Relating Laboratory and Outdoor Exposure of Coatings. Part IV: Mode and Mechanism for Hydrolytic Degradation of Acrylic-Malamine Coatings Exposed to Water Vapor in the Absence of UV Light.
Relating Laboratory and Outdoor Exposure of Coatings.
Part IV: Mode and Mechanism for Hydrolytic Degradation
of Acrylic-Malamine Coatings Exposed to Water Vapor in
the Absence of UV Light.
(7803 K)
Nguyen, T.; Martin, J. W.; Byrd, E.
Journal of Coatings Technology, Vol. 75, No. 941,
37-50, June 2003.
Keywords:
coatings; exposure; degradation; water vapor; acrylic
melamine; heterogeneous; moisture; quantum yield;
relative humidity; humidity
Abstract:
Acrylic-melamme coatings are known to be susceptible to
hydrolysis when exposed to water or humid environments.
The mode and specific pathways for hydrolytic
degradation of acrylic-melamine coatings exposed to
water vapor in the absence of ultraviolet light are
presented. Samples of a partially methylated
melamine-aclylic coating applied to CaF2 substrates were
subjected to five different relative humidity levels
ranging from approximately 0 to 90% at 50 deg C.
Coating degradation was measured with transmission
Fourier transform infrared spectorscopy (FTIR) and
tapping mode atomic force microscopy (AFM). In humid
environments, partially methylated melamine-acrylic
coatings undergo hydrolysis readily, causing
considerable material loss and formation of mainly
primary amines and carboxylic acids. The rate of
hydrolysis increases with increasing RH. Hydrolytic
degradation of acrylic-melamine coatings is an
inhomogeneous process in which pits form, deepen, and
enlarge with exposure. Such localized degradation mode
suggests that hydrolysis of this material is an
autocatalytic progression where acidic degradation
products formed in the pits catalyze and accelerate the
hydrolysis reactions.
