Zinc Oxide Nanoparticle Dispersions as Unique Additives for Coatings

By Charles Hegedus, Frank Pepe, Denise Lindenmuth, Detlef Burgard

The effects of UV exposure on coatings and other polymer systems are well documented. Photodegradation occurs when polymer systems absorb photons causing the molecules to become excited. Excitation leads to chemi­cal reactions which cause degradation. Degradation mechanisms may in­clude chain scission of covalent bonds, formation of free radicals, and ab­straction of hydrogen. Oxygen can enhance degradation by reacting with free radicals to form peroxy radicals and hydroperoxides.

At this point, many re­actions can occur to produce even more free radicals leading to further degradation of the coatings6•7 In general, shorter wavelengths of light, such as ultraviolet light, have the ability to break many polymer covalent bonds. In some cases, such as when clearcoats and stains are applied, underlying substrates also can be degraded by exposure to UV, es­pecially with wood and plastic substrates.

To minimize such failures, coatings that are exposed to UV during their service life are formulated with UV­ resistant polymers, such as acrylics and aliphatic polyurethanes. Although these systems are resistant to UV, they provide minimal UV protection to the under­lying substrate.

Coatings used in more demanding ap­plications, such as automotive, aerospace, and exterior wood, also contain additives which absorb UV to assist in protection. These additives not only protect the coat­ing, but they also can protect the substrate, which may be vulnerable to UV degradation ( e.g., wood, plastics, and textiles).

Typically, these additives are included in coating formulations at concentrations of <5%. Some of the more common chemical classes of organic UV absorbers are benzophenones, benzotriazoles, triazines, malonates, and oxalanilides. However, due to their rel­atively low molecular weight, these additives can mi­grate out of applied coatings, either to the coating sur­face or into the substrate. Also, because they are organic, they are susceptible to a number of degrada­tion mechanisms.