PINHOLES 如何解决PU干法合成革的针孔问题.docx

PINHOLES 如何解决PU干法合成革的针孔问题.docx

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PINHOLES如何解决PU干法合成革的针孔问题

PINHOLES

GASBUBBLES

PlasticFilm

ReleasePaper

ATECHNICALDISCUSSION

for

CASTINGONRELEASEPAPERS

byGaryBlenkhorn

APINHOLEDISCUSSION

Pinholesareoneofthemostcommonproblemsencounteredinthecastingindustry.Highqualitystandards,moresophisticatedtechnologies,andincreasedcompetitionwarrantimmediateattentiontoremedyingthissituation.

Sappi,formerly,TheS.D.WarrenCompany,hasbeeninvolvedinthecastingindustrysinceitsinception.Ourworldwideexperienceandparticipationinallaspectsofcastinghaveenabledustoamassawealthofknowledgeaboutthecastingindustry.Thefollowingdiscussionisacompilationofourlearningsandisintendedtohelpthecasterunderstandwhatpinholesare,whatcausesthem,andhowtoeliminateorminimizethem.

Therearefour（4）partstothisdiscussion:

1.PhysicsofWetting

2.AvoidingtheCausesofPinholes.

3.MinimizingPinholes

4.PinholeIdentification/Recommendations

PART1:

PHYSICSOFSURFACEWETTING

Contrarytopopularopinion,pinholesarenotcausedbytheplasticformulationorthereleasepaper.Aspecificinteractionbetweentheplasticformulationandthereleasepaper,knownaswetting,canhaveamajoraffectontheeliminationorreductionofbubbles,whicharetheprecursorofpinholes.

Inordertousethiswettinginteractionasausefultooltoreducepinholes,itisimportanttounderstandthebasicphysicsbehindthephenomenon.Therearetwoforcesatworkwhichultimatelydeterminethewettabilityofanyliquidonasolid:

SURFACETENSIONandSURFACEENERGY

1.SURFACETENSIONistheinwardforceactingonthesurfaceofaliquid

duetotheattractionforcesbetweenthemoleculesbelowthesurfaceand

themoleculesatthesurface.

DROPOFLIQUIDPLASTIC

SURFACETENSIONFORCES

Thelowertheforceofattractionbetweentheliquidplastic’smolecules,theless

theliquidwillpulltogetherandthebettertheplasticwillwetareleasepaper.

2.SURFACEENERGYistheforceofattractionbetweenthemoleculesofa

solidsurfaceandthemoleculesofanymaterialincontactwithit.

FUNCTIONALSURFACE

surface

energy

forces

ReleasePaper

Inthecaseofareleasepaper,surfaceenergy,releaseandwettingareallrelated.Thelowerthesurfaceenergy,thelessattractionthesurfacehasforothermaterials,therefore,theeasierthereleaseandthepooreraliquidwillwet.Thehigherthesurfaceenergy,thegreatertheattractionforothermaterialsandthereforetheharderthereleaseandthebetteraliquidwillwet.

WETTING

WETTINGistheabilityofaliquidtospreaduniformlyonasolidsurface.

Wettingcanbethoughtofasatugofwarbetweentheforcesofthereleasepaper（surfaceenergy）,tryingtospreadthemoleculesoftheliquidplasticoveritssurface,andtheforcesofliquidplastic（surfacetension）,tryingtoholdtheplastictogetherinadrop.

liquidplasticGOODWETTING

Release

Paper

Whenthesurfacetensionforcesofaliquidplasticarelowerthanthesurfaceenergyforcesofareleasepaper,theliquidplasticwillwetorspreadwellonthereleasepaper.Thegreaterthedifferencebetweentheforces（lowersurfacetensionandhighersurfaceenergy）,thebettertheliquidplasticwillwetthereleasepaper.

liquidplasticPOORWETTING

Release

Paper

Whenthesurfacetensionforcesoftheliquidplasticaregreaterthanthesurfaceenergyforcesofthereleasepaper,theliquidplasticwillnotwetthereleasepaperwell.Thegreaterthedifferenceinforces（highersurfacetensionandlowersurfaceenergy）theworseitwillwet.Thiseffectcanreadilybeseenascrawlingorfeatheringoftheliquidplasticalongthemarginsofthereleasepaper.

Keepingthiswettingconceptinmind,letusnowlookatpinholesandhowwettingaffectsthem.

Firstofall,apinholeisavoidinthesolidplasticfilm.Thevoidiscausedbyagas,usuallyair,butinsomecasesvaporizedmoistureorsolvents.Agasbubbleintheliquidplasticisonlyapotentialpinhole.Wewillrefertogasinaliquidplasticasabubbleandgasinsolidplasticasapinhole.

Ifbubblesexistattheinterfacebetweenaliquidplasticcoatingandareleasepaper,theywillappear（incross-section）somethinglikethediagrambelow.

GASBUBBLES

PlasticFilm

ReleasePaper

Eliminatingthesebubblesfromthefilmabouttoform,mustbedonewhiletheplasticisinaliquidstate.Onceaskinhasformedoverthebacksideoftheplastic,thebubblebecomesapinholeandcannotberemoved.

Thisiswherewettingcomesin.Theonlywaytoremovethebubblesistohavethemriseupthroughtheliquidplasticandbreak,releasingthegasatthebackortopoftheliquid.Tomakethishappen,theliquidplasticmustbeabletowetunderthebubble,otherwisethebubblewillbetrappedattheinterfaceandcannotrise.

Noteherethatneithertheplasticformulationnorthereleasepapercausedthebubbles.Thisiswheretheplasticformulationandthereleasepapercanaffectthenumberofpinholesinthefinishedcastproductandwhyitisessentialto“marry”thecorrectplasticformulationtothecorrectreleasepaper.Theirsurfacetensionandsurfaceenergymustbecompatibleifpinholesaretobeavoided.

page4

PART2:

AVOIDINGTHECAUSEOFPINHOLES

Thebestapproachtomakingapinhole-freefilmistoeliminatetheintroductionofbubbles（theprecursorofpinholes）intotheliquidplastic.Themostcommoncauseofbubblesisairentrapment.Otherlesslikelycausesaremoistureandsolvents.Herearespecificrecommendationsandmeasurestotaketoavoidbubblesofallkinds.

RECOMMENDATIONS:

1.Testrawmaterials（PVC,solvents,etc.）forthepresenceofH2Oandworkwithsupplierstoeliminateit.

2.Avoidsolventmixtures,whichmightcauseafilmtoskinovertooquicklyinthefirstoven,thustrappingsolvents,whichwillvaporizeandformbubbleslater.Asolventwithahigherboilingpointwillusuallyhelp.

3.De-aerateanyformulaswhichhavehadairmechanicallyintroducedduringmixing.Thiscanbedonewithde-aeratingequipmentorinmostcases,bysimplyallowingtheformulationtosetovernightbeforeuse.Thebubbleswillrisetothesurfaceandeitherbreakorcanbedecanted.

4.Amajorsourceofairbubblesisthecoatinghead.Ifyouhaveachoiceofcoatingheads,aKnife-Over-Rollisbest.AReverseRollcoatercausesthehighestfrequencyofbubbles,followedbytheMeadowscoater.

5.WhenusingaKnife-Over-Rollcoater,alwaysrunabackdambehindthecoatingpond（seediagram）.

BACKDAM

KNIFE

KNIFE

POND

POND

GOOD

BAD

Withoutabackdam,thepondofliquidplasticisconstantlyrolling,fallingoverontotherunningreleaseweb,andtrappingairasitfalls.Withabackdam,thisrollingmotioniseliminatedandthechancesoftrappingairaregreatlydiminished.Thehigherthepondiskept,themorepressureisexertedattheinterfacewiththepaperandthelessairwillslipunderthebackdam.

6.Anothermethodtokeepbubblesfromleavingthecoatingheadwiththeplastic,istomountarodjustbehindtheknife（~1.5-2cm）andascloseaspossibletothepaperwebwithoutactuallytouchingit（seediagram）.

ROD

Theroddoesnotneedtorotate.Itspurposeistocreateturbulenceneartheplastic/paperinterface.Theturbulencewillmechanicallybreakbubblesloosefromthereleasepaperallowingthemtoriseupandbreakatthebacksideoftheliquidplastic.

7.Anothervariationonthe6threcommendationistouseasmoothingrodbetweenthecoatingheadandfirstoven.Thisalsomechanicallybreaksbubblesloose.Therodisdrivenandcanrotateineitherdirection.Speedanddirectionwillvaryformulatoformula.Experimentationwillhelpyouadjustitforspecificconditions.

PART3:

MINIMIZINGPINHOLES（ONCEBUBBLESAREPRESENT）

Thissectionrecommendsstepstotaketoimprovewettingsothattheliquidplasticwillwetunderthebubbleallowingitsnaturalbuoyancytoliftittothebacksideofthefilm.

`

1.Alowerviscosityplasticformulationwillwetbetterthanahigherviscosityformulation.

2.Somesolventswillwetsomereleasepapersbetterthanothers.Youwillneedtoexperimenttofindwhichareoptimumforthepaperbeingused.

3.Wettingagentscanbeaddedtoaplasticformulationtoimprovewetting.Theseareusuallysiliconesandworkquitewell.However,anyformulationchangesshouldbecarefullyevaluatedtodeterminethattherearenootheradversesideeffectstothefinishedproduct.

4.Ifyouhavethepossibilityofusinganon-criticalformulationonthefirstpassofareleasepaper,andcriticalformulationsonsubsequentpasses,itwillreducepinholing.Usedreleasepaperwillalwayswetbetterthanunusedorvirginreleasepaper.Thisisbecausethesurfaceenergyofareleasepaperalwaysincreaseswitheachuse.Thelargestincreasewilloccurafterthefirstpass,withsmallerincreasesaftereachsubsequentpass.

5.Pre-curing（drypassing）areleasepaperat160oCorabovewillsignificantlyincreaseitssurfaceenergy.

6.Preheating（notpre-curing）areleasepapersothatitgoesintothecoatingheadwarmwillhelp.Firstofall,warmpaperwetsbetterthancoldpaper.Secondly,theheatinthepaperistransferredtotheliquidplasticandthishelpstolowertheviscosityoftheliquidatthepaperinterface.Andfinally,anybubblespresentattheinterfacearealsowarmed,causingthemtoexpand.Thisgivesthemgreaterbuoyancy,whichcreatesagreaterliftingforcetohelpthemrisefromthesurfaceofthereleasepaper.

7.Applyingheatto