Mechanisms of Color Variation of Automotive Metallic Finishes

Abstract

The color of automotive metallic finishes applied in a basecoat/clearcoat painting process is varied by spraying equipment and/or parameters, even when the same basecoat and clearcoat are used. This color variation results from the differences not only in the orientation, but also in the content and size of flakes in the basecoat film.

During spraying, the flakes are oriented parallel to the substrate surface by the basecoat droplet flow only when the droplet is large enough to penetrate the wet film and spread out on the substrate surface. During drying and baking, the flakes are oriented parallel by the shrinkage of the film thickness caused by the evaporation of volatiles after the film viscosity becomes high enough to prevent the disturbed flake movement by the evaporative convection etc..

The content and size of flakes in the basecoat film are varied by the nonuniformity of flake allotment to the droplets and the selective deposition of the droplets. Small droplets contain only fine flakes or no flakes, and cannot deposit on the substrate easily. Therefore, the deposition of small droplets, namely, the increase of transfer efficiency causes the decrease of both the content and size of flakes.