Water in coatings

摘要

Coatings are applied for protective and esthetic reasons. They have to act as barriers for molecules that should not reach the underlying substrate. In general water is one of the substances that should not accumulate on or in the substrate. For example, wood becomes prone to wood rot and metals start to corrode due to water. Therefore knowledge of the interaction between a coating and water is needed. To understand the coating-water interaction first a technique is needed to detect water migration inside a polymeric matrix without interfering with the process. We have shown that the NMR imaging based on the so-called GARField concept can be used to trace both water and its effect on the polymeric matrix. Experiments were performed with multilayer coatings inspired by automotive applications and nylon-6 films. Depth profiles of water and plasticized polymer could be measured real-time with micrometer scale resolution. On the basis of these depth profiles a kinetic model for water sorption in multilayer coatings was developed, which was based on the sorption isotherm of the base coat and permeability of the top coat. The model predicted a strong asymmetry in uptake and drying (slow uptake and fast drying), which was indeed found in the experiments.