Extrusion assisted by supercritical CO2 : a review on its application to biopolymers

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Extrusion assisted by supercritical CO2 : a review on its

application to biopolymers

Margot Chauvet, Martial Sauceau, Jacques Fages

To cite this version:

Margot Chauvet, Martial Sauceau, Jacques Fages. Extrusion assisted by supercritical CO2 : a

re-view on its application to biopolymers: Rere-view. Journal of Supercritical Fluids, Elsevier, 2017,

Special Issue - 11th International Symposium on Supercritical Fluids, 120 (Part 2), pp.408-420.

�10.1016/j.supflu.2016.05.043�. �hal-01335023v2�

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Extrusion

assisted

by

supercritical

CO

2 :

A

review

on

its

application

to

biopolymers

Margot

Chauvet,

Martial

Sauceau,

Jacques

Fages

∗

CentreRAPSODEE,EcoledesMinesd’Albi,CNRS,UniversitédeToulouse,F-81013Albi,France

Keywords: Biopolymers

Supercriticalfluidextrusion Foaming

Biomedicalapplications Agro-foodapplications Consumerplasticsapplications

a b st r a c t

ExtrusionassistedbysupercriticalCO2(sc-CO2)isanemergingmethodforthemicrocellularfoamingof

polymer.Insteadofbatchfoaming,whichrequiresformationofsingle-phasepolymer/CO2solutionin

longcycletimes,theextrusionassistedbysupercriticalfluidsovercomesthisissuebyprovidingrapid mixinganddissolutionofCO2inthepolymermelt.Becausethesc-CO2issolubleinmanymolten

poly-mersandactsasaremovableplasticizer,itsintroductioninanextruderwillpermitadecreaseofthe processingtemperature.Thistechnicallowstheuseoffragilecomponentlikeactivemoleculeorstarchy andproteinaceousmaterials.Attheendoftheextruder,thepressuredropwillcreateinstabilityand phaseseparationwiththecreationofporosity.Thisreviewisdedicatedtotheextrusionassistedby sc-CO2withdifferenttypesofbiopolymer.Industrialapplicationdomainsincludeagro-food,biomedical,

pharmaceutical,packagingandmanyothers.

1. Introduction

Foamsofpolymersareusedinmanyfieldslikecushioning, insu-lation,packaging,andmedicalpurposes(e.g.scaffolding),because oftheirspecificproperties[1].Aporouspolymermatrixcanbe defined bythesizedistributionofthecells,thecelldensity i.e. thenumberofcellsperunitvolume,andthevolumeexpansion definedasthevolumeoccupiedbyvoidsdividedbythevolume ofthepolymer[2].Itiscombinedwithalowdensityandahigh porosity.

Classicmanufacturingprocessesusechemicalblowingagents (CBAs) like carbonated salts which lead to CO2 upon thermal

decompositionbutleaveresiduesinthefinalproduct.Themain advantageofCBAsliesinthefactthattheycanbeused irrespec-tiveofthepressure.However,theyareexpensiveandrequirehigh temperaturefordecomposition[3].Thisiswhytheuseof phys-icalblowingagents(PBAs)tendstoreplacetheCBAsutilisation. PBAsaregaseoussubstanceslikeHFCs,whichcanbeinjectedunder pressureintoapolymermelt.PBAshavenodecomposition temper-aturerequirements,areducedcost,andgenerallyproducebetter cellmorphology.Howevermostofthesegasescanexhibit delete-riouseffectslikethedepletionoftheozonelayeraswellassome hazardousproperties[3].

∗ Correspondingauthor.

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

Aninterestingalternativeistheuseofsupercriticalfluidsand particularlycarbondioxide (CO2) asPBAinhot-melt extrusion

processesduetotheiruniqueproperties,suchasenvironmental friendliness,non-flammability,andlowcost[3].Thisrelativelynew fieldofresearchhasdevelopedquicklyintherecentyears[4,5]. CO2iswellknownforitscompatibilitywithseveralpolymersin

whichitssolubilitycanberelativelyhighdependinghoweveron thetemperatureandpressureconditions.

Themethodologyusedcanbebasicallydescribedasaninjection ofpressurizedCO2inthebarrelofanextruder(seenextparagraph).

CO2actsasaremovableplasticizerinthemeteringzoneofthe

extruderwhereitmodifiestherheologicalbehaviourofthemelt. Moreover,itactsasanexpansionagentwhenitreturnssuddenly toatmosphericpressureatthedieexit[6].Thispressurequenchis responsibleforthesupersaturationinthemetastablemeltphase leadingtonucleationandgrowthofCO2bubblesandeventually

thefinalporous3D-structure.

Nowadays,becauseoftheshortageinfossilresourcesand envi-ronmentalconcernsoverthewastematerials,theuseofsynthetic polymersisdisputable.Abiobasedpolymerisapolymerderived fromrenewableresources.Therefore,itappearsthattheuseofsuch biopolymerswillreplacethepolymersobtainedfrom petrochem-istryandshowsagreatpotential.

Biodegradablepolymersareanothercategoryofbiopolymers. Jeonetal.publishedin2013areviewonthemicrocellularfoaming ofbiodegradablepolymers[7].Intheirpaper,differentfoaming

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processesareinvestigatedincludingbatch foamingprocess,and injectionfoamingprocess.

Biocompatibilityreferstotheabilityofamaterial–including polymers–to beimplantedinthehumanbody.Inthisreview biocompatiblepolymerswillbeincludedinthegeneric termof biopolymer.

Inthepresentpaper,wehavechosentofocusontheprocessing ofbiopolymersbyextrusionassistedbysupercritical-CO2(sc-CO2)

inseveralapplicationareaswithaspecialemphasisonthe phar-maceuticalandfooddomains.

2. TheprocessofextrusionassistedbysupercriticalCO2

2.1. Theprocess:rationale

Asupercriticalfluidisdefinedasasubstanceforwhichboth pressureandtemperatureareabovethecriticalvalues.The spe-cialcombinationofgas-likeviscosityandliquid-likedensityofa supercriticalfluidmakesitanexcellentsolventorplasticizerfor variousapplicationslikepolymercomposites,microcellular foam-ingorparticleproduction[8,9].Thedensityofsupercriticalfluids canbetunedeasilybysmallchangesinpressurewithinthecritical region.Sc-CO2 ismostoftenusedbecauseitisnon-toxic,

non-flammable,chemicallyinert, anditssupercritical conditionsare easilyreached(Tc=31◦C,Pc=7.38MPa)anditcanberemovedfrom

asystembysimpledepressurization.Itshighsolubilityenhances plasticizingandexpansionofamaterial,withaccompanying mod-ificationofmechanicalandphysicalproperties.Forexample,CO2

decreasestheglasstransitiontemperature,Tg,andtheviscosityof

manypolymerswithoutotherwisechangingtheirpseudo-plastic behaviour[9,10].

Therearemanyadditionaladvantagesofusingsc-CO2withthe

processofextrusion.Notonlythesc-CO2willchangethe

rheologi-calpropertiesofthematerialinsidetheextruderbutitalsoplaysthe roleofanexpansionagent.Thediminutionofviscositywillresult inthelimitationofmechanicalandshearstressesandwillallowa decreaseinoperatingtemperatures.Therefore,itwillbeeasierto handlemoleculeshavingalimitedthermalstability.Moreover,its dissolutioninthepolymerunderpressurewillbeaccompaniedby largevolumeexpansionduringthereturntoatmosphericpressure

[10].

2.2. Experimentaldevicesandmodesofoperation

AccordingtoSauceauetal.[10],theexperimentaldevicesfor theextrusionassistedbyCO2 supercriticalmustallowthesteps

describedhere:

• Dosingandconveyingthecomponents;

• Developmentofrheologicalpropertiesofthemelt(preparation andplasticizing);

• InjectionofpressurizedCO2;

• Homogeneousmixingtoobtainasinglephasemixture; • Nucleation,growthandcoalescenceofporesprovokedand

con-trolledbythethermodynamicinstabilitycreatedbythepressure dropthroughthedie;

• Materialshapingandcoolingatthereturntoatmospheric pres-sure.

Dependingonthenumberofscrewsturninginthebarrel,the extrudersareavailableintwo differenttypes:thesingle-screw extruderorthetwin-screwextruder.Inbothtypes,thepolymer isintroducedintothefeedhopper,conveyedalongthebarreland beginstomeltinthefirstsection,then,itispressurizedinthelast sectionandisforcedthroughthedie.Mostofthetime,themetered amountofphysicalblowingagentisinjectedintotheextrusion bar-relatagasinjectionportbyapositivedisplacementpumpandis mixedintensivelywiththepolymermeltstream.

Attheendoftheextruderscrew,beforethedie,someancillary devicescanbeadded.Astaticmixercanbeinstalledforenhancing bothdistributiveanddispersivemixingandtoimprovethe sorp-tionanddissolutionoftheCO2inthepolymermelt.Matuanaetal. [11,12]andPillaetal.[13,14]addedadiffusion-enhancingdevice (staticmixer OmegaFMX8441S).LeMoigne etal.[15]choseto addastaticmixercomposedoffourelements(SMB-H17/4,Sulzer, Switzerland).Mihaietal.[16–18]addedagearpumpplacedatthe endoftheextrusionlinetopreserveahigh-pressurelevelatthe endoftheextruder.Tocontroltheprocessingpressure,atwo-hole flowrestrictionnozzlecanbemountedlikeinthestudyofRizvi etal.[5]orbyconstrictingthecross-sectionalareaofthechannel inahome-madediebymeansofacentralpinlikeforVighetal.

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wasincreasedordecreasedbytuningtheopeningdegreeofthe restriction,appliedtothemeltflow.

Inthecaseofasingle-screwextruder,likeinFig.1,a restric-tionringcanbelocatedbetweeneachsectiontoincreasetheshear andthepressurelocallyandthuscreatingadynamicmolten poly-merseal,whichpreventsbackflowingofsc-CO2[15,19–23].For

a better mixing, a tandemextrusion system can beused. Park andcoworkers[24–26]useatandemextrusionsystemasshown inFig.2.The systemconsistsofafirst extruderwith a mixing screw,asecondextruderwithacoolingscrew,asyringepumpfor injectingtheblowingagent,agearpump,aheatexchanger con-taininghomogenizingstaticmixers,andafilamentarydie.Thefirst extruderplasticizesthepolymeranddispersestheblowingagent intothepolymermelt.Thegearpumpprovidesflowrates,which areindependentoftemperatureandpressure.Thesecondextruder providesfurthermixingandinitialcoolingofthemelt,andthe heatexchangerremovestheremainingheatfortestingataspecific temperature.

Inthecaseofatwin-screwextruder,itisacommonthingto addapairofreversescrewelements,situatedupstreamfromthe blowingagentinjectionpointforcreatingadynamicmeltsealto maintainahighCO2pressureinthelatterportionoftheextruder [16–18].

2.3. Influenceofthediegeometry

Animportantpointtotakeinconsiderationfortheextrusion foaming is thedie andthepressure droprate induced by this die.Indeed,thepressuredroprateinthenucleationdeviceplays astrong roleindeterminingthefoam celldensity.Thediehas aneffectonthethermodynamicinstabilityinducedinthe poly-mer/gassolutionandthecompetitionbetweencellnucleationand growth.Parketal.[27]showedthatforagivenamountofgas,the celldensityincreasedwithanincreaseofpressuredropratefor polystyrene.Theyalsohighlightedthefactthatregardlessofthe concentrationofCO2,ornucleatingagentinamoltenpolymer,the

diegeometrydeterminesthepressure-droprateandthedie pres-sure,therebydominantlyaffectingboththecelldensityandthe cellmorphology[28].AlaviandRizvi[29]observedsimilarresults withstarch-based foams:thebubbledensity(Nbubble) increased

whiletheaveragebubblediameterdecreasedwithalower noz-zlediameteri.e. ahigherpressuredroprate.Theyalsonoticed thatsamplesunderwenthigherexpansionrateasthenozzleradius

decreasedfrom3to1.5mmandpiecedensitydecreasedby40%. TheyexplainedthatasN_bubbleincreased,theeffectivediffusivity (D_eff)ofCO2isreducedandlessgasescapedout,leavingalarger

amountfordiffusionintothebubbles.Thisledtoan enhanced expansion, althoughthe averagebubble diameterwasreduced becausetheCO2wasdistributedoveragreaternumberofbubbles.

2.4. Importanceoftheoperatingtemperature

Ifthecelldensityismainlydeterminedbythedieandthe pres-suredrop,thefoamexpansionandotherfoam’scharacteristicsare mainlycontrolledbythetemperature,asshownforseveral poly-mers(PLA[13,24,25],PC[26],starch[29],PS[6,30],PP[31],LDPE

[32]).Indeed,during themicrocellularfoamgenerationthrough andafterthedie,thereisalossofCO2,whichlimitsthevolume

expansionbecausetheCO2easilyescapesthroughtheexteriorskin

ofthefoam.Onewaytopreventgasescapefromthefoamisto freezetheskinoftheextrudatebycontrollingthedietemperature. Indeed,thedecreaseofthedietemperaturecanlimitthegas dif-fusionatthesurfaceandfinally,moregasremainsinthefoamto contributetothevolumeexpansion[6].However,ifthe tempera-tureisfurtherdecreased,theexpansiondecreasesbecauseofthe increasedstiffnessofthefrozenskinlayer[6](Fig.3).

Moreover,thepolymermeltshouldbecooledsubstantiallyto increaseitsstrengthinordertopreventcellcoalescenceandto insurea highcelldensity whilekeepingasufficientfluidityfor bubblestogrow[30].Therefore,acompromisefordieandmelt temperaturesmustbefound.Ithastobeemphasizedthat,forsome polymers,tobeabletodecreasethemeltanddietemperatures, thesc-CO2contenthastobeincreased([24,26]).Infact,whenthe

temperatureislowered,thepolymerviscosityincreasedandthe pressureupstreamthediecanbeveryhigh.AddingmoreCO2in

theextruderwillplasticizethepolymerandthusdecreaseboththe viscosityandthepressureallowingtooperateatlowertemperature (Fig.4).

Becausetheexpansionratioisalsolinkedtotheopen-cell con-tent,Parketal.[32],Leeetal.[26]andHuangetal.[33]showed with LDPE/PSandpolycarbonatethat therewasanoptimaldie temperatureformaximizingtheopen-cellcontent(Fig.3).Atlow temperatures(zone3),cellwallsbecametoostiffforcellopening, andtherefore,theopen-cellcontentincreasedasthetemperature wasincreasedinthislow-temperaturerange(betweenzone3and 2).Athighertemperatures,thethicknessofcellwallsgovernedcell

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Fig.4.ExpansionratioandbulkcrystallinityofPLAfoamsasafunctionofprocessing temperatureandCO2concentrationbyWangetal.[24].(Reprintedwithpermission

opening,andtherefore,theopen-cellcontentdecreasedasthe tem-peratureincreasedinthehigh-temperaturerange(betweenzone 2and1).

3. Thedifferenttypesofbiopolymer

In2013, theEuropean plastics consumptionwasabout 46.3 millionstonnes(datafromPlasticsEurope[34]).Theoil-basedand non-biodegradablepolyolefinslikepolypropylene(PP), polyethy-lene(PE)andpolyvinylchloride(PVC)representmorethan50% ofthisconsumption.Theseplasticsfinishoftenaswaste.In2012, for25.2milliontonnesofpost-consumerplasticswastes,62%was recoveredthroughrecyclingandenergyrecoveryprocesseswhile 38%wenttolandfill[34].Inordertodecreasethehighpercentageof wastesendingupinlandfillandtopreservethepetroleumresource, biopolymersaremoreandmoreused.InEurope,theglobal produc-tioncapacitiesofbioplasticsincreasedfrom1492to1622millions tonnesinoneyear(2012–2013)anditisexpectedthanitwillreach

6.7millionstonnesin2018(datafromBio-BasedEconomy[35]). Accordingtodifferentdefinitions,abiopolymer/bioplasticcanbe eitherabiodegradablepolymerorabiobasedpolymer.The biocom-patiblei.e.implantableinthehumanbody,polymersarealsooften incorporatedinthetermbiopolymer.Thereforewehavechosento considerthatabiopolymerisapolymerhavingatleastoneofthe followingthreeproperties:biobased,biodegradableor biocompat-ible.

Biodegradationisanaturalprocessbywhichorganicchemicals intheenvironment areconverted tosimplercompounds, min-eralizedandredistributedthroughelementalcyclessuchasthe carbon,nitrogenandsulphurcycles.Biodegradationcanonlyoccur withinthebiosphereasmicroorganismsplayacentralroleinthe biodegradationprocess[36].Accordingtostandardsdefinition(ISO 472:2013),apolymerundergoesabiodegradationwhenthe degra-dationiscausedbybiologicalactivity,especiallyenzymaticaction, leadingtoasignificantchangeinitschemicalstructure.This degra-dationisan irreversibleprocesscharacterizedmostofthetime byachangeinproperties(integrity,molecularmassorstructure, mechanicalstrength). Forexample,biodegradablepolymersare usefulforvariousapplicationsinmedical,agriculture,drugrelease andpackagingfields.

A biobased polymer is a polymer derived from renewable resourcesaccordingtotheFrenchAgencyoftheEnvironmentand EnergyManagement(ADEME).Itcanalsobesynthesizedbythe polymerisationofnaturalmonomers(Fig.5).Theseproductscan replacepetroleum-basedpolymersinnearlyeveryfunctionfrom packagingandsingleusetodurableproducts.Theyofferthe oppor-tunitytoreducefossilresourcesrequiredtoproducethe21million tonsofplasticannuallyconsumedforpackagingandnon-durable goods,aswellasdivertthe16.7milliontonsofplasticwaste enter-inglandfill[37].

Black[39]definedthebiocompatibilityastheabilityofa poly-mertoperformwithanappropriatehostresponse inaspecific situation.Inthemedicalfield,abiocompatiblematerialmusthave theabilitytoexistincontactwithtissuesofthehumanbody with-outcausingan unacceptabledegreeofharm tothat body[40]. Thebiocompatiblepolymerscanbeusedinseveralapplications liketissueengineering,invasivesensors,drugdeliveryandgene transfectionsystems.

Fig.6 shows the differentpolymer types depending onthe biodegradabilityandthematerialsorigin.Forexample,starchis abiobasedandbiodegradablepolymerwhereaspolystyreneisa non-biodegradableandoil-basedpolymer.

4. Applications

Extrusionassistedbysupercriticalfluidsbroadensitsfieldof applicationandhasbeencalledasarevolutionaryinventioninthe polymerindustry[9].Amongseveraladvantages,theuseofthis technologyallowsadecreaseoftheprocessingtemperatureand thustheuseofthermosensitivecomponents.

4.1. Biocompatiblepolymersusedinthemedicalfield 4.1.1. Enhancingthebioavailabilityofactivemolecules

Combiningextrusionwithsc-CO2allowsusingrelativelyfragile

orthermallysensitivemolecules,likepharmaceuticalmolecules, without any residue in the final material. For instance, these biopolymerfoamscanbelaterusedinfloatingdrugdelivery sys-tems to achieve gastric retention. The use of the injection of pressurizedcarbondioxideinanextruderforanumberof phar-maceuticalgradepolymerslikepolyvinylpyrrolidone-vinylacetate (PVP-VA-64),EudragitE100andethylcellulose(EC).[18–21,39–43]

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Fig.5.Differentwaysforobtainingbiobasedpolymers(adaptedfromRef.[38]).

Fig.6.Biobasedandbiodegradablepolymers(adaptedfromRef.[41]). Table1

Theprocessingconditionsusedfortheextrusionassistedbysc-CO2forenhancedthebioavailabilityofpoorlysolubledrugs.

Matrix. API Extrusion Q(sc-CO2)orsc-CO2(%) Ref.

Screwtype Temperature Speed

PVP-VA-64 – Co-rotatingintermeshingtwin-screw 120–162◦C 100rpm N/A [42]

EudragitEPO100 115–162◦_C

EC20cps 75–190◦C

PVP-VA-64 Itraconazole Co-rotatingintermeshingtwin-screw 110–180◦C 100rpm N/A [43]

EC20cps Acide!-AminoSalicylic Co-rotatingintermeshingtwin-screw 80–130◦_C ₁₀₀_rpm _N/A _[44]

EC20cps Itraconazole Co-rotatingintermeshingtwin-screw 70–180◦C 100rpm N/A [45]

PEO+EudragitEPO Carvedilol Co-rotatingintermeshingtwin-screw 50–140◦C N/A N/A [46]

EudragitE100 Spironolactone Single-screw 110–130◦_C _N/A _2.7–6.3% _[19]

EudragitE100 – Single-screw 110–150◦C 40–80rpm 0.002–0.381% [20]

EudragitE100 – Single-screw 130–150◦C 20–80rpm 0.05–0.1cm3_/s _[21]

EudragitE100 Carvedilol Single-screw 125–130◦_C _5–10_rpm _0.25_cm3_/min _[22]

hasalreadybeenreviewedelsewhere[10]andTable1belowshows thedifferentprocessingconditionsused.

Verrecketal.[42]foundthattheCO2 actsasaplasticizerfor

severalpharmaceuticalpolymers(PVP-VA-64,EudragitE100and EC20cpsgrade)andallowsadecreaseoftheoperating tempera-tures.FortheamorphousPVP-VA64andEudragitE100polymers, theirglasstransitiontemperaturewerenotchangedafterthe extru-sionfoamingandthedissolutionisenhancedthankstoanincrease inspecificsurfaceareaandporosity. ThisprocessalterstheEC 20cpscrystallinity.Inallcases,apost-processingmillingstepis improvedduetoamorphologychange,anincreaseporosityand becausenoplasticizerisleftinthepolymerleavingtheTginthe

productatitsoriginalvalueandthepolymerinitsglassy/brittle state.WhenItraconazolewasblendwiththePVP-VA64at10or 40%wt[43],thedissolutionofItraconazolewascontrolledby

tem-peratureandpressureduringthehotstageextrusionprocess.In thecaseofablendof!-AminoSalicylicAcid(!-ASA)withEC20cps atamassratioof10/90[44],thedecompositionoftheAPIwas reducedthankstotheCO2injection(17%decompositionwithout

theCO2versus5%withtheCO2).Finally,blendingItraconazolewith

EC20cpsat10or40%wtbythisprocess[45]increasedtheinitial wettingandthedrugreleaserate.

Nikitine etal.[20] workedwith theEudragit, a pharmaceu-tical gradepolymer.Foams wereobtained,with porosity range between 65 and 90%. Here, higher temperature enhances the growthphenomenon,consequently,theexpansion rateandthe averagediameterincrease.

ThispolymerwasimplementedwithCarvedilol(CAR),anAPI usedtopreventheartfailures[22,46].Indeed,anamorphoussolid dispersionofCARinEudragitE100ispreferableforarapid

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dissolu-Table2

Theprocessingconditionsusedfortheextrusionassistedbysc-CO2forthescaffoldsfabrication.

Matrix Fillers Extrusion sc-N2(%) Ref.

PCL Chitosan Twin-screw 90–100◦_C ₁₀₀_rpm _0.5 _[49]

tion.Itwasfoundthattheprocessimprovedthedissolutionrateof thedrug.Withaflowrateof0.25cm3_/min_of_{supercritical}_CO₂_with

aCARmasscontentof20%inEudragitE100,100%wasdissolved inlessthan5minagainstonly18%ofdissolutioninthesametime lapsefortheunprocessedcrystallineCAR[22].

Lyonsetal.usedalsopolyethyleneoxide(PEO)blendat differ-entratioswithEudragitandCAR[46].Thedissolutionanalysisstill showedafasterdissolutionoftheCARforthesamplemadewith CO2incomparisonwiththesampleswithoutCO2.

Morerecently,Eudragitwasusedwithanotherpoorly water-solublecrystallineactivepharmaceuticalingredient: sprironolac-tone[19].Like forVerrecketal.[44],itwasobservedthat the extrusionassistedbysc-CO2processcouldlowerthedegreeofAPI

decompositionbecausethesupercriticalfluidenablestodecrease theprocesstemperature.Moreover,thecompletedissolutionof thedrugforthefoamedsamplewasachievedinlessthan10min, whereasonly20%ofdissolutionwasobtainedfortheunprocessed microcrystallinespironolactone.

4.1.2. Scaffoldsforregenerationofhumanbodytissues

Themainpurposeoftissueengineeringistoovercomethelack oftissuedonorsandtheimmunerepulsionbetweenreceptorsand donors.Intheprocessoftissueengineering,cellsareculturedona scaffoldtoformanature-liketissue,whichisthenimplantedinthe defectivepartinthepatientbody.Asuccessfultissueengineering implantlargelydependsontheroleplayedbythree-dimensional porousscaffolds.Theidealscaffoldsshouldbebiodegradableand bioabsorbabletosupportthereplacementofnewtissues.In addi-tion,thescaffoldsmustbebiocompatiblewithoutinflammation orimmunereactionsandpossesspropermechanicalpropertiesto supportthegrowthofnewtissues[47,48].Extrusionassistedby sc-CO2allowscreatingsuchporousstructures.

Recently, Jinget al.[49] studieda newmethod forscaffold production.Thepurposeofthisstudywastoevaluatethe feasi-bilityofapreparationmethodcombiningextrusionfoamingand particulateleachingforthepreparationofhighlyinterconnected three-dimensionalpolymericscaffoldswithcontrolledporesizes. Inthiswork,thepoly("-caprolactone)(PCL),abiocompatibleand biodegradablepolymer,wasusedasthematrixmaterial.Itwas blendedwithpoly(ethyleneoxide)(PEO)asasacrificialpolymer becauseofitswatersolubilityandameltingtemperaturesimilar tothatofPCL.Sodiumchloride(NaCl)wasusedastheparticles. Here,theblowingagentusedwassupercriticalnitrogen(sc-N2)

becauseofitslowsolubilityinmostpolymerscomparedtoCO2

butyieldingtoafinerporousstructureinthefoamingprocess. Chi-tosannanofibres(CSNF)werealsousedinthisstudytocreatea nanofibrousstructureintheporousPCLscaffoldsandtofurther improvethebiocompatibilityofthescaffolds.Theywere intro-ducedintothemicroporesofthescaffoldsbyfreeze-drying(also knownasthermallyinducedphaseseparation).Differentblends weremadewithPCL/PEOratioof70/30,60/40and50/50.These blendsweremixedwith 10%wtofNaCl.Theextrusionfoaming wasmadeinatwin-screwextruderwith0.5%wtofgas,the oper-atingparametersaregiveninTable2.Afterthefoamingoperation, thesamplesweretransferredindeionizedwatertoleachoutthe PEOandNaCl.Thescaffoldswerethensoakedinachitosan solu-tiontointroducechitosannanofibresintheporousPCLscaffolds. Thelaststepwasthelyophilisationofthescaffolds.Afterextrusion

foaming,theyobservedthatthenumberoflargerporesincreased asPEO increasedin theblends.Porouschannelscould beseen throughouttheblends;however,thechannelsbecamelongerwith anincreaseofPEOintheblends.TheauthorsfoundthatNaCl parti-clesimprovetheporosityandporeinterconnectivity.Thechitosan fibresenhancedslightlythecompressivemodulusandincreased thewateruptakerate.

Anothermethod,whichincludesascrewandsc-CO2 forthe

manufacturingofscaffoldsisthemicrocellularinjection mould-ingprocess(commerciallyknownastheMuCell® _Process)._This

process is capableof mass-producing plastics parts with com-plexgeometries,relativelylowporosityandexcellentdimensional stability.Forexample,Mietal.[50]hadproducedPLA/TPU (ther-moplasticpolyurethane)scaffoldswith thisprocess.Theyfound thatthePLA(3001D,NatureWorks)andtheTPU(Elastollan1185A, BASF)areimmisciblebut,inlightofthebiocompatibility,tuneable mechanicalproperties, andporousmicrostructure, thescaffolds manufacturedviathisprocesshavethepotentialtobeusedfor multipletissuetypesinavarietyofmedicalandtissue engineer-ingapplications.Theyalsoinvestigatedtheinjectionfoamingof TPUwithdifferentblowingagents(water,CO2andwater+CO2) [51].Theyobserved thatforthe foamingwith water+CO2,the

watermoleculesnotonlyplasticizedtheTPUmoleculesbutalso grewcellsinitiallynucleatedbyCO2attheadvancingmeltfront

which,combined with fountain flow behaviour (material from thecentre of the part flows outwardto the mouldsurface at the advancing melt front), led to the elimination of the solid skin layer. Zhao et al. [52] chose to workon a blend of PLA andPHBV(Polyhydroxybutyrate-Valerate)withthismethod.The blendsmade with differentweight ratios(100:0, 85:15,70:30, 55:45,and0:100)wereproduced usingboth conventional and microcellularinjectionmouldingprocess. Theincrease ofPHBV contentsignificantlydecreasedthecellsizeandincreasedthecell densityinthemicrocellularspecimens.Moreover,addingPHBV slightlydecreasedthetensilestrengthforbothspecimens. 4.2. Biodegradableorbiobasedpolymersforpackagingandother consumerplasticsapplications

Thebiodegradableandbiobasedpolymerscanbeusedinthe fieldofpackagingorconsumerplasticstoaddressenvironmental concerns.Themainrationaleforfoamingthistypeofmaterialsisto decreasetheirweight.Applicationswithbiodegradableorbiobased polymersarereviewedinthispartandoperatingparametersare listedinTable3.

4.2.1. StructureandporosityofPLAfoams

Thepoly(lacticacid)orpolylactide(PLA)isoneofthemost promisingbiodegradable,biobasedandbiocompatiblepolymer.It ismanufacturedby polymerisation oflactic acid(LA), which is mostlyproducedbyfermentationofglucoseormaltoseobtainedby enzymatichydrolysisofstarchcerealgrains.ThePLAdegrades pri-marilybyhydrolysis,afterseveralmonthsofexposuretomoisture

[53].

NofarandPark[1]recentlypublishedareviewonthe fundamen-talproperties,foamingmechanisms,andprocessingtechnologies forPLAfoams.TheirinvestigationsofPLAfoamingshowedthat enhancedcrystallizationkineticssignificantlyincreasesthe

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expan-Table3

Theprocessingconditionsusedfortheextrusionassistedbysc-CO2forthefoamingofseveralbiodegradableorbio-basedpolymers.

Polymers Additives Extrusion sc-CO2(%) Ref.

Screwtype Temperature Speed PLA2002D ModifiedclayCloisite

30B

Single-screw 150–210◦_C ₈₀_rpm ₅ _[11]

PLA8302D 140–180◦C

PLA2002D Pineflour Single-screw 150–170◦C 80rpm 5 [12]

Talc PP-g-AM

PLA3001D Talc Singlescrew 130–160◦C N/A 4 [13]

Chain-extender

PLA3001D Talc Single-screw 130–150◦C N/A 4 [14]

Ecovio(PLA/PBAT) PBATEcoflex+PLA3001D

PHBV Modifiedclay(Cloisite 30B)

Single-screw 120–160◦C 30–55rpm 1.5–4 [15]

PLA2002D+suspension starch−glycerol–water

PLA-g-MA Co-rotatingtwin-screw N/A 150rpm 7–10 [16]

PLA4032D – Co-rotatingtwin-screw N/A 150rpm 5–9 [17]

PLA2002D PLA8302D

PLA8302D Chain-extender Co-rotatingtwin-screw 110–190◦C 150rpm 5–9 [18]

PLA4032D 130–190◦_C

PLA2002D Chain-extender Tandemline 130–210◦C N/A 5–9 [24]

PLA8051D 115–210◦C

PLA2002D Modifiedclay(Cloisite 30B)

Tandemline 100–140◦_C _5–9 _[25]

PLA8302D Talc Counter-rotatingtwin-screw 100◦C 1.8–9.4 [55]

PLA6300D – Tandemline 110–140◦_C ₅ _[56]

PLA2002D Chain-extender Co-rotatingtwin-screw 115–190◦_C _100–120_rpm _2–8 _[57]

PLA3251D 120–190◦C

PLA8052D 110–190◦_C

PLA2002D TreatedclayNanocor I30P

Co-rotatingtwin-screw 140–170◦_C ₁₀₀_rpm _2–7 _[59]

PLA2002D Modifiedclay(Cloisite 30B)

Single-screw N/A N/A N/A [60]

PLA3001D Oleamide(foaming agent) N/A 130–200◦C 100rpm N/A [61] Hydrotalcite Ultravioletabsorber Pregelatinizedwheat starch+sodium hydroxide Crosslinkingreagent (EPI) Co-rotatingtwin-screw 70◦_C ₁₂₀_rpm ₁ _[62] Nativewheat starch+sodium hydroxide

Cornstarch Crosslinkingreagent (EPI)+Acetylation reagent(Ac) Co-rotatingtwin-screw 60–70◦C 120rpm 1 [63] PVOH Microfibrillated cellulose Tandemline 70–195◦C 100rpm 5–9 [64]

sionratioandthecelldensityoffoamedsamples.Thecrystalnuclei thatareinducedduringfoamprocessingcanincreasePLA inher-entlylowmelt strengththroughthecrystal-to-crystal network. AddingnanoparticlesimprovedthePLAfoamsexpansionandcell nucleationbehaviourbyincreasingitsmeltstrengthandenhancing itsheterogeneouscellnucleationpower.

A patent filed in 2005 [54] describes extruded polylactide foamsblownwithcarbondioxide.Theinventionrelatestoa pro-cessthatcomprisesformingapressurized,moltenmixtureofa melt-processablePLAresincontaining 5–15%by weightof car-bondioxide.Thismoltenmixtureisthenextrudedthroughadie toa regionofreducedpressure.Atthispoint,thecarbon diox-ideexpandsandthePLAresincoolstoformstablefoamhaving atleast70%closedcells.Duringthisprocess,thecarbondioxideis introducedandmaintainedundersupercriticalconditionsfromthe timeofblendingintothemoltenresinuntilitreachestheextrusion die.Veryhighqualityextrudedfoamsarepreparedinthese pro-cess.Densitiesaslowas16–32kg/m3_can_be_obtained_with_good

cellstructure,goodappearanceandconsistentquality.Theoptimal rangeofCO2contentis7–11%inweight.

Thefirstworkaboutthefoamingofpoly(lacticacid)withCO2

nearsupercriticalconditionswasreportedbyReigneretal.[55].In thiswork,theauthorshaveachievedPLAfoamsindensityrangeof 20–25kg/m3_._They_observed_a_very_narrow_processing_window._A

CO2contentbelow7%wtledtoapoorexpansionratiowhereasa

contentofCO2above8.3%wtledtoshrinkageuponageing.Mihai

etal.[16]foundsimilarresultsbyblendingPLAwiththermoplastic starch,withthehelpofacompatibilizeragent(PLA-g-AM).TheCO2

incorporationintotheblendsledtoverypoorlyexpandedfoams untiltheyreachedacriticalCO2 contentaround7%wtatwhich

pointthefoamswerehighlyexpandedandwithasignificant reduc-tionofthedensity.Thedensityfoundbytheauthorswasabout 25kg/m3_between₇_and_10%wt_of_CO

2.Thefoammorphologyfor

thepurePLAandtheblendwascharacterizedbyfinecellsand open-cellstructure.Asignificantcristallinitywasdevelopedduringthe foamingprocess.Theyalsoworkedonthebehaviourofdifferent gradesofPLAwithsupercriticalcarbondioxide[17].Again,inthis study,theCO2contenthastobehigherthan7%wttoobtainhighly

porousstructure(densityaround35kg/m3_)._For_5%_CO₂_,_the_foam

morphologywascoarseandthefoamdensitiesrangedfrom400to 1000kg/m3_whereas_with_7%wt,_densities_ranged_between₃₂_and

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315kg/m3_._The_PLA_with_the_smallest_D-lactic_acid_content

pro-ducedfoamwiththelowestdensity.ThecrystallinityofPLAfoams increasedwith CO2 concentration andL-lacticacidcontent.Lee

etal.[56]studiedtheeffectofextrusionfoamingonPLAwithboth aPBA(CO2)andaCBA(azodicarbonamide).Itwasfoundthatthe

foamdensitieswerelowerwithCO2(<0.10g/cm3with5%wtCO2)

thanwiththeCBA(0.5g/cm3_for_the_lowest_density_with_0.8%wt

CBA).

Adding a chain extender to the polymer can improve the melt strength of PLA. Mihai et al. [18] added between 0 and 2%wtofachain-extender(CE),anepoxy-styrene-acrylicoligomer (CesaExtendOMAN698493fromClariantAdditiveMasterbatches), todifferentPLA,anamorphous(PLA8302D)andasemi-crystalline grade(PLA4032D).Theymanagedtoobtainlow-densityfoamswith thesemi-crystalline PLAwith only5%wt ofCO2 whereas 9%wt

ofCO2 was necessaryto obtain such a low-density foamwith

theamorphousPLA.Thiswasinagreementwith theirprevious study[16,17].At5%wtCO2forthesemi-crystallinePLA,the

den-sitydecreasedfrom65to30kg/m3_with_2%wt_of_CE,_showing_in

thiscasethebenefitofthehighermeltstrength.Athigherblowing agentconcentration,however,low-densityfoamswereobtained regardlessoftheCEconcentration.Similarresultswerefoundby Wangetal.[24],whohaveobservedanincreasedmeltstrength andelasticitybybranchingthePLAwith0.35and0.7%wtCE.These improvementsledtoanincreaseofthecelldensities,whichranged from20to40kg/m3_(depending_on_the_PLA_grade)_with_9%_CO

2.

Theyobservedacloserelationshipbetweencellmorphologyand expansionratio.Followingcellwallsruptures,anopen-cell struc-tureisinducedandresistance fortheblowingagenttodiffuse intotheatmosphereissignificantlyreduced,leadingtoalowfoam expansionratio.Ifaclose-cellstructureisinduced,gaswill dif-fuseintoadjacentcellsandbulkexpansionratiowillincrease.As pointedbyNofarandPark[1],anenhancedcrystallizationkinetics cansignificantlyincreasetheexpansionratioandthecelldensity offoamedPLAsamples.Wangetal.[24],observedthatbyvarying thelengthofthediereservoir,thecrystallizationofthePLAwith 0.7%wtofCEcanbecontrolled.Adiewithalongerisothermal resi-dencetimeinducedhighercrystallinityandhigherexpansionratio overamuchwidertemperaturewindowincomparisonwithadie withnoreservoir.ForPillaetal.[13],theoptimumchain exten-derpercentagewas1%wt,thecelldensitydecreasingabovethisCE content.Unlikethesestudies,forLarsenandNeldin[57],the addi-tionofthechainextender(2%wt)didnotleadtoadecreaseofthe foamdensity.AddingCEisfoundtointroduceinhomogeneitiesat differentlengthscales.Thefoamdensityrangedbetween20and 30kg/m3_with_8%_of_CO

2anddietemperatureaslowas110◦C.

Toobtainfinercellstructureandhighercelldensity,a nucle-atingagentcanbeaddedtofavourheterogeneousnucleation.For Reigneretal.[55],byadding0.5%wtoftalcinPLAbelow7%CO2,the

cellpopulationdensityincreaseda100-foldwhiletheaveragecell sizedroppedfromafewhundredsmicrometresdowntolessthan 60#m.SimilarresultswereobservedbyPillaetal.[13].Inaddition, couplingtalcwithachainextenderledtoadenserandmore uni-formcellstructure.Someauthors[16]didnotobservechangein thefoamstructureandcellsizes.Theyexplainedthisphenomenon bytheirhighblowingagentconcentrations(8%),andthefactthat theheterogeneousnucleationinducedbytalcwassurpassedby theheterogeneousnucleationinducedbytheCO2concentration

fluctuation.

4.2.2. NanocompositesmadeofPLAandsilicatelayeredmaterials OneeffectivewaytoimprovethePLAmechanicalproperties, andparticularlyitsbrittleness,istoaddmultilayersilicate mate-rialslikeclayforthecreationofnanocomposites.Moreover,clay additioncanhaveanimprovingeffectonbarrierpropertiesofa

polymermatrix,duetocreationoftortuouspaththatcontributes todelaywatervapourmoleculespermeation[58],whichcanbe advantageousforpackagingapplications.

Theuseofextrusionassistedbysc-CO2forthepreparationof

polymer/claynanocompositesiswideningbecauseofthe expan-sioneffectontheinterlayerdistanceofclaybytheCO2.ForMatuana

andDiaz[11],anincreaseintheheterogeneousnucleationwith 5%wtclays(montmorilloniteCloisite30B)wasobservedat5%wt sc-CO2.Thisadditionallowedbothhomogeneousandheterogeneous

nucleationstagestooccurduringthefoamingprocess.Finally,they obtainedmicrocellularplasticswithanorderof109_cells/cm3_and

anaveragecellsizeoflessthan10#m.Jiangetal.[59]observed anincreaseintheinterlayerspacingbetweenPLAand2.5%wtclays withthehelpofCO2becauseoftheintercalationofPLAchainsinto

thegalleryspaces.Thisindicatedthattheadditionofsc-CO2was

helpfulintheclaylayerexpansion,owingtoitsgasdiffusionand plasticizingabilityduringcompoundingofnanocomposites. Com-paredwithpurePLA,theelongationatbreakandtensilestrengthfor nanocompositescompoundedwith5wt%CO2wereimprovedby

166%and25%,respectively.Thenucleationeffectwasalsoobserved byKeshktaretal.[25]withblendshavingaclaycontentof0.5,1, 2and5%wtat5or9%wtCO2.Anincreasedcelldensity,expansion

ratioandmeltstrengthwereobservedduetothenanoclayroleas acellnucleationagent.Moreover,withthepresenceofdissolved CO2,claynanoparticles,andshearaction,thePLAcrystallization

kineticswassignificantlyenhanced.

InthecaseofZhaoetal.[60],theychosetopre-foamablendof PLA/nanoclaybeforeusingthismaterialsintheMuCell®_Process.

Indeed,inthemicrocellularinjectionmouldingprocess,itis chal-lengingtoobtainhomogeneousdispersionofnanofillersbecause oftheshortcycletime.Thefirststepwastopreparepelletsof nanocompositeloadedat4%wtclayinextrusion.Then,the pel-letswerepre-foamedwithasingle-screwextrudercoupledwith ahigh-precisionsyringepump.Finally,thefoamsarepelletized andusedinmicrocellularinjectionmouldingwithCO2 orN2 as

physicalblowingagents.Thepelletsofnanocompositewerealso usedinmicrocellularinjectionmouldingwithoutpre-foaming.The authorshaveobservedinWAXDanalysisabetterclayintercalation andexfoliationinthePLAmatrixwiththepre-foaming.The pre-foamingalsoledtosmallercellsizeandcellsbetterdistributed thanthoseofthesampleswithoutpre-foaming.Thispre-foaming stepincreasedthecelldensities,thetensilestrengthand strain-at-break.TGAanalysesshowedthatpre-foamingdidnotcausethe macromolecularweightofPLAtodecreaseorchangesignificantly. 4.2.3. BlendingofPLAwithotherbiopolymers

ThefoamingofthePLAwithanotherbiodegradablepolymer,the PBAT(poly(butyleneadipate-co-terephtalate)),wasalsostudiedin theliterature.Pillaetal.[14]testedtwodifferentblendsat45/55 ratio: a commercially availablecompatibilized PLA/PBAT blend (Ecovio,BASF)andanon-compatibilizedPLA/PBATblendwiththe PLA3001DandthePBATEcoflex.Theyadded0.5%talcas nucleat-ingagent.Thecompatibilizationledtoareductionoftheaverage cellsizeandthevolumeexpansionratiobutincreasedthecell den-sity.Thedietemperaturehadnoeffectonthevolumeexpansion ratiobetween130and150◦_C,_except_for_the_pure_PLA_and

non-compatibilizedblendPLA/PBAT.Forthesematerials,thevolume expansionratioincreasedatlowerdietemperature(between1.6 and1.8at130◦_C_vs._between_1.4_and_1.6_at₁₅₀◦_C)._The_results

showedthattheadditionoftalcinbothblendsdecreasedthe aver-agecellsizeandvolumeexpansionratioandincreasedthecell density buthadvarying effectonthe open cell contentofthe foamedsamples.Thisadditionhadalsoaneffectonthecrystallinity, whichincreases.

Inanotherapproach,MatuanaandDiaz[12]hadstudiedthe influenceofwood-flourparticlesinthefoamingbehaviourofthe

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Table4

TheprocessingconditionsusedfortheextrusionassistedbyCO2forthebiobasedpolymersinthefoodarea.

Materials Extrusion Q(CO2)orCO2(%) Ref.

Cornflour+tomatopowder Co-rotating intermeshing twin-screw

30–130◦_C ₂₅₀_rpm _N/A _[65]

Cornflour+ginsengpowder Cornflour+greentea

Cornmeal+alkalizedcocoapowder Co-rotatingintermeshingtwin-screw 95–120◦_C ₁₅₀_rpm _1% _[66]

Cornstarch Co-rotatingintermeshingtwin-screw 95–135◦C 150rpm 500mL/min [67]

Corngrits+cornfibre Co-rotatingintermeshingtwin-screw 90–120◦C 150rpm 200mL/min [68]

Wheat Co-rotatingintermeshingtwin-screw 40–130◦_C _150–200_rpm ₅₀₀_mL/min _[69]

Cornflour+hull-lessbarley Co-rotatingintermeshingtwin-screw 30–130◦C 250rpm N/A [70]

PLA. Particleshad an effectonthe melt rheology, whichplays an importantrole during cell growth andstabilization. In this study, they added between 0 and 30%wt of wood-flour (pine flour)and0.5%wtoftalc(topromoteheterogeneousnucleation) inaPLAmatrix.TheCO2contentwasapproximately5%wt.They

observedthatmeltindexofPLAdecreasedasthewood-flour con-tentincreasedinthematrix.However,theauthorsfoundawayto counteractthisaspectbyincorporatingvariousconcentrationsof arheologymodifierintothecomposite.Therheologymodifieris alowmolecularweightmaleicanhydride-modifiedpolypropylene (EpoleE-43fromEastmanChemical).Attheend,theyproduced microcellular foamed PLA/wood-flourcomposites with uniform andhomogeneouscellularstructuressimilartothoseachievedin neatPLAfoams.Byaddingthewood-flour,theaveragecellsize increasedfrom 7.4to 11.3#m and thecell population density decreasedfrom1.46×109_to_0.26_×₁₀9_cells/cm3_.

4.2.4. Otherapplications

Kuoetal.[61]workedonadifferentproblematic.They man-agedtocreatePLAfoamswithhighreflectivity.Inthiswork,they compoundedPLA3001,afoamingagentpromoter(oleamide),a nucleatingagent(hydrotalciteLDH-40)andanultravioletabsorber (Tinuvin320)withatwin-screwextruderandpelletizedit.The resultsshowedthattheextrusionassistedbysc-CO2exhibitfaster

manufacturing speed andbetter capability of mass-production thanbatchsupercriticalCO2foamingprocesses.Thedensity,

aver-agecellsize,foamingratio,andreflectivityofPLAfoammadeby supercriticalfluidextrusion(SCFX)canreach0.04g/cm3_,₅_mm,_20,

and99%,respectively.TheymanagedtoobtainLEDlampwith max-imumluminance,viewingangle,andreflectivityof15,200cd/m2_,

123◦_,_and_99%,_{respectively.}

Followingastudyaboutthefoamingofcrosslinkedstarch[62]in whichAyoubandRizvimanagedtoobtainuniformfoam,theyused thesameprocesstoproducemoistureresistantstarch-basedfoams

[63].Inthesestudies,theyfinallyobtainedmicrocellularfoam hav-inganon-porousskinandahighdegreeofuniformityintheircell size.Theadditionofcrosslinkingagenthasbeenfoundtoenhance thefoamuniformity.Theyobservedthataddingtheacetylation agenttothecrosslinkingimpartedsignificantwaterresistanceto theextrudedsamples.Thesecondstudyhasalsodemonstrated thatmodificationsequencehasasignificantimpactonthe struc-turesandpropertiesofdual-modifiedstarches.Reactionconditions employedinmodificationprocessdeterminethedistributionand locationofmodifyinggroups,whichinturndeterminethe proper-tiesofthemodifiedstarch.Inaddition,thiskindofmaterialcould becontrolledtoproduceparticleswithmorphologyandproperties usefulforgreenplasticsindustry.

Thepoly(vinylalcohol)(PVOH)isawatersolubleand biodegrad-ablepolymerwhichiscompatiblewithmanyorganicandinorganic materials.Itcanbeusedinmanyfields(drugdelivery,packaging, heavymetalionsadsorption,andinacousticnoisereduction).Zhao etal.[64]havemixedPVOHwithmicrofibrillatedcellulose(MFC) whichisabiodegradablenanofillerwithhighaspectratio,stiffness

andstrength.Theychosetoworkwithtwophysicalblowingagents (PBA):sc-CO2 andwater.The foamingexperimentsweremade

onatandemlinewith22.5%wtwaterinthepurePVOH(Mowiol 23–88ofKuarayCompany)and12.5%wtwaterinthecomposite PVOH/MFC.ThecontentofMFC(KY100GbyDaicelFineChem)was 0,0.05or0.1%wt.Whenwaterwasusedaloneasblowingagent,the nucleationduringthePVOHextrusionfoamingwasgreatly weak-ened.Whenthesc-CO2 andthewaterwerecoupled,thewater

facilitatedtheCO2processingandimprovedtheextrusion

foam-ingprocess.Thisisexplainbythefactthatwaterplasticizesthe PVOHandtherebyincreasesthelowsolubilityofCO2while

sc-CO2promotescelldensitythatcannotbeachievedbyusingwater

alone.TheMFCinthePVOHaffectedthemeltstrengthand crys-tallinity.Duringthefoaming,itservedascellnucleatingagentand influencedcellnucleationandcellgrowthbehaviourthrough crys-tallization.Likeinmanyotherspublications,thecelldensitywas increasedwithincreasedCO2contentanddecreaseddie

tempera-tures.

InthesameopticasKeshktaretal.[25],LeMoigneetal.[15], studiedtheuseofSCFXtoimprovetheclay(Cloisite30B) disper-sioninthepoly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV) polymer.Inthispublication,theyobservedthattheprior prepa-rationofamasterbatchwasa necessarystep toobtained good claydispersionandlimitedPHBVdegradationduringthe extru-sionfoamingprocess.Topreparethismasterbatch,theyusedthe methodofmelt intercalationbyusingaco-rotatingtwin-screw extruderwith10or20%wtofCloisite30B.Themasterbatchwas thendilutedat2.5%wtofclaybeforeSCFX.Herethesc-CO2

con-tent hada great importance. Theyfound a narrow window of sc-CO2massfractioninwhichthegoodclaydispersionappeared

tofavourhomogeneousnucleationwhilelimitingthecoalescence ofthepores.Itallowedobtainingnanobiocompositefoamswith betterhomogeneityandporosityupto50%.Nevertheless,the crys-tallizationofthePHBVuponthefoamingprocessshouldhamper thediffusionofthesc-CO2withinthematrixandhencethe

nucle-ationandgrowthoftheporeswhich,inturn,limitthehomogeneity andultimateporosityofthefoams.

4.3. Biobasedpolymersusedinthefoodarea

Duetotheubiquitouspresenceofwaterinfood,thesteam extru-sionisusedinthisdomainwithstarchyorproteinaceousmaterials. Inthismethod,thewateractsasplasticizerandblowingagentbut involveshightemperature(130–170◦_C),_shear_and_low_moisture

content(13–20%).Theseconditionspreventtheuseofheat sen-sitiveingredients.Toavoidthedrawbacksofthistechnique,the injectionofCO2canbeused.SomestudiesfromRyuetal.[65–70]

arededicatedtotheinjectionofpressurizedCO2inthebarrelof

anextruder(between1to5MPa).Withtheinjectionof pressur-izedCO2,theextrusiontemperatureisdecreased(below100◦C)

andpreventsthedegradationofthenutrients.Theproductsmade canbeusedinthefoodindustry,especiallyinsnackfoodswith

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Table5

Theprocessingconditionsusedfortheextrusionassistedbysc-CO2forthebiobasedpolymerinthefoodarea.

Materials Extrusion Q(sc-CO2)orsc-CO2(%) Ref.

Flour,sugar,wheyproteinconcentrate,salt.... Co-rotatingtwin-screw 36–38◦_C _N/A _N/A _[74]

Wheatflour,water,salt,sugar,dryskimmilk, vitalwheatgluten...

Co-rotatingtwin-screw 31–37◦C 39rpm N/A [75]

Waxyriceflour,ricebran,soyprotein concentrate

Co-rotatingtwin-screw 80◦_C ₁₂₀_rpm _7.6_×₁₀−5_kg/s _[77]

Wheyproteinconcentrate,pregelatinizedcorn starch,prebioticfibre...

Co-rotatingtwin-screw 70–80◦C 120rpm 7.6×10−5kg/s [78]

Waxyriceflour,soyproteinconcentrate, toasteddefattedsoyflour...

Co-rotatingtwin-screw 80◦C 120rpm 7.6×10−5kg/s [79]

Milkproteinconcentrate,applepomace,grape pomace,pre-gelatiniedstarch....

Co-rotatingtwin-screw 86–94◦_C _135–180_rpm _7.6_×₁₀−5_kg/s _[80]

Wheyproteinconcentrate,masaflour, pre-gelatinizedstarch,oatfibre,vegetable shortening

Co-rotatingtwin-screw 40–75◦C 130rpm 0.3–0.7% [81]

Wheyproteinconcentrate,pre-gelatinized cornstarch...

Co-rotatingtwin-screw 25–90◦C 180rpm 1% [82]

Co-rotatingtwin-screw 25–90◦C 180rpm 1% [83]

Co-rotatingtwin-screw 90◦_C ₁₈₀_rpm _2% _[84]

Wheyproteinconcentrate,cornoil,unsalted creambutter(foranhydrousbutteroil preparation)

Co-rotatingtwin-screw 90◦C 180rpm 1% [85]

nutritionalorantioxidantproperties.Thematerialsusedandthe processingconditionsareshowninTable4.

NotonlythepressurizedCO2allowsadecreaseoftheoperating

temperature,butalsotheinjectionofsc-CO2ispreferablefora

betterexpansionrateandviscosityreduction.Sauceauetal.[10]

reviewedrecentlytheusedofsc-CO2extrusioninthefieldoffood

area.ThemainrecentapplicationsconcernedbytheSCFXinthe foodareahavebeendetailedhereandtheprocessingconditions arelistedinTable5.

4.3.1. Starchyproducts

In1992,RizviandMulvaney[71]patentedanovelextrusion technologyforproducinghighlyexpandedstarchwithsc-CO2used

asanexpansionagentandplasticizer.Thispatentwasextendedto otherapplicationslikecerealsandflourproducts[5,72,73]witha porousstructuredifferingfromtheoneobtainedbysteam extru-sion.Thenewporousstructurewasgenerallyclosed,withafine andregularporosity,andsometimesasmoothersurfacestate(non porousskin).Thiswasexplainedbyahighernucleationrate asso-ciatedwithmorerapidnucleationlimitinggasdiffusion.

Forexample,Hicsamazetal.[74]showedthatSCFX technol-ogycouldbeusedtocontinuouslyproduceyeast-freedoughusing CO2asaswellinggas.TheSCFX-leaveneddoughwasfoundtohave

rheologicalpropertiescomparabletothecommercialdoughand withoutethanolreleased.Theleaveneddoughcouldbeproduced bySCFXprocessinabout2min,eliminatingtheneedforproofing andholdingtime.Ruttarattanamongkoletal.[75]continuedthis previousworkbuttheobjectivewastoobtainbreadswithsimilar densitytotheoneofconventionalyeast-leavenedand commer-cialbreadproducts.Thedoughwasmadeofwheatflour,water, salt,sugar,dryskimmilk,vitalwheatglutenandothersingredients basedonapreliminarystudy[76].Thebreadobtainedhadadensity between0.19and0.45g/cm3_._It_had_similar_qualities_to

commer-cialproducts.Thisprocessreducesthetimeandspacerequiredto producebreadandotherexpandedbakedgoods.

4.3.2. Fibrousandproteinaceousproducts

AnotherapplicationofSCFXprocessistheexpansionand pro-ductionoffortifiedcrisps.Paramanetal.[77]evaluatedthisprocess

ofSCFXforrice-basedexpandedproducts.Theyobservedthatthe additionofricebran,soyproteinconcentrate,distilled monoglyc-eride,saltandmicronutrientpremixinthericeflourcanproduce protein,fibreandmicronutrientsfortifiedpuffedricewithbalance nutritionalprofile.Theadditionof22.5%ofsoyproteinimproved theproteinamountinthefinalproduct from5.8to21.5%.The presenceofvitaminAandCismaintainedduetolower temper-atureandshear.The final product hadadensity of 0.31g/cm3

andwasverycrispywithgoodtexturalcharacteristics.Theyalso usedthismethodtoproducewheyproteincrispsthatcontained ahighamountofproteinsandprebioticfibres[78].Aformulation containing60%wheyproteinconcentrates(WPC-80)and8% pre-bioticsolublefibresresultedinproteincrispswithgoodexpansion characteristicsandcrispiness.InthecaseofSharifetal.[79],the crispsweremadewithrice-soyandwerefortifiedwithsoy pro-teinconcentrateandtoasteddefattedsoyflour.Theyobservedan increaseoftheproteincontentandanimprovementoftheamino acidbalanceinthefood,providingamorenutritiousproduct.More recently,Sunetal.[80]investigatedthepotentialofincorporating fruitpomace(fruitresidues)andconcentratedwheyinhighprotein products.Theyfoundthattheadditionofpomaceandwheydidnot affecttheoveralltexturalqualityoffinalextrudates.Moreover,the fruitpomaceadditionimprovedthedietaryfibrecontentofthefinal products.Theymanagedtoobtainproteinextrudatepiecedensity rangingbetween0.24and0.31g/cm3_._Cho_and_Rizvi_[81]_developed

aneffectiveprocessforexpandedSCFXchipswithhighnutrient concentration.Theymanagedtoproducewiththisprocesshealthy snackchipscontainingupto60%wtproteincontentwithoutany chemicalmodification.Theyobtaineduniformlyexpandedcellular structurewhenexpansionoperationwasperformedbelowwhey proteindenaturationtemperature.Thetexturalpropertiesofbaked andfriedproductswerecomparabletocommercialextrudedor friedchipproducts.Thesc-CO2contentwasthecriticalparameter

tocontroltheexpansionandtextureofthefinalproduct. 4.3.3. Texturizedproducts

Wheyproteins(WP)areusedinmanyfoodapplicationsbecause theyhavetheabilitytogeluponheatingandtoprovidedesirable foodtexture.ThemostcommontechniqueusedtotexturizeWPs

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isthermoplasticextrusion. Ingeneral,functionalityofWPs can bealteredbyheating,addingsalts,adjustingthepH,and shear-ing.Heat-inducedgelationofWPshasbeenextensivelystudied andusedto add texture tofoodproducts. In2008, Manoi and Rizvi[82]investigatedthemodificationofwheyprotein concen-trates(WPC)byusingtheextrusionassistedbysupercriticalCO2.

ItwashypothesizedthatreactiveSCFXprocessinhighlyalkaline oracidicenvironmentcombinedwithcontrolledshearandheatin thepresenceofmineralsalts(CaCl2andNaCl)andsc-CO2would

favourablyalterthegellingandfunctionalpropertiesofWPC.The resultsobtainedbyManoiandRizviconfirmedthishypothesisas theygeneratednewWPingredientswithuniquegellingand func-tionalproperties,whichmayopenupanewavenueforutilisation ofWPasathickeningorgellingagentinfoodformulations. Tex-turizedproteins(tWPC)samplesproducedunderacidic(pH=2.89) andalkaline(pH=8.16)conditionswithsc-CO2exhibitedhigh

sta-bility ofrheological properties over a wide temperature range (25–85◦_C)._The_water_holding_capacity_of_pH-treated_WPC_samples

wasincreasedwithsc-CO2.In2009,thesameauthors[83]

inves-tigatedthemechanismsofinteractionsofWPCinthesameacidic (pH=2.89)andalkaline(8.16)conditionstoelucidatetheir influ-encesontheselectedphysicochemicalpropertiesofthefinaltWPC products.TheyfoundthatthemechanismofinteractionsinWP dur-ingtheprocessishighlydependingonpH.Approximately30%and 80%ofproteinsinthetWPCproducedatpH2.89and8.16, respec-tively,becameinsolubleinthestandardbuffer.Inastudyof2012 followingtheworkonwheyproteins,Mustaphaetal.[84]studied thetexturizationeffectoftheextrusionassistedbysupercritical CO2onthesurfacehydrophobicityofthewheyprotein

concen-trate.Theyfoundthatthisprocesscouldformcold-setproteingels andemulsiongelsthatcanbeusedasgellingandemulsifying ingre-dientforuseinmanyfoodapplications.Ruttarattanamongkoletal.

[85]hadthenstudiedtheeffectoftheincorporationoftWPCinan emulsioncontainingliquidoil(cornoilorbutteroil)bySCFX pro-cess.Theyobservedthatthisincorporationinanaqueousphase retardedthedropletcoalescence.Theyproducedfreshlyprepared emulsionswithmonomodalandnarrowdropletsizedistribution. Emulsionswithhigherelasticmoduluswereobtainedbyincreasing theoilconcentrationduetothedropletsrepulsionand deforma-tion.Thestoragetemperaturesandoilcontentsmarkedlyaffected thestabilityandrheologicalbehavioursofemulsionscontaining crystallisablebutteroil.

5. Conclusion

Thisoverviewenlightensthegrowinguseofextrusionassisted bysupercriticalfluidwithdifferentkindsofbiopolymer. Tempera-tureappearstobethefirstandmostimportantparametertomaster themanufactureofhighlyporousmaterial.Withadecreaseinthe melttemperature,anincreaseintheoverallporosityisobserved.A higherCO2contentallowstodecreasetheoperatingtemperature.

Inaddition,alotofparametersdoinfluencethepropertiesofthe foamsmadewithbio-basedorbio-degradablepolymers.WithPLA, forexample,theadditionofclayorchain-extenderswillchangethe porosity.

Forbiocompatiblepolymersusedinpharmaceuticalproducts, thistechniqueallowstheuseofsensitivemoleculesbecauseofthe plasticizingeffectoftheCO2andthecorrelativedecreaseof

oper-atingtemperature,pressure,viscosityandshearstresses.Dueto thefineporousstructureinducedatthedieexit,thisprocessopens newapplicationareas.Themanufactureofscaffoldscouldbenefit fromthistechniquetoo.

Inthefoodarea,theSCFXhelpstocreateexpandedfoodwith improvednutritionalpropertiesandallowsoperatingatlower tem-peratureincomparisonwiththemoreclassicsteamextrusion.

Thereareobviouslystillseveralchallengestobeovercome,asfor instancetheneedtomoveawayfromfabricatingmicrocellularto nanocellularfoamstoreachsuperiorproperties(thermal, mechan-ical,electrical,etc.)[86].Buttheextrusionassistedbysupercritical fluidhasalreadyproventobeaverypromisingmethodtocreate biopolymerfoamswithabroadspectrumofapplicationinmany fields.

Acknowledgements

WeacknowledgethefinancialhelpoftheRégionMidi-Pyrénées, Franceinformofadoctoralscholarshiptothefirstauthor(MC). References

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