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.

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

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.

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

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

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

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

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

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

Test Method B Fixed Alignment Type II (Mechanical)

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

Test Method D Self-Aligning Type IV (Pneumatic)

Test Method E Self-Aligning Type V (Hydraulic)

Test Method F Self-Aligning Type VI (Hydraulic)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Types of Breaks 1 2 3 4 5

What is Acceptable?

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

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

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

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

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.