The Development of Novel Nitroxide Additives to Protect against Free Radical Damage, Indicate Lifetime and Signal Damage Levels in Polymers and Coatings

In this study a number of profluorescent nitroxides were developed as additives able to signal radical-mediated damage in coating materials. These additives possess a very low fluorescence quantum yield, however when these nitroxides capture the free radicals generated during materials failure, they are converted to strongly fluorescent species. Scavenging these radicals protects the material, and the level of fluorescence generated signals the history of damage exposure. This technique compares favourably with other methods of monitoring degradation, providing an indication of damage well before other techniques show any response. When applied to the monitoring of crosslinked polyester coating resins, differentiation in the oxidative stability of the resins could be detected after as little as 200 minutes where other monitoring techniques require up to 300 hours of accelerated degradation at 95° C. A novel mono-isoindoline nitroxide additive containing a 9,10-bis(phenylethynyl)anthracene structural core was developed and shown to be suitable for epoxy-resins and polyurethane coatings relevant to corrosion protection. Significantly, in addition to lifetime indication, the profluorescent nitroxide additives show potential in highlighting impact damage to surfaces including composite panels.