Quantitative study of water transport during the hydrolysis of polymer coatings exposed to water vapor
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Tinh Nguyen, Eric Byrd, and Jon Martin Materials and Construction Research Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 15 April 2003; accepted 30 June 2003)
Thermoset acrylic–melamine resins are widely used for automobile exterior coatings. These materials are formulated by reacting an acrylic polyol with an alkylated melamine. Because the reactions are reversible, acrylic–melamine coatings tend to hydrolyze during exposures in moist environments. During hydrolysis, water in the coating film is consumed. To keep the moisture content in the film in equilibrium, water must be transported from regions of high water concentration to regions of low water concentration. An approach based on Fourier transform infrared (FTIR) spectroscopy analysis of the coating degradation fitted to a transport model is presented to estimate the diffusion coefficients and velocities of water transport during the hydrolysis of an acrylic–melamine coating exposed to different relative humidities (RHs). Theoretical prediction agreed well with the experimental FTIR data of coating hydrolytic degradation. Generally, both the diffusion coefficient and velocity of water transport in the coating increased with increasing RH. Since water transport resulting from the hydrolysis reactions is a very slow and complex process, the approach presented here provides a viable means for obtaining valuable data for quantitative analyses of coating hydrolytic degradation at different RHs.
I. INTRODUCTION
Thermoset acrylic–melamine resins are widely used for automobile exterior coatings. These materials are formulated by reacting an acrylic polyol with an alkylated melamine. Because the reactions are reversible, acrylic–melamine coatings are susceptible to degradation during outdoor exposure. In water, acid rain environments, and humid conditions, acrylic–melamine coatings undergo hydrolysis at the cross-link and alkoxy ether bonds.1–3 A particular attribute of hydrolysis is the localized loss of material, resulting in pitting (etching) of the coating surface. During hydrolysis, water in the coating film is consumed. To keep the moisture content in the film in equilibrium, water must be transported from regions of high water concentration to regions of low water concentration. Some transport data of water in coatings and polymers exposed to nonreaction moisture environments are available.4–6 However, data on water diffusion in coatings and polymers during hydrolytic degradation do not exist. In this study, water transport properties during the hydrolysis of a partially methylated acrylic– melamine coating exposed to different relative humidities (RHs) were derived by fitting molecular degradation 2268
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J. Mater. Res., Vol. 18, No. 9, Sep 2003 Downloaded: 03 Dec 2014
data obtained by Fourier transform infrared (FTIR) spectroscopy to a transport model. Since water transport induced by the hydrolysis reactions is a very slow and complex process, the approach presented in this
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