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.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
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