Confocal Raman microscopy study of several factors known to participate in overcoming O2 inhibition

The free-radical photopolymerization (FRP) of coatings suffers from inhibition by atmospheric oxygen which reacts with excited species and radicals.1 During the process, oxygen diffuses from the atmosphere into the film and leads to inhibition period, longer irradiation time, incomplete conversion of the coating and heterogeneity in depth conversion with a possibly sticky surface.2 Indeed, the top first microns have minimal conversion while in lower depths the curing is almost complete. Confocal Raman Microscopy (CRM) is a powerful characterization technique whereby the deleterious effects of oxygen on the polymer structure can be studied by performing depth profiling measurements.2 Therefore in this study, an experimental procedure has been developed in which optical effects, which occur scanning in depth and could jeopardize data, are minimized. In this way, CRM was performed according this procedure to study some factors which are known to participate in overcoming oxygen inhibition: the influence of the light intensity or the influence of an oxygen scavenger. This study clearly evidences the potential of CRM to study oxygen inhibition in FRP bringing additional information–inhibited layer, conversion profile–which cannot be determined by commonly used experimental techniques such as real time Fourier transform infrared spectroscopy (RT-FTIR) or even by attenuated total reflection spectroscopy (ATR).