Rheology and roll coating dynamics of metallic basecoats for automotive pre-coated metal sheet

Abstract

Rheological behavior and coating dynamics of solvent-based metallic basecoats using 3-roll coaters were investigated for the application process based on pre-coated metal (PCM) sheet technology that can promisingly diminish the environmental pollutants in automotive OEM line. Rheological properties of the metallic basecoats related to optical appearance and color variations were measured by altering the content of aluminum (Al) flake and temperature of the coating solution. The results showed that the basecoats exhibited a very weak shear-thinning behavior regardless of Al flake contents in this study. However, their elastic behaviors were intensified with increasing the Al flake content. From the 3-roll coating experiments with forward and reverse coating modes, the operability windows in the coating process, demarcating the uniform coating flow from defects, were established. In particular, it was possible to elucidate the relationship between the capillary number and speed ratio conditions for the desired coating thickness in roll coating systems accompanied with reverse coating mode between the pick-up and metering rolls. It was verified from the cross-sectional images of coating layers that most of Al flakes inside the basecoat layer were oriented parallel to the coating surface under the continuous roll coating flows.