Relating Laboratory and Outdoor Exposure of Coatings – ll. Effects of Relative Humidity on Photodegradation and the Apparent Quantum Yield of Acrylic-Melamine Coatings

By Tinh Nguyen, Jonathan Martin, Eric Byrd, Ned Embree­

The effect of relative humidity (RH) from < <1 %to 90% on the photodegradation and quantum efficiency for a partially-methylated melamine acrylic coating exposed to UV/50°C condition has been investigated. The UV source is supplied by two 1000 W Xenon arc solar simulators and the relative humidities are provided by specially de­signed humidity generators, which control relative humidity in the Oto 90% range to within < 3% of the measured values.

Radiation absorbed in the coating and degradation of the films are measured by UV-visible and Fourier transform infrared spec­troscopies, respectively. The degradation at a par­ticular RH/UV condition consists of four different modes: reactions taken place during post curing, hydrolysis due to water in the film at a particular RH, photodegradation, and moisture-enhanced photodegradation.

Total degradation, hydrolysis, and moisture-enhanced photodegradation increase with increasing RH. At low relative humidities, photodegradation is an important degradation mode but hydrolysis dominates the degradation at high RH levels. Moisture in the film is found to increase the quantum efficiency of acrylic melamine coating photodegradation.