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The UV Blocker - Absorber

Epoxy Resin Protection Page

Your Host and Tour Guide:

Paul Oman, MS, MBA - Progressive Epoxy Polymers, Inc. (floor epoxies, marine epoxies, underwater epoxies, repair epoxies)

Member: NACE (National Assoc. of Corrosion Engineers), SSPC (Soc. of Protective Coatings)

"Professionals helping Professionals"

As a 'hands on' epoxy vendor (and current owner of more than 20 canoes, kayaks and small sailboats) I've very interested in keeping keeping my painted, varnished, and epoxied surfaces as bright and shiny for as possible. As an epoxy 'salesman' I'm also interested in keeping epoxies from yellowing due to UV (sun) exposure. Clear epoxies will turn yellow, blue epoxies turn greenish etc., within a few days or weeks.

I like to test and experiment and I have access to all sorts of products to try alone and in combination (and I don't mind buying products like everyone else, just to test). I also have regular contact with the technical folks at the 4 or 5 different companies that make our epoxies and polyurethanes, so over the years I've been sort of self educated about UV protection.

I have yet to find any useful primer on clear coats and UV protection anywhere, so I guess it is up to me to offer up what I know about the subject. So, here goes.....

I have done tests on the best way to maintain a 'varnish' like clear, exterior finish (CLICK HERE FOR THAT PAGE). It basically comes down to sealing the surface with epoxy and then coating with varnish (which seems to have natural UV blockers - but only lasts a year or two) or some other product with UV blocking and/or absorbing chemical additives.


I was surprised to learn from my suppliers that most clear coat products have very little UV protectors added to them (many have a "UV Package" in them that does have a small/token amount of UV protecting chemicals). I suspect it has to do with cost. When I had one of my formulators take our 2 part acrylic polyurethane clear coat (which I always just assumed, incorrectly, had UV blockers/absorbers) and and the maximum amount of UV blockers and absorbers to it, it added over 20% to the manufacturing cost of an already expensive, high end coating.

As far as I can tell, our product, Acrylic Poly UV Plus™, and auto clear coat (which has a very similar composition) are the two high performance clear coat products with max. UV protection. NOTE that this product is classified as a floor coating. It's VOC level is too high for sale in Southern California.

Note that 2 part urethanes can be tricky/difficult to brush apply. They set up quickly on applied surfaces and can leave brush marks, even when special 'reducers' are used. If using a brush, have our 'slow' SU 93 reducer (see below) on hand, use a quality brush, and test your application first, in case you're not happy with the brushed on result (such as on boat hulls, poured on epoxy table tops, etc.).

Cycloaliphatic epoxies are one small group of epoxies using a very high end curing agent (we carry several including a cyclo based marine epoxy - Premium No Blush). When I researched the internet for cycloaliphatic epoxies all the references were about high power electrical line insulators and how the electrical industry was switching from glass insulators to cycloaliphatic epoxy insulators because they found the cycloaliphatic insulators suitable for the exposed weathering and UV they received. I can state that the cyclo-based epoxies will yellow, but more slowly. If you type cycloaliphatic epoxy into a web search engine you'll still find many of the electrical industry posts, although they are now mixed in with lots of non-electrical related sites too.


The only other thing I can offer is that I did put some epoxy paint on both sides of what I assume was ordinary plate glass. The epoxy that was on the outside of the glass, yellowed as expected but the epoxy on the inside of the glass had little or no yellowing. I assume that what I think is regular glass, does (to my surprise) provide some degree of UV protection.

I also recently purchased a UV protection product designed for outdoor fabric garden ornaments. I tried it on an epoxy surface and was not impressed.

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Now for a more technical discussion of UV blocking chemicals. The following textcan be found on the WoodenBoat Forum in the Bulding/Repair Section in a thread tagged "Appropriate UV-shield for epoxied deck." It is a post dated 12-28-05 by a gentleman listed as J. Blazy, who is obviously an expert and much more of an expert than I.

Please allow me to provide some minor exterior finish Chemistry 101 from back in my days as a polymer scientist developing UV blockers in clear coatings for a 100% solids polymer company:

UV protection in the best modern boat varnishes would a synergistic combination of two additives: Tetratraziole-based UV blockers and Hindered Amine Light Stabilizers (HALS).

The best and largest supplier of these additives are from Ciba-Geigy (Five Year Clear mentions them on their website also), and both additives are deep yellow in color, yet crystal clear liquids, not milky (some will say that the milkiness of the varnish is the UV additives - not true).

These additives are used in a 2 to 1 ratio together, where the overall formula will have at most (very expensive formula loading) 2% by weight Tinuvin (Ciba's brand name) 292 (HALS) and 1% by weight of any of the 400 series of Tinuvin tetratraziole UV blockers. (HALS do not block UV, but "mop-up" the excess free-radicals created from UV oxidation, while the Tetratraziole UV blockers absorb the UV light similar to opaque pigment, but are clear additives)

This is the first dead giveaway to me of the effectiveness of any given clear marine varnish - How "yellow" or amber the coating is - the more yellow, the better UV protection. The Ciba additives are also very expensive, so you get what you pay for - i.e. if the varnish is cheap, it'll likely have very little UV protection, and vise versa (hopefully).

This is highly dependent upon the elasticity of the given formula, though. You could have the most expensive, highest loaded UV blocking package in a given formula, but if the urethanes cross-link too tightly, you'll get brittleness and cracks. This is perhaps the advantage of the one-part varnishes that are tried and true, however it is possible to formulate 2 part, elastic varnishes quite well, and thats why they are also high-performance (they spray better, etc).

All this is also dependent upon the mil thickness of the final coating - Too thin, and the sunlight easily penetrates past even the best blockers - too thick and you risk polymer shrinkage cracks.

And one final observation - even the best UV protection in the best aliphatice urethane formula will eventually "extinguish" - which is chemist-speak for the "burning out" of these expensive UV blockers/HALS over the years. This is why you want to apply the maximum safe mil thickness, so that as the surface of the coating extinguishes, the sub layer is going to be "fresh"...

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