1. Pull-Off Adhesion Testing Presented by: David Beamish,
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.

2. Learning Outcomes At the end of this webinar you will be able to:
Define pull-off strength
Explain the difference between Adhesion and Cohesion
Describe several types of pull testers
Understand requirements of ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
Conduct a pull test
Understand differences when testing on concrete
Analyze and report test results
List common pitfalls

3. Introduction
This 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.

4. Adhesion A complex phenomenon related to physical and chemical forces
The ability of a coating to adhere to the substrate or to a previous coating
There are many ways to evaluate degree of bonding. But there is no single test to accurately and quantitatively determine adhesion
Many factors should be taken into account. Conservative evaluations are recommended

5. What is Being Measured? Coating strength at different planes
Adhesion Strength - Bond strength between the substrate and the coating and/or the coating layer to one another
Cohesion Strength - Inner-strength of a coating layer
Adhesion strength is highly variable
No industry wide adhesion acceptance value
Minimum adhesion values may be on a PDS or in a specification

6. The Pull-Off Test
A 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

7. Pull-Off Strength Test Method
For metal substrates use:
ASTM D 4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
For concrete substrates use:
ASTM D 7234, Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers

8. Portable Pull-off Adhesion Testers
Mechanical (twist by hand)
Hydraulic (oil pressure)
Pneumatic (air pressure)
Widely used by coatings manufacturers, specifiers, inspectors, coating specialists.

9. Testing Equipment The following 5 devices are described:
Method A: Discontinued
Method 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 results

10. Test Method B
Fixed Alignment Type II (Mechanical)

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

12. Test Method D
Self-Aligning Type IV (Pneumatic)

13. Test Method E
Self-Aligning Type V (Hydraulic)

14. Test Method F
Self-Aligning Type VI (Hydraulic)

15. ASTM D 4541 Overview Select and clean the test site
Prepare the test dollies
Apply adhesive to dolly and/or surface and allow to cure
Score around the dolly through to the substrate (optional)
Connect an adhesion tester and pull to failure or to a specified max force
Record the final pulling force and qualify the nature of the failure

16. Apparatus
Adhesion Tester, commercially available, examples are listed in Annex A1 - A5
a. Loading Fixtures
b. Detaching Assembly
c. Base (if needed by the adhesion tester)
d. Means of pulling the loading fixtures vertically
e. Timer to maintain 150 psi/s in 100s
f. Force indicator

17. Apparatus Solvent Sandpaper Adhesive Clamps Cotton Swabs
Circular Hole Saw (optional)

18. 1 – Select and Clean Test Site
A flat surface large enough to accommodate the specified number of replicate test dollies
For statistical purposes 3 test sites within a specified area are typically required
A rigid surface that can support the counter force
Steel substrates less than 3.2 mm (⅛”) thickness usually reduce pull-off strength

19. 1 – Select and Clean Test Site
The surface should be lightly abraded to promote adhesion of the adhesive
Lightly roughen with fine sandpaper (400 grit or finer) or abrasive pad, especially smooth or glossy surfaces
Care must be taken to prevent damage or significant loss of coating thickness
Perform a SSPC-SP 1, Solvent Cleaning
Remove residual dust with a solvent that does not degrade the coating

20. Loading Fixture Design
Fixed Alignment Type II
Self-Aligning
Type III
Self-Aligning
Type V
Self-Aligning
Type IV
Known as: -Loading Fixtures, or
-Dollies, or
-Studs, or
-Pull Stubs
Self-Aligning
Type VI

21. 2 - Prepare the Loading Fixtures
Follow the manufacturer’s instructions
Failures 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

22. 3 - Apply Adhesive Identify a suitable glue:
Cyanoacrylates (super glues) cure quickly and are commonly used on some coatings with low bond strengths
2-part epoxies when stronger bonds are required

23. 3 - Apply Adhesive
Prepare and apply the adhesive according to the manufacturer’s instructions
Note: Adding about 1% of #5 glass beads to the adhesive helps with test dolly alignment
Apply adhesive over the entire surface area of the dolly
Place dolly onto the surface
Remove excess adhesive from around the dolly with a Q-Tip without moving the dolly

24. 3 - Apply Adhesive
While curing, a constant pressure should be maintained on the dolly using:
Magnetic or mechanical clamping systems
Masking tape

25. 4 - Score around the dolly
Scoring is a method of cutting around the perimeter of the dolly down to the substrate
Should only be done for:
coatings greater than 500 µm (20 mils) thick
reinforced coatings
elastomeric coatings
Avoid twisting or torquing the test area
A template made out of plywood with a hole is an effective method to avoid cutting tool movement

26. 5 – Pull the Dolly
Attach the apparatus and set the force indicator to zero
Increase the load to the dolly in a smooth, continuous manner
Not faster than 1 MPa/s (150 psi/s)
Not longer than 100 seconds

27. 6 – Record the Results General test description
Temperature and Humidity
Equipment selected
Scoring if performed
Date, time, operator
Record the maximum pull force
At failure, or
The maximum force applied

28. 6 – Record the Results Report the Type and Location of the Break
Adhesion Break: A break between coating layers or between the substrate and first coating layer
Cohesion Break: A break within a single coating layer
Glue Break: Coating adhesion and/or cohesion strength exceeds bonding strength of the adhesive
If multiple locations of break occur, estimate the % of each
e.g., 75% cohesion within primer; 25% adhesion between primer and intermediate coats

29. Types of Breaks

30. What is Acceptable?

31. ASTM D 7234 Concrete Test Differences
Larger 2” (50 mm) diameter loading fixtures
Scoring is generally performed
Slower loading rate
Failure is typically within the concrete itself
SSPC SP13/NACE No. 6, Appendix A1.6 describes procedures for testing concrete coating adhesion

32. Cautions Use D 4541 charts to select a test device
Verify test device has current calibration
The substrate must be rigid
Adhesive should not damage the coating
Prepare the contact surfaces
Mix and apply the adhesive carefully
Perform testing in triplicate

33. Summary
Pull-off adhesion testing is an excellent method for verification that the coating has created a mechanical bond with the substrate

34. Acknowledgements -SSPC www.sspc.org  Market Place
-ASTM  Standards
-Bill Corbett, KTA, Coating Adhesion Testing Methods and Equipment Webinar
-Elcometer, SEMicro, DeFelsko and DFT Instruments - Equipment Suppliers

35. Pull-Off Adhesion Testing Presented by: David Beamish, DeFelsko Corporation
The 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.