Watching Paint Dry with Passive Microrheology
Author: Selwin M. Varghese
Publisher:
Published: 2017
Total Pages:
ISBN-13:
DOWNLOAD EBOOKCoatings are complex fluids that are present in every aspect of our lives, and are primarily used to protect and decorate materials, such as automobiles, houses, and industrial structures. The U.S automotive coating industry is expected to have revenues of $3.5 billion by 2025, according to a report by Global Market Insights.The drying step in the coating application process is crucial to creating a well performing product. For paints used in automobiles, drying involves a "flash" stage after which the product is cured. During flash, solvent evaporation occurs in paints exposed to the atmosphere. Paint remains fluidic for a portion of the flash step and semi-solid for the remaining portion, thereby allowing defects to form and solidify during flash. The rheological properties, and how these properties relate to flows during drying, are crucial to controlling defect formation during flash and ultimate performance of the coating. This work studies the transient rheological behavior of a drying thin film of paint during flash.We used passive microrheology to probe both steady (non-drying) and unsteady (drying) conditions and determined how parameters such as film thickness affect the properties of the film. We found that microrheology can be conducted in both clear and cloudy coatings, using fluorescent micro particles. An algorithm was developed to account for convection while drying. Drying behavior could be observed from the decrease in logarithmic slopes of the mean squared displacement (MSD), which indicated an increase in viscosity over the course of the drying period. Using a confocal microscope, non-drying and drying measurements were conducted in 3D. And finally it was discovered that drying could be observed as a function of film thickness for 100 and 150 μm wet films.
