Improving appearance of 3-coat-1-bake multilayer films on automotive bodies through solvent composition design

•Appearance of 3-coat-1-bake film deteriorates when the volume shrinkage of the clearcoat layer exceeds the flow and leveling during the drying.•Residual high-boiling-point solvent in the wet film degrades the appearance.•The appearance is improved by using low-boiling-point ether-ester or ether-alcohol type solvents for the clearcoat. 3-coat-1-bake (3C1B) is a wet-on-wet-on-wet coating method in which three layers, consisting of a primer-surfacer, basecoat, and clearcoat, are coated consecutively without baking. The 3C1B method is widely applied to automobile paint because of its low energy consumption. However, the appearance of the coated surface is usually inferior to films obtained by the 2-coat-1-bake coating method. The main cause of this difference is presumed to be the waviness of the substrate surface and the waviness of the interfaces between the primer-surfacer/basecoat and the basecoat/clearcoat layers, which are telegraphed to the coating surface. The waviness of the coating surface increases as the coating film shrinks, and decreases as the surface of the coating film flows. Increases in waviness occur as the shrinkage process of the coating film proceeds. Conversely, waviness decreases as the flow and leveling process proceeds. The degree of waviness depends on the balance between volume shrinkage and flow/leveling. Consequently, reducing the shrinkage is an effective means of improving the appearance of a 3C1B coating film. Therefore, if the level of high-boiling-point solvent, which is used for leveling, remains significant while the flow and leveling process is stopped by the extent of the cure, film shrinkage caused by the evaporation of the remaining solvent might telegraph the waviness of the substrate and interfaces to the surface and degrades appearance. This study verified this assumption. When preparing a 3C1B coating film using a waterborne primer-surfacer, waterborne basecoat, and solventborne clearcoat, the solvent that remains in significant quantities while the flow and leveling process is stopped, is mainly an ether-ester-type high-boiling-point organic solvent mixed with the clearcoat as a leveling agent. This solvent was replaced by a low-boiling-point ether-ester-type solvent or an ether-alcohol-type solvent. As a result, in addition to lowering the boiling point, the shrinkage after end of the flow and leveling process was decreased and the appearance was improved. Thus, it was concluded that the origin of the poor surfa