Spraying automotive paints and coatings, and the application of paper coatings, are typical examples of industrial processes where the characterization of elongational properties is essential to optimizing product properties or production processes. Elongation flows occur in many industrial production and working processes where product flows experience cross sectional changes or are diverted.
Paints and coatings are usually highly structured fluids that consist of several different components, including additives that can be used to modify the surface tension, optimize the thixotropic behavior, or improve finished appearance of the paint film. All these components contribute to the flow behavior of the final paint product.
The application behavior of automotive coatings often cannot be sufficiently characterized with traditional shear experiments because products with similar shear viscosities can have very different elongation properties; in a shear flow the flow lines run parallel, but in an elongational flow they converge. Elongational flows and elongation properties of substances therefore cannot be simulated and analyzed with rotational rheometers.
With an extensional rheometer, a liquid filament is created that is stretched under the influence of surface tension. Using the decrease in the filament diameter as a function of time and the life of the filament, it is possible to characterize the elongational behavior of low viscosity to pasty liquids in a simple way. The rheological properties of typical water-based automotive coatings and paper coatings are determined by the thickeners used and their interactions with the other recipe components.
Read The Influence of Thickeners on the Application Method of Automotive Coatings and Paper Coatings – Rheological Investigations with the HAAKE CaBER 1, which discusses the rheological results of elongational and shear experiments on three different classes of thickeners: Acrylate thickener, associative thickener, and CMC (CarboxyMethylCellulose) thickener. In the experiment described in this application note, the various thickeners displayed characteristic differences. The results indicate that the behavior of automotive and paper coatings cannot be predicted by the simple characterization of the pure thickener. The properties of these complex formulations are controlled by the balance of the interactions between the dissolved thickeners and the pigments or the binder.
Read more details about this study in Part 2 of this post, to be published on Thursday.
Additional recommended reading:
- Investigating the Thixotropic and Shear Recovery Behavior of Paints and Coatings Using the Thermo Scientific HAAKE Viscotester iQ
- Do Powder Coatings Have the Same Rheological Properties During and After the Cure?
- How Do Powder Coatings Compare with Paint?
- Metal Finishing with Powder Coatings