Highefficiencygreenpreparationtechnologyofmelt

differentialelectrospinning

Abstract:Nanofibershavemanypotentialapplicationsinavarietyofareas,suchasbiomedicine,high-efficiencyfiltrationandbiochemicalprotection.Electrospinningisconsideredasoneofthebesttechnologiesfornanofiberpreparationbybothindustryandacademia.Comparedwithsolutionelectrospinning,meltelectrospinning,whichdoesnotrequiretheuseofsolvents,hastheadvantagesofleavingnotoxicsolventresiduesandnoneedforsolventrecoveryordisposal,andhasobviousadvantagesinthelarge-scalepreparationofnanofibers．However,itisdifficulttofabricatenanofibersonalargescalebyconventionalmeltelectrospinningduetothehighviscosityandlowelectricalconductivityofmaterials.Meltdifferentialelectrospinningwasfirstproposedbyourteamabouttenyearsago,andaseriesofstudiesonprocesses,equipment,materialsandapplicationsofmeltdifferentialelectrospinninghavebeenconducted．Atpresent,nanofibershavingadiameterofabout500nmcanbepreparedcontrollablyandefficientlybymeltdifferentialelectrospinningandtheworld'sfirstmeltdifferentialelectrospinningnanofiberindustrializedproductionlinehasbeenestablished．Inthispaper,ourresearchresultsandthelatestdevelopmentsinmeltdifferentialelectrospinningareintroducedfromthreeaspects:themechanismofmeltdifferentialelectrospinning,thekeytechnologyofmeltdifferentialelectrospinning,andthegreenbatchproductionofnanofibersandtheirapplications.

Keywords：electrospinning;meltdifferential;nanofiber

1.Introduction

Asahigh-performanceone-dimensionalnanomaterial,nanofibershavebroadapplicationprospectsinthefieldsofelectronics[1],environment[2],energy[3],andbiomedicine[4].Thedemandforequipmentismoreandmoreurgent.Amongthe60majorscientificandtechnologicalproblemsreleasedbytheAssociation,thebatchproductionofnanofibersislistedasoneofthefivemajorproblemsinthefieldofadvancedmaterials.Thecurrentmethodsforpreparingnanofibersmainlyincludeelectrospinning,templatesynthesis,meltblown,phaseseparation,etc.[5],amongwhichtheelectrospinningmethodhassimpleequipment.Theadvantagesofsimpleandcontrollableprocesses,widerangeofrawmaterialsandeasyindustrializationhaveattractedwidespreadattentioninthescientificandindustrialcircles.

Electrospinningisdividedintosolutionelectrospinningandmeltelectrospinningbasedonthepresenceofsolventsinthespinningrawmaterials.Inthe1990s,Renekerreportedthefeasibilityofpreparingnanofibersbysolutionelectrospinning,whichledtotheupsurgeofpreparingnanofibersbythismethod.Aftermorethan20yearsofdevelopment,nanofibersofmorethan200materialshavebeensuccessfullypreparedusingsolutionelectrospinningtechnology,andsolutionelectrospinningequipmenthasalsoinitiallyachievedcommercialapplications[6].However,mostofthesolventsusedinsolutionelectrospinning,suchaschloroformanddichloromethane,aretoxic.Inthebatchpreparationprocess,thepreparation,removalandrecyclingoftoxicsolventswillincreasethecostofequipment.Leakageoftoxicsolventswillcausesafetyproductionaccidentsandenvironmentalpollution.Inaddition,thermoplasticmaterialssuchaspolypropylene(PP),polyethylene(PE),andpolysulfide(PPS),whicharecommonlyusedintheindustry,donothaveasuitablesolventatroomtemperature[7].Soitisdifficulttopreparethroughsolutionpreparationofnanofibers.Therefore,the"zerosolvent"meltelectrospinningtechnologyhasgraduallyattractedresearchers'attention.

Forthebatchpreparationofmelt-spinningnanofibers,AachenUniversityofTechnologyhasdevelopedasmall-scalemelt-spinningtestdevicewith64needles[8],butithastheproblemsoflowefficiencyandeasyneedleclogging.Basedontheproposednewprincipleofmelt-differentialelectrospinning,ourteamhassetuptheworld'sfirstmelt-differentialnanofiberproductionline,withaproductioncapacityof300to600g/h[9],andmovedtowardsacontrolledandefficientgreenpreparationofnanofibersabigstep.

Theteamfirstproposedthemeltdifferentialelectrospinningtechnologyin2008.Aftermorethantenyears,ithascarriedoutsystematicresearchonprocesses,equipment,rawmaterialsandotheraspects,andrealizedthebatchpreparationofmeltnanofibers.Thewideapplicationoffiberlaidthefoundationforthefurtherresearchers'recognitionandaffirmation.Thisarticlewillintroducetheresearchresultsandlatestprogressofmeltdifferentialelectrospinningfromthreeaspects:meltdifferentialelectrospinningmechanism,keytechnologies,nanofiberbatchgreenmanufacturingandapplication.

2.Differentialprincipleofelectrostaticmeltspinning

Ourteaminnovativelyproposedanumbrella-shapeddifferentialnozzle.Basedontheresearchonthemeltelectrospinningfiberthinningmechanism,thedynamicevolutionofthejetandthecontrolmechanismofthejetspacing,thenanofiberpreparationefficiencywasimproved.Thenumberofjetsperunitareaincreasedby80times[10.]

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Fig.1Nozzleofthemeltdifferentialelectrospinningdevice

Meltdifferentialelectrostaticspinningnozzleatpresent

Themainformsofexistingmeltelectrostaticspinningnozzlesarecapillaryneedles,discnozzles,slitnozzles,andmeltdifferentialnozzles.Theoutputofcapillaryneedlesislow,onlyabout1.38g/h.Inaddition,thereisabigproblemthattheneedlesareeasilyclogged.Thediscspinningmethodandtheslitspinningmethodincreasethenumberofjetsperunittoacertainextent.However,thereisanunevendistributionofmicro-flow.Sonoindustrializationhasbeenreported.

Inspiredbythenaturalshuntingphenomenonofwaterfallsinnature,ourteaminnovativelyproposedanumbrella-shapeddifferentialnozzle.Throughthedesignofthemicro-channel,thepolymermeltwasdividedandthinnedseveraltimestoachievethepolymermeltjetattheendofthenozzle.NozzlestructureandspinningprincipleareshowninFig.1.Undertheactionofahigh-voltageelectricfield,thepolymermeltwillformacircleofTaylorconeattheendoftheumbrellanozzle.Whentheelectricfieldforceisgreaterthanthesumofthesurfacetensionandtheviscosityofthepolymermelt,themeltjetwillformthetipoftheTaylorconeandspraytothereceiverbytheoppositepotential.Intheprocessofmovingtowardthereceiver,themeltjetisdrawnandrefinedbytheelectricfieldforce[11].Whilecontinuouslytransmittingheattothesurroundingenvironment,itisgraduallycoolingandsolidifyingtofibers.

2.2Tug-of-warfiberrefiningmechanism

Fiberdrawingandrefinementisoneofthemostimportantprocessesinmeltelectrospinning,whichhasahugeimpactonfiberdiameterandmorphology.However,theprocessofmelt-spinningdrawingisextremelycomplicated,whichinvolvestheinfluenceofmanyfactorssuchaselectricfieldforce,electrostaticrepulsion,meltconductivity,meltviscosityandtemperature[12].

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Fig.2Fiberrefiningmechanismofmeltelectrospinning

Ourteamfirstusedthephenolicmicrosphereparticletracertorevealthebasicphysicalprocessofmeltelectrospinningjetdraft,whichmainlyincludesthestraight-fallstage,theunstablestageandthesolidificationstage.Thenthedissipativeparticledynamicsmethodwasusedtosimulatethethinningprocessofthemeltelectrospinningfiber[12].Thesimulationresultsandexperimentalresultscanbewellmatched.Thecorrectnessofthetug-of-wareffecthasbeenverified(Fig.2).Thediscoveryofthetug-of-effectfiberrefinementmechanismhelpstodeepentheresearchers'understandingoftheelectro-spinningelectrojetdraftingandrefinement,buttheunderstandingofthepolymermeltelectricalmechanismneedstobeimproved.

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Fig.3Fiberrefiningmechanismoftug-of-warfiber

3.Keytechnologyofmeltdifferentialelectrostaticspinning

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hereisnoobvious"whippingeffect"inthemeltelectrospinningprocess.Thediameterofthepreparedfibersisgenerallyuptothemicronlevel.However,onlythefibersarerefinedtothenano-scalerange.Inordertorealizetheadvantagesofhighspecificsurfaceareaofnanofibersandrealizetheirhighaddedvalue,realizedthecontrollablepreparationofmeltnanofibersintherangeof500nmthroughtheseresearchesontheplasticizationsysteminsulationtechnology,multi-fieldelectrodedraftingtechnologyandairflow-assistedrefinementtechnology[13].

Insulationtechnologyofplasticizersystem

Inthemeltelectrospinningprocess,inordertoensurethatthepolymerfluidisinacontrolledflowstate,theaccuracyandstabilityoftheheatingsystemarerequiredtobeveryhigh.Theexistingmainheatingmethodsincludeelectricheating,hotairheating,thermalcirculatingfluidheating,andlaserheating.Electricheatinghasthecharacteristicsofprecisetemperaturecontrolandsimpletemperaturecontrolequipment[14].However,thetraditionalmeltelectrostaticspinninguseshighvoltagestaticelectricityonthespinningnozzle.Theproblemsofelectrostaticinterferenceandelectrostaticbreakdownseriouslyaffectthenormaloperationoftheelectricheatingsystem.Thisphenomenonisalsoaproblemthathaslongpuzzledresearchersinthefieldofmeltelectrospinning,resultinginslowprogressinresearchonmeltelectrospinning.Althoughhotair,thermalcirculatingfluid,andlaserheatingmethodscanavoidelectrostaticinterferenceandelectrostaticbreakdown,italsobringsproblemssuchasinaccuratetemperaturecontrolandcomplicatedtemperaturecontrolsystem[15].

Ourteambrokethroughthetraditionalmeltelectrostaticspinninghighvoltageelectrostaticloadingmode,innovativelyloadedhighvoltageelectrostaticonthefiberreceivingend,andsolvedtheproblemofinterferenceofhighvoltageelectrostaticontheelectricheatingsystem.Theaccuratecontrolofthemeltingtemperaturereducesthecomplexityoftheheatingsystemandlaysthefoundationfortheindustrial

preparationofmeltelectrostaticspinning.ThisbreakthroughwashighlyevaluatedbyJapanesespinningexpertTakasakiGreen,andcommented:"Yang'sresearchteamusedanextrusionplasticizingshuntmechanism.Thenozzlewasgrounded,andthecollectiondeviceappliedhighvoltageelectricitytosolvethedangerofhighvoltagestaticelectricity[16].".

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Fig.4Schematicdiagramofrefinementassistedbyairsuction

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Multistageelectricfielddraftingtechnology

Intheprocessofelectrostaticspinning,therefiningeffectofthehigh-voltageelectrostaticfieldonthefiberismainlyreflectedintwoaspects:First,thenetchargeinducedbythehigh-voltagepotentialissubjectedtotheelectricfieldforcedirectedtothereceiverintheelectricfield,andthenthefiberisstretchedandrefined.Thisiscalledthetug-of-wareffect.Thesecondisthecoulombrepulsionbetweenthenetchargesonthefibersurfacetorefinethefiber,whichisthefloggingeffect[17].Polymermeltshaveweakconductivityandlessnetchargeonthefibersurface.Sothetug-of-wareffectisthemainrefinement.Inordertoachievesufficientdrafting,itisnecessarytoincreasetheelectricfieldstrengthandincreasethespinningdistance.Butexcessiveelectricfieldstrengthwillcauseproblemssuchaselectrostaticbreakdown.

Ourteamproposedamulti-stageelectricfielddraftingtechnique.Thearrangementofthemulti-stageelectrodeplatesandtheelectricfielddistributioninthespinningspaceareshowninFig.4.Theprincipleofthemulti-stageelectricfielddraftingtechnologyis:betweentheair-assistedmeltelectrospinningnozzleandthereceivingdevice,acenterholeelectrodeplate(Fig.4)isinnovativelyadded[18],sothatthecenterholeisafirst-levelelectricfieldwhichisformedbetweentheelectrodeplateandthedifferentialnozzle.Asecond-levelelectricfieldisformedbetweenthereceivingplateandthereceivingelectrodeplate.Themultiplejetspassthroughthefirst-levelelectricfieldandthesecond-levelelectricfieldinsequenceundertheconstraintoftheairflow.refinementofdrafting(Fig.5).Inaddition,accordingtotheactualsituation,apluralityofcenterperforatedelectrodeplatescanbeflexiblyarrangedbetweenthedifferentialnozzleandthereceivingdevice.Thecentralperforatedelectrodenotonlyrealizesthejetbutalsobreaksthroughtherestrictionthatthereceivingdevicemustapplyhighvoltageelectricfieldintraditionalmeltelectrospinning,whichisbeneficialtothediversificationofthereceivingdeviceandlaysthefoundationforthepreparationoffibermembraneswithdifferentmorphologies[19].

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Fig.5Schematicdiagramsofmultistepelectricfield

3.3Airassistedrefiningtechnology

Airflow-assistedrefinementoffibertechnologyiswidelyusedinvariousfiberproductionprocesses,suchasmeltblown,meltspinning,andspunbonding.Researchonmeltelectrospinninghasfoundthatthefinenessoffibersrefinedwithhotairflowisnearly20timessmallerthanthatoffibersnotrefinedwithairflow.Although

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air-assistinghasasignificantrefinementeffectonasinglejet,inamulti-jet,thehigh-speedairstreamhasdifferenteffectsoneachjet,resultinginawiderangeoffiberdiameterdistributionandfibertangles.Ourteamusestheadvantagesofauxiliaryairflowtorefinethefiber,andcombinesthestructuralcharacteristicsofthemeltdifferentialnozzletousethenegative pressureairflowauxiliarythinningtechnology.

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Fig.6Schematicdiagramsofrefinementassistedbyairsuction

TheschematicdiagramofthedeviceisshowninFig.6.Theprincipleofthenegative-pressureairflow-assistedrefinementtechnologyis:thenegative-pressureejectorisinstalledatthehollowelectrodeplate,andthemeltjetisfirstdrawnfromthehigh-voltagepotentialdifferencebetweenthehollowelectrodeplateandthenozzle,andthengeneratedbythenegative-pressureejector.Undertheactionofthesuctionwindflowfield,itgraduallyrefinesandentersthecenterflowchannelofthenegativepressureejector.Insidethenegativepressureejector,theflowvelocitygraduallyincreaseswiththechangeofthecrosssectionoftheflowchannelandthehigh-pressureairflow.Thecrosssectionoftheflowchannelattheinjectorthroatisminimized.Thefibersaredrawnandrefinedquicklyduetotheultra-high-speedairflowgeneratedbygasexpansion,therebyachievingthepreparationofnanofibers.Negativepressureairflowisagentlegradualprocessoffiberrefinement,anditwillnotaffectthemorphologyoftheTaylorconeandtheinitialstageofthejet.Therefore,thepreparedfiberhasasmalldeviationindiameterandgoodfibermorphology.

4.Conclusionsandprospects

Ourteamaimsattheefficientandgreenpreparationofnanofibers,breaksthroughthetraditionalnozzledesignideas.Wecreativelyproposeanewprincipleofdifferentialelectrostaticspinning.Throughmechanismdisclosureandprocesscontrol,thenumberofjetsperunitareareaches80timesthenumberofcapillaryjets.Thefiberproductionefficiencyreaches500to1000timesoftheelectrospinningofcapillarysolution.Throughkeytechnologiessuchasairdrafting,ithassolvedtheproblemssuchaslargemeltviscosityandweakelectricalconductivity,andachievesnanofibersinthe500nmrange.Byusingmodulardesignandmicro-channeloptimization,theworld'sfirstmelt-differentialelectrostaticspinningnanofiberindustrializedproductionlinehasbeenestablished,realizingthegreenindustrializedproductionofnanofibersandlayingasolidfoundationforthewideapplicationofnanofibers.Intermsofapplication,ourteamhasexploredthemechanismofnanofibersaccordingtothespecificconditionsindifferentfields.Wecancontrolnanofiberproductswithspecificdiameters,shapesandstructures.Amongthem,nanofibermembranemasksandairfiltershaveobtainedpreliminarycommercialgeneralization.

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