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Poor adhesion between urethane top coat and epoxy primer


Q. I am using a liquid epoxy primer and liquid urethane which are exhibiting inconsistent adhesion to each other. I am measuring adhesion with a 3 mm crosshatch tool, and an "X" scribe.

The part measures approximately 48" x 72". I will perform adhesion test in an area and pass the test. Perform the test again in another area and fail. Film thickness in both areas will be about the same.

What should I be looking for that could create an intercoat adhesion problem of this nature?

Kelly Loch
captive finishing shop - Syracuse, New York


A. Hi,

In the areas where you are having the failures is the failure to the base substrate or is it to the primer. If the failure is to the substrate, it could be pretreatment related. From a pretreatment standpoint things to look at would be insufficient cleaning, powdery coating, poor rinsing.

Dependent on part configuration, rinsing procedures, and/or nozzle pattern it is possible that a portion of the part could be adequately prepared and a portion inadequately.

If the failure is to the primer level, sorry you had to read the above.

Dan Zinman

April 17, 2012

A. There needs to be a mechanical 'key' to enable adhesion of a polyurethane to an epoxy unless it is being applied within the recoat 'window' as per the manufacturer's Product Data Sheet. On mechanically prepared surfaces, the finer the sanded profile on the epoxy, the more 'polished' is the surface and the less profile available for the polyurethane to 'key' into for adhesion. Also, any contamination, perhaps from the atmosphere or an airline or your hands will inhibit adhesion - even sweat. This is more often revealed by 'patchy' adhesion. If your poor localised adhesion was actually a product failure, it would not be localised but generally all over the surface. Some so called wax and grease removers actually contaminate the surface. You should wash carefully with the same solvent you are using in the polyurethane

Steve Dyer
Coating Engineer - Perth, Australia

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