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5/19/2015 TheSurfaceTensionTestIsitWorthResurrecting?

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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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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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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).



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.

PracticingOilAnalysis(9/2002)

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Download - The Surface Tension Test - Is It Worth Resurrecting

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