Mode, Mechanism, and Model for Hydrolysis of Acrylic-Melamine Coatings in the Absence of UV Light

摘要

Acrylic-melamine coatings are used extensively for automobile topcoat/base coat systems. However, these materials undergo hydrolysis when they are exposed to water or humid environments. The mode and mechanism of degradation and a physics-based model for predicting the course and rate of hydrolysis of this type of coating in the absence of ultraviolet (UV) light are presented. The results are from experiments conducted using a model acrylic-melamine coating exposed to a wide range of relative humidities (RH) and temperatures. The coating was prepared by reacting a partially-methylated melamine resin and hydroxy-terminated acrylic resin. Specimens of cured film applied to a CaF2 substrate were subjected to five different RH levels ranging from approximately 0 % to 90 % at four different temperatures using a specially-designed exposure cell. The humidity of the sample environment was controlled to within 3 % of the nominal RH by mixing dry air and moisture-saturated air in the proper proportions. Coating degradation as a function exposure time was measured with transmission Fourier transform infrared spectroscopy and tapping mode atomic force microscopy. In humid conditions, the coating hydrolysis resulted in considerable material loss and the formation of various new groups in the film, including primary amines and carboxylic acids. The rate of degradation increased with increasing RH. The primary reactions responsible for material loss were hydrolysis reactions, which occurred at the crosslinks and melamine methoxy groups. Oxidation of monosubstituted amino methylols, which were the dominant products formed during hydrolysis, generated primary amines and formaldehyde. The latter was further oxidized to acids. The hydrolytic degradation was an inhomogeneous process, in which pits formed, deepened and widened with exposure time. Such localized degradation suggests that the hydrolysis was an auto-catalytic process in which hydrolysis products accumulating in the pits catalyzed and accelerated the hydrolysis reactions. A mathematical model was derived for the formation of primary amines, assuming first-order kinetics. Experimental results agree well with theoretical prediction for all relative humidities. Information on the nature of the regions where hydrolysis occurs, the degradation mode, specific mechanism, and physics-based model presented in this study should help to design more hydrolytically-stable coatings.