Hiromi Harakawa

The rheological properties of an aqueous acrylic dispersion suitable for automotive waterborne basecoats

Superior film appearance as well as performance is required of automotive top-coatings applied using the method of two-coat one-bake (2C1B) in the case of metallic finishes. The basecoats, however, dominate the final film appearance, that is, the metallic effect and the film surface smoothness. Waterborne metallic basecoats are rather difficult to maintain a high level of workability and appearance under various spraying conditions involving a wider range of temperature and humidity. Both the aluminum flake orientation and the surface smoothness of basecoats, which involve opposite requirements especially in a waterborne system, should be superior before clearcoats are over-coated. In this paper, we will present a means of incorporating them. The studied model basecoats are composed of three materials, a specific water-dispersible acrylic resin, an internally crosslinked latex and a melamineformaldehyde resin. The viscosity versus non-volatile content relationship and the viscoelastic behavior of the model paints are discussed, in relation to the actual spraying workability and the final appearance.