Paper Number
BC6 

Session
Blends, Composites and Multiphase Systems

Title
Advances in characterization of rheology at high strain rates related to paint atomization

Presentation Date and Time
October 7, 2014 (Tuesday) 10:00

Track / Room
Track 5 / Washington B

Authors (Click on name to view author profile)

1. Houze, Eric C. (Axalta Coating Systems)
2. Koerner, Michael R. (Axalta Coating Systems)
3. Moore, John R. (Axalta Coating Systems)
4. McKinley, Gareth H. (Massachusetts Institute of Technology, Department of Mechanical Engineering)
5. Keshavarz, Bavand (Massachusetts Institute of Technology, Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Eric C. Houze¹, Michael R. Koerner², John R. Moore¹, Gareth H. McKinley³, and Bavand Keshavarz^3
1Axalta Coating Systems, Wilmington, DE 19803; ²Axalta Coating Systems, Mount Clemens, MI 48043; ³Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307

Speaker / Presenter 
Houze, Eric C.

Text of Abstract
Paint atomization is a nonlinear time-dependent process that involves very high shear and strain rates. Historically, industry researchers have used high shear viscosity as a predictor for atomization quality because there were no commercially available instruments capable of measuring extensional viscosity of ready-to-spray paints under realistic conditions. This sometimes leads to poor predictions of the suitability of formulations for spray application. Studies using the Capillary Breakup Extensional Rheometer (CaBER) have been previously used to assess capillary thinning and filament formation dynamics. This can often be related to the droplet formation process during paint spraying but is often limited as a result of relatively low relaxation times observed in commercial paints. More recently we have developed a jet-based approach to measure extensional rheology. Stroboscopic recordings of the thinning dynamics in a periodically forced free-stream paint jet can be analyzed to reveal local domains of visco-capillary and elasto-capillary behavior. Fitting these models to time-resolved measurements of the jet diameter provides characteristic measures of the paint rheology (specifically the viscosity and fluid relaxation time) at high deformation rates. Characteristic relaxation times of waterborne paints can be successfully determined down to less than 60 µs using this jet rheometry approach. We explain the test methodology used to characterize the paint rheology and share data collected from both model fluids and commercial coatings. To relate these measurements to heuristic assessments of paint ‘sprayability’ we use strobed shadowgraphy methods to collect droplet particle size distributions in a spraying process. We show that the droplet size distribution in our paint sprays is well described by a single gamma distribution as described by Villermaux et al for Newtonian fluids.