Hydrolysis-Controlled Transport of Water in Polymeric Coatings
Sanghun Lee, Tinh Nguyen, Jonathan W. Martin, E Byrd
Thermoset acrylic melamine coatings are widely used for exterior structures. 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 to moist environments. During hydrolysis, water in the coating is consumed. In order to keep the moisture content in the film in equilibrium, water must be transported from the regions of high water concentration to the regions of low water concentration. An approach based on FTIR analysis of coating degradation fitted to a transport model is presented to estimate the diffusion coefficients and velocities of water during hydrolysis of acrylic melamine coatings exposed to different relative humidities. Theoretical prediction agreed very well with the experimental FTIR data of coating degradation. Generally, both the diffusion coefficient and velocity of water increased with increasing relative humidity, regardless of oxidation or chain scission of the coating. Since the transports during chemical reactions in coatings are very complex, the approach presented here provides a viable means for obtaining valuable data for quantitative analyses of coating hydrolysis at different relative humidities.