Using nitroxide decay to study the photooxidation kinetics of automotive topcoat enamels.

Free radical scavenging by nitroxide dopant is used to quantify the photoinitiation rate of free radicals in acrylic/melamine and polyester/urethane coatings during photolysis under "near ambient" exposure conditions. Photoinitiation rate measurements on weathered coatings reveal that acrylic/melamine coatings photooxidize non-autocatalytically, while polyester/urethane coatings photooxidize autocatalytically. The decomposition of hydroperoxide photolysis products by melamine crosslinker is claimed to account for this difference in photooxidation kinetics.

Fluorescence detection of free radicals by nitroxide scavenging.

The fluorescence quantum yield of 4-(1-napthoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl (I) in acetonitrile and hexane is 55 and 30-fold lower, respectively, than those of diamagnetic analogs. Experiments described herein demonstrate that this property makes possible the fluorescence detection of radical scavenging reactions in which the paramagnetic nitroxide-substituted naphthalene is converted to a diamagnetic N-alkoxy derivative. 2-Cyanopropyl free radicals were generated by the thermal decomposition of azobisisobutyronitrile (AIBN) in cyclohexane or in acetonitrile containing I. The fluorescence intensity of the sample increased proportionally to the decrease in its ESR signal intensity, indicating the conversion of the paramagnetic nitroxide to the diamagnetic product. The linear relationship between the increase in fluorescence intensity and decrease in ESR signal intensity shows that the changes in the fluorescence intensity can serve as a sensitive means for optically detecting radicals.