Water transport in pigmented waterborne coatings studied with NMR

A critical feature of a waterborne coating is its sensitivity to water that may affect its durability. At the Eindhoven University of Technology, the group Transport in Permeable Media (TPM) focuses on water uptake by coatings and the influence of the substrate on the uptake process. Water uptake in coatings, applied on various substrates, is monitored with nuclear magnetic resonance (NMR) and compared with wet-cup and DVS measurements. This gives the possibility to study real-time and non-destructively water migration inside a sample. The key question for water uptake by coatings is: which substance of a coating promotes the uptake of water? With help of NMR imaging and relaxometry, we found that the dispersant of the pigment is the most important factor according to water uptake in waterborne coatings. So, if the dispersant of pigments is important, what is the relation between permeability and
pigmentation? We found that the dispersant creates a layer around pigments which facilitates water transport. The effect of this layer is much stronger in case of smaller pigments compared to larger ones, if in both cases a similar pigment to volume concentration (PVC) is used. As coating layers are meant to protect an underlying substrate, the effect of a coating layer on the water uptake characteristics of a substrate is also studied. The experiments with coatings on wood show that a waterborne coating keeps liquid water out of a wood substrate, but water can still penetrate the substrate by diffusion through cell walls. The uptake rate decreases with the time between application and the experiment. This time period can take several months.