Comparison of NMR and confocal Raman microscopy as coatings research tools

Recently, several new experimental techniques have been developed to measure the spatial and temporal evolution of a drying coating film. One of these techniques is nuclear magnetic resonance (NMR) using a very high magnetic field gradient. An advantage of this technique is the ability to image a nontransparent coating film perpendicular to its surface with a resolution better than 10 μ m (depth profiling). Another, well known technique is confocal Raman microscopy (CRM). This paper deals with a comparative study of such a microimaging NMR setup and confocal Raman microscopy for two alkyd coatings, one organic solvent based and another water based. The NMR and CRM results show a good similarity, when comparing NMR profiles with the disappearance of the double bonds (CRM-detectable) of the unsaturated fatty acids. This indicates that the disappearance of the double bonds in the fatty acid side chains of the alkyd molecules is directly connected with the formation of cross links by these fatty acid entities. This oxidative cross linking results in an increase of viscosity and therefore in a change of the measured NMR relaxation times.