Squeegee coating is a wet coating technique widely used in the preparation of functional coatings by uniformly coating a slurry containing solid particles such as ceramics, polymers, or metal oxides onto a substrate, which is subsequently dried and heat treated to form a coating. In the preparation of porous coatings, scraper coating has become one of the mainstream methods due to its easy operation and strong controllability of coating thickness. The core of this is to form a specific size, distribution and connectivity pore structure in the coating by controlling the slurry composition and coating process parameters, which directly affect the specific surface area, permeability, adsorption and mechanical properties of the coating.
Influencing factors and control mechanism of porosity
Coating porosity is defined as the percentage of pore volume in the total volume of the coating, and its control is a multi-factor coupling process. The main influencing factors can be divided into two categories: slurry formulation parameters and coating process parameters.
In terms of slurry formulation, the particle size distribution, morphology and concentration of solid particles are the basis. A wider particle size distribution tends to help create more gap pores of varying sizes as particles accumulate. The type and amount of binder affect the connection strength and pore retention ability between particles. Too much binder can clog pores, while too little can lead to insufficient coating strength. In addition, pore-forming agents (such as polymer microspheres, starch, etc.) are often added, which decompose or volatilize during heat treatment, leaving preset pores. The relationship between porosity P and these factors can be approximated as:
P ≈ (Vpore / (Vsolid + Vbinder + Vpore)) × 100%
where VporeIt is mainly contributed by the particle accumulation gap and the decomposition volume of the pore-forming agent.
In terms of process parameters, the height of the scraper gap directly determines the thickness of the wet film, which in turn affects the thickness and pore structure of the dry film after drying and shrinkage. The coating speed affects the shear rate and leveling behavior of the slurry under the knife, and too fast speed may lead to defects, and too slow can easily cause premature evaporation of solvent. Drying rate is particularly critical, as slow drying is conducive to uniform particle arrangement and pore formation, while rapid drying may cause cracks or closed pores due to uneven stress.
Process parameters
In order to achieve precise control of porosity, the following key parameters need to be optimized and matched by the system.
| Control parameters | The main direction of influence on porosity |
| Scraper clearance height | Increasing the gap usually increases the coating thickness, potentially increasing the total pore volume, but needs to match the rheology of the slurry. |
| Solid content of slurry | The solids content decreases, the particles accumulate more loosely after drying, and the porosity tends to increase, but strength may be sacrificed. |
| Types and contents of pore-forming agents | The content of the pore-forming agent is positively correlated with the pore volume left after decomposition, and its decomposition temperature needs to be compatible with the process. |
| Drying temperature curve | Gradient heating is conducive to the gentle removal of solvents and the decomposition of pore-forming agents, avoiding pore collapse and maintaining pore connectivity. |
| Slurry viscosity | Moderate viscosity ensures uniform coating and stable suspension of particles, and too high or too low may lead to uneven pore distribution. |
In practice, design experimental methods, such as response surface method, are usually used to establish a quantitative model between the above parameters and the porosity, average pore size and distribution of the final coating, so as to determine the optimal process window.
Characterization methods
Accurate characterization of the prepared porous coatings is a prerequisite for verifying process effectiveness and achieving quality control. Commonly used porosity characterization methods include:
1. Archimedes Method (Liquid Impregnation Method): Calculates the porosity of the opening porosity by measuring the change in the quality of the coating before and after impregnation. This method is easy to operate but not sensitive to closed cells.
2. Gas adsorption method (such as BET method): Calculate the specific surface area and pore size distribution by nitrogen adsorption isotherms, especially suitable for nanoscale and mesoporous analysis.
3. Microscopic imaging: Use a scanning electron microscope to directly observe the coating section, and quantitatively or quantitatively analyze the pore morphology, size and distribution through image analysis software.
It is recommended to use a combination of methods in production to fully evaluate the porosity properties of coatings. At the same time, porosity should be tested in relation to the coating's critical application properties, such as penetration flux, filtration efficiency, or conductivity, and internal quality control standards should be established.
Conclusion
Controlling the porosity of porous coatings during scraper coating is a systematic project involving slurry science, rheology, and drying kinetics. By carefully designing the slurry formula and precisely controlling the process parameters such as scraper gap, coating speed and drying conditions, the porosity, pore size distribution and pore connectivity of the coating can be effectively controlled. Future research trends may focus on developing more environmentally friendly water-based slurry systems, using in-situ monitoring technology to provide real-time feedback on the status of the coating formation process, and optimizing complex multivariable process parameters through machine learning algorithms to further improve the consistency and designability of porous coating performance.