High-viscosity slurries are widely used in energy storage, functional coatings, and composites. The core challenge of the coating process is that the high cohesion and yield stress within the slurry cause its leveling and spreading behavior on the substrate to differ significantly from that of low-viscosity fluids. Squeegee coating is a classic pre-dose coating technique that precisely controls wet film thickness by adjusting the gap between the squeegee and the substrate, especially for high-viscosity non-Newtonian fluids. The key to process success is to understand the synergistic relationship between the rheological properties of the slurry and the parameters of the coater and to set it up systematically.
Rheological properties of slurry
High-viscosity slurries typically exhibit non-Newtonian fluid behaviors such as shear thinning (pseudoplastic) or Bingham plasticity. The apparent viscosity (η) varies with the shear rate (γ̇), which is commonly described by the power law model: τ = K * γ̇^n, where τ is the shear stress, K is the consistency coefficient, and n is the flow index (n<1 represents shear thinning). During the coating process, high shear is generated in the gap between the scraper and the substrate, and the viscosity of the slurry is temporarily reduced, which is conducive to spreading. After the shear stops, the viscosity is restored, which helps to fix the coating shape and prevent sagging. Therefore, the parameter setting needs to revolve around "using the shear field" and "controlling the relaxation process".
Parameter setting tips
Coating parameter setting is a multivariate optimization process that takes into account the slurry characteristics, target coating thickness, and substrate properties.
Scraper clearance and angle
The scraper clearance (H) is the most direct parameter for controlling the thickness of the wet film. For high-viscosity slurries, the actual wet film thickness (h) is usually less than the theoretical gap, and the relationship can be approximated as h ≈ k * H, where k is the transfer coefficient related to the viscosity and coating speed of the slurry (0<k<1). The gap setting should be slightly larger than the target dry film thickness divided by the estimated wet film thickness of the slurry solid content. The scraper angle (θ) affects the shear strength and pressure distribution of the slurry, and a smaller angle (e.g., 30°-45°) can provide a gentler shear field and reduce the damage to the functional particle structure in the slurry, but may require higher coating pressure to ensure filling.
Coating speed and substrate tension
The coating speed (v) directly affects the production efficiency and shear rate (γ̇≈ v/h). Increased speed enhances shear thinning and reduces operating viscosity, but too high a speed can lead to coating defects (e.g., streaking, drawing) or insufficient slurry supply. For high-viscosity slurries, medium and low speeds should be used at the beginning, and gradually increased after the process is stable. The tension of the substrate needs to be kept uniform and stable, insufficient tension can easily cause the substrate to wrinkle, and excessive tension may lead to tensile deformation of the substrate, affecting the uniformity of the coating.
Feeding and back pressure control
A stable slurry supply is a prerequisite for a uniform coating. It is recommended to use a precise dosing pump or pressure tank to ensure a stable, full slurry "meniscus" is formed before the scraper. For slurries that are very easy to settle, the feeding system should have a gentle stirring function. Back pressure refers to the static pressure of the slurry upstream of the scraper, which is controlled by adjusting the feed rate or using a baffle. Moderate back pressure helps the slurry fill the scraper gap adequately, but too high a back pressure can cause the slurry to spill out within the gap, creating edge thickening or "ink buildup".
Drying conditions are connected
The wet film after application should be immediately put into the drying procedure. For high-viscosity slurries, due to the slow migration rate of solvents, gradient heating should be used for initial drying to avoid bubbles or cracks caused by too fast surface crusting, resulting in the inability of the internal solvent to escape. The temperature and wind speed settings in the first area of the drying oven should match the coating speed to ensure that the wet film has been preliminarily shaped before entering the high temperature area.
Common problems and countermeasures
Parameter settings should follow the principle of "single variable, gradual adjustment". It is recommended to start with the reference parameters, prioritize fixing the scraper clearance and angle, optimizing the coating speed and feed pressure, and finally adjusting the tension and drying conditions. Frequently asked questions and adjustment directions are shown in the table below.
| Coating phenomenon | Possible causes and adjustment directions |
| Longitudinal stripes | contamination or damage to the blade edge of the scraper; Slurry aggregate blockage; Reduce viscosity or improve shear. |
| Uneven coating thickness | substrate tension fluctuations; scraper parallelism deviation; The feeding pressure is unstable. |
| Edges thickened | the back pressure at both ends of the scraper is too high; Reduce the gap or reduce the feeding pressure appropriately. |
| Wet film drawing | The elastic composition of the slurry is too high; the coating speed is too fast; Increase the temperature or adjust the recipe appropriately. |
| Bubble embedding | the slurry is not completely defoamy; Air is caught in the feeding process; Reduce the feed flow rate or increase the defoaming process. |
Summary
Squeegee coating of high-viscosity slurries is a technique that requires strict process control. The core of success lies in using the rheological properties of the slurry as the basis for parameter setting, and by finely controlling key variables such as scraper clearance, angle, coating speed, feeding, and back pressure, the shear field of the coating and the relaxation characteristics of the slurry are balanced. Continuous process monitoring and phenomenon-based parameter fine-tuning are necessary to achieve uniform, stable, and high-performance coatings.