Magic‐angle ¹³C‐NMR of cured and degraded acrylic copolymer/melamine formaldehyde coatings

Magic‐angle carbon‐13 NMR has been used to probe the structure and dynamics of acrylic copolymer melamine formaldehyde crosslinked coatings. Changes in chemical composition that occur in conventional accelerated weathering tests were found to be dominated by hydrolysis of acrylic‐melamine crosslinks and subsequent formation of melamine‐melamine crosslinks. Evidence for photo‐oxidation was also observed. These results are in substantial agreement with infrared measurements also made on these coatings. Gated high‐power decoupling experiments were used to determine relative mobilities of the different carbon resonances as a function of acrylic copolymer composition and as a function of degradation. It was found that motion of the side‐chain carbons on the acrylic copolymer was not sensitive either to the chemical composition of the coating or to the extent of degradation. Mobilities of the main‐chain carbons of the acrylic copolymer decrease with increasing glass transition temperature of the acrylic copolymer. For acrylic copolymers containing styrene, it is found that the main‐chain carbon mobilities decrease with weathering. The melamine triazine ring becomes very rigid after degradation consistent with the formation of short melamine‐melamine crosslinks.