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Melting Efficiency for Various Polylactide Resins in a Co-rotating Intermeshing Twin Screw Extruder NickKnowlton1,AugieMachado2,BrianHaight2

1NatureWorksLLC,Minnetonka,MN,USA2LeistritzExtrusion,Somerville,NJ,USA

Abstract: TherearemanygradesofpolylactideresinsmarketedbyNatureWorksLLCundertheIngeo™tradename.Forcommercialandtechnicalreasons,Ingeomaybesuppliedasaneatresinorwiththeadditionofexternallubricant.Theappliedlubricantimprovespelletflowthroughconveyingsystems,silos,anddryerswithminimalinfluenceonphysicalproperties.Severalstudieshavebeenperformedcomparingthedifferencesinmeltingbehavior,powerload,andmelttemperatureinsinglescrewextrusion,buttodatenostudyhascharacterizedthesameparametersintwinscrewextrusion.

LeistritzExtrusionandNatureWorksLLChaveconductedexperimentsthatexaminetheeffectofexternallyappliedlubricantsonthemeltingperformanceinatwinscrewextruderwithmultiplescrewconfigurationsanddifferingoperatingconditions(i.e.rotationalscrewspeed),denotingthelocationoftheonsetofmelting,powerconsumption,andmelttemperatureforlubricatedandunlubricatedhighmolecularweight,lowmeltflowrate(MFR),formulations.

Introduction: NatureWorksLLCandLeistritzExtrusionhavecollaboratedtoconductexperimentsexaminingtheeffectsofexternallyappliedethylene-bis-stearamide(EBS)atlevelslessthantwopercentbyweightonthemeltingperformanceofIngeopolylactide(PLA)resinsinaLeistritz27-mmZSEMAXXtwinscrewextruder.Anaturallyadvancedmaterialscompany,NatureWorksoffersafamilyofcommerciallyavailableperformancematerialsderivedfromlocallyabundantandsustainablenaturalresources.1SomeofNatureWorksIngeoresinsareprovidedwithasurfacelubricanttoreducethefrictionandpreventstickinginconveyingsystems,silos,anddryers.

Twinscrewextruders(TSEs)suchastheLeistritz27-mmZSEMAXX,showninfigure1,areapreferredmanufacturingmethodforcompoundingbioplastics,includingPLA.TSEsutilizemodularbarrelsandsegmentedscrewsassembledonsplinedshafts.TSEmotorstransmitpowerintothegearbox/shaftsandrotatingscrewswhichimpartshearenergyintothematerialsbeingprocessed.ThemodularityofTSEsallowsforawiderangeandrefinementofmanyprocessingapplications.

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Figure1-LeistritzZSE27MAXXtwinscrewextruderusedforprocessing.

OptimalprocessingofPLAinatwinscrewextruderrequiresproperlyaddressingtheheatandshearsensitivityoftheneatPLA,aswellasitstorquerequirements.Poorprocessingpracticessuchashighpressure,highmelttemperature,excessivemoisturecontent,andincreasedresidencetimecanresultinhydrolyticdegradationandreducedmechanicalproperties.3Useofasurfacelubricantprovidesmanybenefitsinconveyingandtransportation,butalsohasimplicationsonthemeltingperformanceofPLAinatwinscrewextruder.ExplicitlyengineeringaTSEtoaccountforthedifferencesinprocessingcanincreasetheefficiencyofmelting,improvematerialproperties,improvethroughput,andreducetoolwear.AfewkeyfactorsthatinfluencethemeltingperformanceofPLAinatwinscrewextruderinclude:

1.) Levelofsurfacelubricantused2.) Barreltemperaturesetpoints3.) Lengthofthemeltingregion4.) Pellettopelletandpellettometalfriction5.) Screwrotationspeed6.) Screwelementandpelletgeometry

Thefactorsthatcanbecontrolledthroughthedesignofatwinscrewextruderarethetemperaturesetpoints,length,anddesignofthemeltingregion,screwspeed,andscrewelementgeometry.TheinfluenceofthesevariousparametersinaTSEprocesscanbeobservedinpolylactideintheformofmelttemperature,overalltorque,energyinput,onsetofmelting,andmolecularweightdegradation.Thesevaluescanbemeasuredinlineorthroughanalyticalstudiesonthefinalproduct.AnalysisofthespecificenergyisparticularlyusefulwhenevaluatingthemixingperformanceofaTSEandlookingfordrasticdifferencesinprocessing.Thespecificenergycanbecalculatedthroughcombinationofequations1and2below.

𝑘𝑊 𝑎𝑝𝑝𝑙𝑖𝑒𝑑 =0.97 𝑔𝑒𝑎𝑟𝑏𝑜𝑥 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 ∗ 𝑘𝑊 𝑚𝑜𝑡𝑜𝑟 𝑟𝑎𝑡𝑖𝑛𝑔 ∗% 𝑡𝑜𝑟𝑞𝑢𝑒 ∗ 𝑠𝑐𝑟𝑒𝑤 𝑅𝑃𝑀 𝑟𝑢𝑛𝑛𝑖𝑛𝑔

𝑆𝑐𝑟𝑒𝑤𝑠 𝑀𝑎𝑥 𝑅𝑃𝑀

Eq.1

𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 =𝑘𝑊 (𝑎𝑝𝑝𝑙𝑖𝑒𝑑)

𝑘𝑔/ℎ𝑟

Eq.2

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Forthepurposeofthisstudy,atwinscrewextrudercanbeviewedasvariousregionswithuniquetasks.Asshownintheimagebelow,theseprocesssectionsincludethemeltingregion,themixingandconveyingregion,andfinallyaventanddischargeregion.Inthemeltingregion,thepelletsmustabsorbheatfromthebarrelsandenergyofthescrew.Thiswillallowthepelletstosoftenandbegintoformaviscousmelt.Asthepolymerreachesthefirstsetofmixingblocks,theyshouldbesoftenoughtomechanicallydeformwithoutcrackingorcrunching.Audiblecrunchingpresentintheprocessisindicativeofhigherlevelsofmechanicalenergy,whichleadtohighermotorloadsandinessence,wastedenergy.Theformationofaviscousmeltinthemeltingregionwillalsohelpmixing,whichisessentialforahomogenouspolymermelt.Aftermelting,additionalconveyingandmixingwilloccurandmaterialswithviscositymismatchsuchasfillersandliquidsmaybeaddedthroughtheuseofasidestufferorliquidpump.Thelatterregionincludesameltseal,conveyingandventing,andpump/discharge.Afterthepolymerleavesthedischargeregionofthescrews,downstreamprocessessuchaspelletization,sheetextrusion,meltpumps,etc.areperformed.

Figure2-Co-rotatingscrewsinatwinscrewextrudersplitintovariousseparateregions.Thesewillbereferredtoasthemelting,mixing/conveying,andpumping/dischargeregions.2

Methods and Equipment: Twinscrewextrusion(TSE)experimentsprocessingNatureWorksLLCIngeo™Biopolymer4032Dwereperformedusingthethreescrewdesignsinfigure3below.NeatIngeo4032DpelletsaswellaspelletswithmediumandhighlevelsofexternallyappliedsurfacelubricantwereprocessedonaLeistritzZSE27MAXXextruderwith28.3mmdiameterscrews,5.7mmflightdepth,1.66OD/ID,torqueratingof304NMforbothscrews,and1200maxrpm.

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Figure3-Threescrewdesignsusedoverthecourseofthisstudy.

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Closeattentionwasgiventothemelttemperature,meltpressure,specificenergy,overalltorque,residencetime,andonsetofmelting.Theparametersweremeasuredandloggedandthespecificenergycalculatedusingastateofthearttwinscrewextrusioncontrolsystem.Animageofthetwomelttemperatureprobesandtwopressuretransducersusedinthisstudyareshowninfigure4belowwheretheinternal(deep)meltprobewasfixedat0.25”meltpenetration.

Figure4-Locationsofthetwomeltpressuretransducers,thesurface(shallow)meltprobe,andinternal(deep)meltprobeattheendofthe27-mmTSEused.

TheonsetofmeltingofPLAwasdeterminedbyremovingtheventatzone3,whichimmediatelyfollowsthekneadingblockspresentinzone2.Animageofthemeltextrudatefreelyflowingoutoftheextrudercanbeobservedbelow.Afterremovalfromtheextruder,thecollectedpolymerwasimmediatelyquenchedinroomtemperaturewatertopreservethemeltimage.

Figure5-MeltextrudatecollectedfromhighlylubricatedPLAatzone3ventforscrewdesign1.

Figure1-Imageofmeltextrudateremovedfromzone3onscrewdesign#1.ThepolymerextrudateisthatofhighEBScontent.

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Results and Discussion:

Figure6-Meltingregionofscrewdesign1usingGFFscrewelementsandkneadingblocksinzone2.

ExaminingtheLubricant-ScrewDesigns1&2:

Screwdesign1istypicalofascrewoptimizedformeltingandmixingofunlubricatedpolylactide.Themeltingregion,asshownintheimageabove,consistsofthreeGFF2-40-30elements,threeGFA2-40-30elements,andfourkneadingblocksvaryingfrom30°to90°forwardtwistbetweeneachdivision.ThisisarelativelyshortmeltingzoneforPLAthatisadvantageouswhenprocessingunlubricatedpolymerduetothehighfrictionandshearenergyabsorbedbythepolymer.TheadditionofthesurfacelubricantEBSalteredthepellet-to-pelletandpellet-to-metalfrictionexperiencedintheextruder.Thereductionoffrictionfromsurfacelubricantinthisdesignreducedtheshearenergyandfrictionalheatappliedtopellets.Asaresult,themeltingefficiencyofthePLApelletswasreducedandthepolymerwastoosolidasitapproachedthefirstsetofkneadingblocks.Thiswasobservedintheformoftorquespikesandaudiblepelletcrunching.Photosofthemeltextrudateforvariouslevelsofsurfacelubricantcanbefoundinfigure7.Onthefarleft,neatIngeo™4032Dshowsalmostcomplete,uniformmeltingwhilethehighlylubricatedPLAontherightshowsahighportionofunmeltedpellets.Thephysicalcrackingofthepelletsisnotdesiredinatwinscrewextrusionprocessasitisanindicationofinefficientmelting,excessivetoolwear,lowerthroughput,torqueoverload,andincreasedprobabilityofcatastrophiceventssuchasbrokenelementsandshafts.

Figure7-Imagesofmeltextrudateremovedfromzone3.Fromlefttoright:unlubricatedIngeo™4032D,mediumlevelofsurfacelubricant,andhighlevelofsurfacelubricant.

Thelearningsfromscrewdesign1wereusedtoengineerarevisedtemperatureprofileandscrewconfiguration,referredtohereasscrewdesign2.Inscrewdesign2,themeltingregionwas

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extendedby30-mmtoallowforincreasedheatingandsofteningpriortothefirstsetofkneadingblocks.Also,theGFFscrewelementsusedinthefeedthroatofdesign1werereplacedwithGFAelementsofequivalentpitchandlength.Asshowninfigure8,GFF’sarefreelycut,non-self-wipingelementswhileGFAsareclosely-meshing,self-wipingelements.TheuseofGFAelementsreducedthefreevolumeandincreasedtheenergyinputintothepolymerthroughadditionalpellettoscrewsurfacecontact,increasedpellettopelletfriction,andadditionalcontactwiththeheatedbarrelwall.4Thetemperatureprofileearlyinthescrewwasalsoincreasedfrom210°Ctotemperaturesashighas222°Cinthemeltingregion.Theincreasedtemperatureinthemeltingregioncausedthepolymertomeltfasterandreducedtheoverallloadonthescrews.

Figure8-SchematicofGFFandGFAelementsusedinscrewdesigns1and2,respectively.Alsoincludedarethetemperatureprofilesusedforprocessingat35kg/hrand350RPM.

Acomparisonofthespecificenergyinputwhileoperatingat350RPMand35kg/hrwithamediumlevelofsurfacelubricantinscrewdesigns1and2isincludedinfigure9below.Itcanbeseenthatanincreaseintemperaturesetpointsinthemeltingzoneandextensionofthemeltingzonereducedtheloadontheextruder.TheaudiblecrunchingsoundsthatwereexperiencedwhenprocessinglubricatedPLAwereeliminatedusingscrewdesign2.Thetemperatureprofileforscrewdesign2alsoefficientlymanagedthefinalmelttemperature.Thisprovidesadditionalbenefitssuchasimprovedmeltstrengthfordownstreamprocessessuchassheetextrusion,reduceddegradation,andefficientmixingthroughoutthescrew.Onethingtonoteistheextensionofthemeltingzonerequiredtheatmosphericventtobemovedonebarreldownstreamformzone3tozone4.However,relocationofthemixingblocksandincreasedviscosityalongtheprocesslengthisstillexpectedtoprovidequalitydispersionofadditives.

Figure9-Specificenergyandmelttemperaturewhileprocessingmediumleveloflubricantat350RPMand35kg/hronaLeistritz27-mmZSEMAXX.Melttemperaturemeasuredwithshallow,surfacethermocouple.

Figure 4: GFA element Figure 5: GFF element

Zone1 Zone2 Zone3 Zone4 Zone5 Zone6 Zone7 Zone8 Zone9 Zone10ScrewDesign1 210°C 210°C 210°C 210°C 210°C 210°C 210°C 210°C 210°C 210°CScrewDesign2 220°C 222°C 222°C 212°C 212°C 212°C 202°C 202°C 202°C 210°C

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OptimizingtheProcess-ScrewDesigns2&3

Furtheroptimizationofscrewdesign2wastargetedwithvariedscrewspeeds,temperaturesetpoints,lubricantlevels,andamodificationthatresultedinscrewdesign3.Inscrewdesign3,themeltingregionwasextendedbyanadditional60mmbeyondthatofscrewdesign2.Theconveyingelementsinthemeltingregionofdesign3werealteredtoincludetwofewer30mm,40°pitchedelementsandinsteadusedfouradditional30-mm,20°pitchedelements.Theabilitytoextendthescrewanadditional60mmwilldependontheprocessbeingused.Insomecases,thismaynotbeanoptionduetoadditionalfeedersand/ormixingnecessaryinthemixingandconveyingportionofthescrew.However,thisdesignfurtherimprovedthemeltingpriortomixingbyincreasedpellettometalsurfaceareaandincreasedresidencetimeintheheatedbarrelpriortothefirstsetofkneadingblocks.

Thetemperatureprofilesandfeedratesforscrews2and3werealteredbeyondtheworkperformedinscrews1and2.Theincreasedtemperatureisdesignedtoincreasemeltingearlyinthescrew,managemelttemperaturethroughouttheprocess,decreasethetorqueonthescrewshafts,andincreasetheoverallthroughput.Thefeedratesforallmaterialswasincreasedto45kg/hrduetoimprovedmeltingperformanceofthescrews.

Figure10-Temperatureprofilesusedforacomparisonandoptimizationofscrewdesigns2and3.

ThefinalmelttemperatureofIngeo4032Dwasmeasuredatthebarrelsurfaceaswellas0.25”(6.4mm)intothemeltwiththeuseofavariablemeltprobe.Therecordedtemperaturesfromthisprocessatscrewspeedsvaryingfrom325to600RPMcanbeobservedinfigures11and12.Asexpected,theinternalmelttemperatureissignificantlyhigherthanthesurfacetemperatureinbothscrewdesignsduetoreducedinfluenceofheattransferfromthebarrel.Thedifferencesinmelttemperatureoflubricatedandunlubricatedfeedscanbeexplainedbydecreasedmeltingperformance.Thereductionoffrictionalheatearlyinthescrewresultsinacoolermeltthroughouttheextruder.Onethingtonoteisthemaximummelttemperatureobservedinscrewdesign2was243°Cwhilethemaximumtemperatureindesign3was240°C.Thedifferenceof3°Cmayseemmarginal,however,itmayhaveasignificantinfluenceondegradationinprocesseswhereregrindisincorporatedandthepolymermaybeexposedtothesetemperatures5-10times.Itisrecommendedthateachprocessbeoptimizedwithcloseattentionpaidtothemolecularweight,melttemperature,torqueload,andmechanicalpropertiesforanyprocess.

Zone1 Zone2 Zone3 Zone4 Zone5 Zone6 Zone7 Zone8 Zone9 Zone10ScrewDesign2 220°C 240°C 230°C 230°C 220°C 210°C 210°C 210°C 210°C 210°CScrewDesign3 220°C 240°C 230°C 230°C 220°C 210°C 210°C 210°C 210°C 210°C

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Figure11-MelttemperatureforshallowanddeepthermocouplemeasuredonlubricatedandunlubricatedPLAatthediepriortodischargeinscrewdesign2.

Figure12-MelttemperatureforshallowanddeepthermocouplemeasuredonlubricatedandunlubricatedPLAatthediepriortodischargeinscrewdesign3.

Inadditiontomonitoringthemelttemperature,closeattentionwaspaidtothespecificenergyinputinscrewdesigns2and3.Infigure13itisclearthatscrewdesign2isaddingmoreenergytothepolymermelt.Onewouldexpectthatdesign2isprovidingmoreefficientmixingwhiledesign3isalowerworkscrew.Eachofthesedesignsprovidesbenefitsfordifferentapplications.Whencompoundingaperformanceenhancingmaterialsuchasanimpactmodifier,screwdesign2maybepreferredduetoenhanceddispersivemixing.However,screwdesign3maybebeneficialforcompoundinginacolorantpowderorotherhighlyhygroscopicmaterialwherelowresidencetime,increasedthroughput,andlowmelttemperaturesaredesired.

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Figure13-Specificenergyvsextruderspeedfordesigns2and3withlubricatedandunlubricatedPLA.

Foreachoftheconditionsappliedtoscrewdesigns2and3,thepolymerextrudatewasstrandcutandanalyticaltestingperformed.Itwasconcludedthateachofthescrewdesignsandprocessconditionsresultedingreaterthan92%molecularweightretentionafterasinglepass,thesurfacelubricantlevelwasmaintainedconstant,themelttemperaturedidnotexceed245°C,pressuredidnotexceed300psig,andresidencetimewasheldmaintainedbetween15and20seconds.Thesurfacelubricantusedinthestudieshasbeentestedextensivelyanddoesnotsignificantlyinfluencethemechanicalproperties,rheology,ordegradationofpolylactideattheconcentrationsused.Screwdesigns2and3offerviableoptionsformeltingandprocessinglubricatedPLAbutshouldbefurtheroptimizedbasedondesiredthroughput,mixingefficiency,andpolymermechanicalproperties.

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Conclusions: TheprocessingofNatureWorks’Ingeo™4032DwasobservedwithandwithouttheapplicationofEBSsurfacelubricantwhileprocessingwiththreedifferentscrewconfigurationsinaLeistritz27-mmZSEMAXX.KeydifferenceswerenotedinthemeltingbehavioroflubricatedPLAincludingdelayedmelting,reducedloadontheextruder,andareductionofmelttemperature.Inscrewdesign1,unlubricatedPLAexhibitedqualitymeltingbehaviorwhilelubricatedPLAcausedtorquespikesandunmeltedpelletsimmediatelyfollowingthefirstsetofkneadingblocks.Indesigns2and3,optimizedforprocessinglubricatedPLA,qualityproductwasobservedunderallprocessingconditionstested.Thiswasdeterminedasgreaterthan92%molecularweightretention,melttemperaturesoflessthan245°C,specificenergyinputof0.15to0.21kW-hr/kg,andsufficientmelting/softeningpriortofurtherprocessing.Forthegeneralpractitioner,thisworksuggestssomepracticalguidelinesforoptimizingatwinscrewcompoundingprocesswhenPLAmakesupamajorportionoftheblend.Ifmotorloadsseemexcessivelyhighorthereisaudiblecrackingandgrindingofpelletscomingfromtheextruder,theprocess,andthereforeproduct,wouldbenefitfromachangeinthescrewdesign.Increasingtheheattransferintothepelletbedpriortothefirstkneadingsectiongenerallywillleadtolowerenergyconsumption,improvedprocessstabilityandreducedmelttemperature.ThisworkperformedbyNatureWorksLLCandLeistritzExtrusionhasidentifiedmultiplesolutionstoimproveoptimizetheprocessingoflubricatedIngeo™resingrades,suchasextendingthemeltingregionofthescrew,alteringtemperatureprofiles,improvingthemodesofheattransferthroughincreasedsurfaceareaandfrictional/shearheat.

References and Notes:

1.) "About NatureWorks." About NatureWorks. N.p., n.d. Web. 10 May 2016. 2.) Martin, Charlie. "Twin Screw Extrusion for Pharmaceutical Processes." - Springer. American

Association of Pharmaceutical Scientists 2013, n.d. Web. 09 May 2016. 3.) Garlotta, Donald. "A Literature Review of Poly(Lactic Acid)." ResearchGate. Journal of

Polymers and the Environment, Vol. 9, No. 2, April 2001 (q 2002), n.d. Web. 09 May 2016. 4.) "ZSE MAXX SERIES Co-rotating Twin Screw Extruders." Leistritz Extrusion. Web. 13 May

2016