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Tackifiers are high molecular weightpolymers dissolved in oil that contributetack to formulated lubricating oils. Oneproblem that has emerged in theindustry is the inability to measure thedegree of tackiness in polymer-oilsolutions. Several methods have beenput forth to quantify tack including stringlength and the rotating disk method. The

degree of tackiness is related to theamount of internal energy or cohesiveenergy of the fluid. A simple, inexpensivemethod was developed to quantify thetackiness of an oil solution by measuringthe force required to pull a known massfrom the solution. The force wascorrelated with other fluid propertiesincluding viscosity, contact angle, and

capillary height. A linear relationship hasbeen shown between string length andpull-off force and between viscosity andpull-off force.

KEYWORDS: Tackifier, Friction, Adhesion, Probe Tack

Quantitative evaluation oftackiness in polymer-oilsolutions using modified probe tack method Daniel M Vargo, Oriol Ribera Serra and Brian M LipowskiFunctional Products Inc.

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INTRODUCTIONTackifiers are important in the lubricationof many processes. They may be used toprovide adherence in way oils and chainlubricants, stringiness to greases, andanti-mist properties to metalworkingfluids (1). Tackifiers are typically polymericadditives that impart tack or stringinessto a lubricant. Tack is considered acomposite property; the ability of amaterial to function as a tackifier isdetermined by its cohesive and adhesiveforces, viscosity and other factors such asthe molecular weight and concentrationof the polymeric additives used in theformulation of such additives. Tackifiershave high cohesive and adhesive forces.High cohesive forces allow the tackifierto remain together as a single mass whilehigh adhesive forces cause the tackifierto remain on the surfaces to belubricated (2).

Due to the many factors that influencetack, it has been difficult to quantitativelydetermine how tacky a particular solutionis (3). Several methods have beendeveloped which are able to measuresome, but not all, of the relevant drivingforces of tackiness.

Current Test MethodsTest methods for measuring tackiness aregenerally most suited to the adhesivesmarket including pressure sensitiveadhesive tapes and adhesive coatings.Several organisations provide teststandards to the adhesives marketincluding the American Society of Testingand Materials (ASTM), the PressureSensitive Tape Council (PSTC), theEuropean Association of the Self-Adhesive Labelling Industry (FINAT), theBritish Standards Institution (BSI) and theTag and Label Manufactures Institute(TLMI). The test methods currently usedfor the pressure sensitive tapes marketinclude probe tack (ASTM D2979), looptack (BS EN 1719, TLMI LIB 1/2), rollingball tack (ASTM D3121, BS EN 1721) aswell as tests for double-sided tapes (BS 7116) (4).

These methods use a technique wherethe adhesive is coated onto a solidsupport or applied on a tape and thenplaced in contact with a second surface.The force required to separate thesurfaces is then measured as an

indication of adhesiveness or tackiness.These tests are useful for comparingadhesives to one another but are notsuitable for use in the lubricant tackifierindustry. This is due to the fact that thesetests measure pressure sensitive tackinessrather than adhesiveness as it relates tocling or adherence of an oil solution to ametal part. Cohesion is also an importantproperty that is generally not assessedusing test methods desi`gned for theadhesives industry. Cohesion provides thestring-forming ability of a tackifiersolution due to the interaction of theindividual polymer molecules of thesolution.

Further test methods have beendeveloped specifically for use in thelubricant tackifier industry. One suchmethod is the Brookfield spindle method.This method determines the amount ofoil left on the surface of a Brookfieldspindle. The spindle is submerged in thetackifier in oil solution then spun for 10minutes at a high rpm and then theweight of the spindle and adheredtackifier solution is recorded. The amountof tackifier left on the spindle is anindicator of its ability to adhere to ametal surface. Depending upon theconcentration, base oil properties andthe temperature the effectiveness of atackifier can be assessed relative to eachother (5). Another test is the open (orductless) siphon test method. In this test,a capillary tube attached to a vacuumpump is used to withdraw a dilutetackifier solution from a graduatedcylinder. The tackiness is quantified bythe maximum length of the polymerstring measured before the string breaks(1). Neither of these tests has beenstandardised by ASTM or any testingbody.

In this study, the pressure sensitive tackof adhesives using an inverted probemachine (ASTM D2979) method used inthe adhesives industry is modified tomake this test more suitable for thelubricant industry. In the standard testthe force required to remove theadhesive from a solid surface shortly afterit has been in contact after a shortperiod of time is measured using aninverted probe machine. The adhesive isremoved from the solid surface at aconstant rate and the maximum forcerequired to break the adhesive bond is

measured. A further modification of thetest procedure is required because ofdifferences in the rheological propertiesand the expected pull-off force (3). Thestandard ASTM test is also simplified inorder to eliminate the need for expensivetest equipment.

A test similar to the ASTM probe tacktest has been used in greases todetermine the pull-off force (6). Theexpected pull-off force in a grease ismuch higher than for a lubricant tackifiersolution.

Adhesion and CohesionIn many industrial applications thelubricating oil must not drip or form amist when bearings or machine surfacesare in motion; the addition of a tackifierwill decrease the tendency of a lubricantto do so. Oil mists have been associatedwith various health issues in plantworkers so the impetus is to lower themisting of oils in the workplace (7). Toalleviate oil mists, a tackifier can beadded to the oil. Cohesion is determinedby the attractive forces between themolecules of a substance that tends tohold the substance together. Materialswith high cohesive energies are able toresist separation of the oil into separatesmall droplets thus the mist does notform (7). Adhesion is determined by theattractive forces between dissimilarmolecules and causes one material tostay in place on another.

Adding a tackifier to a lubricant packagewill tend to increase the cohesivenessand adhesiveness of the lubricantwithout substantially increasing itsviscosity. The cohesive forces within atackifier result in the string formingability that is a key component oftackiness. Cohesion also drives the elasticnature of these materials. Adhesion isalso increased when using a tackifier.Higher adhesiveness is required to makea lubricant stick to bearing surfaces athigh speeds than at low speeds. At lowspeeds, greater cohesiveness is requiredto keep the lubricant from beingsqueezed out from between the bearingsurfaces (8).

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Contact AngleCohesion and adhesion are important tothe performance characteristics oftackifiers in the lubricant industry. Oneway to measure the relative strength ofthese two forces is by determining thecontact angle.

Cohesive forces between molecules causethe surface of a liquid to contract to thesmallest possible surface area. This generaleffect is called surface tension. Moleculeson the surface are pulled inward bycohesive forces, reducing the surface area.Molecules inside the liquid are surroundedby other liquid molecules on all sides andtherefore experience zero net force (9).

Interfacial tension is proportional to thestrength of the cohesive force, whichvaries with the type of liquid and thesurface that it is in contact with. Interfacialtension, γ, is defined to be the force, F,per unit length, L, exerted by a stretchedliquid membrane, as shown in Equation 1.

(1)

The contact angle, θ, of a droplet isdefined as the angle within the dropletbetween a tangent line drawn on thedroplet surface at the solid-liquidinterface and the solid surface, as shownin Figure 1. A θ of less than 90° indicateswetting behaviour while a θ of greaterthan 90° indicates non-wettingbehaviour.

The relative strengths of the cohesive andadhesive forces of the droplet determinethe shape of the droplet. A material thatis more cohesive than adhesive will showmore non-wetting behaviour, i.e. thecontact angle will be larger. The forcesbetween the molecules of the drop arestronger than the forces between themolecule and the surface which results indroplet molecules that are more stablewhen interacting with other dropletmolecules rather than the surfacemolecules (10).

Capillary Action and Surface TensionThe adhesiveness of a tackifier is relatedto the surface tension which can bedetermined from the contact angle andcapillary height. Liquids in contact withconfined spaces such as small pores willfill these spaces without an externalforce, even against the force of gravity.The cohesive forces between themolecules of the fluid and the adhesiveforces between the fluid molecules andsurface molecules create the drivingpressure that will force the fluid into thecapillary space (11).

The Lucas-Washburn equation describesthe rate of fluid flow through acylindrical capillary of radius r as afunction of the driving pressure. Makingthe assumptions that flow is laminarviscous and incompressible and that thecapillary is much longer than it is wide,Washburn applies Poiseuille’s Law for thepressure drop in a fluid flowing througha cylinder to derive Equation 2.

(2)

where η is viscosity and ΣP is the sum ofatmospheric pressure (zero if the ends ofthe capillary are open), hydrostaticpressure, and capillary pressure. ϵ is thecoefficient of slip, taken to be zero for afully wettable surface (11). Capillarypressure is given by Equation 3.

(3)

where γ is interfacial energy and θ is thesolid-liquid contact angle.

If a capillary tube is placed vertically intoa liquid capillary action will raise orsuppress the liquid inside the tubedepending on the materials at theinterface. The effect depends on therelative strength of the cohesive andadhesive forces and, thus, the contactangle. If θ is less than 90°, then the fluidwill be raised; if θ is greater than 90°, itwill be suppressed.

In the cases of horizontal and verticalcapillaries, where hydrostatic andatmospheric pressure are negligible, the

surface tension σ for non-steady stateconditions is given by Equation 4.

(4)

where h is the height in the capillary, µ isthe dynamic viscosity, r is the radius ofthe capillary tube, θ is the contact angle,and t is the time it takes the solution torise in the capillary (12).

However, when evaluating polymersolutions some assumptions need to bemade in regard to time. For long liquidrise times in a capillary tube the Lucas-Washburn equation is not the bestmethod to determine the surface tensionof a fluid as the equation predicts acontinuous rise in height. In reality, theliquid height will eventually stop rising asan equilibrium is reached between thecapillary force and the force of gravity (12).

Zhmud (13) derives an equation andsolutions for different time intervalsspecifically over long time intervals. Thisequation was modified by the Lambertfunction to describe the behavior usingan inverse exponential function. Themodified equation results in anequilibrium height that a liquid will reachin a capillary tube under the force ofgravity when solutions are considered atinfinite time. The surface tension of theliquid can be calculated from steady statecapillary height using Equation 5.

(5)

where ρ is the polymer solution densityand ϕ is the inclination of the capillarytube from the horizontal plane.

EXPERIMENTAL METHODSTackifier solutions were prepared with atotal polymer concentration of 3%(w/w). Solutions of an olefin copolymer(OCP), polybutadiene (PBR), naturalrubber (NR) and polyisobutylenes (PIB)were used as shown in Table 1. Solutionswith more than one component arelisted with the major component first.The PIBs used have viscosity averagemolecular weights ranging from 1000 to4000 kDa.

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Figure 1: Contact angle of a liquid droplet on asolid surface.

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Solutions were prepared by dissolvingpolymer in the diluent at 95°C with lowshear mixing to avoid shear degradationof the polymer.

Modified Probe Tack TestA small dish having a radius of 7.3cmwas used to perform this test. The centreof the dish had three raised ridges ofabout 1mm height. A 50 gram hookedweight was placed on three small ledgesat the bottom of the dish in order tominimise the capillary force generatedwhen liquids are placed in confinedspaces. The polymer test solution wasadded to the dish so that the weight wassubmerged to a depth of 3mm. A hand-held spring scale was attached to thehook. A steady upward force was appliedto the weight, normal to the surface ofthe liquid, over a 3 second period. Theweight registering on the scale wasrecorded by a camera. The mass of theweight was subtracted from the weightregistering on the scale. This yields thepull-off force required to remove themass from the polymer solution. Thisoperation was repeated 10 times foreach polymer solution. The highest massrecorded on the scale is recorded justbefore the weight is lifted from thesolution. From the maximum mass thepull-off force, F, can be calculated usingEquation 8.

(8)

where m is the maximum mass recordedon the spring scale and a is acceleration.In this case acceleration is taken to beonly the acceleration due to gravity asthe weight is stationary until enoughforce is applied to remove it from thetackifier solution.

Table 1: Properties of the 3% (w/w) polymersolutions prepared for use in this study.

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Capillary TestA borosilicate glass capillary tube havinga radius of 0.35 mm was used for themeasurement of capillary height of thepolymer solutions. The tubes werelowered vertically into a polymer solutionto a depth of one mm and the distancethat the solution travelled up the tubewas measured from the surface of thetest solution. A time of 600 seconds wasallowed to reach steady state.

Contact angleContact angles of three of the polymersolutions on borosilicate glass weremeasured using a contact-anglegoniometer.

RESULTSModified Probe Tack TestBased on the results of this study, it canbe shown that the pull-off force followsthe same trend as the string lengths asmeasured by the ductless siphonmethod. Both tests provide a measure ofthe cohesive energy of the tackifiersolution. Materials with high cohesiveenergies are able to resist separation ofthe material into separate droplets whichwould result in their removal from thesurfaces to be lubricated.

In the modified probe tack test, thecohesive energy of the tackifier resiststhe separation of the layers of fluidbetween the bottom of the weight andthe dish. The higher the cohesive forcebetween the layers of fluid within thetackifier solution, the more force must beapplied to separate them, resulting in ahigher pull-off force. Another point ofseparation that could occur during pull-off would be the breaking of theadhesive forces between the tackifiersolution and the weight or the dish. Thisis not observed to be the case; there is afilm of tackifier that covers the bottom ofthe weight after it has been removed.Further, strings of tackifier solution formas the weight is lifted from the dish.

As shown in previous studies, an increasein molecular weight of the polymerimproves its performance as a tackifier.This has been confirmed in this studywhere an increase in molecular weightcorresponds to a higher pull-off force. Thisalso correlates to an increase in tackifier

performance as measured by the ductlesssiphon test, as shown in Figure 2.

The string formed during the ductlesssiphon test is held together via thecohesive forces within the string. As aresult of increased cohesive force alonger string can be formed as thematerial is able to hold itself together toan increased height. It has also been determined that the pull-off force follows the same trend as theviscosity, as shown in Figure 3. Viscosityis another parameter that determineshow well a tackifier will perform.

Viscosity is determined by several factors.In polymer solutions, the large polymermaterials must untangle and move paston anther in order for the fluid to flow.

Another important factor is the cohesiveforce between the molecules themselves.For one molecular layer to flow pastanother, the cohesive force betweenthose molecules must be overcome. As aresult, the layer in motion will experiencea drag force from the next layer whichwill resist flow. Increasing the cohesiveforces will result in higher drag forces,i.e. more resistance to flow and higherviscosity.

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Figure 2: Pull-off force from the modified probe tack test and string length from the ductless siphon testcorrelate. An increase in polymer molecular weight for PIB samples (E-I) also correlates to improved tackifierperformance. Values for pull-off force were averaged and the standard deviation is shown.

Figure 3: Pull-off force from the modified probe tack test and viscosity show a correlation as both propertiesare dependent on the cohesive forces within the tackifier solution.

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Capillary TestBased on the data obtained from thecapillary test.

Table 2, it can be shown that thecalculated surface tension values showan inverse correlation to both the stringlength and the pull-off force of thepolymer solutions. The capillary height isdependent on the adhesive strength ofthe material. As a material is better ableto adhere to the surface of the capillary,the higher it will be able to rise in thetube against the force of gravity.

Tackifiers require some adhesive strength,as well as high cohesive strength, inorder to stay in place during lubrication.The capillary test is a measure ofadhesive strength rather than cohesivestrength as the modified probe tack testand ductless siphon tests are. As theadhesive strength of the materialincreases, the surface tension increases,but the cohesive force decreases. There isa trade-off between increasing cohesiveforces and decreasing adhesive forces asshown by the inverse correlationbetween the pull-off force and thesurface tension calculated from the dataobtained using the capillary test.

The capillary test shows that as thesurface tension increases, i.e. theadhesive forces also increase, thesolutions are becoming less effectivetackifiers. In order for a tackifier to beeffective it must stay in place on the partsurface and it must also have highcohesive energy in order for the tackifierto not be removed easily. A balance ofadhesiveness and cohesiveness isrequired for the best performance of atackifier.

CONCLUSIONSA correlation exists between the pull-offforce and string length as measured bythe ductless siphon test of a tackifiersolution. There is also a correlationbetween the pull-off force and thesolution viscosity. Both the string lengthand viscosity of a tackifier solution aredependent on the cohesiveness of thematerial. Cohesiveness is partlyresponsible for the property of alubricant known as tack. Anotherproperty that determines how well atackifier performs is the adhesiveness ofthe solution. The capillary height of atackifier solution is related to itsadhesiveness. An inverse relationshipbetween the adhesiveness as determinedby the capillary test and the cohesivenessas determined by the probe tack test of atackifier solution has been demonstrated.

Tack is a composite property andtherefore must be measured indirectly.Multiple tests are necessary tounderstand how well a tackifier willperform as a lubricant additive. The pull-off test and the ductless siphon testquantify only a portion of tackifierbehaviour, the cohesiveness. The additionof the capillary test allows anunderstanding of another importantproperty of a tackifier, the adhesiveness.

The pull-off and capillary test used in thisstudy are relatively quick and simple toperform and require minimal equipment.Potential tackifiers can be quantitativelyevaluated and judgments can be madeabout their performance. Based on theresults of this study, a potential tackifiershould have a high pull-off force and alow capillary height. Combined withprevious tests such as the ductless siphon

method and knowledge of the polymermolecular weight, a tackifier solution canbe developed and evaluated more readily.

A similar method to determine pull offforce in grease using expensive equipmentwas previously developed. Further workusing the simplified probe tack methoddeveloped in this paper will be performedto determine its suitability for the charac-terisation of tackiness in grease.

Daniel M Vargo, Oriol Ribera Serraand Brian M Lipowski

Functional Products Inc.8282 Bavaria Road EastMacedonia, Ohio 44056.330-963-3060

References1. Levin, Victor J., Stepan, Robert J and Leonov, Arkady L.

Evaluating Tackiness of Polymer-Containing Lubricantsby Open Siphon Method: Experiments, Theory, andObservations. [book auth.] Leslie R. Rudnick. LubricantAdditives Chemistry and Applications 2nd Edition.Boca Raton : CRC Press, 2009, pp. 357-375.

2. Vargo, D. M. Biobased Polymer Additives forEnvironmentally Friendly Lubricants - Tackifiers. Societyof Tribologists and Lubrication Engineers. Cleveland :s.n., 2013.

3. The effect of the nonlinear viscoelastic properties ontack. Verdier, C and Piau,

J.-M. Grenoble : s.n., 2003, Journal of Polymer SciencePart B-Polymer Physics, Vol. 41, pp. 3139-3149.

4. Roberts, R A. Review of Methods for the Measurementof Tack. Failure Criteria and their application to Visco-elastic/Visco-plastic materials. 5, 1997.

5. Addapt Chemicals BV. Addapt Chemicals BV website.[Online] 2013. http://www.addapt-chem.com/assets/

tackifiers.pdf.6. Lugt, Piet M. Grease Lubrication in Rolling Bearings.

Chichester : John Wiley & Sons, Ltd, 2013. 978-1-118-35391-2.

7. Howell, J.K., Lucke, W.E. and White, E.M. Health andSafety Aspects in the use of metalworking fluids.[book auth.] J.P. Byers. Metalworking Fluids, SecondEdition. Boca Raton : CRC Press, 2006, pp. 337-376.

8. www.tpub.com Integrated Publishing. www.tpub.comIntegrated Publishing website. [Online] 2013.http://www.tpub.com/engine3/en32-38.htm.

9. Surface Tension, Viscosity, and Poiseuille's Law. [bookauth.] J.D. Wilson, Anthony J Buffa and Bo Lou.College Physics, Fifth Edition. Upper Saddle River :Prentice Hall, 2003, pp. 331-335.

10. Adhesion. Nelson, G L. Conshohocken : ASTMInternational, 1995, MNL17-14TH-EB, Vol. 44.

11. Washburn, E.W. The Dynamics of Capillary Flow.Physical Review. 1921, Vol. 17, p. 273.

12. Fries, N and Dreyer, M. An Analytic solution ofcapillary rise restrained by gravity. Journal of Colloidand Interface Science. 2008, 320, pp. 259-263.

13. B.V., Zhmud, Tiberg, F and Hallstensson, J. 2, 2000, J. Colloid Interface Sci., Vol. 228, pp. 263-269.

LINKSdvargo@functionalproducts.comwww.functionalproducts.com

Table 2: Experimental data obtained from the capillary test including steady state capillary height and thecalculated surface tension values.

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