Durability problems with polyurethanes

[sdold]

Hey guys, is anyone here familiar with polyurethanes? I recently coated some parts with what is supposed to be polyurethane, and followed the instructions closely regarding recommended curing (450 degrees for 15 minutes, it's a .060 piece of aluminum, not a big heat sink).

After curing, the coating is easily softened with gasoline (aviation gasoline), but similar parts that have been coated with a polyester are not as easily softened and seem more durable.

The polyurethane looks a little better, flows better (less orange peel) but I'm wondering about the fact that gasoline softens it. I thought polyurethanes were supposed to be more chemical-resistant.

MEK softens it up good too.

Does this sound like it's really polyurethane? I'm wondering if it's something different, or if I'm expecting too much.

Steve

[tavo1765]

Most likley the parts are not fully cured, go with a 400f for 35 mins, you dont need to go that high with temp unless your coating thick or large heavy parts, But eventually with time the gas with dull it. I do alot of offroad applications and over time they just need to be coated with clear to bring them back to a shine.

[sdold]

Originally Posted by tavo1765

go with a 400f for 35 mins

Thanks, I'll give that a try and report back.

[DALE]

The highest chemical resistance is with Epoxy powders.These are limited in color and not UV stable. Go with a Epoxy-polyester hybrid. The 400F for 35 min is a better cure cycle in my opinion.If you can leave it in the oven and just turn oven off after curing. Allow it to sit and cool very slowly inside the oven. Then run a test and see what you get.
Dale

[sdold]

Thanks Dale, I'll try a hybrid and see how that works. I don't have it in Red but I have some in something else and I'll make some comparisons with some other types. Yeah, I think you are right about the epoxy, a friend did some gas caps in epoxy and it's chalking a little after a couple of months.

Oddly enough, I tried some samples today of the polyurethane that I had trouble with, at 10, 20, 30 and 40 minutes. All of them showed good resistance to aviation fuel afterwards, so maybe tavo1765 is correct in that the original parts might not have been fully cured.

Powdercoating is so cool, it's hard not to take stuff out of the oven, I just gotta look at the finished product But I'm getting better at leaving in to fully cure.