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Old 07-31-09, 07:12 AM
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meanwhile
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Originally Posted by Falanx View Post
The zinc's for the galvanic protection of the underlying steel, yes.

The Epoxy isn't for toughness at all. it's there because it's easy to apply and is porous, so that he zinc can electrically contact the steel, otherwise it'd be worthless. Epoxy isn't particularly tough at all. It's rather brittle, no matter its formulation. What it is is *rigid*.
You've been awfully good - but can I ask you to disambiguate one more thing? In this case are you talking about an epoxy that contains zinc, or epoxy as a second layer over a zinc primer? If as a second layer, surely the zinc is already in contact... so you're talking about an epoxy with zinc in, yes?

The PU is for the toughness. Neither PU or epoxy systems without the correct additives are particularly UV resisting. PU especially yellows horrendously (electronics conformal coatings which are essentially pure PUs demsontrate this well) as it crosslinks to terminal brittleness. The additives are what make both UV resistant, but naturally PU starts off tougher and more impermeable. I say *more*, and remind you of that. Neither *are*. So it takes less additive to keep it more tough, if you see what I mean.
Very clear.

Yes, there are a number of UV resistant epoxies, but the aerospace sector tends to throw PU down on top because it's tougher, not more UV resistant.

And yes, you can put an epoxy topcoat down on top of the zinc epoxy primer, although it won't be as damage tolerant as PU.

Hope that helps :-)
Very clear. Thanks!
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