Development of a Rheometer Using Laser Trapping Method and Measurements of Viscosity in Multilayer Coatings

Abstract

Viscosity is a key property controlling the appearance of automotive coatings. However, the direct measurements of the viscosity in automotive coatings have not been achieved, since the automotive coating is a complex system consisting of a multilayer where mass transfer occurs between each thin layers. Using the laser trapping method, we have developed a rheometer to realize non-destructive viscosity measurements of the multilayers. Applying the new rheometer to automotive coatings, the viscosity has been estimated for (A) a clearcoat itself, (B) a clearcoat on a basecoat, and (C) a clearcoat on a basecoat and a surfacer. The results demonstrate an increase of viscosity in the order of (A) < (B) < (C), depending on the thickness of the lower layers. The viscosity increase suggests that the concentration of the clearcoat is likely to increase due to the diffusion of the solvent into the lower layers.