PHOTODEGRADATION AND PHOTOSTABILIZATION IN ORGANIC COATINGS CONTAINING A HINDERED AMINE LIGHT STABILIZER .6. ESR MEASUREMENTS OF NITROXIDE KINETICS AND MECHANISM OF STABILIZATION

The concentration of nitroxide has been measured in an acrylic melamine coating containing different hindered amine light stabilizers as a function of exposure harshness, time and stabilizer concentration using electron spin resonance spectroscopy. The four hindered amines are based on 2,2,6,6-tetramethylpiperidine and have the following substituents on the piperidine nitrogen: H, CH3, CH2CH2-polymer, and COCH5. Comparisons of nitroxide formation and decay kinetics with infrared spectroscopic measurements of hindered amine effectiveness reveal that a major factor in the ability of hindered amine to inhibit photo-oxidation in acrylic melamine coatings is the rate of conversion of the amine functionality to nitroxide. Conversion is fastest for the z.lbond2; group. The conversion of z.lbond2; to nitroxide is also fast and proceeds through the formation of z.lbond2; groups by at least two mechanisms. By contrast, the conversion of z.lbond2;-polymer and z.lbond2; to nitroxide is roughly an order of magnitude slower than that for z.lbond2; accounting for the decreased effectiveness of these stabilizers. © 1990.