(426g) Effect of Time-Dependent Thixotropic Fluids on Slot-Die Coater Flow Dynamics, Viscosity Recovery, and Operating Conditions

Slot-die coating is a precise, scalable, and pre-metered technique widely used to deposit thin, uniform films onto substrates. It is suitable for applications across industries such as photographic film, battery electrodes, photovoltaics, transparent conductive films, and catalyst layer deposition in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). Because of its versatility, slot-die coating is commonly employed for a broad range of complex inks, many of which are multicomponent mixtures comprising polymer solutions, alcohols, metallic solids, and colloidal suspensions. These complex formulations form microscale networks of particles and entangled polymer chains that give rise to diverse non-Newtonian behaviors, including shear-thinning, extensional thickening, viscoelasticity, viscoplasticity, and thixotropy.

In this presentation, we investigate the thixotropic behavior of platinum-on-carbon (Pt/C) ink used in proton exchange membrane fuel cells (PEMFCs) and its impact on flow dynamics, wet-film viscosity recovery, and slot-die coating windows using Goma, a finite-element multiphysics simulation code. To evaluate the influence of time-dependent viscosity on operating conditions, we compare two rheological models: a steady-state Carreau–Yasuda model and a time-dependent first-order structural kinetics model.

Simulations for the baseline PT-C ink reveal that while overall flow characteristics within the die and coating gap remain similar between models, the thixotropic model exhibits markedly reduced viscosity recovery. Further simulations for a fluid of slow structure recovery and breakdown rates show a significantly altered vortex formation within the die. This changes the viscosity distribution throughout the slot and wet film and leads to the development of a persistent viscous strand. These structural effects culminate in a substantial increase of up to 15% in the vacuum pressure required to maintain stable, defect-free coating.

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