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October 28 - November 1, 2019

Why would a pull-off adhesion test give a high value (no failure) for a solventless epoxy that later showed poor adhesion in service?

Selected Answers

From Jeff Longmore of Thin Film Technology, Inc. on November 1, 2019:
The coating formulation would play a critical part in premature failure. For example, excessive water-solubles in the resin system or pigmentation invite transport of water through the film to degrade adhesion at a steel substrate. Deviation from correct stoichiometry may also invite water penetration through excess or unbound hydrophilic curing agents.

From Ricardo Márceles of Pintuco on October 28, 2019:
An adhesion test, in itself, is not decisive to say that the product will work excellently. There are several factors that affect it, such as: the roughness profile, the degree of surface preparation, and the presence of soluble contaminants.  Also consider profile vs. thickness ratio, ambient operating conditions to which it will be subjected, thermal extenders, etc.  Adhesion values, like the others, are parameters that indicate the probability that the product will behave well, but that added to the others, also remember that the product continues crossing over time and that cross-linking generates a shrinkage of the coating and this can be subtle or very strong and will depend on temperatures, chemical medium, profile vs. thickness and curing speed. If an accelerator is added, for example, this becomes more critical.

From Jon Cavallo of Jon R. Cavallo PE LLC on October 28, 2019:
I am assuming that the question has to do with pull-off adhesion tests performed in accordance with ASTM D4541. This standard specifically prohibits cutting of the coating on the periphery of the adhesion "dolly" unless specifically directed in writing, If you don't cut around the "dolly" prior to performing the adhesion pull-off test, you may be measuring the cohesive film strength of the coating rather than its adhesion to underlying coatings or the substrate. The test may result in an erroneously high number even if the adhesion of the top coat to the underlying layer or substrate is poor. That is one of the main reasons that knife adhesion testing was standardized (ASTM D6677).

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Tagged categories: Adhesion; Asia Pacific; Coating Application; EMEA (Europe, Middle East and Africa); Latin America; North America

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