Effects of Relative Humidity on Photodegradation of Acrylic Melamine Coatings: A Quantitative Study.
Effects of Relative Humidity on Photodegradation of
Acrylic Melamine Coatings: A Quantitative Study.
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.
coatings; humidity; degradation; FT-IR;
photodegradation; quantum yield; moisture; relative
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