Mechanism of Acid Rain Etching of Acrylic-Melamine Clearcoats

Hydrolysis Rates of Acid-Epoxy and Melamine Resin Crosslink

Abstract

Using butyl-2-methylpropionate (BP) and monomeric butoxymethylmelamine (MB) as model compounds for acid-epoxy crosslink (ester bonding) and melamine crosslink respectively, hydrolysis rates of these crosslinks in 70 wt% sulfuric acid have been estimated. BP was dissolved in 70 wt% sulfuric acid at 40°C. The solution was neutralized to stop the hydrolysis after predetermined time. The concentration of 1-butanol in the solution was measured with a gas chromatography. MB was hydrolyzed at -18 and 0°C, and treated in the same manner described above. Hydrolysis rate constants were determined from 1-butanol formation by fitting to the first-order reaction kinetics.
The hydrolysis of both compounds was first order reaction with respect to 1-butanol formation. The hydrolysis rate constant of BP at 40°C was 2. 5×10^-4s^-1, while those of MB at -18 and 0°C were 1. 0 × 10^-1 and 2. 8 × 10^-1s^-1, respectively. An Arrhenius' approximation gave the rate constant for MB at 40°C to be 1. 7 s^-1. The hydrolysis rate constant in ester bonding was less than 1/6600 of that of melamine crosslink. It was confirmed that acid-epoxy coatings have excellent resistance to acid rain.