UV Protection — Aliphatic Urethane
One of the most important industrial and commercial painting features the PennCoat looks for in exterior topcoats is protection from the Sun’s powerful ultraviolet (UV) radiation. In today’s article, we’ll examine UV in some detail and then revisit the properties of aliphatic urethane topcoats and see how it stands up to UV rays.
Ultraviolet Light
UV light spans wavelengths from 400 to 10 nanometers, shorter than visible light wavelengths but longer than those for X-rays. Humans can’t see UV light, although some animals can. Because it vibrates at a higher frequency than does visible light, it carries more energy per photon and in high amounts can damage paints and people. UV takes its toll on exterior structures by making materials brittle and causing color to fade. In humans, it’s been linked to eye problems and skin cancer.
Actually, scientists define a number of subtypes of UV radiation. For example UV A has a wavelength range of 400-315 nm, UV B’s range is 315 – 280 nm and so forth, all the way to Extreme UV with wavelengths between 121 and 10 nm. The energy per photon throughout the UV range starts at 3.10 electron volts and goes as high as 124 eV. Much, but not all, UV radiation is absorbed by Earth’s ozone layer, and one reason PennCoat favors coatings with low volatile organic compound content is to help protect the delicate ozone layer. Without it, we all would fry. On the other hand, a little UV is good — we humans use it to make Vitamin D.
Free Radicals
Paints and coatings protect substrates from UV degradation through the use of specific additives that absorb UV light and render it harmless. Normally, the relatively high energy of UV photons is enough to partially strip electrons from the valence shell of organic compounds, creating reactive free radicals. These partially ionized molecules contribute to aging — in humans and paints — by stimulating chemical reactions (specifically, oxidation) that then break down a substrate. Paint and coatings fortified with antioxidant UV stabilizers absorb UV rays and thus prevent them from forming free radicals. This increases the working life of the topcoat and slows degradation.
Aliphatic Urethane Top Coat
One of our most commonly used aliphatic urethanes is a multi-part aliphatic urethane system containing a resin (blocked cycloaliphatic diamine), a hardener, and a colorant (dipropylene glycol monomethyl ether acetate + coloring oxides). A top grit is also available.
The resin part contains HDI, or hexamethylene diisocyanate, C8H12N2O2, which is widely used in paints and coatings because of its resistance to UV light. Our preferred aliphatic urethane’s UV resistance is rated as “excellent,” although it doesn’t specify which testing method was used. In general, exterior coatings are tested by applying them to substrates and then subjecting them to high-intensity UV lamps. In this way, testers can induce over a decade of weathering in only a year. And commercial paints that perform will in this test are often granted the privilege of becoming a commercial painting top coat for most painting contractors.