COATINGS ON POROUS SUBSTRATES
Coatings are often applied on porous substrates, e.g., wood, stone, or gypsum layers. The type and porosity of the substrate influences the coating performance. One example is the adhesion, which generally increases when the depth of penetration of the coating into the substrate is higher. Until recently, no techniques were available to monitor the drying process in a combination of coating and substrate. We have studied using a NMR with a high spatial resolution (about  5 µm.) in the drying of a coating both in the coating itself as in the porous substrate

First a  water-borne alkyd emulsion was  investigated on the gypsum substrate. The profiles of the NMR measurements are plotted in figure 1.



Figure 1: A water-borne alkyd emulsion on a gypsum substrate. The dashed profiles show the penetration and evaporation of the solvent.
These profiles are given at t = 0; 0:3; 0:6; 1:0; 1:3 h. The solid profiles show the curing of the coating. These profiles
are given at t = 8; 16; 49; 65 h after application. The horizontal line shows the signal from the hydrated gypsum layer
, both before and after paint application.

At the left side the coating is located and at the right side the gypsum is located. The surface of the coating and the coating/substrate interface are denoted by the two vertical lines. The measurements show that the penetration of the water phase into the gypsum layer is almost instantaneous,and wets the gypsum layer. In practical applications the gypsum substrate is thicker, and as a result the water uptake would be even higher. Directly after this water uptake, the water present in the coating and substrate starts to evaporate. This process is indicated by the dashed profiles in figure 1. The signal intensities from the gypsum layer before application of the paint and after the evaporation stage are the same. Therefore, it can be concluded that the resin has not penetrated the substrate, except possibly at the interface with a penetration depth smaller than the experimental resolution.

Secondly a solvent-borne alkyd was also applied on a thin layer pine wood with a thickness of approximately 0.5 mm. The results of the measurements are given in figure 2.


Figure 2: Solvent-borne alkyd coating applied on a wooden substrate. The dashed NMR profiles show the penetration
 and evaporation of the coating. These profiles are given at t = 0:3; 0:6; 1; 1:3; 1:6; 3:3 h. The solid profiles
 are given every 1.7 h after the last dashed profile at t = 3:3 h. The vertical line shows the wood surface.


 The surface of the coating is located at the left side of the profile and the vertical line shows the surface of the wood. The solvent and resin both penetrate the wood almost instantaneously after application; after the first profile no changes are observed in the deeper region of the wood. In this case the depth of penetration into the wood is approximately equal to the thickness of the pine wood. After the penetration of the substrate the solvent starts to evaporate. This stage is indicated by the dashed profiles in figure 2. A curing front develops,which slowly moves down into the wood.
 

          S.J.F. Erich, O.C.G. Adan, L. Pel, H.P. Huinink, K. Kopinga, NMR Imaging of Coatings on Porous Substrates, Chemistry of Material 18, 4500 (2006).
         
          S.J.F. Erich, NMR imaging of curing processes in alkyd coatings, Ph.D. thesis, Eindhoven University of Technology, the Netherlands (2006).