Improving the appearance of 3-coat-1-bake multilayer films on automotive bodies

3-coat-1-bake is a coating process which primer-surfacer, basecoat, and clearcoat sprayed on an e-coat substrate by a wet-on-wet technique. However the appearance of the 3-coat-1-bake films has been inferior to that of the conventional 2-coat-1-bake films. Therefore, 3-coat-1-bake coating process has been introduced to only economy-class automobiles. In this study, the surface unevenness is discussed as a quantitative parameter of appearance. We assumed that the surface unevenness of the 3-coat-1-bake films is mainly occurred by not only telegraphing of substrate unevenness but also telegraphing of interface unevenness between coating layers. In this paper, we verify this assumption by focusing on telegraphing of interface unevenness between coating layers and examine to reduce the surface unevenness.　To reduce the surface unevenness, it is effective to decrease shrinkage of wet film and/or to minimize interface unevenness between coating layers. The surface unevenness reduced when selecting an isocyanate hardener to the basecoat instead of a melamine resin. Because selecting an isocyanate hardener decreased shrinkage of the basecoat film after loss of the macroscopic flow of the clearcoat film. The surface unevenness greatly reduced in the case of basecoat with a low Tg (glass transition temperature) acrylic resin. A low Tg acrylic resin in the basecoat promotes levelling better. Therefore, this minimized interface unevenness between the basecoat and the clearcoat layers at loss of the macroscopic flow of the clearcoat film. It is verified that to decrease shrinkage of wet film, it is effective to select isocyanate hardener. It is also verified that to minimize interface unevenness between coating layers it is effective to use acrylic resin with low Tg.