Novel techniques to investigate the impact of cellulose esters on the rheological properties and appearance in automotive basecoat systems

Abstract

The viscous response of a coating formulation changes over several orders of magnitude after application onto a substrate. Furthermore, the performance and application attributes for a refinish basecoat formulation are slightly different from those of an OEM basecoat. The absence of a high temperature baking step during the refinish process necessitates that the paint exhibit very good antisag behavior as well as quick ‘dry to touch’ without compromising the flow and leveling properties of the coating as it dries. As most paint formulations are multicomponent in nature, it is always a challenge to precisely measure the impact of specific formulation components on the collective performance attributes of the applied coating. In this work, the focus has been to develop novel techniques that can be used to demonstrate a more quantitative measure of some of the performance attributes that rheological additives like cellulose esters can provide to automotive coatings. A series of pigmented basecoat formulations were prepared which contained cellulose acetate butyrates (CABs) of varying molecular weights at different levels based on the total solids of the coating formulation. The nonvolatile content of the formulations was also varied. The viscoelastic behavior of a typical automotive basecoat formulation during the drying process was then investigated using a novel rheological technique. Complex viscosity data (including storage and loss moduli as well as tan delta) were determined as a function of drying time and then compared to the macroscopic properties typically associated with a coating film as it dries. Thermogravimetric analysis (TGA) was also used to correlate the rheology of the metallic basecoat formulations with drying behavior of the coating. The final appearance of the coating was investigated by several microscopic techniques such as Laser Scanning Confocal Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, and Surface Profilometery, and attempts were made to correlate bulk measurements like ‘flop index’ with the microstructure of the coating.