Adhesion to Plastics

Question:

Recently, I have experienced some sporadic paint adhesion failures. My substrate is plastic and the topcoat is a low-solids 1K/2K solvent-borne technology. The adhesion failures occur between substrate/primer and the primer itself (splitting). I checked the power wash with the supplier to ensure the detergent/virgin DI water performs within specs. Everything checked out. My bake chart checked out with proper part temperature as recommended by the paint supplier. The failures occurred across different colors. Can you suggest anything else I can do eliminate the issue? I also use another paint supplier for other product but experience no problem. There is no contamination between the two suppliers. The substrate that has adhesion problems is molded in-house. No release agent is used at all for these parts. L.T.

Answer:

I believe your problem is incompatibility of the low-solids topcoat with the primer. The solvent in the topcoat is attacking the primer causing it to lift from the substrate. The splitting of the primer, that is disintegration of the primer film with part of the film adhering to the substrate and part of the film adhering to the topcoat, can also be caused by solvent attack.

One suggestion is to change the coating schedule by allowing more time for the primer to cure before topcoating, if it is air drying. If the primer is baked, increase the baking time. If that doesn’t help, you must use a more solvent-resistant primer. Another suggestion is to use a topcoat containing less aggressive solvents.