the surface tension test - is it worth resurrecting
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The Surface Tension Test - Is It Worth ResurrectingTRANSCRIPT
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5/19/2015 TheSurfaceTensionTestIsitWorthResurrecting?
http://www.machinerylubrication.com/Read/376/surfacetensiontest 1/5
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TheSurfaceTensionTestIsitWorthResurrecting?
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Inallofmyyearsinoilanalysis,Icannotrecallasingletimewhendatafromasurfacetension(ST)orinterfacialtensiontest(IFT)appearedonaroutineanalyticalreport,asidefromtransformeroilanalysis,forwhichthetestisconsideredroutine.Thereasonthisissosurprisingisthatmanystudieshavereportedthatchangesinanoilssurfacetensiontobetheearliestsignofcontamination,sludgepotentialandoxidation.QuotingfromthebookLubricationofIndustrialandMarineMachinery(1943)byWilliamG.Forbes,Theinterfacialtensiontestisthemostvaluablesingletestthatcanbeusedtoevaluateaturbineoil.Itisgenerallyagreedthatwhentheinterfacialtensionisbetween15and20dynes/cm,depositsmayormaynotbeforminginthesystem.Safepracticewouldcallforanoilchangeinthisrange.
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5/19/2015 TheSurfaceTensionTestIsitWorthResurrecting?
http://www.machinerylubrication.com/Read/376/surfacetensiontest 2/5
Figure1.IntermolecularForcesasTheyEffectInterfacialTension
Whatissurfacetension?Considerthelayerofmoleculesonanoilssurfacethatareincontactwithair.AsseeninFigure1,themoleculesaremorestronglyattractedfrombelowbecausethemoleculesintheairareonaveragefurtherapart(theattractionisinverselyproportionaltothedistancebetweenthemolecules).Asimilarimbalanceoccursbetweentwoimmiscibleliquidssuchasoilandwaterinthiscasethephenomenoniscalledinterfacialtension.Attheairoilinterface,thisimbalancecreatesaskinlikemembraneattheoilssurfacecausingtheoiltowanttoformthesmallestpossiblesurfacearea.Duetothetendencyofthesurfacetowanttocontract,itbehavesasifitwereinastateoftension.Forthesamereason,adropofwaterfallingthroughairtendstobesphericalbecauseaspherehastheminimumsurfacetovolumeratio.
MeasurementTechniquesThemostcommonmethodofmeasuringsurface/interfacialtensioninvolvestheuseofatorsionbalanceknownastheDuNouyinstrument(tensiometer),describedinASTMD971:99aandASTMD1331.Inthetest(Figures2and3),theforcerequiredtoliftahorizontalplatinumwireringawayfromtheoilssurfaceismeasuredeitherdirectly(surfacetension)oratanoil/waterinterface(interfacialtension).Theunitofmeasureforsurfacetensionisdynespercentimeter(equaltomN/m).
Figure2.HowtheDuNouyMethodWorks
Clickheretoseefigure3.
Anothermethodtodetermineinterfacialtensionisusedtocharacterizethedegradationofelectricalinsulatingoilsduetooxidationorcontamination.ItinvolvestheuseofadropweightprocedureandisdescribedinASTMD2285:99.Withanyoftheprescribedmethods,testaccuracyisaffectedbyanumberoffactorsincludingcleanlinessofthetestapparatus,agitationofthefluid(requiredtoresuspendsolids)andtheaccidentalcontaminationofthefluidbydirtycontainersandlaboratoryglassware.Ingeneral,goodreproducibilityofthesetestscanbedifficulttoobtainunlesscareistakentoensurequalitylabtestprocedures.
InterfacialTensionofBaseOilsExceptingsomesyntheticfluids,baseoils(forexample,mineraloilsandpolyalphaolefin)arehydrophobic(waterrepelling).Theyarenonpolarunlikewaterandmostothersubstancesinthemachinesenvironment.Polardislikesnonpolar,likewaterrunningoffaducksback.Inthecaseofoilandwater,oppositesdonotattract.
Purewaterhashighsurfacetensionduetoitshighpolarity.Likewise,apuremineralbaseoilhasahighsurfacetensionduetoitshighnonpolarity.
Thiscanbeobservedbysimplyplacingadropofpurebaseoil(oreventurbineoil)onthesurfaceofaglassofwater.Theoilwillliftuponthewaterandassumealensshape,becauseitistryingtodisassociatefromthewaterbyreducingitsareaofcontact.However,anagedandcontaminatedoilwillexhibitgreatercontact
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5/19/2015 TheSurfaceTensionTestIsitWorthResurrecting?
http://www.machinerylubrication.com/Read/376/surfacetensiontest 3/5
area(knownasthespreadingcoefficient)withthewaterandbemorepronetoflattenout.Thisisduetothepresenceofsurfaceactivehydrophilic(waterloving)impuritiesintheoilthatadsorbtotheoilwaterinterface.Becauseitcantaketimeforhydrophilicimpuritiestoarriveattheinterface,thelongeroilandwaterremainincontact,thelowertheIFT.TestingofIFTusuallycallsfora30secondagingoftheinterfacebeforemeasurementbegins.
OnecouldsaythatthepurityofabaseoilcanbecharacterizedbyitsIFTbecauseIFTisproportionaltotheconcentrationandstrengthofpolar,surfaceactiveimpurities(forexample,compoundsofoxygenandsulfur,fattyacidsandalcohols).Hence,aGroupIIImineraloilwillhaveahigherIFTthanaGroupII.Likewise,aGroupIIwillbehigherthanaGroupI.IncreasingviscositycorrespondstoincreasingIFT.However,increasedtemperaturedecreasestheinterfacialtensionbetweenoilandwater.Onecouldsaythatatsomeelevatedtemperature,allsubstancesbecomemiscible.
TheInfluenceofAdditivesonIFT,OilWaterEmulsionsandFoamManyadditivesaresurfaceactivebydesignandreduceIFTbydefault.Somearemetalwetting(AW,EPandrustinhibitorsforinstance)andothersareparticleenveloping(forexample,dispersants,detergentsandmetaldeactivators).Emulsifyingagents,usedinsomehydraulicfluidsandmetalworkingfluids,areintendedtoreduceinterfacialsurfacetensiontoformoilwateremulsions.
Lubricantsformulatedwithhighdosesofsurfaceactiveadditiveswillbemorepronetodissolveandemulsifywaterduringservice.Infact,thehighertheconcentrationoftheseadditives,thehighertheconcentrationofwaterthatcanbeemulsified.Forinstance,whenasufficientlyhighconcentrationofsurfaceactiveagentsarepresentineithertheoilorthewaterphasesreducingtheIFTto2dynes/cmorless,eventheslightestagitationcandisrupttheinterfaceandleadtoemulsification.
Aswaterdropletsizedecreases,moresurfaceareaisexposedtosurfaceactiveemulsifyingagents.Thishighinterfacialsurfaceareaeffectivelyconsumesthehydrophilicemulsifyingagents.Ifthesepolaragentsareoflimitedsupply,waterdropletsaremorepronetostaylargerandpooledtogether(reducingtheeffectiveinterfacialarea),whichinturnreducesthestabilityoftheemulsion.Inastableoilwateremulsion,thewaterdropletsizeistypicallyonetosixmicrons.
Foamsareessentiallyemulsionsofairairglobulesseparatedbythinwallsofoil.Thesewallscanbeathickas1mmorasthinas0.01micron.Highlypureliquids,suchasbaseoilsandwater,donotfoam.However,theenrichmentoflubricantswithsurfaceactiveagents,includingmanyadditives,cannegativelyimpacttherateatwhichairdetrainsfromlubricantsincreasing,thetendencyforfoamtoformwhentheairreachesthesurfaces,andincreasingthestabilityoffoam.Manyotherfactorsinfluencethisaswell,includingviscosity,agitation,temperatureanddefoamantadditives.AnotherinterestinginfluenceonIFTisdissolvedair.Thehighertheconcentrationofdissolvedair(andothergases),whichinturnbeinfluencedbytemperatureandfluidpressure,thelowertheIFTandST.
ConditionMonitoringUsingIFTThehealthandconditionoflubricatingoilscan,toalargedegree,bemeasuredbyinterfacialtension.Whenoilsdegradeandbecomecontaminated,IFTgoesdown.Forinstance,alubricantwithanIFTsimilartonewoilisprobablystillserviceable.However,therootcauseofadownwardexcursionoftheIFTmaybehardtodiagnosisbecausethetestdoesnotcharacterizethecause,onlytheeffect.However,itatleastindicatesaproblemexists,promptingfurtherstudy,makingIFTapotentiallyeffectivescreeningtool.ThefollowingareoilanalysisandconditionmonitoringstrategiesfortheuseofIFTtestingwithinservicelubricants:
OxidationStabilityWhenoilsoxidizetheyformsolubleandinsolublehydrophiliccompounds.ManyofthesecompoundscanaltertheIFTofalubricantlongbeforethereisanynoticeablechangeinacidnumberorviscosity(Figure4).
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5/19/2015 TheSurfaceTensionTestIsitWorthResurrecting?
http://www.machinerylubrication.com/Read/376/surfacetensiontest 4/5
Figure4.ComparingOilOxidationSensitivityUsingInterfacialTensionandNeutralizationNumber
ThisincipientindicationofoiloxidationissimilartowhatisobservedwithRotatingPressureVesselOxidationTest(RPVOT),differentialscanningcalorimetryandcyclicvoltametry.Forinstance,whenaturbineoilssurfacetensiondropstowithintherangeof15to20dynes/cm,thereisriskofsludgeanddepositformation.EvenphotocatalyticreactionsofoilfromexposuretosunlightcanlowerIFT.
ContaminationThereareagreatmanycontaminantsthatcansharplychangeanoilsIFT.Theseincludewashdowndetergents/soaps,terraindust,processchemicals,antifreeze,degreasers,surfactants,etc.Manyofthesecontaminantsareinvisibletootheroilanalysisinstrumentssuchasparticlecountersandspectrometers.
CrossContaminationLubricantsthatbecomecrosscontaminatedwithotherincompatiblelubricantsmayshowamarkedchangeinIFT.Forinstance,thismightoccurwhengearoilsormotoroilsaremixedwithhydraulicfluidsorturbineoils.GreasethathasbeenallowedtomixwithalubricatingoilcanchangeIFTaswell.
AdditiveDepletionThedepletionofsurfaceactiveadditives(detergents,antiwearagents,rustinhibitors,etc.)bymasstransfermayactuallyraisetheIFT.Ingeneral,however,testingofusedoilsisbestsuitedforthosefluidswithlowconcentrationsofpolaradditivesandahighoriginalIFT.Examplesincludeturbineoils,hydraulicfluidsandR&Obearingoils.
LeakageAsuddenincreaseinoilleakagemaybeduetoachangeinIFT.Inmanycases,therootcausecouldbeanincreaseinoperatingtemperature.BecauseIFTdecreaseslinearlywithincreaseintemperature,hightemperaturecanleadtoleakagebyareductioninsurfacetension(sealmeniscus,capillaryflow)andaloweringofviscosity.Bothcanleadtosuddenorincreasedleakage.
FieldTestingofIFTThequantitativeanalysisofIFTinnewandusedlubricantsmaynotalwaysbearequirement.Justasuserscangainconsiderableknowledgeofoilpropertiesfromsimplefieldtestssuchascrackletest,patchtest,blottertest,etc.,therearewaystoinvestigatechangesinIFTaswell.Onemethodhasalreadybeenmentionedexaminethespreadabilityofanoildropplacedonthesurfaceofwater.Compareusedoiltonewoil.Evensimplecapillarytubesusedforobtainingbloodspecimenscanbeuseful.Alternatively,considerusingablenderbymixingoilandwatertoobservetheoilsdemulsifyingproperties.Afterall,testsfordemulsifyingproperties(ASTMD1401,D2711),foam(ASTMD892)andairrelease(ASTMD3427)areallbloodbrotherstosurfacetensionandinterfacialtensiontests.
Surfacetensiontestingdatesbacktotheearlydaysoflubrication.Today,theremaybeareneweddemandforthisimportantbutscarcelydeployedoilanalysistool.
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