Studies of photopolymer heterogeneties by Confocal Raman Microscopy
Résumé
Photopolymers are used in a wide range of industrial applications (graphic arts, microelectronic, coatings...). Oxygen inhibition is most of the time an issue in free radical photopolymerization performed in air. As a result, the structure of the cured polymer coating shows heterogeneities across the depth of the film: a gradient of conversion is created leading to poor final surface properties (scratch or chemical resistance). The depth characterization of photopolymer films by Confocal Raman Microscopy (CRM) is important to assess the extent of the effect of oxygen inhibition but also to correlate the curing state of the sample with its mechanical properties. Nevertheless the depth profile determination could be tricky for thick samples due to optical issue like refraction effects. An adequate preparation of the sample combined with the use of an immersion objective allows us to study these specific samples by CRM. After detailing the procedure used, some examples of the studies performed with this setup will be shown such as the influence of the photoinitiator nature or additives known to reduce oxygen inhibition on the through cure of acrylate coating.