What UV does to a binder
Ultraviolet photons carry enough energy to excite or break bonds in many polymers. In the presence of oxygen this starts photo-oxidation: radicals form, chains break (scission), carbonyl groups and colored by-products build up, and the surface erodes. Pigment particles, no longer held by binder, wash or rub off as chalk.
Why aromatic systems yellow
Aromatic structures, including bisphenol A in epoxies and MDI in many polyureas and polyurethanes, absorb UV strongly and form colored oxidation products. Studies of aromatic polyurea and polyurethane coatings report yellowing and gloss loss within days to weeks of UV exposure, often while the coating continues to protect the substrate underneath. Our research digest summarizes the evidence.
| Binder type | Typical UV behavior |
|---|---|
| Aromatic epoxy | Chalks and ambers quickly outdoors |
| Aromatic polyurea / polyurethane | Yellows and loses gloss early; bulk properties often hold up longer |
| Aliphatic polyurethane, polyaspartic | Excellent gloss and color retention |
| Acrylics | Very good weathering |
Stabilizers that help
- UV absorbers such as hydroxyphenyl-triazines convert absorbed UV into harmless heat, protecting the binder and underlying layers.
- HALS (hindered amine light stabilizers) trap the radicals that UV creates and regenerate in a cycle, giving long-lasting protection. Basic HALS can interfere with acid-catalyzed systems, so special grades are used there.
- Pigments, notably carbon black and iron oxides, screen UV in the film.
Specifying for sunlight
- Decide whether appearance matters or only protection.
- If appearance matters, topcoat aromatic systems with an aliphatic polyurethane or polyaspartic.
- Ask for accelerated weathering data such as ASTM G154 gloss and color retention, and compare like with like.
- Expect darker colors and clears to show change sooner.