GLUING

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Content

1. Introduction 5

2. SolventBonding 7

2.1. Principle 8

2.2. Solvents 8

2.3 Procedure 8

2.4 Designforsolventbonding 9

3. AdhesiveBonding 11

3.1. Principle 12

3.2 Adhesivetypes 13

3.3 Compatibleadhesivesandsubstrates 14

3.4 Adhesiveproperties 16

3.5 Pre-treatments 18

3.6 RecommendationsforDSMproducts 19

3.7 Designforadhesivebonding 24

3.8 Quickadhesivetest 25

3.9 Howtoselectthebestadhesive? 25

4. Double-SidedTape 27

5. Surfacewetting 29

6. Cleaning 33

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1. Introduction

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Gluingforassemblyofplasticpartsisaneffectivemethodofmakingpermanentconnections.Thismethodproducesestheticcleanlookingjointswithlowweightandsufficientlystrongconnections.Thisisaveryeffectivejoiningmethodforheatsensitiveplasticsthatwouldnormallydeformifwelded.

Inordertoachieveastrongbond,itisimportantthattheselectionoftheglue,theapplicationtechniqueandtheshapeoftheconnectionareallintegralpartsofthedesignprocess.

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2.SolventBonding

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2.1.PrincipleSolventbondingorsolventweldingisaprocessinwhichthesurfacesofthepartstobejoinedaretreatedwithasolvent.Thisswellsandsoftensthesurfaceandbyapplyingpressuretothejointandwiththeevaporationofthesolvent,thetwosurfacesbond.Adhesivesarenotused.

Specificadvantagesofsolventbondingare:

− homogeneousdistributionofmechanicalloads

− goodesthetics/nospecialrequirementstohidethebond

− economicassembly− lowweight,noheavyscrews,boltsandnuts− abletojoinheatsensitiveconstructionsor

materialswhichweldingwoulddistortordestroy

− goodsealingandinsulatingproperties.

Therearethreedifferentgluingprocesses:solventbonding,adhesionbondingandbondingwithdoublesidedtape.Thesewillbediscussedinmoredetailinthechapters2,3and4.Surfacewettingandcleaning,whichareimportantforallgluingprocesses,arediscussedinthechapters5and6.

Potentiallimitationsare:

− entrapmentofsolventinthejoint− stresscrackingorcrazing− dissimilarmaterialscanonlybejoinedif

botharesolubleinacommonsolventorinamixtureofsolvents

− differencesinthermalexpansionofcomponentsarenotcompensatedinathickadhesivelayerifdissimilarmaterialsarebonded

− reproducibility/processcontrol− curingtime− nodisassemblypossible− assemblyhazardssuchasfireortoxicity.

2.2.SolventsSuitablesolventsforbondingselectedDSMproductsaregiveninthetable1.ArniteandArnitelaregenerallybondedbyothertechniquessuchasadhesivebonding.

Differentsolventscanbemixedtoproduceamixturewithoptimalproperties.Forinstance,iftwodissimilarmaterialsaretobejoined,amixtureoftwomisciblesolventsspecifictothedifferentpolymerscanbeused.

Aslurrymadeofsolventandupto25%ofthebaseresincanbeusedtoproduceasmoothfilledjointwhenthematingpartsdonotfitperfectly.Addingbaseresinmakesthesolventeasiertouse.

ItisimportanttoconsulttheMaterialSafetyDataSheetofthesolventused,forhealthandsafetyinformationandforproperhandlingandprotectionequipment.

2.3ProcedureGoodwettingofthesurfacewiththesolventisarequisitetoachievegoodsolventbonding.Chapter5describesthekeyparametersthatareinvolvedinsurfacewetting.

Thematingsurfacesmustbecleanandfreeofgreasebeforebonding.Cleaningwithasuitablesolventmaybenecessary,seepar.6.

Partsthathaveasinglejoiningsurfacearesimplypressedagainstaspongeorfeltpadthathasbeenimpregnatedwithsolvent.Thequantityofsolventusedshouldbekepttoaminimumtoavoiddripsandcrazing.Morecomplexmultiplanejoiningsurfacesrequirecontouredsolventapplicatorsmadefromwoodorametal.Itmaybenecessarytoallowafewsecondstoensuresufficientswelling.Thepartsarethenclampedtogetherwithamoderatepressure.Thepartsareremovedfromtheclampingequipmentandmustnotbeusedforaperiodof24to48hourstoensurethatfullstrengthhasbeenattained.Heatcanbeusedtoacceleratetheoverallrateofevaporationandreducethecycletime.

Table1SuitablesolventsforsomeDSMproducts.

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2.4DesignforsolventbondingTheloadontheassemblycanbeappliedinseveralwaysasindicatedinfigure1.

Generaldesignguidelinesare:

− designforlap-shearloads− maximizethebondingsurface;for

instance,useascarfedoradovetailjoint− avoidstressconcentrationsatthick-thin

sections− ensurethatthereissufficientventing.

Scarfordovetailjointsshouldberelativelyshallow,sothatsolvententrapmentisavoided.Entrappedsolventcancausecrazingovertimeandleadtopartfailure.Thepartsshouldthereforebemoldedwithaminimumofinternalstress.

Gatesshouldbelocatedawayfromtheareastobebonded.Cautionshouldalsobeexercizedwhenworkingwith“closed”parts,toavoidgettingsolventtrappedinsidethepart.

Thedesignershouldtakeaccountofthefactthatthematerialstrengthinthebondbetweentwopartsmadeofglassfibrereinforcedmaterialswillneverexceedthematerialstrengthofthematrixmaterial,becausetheglassfibresdonotbridgethegapbetweenthetwoparts.

Figure1Theloadcanbeappliedinseveralways.

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3.Adhesive Bonding

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3.1.PrincipleThemaincriteriaforachievinggoodadhesivebondingaresurfacewettingandcuringoftheadhesive.Importantvariablesfortheapplicationofanadhesiveanddistributiononasubstratearesurfacewetting,adhesiveviscosityandchemicalresistanceofthesubstratetotheadhesive.Theprinciplesofsurfacewettingaredescribedinmoredetailinparagraph5.

Ingeneral,adhesionisbasedonvariousmechanismsasshown(seefigure2).

- Mechanicalinterlockingcancontributetothestrengthofthebondifthesubstratesurfaceisrough,thusenablingthegluetoflowintothemicroholes.

- Thesamephysicalforcesofthewettingprocessalsoplayanimportantrolefortheadhesionofthecuredgluetothesubstrate.Agoodadhesionbetweenthemoleculesofthesubstrateandthegluecontributestothebondingstrength.Whenpolairgroups(e.g.–COOH,-C=O,-NH,-NH2)arepresentatthesurfaceofasubstrateastronginteractionispossiblebetweentheglueandthesubstrate.

- Insomecasesthereisachemicalreactionbetweenthereactivegroupsoftheglueandthesubstrate.Veryhighbondingstrengthcanbeachievedwitharelativelysmallamountofthesechemicalbonds.

- Molecularinterdiffusioncanalsooccuronamolecularlevel.An“interpenetratingnetwork”.Thiscouldhappenwhenthesurfaceofaplasticdissolvesintheglue.Akindofthirdphaseisformedbetweenthesubstrateandthegluewherethepolymerchainsaremixedonamolecularlevel(interdiffusion).

Molecularinterdiffusionislimitedbycrystallites,thereforeitismoredifficulttoachievegoodadhesiononsemi-crystallinethermoplasticscomparedtoamorphousones.Adhesiononnon-polarthermoplastics,e.g.polyolefins,willimproveconsiderablywhenthesurfaceispretreatedusingcorona,UV,plasmaorflametreatments.

Poorbondingoccurswhentheadhesivelayerdoesnotstickproperlytothesubstrate.Pretreatmentmaybehelpful,e.g.cleaning,degreasingandsanding,seeparagraphs3.5and5.

Specificadvantagesofadhesivebondingare:

- applicationonvarioussubstrateslikethermoplastics,thermosets,elastomersandmetals

- homogeneousdistributionofmechanicalloads

- differencesinthermalexpansionofcomponentscanbecompensatedbyusingathickadhesivelayer

- goodesthetics/nospecialrequirementstohidethebond

- economicassembly- lowweight,noheavyscrews,boltsandnuts- abletojoinheatsensitiveconstructionsor

materials,whichweldingwoulddistortordestroy

- nothermalstressesintroduced- goodsealingandinsulatingproperties- inmanycasescheaper(nohigh

investmentcosts,noadditionalcostsofmetalparts).

Potentiallimitationsare:

- longtermbehaviormaynotbeverygood- stresscrackingorcrazingoftheplastic

mayoccur- dissimilarmaterialscanonlybejoinedif

botharecompatiblewiththeadhesive- reproducibility/processcontrol- curingtimecanbelong,dependingonthe

adhesive- nodisassemblypossible- assemblyhazardssuchasfireortoxicity- sometimesrequiresacomplexprocess

(pretreatment,specialequipment,curing)

Figure2Differentadhesionmechanisms.

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3.2AdhesivetypesAwidevarietyofadhesivesarecommerciallyavailable.

EpoxyVariousepoxyadhesivesareavailable,withdifferentcharacteristicsandproperties.Thevariouscuringmechanismsare:

- 2componenthotorcoldcuring- 1componenthotcuring- UV-curing.

Standardepoxyadhesivesareverystrongbutbrittleandshowlowpeelstrength.Toimprovetoughness,modifiedepoxyadhesiveshavebeendeveloped.Theusetemperaturevariesbetween-40°Cand80°C(-40°F-180°F)forcoldcuringsystems.Hotcuringepoxiescannormallybeusedupto150°C(300°F).Epoxieshaveagoodmoistureresistance.

Ingeneral,largedeviationsarefoundinlap-shearbondingstrength,dependingontheparticularcombinationofadhesiveandmaterial.

Withsomeplastics,pretreatmentcangiveaconsiderableimprovement.Oilsandgreasenegativelyaffecttheadhesionofepoxies.

PolyurethanePolyurethaneadhesivesarerelativelyinexpensiveandshowgoodadhesion.Varietiesexistfromelastomerictorigid.Severaltypesofcuringmechanismsareavailable:

- 1componentthermosetting- 2componentcatalyzed- reactivehotmelts.

Polyurethaneadhesivesaretoughandshowahighpeelstrength.Theycanbeusedattemperaturesbetween-80°Cand100°C(-110°F-210°F).

Adhesiononengineeringplasticsisgood.Degreasingisoftensufficienttoobtaintherequiredbondingstrength.

AcrylicAcrylicsareflexibleandtough.Fastcuringtakesplaceatroomtemperature.CareshouldbetakenwhenjoiningamorphousthermoplasticssuchasPC,asenvironmentalstresscrackingmayoccur.

Severalsystemsareavailable:

- 1componentUV-curingusedfor transparentplastics- 2componentpremix- 2componentno-mix

Usetemperatureisbetween-55°Cand120°C(-70°F-250°F).Acrylicsshowexcellentpeelstrengthandaretough.

Goodadhesionisobtainedonamorphousthermoplastics.Pretreatmentmayimprovethelapshearbondingstrengthconsiderably.

CyanoacrylateCyano-acrylicsarefastcuringsystems(underthecatalyticeffectofmoisturefromtheaironthesurface,ananionicreaction)butratherbrittle,whichresultsinlowpeelstrengthandimpactpropertiesinthejoint.Rubbermodifiedcyanoacrylicshavebeendevelopedtoimprovetoughness.Cyanoacrylateadhesivesarespeciallydevelopedforsmallsurfacesandarenotsuitedforoutdoorapplications.

Averyhighlapshearbondingstrengthcanbeobtainedwithmostengineeringthermoplastics.Unfilledpolyesters(ArnitePETandPBT)showmoderateresults.Effectiveprimersareavailabletoimprovethebondingstrengthonpolyolefins.

SiliconeSiliconeadhesivesreactunderthecatalyticeffectofwater.Humidityintheairorsomemoistureonthesurfaceofthepartsissufficient.Thereactiontimesarerelativelylong,comparedtocyano-acrylics.Siliconeadhesivesofferahighelasticity.Siliconeadhesivesshowverygoodweatheringandtemperatureresistance.

MS(Modifiedsilanepolymer)ModifiedsilaneadhesivesachievegoodadhesiontovarioussubstratesandhavegoodUVresistance.

UVCureUVcurableadhesivesuseultravioletlighttoinitiatepolymerizationandcontainnosolvents.Curingtimeisshort,typically3to10seconds.UVcurableadhesiveshaveahighbondstrengthandcaneasilybeappliedtotransparentmaterialslikepolycarbonate.(However,thisdoesnotapplywhenUV-blockershavebeenadded).

ContactglueThesearesolventbasedadhesivesthatareformulatedwithdifferentrawmaterialgroups,includingnaturalandsyntheticrubbersandsuitableresincombinations(Naphtas,ketones,estersoraromatics).Adhesivefilmswillbeformeduponevaporationofsolvents.Assembliesmaybemadebycontactbonding(adhesiveapplicationtobothsurfaces)orwetbonding(appliedtooneofthebondsurfaces).Mostofthecontactadhesivesarebasedonpolychloroprenerubber.Theydisplaygoodinitialstrengthsandachievehighstrengthsonnumeroussubstrates.Thetypicalcontactpressureduringassembyis0.5MPa.

HotmeltHotmeltadhesivesarethermoplastics,availableaspellets,orinblock,tapeorfoilshape.Theadhesiveisheatedaboveitsmeltingtemperatureandappliedtothesurfacestobebondedwithspecialequipmentlikerollers,nozzlesorcalandars.Thebondisformedafterthemeltcoolstoasolid.Theoperatingequipmenthastooperatefastforeffectivebonding.Theseadhesivesarefairlyviscous,solventfreeandhavegoodgapfillingabilities.

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3.3CompatibleadhesivesandsubstratesFigure3givesanimpressionofgoodsolutionsforadhesionbondingofplastics.Thematrixwithsubstratesandadhesivesgivesanindicationofcompatiblecombinations.Combinationsthatdeliverabondwithahighstrengthhavebeenindicatedbyablackrectangle.Goodbondsareindicatedbyablackandwhiterectangle,andmoderatebondsareindicatedbyawhiterectangle.

Figure3Compatibilityofsubstratesandgluetypes(CourtesyHenkel)

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3.4AdhesivepropertiesEachadhesivetypeischaracterizedbyasetoftypicalproperties,asdemonstratedbyfigure4.

- Theadhesiontimerequiredtoreachthedesiredbondingstrengthhighlydependsonthetypeofadhesive,andmayvarybetweenafewsecondstoupto24hours,seefigure4a.Theminimumbondingstrengthof1N/mm2shouldbereachedbythen.

- Thetimerequiredforcompletecuringuptothemaximumstrengthmaytakeconsiderablylonger,seefigure4b.

- Thedesignworkingtemperatureoftheconstructionshouldnotexceedthemaximumallowableusetemperatureoftheadhesivegiveninfigure4c.

- Figure4dgivesanimpressionoftheshearstrengthofthedifferentadhesivetypes.

- Thegapfillingcapacityinmillimetresforthedifferentadhesivesisshowninfigure4e.

- Theflexibilityoftheadhesiveisimportantforconstructionsthataresubjectedtolargestrains,seefigure4f.

- Figure4ggivesanimpressionoftheresistanceagainstenvironmentalinfluence

Figure4Adhesiveproperties(CourtesyHenkel)

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3.5Pre-treatmentsThesurfacestobeadhesionbondedmustbecleanandfreeofoils,grease,mold-releaseagentsandotherforeignmaterialsforgoodadhesionoftheadhesive,soitmaybenecessarytocleantheparts(seepar.6).

Gluingshouldbedoneinadustfreeenvironment.

Apartfromcleaning,severalotherpretreatmentsexisttoenhanceadhesionoftheadhesivetothesubstrate:

- Flamingisasimpleandwidelyusedprocess,inwhichagasflameismovedafewcentimetresabovethesurfaceofthepart.Thespeedwithwhichtheflameismovedisintheorderof0.1m/sec.Propane,butaneandnaturalgascanbeusedtofueltheflame.

- Coronatreatmentisanelectricaldischargeprocess.Ahighvoltage,highfrequencyelectrodeismovedoverthesurfaceofthepartatadistanceof1to2mm,activatingthesurfacethroughoxidation.Asasideeffect,ozoneisformed.Thisprocessisnormallyusedforpartswithflatsurfacesandisespeciallysuitableforsheetmaterial.

- Lowpressureplasmatreatmentisspecificallysuitedforcomplexparts,withsurfacesthatcannoteasilybereachedinflameorcoronatreatments.Inthisbatchwiseprocess,thepartsareexposedtogasdischargeatlowpressure.

- In-lineplasmatreatmentbyOpenair®atmospheric-pressureplasmatechnologywasdevelopedbyPlasmatreatGmbH,Steinhagen.Thisplasmaprocess,patentedasearlyas1995,is

characterized,amongstothers,bythefactthattheplasmabeam,whichinthiscaseemergesfromjets(seefigure5),iselectricallyneutral.Itsintensityissohighthattreatmentspeedsofseveral100m/mincanbeachieved.Indoingso,theplasticsurfacesheatupbylessthan20°C.Thejetscanbeusedforthemostdiverseparts(3-dimensionalworkpieceswithgroovesorundercutsetc.).Theyarealsocompatiblewithrobotsandcanbeintegratedintoexistingproductlines.TheOpenair®systembringsaboutmultipleeffects:itactivatesthesurfacebyselectiveoxidationprocessesandincreasesitsfreesurfaceenergybyasignificantfactor.Inthisway,valuesofmorethan72mN/marepossibleonmanyplastics.Atthesametimethesurfaceisstaticallydischargedandcleanedverythoroughly.Moreover,byaddingaprecursor,selectivenanocoatingsmayalsobeappliedinordertoinfluenceproductpropertiesinparticularways.Thismakesitpossibletoproduceplasticswithsurfacesparticularlyreceptivetoadhesivebonding.

Plasmatreatmentmayresultinbetteradhesion,but(atmospheric)plasmaequipmentisgenerally2to3timesmoreexpensivethancoronaequipment.

Thesurfaceisdriedbytheatmosphericplasma,sothatcompletecuringmayrequiresomemoretimeincasecyanoacrylateadhesiveisused.Fiveminuteintervalsbetweentheplasmatreatmentandthegluingoperationaregenerallysufficienttoadmitmoisturefromtheairforthepolymerizationreaction.

- Priming.- Sanding.

Figure5Plasmajets(ImagesprovidedbycourtesyofPlasmatreatGmbH)

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3.6RecommendationsforDSMproductsInthischapterrecommendationsaregivenforDSMproductsandtheircompatibilitywithsomeadhesivetypesisshownonthebasisofshearteststhatweredoneonsmallstrips.

SheartestsThesheartestswereconductedaccordingtoISO4587,onstripswithathicknessof1to3mm,awidthof25mmandanoverlapof12.5mm.19differentDSMengineeringplasticsgrades-representingmaterialclusters-weretested,incombinationwithfourdifferentadhesivesgradesfromHenkel:

- cyanoacrylate(Loctite406)- 2componentepoxy(LoctiteHysol9466)- 2componentpolyurethane (Terokal4310-700)- modifiedsilanepolymer(TerostatMS-937)

TheAkulonandStanylstripswerebroughtinthemoiststateinadvanceofthetestbyacceleratedconditioningat70°Cand62%relativehumidity.

Threedifferentpretreatmentprocedureswerefollowed:

- cleaningbywipingwithIPA(isopropylalcohol)

- cleaningbywipingwithIPA,followedbyacoronatreatmentfor30seconds

- cleaningbywipingwithIPA,rougheningwithaScotchBritegeneralpurposehandpad,type3M7447(veryfine),followedbywipingwithIPA

Thethicknessoftheadhesivelayerwasadjustedusingthinmetalwireasaspacer,asfollows:

Gapsizecyanoacrylate(Loctite406) 0.05mm2componentepoxy(LoctiteHysol9466) 0.1mm2componentpolyurethane(Terokal4310-700) 0.1mmmodifiedsilanepolymer(TerostatMS-937) 2mm

Figure6Localhighbendingstressesareatpresentneartheoverlapduringasheartest

Figure7Peaksintheshearstressandthenormalstressarepresentattheedgesoftheoverlap

Thecuringtimebeforetestingwassevendays.Allmaterialsweretestedwithatensilespeedof10mm/min,exceptthelowmodulusArnitelTPE,whichwastestedat50mm/min.

MechanicalconsiderationsaboutsheartestsTheinterpretationofsheartestresultsisnotasimplematter.Theflexiblestripsarebentduringthetest,whichresultsinadditionalbendingstresses.Localtensilestressescaneasilybetwiceashighasthenominalstress.Figure6givesanimpressionofthehighlocaltensilestressesattheedgesoftheoverlap.Thesehighstressesmaycausebreakageof

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thestripsbeforetheadhesivefails.Severalstripsdidindeedbreakduringthesheartests,beforecohesiveoradhesivefailureofthegluelayercouldtakeplace.Thesebrokenstripshavebeenindicatedbyanasteriskinthefigures8to13.

Moremore,theshearstressisnotuniformoverthetotallengthoftheoverlap,asdemonstratedbythestresscalculationinfigure6.Stressconcentrationsarepresentattheedges,indicatedbythebluelineinfigure7.Anormalstressisalsopresent,withpeaksattheedgesoftheoverlap,indicatedbytheredlineinfigure7.

Figures6and7clearlyshowthatsheartestsonlyprovidearoughrankingforadhesiveperformance,andshouldnotbeusedforprecisecalculations.

Akulon(PA6andPA66)Cyanoacrylate,epoxy,polyurethane,siliconeandhotmeltadhesivesaresuitableforbondingAkulonPA6andPA66.

SheartestswereperformedonfourdifferentAkulongrades,seefigure8.Ahighshearstrengthwasfoundforcyanoacrylateadhesive,indicatedbythebluebarsinfigure8.Rougheningofthesurfacegavenosignificantimprovementforcyanoacrylateadhesive.

Theresultsfor2-componentepoxy(theyellowbarsinfigure8)andthe2-componentpolyurethane(thegreybarsinfigure5)werealsogood,especiallyafterrougheningorcoronatreatment.Thecoronatreatmentgenerallygavesomewhatbetterresultsthantherougheningtreatment.

Asexpected,thestrengthofmodifiedsilaneadhesiveisrelativelylow,asindicatedbythegreenbarsinfigure8.Thiselasticadhesivehasexcellentgap-fillingcapacities(agapsizeof2mmwaschoseninthetestforthisadhesive)andmustberegardedasanelasticsealant/adhesive.

TheshearstrengthoftheunfilledAkulonF223-Dwasconsiderablylowerthanthe

strengthofthethreeglass-filledmaterials,whichisunderstandable,asthestiffnessofthismaterialismuchlower,sothatthestripswillbebentmoreduringthetestandtheadhesivelayerwillbeloadedmoreeasilyinanunfavourablepeelmode.

Stanyl(PA46)Althoughmanydifferentgluescanbeappliedtopolyamides,onlyafewarerecommendedforuseinStanylathightemperaturesof120°C–150°C.AselectionofpossibleadhesivesforStanylislistedintable2.

ApplicablepretreatmentsforStanylare:

- Abradingthesurfacewithmediumgrit(80150)emerypaperorgritblasting(especiallyeffectiveforpolyurethanesandacrylates)

- Etchingthesurface(3minutesat20°C)withamixtureofsulphuricacid(90%),potassiumdichromate(4%)andwater(6%)

- Primingthesurfacebymeansofa

Figure8

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mixtureofresorcinol,ethanolandp-tolueensulfonacid,anitrilphenolbasedsolutionorbymeansofaresinbasedonresorcinformaldehyde

- PlasmaorUV/ozonepretreatment(especiallyeffectiveincombinationwithgluesbasedonepoxies)

TheadhesiveformstheweakestlinkinagluedStanylcomponent,duetothelowertemperatureresistanceoftheadhesive.ConsequentlyadhesivebondingisnotapreferredjoiningtechniqueforStanyl.Morestablesystemsareachievedusingweldingtechniquesormechanicalfasteners.

ThestrengthofgluedStanylpartsdependson:

- Themoisturecontentofthepolyamideparts:dryasmoldedpartsgivehigherstrengthsthanconditionedparts.

- Environmentalconditions(chemicalattack),sizeandkindofloading,sizeofthegapbetweenthematingparts.

- Theapplicationofapretreatment.

SheartestswereconductedonthreedifferentStanylgrades,seefigure9.Thecyanoacrylateadhesiveshowedahighshearstrength,indicatedbythebluebarsinfigure9.Rougheningofthesurfacedidnotimprovetheresultsforcyanoacrylateadhesive,onthecontrary,thestrengthwasevenlower.

The2-componentepoxy(theyellowbarsinfigure9)providedmoderateresultsifacleaningoperationwasonlyperformed.Rougheningalreadygavesomeimprovement,butthebestresultsarefoundforcoronatreatment.

The2-componentpolyurethane(thegreybarsinfigure9)producedmoderatetogoodresults.Rougheningorcoronatreatmentdonotresultinabigimprovement.

Table2HightemperatureresistanceadhesivesforStanyl.

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Thestrengthofmodifiedsilaneadhesiveisrelativelylow,asexpected(seethegreenbarsinfigure9).Thisadhesivehasexcellentgap-fillingcapacities(agapsizeof2mmwaschoseninthetestforthisadhesive)andmustberegardedasanelasticsealant/adhesive.

OnaveragetheunfilledStanylTW341hasalowershearstrengththanthetwoglassfilledStanyltypes,justaswasfoundforunfilledAkulon,buttheresultislesssignificant.Arnite(PBTandPET)Ethylcyanacrylate,methacrylatelastomer,ethyl,methyl,polyethane,epoxyandsiliconetypeadhesivesaresuitableforArnitePBTandPET.Hotmeltadhesivescanalsobeapplied.Theareatobejoinedshouldbelightlyroughenedandfreeofgrease.Acoronatreatmentisevenmoreeffectivethanroughening.Theadhesionstrengthobtained,however,willbebelowthespectfiedproductstrength.

SheartestsconductedonfourdifferentArnitegradesshowedahighshearstrengthforcyanoacrylateadhesive,seethebluebarsinfigure10.Thestrengthcanevenbeimprovedconsiderablybyrougheningofthesurface.

Thestrengthof2-componentepoxy(theyellowbarsinfigure10)providedlowtomoderateresultsifonlyacleaningoperationwasperformed.Rougheningalreadygaveasignificantimprovement,butthebestresultswereattainedwiththecoronatreatment.

The2-componentpolyethane(thegreybarsinfigure10)producedmoderatetogoodresults.Rougheningandespeciallycoronatreatmentbringsabigimprovement.

Thestrengthofmodifiedsilaneadhesiveisrelativelylow,asexpected(seegreenbarsinfigure10).MS-adhesiveexcellentgap-fillingcapacities(agapsizeof2mmwaschoseninthetestforthisadhesive)andcanberegardedasanelasticsealant/adhesive.

Arnitel(TPE)GoodbondingresultscanbeachievedonArnitelcomponentswithpolyurethaneadhesives.Normallytwo-componentsystemsareused,withisocyanateordi-isocyanatehardeners.Arnitelcanbelaminated(e.g.tofabric)withaTPU(thermoplasticPolyurethane)hotmeltadhesive.Thehightemperatureduringmeltingoftheadhesiveactivatesthehardenerinthehotmelt.

SheartestshavebeendoneonfourdifferentArnitegrades,asshowninfigure11.The

gradesUM551-VandVT3108areharderandstifferthanthegradesEM400andPL380,andhadahighershearstrength.Thedeflectionofthesestifferteststripsundertheloadwillbelower,sothattheadhesivelayerwillbenotbeloadedinanunfavorablepeelmode.

Agoodshearstrengthwasfoundforcyanoacrylate(thebluebarsinfigure11)afteracleaningoperationonly.Thestrengthcouldnotbeimprovedsignificantlybyrougheningofthesurface.

Theshearstrengthofthe2-componentepoxy(theyellowbarsinfigure11)isquitelowaftercleaning,butcanbeimprovedconsiderablybyarougheningoperation.Acoronatreatmentgivesevenbetterresults.

The2-componentpolyurethane(thegreybarsinfigure11)providedresultsthatarecomparablewiththoseofcyano-acrylate.Rougheningorcoronatreatmentgavenosignificantimprovement.

Thestrengthofmodifiedsilaneadhesiveisrelativelylow,asexpected,shownbythegreenbarsinfigure11.Thiskindofadhesivehasexcellentgap-fillingcapacities(agapsizeof2mmwaschoseninthetestforthisadhesive)andshouldberegardedasanelasticsealant/adhesive.

Figure9

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Figure10

Figure11

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3.7DesignforadhesivebondingTheloadontheassemblycanbeappliedinasimilarwaytosolventbonding,seefigure1.

Thinlayersareadvisableincaseoflap-shear.Peelandsplitloadsarebesttakenupbyathicklayerofadhesive.

Generaldesignguidelinesare:- designforlap-shearloads- maximizethebondingsurface;for

instance,useascarfedoradovetailjoint- avoidstressconcentrationsatthick-thin

sections- ensuresufficientventingonsubstrate.

Recommendedjointdesignsaregiveninfigurein14.HermeticsealsrequiredforcontainersandbottlesmaybeachievedwiththedesignsshowninfigureAandB.JointCismoreuniversal.

Toensuresuccessfuljoiningwithadhesivesitisimportanttoknowthefunctionalrequirementsoftheassemblyandthepossibilitiesandlimitationsoftheadhesiveincombinationwiththesubstrate.

Thefollowingchecklistmayproveuseful:

- product:designjointsspecificallyforadhesives

- mechanicalload:lap-shear,peel,splitortensile

- lifeofjoint:usetemperature,environment,relativehumidity

- thermoplasticsubstrate:mechanicalproperties,wetting,moistureabsorption

- adhesive:temperatureandchemicalresistance

- pretreatment:cleaning,etching,sanding,oxidation,primer

- safety:MSDS(MaterialSafetyDataSheet)chart.

- process:numbers,curingconditions,applicationtechniques

Themoisturecontentofpolyamidesdoesnotstronglyinfluencethebondstrength.However,itisadvisabletoconductsomebondingtestswithconditionedpartspriortoproduction.

Figure14Designsforadhesionbonding

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3.8QuickadhesivetestThecompatibilityofasubstrateandanadhesivecanquicklybetestedasfollows.

1.Dispenseadropoftheadhesiveontothesampleandcureaccordingly.Whentestingmultipleadhesives,applyeachadhesivetothesubstrateandcurethemallatonce.Labeleachadhesive.

2.Allowtheparttocoolforapproximately1-2minutes(theadhesivemayproducedifferentresultswhenwarm).

3.Conducttheadhesiontest:Picktheadhesiveoffwiththetipofadentalpickorasafetyrazor.Rateproductsfortheiradhesiononthespecificsubstrate.Evaluatingadhesionissomewhatsubjectiveandmayvaryfrompersontoperson.Havingmorethanonepersonperformthetestandusingacompositeratingminimizesthiserror.

Recordresults.Morerigorouspicktestsmayincludeexposureofcuredadhesivetohotorcoldtemperaturesortowaterorothersolventsifadhesionunderthoseconditionsisimportant.

3.9Howtoselectthebestadhesive?Step1.AdhesionThefirstandmostbasicpropertythatalladhesivesmusthaveisgoodadhesiontoallofthesubstratesthatneedtobebonded.Thesubstrateselectorguideinfigure3maybeagoodplacetostarttheselectionprocess.Whenasinglesubstratematerialistobebonded(forexamplePA6toPA6)theprocessofselectingpossibleadhesivesisstraightforward.Whentwodifferentsubstratesaretobebonded,i.e.,PA6toPBTarangeofpossibilitiesshouldbeevaluated.Afterafewpicktests(seeparagraph3.8)itwillbeobviouswhichsubstrateismoredifficulttobondandchoicescanbeoptimizedforthatsubstrate.

Step2.ViscosityIndeterminingviscosity,considerationshouldbegiventohowtheadhesive

mustflow(ornotflow)onthepartaftertheadhesiveisapplied.Partgeometry,processgeometryandassemblyspeedsandmethodsshouldallbeconsideredwhenselectingaviscosity.

Step3.PhysicalpropertiesIfanapplicationrequiresanadhesivewithaspecialsetofphysicalproperties,thisstepshouldbeconsideredbeforeviscosity.Productdatasheetsoftheadhesivesincludeamoreextensivelistofproductpropertiesi.e.,hardness,tensileproperties,shrinkageandwaterabsorption.

Figure4dshowsthatgluescanbedevidedintotwogroups.Epoxy,acrylic,cyanoacrylate,polyurethaneandcontactgluesarebonderswithhighstrengths(figure4d)andrelativelyshortcuringtimes(figures4aand4b).FlexiblePU,MS-polymerandsiliconeglueshaveahighgapfillingcapacity(figure4e)andareveryflexible(figure4f),buttheyhavearelativelylowstrength.

Thefigures4cand4ggivenanimpressionoftheusetemperaturerangeandtheenvironmentalresistance.Modifiedsilaneadhesivescanbeusedinawiderange,butepoxyandacrylicadhesivesalsohaveexcellenttemperatureresistance.Theenvironmentalresistanceofepoxiesandsiliconesisverygood.

Step5.CuringequipmentLightcuringadhesivesareformulatedtoreactoptimallywithspecifictypesoflight.Bestcureresultsareobtainedwhentheoutputofthecuringlampismatchedwiththeabsorptionspectrumoftheadhesive.

Step6.PriceThepriceperkiloofgluesvariesconsiderably,asdemonstratedbyfigure15.Thecheapestgluetypethatfulfilsallthetechnicalrequirementsshouldbeselected.

Step7.UsetestingFinaladhesiveselectionshouldincludeasoundevaluationofsuitableadhesivesbyapplicationandcureofadhesivetoactualpartssubjectedtoreal-lifestressconditions.Usetestingshouldalsoincludeprocessvalidationtests,whichincludethe

Figure15Roughpriceindexperkiloofdifferentadhesivetypes.Theadhesivesatthebottomofthepyramidarethehigh-volumetypes.

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productionofpartsusingplannedadhesiveapplicationandcuringmethods.Adhesiveusetestingshouldincludestressessomewhathigherthanthoseexpectedforassembledparts.Theseverityofthistestingisbestdeterminedbypartdesigners.

DSM’sTechnicalServiceDepartmentforengineeringplasticsmaybeconsultedforallgluingadvices.

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4.Double-Sided Tape

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Double-sidedcoatedtapesareadhesive-coatedonbothsidesofpaper,filmortissue.Thisincreasestheadhesive’sdimensionalstabilityforeasyhandlingandapplication.Double-sidedpressuresensitivetapesareavailablewithavarietyofcarriers,adhesivesandloadbearingcapabilities.

Specificadvantagesofdouble-sidedtapesare:

- homogeneousdistributionofmechanicalloads

- dampenvibrationsandnoise- absorbimpact- joindissimilarmaterials- resistplasticizermigration,avoidingstress crackingproblems- goodesthetics/nospecialrequirements tohidethebond- economicassembly/minimalapplication training/noinvestmentinmajorequipment- lowweight,noheavyboltsandnuts- differencesinthermalexpansionof componentscanbecompensatedbya thickadhesivelayer- abilitytojoinheatsensitiveconstructions ormaterials,whichweldingwoulddistort ordestroy

Potentiallimitationsare:

- stresscrackingorcrazingcausedbytheadhesive

- reproducibility/processcontrol- disassembly.

Adhesivetransfertapeconsistsofapressure-sensitiveadhesivepre-appliedtoaspecialreleaseliner.Thetapeissimplyappliedtoasurfaceandthelinerispeeledoff.Thisleavesaclean,drystripofacrylicadhesiveforjoiningoflightweightmaterials.

Pressure-sensitivetapesrequireacleansurfaceforoptimalstrength.Cleaningofthepartmaybenecessarytoremoveoil,grease,moldreleaseagentandotherforeignmaterials,seepar6.

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5. Surfacewetting

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Thewettingofthesurfaceischaracterizedbythecontactangleoftheliquidwiththesurface,asshowninfigure16.Ifthecontactangleis90°orlarger,wettingwillbeincomplete.

Thesurfacecontactangleinitsturnisdeterminedbythesurfacetensionoftheliquidandthesurfacefreeenergyoftheplastic.Goodwettinganddistributionofthesolventortheadhesiveonthesubstratecanbeachievedonlywhenthesurfacetensionoftheliquidislowerthanthesurfacefreeenergyofthesubstrate.

Thesurfacefreeenergyofthesubstratecanbemeasuredinvariousdifferentways.Oneprocedureistomeasurethecontactangleofadropofde-ionizedwaterthatisappliedtothesubstratewithahand-heldmeasuringdeviceasshowninfigure17.ThemeasuringresultscanberepresentedonaPC-screen,seefigure18.

Figure16Therelationbetweenthesurfacecontactangleandwettingofthesurface.

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Figure17Hand-heldmeasuringdevice,enablingmeasurementofthecontactangleonthesub-stratewithoutanysamplepreparation.Thesystemlooksfromthesideontheapplieddropletandmeasuresinthiswaythecontactangle,thebase,theheightandthevolumeofthedroplet,inbothstaticanddynamicmode(=measurementasafunctionoftime).(ImagesprovidedbycourtesyofRycobel-Belgium)

Figure18Screenpresentationofthecontactanglemeasurements.Thecontactangle,thedropwidthandthedropheightarepresentedasafunctionoftime.

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Figure19ShowssometypicalsurfaceenergyvaluesofDSMpolymersthatweremeasuredinthisway.Thevalueswereallhighenoughtoensuregoodwettingofthesurfacewithanadhesive.Ifmuchlowersurfaceenergiesarefound,thiscouldbeanindicationthatthesurfaceisnotclean.

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6.Cleaning

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Thesurfacesofpartsthatmustbejoinedtogetherbyagluingprocess,mustbecleanandfreeofforeignmaterials,suchasdirtparticles,oil,greaseormoldreleaseagentinordertoachieveastrongbond.

Iftheplasticpelletsdonotcontainamoldreleaseagentandifnomoldreleaseagenthasbeensprayedinthemoldduringtheinjectionmoldingprocessandthepartswerenottouchedwithbarehands,cleaninginanairbathtoremovedustparticlesmaysuffice.

Theuseofamoldreleaseagentcanbeavoidedbydesigningthepartswithgenerousreleaseangles.

Ifnecessary,persistentcontaminantscanberemovedbywashinginasuitablesolvent.Tables3to6givesomeexamplesofsolventsthatmaybeusedforDSM’sthermoplastics,assumingthatthepartswillnotbeexposedtothesecleaningsolventsformorethan10minutes.

Atmosphericplasmaenablesbothacleaningandanactivationeffectonthepolymersurfacewherethereisalowlevelofcontamination.

Somesolventsmaycauseenvironmentalstresscrackinginpolycarbonateandpolycarbonateblendpartswhicharesubjectedtointernalorexternalstresses.Chlorinatedandaromaticsolvents,aswellasketones,shouldthereforebeavoidedforthesepolymers.

ItisimportanttoconsulttheMaterialSafetyDataSheetofthesolventused,forhealthandsafetyinformationandforproperhandlingandprotectiveequipment.

Anautomatedcleaninglinemaybeusefultospeedupthecleaningprocessandimprovequalitycontrol.Anultrasonicbathorasprayinginstallationcouldbeconsidered.

Table3CleaningsolventsforAkulon(PA6&PA66).

Table4CleaningsolventforStanyl(PA46).

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Table5CleaningsolventforArnite(PBTandPET).

Table6CleaningsolventsforArnitelTPE.