Evaluation and Reporting of Pull-Off Test Results

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Pull-off adhesion testing of protective coatings applied to steel and other substrates is widely used to assess the coating process and to determine if the coating is fit for service, both in new construction and for repairs to existing structures. The result of pull of adhesion testing is often critical to the acceptance or rejection of a coating process, as the adhesion value quoted by the paint manufacturer can be adversely affected by aspects of the coating process. Low adhesion values are indicative of premature failure of the coating in service and are often the result of inadequate surface preparation of the substrate before the coating is applied.

ASTM D4541 and BS EN ISO 16276-1 describe several different test apparatus; however, the basic approach of gluing a test dolly to the coated surface and then exerting a perpendicular force to the surface in an effort to remove both the dolly and the coating from the substrate is common to all these standards.

TEST METHOD

For a valid pull test, the coating must cover at least half of the area of the dolly face. If the glue fails, and no coating is present on the dolly, or it covers less than half of the dolly face area, the pull test is invalid and should be repeated. However, if the glue fails at a value that’s above the specification, then it can be reported the adhesion exceeded the specification for this individual test, and is a valid result.

When the coating has failed within the layer, leaving the same coating on both the dolly and the test panel, it is known as a cohesive failure.

Adhesive failures occur when either the coating has failed at the interference with another coating, (leaving coating on the dolly and another coating on the substrate), or when the coating has failed at the substrate, (leaving coating on the dolly, and a bare substrate).

It is possible for both adhesive and cohesive failures to occur during one test.
Many National and International Standards require the user to record both the pull-off force and the nature of the failure as an estimated percentage to the nearest 10%.

Codes are used to denote the layer or layers in which the failure occurred.

For example, if the pull off force was 20 MPa, and upon reviewing the dolly and surface there is a 50% cohesive failure of the bottom coat, and a 50% adhesive failure between the first and second coats, (usually identified by the colours on the dolly and the coating system) then the result is recorded as 20MPa, 50% B, 50% B/C.

Once the dolly and substrate have been assessed, the dolly diameter, the pull rate, whether you did or did not cut around the dolly, the pull off force, and the percentage cover of the failure should all be recorded.

Several aspects of the test method were assessed in an attempt to determine the significance of the aspect to the test results. These aspects included the mix of the epoxy glue, different types of glue, surface preparation, the design of the dolly, temperature of the cure and the test, and the cutting of the coating or not. The difference between manual and automatic pull-off tester operation was also investigated. You can see test results for test questions for changes.


Table1. Test results at different curing temperatures


Table2. Test results for different ratios of two component epoxy adhesive

This paper evaluates the effects of any deviation from the required method in several aspects of the test. Each aspect listed below is examined in turn, the results tabulated and the potential effect on a valid test discussed.

Adhesive Mixing
Comparison Of Two Adhesive Types
Surface Preparation
Variation In Loading Fixture (Dolly) Design
Temperature Of Adhesive Curing And Pull Testing
To Cut Or Not To Cut
Manual Vs. Automatic Adhesion Testers

Pull-Off Adhesion Test

Equipment:

  • Elcometer 106 Adhesion Tester

-Test dollies
-Operating instructions

  • Defelsko PosiTest AT with test dollies

-Test dollies
-Operating instructions

  • Light grit sand paper
  • Adhesive
  • Photos of dollies pulled from coated surface (4) numbered 1 through 4
  • Test panel

Assignment

Perform adhesion test using the instruments provided. Perform these test on the sample panels provided and document observations in the chart below. Describe location of failure using the key below:

A=Substrate
B=First Coat
C=Second Coat
D=Third Coat
Y=Adhesive
Z=Dolly

Apply test dollies (1 for Elcometer 106 and 1 for Defelsko AT) to the test panel using supplied adhesive; allow to cure before performing test. Use proper procedure to apply test dollies.

Pull-Off Adhesion Test Results

Test Location Test Type Value 
(psi)		 Adhesion defect % Cohesion defect % Glue defect % Defect Location
Test Panel #1 Elcometer 106 200 %10 D-C
%90 Y
Test Panel #2 Defelsko PosiTest AT 700 %20 C
%80 A-B

Table3. Test results with different pull-off devices

REFERANCES:
1.            Elcometer, Pull-off Adhesion Testing Of Coatings – Improve Your Technique, date of access: 6 January 2017, http://www.elcometer.com/images/stories/PDFs/Pull-Off_Adhesion_Testing_of_Coatings_Improve_Your_Technique.pdf
2.            Elcometer, Assessing the Result; Glue, Adhesive and Cohesive Failures, date of access: 6 January 2017, http://www.elcometer.com/en/assessing-the-result-glue-adhesive-and-cohesive-failures.html
3.            Defelsko, Dolly Preparation for Pull-Off Adhesion Testing, date of access: 6 January 2017, http://www.defelsko.com/technotes/DollyPreparation.htm