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A-Z of organic UV stabilizers.

One of the most destructive forces from Mother Nature, the combination of ultraviolet (UV) light, water, wind erosion, and freeze/thaw cycles, can quickly destroy an unprotected coating. So, to ensure an extended coating life and the highest performance, it is necessary to protect it. The two most common ways to protect the coating from the destructive UV forces are the use of a UV absorber and hindered amine light stabilizers (HALS).

The earth's atmosphere absorbs most of the radiation below 290 nanometers. Only about 6% of the energy reaching the earth's surface is in the UV spectrum, but this energy-rich UV light accounts for most of the degradation seen in coatings.

There are three aspects of a UV protection system: extinction, photopermanence, and absorptive range. Extinction refers to how much light is absorbed at a given wavelength. Photopermanence refers to how long the UV absorber will last, the resistance to the degradation of the UV absorber. The absorptive range is the wavelengths covered by the UV absorber.

UV absorbers prevent polymer degradation by absorbing light and converting it to heat. UV absorbers use a Tautomeric mechanism which allows long-term activity. The Enol-Keto mode of action allows energy dissipation by converting the absorbed UV light to heat (see Figure 1). Beer's law also applies to absorbing light. By either increasing the concentration of the UV absorber or increasing the film thickness at the same concentration, you have increased the capability of the UV absorber to prevent UV light from getting to the substrate.




The main chemistries of UV absorbers are hydroxy benzophenone, hydroxy phenyl benzotriazoles (BTZ), and hydroxy phenyl triazines (triazines). While they have been around for a while, newer developments have increased the performance of these chemistries, leading to longer photopermanence and higher extinction. Triazines have stronger intramolecular hydrogen bonds than BTZ products and this leads to increased photopermanence. These bonds also resist breaking and forming intermolecular hydrogen bonds in the presence of strong polar solvents. UV absorbers are less effective in pigmented systems because the UV light will not penetrate into the coating due to the pigments. Many pigments, including titanium dioxide (Ti[0.sub.2]), will absorb in the UV range, but Ti[O.sub.2] will help create free radicals, which will quickly destroy a coating.

Hindered amine light stabilizers operate differently than UV absorbers, as shown in Figure 2. When free radicals are generated, they will react and quench the free radical, thus preventing the destruction of the polymer backbone. The cyclical mechanism of HALS (Figure 3) allows long-term activity. Another important feature of HALS is the effect with UV absorbers.

The main concern with HALS is that they tend to migrate out of the film. They can migrate to the surface and be washed away by rain or during cleaning of the coating. They can also migrate into organic substrates such as plastics or rubbers. HALS can contain hydroxyl groups and can react with crosslink agents (melamine, isocyanate, etc.). This process lowers their migration, but being bound also lowers their mobility and access to free radicals. The newer HALS agents are designed to greatly lower migration and prolong their effectiveness in the coating. HALS are also quite hydrophobic and incorporating them in water-based systems can be difficult. One method used is micro encapsulation which makes incorporation into the coating significantly easier.

A combination of UV absorbers and HALS gives the coating maximum UV protection. The UV absorber will absorb UV light (300-410 nm) in the film preventing free radical formation. UV absorbers have only limited capabilities at the surface but protect deeper parts of the coating and also the surface of the substrate. If free radicals are formed, HALS scavenge and deactivate them, preventing further damage to the coatings. HALS work at both the surface and in the bulk coating, providing additional protection.

By Mike Praw, BASF Corporation Mike Praw is group leader, technical marketing for resins at BASF Corporation;
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Author:Praw, Mike
Publication:JCT CoatingsTech
Date:Apr 1, 2012
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