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
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: 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
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
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.
An extensive description can be found in:
S.J.F. Erich, O.C.G. Adan,
L. Pel, H.P. Huinink, K. Kopinga, NMR
Imaging of Coatings on Porous Substrates, Chemistry of Material18,
S.J.F. Erich, NMR
imaging of curing
in alkyd coatings, Ph.D. thesis, Eindhoven University of Technology,