Pull-off Adhesion Testing Of Coatings – Improve Your Technique

 

 

A recent study by Elcometer investigates the effects of deviating from the specified Pull-Off Adhesion Test Method.

 

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. A measure of the adhesion of the coating system is the force at which the coating fails and the type of failure obtained.

 

TEST METHOD


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.

 

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
 
 
RESULTS
 
  • From the choice of adhesive, through the preparation of the coating surface and the test dolly, to the decision to cut through the coating or not, the test needs to be precise and consistent.
  • Different adhesives have different operating constraints. It must be noted that the bond strength of the cured adhesive must be greater than the bond strength of the coating, either to the substrate (adhesive failure), to the coating beneath (also adhesive failure) or within a single layer (cohesive failure).
  • As with coating processes, the preparation and cleaning of the surface of the coating and the face of the dolly is crucial to optimising the adhesion of the dolly to the surface and therefore increasing the probability of a coating adhesion failure rather than a glue failure.
  • The design of the dolly is significant in the adhesion values obtained but further work is required to determine why this is the case.

 

 


For a copy of the full report, methods and analysis please click here.

To see Elcometer's series of videos on Adhesive Testing, please click here.