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Bill CorbettKTA-Tator, Inc.
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Coating Adhesion Testing Methods and EquipmentWebinar Content:
Why Measure Adhesion Properties?What is Being Measured?Selecting an Adhesion Testing MethodMeasuring Adhesion by the Tape TestMeasuring Adhesion by the Knife TestMeasuring Pull-off Adhesion
Fixed Alignment – Mechanical Self-Aligning – Hydraulic (3 devices) Self Aligning – Pneumatic
Special Requirements for Testing Concrete Coating Adhesion
Common Pitfalls When Testing Coating AdhesionReporting
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Learning OutcomesCompletion of this webinar will enable the
participant to:
Explain the purpose of testing the adhesion properties of coatings
Describe the two types of adhesion testing mechanisms
List four standard test methods used to test the adhesion of coating
systems
Describe the difference between shear adhesion and tensile
adhesion
Measure the adhesion of coating systems using the tape method
Measure the adhesion of coating systems using the knife method
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Learning Outcomes, continuedDescribe the difference between fixed alignment and self aligning adhesion
testers
Explain the differences between testing the pull-off strength of coatings
applied to steel and concrete surfaces
Measure the tensile (pull-off) strength of coating systems using mechanical,
hydraulic and pneumatically-operated testing devices
List common pitfalls associated with adhesion testing of coatings
Evaluate potential locations of break, including adhesion, cohesion and glue
Describe the reporting requirements for adhesion testing
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Why Measure Adhesion Properties?Determine
specification complianceCommon in-process
inspection of TSCDetermine
maintenance strategy for existing coating systems
Adhesion data is not typically a predictor of coating performance
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What is Being Measured?Measuring Coating Strength at Different
PlanesAdhesion Strength
Bond strength between the substrate and the coating and/or the coating layers to one another
Cohesion Strength Inner-strength of a coating layer
Adhesion strength is highly variableNo industry-wide adhesion acceptance valueMinimum adhesion values may be on a PDS or
in a specification
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Selecting a Test Method
ASTM Standard
Method
D 3359-09 Adhesion By Tape Test
D 6677-07 Knife Adhesion
D 4541-09 Pull-off Strength of Coatings Using Portable Adhesion Testers
D 7234-05 Pull-off Strength of Coatings on Concrete Using Portable Pull-off Adhesion Testers
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Selecting a Test MethodTape and Knife Adhesion require minimal
equipment and can be conducted quicklyEvaluate “shear” or “peel” strength (peel-back)
Pull-off adhesion requires specialized equipment and the testing takes comparatively longer to performEvaluates pull-off strength (resistance to
perpendicular pull)Cannot compare data, since testing mechanisms
are differentAdhesion testing is destructive to the coating film
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Measuring Adhesion By Tape Test
EquipmentSteel ruler or guideUtility knifeBrushAdhesive tapeMagnifierASTM Test MethodCross-cut Tester
(alternative to knife and guide)
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Measuring Adhesion By Tape TestStep 1: Select test area(s) and conduct triplicate
tests per areaStep 2: Determine the thickness of the coating
systemStep 3: Select the test method based on the
coating thicknessMethod A (X-cut): Coating thickness > 5 milsMethod B (Cross-cut): Coating thickness < 5 mils
Up to 2 mils: Use 1mm spacing between incisions (11 cuts)
From 2-5 mils: Use 2mm spacing between incisions (6 cuts)
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Measuring Adhesion By Tape TestMethod A (X-cut)Make a single 1.5”
cut through the coating system. Avoid multiple passes.
Make a second cut to form an “X”
30-45° angle at intersection of the “X”
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Measuring Adhesion By Tape TestMethod A (X-cut)Remove cutting debrisRemove 2 laps of tape
(discard)Apply 3” piece of tape
to X-cutCover all of X-cutAssure good tape
contactRemove tape within
90 +/- 30 secondsRemove tape 180°
back across X-cut (do not pull upwards)
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Measuring Adhesion By Tape TestMethod A (X-cut)
Rating Description
5A No peeling or removal
4A Trace peeling or removal along the incisions
3A Jagged removal along the incisions up to 1/16” on either side
2A Jagged removal along the incisions up to 1/8” on either side
1A Removal of most of the coating from the area of the “X” under the tape
0A Removal of coating beyond the area of the “X”
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Measuring Adhesion By Tape TestMethod B (Cross-cut)Make series of 6 or
11 parallel incisions through the coating system. Avoid multiple passes.
Make a second series of incisions perpendicular to first series
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Measuring Adhesion By Tape TestMethod B (Cross-cut)Remove cutting debrisRemove 2 laps of tape
(discard)Apply 3” piece of tape
to gridCover all of gridAssure good tape
contactRemove tape within
90 +/- 30 secondsRemove tape 180°
back across cross-cut (do not pull upwards)
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Measuring Adhesion By Tape TestMethod B (Cross-cut)Ratin
g% Description
5B 0% Edges of cuts completely smooth
4B <5% Small flakes of coating detached at intersections
3B 6-15% Small flakes of coating detached along edges & at intersections
2B 16-35%
Coating flaked along edges and parts of squares
1B 36-65%
Coating flaked along edges in large ribbons and whole squares detached
0B >65% Flaking & detaching worse than 1B
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Measuring Adhesion By Knife TestEquipment
Steel ruler or guideUtility knifeASTM Test Method
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Measuring Adhesion By Knife TestSelect test areaMake a single 1.5”
cut through the coating system. Avoid multiple passes.
Make a second cut to form an “X”
30-45° angle at intersection of the “X”
Attempt to lift coating with tip of knife blade
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Measuring Adhesion By Knife Test
Rating
Description
10 Coating is extremely difficult to remove. Fragments no larger than 1/32” x 1/32” removed with great difficulty
8 Coating is difficult to remove. Coating chips from 1/16” x 1/16” to 1/8” x 1/8” removed with difficulty
6 Coating is somewhat difficult to remove. Coating chips from 1/8” x 1/8” to 1/4” x 1/4” removed with slight difficulty
4 Coating is somewhat difficult to remove. Coating chips in excess of 1/4” x 1/4” removed using light pressure with knife blade
2 Coating is easy to remove. Once started with the knife blade, the coating can be grasped with fingers and easily peeled to a length of at least 1/4”
0 Coating is easy to remove. Once started with the knife blade, the coating can be grasped with fingers and easily peeled to a length greater than 1/4”
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Pitfalls to AvoidASTM D3359 is for coatings on metallic surfacesDull knife blades can produce erroneous resultsMaintain proper spacing between incisions when
employing the cross-cut tape adhesion methodWhen employing the X-cut methods (tape or
knife adhesion procedures), maintain proper angle at intersection of “X.” The legs of the “X” should be ~ 1” apart (width of adhesive tape)
Report tape used. Store tape properlyTape may not be effective in cold temperatures
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What Tape Should be Used?Note 5 in the current standard lists
Permacel 99 as suitablePermacel 99 discontinued in 2009No immediate replacementIntertape Polymer Group LA26 will
be investigated (backing, adhesive, thickness & adhesion to steel similar)
Section 5.3 indicates 1” (25 mm) width and adhesion strength as agreed is acceptable
Exercise caution when comparing data using different tapes/batches of tape
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ReportingSubstrate type and coating system; method of
cureEnvironmental conditions (temperature and RH)No. of tests performedManufacturer and type of tape employed (when
used)Mean and range of ratingsLocation of Break (between coats, within a coat,
between substrate surface and primer, etc.)
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Measuring Pull-off AdhesionASTM D4541, “Pull-off Strength of Coatings
Using Portable Adhesion Testers”Annex A1: Fixed Alignment Type II (mechanical)Annex A2: Self Aligning Type III (hydraulic)Annex A3: Self Aligning Type IV (pneumatic)Annex A4: Self Aligning Type V( hydraulic)Annex A5: Self Aligning Type VI (hydraulic)
Note: Different testers may yield different pull-off values
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Preparing the Test SurfacesRemove dirt,
chalking, oil or other interfering materials
Lightly roughen smooth or glossy surfaces using fine grit paper or abrasive pad
Remove any residual dust using a compatible solvent
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Loading Fixture Design
Fixed Alignment Type II
Self-AligningType III
Self-AligningType VI
Self-AligningType VSelf-Aligning
Type IV
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Preparing the Loading FixturesLoading fixtures for
Type II and Type IV testers are available pre-abraded
Loading fixtures for other testers may need to be abraded to enhance bond to surface (recommended)
Clean fixtures before use
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Selecting an AdhesiveHigh solids epoxy
adhesivesCyanoacrylate
adhesiveEnsure compatibility
with coatingUV curable adhesive
available with Type IV tester (requires special loading fixtures with transparent base)
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Attaching the Loading Fixtures
Apply adhesive to the loading fixture, surface, or both
Addition of No. 5 glass beads (1%) assists with fixture alignment
Attach the loading fixture to the coated surfaces
Push downward; do not twist
Remove excess adhesive
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Attaching the Loading Fixtures(Type III Tester)Insert Teflon® plugApply adhesive to the
loading fixture, surface, or both
Attach the loading fixture to the coated surfaces
Push downward; do not twist
Remove Teflon® plugRemove excess adhesive
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Attaching the Loading FixturesSecure loading
fixture until adhesive cures
Use magnetic or mechanical fixture holder or tape
If using tape, ensure it doesn’t “relax” during adhesive cure
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Scoring Around Loading FixtureLight scoring can be
performed to remove excess adhesive
Scoring of the coating is not recommended
If scoring is performed, must be done carefully
Scoring (if done) must be reported with test results
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Measuring Pull-off Adhesion Using the Type II Tester
Model No. Range
106/1 0-500 psi
106/2 (shown)
0-1000 psi
106/3 0-2000 psi (uses wrench)
106/4 0-3200 psi (uses wrench)
106/4A Use w/ 2” diameter pull stubs(uses wrench)
106/5 0-30 psi
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Measuring Pull-off Adhesion Using the Type II (fixed alignment) Tester
1. Lower the jaw by turning the hand wheel (nut) counterclockwise
2. Slide the jaw opening over the head of the loading fixture
3. Verify levelness4. Set the drag pin to
zero (push downward)
1
43
2
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Measuring Pull-off Adhesion Using the Type II (fixed alignment) Tester5. Rotate hand wheel
(nut) clockwise to increase load (<150 psi/second)
6. Break (or maximum force) should occur in 100 seconds or less
7. Record the value corresponding to the bottom of the drag pin, E.g. 6 is 600 psi
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Measuring Pull-off Adhesion using the Type III (hydraulic) TesterInsert pin through center
hole of stainless steel loading fixture (may need to loosen black threaded hand lever)
Pull upwards on the quick connect and allow the locking ring to engage the outside flange of the loading fixture.
Pull upwards to verify locking ring is engaged
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Measuring Pull-off Adhesion using the Type III (hydraulic) TesterReturn red “sweep
needle” to zero (or zero set the digital display)
Rotate the black threaded handle clockwise until break occurs or maximum pressure is achieved
Read pressure from analog or digital display
Loading fixtures (stainless steel) can be cleaned with heat clamp
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Measuring Pull-off Adhesion using the Type IV (pneumatic) TesterDevice uses air
pressure inside a piston to provide a constant, perpendicular pull on a threaded loading fixture
Piston pressure is converted to psi (or Kpa) using conversion charts based on piston size
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Measuring Pull-off Adhesion using the Type IV (pneumatic) Tester
Piston Range
F-1 0-500 psi
F-2 0-1,000 psi
F-4 0-2,000 psi
F-8 0-4,000 psi
F-16 0-8,000 psi
F-8/12 0-4,000/0-10,000 psi
• Round base with pigtail hose• Silicone rubber gasket (lifted upward)• Round threaded top plate
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Measuring Pull-off Adhesion using the Type IV (pneumatic) TesterConnect piston hose
to the main air hose (brass coupling)
Push air hose connection into the base unit
Insert CO2 cartridge into sleeve
Thread the CO2 sleeve onto the base unit
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Measuring Pull-off Adhesion using the Type IV (pneumatic) Tester
Place piston over the loading fixture
Thread top plate onto the fixture until contact is made with the piston, then reverse the top plate ¼-1/2 turn to allow silicone gasket to raise
Close the Rate Valve (don’t over-tighten)
Rate Valve
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Measuring Pull-off Adhesion using the Type IV (pneumatic) TesterTurn the Power switch
to the “on” positionPush the peak reset
button Push in and hold the
“Run” button, while slowly opening the Rate Valve until pressure increases at ~2 psi/second
Do not exceed 6 psi/ second
On/off PeakReset
Run
Rate valve
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Measuring Pull-off Adhesion using the Type IV (pneumatic) TesterRead the Piston Pressure
from the digital displayConvert the Piston
Pressure to psi (or Kpa) using the conversion chart corresponding to the piston used and diameter of the loading fixture
Example: 46 psi piston pressure = 937.3 psi adhesion strength (F4 piston and 0.5” diameter loading fixture)
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Measuring Pull-off Adhesion using the Type V (hydraulic) TesterType V uses
hydraulic pressure to apply perpendicular force to the loading fixture
Pull-off strength is read directly from digital display
Manual version isshown
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Measuring Pull-off Adhesion using the Type V (hydraulic) TesterOpen the pressure
relief valve and push actuator handle into assembly
Power-up tester (green)
Verify LCD reads zero
Select pull-stub size (*)
Select units (**)* **
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Measuring Pull-off Adhesion using the Type V (hydraulic) TesterPosition actuator over
loading fixture. Attach quick connect coupling to head of fixture
Close the pressure relief valve
Pump lever until digital display indicates pressure is applied
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Measuring Pull-off Adhesion using the Type V (hydraulic) TesterContinue pumping
uniformly (<150 psi/ second)
Monitor pull rate using LCD display (psi/s)
Display holds maximum pressure of pull-off
Record value, or press memory pad (*) to store value
Depress green button to zero display *
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Measuring Pull-off Adhesion using the Type V (hydraulic) TesterAutomatic version
applies hydraulic pressure to actuator without pumping of a lever
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Measuring Pull-off Adhesion using the Type VI (hydraulic) Tester
Type VI tester uses hydraulic pressure to apply perpendicular force to the loading fixture
Pull-off strength is read directly from the gage dial (up to ~ 2,470 psi)
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Measuring Pull-off Adhesion using the Type VI (hydraulic) Tester
Type VI tester consists of a turning crank/pressure gage & testing head
Verify crank and tightening wheel are in start position
Press 4 pistons against even surface to return oil to the pump
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Measuring Pull-off Adhesion using the Type VI (hydraulic) TesterLift outer ring of the
quick release coupling; mount the testing head to the loading fixture
Release the quick connect and listen for “click,” indicating it is locked into the groove
Return the red “sweep needle” to zero
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Measuring Pull-off Adhesion using the Type VI (hydraulic) TesterTurn the tightening
wheel at base of pump until piston legs contact surface and gage dial initiates movement
Turn hand crank smoothly & evenly until a break occurs or the maximum pressure is reached
Read pressure from gage dial (psi x 100 on red dial)
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Special Requirements for Pull-off Adhesion of Coatings on ConcreteStandard Test Method ASTM D7234-05 SSPC SP13/NACE No. 6, Appendix A1.6
describes procedures for testing concrete coating adhesion
Testing procedures are similar; minor differences in equipment and test procedures
This section is not comprehensiveSSPC CCI (Concrete Coating Inspector)
Training Program contains more detailFour considerations described
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Special Requirements for Pull-off Adhesion of Coatings on Concrete1. Scoring the coating
Thick film coatings (>20 mils) have lateral bond strength
May produce misleading pull-off values (false, high)Scoring through coating (down to the concrete
surface) is typically performedScoring is done prior to attachment of loading
fixture (except if loading fixture is square)Scoring coatings <20 mils may also be requiredWhen attaching loading fixture, verify adhesive
does not flow into groove
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Special Requirements for Pull-off Adhesion of Coatings on Concrete2. Loading Fixture Size
Unlike steel, concrete is non-uniform and contains cement and aggregate
Round loading fixture diameter should be 2” (50 mm) to ensure test is performed over a representative area
Square loading fixtures are used with some testers
3. Loading Rate The pressure loading rate should be 30 psi/second;
the test should be completed in 5-30 seconds
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Special Requirements for Pull-off Adhesion of Coatings on Concrete4. Compressive Vs. Tensile Strength
Concrete has high compressive strength (i.e., 4000 psi), but low tensile strength (i.e., 400 psi)
Low pull-off values common Plane of fracture commonly within the
concrete When fracture occurs in the concrete, report
the minimum coating tensile adhesion properties; actual coating tensile strength remains unknown
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Recording the Type and Location of BreakAdhesion 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 adhesive
If multiple locations of break occur, estimate the percentage of each (e.g., 75% cohesion within primer; 25% adhesion between primer and intermediate coats)
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Illustration of Various Illustration of Various Locations of BreakLocations of Break
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Recording the Type and Location of BreakDisregard any test where glue break
represents more than 50%If glue break occurs and the pull-off value is
greater than the acceptance value, re-attachment is not necessary (report “pass with pull-off strength > X psi”)
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Pitfalls to AvoidDetermine which tester to use. Different testers
generate different pull-off values on the same coatingVerify testing equipment has current calibrationPerform testing in triplicateThe rigidity of the substrate can affect the testingClean the surface and the loading fixtures to reduce the
possibility of glue failures. Use care when abrading the surface to prevent introduction of surface flaws
Verify adhesive is compatible with the coatingAllow the adhesive to cure. Conducting pull tests before
adhesive is cured often results in glue breaksVerify perpendicular pullSupport pull-off device on vertical surfaces
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ReportingGeneral Nature of Test
Substrate type and coating system (thickness, type, surface orientation)
Field or laboratory testingAir temperature and relative humidityTesting apparatus employed; loading fixture type/dimensionsAdhesive type employedNo. of tests performedLoad applied (pull-off values, in psi, KPa or MPa)Mean and range of pull-off valuesLocation of break (between coats, within a coat, between
substrate surface and primer, etc.)Note scoring, if employed (and any other deviations from
procedure)
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THE ENDTHE END