(522i) In-Situ Microrheology of Drying Paint

Paint formulation and drying conditions have a strong influence on the final performances of the coating, yet there are few measurable variables between the formulation stage to the final film performance. The fast drying of thin films of paint, such as those used in automotive coatings, results in higher potential for fluid instabilities that often lead to defect formation in the final coating. The rheological properties of paint prior to and during drying is key in understanding the transition from a fluid coating to a solid film and is necessary for defining the connection between formulation and drying conditions, defect formation, and consequently the final paint performance. Passive microrheology, tracking the Brownian motion of passive tracers in the fluid, is used to characterize the properties of coating formulations in both quiescent liquids and during drying where the evaporative flux can cause partitioning and even strong Marangoni stresses that result in internal convection, leading to superimposition of the fluid kinematics on the random fluctuations of the probe particles. In this work, we implement a de-drifting algorithm to subtract the substantial influence of convection, allowing us to probe the time-evolved rheological properties of a drying film, by removal of the convective component of the initial measured particle Mean Square Displacements. We test thin films of model automotive paints in both the quiescent state and during drying in a range of different conditions, such as composition, molecular weight and polymer concentration. The local shear thinning resulting from drying-induced convection directly influences the local rheology and final behavior of the final coating.