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Effects of Relative Humidity on Photodegradation of Acrylic Melamine Coatings: A Quantitative Study.


pdf icon Effects of Relative Humidity on Photodegradation of Acrylic Melamine Coatings: A Quantitative Study. (321 K)
Nguyen, T.; Martin, J. W.; Byrd, W. E.; Embree, N.

Fall Meeting. Volume 83. American Chemical Society. PSME. Proceedings. August 20-24, 2000, Washington, DC, 118-119 pp, 2000.

Keywords:

coatings; humidity; degradation; FT-IR; photodegradation; quantum yield; moisture; relative humidity

Abstract:

Service life prediction methodology based on correlating short-term performance with outdoor data is not a fundamentally sound approach because the weather never duplicates itself. Experiments having over 200 different combinations of exposure conditions of UV light, temperature, and relative humidity have been conducted to generate experimental data for validating the reliability-theory approach to predict the service life of polymeric coatings. The UV source was supplied by 1000 W xenon arc solar simulators, and relative humidities (RH) were provided by special-design humidity generators, which controlled RHs in the 0% to 90% range to within 3%. Radiation absorbed in the film and coating degradation were measured by UV-visible and FTIR spectroscopies, respectively. The effects of relative humidity (< 4%, 20%, 40%, 70% and 90%) on the phtodegradation and quantum efficiency for a partially-methylated melamine acrylic coating exposed to UV/50 deg C condition are reported in this paper. Both the total degradation and dark hydrolysis rates increased with increasing RH. The total degradation of the coating was found to consist of four different processes: post curing reactions, dark hydrolysis, photolysis, and moisture-enhanced photolysis. The effects of RH on each process were quqntified, which showed that moisture enhanced both the magnitudes and rates of the photo-oxidation, chain seission, and ester side chain cleavage of the coating. Formaldehyde released during hydrolysis is believed to be responsible for the enhancement. Further, moisture enhances the apparent quantum efficiency of acrylic melamine coatings.