NMR imaging of coatings on porous substrates

Coatings are often applied on porous substrates, for example, wood, stone, or gypsum layers. The type and porosity of the substrate influences the coating performance. Until recently, no techniques were available to monitor the drying process in-depth as a function of time with a high spatial resolution (about 5 mu m) in nontransparent coating systems. In the study presented in this paper, we show that with high resolution NMR imaging the drying process, consisting of penetration and evaporation of solvent and subsequent curing (chemical cross-linking) on and inside the substrate (wood and gypsum), can be monitored. The drying of a waterborne alkyd emulsion on a gypsum substrate was investigated. The curing of the emulsion was studied for both glass and gypsum substrates as a function of catalyst concentration, in this case cobalt based. Curing was not only observed at the coating surface, but also, in the case of gypsum, it was observed at the coating/substrate interface. On both substrates a concentration dependence of the catalyst concentration was observed. On the gypsum substrate the speed of the observed curing front was always higher than on glass. This indicates that part of the Ca2+ ions originating from the gypsum might act as a secondary drier after migration to the coating. The drying of a commercially available solvent-borne alkyd coating was monitored on gypsum and pine wood. The measurements showed that the coating completely penetrates the substrate and starts to cure inside. The results stress that to optimize the coating performance one should explicitly take the substrate into account.