Pull-Off Adhesion Testing

Presented by: David Beamish, DeFelsko

Corporation

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

2

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.

3

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

4

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

5

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

6

Pull-Off Strength Test Method

• For metal substrates use:• ASTM D 4541, Standard Test Method

for Pull-Off Strength of CoatingsUsing 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

7

Portable Pull-off Adhesion Testers

– Mechanical (twist by hand)– Hydraulic (oil pressure)– Pneumatic (air pressure)

• Widely used by coatings manufacturers, specifiers, inspectors, coating specialists.

8

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

9

Test Method B• Fixed Alignment Type II (Mechanical)

10

Test Method C

• Self-Aligning Type III (Hydraulic)

• Load is applied through the center of the loading dolly by a hydraulic piston and pin

11

Test Method D• Self-Aligning Type IV (Pneumatic)

12

Test Method E• Self-Aligning Type V (Hydraulic)

13

Test Method F• Self-Aligning Type VI (Hydraulic)

14

ASTM D 4541 Overview1. Select and clean the test site

2. Prepare the test dollies

3. Apply adhesive to dolly and/or surface and allow to cure

4. Score around the dolly throughto the substrate (optional)

5. Connect an adhesion tester and pull to failure or to a specified max force

6. Record the final pulling force and qualify the natureof the failure 15

Apparatus1. Adhesion Tester, commercially

available, examples are listed in Annex A1 - A5

16

a. 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

Apparatus2. Solvent3. Sandpaper4. Adhesive5. Clamps6. Cotton Swabs7. Circular Hole Saw (optional)

17

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

18

• 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

1 – Select and Clean Test Site

19

Loading Fixture Design

Self-AligningType VI

Fixed Alignment

Type II Self-AligningType III

Self-AligningType V

Self-AligningType IV

Known as: -Loading Fixtures, or

-Dollies, or -Studs, or -Pull Stubs 20

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

21

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

22

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 adhesivefrom around the dolly witha Q-Tip without movingthe dolly

23

3 - Apply Adhesive• While curing, a constant pressure should be

maintained on the dolly using:• Magnetic or mechanical clamping systems• Masking tape

24

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

25

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

26

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

27

6 – Record the Results

• 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

• Report the Type and Location of the Break

28

Types of Breaks

1 2 3 4 529

What is Acceptable?

30

ASTM D 7234Concrete 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

31

Cautions1. Use D 4541 charts to select a test

device2. Verify test device has current

calibration3. The substrate must be rigid4. Adhesive should not damage the

coating5. Prepare the contact surfaces6. Mix and apply the adhesive carefully7. Perform testing in triplicate

32

Summary• Pull-off adhesion testing is an

excellent method for verification that the coating has created a mechanical bond with the substrate

33

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 34

Pull-Off Adhesion Testing

Presented by: David Beamish, DeFelsko Corporation

The End