Confocal Raman microscopy study of several factors known to influence the oxygen inhibition of acrylate photopolymerization under LED

• Effect of physical and chemical factors known to participate in reducing O2 inhibition in FRP Depth conversion profile Confocal Raman microscopy.
• An increase in the initial concentration of photoinitiator enhances the surface conversion and greatly reduces the oxygen affected layer thickness.
• A change in light irradiance influences more the oxygen affected layer thickness than the surface conversion.
• An increase in the formulation viscosity reduces the depth heterogeneities in conversion.
• A tack-free film can be obtained using triphenylphosphine as oxygen scavenger.

The free-radical photopolymerization (FRP) suffers from inhibition by atmospheric oxygen which reacts with excited species and radicals leading to inhibition period, longer irradiation time, incomplete conversion of the coating and heterogeneity in depth conversion with a possibly tacky surface. A study of the influence of several factors that affect oxygen inhibition on the depth conversion was performed by Confocal Raman Microscopy (CRM). Increasing the photoinitiator content is shown to increase the surface conversion and decrease the oxygen affected layer thickness. Interestingly, increasing the irradiance mostly affects the oxygen affected layer. The viscosity of the resin was also found to be important to reduce the depth heterogeneities in conversion. Finally, triphenylphosphine was used to highlight the decrease of the oxygen inhibition by using an oxygen scavenger. By selection of the reactive diluent, a tack-free sample surface was obtained under visible light in open-air atmosphere. This study clearly evidences the potential of CRM to assess oxygen inhibition in FRP bringing additional information – oxygen affected layer, conversion profile – which cannot be determined by commonly used experimental techniques such as real time Fourier transform infrared spectroscopy or even by attenuated total reflection spectroscopy.

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