ABSTRACT
A latex paint film on wood can be looked upon as a composite consisting of an acrylic copolymer, pigments and additives.
In the supplied paint half the volume consists of water. When painted water evaporates and a paint film is created.
However, the latex paint film is less homogenous and has a higher water diffusion through the film than a solvent cast polymer.
By using ESEM (Environmental Scanning Electron Microscope) and SEM (Scanning Electron Microscope) we have shown that the latex paint film has many coned holes through the film. The number of holes and the hole area increases with age under environmental degradation. We also found that smaller latex particle sizes (0.1 μm) had half the number of small holes compared to films with larger latex particle sizes (0.4 μm). The water diffusion through the paint films at 85RH complied with the number of small holes (less than 2.5 μm diameter). This was measured by CT (Computed Tomography).
It was also shown that the rough surface and holes developed during the first part of the coalescence step was due to latex particle clustering. It is believed that the clustering of latex particles is already present in the paint can.
The fast diffusion through the film is known as the Knudsen diffusion or slip flow. The diffusion can be several orders faster from the direction of the airside than from the opposite woodside of the paint film.
