Three-Dimensional Technique for Measuring Sag in Drying Coatings

Coatings are found in nearly every aspect of modern society including architectural, automotive, and consumer goods applications. Undesired coating defects often arise during the flash stage of the application process which constitutes the roughly 10-minute interval immediately following spray application for automotive coatings. Fundamental understanding of the transient rheology of these systems is essential for predicting and avoiding defects such as sag. Sag refers to excessive gravity-driven flow that can occur following coating application, resulting in a nonuniform surface texture and visually undesirable appearance. We used a new technique, called Variable Angle Inspection Microscopy (VAIM), to non-invasively measure flow through the volume of an arbitrarily oriented thin film. Initial benchmarking measurements in the absence of drying tracked the velocity of silica probe particles in ~140 µm thick films of known viscosity, much greater than water, at incline angles between 5° and 10°. Probe particles were tracked at speeds as high as ~100 µm/s deep into the film. The sag flow field was well-resolved in ~10 µm thick slabs and in general the VAIM measurements were highly reproducible. Complementary profilometer measurements of film thinning were utilized to predict sag velocities with a known model. The model predictions show excellent agreement with our measurements, which validates the effectiveness of this new method in relating material properties and flow regimes. Thereby, this work will assist in accelerating formulation efforts including the development of energy efficient coating systems for automotive applications.