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Coating Adhesion Testing


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Pull-Off Adhesion Testing

Presented by: David Beamish, DeFelsko Corporation

This one-hour webinar describes pull-off adhesion testing, one of the most popular methods of determining the bond strength of coatings applied to rigid substrates.

2Pull-Off Adhesion Testing

Presented by: David Beamish, DeFelsko Corporation

This one-hour webinar describes pull-off adhesion testing, one of the most popular methods of determining the bond strength of coatings applied to rigid substrates.

3Learning OutcomesAt the end of this webinar you will be able to:Define pull-off strengthExplain the difference between Adhesion and CohesionDescribe several types of pull testersUnderstand requirements of ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers

Conduct a pull testUnderstand differences when testing on concreteAnalyze and report test resultsList common pitfalls44IntroductionThis webinar will discuss the method for evaluating the pull-off strength (commonly referred to as adhesion) of a coating system applied to a metal substrate.

55AdhesionA complex phenomenon related to physical and chemical forces The ability of a coating to adhere to the substrate or to a previous coatingThere are many ways to evaluate degree of bonding. But there is no single test to accurately and quantitatively determine adhesionMany factors should be taken into account.Conservative evaluations are recommended66What is Being Measured?Coating strength at different planesAdhesion Strength - Bond strength between the substrate and the coating and/or the coating layer to one anotherCohesion Strength - Inner-strength of a coating layerAdhesion strength is highly variableNo industry wide adhesion acceptance valueMinimum adhesion values may be on a PDS or in a specification77The Pull-Off TestA loading fixture (dolly) is glued to the coating, then pulled by a portable tester.Determines:greatest tensile force that a surface area can bear before material is detached, or whether the surface remains intact at a prescribed force (pass/fail).Scratch or knife adhesion results may not be comparable

88Pull-Off Strength Test MethodFor metal substrates use:ASTM D 4541, Standard Test Methodfor Pull-Off Strength of CoatingsUsing Portable Adhesion TestersFor concrete substrates use:ASTM D 7234, Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers

99Portable Pull-off Adhesion TestersMechanical (twist by hand)Hydraulic (oil pressure)Pneumatic (air pressure)Widely used by coatings manufacturers, specifiers, inspectors, coating specialists.1010Testing EquipmentThe following 5 devices are described:Method A: DiscontinuedMethod B: Fixed Alignment Type II (Mechanical)Method C: Self-Aligning Type III (Hydraulic)Method D: Self-Aligning Type IV (Pneumatic)Method E: Self-Aligning Type V (Hydraulic)Method F: Self-Aligning Type VI (Hydraulic)

Results obtained by each test method may give different results11Test Method BFixed Alignment Type II (Mechanical)

12Test Method CSelf-Aligning Type III (Hydraulic)Load is applied through the center of the loading dolly by a hydraulic piston and pin

1313Test Method DSelf-Aligning Type IV (Pneumatic)

14Test Method ESelf-Aligning Type V (Hydraulic)

15Test Method FSelf-Aligning Type VI (Hydraulic)

16ASTM D 4541 OverviewSelect and clean the test sitePrepare the test dolliesApply adhesive to dolly and/or surface and allow to cureScore around the dolly throughto the substrate (optional)Connect an adhesion tester and pull to failure or to a specified max forceRecord the final pulling force and qualify the natureof the failure

1717ApparatusAdhesion Tester, commercially available, examples are listed in Annex A1 - A518a.Loading Fixturesb.Detaching Assemblyc.Base (if needed by the adhesion tester)d.Means of pulling the loading fixtures verticallye.Timer to maintain 150 psi/s in 100sf.Force indicator

ApparatusSolventSandpaperAdhesiveClampsCotton SwabsCircular Hole Saw (optional)191 Select and Clean Test SiteA flat surface large enough to accommodate the specified number of replicate test dolliesFor statistical purposes 3 test sites within a specified area are typically requiredA rigid surface that can support the counter forceSteel substrates less than 3.2 mm () thickness usually reduce pull-off strength2020The surface should be lightly abraded to promote adhesion of the adhesiveLightly roughen with fine sandpaper (400 grit or finer) or abrasive pad, especially smooth or glossy surfacesCare must be taken to prevent damage or significant loss of coating thicknessPerform a SSPC-SP 1, Solvent CleaningRemove residual dust with a solvent that does not degrade the coating1 Select and Clean Test Site

2121Loading Fixture DesignSelf-AligningType VI

Fixed Alignment Type IISelf-AligningType IIISelf-AligningType VSelf-AligningType IVKnown as: -Loading Fixtures, or -Dollies, or -Studs, or -Pull Stubs222 - Prepare the Loading FixturesFollow the manufacturers instructionsFailures at the dolly-adhesive interface may be avoided by treating the dolly surface in accordance with ASTM Guide D2651, Standard Guide for Preparation of Metal Surface for Adhesive Bonding

23233 - Apply AdhesiveIdentify a suitable glue:Cyanoacrylates (super glues) cure quickly and are commonly used on some coatings with low bond strengths2-part epoxies when stronger bonds are required

243 - Apply AdhesivePrepare and apply the adhesive according to the manufacturers instructionsNote: Adding about 1% of #5 glass beads to the adhesive helps with test dolly alignmentApply adhesive over the entire surface area of the dollyPlace dolly onto the surfaceRemove excess adhesivefrom around the dolly witha Q-Tip without movingthe dolly

253 - Apply AdhesiveWhile curing, a constant pressure should be maintained on the dolly using:Magnetic or mechanical clamping systemsMasking tape

264 - Score around the dollyScoring is a method of cutting around the perimeter of the dolly down to the substrateShould only be done for: coatings greater than 500 m (20 mils) thickreinforced coatingselastomeric coatingsAvoid twisting or torquing the test areaA template made out of plywood with a hole is an effective method to avoid cutting tool movement

27275 Pull the DollyAttach the apparatus and set the force indicator to zeroIncrease the load to the dolly in a smooth, continuous mannerNot faster than 1 MPa/s (150 psi/s)Not longer than 100 seconds

28286 Record the ResultsGeneral test descriptionTemperature and HumidityEquipment selectedScoring if performedDate, time, operatorRecord the maximum pull forceAt failure, orThe maximum force applied

296 Record the ResultsAdhesion Break: A break between coating layers or between the substrate and first coating layerCohesion Break: A break within a single coating layerGlue Break: Coating adhesion and/or cohesion strength exceeds bonding strength of the adhesiveIf multiple locations of break occur, estimate the % of eache.g., 75% cohesion within primer; 25% adhesion between primer and intermediate coats

Report the Type and Location of the Break30

Types of Breaks

1234531What is Acceptable?

32ASTM D 7234Concrete Test DifferencesLarger 2 (50 mm) diameter loading fixturesScoring is generally performedSlower loading rateFailure is typically within the concrete itselfSSPC SP13/NACE No. 6, Appendix A1.6 describes procedures for testing concrete coating adhesion

33CautionsUse D 4541 charts to select a test deviceVerify test device has current calibrationThe substrate must be rigidAdhesive should not damage the coatingPrepare the contact surfacesMix and apply the adhesive carefullyPerform testing in triplicate34SummaryPull-off adhesion testing is an excellent method for verification that the coating has created a mechanical bond with the substrate

3535Acknowledgements-SSPC Market Place-ASTM Standards-Bill Corbett, KTA, Coating Adhesion Testing Methods and Equipment Webinar-Elcometer, SEMicro, DeFelsko and DFT Instruments - Equipment Suppliers 36Pull-Off Adhesion Testing

Presented by: David Beamish, DeFelsko CorporationThe End

This one-hour webinar describes pull-off adhesion testing, one of the most popular methods of determining the bond strength of coatings applied to rigid substrates.



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