(583a) Impact of Resin Molecular Weight on Drying Kinetics and Sag of Coatings

Automotive coatings generally have a dual role, to both protect and decorate, and coating defects tend to compromise both functions. Coating defects often arise during the flash stage of the manufacturing process, which is the roughly 10-minute interval immediately following application of the coating. Following application, the solvents begin to evaporate and the coating experiences rapid changes in both rheology and kinematics, both of which will contribute to the formation of defects. This presentation will summarize a project focused on measuring the impact of resin molecular weight and solvent choice on drying kinetics and sag defect velocity in polymer films. These films, ranging in thickness from ~60 micrometer to ~120 micrometer were formulated with 45% by weight polymer resin in one of two solvent packages with different relative evaporation rates (RER). Results from these measurements showed that for fixed molecular weight the drying kinetics increased by approximately two-fold for the high RER solvent, whereas the apparent diffusivity tended to increase with increasing polymer molecular weight. Films formulated from higher molecular weight resins had greater initial viscosities and thicknesses for identical draw down blade clearance. By extension, the higher apparent diffusivities at greater molecular weights were attributed to effects of prolonged evaporation times for the thicker films. Flow field measurements showed an increase in sag velocity for thinner and less viscous films, which was somewhat surprising both because a thinner film will experience lower gravitational stress and quicker drying times as compared to a thicker film. From these data we conclude that formulating a coating with higher molecular weight resin, although likely to increase drying time, will tend to deter sag because of the large impact of viscosity on these phenomena.