Novel techniques to investigate the impact of cellulose esters on the rheological properties and appearance in automotive basecoat systems |
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Authors: | Deepanjan Bhattacharya Kab Seo Louis T Germinario Mark D Clark Kevin W McCreight Chip Williams |
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Affiliation: | (1) Research Laboratories, Eastman Chemical Company, Kingsport, TN 37662, USA |
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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.
Presented at the 2006 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans,
LA, on November 1–3, 2006. |
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Keywords: | Flake orientation Drying rheology Cellulose esters Automotive basecoats Film shrinkage |
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