Improving the appearance of multilayer films on automotive bodies
Scientists recently clarified the mechanism leading to unevenness in 3-coat-1-bake multilayer films and improved the appearance by using an acrylic resin with a low glass transition temperature in the basecoat.
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 a new study, the surface unevenness is discussed as a quantitative parameter of appearance.
Telegraphing of interface unevenness between coating layers
The scientists 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 their paper, the scientists 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 minimise 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.
Minimisation of surface unevenness with acrylic resin
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 minimised 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 minimise interface unevenness between coating layers it is effective to use acrylic resin with low Tg.
The study was published in: Progress in Organic Coatings Volume 123, October 2018, Pages 299-305.
Book tip:
If you want to learn more about automotive coatings, the reference book Automotive Coatings by Ulrich Poth is the benchmark for the industry. This book deals with the composition and the production of the most difficult components as well as their physical and application technology characteristics.
