Problem Solving Forum
October 31 - November 4, 2022
Why would a pull-off adhesion test give a high value (no failure) for a solventless epoxy that later showed poor adhesion in service?
Ronald Thorn of International Paint on
November 4, 2022:
It could depend on the mode of failure, how the te ...read more It could depend on the mode of failure, how the test was carried out and the atmosphere in service. If the coating scheme was 2 (or more) layers of solvent free epoxy, then there can be an interfacial "suction effect" - akin to 2 plates of glass sticking together. Once exposed in service, then moisture or tank contents could expose the reduced adhesion, resulting in a lower adhesion rating in service. This could be exacerbated if the original pull-off was carried out without cutting around the test dollies, which can be optional if agreed by all parties in adfvance. From memory, the 2 plates of glass effect and the effect of not cutting around the dollies is discussed in the ASTM.
Jon Cavallo of Jon R. Cavallo PE LLC on
November 5, 2022:
Really simple and common cause of this apparent ph ...read more Really simple and common cause of this apparent phenomenon... If you do NOT cut around the adhesion dolly before pulling it, (which is the specified practice if you are following ASTM 4541) you will be measuring a combination of the adhesive strength of the top coat to whatever it is under it and the cohesive strength of the topcoat (which is often HUGE when compared to the adhesion of the topcoat to whatever is under it). This has happened to me over the years in my work as a failure analyst. It is common to see the cohesive strength of the uncut dolly pull to measure over 1000 psi, yet using a simple "tap test" (which we are finally working on an ASTM Standard for) the top coat will readily pop off of the underlying coat or substrate. By the bye, this effect is very uncommon if ISO Standard 4624 is followed, because that standard REQUIRES cutting around the dolly before pulling unless specifically called for. Hope this helps.