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Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2005

Wet Tape Adhesion Test

Q. I work at Martin-Baker America in Johnstown, PA. We are a manufacturin /assembly plant. We do wet coating as part of our process. We spray low VOC polyurethane and epoxy paint and epoxy primers with HVLP equipment. All of our top coated parts must pass both a dry and a wet tape test. Occasionally we have a group of parts fail the wet tape test. What causes parts to fail wet tape test? I would appreciate any help that you could provide on this matter.

A. The wet tape adhesion test is more sensitive than the dry adhesion test and for this reason I prefer it. If I am in a situation where I don't know if one coating will adhere to another, the wet tape test is more likely to warn me of a problem.

I have not seen a scientific explanation, but in my opinion when you immerse the painted substrate into water for several hours, some of the water, in molecular form, migrates into the coating and swells it. Even though it appears to the naked eye as if a paint film is impervious, in microscopic terms it is somewhat porous. Therefore, the coating will be forced to expand, even if only minutely to allow the water molecules to migrate inwards. If under dry conditions the adhesion between two layers of paint is somewhat weak, the expansion between the layers will weaken it even further. If the water manages to migrate to the interface between the two coatings then, if the intercoat adhesion is weak, the water will find it easier to migrate between the two layers rather through to the second coating. In other words, it will take the path of least resistance. If intercoat adhesion is excellent and the bond between the two layers is tight, the water will not be able to get between them. By the way, when you remove the painted surface from the water and let it dry, the coating might contract again, even further weakening the bond.

I hope this helps.

Best wishes,

Ron Joseph

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