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July 1 - July 7, 2013

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

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Selected Answers

From Mike Moran of Kiewit Power Enbgineers on August 15, 2013:
Thick epoxy coatings have lateral strength/self supporting. The epoxy can be disbonded; however, it takes a lot of force to break the coating with the pull-off.

From S. Matsumoto of Abraco on August 14, 2013:
Solventless epoxy coatings shrink about 5% when cured. The relationship between WFT and DFT shows this easily. If the coating's adhesion is not resistant to shrink tension, detachment could occur.

From PARVEZ SHAIKH of SANDVIK ASIA PVT. LTD. on August 1, 2013:
When it comes to service, the question involves the exposure condition to which the coating was exposed. Was the coating designed for that exposure?  If not, the chance of failure is very high.  To clarify, we have to get the data detailing the exposure conditions.

From leoncio ucancial of hempel-glucom on July 6, 2013:
The first point is to evaluate the conditions of surface preparation. If the structure will be in immersion service,  surface preparation should be SSPC-SP 5, with a profile of 3 mils mínimum. If the environment i satmospheric, surface preparation should be SSPC-SP 6 or SP10, with a profile of 1.5 to 2.5 mils.

The second point involves the coating systems. Failure of the adhesión could be caused by improper mixing (resin mixing ratio-catalyst).

 The third point is  evaluating the climatic conditions where this painting occurred and evaluating the adhesión test. It should be noted that with epoxy resins at low temperatures, the chemical reaction is slower

From Bonny Njimogu of Construction Specialist Services Ltd. on July 3, 2013:
There could be two major reasons that result in such failures. 1)The tack or primer coat was not applied to the substrate prior to applying the topping. 2) The water level below the substrate might be low during the installation and testing period. Moisture absorption and capillary action at the high water level could affect the substrate, thereby resulting in failure.

From Stefan Danek of Defence Scienece technology Organisation on July 2, 2013:
First, one has to consider whether the pull-off adhesion test was carried out on the same sample that failed later in service, i.e., is the test sample representative of the in-service coating? This will address the very valid point raised by Atanas on whether there is likely to have been a difference in surface preparation (surface preparation and substrate cleanliness, including soluble salts) and paint application (paint sample, operator, equipment, application conditions, DFT, dew point, etc.) If the test sample is not representative of the in-service sample, then there could be any of a number of reasons for the difference as discussed above. Second, in the laboratory, the results obtained from dry adhesion tests can often be superior to wet adhesion tests. Typically, this involves cross hatch testing rather than pull-off adhesion testing, and is used  for coating systems that are relatively thin (less than 5 mils), for example, as applied in the aerospace industry. This can be very critical for exterior coatings exposed to weather, particularly when substrates can be subjected to periods of prolonged wetness. By dry adhesion, I mean undertaking the adhesion test after the coating has cured under standard conditions. Wet adhesion refers to adhesion tests undertaken on the cured paint film following immersion in water. Some specifications require adhesion to be remeasured following immersion in water at ambient temperature for 24 hours. I prefer immersion in water at 122 F (50 C) for 7 days. Coatings will fail wet adhesion tests if they are either unsuited to the application or have been applied over substrates which  contain soluble surface salts which can lead to osmotic blistering, even in epoxy systems applied at 6 -8 mils. The question is what to what climatic conditions was the failed sample exposed in service?

From Atanas Cholakov of ACT on July 1, 2013:
If the test is carried out on a test panel, is the coating been applied in  the same climatic conditions as on the work piece? Often, there is a difference. A pull-off on the work piece itself can give a better indication of adhesion. Coating  application might have been compromised (surface profile, cleanliness). Is the coating being properly selected for the service environment and conditions, and what does 'later' mean in elapsed service life?

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Tagged categories: Adhesion; Performance testing; Solvents

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