Determination of hydroperoxide concentrations in cross-linked polymer coatings containing hindered amine light stabilizers

Abstract

A procedure for measuring hydroperoxide concentrations in unstabilized, cross-linked polymer coatings as a function of weathering time has been extended for use on coatings containing hindered amine light stabilizers (HALS). The modification involves electron spin resonance spectroscopic measurements of the nitroxide concentration in HALS stabilized coatings. Hydroperoxide concentrations are determined by correcting iodometric titration values for the contribution from nitroxide reduction by I −. The correction is usually small. This procedure has been used to compare hydroperoxide concentrations formed during weathering of unstabilized and HALS stabilized acrylic melamine and urethane cross-linked coatings. The reduction in hydroperoxide level with the addition of HALS is consistent with longer-term infrared spectroscopic measurements of carbonyl growth in these coatings. The results strongly suggest that measurements of hydroperoxides formed during controlled ‘outdoor-like’ exposures can be used for the rapid evaluation of stabilized coating weatherability and the selection of effective HALS additives.