Relating Laboratory and Outdoor Exposure of Coatings – IV. Mode and Mechanism for Hydrolytic Degradation of Acrylic-Melamine Coatings Exposed to Water Vapor in the Absence of UV Light

By Tinh Nguyen, Jon Martin, Eric Byrd

Thermoset acrylic-melamine resins are widely used for automobile exterior coatings. These materials
are formed by reacting an acrylic polyol with an alkylated melamine. However, the ether crosslinks of
acrylic-melamine coatings are known to be susceptible to hydrolysis when exposed to moist environments.
Under outdoor and artificial acid rain environments, acrylic-melamine coatings undergo etching, which is
primarily a result of acid hydrolysis of the crosslinks. A particular attribute of etching is the localized loss of
material, resulting in pitting of the coating surface. In the presence of ultraviolet (UV) light, the hydrolysis
process becomes more complex. For example, the hydrolysis in Miami, FL, has been observed to be faster
than that in Phoenix, AZ. Numerous studies using controlled environments also showed that, in the presence
of UV light, the hydrolysis rate is greater than the sum of dark hydrolysis and photolysis combined.

The enhanced degradation, which follows closely with the hydrolysis rate and relative humidity (RH) levels, not only occurs at the acrylic-melamine crosslink and in the melamine chains but also on the acrylic resin structure. Enhanced degradation in UV /humidity conditions has been variously attributed to melamine excited state chemistry/ oxidized products of formaldehyde, catalysis by carboxylic acids produced by the photooxidation, and chromophoric activity of formaldehyde molecules released from the hydrolysis reactions. melamine chains but also on the acrylic resin structure. Enhanced degradation in UV /humidity conditions has been variously attributed to melamine excited state chemistry/ oxidized products of formaldehyde,9 catalysis by carboxylic acids produced by the photooxidation, and chromophoric activity of formaldehyde molecules released from the hydrolysis reactions.