(504h) Flow Kinematics of Acrylic-Based Model Clear Coat Systems Provide Real Time Measurements and Predictions of Product Performance Via Particle Tracking

Automotive coatings generally have dual role, which is to protect and decorate. In industrial topcoat applications, coating defects often arise during the flash stage which is roughly the 10-minute interval immediately following application. During flash, the solvents evaporate, and polymer crosslinking can be initiated. Fundamental understanding of the rheology of such complex systems is crucial for avoiding defects such as sag. Sag is a gravity driven phenomena that leads to material accumulation downstream and results in a non-uniform surface appearance. Our benchmarking experiments utilized a new technique, called Variable Angle Inspection Microscopy (VAIM), to non-invasively measure sag through the volume of an arbitrarily oriented thin film. Flow kinematics were investigated by measuring the velocity of silica particles in non-evaporative systems of known viscosities. The coating films were ~140 µm thick oriented at angles between 5° and 10° relative to gravity. Probe particles were tracked at speeds as high as ~100 µm/s. The flow field was well-resolved in ~10 µm thick slabs. Generally, VAIM measurements were highly reducible. Velocity profiles were predicted with a known model. The agreement between the measured and model velocities validated the effectiveness of this new method in relating material properties and flow regimes. Ongoing effort works to explore complex systems of acrylic based model clearcoats. Polymer molecular weight and initial solid content were observed to affect the rate of solvent evaporation. Evaporation of the film, represented by the decay in thickness over time, showed an exponential decrease throughout the dehydration process. The rate was observed to be inversely proportional to the polymer molecular weight. Evaporation rate was highest for the lowest molecular weight polymer. The added complexity in evaporative systems is predicted to affect the kinematics of falling liquid films. Tracking probe particles throughout the volume of film via VAIM will provide fundamental understanding of the drying process where particle velocities are expected to decrease in regions of film where polymer networks originate and propagate.