Q: We are facing failed wet adhesion tests on painted parts. Can you tell me the root cause of these failures? PG
A: You did not tell me the nature of the substrate and the type of paint you used. Also, you didn't tell me what you were testing, adhesion to pretreated substrates or intercoat adhesion. So I will answer both cases. During wet adhesion testing, immersion causes water to be absorbed by the paint film and cause it to swell. This swelling can separate the film from the substrate. This occurs in intercoat adhesion and adhesion to pretreated substrates.
In the case of intercoat bonding, the two paint films can swell at different rates. If the compositions of these films are not the same, they may have different absorption and swelling rates. If they are the same, the water may not have been absorbed by the underlying membrane to the same extent, and the degree of swelling may be different. Again, these differences will cause the film to separate and lose adhesion. In the event of binder loss to a pretreated substrate, the paint film can follow the same pattern by swelling and detaching from the substrate. However, keep in mind that the single largest cause of loss of adhesion to pretreated substrates is improper surface preparation. This rule applies to both cases above and may be the real reason for wet adhesion test failure.
