¹⁵N CP/MAS solid-state NMR spectroscopy of a ¹⁵N-enriched hindered amine light stabilizer photolyzed in acrylic/melamine and acrylic/urethane coatings

Solid-state cross-polarization magic angle spinning ¹⁵N nuclear magnetic resonance spectroscopy has been used to follow the chemistry of a ¹⁵N-enriched hindered amine light stabilizer and its corresponding nitroxide in an acrylic/melamine and an acrylic/urethane coating system, as a function of ultraviolet light exposure. The two coating systems are based on the same acrylate copolymer. Samples were photolyzed in a QUV accelerated weathering tester equipped with FS-340 UV-A fluorescent bulbs. The tester was operated in the `light only' exposure mode at an air temperature of 40 °C and a dew point of 25 °C. Spectra reveal the formation of HALS based amino ether derivatives and hydroxylamine. No evidence for the accumulation of HALS decomposition products or non-reactive HALS products was found. Direct evidence that, the amino ether derivatives formed in the two coatings system are distinctly different, was found. One amino ether derivative was unambiguously identified as a methyl adduct. The formation of methyl radicals during the photolysis of both coatings was demonstrated.