Spectroscopic ellipsometric characterization of an aqueous polyacrylic dispersion on steel

Visual Spectroscopic Ellipsometry (VISSE) was used to analyze thin organic coatings on steel. VISSE allows a fast and non-destructive determination of the coating's thickness and optical constants. Therefore, an optical model was devised to describe the steel/polyacrylic system. To test this model, the measured color of the system was compared to the one generated by the optical model. This comparison revealed that the color originates from interference. Besides thickness and optical constants of the layer, it is equally important to obtain information about how the polyacrylic film is formed on the substrate. Subsequently, the optical model was then used to study the formation of a thin organic layer from an aqueous polyacrylic dispersion at three different temperatures (23 °C, 33 °C and 60 °C). VISSE measurements showed a decrease in layer thickness and an increase in the refractive index of the layer as a function of the drying time for temperatures above the Minimal Film forming Temperature. In order to determine the amount of voids in the acrylic layer, the model was adapted to include a Bruggemann Effective Medium Approximation. It allowed the extraction of the amount of air at every instant during the drying of the layer. It is thus possible to study a polyacrylic layer on a steel substrate using VISSE.