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Adhesion problem with polyurethene paint

September 9, 2008

Sir,
We are painting one of our fordged component as;
First with epoxy bicomponent primer, thickness around 40 microns and then with bicomponent polyurethene paint, thickness around 40 microns.
Paint fails during the hatch test.
pretreatment is as follows;
Manually cleaning with wire brush.
Trichloroethylene dip
Cold phosphating
Primer application
Drying time of 24 hrs.
First coat of polyurethene paint
Drying time of 4 hrs.
Second coat of P.U. paint
Drying time of 8 hrs.
Is there any problem in cleaning steps, as components has got the porosity and oil remains on the surface to be treated ?
If, so what is the best method to remove oil from the surface, we can not go for the hot degreasing or acid pickling as the components one part is machined and has to be kept intact.
Please guide us.
Thanks.
V.Keer

Vijay Keer
Product Designer - Pune, Maharashtra, India

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First of two simultaneous responses --

The cause is most likely the oil that is left on the item.
Washing/wipe/bushed with any solvent, the solvent becomes contaminated and leaves some oil behind. Vapour degreasing ensures a clean solvent arrives at the item to clean it. Have you tried powder coating where machining can take place after baking?

Terry Hickling
Birmingham, United Kingdom
September 10, 2008

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Second of two simultaneous responses -- September 11, 2008

Vijay,

You have pretty much answered your own query. If there is oil left on the surface before phosphate and paint you will always have adhesion problems. You must make sure all oil and grease is removed from the parts before processing. If you can't use hot degreasing then you will need to manualy degrease. Don't use the chlorinated solvents such as Trichloroethylene as this may be very hazardous for you and your colleagues. Try one of the isoparaffinic hydrocarbon degreasers on the market (most of your local chemical suppliers will be able to help you out with an appropriate product).

After degreasing have an inspection operation to ensure the complete removal of the oil. If it hasn't been completely removed then you have to repeat the degreasing operation until the parts are absolutely clean.

Remember, if there is any oil or grease on the parts before you phosphate or paint then you will have adhesion problems.

Brian Terry
Aerospace - Yeovil, Somerset, UK