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 resoln. (about 5 mm) in nontransparent coating systems. In the study presented , we show that with high resoln. NMR imaging the drying process, consisting of penetration and evapn. of solvent and subsequent curing (chem. crosslinking) on and inside the substrate (wood and gypsum), can be monitored. The drying of a waterborne alkyd emulsion on a gypsum substrate was studied. The curing of the emulsion was studied for both glass and gypsum substrates as a function of catalyst concn., in this case cobalt based. Curing was not only obsd. at the coating surface, but also, in the case of gypsum, it was obsd. at the coating/substrate interface. On both substrates a concn. dependence of the catalyst concn. was obsd. On the gypsum substrate the speed of the obsd. 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 com. 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. [on SciFinder (R)]