UV protection of Polyurethane

[sailalex]

I need some help in understanding whether polyurethane has UV protection or not. I have heard that it can go years without a problem but,on the other hand, others say it has no UV protection and several coats of varnish is necessary to get UV protection.I have been of the opinion that it has no protection otherwise why would people use varnish? Are there downsides to using polyurethane as a finish?This is not an academic question because I am building a boat with another guy and we are on oposite sides of this question; can you all help?

[ken mcclure]

If memory serves (which it usually doesn't) the polyurethane needs additives to give it UV protection. Try calling manufacturers and asking them the question. They'll know if theirs has it, and can tell you how it all works.

Now that you've brought it up, you'll have to file a report when you get the answers. tongue.gif

[Scott Rosen]

What do you mean by "polyurathane"? That word covers a lot of different products.

[NormMessinger]

Where ever did that chemist go off too?

UV protection has to be added to any clear coating system. (Wild generalization.) That is one of the differences between interior and exterior coatings. The more that is added the greater the cost of the product.

Most of the hardware store polyeurothanes (he could have spelled it as well) are not, as Scott suggests. Choose the proper "polyurethane" and no additional additives will be necessary.

And if that isn't the truth, blame the chemist for running out on us. Ya suppose got bored or angry with us?

[T. Crisp]

Standard polurethane is an interior coating. Polyurethanes with UV protection added are either labeled "for exterior use" or as "spar" coatings.

[John Blazy]

Hi Norm - been a while since I posted, and you brought me out cuz I was a chemist, and headed up UV additives research, so I am the surrogate "Chemist" at this point.

Normal (interior) Polyurethane does not have these additives. Some protection is created simply from the film, but not much. Exterior PU's should have Hindered Amine Light Stabilizers (HALS) coupled with UV blockers called Tetratrazioles (If I can remember the name correctly). These additives can be added to any clear film finish - varnish, PU, Acrylic Urethane etc. but only for those designated "exterior", although some "exterior" finishes may not have these additives - they are expensive, and cheapo companies probably have very little loading of them.
HALS are free-radical absorbers. Free radicals are sheared electrons that come off of a polymers molecular chains due to bombardment of UV light from sun. Enough electrons lost, and the chains breakdown. UV Blockers (tetratrazioles) are non-pigment (clear) blockers of the actual UV light. The best blockers of UV light is pigment itself, white being best, but in clearcoats, a little higher-tech additive is required (can't remember how it worked, but it blocked UV quite well).
Both types of additives are meant to be used together.

I used them in 1-1/2% to 2-1/2% loadings and that is a lot - which is why certain clears are more expensive. You still need an elastic formula though, for longevity in a clear. These additives are great for protecting epoxy from blush, which is why you need to overcoat epoxy with marine varnish.
Cya - JB

[Buddy]

Both one and two part clear ployurethanes can have tem, but the label has to say so- and then that's no real answer as to how much of this expensive stuff is in the pot- how much beef in the stew. Price is not necessarily the best indicatore, but brand name/ cost is a guide.

For instance, Interlux Schooner Varnish is not a polyurethnae but has "high" UV protection. Their Goldspar one part urethane does have UV protection.
Interlux's Interthane Plus Clear two part poly urethane has no UV.

Bristol Finish is amber tinted to match Captains Varnish ( the old standard), but is a modern two part urethane with heavy UV protection. Techincally is an acrylic urethane, not a true liner polyurethane as above and is a tad softer, less abrasion resistant. But is is easier than LPU ( but not as easy as oil based varnishes) to brush on and level like regular varnish. Concocted for the sole purpose of getting the brightwork on boats to deliver the same hard wearing, hard to weather performance of that same boats two part polyurethane painted topsides and decks.

I've used Interux for the painted areas and Bristol Finish for the bright work and have been pleased.
Both are more expensive to buy, and more effort to apply than "oil" products, but much longer lasting and easier to maintain. The same boat hase one part Brightside decks, and they do get dinged noticably more. When I overall repaint, I'm going to strip and get a custom mixed LPU for the decks too. They are that much better and easier to keep "new".

[NormMessinger]

Thanks, John and a satisfactory substitue ye be. The job of first string quart-er back is yours if you want it.

[John Blazy]

"The Chemist" is much smarter and more experienced than I am though. The cool part is that I have years of experience behind a spray gun too, - not sure if the Chemist does, but he likely does. Everyone can bring much to the table I'm sure.

[sailalex]

Thanks for all your responses; seems as if a polyurethane (exterior) has UV additives , it is okay. But for how long? If it is the same as varnish in terms of UV protection, then won't I be left with with applying additional coats of polyurethane in the future just as I would be applying additional coats of varnish? If so, what's the advantage? Seems as if a coat of polyurethane followed by varnish when needed into the future, would be just as good.
Thanks again for your help.