NMR depth profiling of drying alkyd coatings with different catalysts

Abstract

Cobalt based catalyst are widely used for curing of alkyd resins. The concern on the carcinogenicity of cobalt based catalyst is steadily growing. Therefore, possible alternative catalyst systems are investigated, e.g. based on Mn or Fe. Knowledge of the influence of these catalysts on the spatial and temporal evolution of the solidification of a coating is required to understand the mechanisms involved and to optimize the performance of catalyst. We have studied the curing of alkyd coatings with cobalt (Co) and manganese (Mn) based catalysts, using a high gradient nuclear magnetic resonance (NMR) setup with a spatial resolution of 5 μ m and Confocal Raman Microscopy (CRM). NMR probes the hydrogen density and the mobility of the hydrogen atoms attached to the polymers, which decreases with increased degree of cross-linking. CRM was used to obtain chemical information by probing the amount of double bonds in the fatty acid side chains of the alkyd molecules. The results indicate that the CRM profiles directly correspond with the observed NMR profiles, which indicates that NMR is a useful tool for depth profiling of coatings during curing. NMR measurements indicate that the curing process of an alkyd coating is strongly influenced by the type of catalyst. In the case of Co a sharp cross-linking front was observed, whereas no front was observed in the presence of a Mn catalyst, even at high Mn concentrations. The front formation in the presence of Co indicates that the oxygen transport limits the curing process, whereas the reactivity of the catalyst is the limiting factor in case of Mn.