Sulfuric acid distribution in acrylic-melamine coatings immersed in 70 wt.% sulfuric acid at 60°C was examined with an electron probe microanalyzer. Melamine derivatives extracted from the coatings were determined by UV spectra of the acid phase. It was found that the region containing the sulfuric acid had a distinct boundary expanding at a constant rate and that the sulfuric acid extracted water-insoluble melamine derivatives. The extraction rate was proportional to the permeation rate. These results indicate that the etching is caused by the extraction of the water-insoluble derivatives by the concentrated sulfuric acid and that the diffusion of sulfuric acid molecules at the boundary is controlled by hydrolysis of cross-links.
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