theuseofastaticmixerexertsamajor modificationontheflowbehaviour:itimprovesmixingbyenhancingdispersion.Inaddition,thestructureofthe manufacturedproductswasstudied:thestaticmixerledtomorehomogeneousporousstructure.Thebroadrange ofCO2incorporation(upto25%,w/w)intothemeltledtothemanufactureoffoamswithadjustableporosityfrom

15to70%.

Keywords: Hot-meltextrusion;Supercriticalcarbondioxide;Residencetimedistribution;Staticmixer;Raman spec-troscopy

1. Introduction

Residencetimedistribution(RTD)measurementisaclassic methodusedinchemicalengineering.RTDfunctionisa prob-abilitydistributionfunctionthatmonitorsthetimeeachfluid elementspendsinsideaprocessoperation.Ithasbeenused bychemicalengineersinextrusionprocessesformorethan 40yearstocharacteriseandmodelthemixingandtheflow withintheextruderbarrel(PintoandTadmor,1970).

Single-screwextrudersarewidelyusedinseveral indus-triesbecauseoftheirsimplicityandrobustness.Processing polymersbyhot-meltextrusionwithsuchextrudersisa com-montechniquepresentinseveralindustrialsectors.Although moderngeometrieshaverecentlyimprovedthemixing capac-ities of single-screw machines (Bi and Jiang, 2009; Wang etal.,2010)thepoormixingabilityremainsoneofthemajor

∗_{Corresponding}_author_at:_Ecole_des_Mines_d’Albi,_Campus_Jarlard,_F-81013_Albi,_France._Tel.:₊₃₃₅₆₃₄₉₃_141;_fax:₊₃₃₅₆₃₄₉_3025.

E-mailaddress:Jacques.Fages@mines-albi.fr(J.Fages).

i d il i di i df b d b

1 _Present_address:_Feyecon,_Inc.₁₃₈₂_GS_Weesp,_The_Netherlands.

drawbacks of such process. Therefore, the study of the residence time distribution to better understand the flow behaviourintheextruderisausefulmethodwhichhasbeen welldocumented(Seker,2005).

Theadditionofstaticmixingelementsjustbeforethedie isacommontechniquetoensureabetterhomogeneityofthe meltenteringthedie.Thakuretal.(2003)havereviewedthe useofsuchmixersintheprocessindustries.Another tech-niquetoimproveandenlargetheapplicationofsingle-screw extrusionistheadditionofsupercriticalcarbondioxideinthe barreltoactasaremovableplasticiseraswellasafoaming agent (MulvaneyandRizvi,1993;Sauceauetal.,2011).This additionexertsalsoaninfluenceontheglobalpolymerflow

(SinghandRizvi,1998;Nikitineetal.,2009).

In-linespectroscopicanalysisisdevelopingquicklymainly becauseitavoidstediousproceduresofsamplingandoff-line

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analyses(Barnesetal.,2007).ArecenttechniqueofRTD deter-minationby Ramanspectroscopy (Saerensetal.,2011)has beenimplemented.

Thisstudydealswithholt-meltextrusionofapolyamide polymerinasingle-screwextruder.Itaimsatelucidatingthe influenceofboththeadditionofastaticmixerand supercrit-icalCO2injectiononthemeltflowbehaviour.Atfirst,afew

theoreticalconceptsneededforthestudyofthedistribution of residencetimesaregiven.Thedevelopmentofthe RTD methodisthendetailedbeforepresentingtheexperimental studydescribing(i)theeffectoftheoperatingparameterson themeltflowand(ii)thecharacterisationoftheCO2-foamed

polymersamplesproducedasafunctionoftheadditionofa staticmixingelement.

2. Theory

(

Danckwerts,

1953;

Villermaux,

1993

)

ThefunctionE(t)isanormalisedexitagedistributionandis definedsuchthat:

!

∞ 0

E(t)dt=1 (1)

E(t)dt is the fractionof the fluid in the exit flow having a durationoftinsidetheextruder.Thecumulativeand dimen-sionlesscurveF(t)isdefinedby:

!

t 0

E(t)dt=F(t) (2)

Theaverageresidencetimetmoy(equaltothefirstorderraw moment!1)istheaveragetimespentbythefluidelements

insidetheextruderandisdefinedasthefirstmomentofthe functionE(t):

tmoy=

!

∞

0

t·E(t)dt=!1 (3)

tmoycanbecomparedto ¯t,thegeometricresidencetime,i.e. the extruder volumefilledwithpolymer, V,dividedby the volumetricflowrateQv.

¯t = V

Qv (4)

Without any dead orstagnant zones, thesetwo times are equal.tmoy< ¯tmayindicatethepresenceofastagnant

vol-umeexchangingmatterslowlywiththemainstream.This leadsto adrag in the RTD curve.On the contrary tmoy> ¯t

issymptomaticofchannellingorincreasedaxialdispersion. Channellingmayleadtotheapparitionofasmallpeakbefore themainstreampeak.

Thevariance"2,thesecondordercentralmomentofE(t),

givesthedegreeofdispersionaroundthemeanand charac-terisesthespreadingoftheRTDcurve.

"2=

!

∞ 0

((t−tmoy)2)·E(t)dt (5)

A dimensionlessnormalised variance VarC can beused to knowwhetherornot,aparametercanmodifytheflow.Itis definedas:

VarC= "

2

t2moy

(6)

Theonsettimetapp (atwhichthe beginningofthe peakis detected)canbeusefulintheratiotapp/tmoy.Itisequalto1for aplugflowandto0inaperfectlymixedreactor.Itwas eval-uatedto0.75forasingle-screwextruderand0.6forastatic mixerofKenicstype(Thakuretal.,2003).

Dimensionlessparametersweredefinedasfollows:(Ward

etal.,1996).

tadim=

t

¯t (7)

Eadim=E(t)· ¯t (8)

3. Materials

and

methods

3.1. Extruder

Hot-melt extrusion was performed using a single-screw Rheoscamextruder,whichhasa30mm-screwdiameterand alengthtodiameterratio(L/D)of35(Scamex,France)already describedindetailselsewhere(Nikitineetal.,2010).Unless otherwise specified,extrusiontemperature was 220◦_C _and

rotation speed of the screw was 40rpm. Carbon dioxide, pumpedfromacylinderbyasyringepump(260D,ISCO),is injectedintheextruderbarrelatalengthtodiameterratioof 20fromthefeedhopperatthesamepressureasthatofthe extruder.Thepumprunsinaconstantvolumetricflowrate mode.ThemaximumCO2flowrateisreachedwhen

desta-bilisation ofthe polymer flowoccurs.The diameterof the cylindricaldieis2mmanditslengthis8mm.Thestaticmixer usedcontainsfourelementswithadiameterof17mm(SMB-H 17/4,Sulzer,Switzerland).

3.2. Polymer

Polyamide PA 11, commercial name Rilsan® _(BMFO _grade,

Arkema, France), is a bio-sourced polymer amino-11-undecanoicacidobtainedfromcastoroil.Itisalinearpolymer with anumber average molar mass of 8700gmol−1 _and _a

weightaveragemolarmassof20,010gmol−1_._In_the_following,

itwillbedesignatedbyPA.

3.3. Tracer

A Raman spectrometer (Kaiser Optical Systems, model Rxn2TM_cart_version)_equipped_with_an_extruder_immersion

probehas beenusedto monitorin-linethe tracer concen-trationinthemelt.Theprobewasplacedjustbeforethedie usingalocationinitiallydevotedtoapressuregauge.Thelaser wavelengthwas785nm,andthepowerwas400mW.Lightwas directedontothepolymermeltstreamcontainingthetracer. Thescatteredlight,whichisdifferentfromtheincident sig-nalandwhosewavelengthsarecharacteristicofthesample, ismonitored.Wehavechosentoperformpulseinjectionsof tracerwithaverysmalltracermasscompared tothetotal massintheextruder.

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Fig.1–Electrostaticloadofthematerialsversusrotation time.

3.4. RTDdetermination

Titaniumoxide(TiO2)initsanatasepolymorphwaschosenas

atracerandpurchasedfromRiedeldeHaën(Seelze,Germany). Asmallquantityoftracer,typically0.5%,w/wispreviously mixedwiththe polymergranules. Thequality ofthe coat-ingcanbecheckedbythekineticsoftriboelectrificationofPA, TiO2andamixtureofbothcomponentsafterbeingplacedina

stainlesssteelrotativedrumfor10min(Fig.1).TheTiO2

pow-derbeingagoodconductorcoatsthepolymergranuleswhich areinsulating,asdemonstratedbythelowlevelofelectrical chargeofthemixture.Indeed,theelectricpotentialofthe mix-ture(−340V)ismuchlessthanthepotentialmeasuredwith thepolymerwithouttracer(−1800V).Similarly,therelaxation timedropsfrommorethan20min(PAonly)to160ms (mix-ture).Inaddition,TiO2isafinepowder(d50=0.495!m)that

canfolloweasilyallthelinesofflowwithintheextruder bar-rel.Apreviousstudy(Nikitineetal.,2009)hadindeedrevealed theimportanceofthefinenessofthetracer,toolargeparticles beingsubjecttochannelling.Thismethodhasallowedtouse asimplephysicalmixture,withoutanyotherprocessingstep. Asmallquantityofmixtureistheninjectedinthehopper ontothefirstscrewthreadandthencoveredwithpolymer. Thesignalwasrecordedovertimewithanexposuretimeof 5sandspectrumacquisitionwascarriedoutevery10s.These twoparameterswerefirsttested.Theexposuretimewas cho-sentogetaRamanspectrumwhoseintensitywashighenough butnottoonoisy.Thefrequencyofacquisitionwaschosenin ordertogetenoughmeasurementpointsforthe RTD func-tion tobe precise enough whilebeing consistent withthe exposuretime.RamanspectraofpurePAandpureTiO2are

radicallydifferent,asshowninFig.2,thepolymerpeaksbeing obtainedathighwavenumbers(>1000cm−1₎_while_the _TiO₂

peakswereobservedatlowwavenumbers(<700cm−1_)._The

maximumamountofTiO2thatcanbeintroducedwithout

dis-turbingtheflowishowevertoosmalltogivedetectablepeaks intheRamansignals.ButtheTiO2beingwhite,itabsorbs

sig-nificantlythemainsignalandreducestheintensityofthePA peaksaccordingtoitsconcentration.Fig.3showsthevariation intheintensityofthemainpolymerpeakasafunctionoftime inthepresenceoftracer.Thispeakintensitywashence cho-senfortheRTDdetermination.Thiseffectofloweringpeak intensitywasfirstobservedbyVannetal.(2009)withnylon 6.6.

Thetracerconcentrationcm(t)wasdeducedfromthepeak intensityI(t)measuredbytheprobebyassumingadirect pro-portionalityandexpressedinarbitraryunits(au):

cm(t)=K′·(IPA−I(t)) (9)

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Fig.3–MainPApeakintensityinthepresenceofTiO2asafunctionoftime. IPAis the intensity ofpurePA signal(baseline)and K′ is a

constantcharacteristicofthepolymernature,oncethe expo-suretimeisfixed.ConsequentlytheRTDfunctionE(t)canbe deducedfromthissignalbythefollowingequation:

E(t)=

"

∞cm(t) 0 cm(t)dt

=

"

∞IPA−I(t) 0 (IPA−I(t))dt

(10)

Inaddition,thetotalmassoftracermtraccanbewrittenas:

mtrac=

!

∞ 0 cm(t)Qvdt (11) Therefore: mtrac=K′QvSpic (12)

whereSpicisthepeakintensitysurfaceandcanbedefinedby:

Spic=

!

∞

0

I(t)dt (13)

3.5. Samplecharacterisation

Waterandheliumpycnometry(Micromeritics,AccuPYC1330) were used to determine respectively, overall and closed porosity.Thecharacterisationoftheporositystructurewas alsostudiedbyenvironmentalscanningelectronmicroscopy (ESEM,FEG,Philips).

Table1–Polymerandtracermassesusedin experimentsforthestudyofthepulse.

Run# mPA(g) mTiO2(mg) Massratio

TiO2/PA(%) 1 1.0551 27.7 2.63 2 2.1536 93.7 4.35 3 3.1616 173.9 5.50 4 4.2607 188.3 4.42 5 5.6771 178.8 3.15 6 2.1124 113.0 5.35 7 2.2821 58.2 2.55 8 2.0349 22.6 1.11

4. Results

and

discussion

4.1. SettinguptheRTDmeasurementandvalidityof theresultsobtained

Inadditiontothefrequencymeasurementandexposuretime, twomoreparametersmustbefixed:thewidthofthepulse, whichislinkedtothemassoftheinjectedmixture,andthe tracerconcentrationinthepulse.

4.1.1. Influenceofthemassofthepulse

Table1givestheexperimentalconditionsof5runs(1–5)done

toelucidatetheroleofthetotalmassinjected.

Screwspeed,andthusflowrate,werekeptconstantinall5 runs.Thelasttworunswerehoweverperformedwithanother polymerbatch,whichinducedachangeintheintensityand

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Table2–InfluenceofpulsemassonRTDdata.

Run# Spic(aus×104) ¯t (min) tapp(min) tmoy(min) "2(min2) VarC tapp/tmoy

1 2.96 3.9 2.6 4.2 1.3 0.074 0.61

2 4.67 3.9 2.6 4.6 1.5 0.070 0.57

3 6.81 3.9 2.4 4.6 2.1 0.101 0.53

4 4.34 4.4 2.9 4.8 1.2 0.053 0.59

5 4.96 4.2 2.9 5.1 2.0 0.078 0.58

Table3–InfluenceofthetracermassinthepulseonRTDdata.

Run# Spic(aus×104) ¯t (min) tapp(min) "2(min2) VarC tapp/tmoy

6 2.59 3.9 2.6 1.3 0.069 0.61

7 1.59 3.9 2.5 1.2 0.069 0.62

8 0.81 3.9 2.6 0.8 0.050 0.65

thereforeinthepeakarea,buttheshapeoftheRTDcurves remainedunchanged(Fig.4).

Table2givesthevaluesofthemainparameters

character-isingtheRTD.Bycomparingthefirstthreeexperimentsonone handandthelasttwoontheotherhand,itisclearthatthe varianceincreases.Logically,thelargertheimpulse,thewider thepeak.Inaddition,thenormalisedvariance,whichis char-acteristicoftheflowtype,ischangedbyincreasingmasspulse, indicatingasensitivityoftheRTDtothisparameterprobably duetothedecreaseofthequalityofthepulse.Atotalmass ofinjectedmixtureof2gwasthereforeconsideredoptimalto remaininpulsemode,whilemaintainingtheintensityand thusthequalityofthesignal.

4.1.2. InfluenceofthemassofTiO2inthepulse

In this section and in the following, the laser exposure time and the frequency acquisition time were reduced to respectively5and10sinordertoincreasethenumberof mea-surementpointsinthepeak.Thetotalinjectionmassbeing fixedat2g,theinfluenceofthepercentageofTiO2wasstudied

(Table1,runs6–8).Foragivenflowrateandthesamegrade

ofthepolymer,Fig.5showsaclearlinearcorrelationbetween thepeakareaandthequantityofTiO2,thusvalidatingthe

hypothesismadeinEq.(9).

Moreover,whentheamountofTiO2increasesthesignal

intensityincreases,andsodoesthepeakwidth(i.e.the vari-anceincreases,see Table3).However,the variation ofthe normalisedvarianceseemstobelimited.Itcanbethus consid-eredthattheoutputsignalisslightlyinfluencedbythetracer massintherangestudied.Thesethreerunsbeingdonein verycloseexperimentalconditionsgivingsimilarRTDcurves, theywere usedtoevaluatethe experimentalerrorontmoy.

Fig.5–PeaksurfaceasafunctionofTiO2mass.

Table4–Experimentsforthestudyofoperating parameters.

Run# Staticmixer N(min−1₎ _CO₂_(ml_min−1₎

9 No 30 0 10 No 40 0 11 No 60 0 12 No 80 0 13 Yes 40 0 14 Yes 40 0 15 Yes 40 0.7 16 Yes 40 1.0

Thestandarddeviationofthethreemeasurementswas17s or0.29min.

Inconclusiontotheseexperiments,itwasdecidedtofixa TiO2ratioof2%onatotalmassof2g(i.e.40mg)inallthe

following experiments.Thesefiguresappeartobethe best compromisebetweenthequalityofthesignalandtherespect ofapulsemode.

4.2. InfluenceofoperatingparametersonRTD 4.2.1. Screwspeed

Four experimentshavebeen carriedoutat differentscrew speed(runs9–12inTable4).First,wecheckedthatthe con-stantK′_defined_in_Eq.₍₁₂₎_,_did_not_change_with_screw_speed.

Table5showsthevaluestakenbyK′_as_a_function_of_the_screw

speed,whicharealwaysclosetoK′₌_8.10−8_g_cm−3_au−1_.

TheexperimentalresultsofRTDforruns9–12aregiven

in Table6 and illustratedin Fig.6.When the screwspeed

increases,thetimeofonsetdiminishesandsodoesthe aver-ageresidencetime.Thisisaclassicresultalreadydescribed intheliterature(seeforinstanceBiandJiang,2009).In addi-tion,thepeaknarrowswithlessdrag,asdemonstratedbythe decreaseofthevariance.Table6showsdifferencesbetween thegeometricresidencetimeandmeanresidencetime,which aresmallerorequaltotheexperimentalerror(17s).Themean residencetimeisthereforenotsignificantlygreaterthanthe geometricresidencetime.

Inallexperiments,thevarianceisofthesameorderof mag-nitude,indicatingthatthescrewspeedhasnoinfluenceonthe flowbehaviour.Thisisconfirmedbytheverysimilarshapeof

Table5–K′_values_as_a_function_of_screw_speed.

Run# 9 10 11 12

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Table6–RTDdata.

Run# ¯t (min) tapp(min) tmoy(min) "2(min2) VarC t−tmoy(min) tapp/tmoy

9 5.3 3.4 5.5 1.9 0.064 0.2 0.63 10 3.9 2.5 4.2 1.2 0.071 0.3 0.60 11 2.4 1.6 2.7 0.5 0.066 0.2 0.59 12 1.8 1.1 1.9 0.3 0.075 0.1 0.59 13 4.86 3.02 6.36 3.68 0.091 1.70 0.47 14 4.86 2.77 5.94 2.58 0.073 1.20 0.47 15 6.20 3.97 7.78 5.28 0.087 1.78 0.51 16 6.20 4.13 8.03 4.61 0.071 2.02 0.51 0 2 4 6 8 10 −0.2 0 0.2 0.4 0.6 0.8 1 1.2

Time (min)

F

(−

)

0 2 4 6 8 10 −0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (min)

E (s

− 1

)

30 rpm (run #9) 40 rpm (run #10) 60 rpm (run #11) 80 rpm (run #12) 30 rpm (run #9) 40 rpm (run #10) 60 rpm (run #11) 80 rpm (run #12)

(b)

(a)

Fig.6–InfluenceofscrewspeedonRTD.Differential(a)andcumulative(b)curves. dimensionlesscurvesshowninFig.7.Inaddition,theratio

tapp/tmoyhardlyvariesandremainscloseto0.6.

4.2.2. Influenceofthestaticmixer

A static mixer element (Sulzer, SMR-H, 17/4) was added betweenthescrewandthedietoimprovethemixing qual-ityand thus minimising the plugfloweffect (runs13 and

14 in Table 4). As shown by the increase of the several

characteristictimes,thecurvesareshiftedtohighertimesdue totheincreaseofthefreevolume(runs13and14versus10in

Table6).Theratiotapp/tmoyismuchsmallerinthepresenceof

themixer:0.47insteadof0.60.Thisshowstheevolutionofthe flow,evidencingabettermixingeffect.Inaddition,themuch largerdifferencetmoy− ¯t couldbeexplainedbytheincreaseof

thedistributivemixing,whichthenincreasestheresidence timeasexplainedinSection2.Fig.8confirmsthatthemixer

0 0.5 1 1.5 2 2.5 −0.2 0 0.2 0.4 0.6 0.8 1 1.2 t_adim (−) F (− ) 0 0.5 1 1.5 2 2.5 −0.5 0 0.5 1 1.5 2 2.5 3 3.5 t_adim (−) E adim ( − ) 30 rpm (run #9) 40 rpm (run #10) 60 rpm (run #11) 80 rpm (run #12) 30 rpm (run #9) 40 rpm (run #10) 60 rpm (run #11) 80 rpm (run #12) (b) (a)

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Fig.8–InfluenceofthepresenceofthestaticmixerondimensionlessnormalisedRTD.Differential(a)andcumulative(b) curves.

doeschangetheflow.ThedimensionlessRTDcurvebroadens andexhibitsastrongerdragandisclosertothatofastirred reactor.

4.2.3. InfluenceofCO2injection

AstudyofCO2injectiononthemeltflowinthepresenceof

astaticmixingelementwasalsoperformed(runs15and16

inTable4).TheadditionofCO2changesthethroughputdue

probablytoitsplasticisingeffect(runs15and16versus13and

14inTable6).Indeed,themeanresidencetimeaswellasits

standarddeviationroseduetothemass-flowreduction.Itis thereforenecessarytousedimensionlesstimegraphsfora propercomparison.Finally,itappearsthatCO2injectiondoes

notaffecttheflow(Fig.9).

4.3. CharacterisationofCO2-foamedsamples

4.3.1. Withoutstaticmixer

Inthisexperiment,CO2wasintroducedat5MPa.Fig.10shows

thatupto7%ofCO2canbeintroducedwhileitssolubilityin

PAhasbeenmeasuredusingamagneticsuspensionbalance (datanotshown)at2.2%.Belowthislimit,largeporescanbe

observedwhilesmallerporeswithamoreregular distribu-tionappearedbeyondthesolubilitylimit.Fromthisvalueand above,overallporositydecreasesfrom80%to70%,atthesame timeclosedporosityincreasesfrom25%to60%.Atthehighest CO2ratio,mostoftheporosityisclosed.

4.3.2. Withstaticmixer

Usingastaticmixerenlargesdramaticallythequantityof car-bondioxide,whichcanbeintroducedintothepolymermelt asshowninFig.11.ThisadditionfavoursCO2incorporation

throughtheincreaseinthepressureinsidethebarrelatwhich CO2mustbeinjected.Inthisexperiment,thepressurewas

12MPacorrespondingtoaCO2solubilityof5.0%.The

high-estCO2massratioachievedwas27%,morethan5timesthe

solubility.

Belowthesolubility,highporosity,withlarge inhomoge-neousporeswasobserved.Homogeneousnucleationoccurred (Parketal.,1995),followedbygrowthandcoalescenceofthe poresduringthetemperaturedropundergonebythesamples afterthedie.ForintermediatemassfractionsofCO2,smalland

homogeneousporesinthecentresurroundedbylargepores near thesamplecircumferencecouldbeseen.AthighCO2

0 0.5 1 1.5 2 2.5 −0.2 0 0.2 0.4 0.6 0.8 1 1.2 t_adim (−) F ( − ) 0 0.5 1 1.5 2 2.5 −0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 t_adim (−) E adim ( − ) without CO₂ (run #13) CO₂: 0.7 ml/min (run #15) CO₂: 1 ml/min (run #16) without CO₂ (run #13) CO₂: 0.7 ml/min (run #15) CO₂: 1 ml/min (run #16) (b) (a)

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Fig.10–GlobalandclosedporosityasafunctionofCO2massfraction.

Fig.11–GlobalandclosedporosityasafunctionofCO2massfractioninthepresenceofastaticmixingelement.

ratios,beyond15%,aseveredropintheglobalporositywas observedfrom70%tolessthan20%.Thiswasattributedto astrongcoolingeffectdueto thedepressurisationof large quantities ofCO2, whichpreventspores fromgrowing and

coalescing. Thisexplains alsowhy most ofthe porosity is closed,andahomogeneousappearancewasobtained.

Theuseofastaticmixerinsertedbetweenthescrewtip and the dieis thereforean efficienttooltoget a homoge-neousfoamystructure.ItallowsagreateramountofCO2to

beinjectedintheextruderbarrel,whichcontrolstheporesize anddistribution.

5. Conclusion

Ramanspectroscopyisaquickandefficienttechniqueforthe RTDdeterminationinanextruderavoidinganysamplingand subsequentanalyses.Theinfluenceofdifferentparameters ontheflowbehaviourhasbeenelucidated.Nevertheless,the

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settingupanddevelopmentphasecanbetedious,especially fortheRamanexposuretimespecificationandthe determina-tionofthegaplengthbetweentwomeasurements.Thechoice ofapropertracertogetausableRamanresponseaswellasthe modeofinjectionarealsocrucial.WedemonstratedthatTiO2

canbeagoodtracerprovideditistheattenuationofthe poly-merpeaksurfacewhichisthechosenparametertodetermine theRTDcurves.

Theuseofastaticmixerexertsamajormodificationon theflowbehaviour:itimprovesmixingbyenhancing disper-sion.Ontheotherhand,screwspeedandcompressedcarbon dioxideadditionplayonlyaroleonthethroughputandmean residencetimewithoutdisturbingthedispersionandleaving theflowbehaviourunchanged.Inaddition,theuseofastatic mixergivesamorehomogeneousfoamystructureandallows theincorporationofmuchlargerCO2quantitiesinthe

poly-mermelt,upto25%(w/w).Finally,thetuningofCO2injection

allowedthecontroloftheporositywithinarangeof15–70%. Togofurther, aflowmodelling,withandwithout static mixer,appearsnecessary.Severalmodellingstrategy using Markovchains(Ponomarevetal.,2012)orclassic chemical engineeringreactormodelling(Nikitineetal.,2009)couldbe used.

Acknowledgements

AuthorsgratefullyacknowledgetheArkemacompanyfor pro-viding the polymer and the financial support of the ANR (Frenchnationalagencyforresearch).

References

Barnes,S.E.,Sibley,M.G.,Edwards,H.G.M.,Coates,P.D.,2007.

Processmonitoringofpolymermeltsusingin-line

spectroscopy.Trans.Inst.Meas.Control29(5),453–465.

Bi,C.,Jiang,B.,2009.Studyofresidencetimedistributionina

reciprocatingsingle-screwpin-barrelextruder.J.Mater.

Process.Technol.209,4147–4153.

Danckwerts,P.V.,1953.Continuousflowsystems.Distributionof

residencetimes.Chem.Eng.Sci.2(1),1–13.

Mulvaney,S.J.,Rizvi,S.S.H.,1993.Extrusionprocessingwith

supercriticalfluids.FoodTechnol.47(12),74–82.

Nikitine,C.,Rodier,E.,Sauceau,M.,Fages,J.,2009.Residencetime

distributionofapharmaceuticalgradepolymer/supercritical

CO2meltinasinglescrewextrusionprocess.Chem.Eng.Res.

Des.87(6),809–816.

Nikitine,C.,Rodier,E.,Sauceau,M.,Letourneau,J.-J.,Fages,J., 2010.Controllingthestructureofaporouspolymerby

couplingsupercriticalCO2andsinglescrewextrusionprocess.

J.Appl.Polym.Sci.115,981–990.

Park,C.B.,Baldwin,D.F.,Suh,N.P.,1995.Effectofthepressure

droprateoncellnucleationincontinuousprocessingof

microcellularpolymers.Polym.Eng.Sci.35(5),

432–440.

Pinto,G.,Tadmor,Z.,1970.Mixingandresidencetimedistribution

inmeltscrewextruders.Polym.Eng.Sci.10,279–288.

Ponomarev,D.,Nikitine,C.,Sauceau,M.,Rodier,E.,Mizonov,V., Fages,J.,2012.Modellingnon-homogeneousflowand

residencetimedistributioninasingle-screwextruderby

meansofMarkovchains.J.Math.Chem.50(8),2141–2154.

Saerens,L.,Dierickx,L.,Lenain,B.,Vervaet,C.,Remon,J.P.,De Beer,T.,2011.Ramanspectroscopyforthein-line

polymer-drugquantificationandsolidstatecharacterization

duringapharmaceuticalhot-meltextrusionprocess.Eur.J.

Pharm.Biopharm.77,158–163.

Sauceau,M.,Fages,J.,Common,A.,Nikitine,C.,Rodier,E.,2011.

Newchallengesinpolymerfoaming:areviewofextrusion

processesassistedbysupercriticalcarbondioxide.Prog.

Polym.Sci.36,749–766.

Seker,M.,2005.Residencetimedistributionsofstarchwithhigh

moisturecontentinasingle-screwextruder.J.FoodEng.67,

317–324.

Singh,B.,Rizvi,S.S.H.,1998.Residencetimedistribution(RTD)

andgoodnessofmixing(GM)duringCO2-injectionin

twin-screwextrusion–PartI:RTDstudies.J.FoodProcessEng.

21(2),91–110.

Thakur,R.,Vial,C.,Nigam,K.,Nauman,E.,Djelveh,G.,2003.

Staticmixersintheprocessindustries–areview.Chem.Eng.

Res.Des.81(7),787–826.

Vann,B.C.,Angel,S.M.,Hendrix,J.E.,Bartick,E.G.,Morgan,S.L., 2009.Analysisoftitaniumdioxideinsyntheticfibersusing

Ramanmicrospectroscopy.Appl.Spectrosc.63(4),

407–411.

Villermaux,J.,1993.GéniedelaRéactionChimique:Conception

etFonctionnementdesRéacteurs,2nded.Lavoisier,Tecet

Doc,Paris.

Wang,C.,Bussmann,M.,Park,C.B.,2010.Numericalinvestigation

oftheeffectofscrewgeometryonthemixingofaviscous

polymermelt.J.Appl.Polym.Sci.117(2),775–784.

Ward,N.,Edwards,H.,Johnson,A.,Fleming,D.,Coates,P.,1996.

ApplicationofRamanspectroscopyfordeterminingresidence

timedistributionsinextruderreactors.Appl.Spectrosc.50(6),