Summary for poor adhesion between epoxy primer and urethane top coat Discussion
Poor adhesion between epoxy primer and urethane top coat is the central question here: an aerospace coatings engineer asks whether an over-thick urethane topcoat could cause primer-to-substrate delamination on a properly prepared, properly dried epoxy primer. The poster says the customer blamed a 0.0025" topcoat thickness for major delamination at the primer/substrate interface, even though surface prep and contamination are the usual suspects. The thread excerpt provided does not include any replies, so no consensus or named expert opinion is available in the source shown. Based on the question alone, the issue being investigated is whether topcoat build can somehow drive failure at the primer interface, versus the more common explanation of substrate preparation or contamination.
Q. Hello All,
I work for an aerospace manufacturer and we had an issue with our coating system at our customer. We had some parts that suffered major delamination at the Primer to Substrate Interface. Normally this is due to surface preparation or contamination. In this case the customer is stating that the topcoat over thickness (Topcoat thickness was 0.0025″) is the root cause of the primer to substrate delamination. I have been in coatings for 20 years and have not seen that being an issue before, but I do not claim to have seen it all.
My question is: Is there any validity to this theory? Can over-thickness of the topcoat cause primer delamination on a properly dried primer on a properly prepared surface?
Materials:
Epoxy Primer: MIL-PRF-85582
[from DLA]
Type I Class C2 (PPG/DEFT 44-GN-72)
0.0006-0.0009 Thickness, 2 hr dry time prior to Topcoat
Urethane Topcoat: , MIL-PRF-22750 Type II (PPG/DEFT 01W081F)
0.0011-0.0015 Thickness
Ted Moore
– Shelby, North Carolina
⇩ Similar, related, Q&As — oldest first ⇩
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
1996
1996
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
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
April 17, 2012