A depth-resolved look at the network development in alkyd coatings by confocal Raman microspectroscopy

The formation of molecular networks related to the consumption of unsaturated carbon-carbon double bonds (CC) during oxidative drying of alkyd coating films incorporating unsaturated fatty acids was studied. The concentration of CC bonds was measured as a function of drying time and distance from the exposed film surface (depth) using confocal Raman microspectroscopy (CRM). The change in spatial distribution of the CC double bond concentration across the film cross section provides information on the kinetics of the oxidative cross-linking process in the alkyd films. It was found that the CC bond consumption is not homogeneous across the depth of the drying film. The results obtained allowed us to quantitatively monitor the progress of the drying process and the movement of the ‘drying front’ within the coating films. The drying profiles suggest that oxygen penetration into the coating film is a rate-limiting factor in the drying process. Depth profiles during the film forming process develop due to local variations in the oxygen solubility, diffusion coefficient of oxygen, and available amount of double bonds for cross-linking. The influence of several industrially relevant factors, like oil length of the alkyd resin, thickener, solvent, and drier on the film formation process is discussed. Depth resolution of the analytical approach and spatial accuracy of confocal Raman microspectroscopy are also treated.