Thixotropic Rebuild Kinetics Governing High Speed Slot Die Coating Instabilities
Thixotropic rebuild delay shifts slot die bead rupture limits during high speed web acceleration.
Micro Ribbing Defect
Meaning
Periodic surface-pattern irregularities appearing as continuous lengthwise ridges along a coated wet film constitute a common instability in slot-die processes. When slurry flow fails to stabilize in the coating bead, a micro ribbing defect arises, showing thin lines of alternating thickness in the coating direction. This phenomenon occurs when the capillary number exceeds a critical threshold for the given gap-to-thickness ratio.
The resulting uneven wet thickness causes non-uniform drying rates and localized density variations.
Capillary Instability
The balance between viscous forces dragging the fluid and surface tension holding the meniscus determines the onset of these lines. Low slurry viscosity or high coating speed increases the likelihood of meniscus deformation.
Drying Disturbance
During the solvent evaporation phase, the thin regions between the ribs dry faster than the thicker peaks. This differential drying creates internal stresses that lead to micro-cracking and poor adhesion of the active layer to the current collector foil. Such cracks act as starting points for mechanical delamination during the subsequent cell assembly processes.
Mitigation Protocol
Prevention of this problem requires reducing the coating speed, increasing the wet thickness, or adjusting the slurry formulation to change its surface tension. Optimizing the lip gap also helps stabilize the meniscus against the pressure fluctuations of the slurry feed pump.