In the R&D and quality control of materials science, obtaining a film layer with accurate thickness and good uniformity is the basis for subsequent spectral analysis, mechanical property testing, and durability evaluation. Laboratory film applicators with appropriate scraping equipment provide a technical platform for reproducible preparation of film layers. This paper discusses how to improve the precision of laboratory coating film from the dimensions of equipment selection, process parameter control, and common problem handling.
Functional positioning of the coating machine
The main function of laboratory coating machines is to provide a stable and adjustable coating platform to eliminate the instability caused by human operation. By replacing manual scraping with mechanical motion, the applicator ensures consistent scraping speed, angle, and pressure, which is important for obtaining reproducible experimental data. Usually, the film coating machine integrates a vacuum adsorption platform to fix the flexible or easily deformed substrate to ensure the flatness of the substrate during the coating process.
Types of film applicators and selection basis
To prepare a precise film layer, it is necessary to select the appropriate film applicator according to the rheological characteristics of the material and the target film thickness range. The following table compares the characteristics and applicable scenarios of two commonly used laboratory applicators.
| Applicator type | Features and applicable scenarios |
|---|---|
| Wire rod (wire wound type) | Based on the Mayer principle, the film thickness is determined by the winding gap and is suitable for low to medium viscosity systems, and ultra-thin coatings (up to 1.5 μm) can be prepared. |
| Scraper (gap type) | By adjusting the gap between the scraper and the substrate to control the film thickness, it is suitable for high viscosity and high volatilization rate systems, and is not easy to cause scratches. |
In the specific selection, if the slurry viscosity is high or the solvent volatilizes quickly, the scraper applicator can usually provide a better leveling effect. For samples that require precise control of ultra-thin thicknesses in the nano or micron scale, wire stick applicators are more suitable.
Precise control of key process parameters for film making
Obtaining precise film layers is not only about choosing the right equipment, but also about the fine control of process parameters. These parameters interact with each other and together determine the final film formation quality.
1. Coating speed and shear force
Although the coating speed itself does not directly change the theoretical wet film thickness for wire rods and scrapers, it affects the leveling behavior of the coating and the final film uniformity. Too fast speed may cause streaks or bubbles on the surface of the coating; Slowing down can lead to thickening of the edges. For non-Newtonian fluids, changes in shear rate can significantly affect viscosity and, in turn, leveling.
2. Environment and substrate status
Ambient temperature and humidity directly affect the drying rate and rheological behavior of coatings. The surface energy, cleanliness and flatness of the substrate are also critical. For example, if the surface tension of the substrate is too low, the paint may not spread, resulting in shrinkage. Cleaning the substrate with an ionized air purge or adhesive rag before coating helps reduce particle contaminants adsorbed by static electricity.
3. Solid content and viscosity of the coating
The thickness of the final dry film is determined by the thickness of the wet film and the volumetric solids content of the coating, and the relationship can be expressed as:
δdry = δWet · φ
Among them, δdryIt is the thickness of the dry film, δWetis the thickness of the wet film, φ is the volumetric solids content of the coating. If the solvent volatilizes during the preparation process, the solid content of the coating will increase, resulting in the actual dry film thickness being higher than the theoretical calculated value. Therefore, for systems containing volatile solvents, the coating operation time should be minimized or operated in a closed system.
Analysis and countermeasures of common coating defects
During the preparation of coating films, various defects may be encountered. Analyzing its causes and taking corresponding measures is a necessary link to improve the quality of the film layer.
| defect phenomenon | Possible causes and direction of investigation |
|---|---|
| Orange peel | Coating leveling is not good. It can check whether the viscosity of the coating is too high, whether the volatilization rate of the solvent is too fast, or whether the thickness of the coating film is too thin. |
| Shrinking hole (fisheye) | The substrate or coating is contaminated with low surface energy substances such as oil stains and silicone oil. Check the cleanliness of the substrate and whether the compressed air source contains oil. |
| Pinholes/bubbles | The solvent volatilizes too quickly or is stirred into bubbles. The solvent formulation can be adjusted to extend the flash drying time, or to stand for defoaming before application. |
| Uneven thickness | The substrate is uneven, the coating speed is unstable, or the scraper gap is not parallel. Check the status of the vacuum adsorption platform and calibrate the applicator level. |
| Whitting/loss of light | The ambient humidity is too high, and the solvent volatilizes and absorbs heat, causing water vapor to condense into the coating film. Control the temperature and humidity of the environment within an appropriate range. |
A systematic approach to improving film thickness accuracy
For demanding applications, such as flexible electronics or optical films, the mechanical precision of the applicator itself may not be sufficient. At this time, the concept of process monitoring and closed-loop control needs to be introduced. For example, the thickness tolerance can be controlled within a small range by monitoring the film thickness in real time through an online thickness gauge (such as a white light interferometric thickness sensor) and feeding the data back to the coating system to automatically adjust the coating pressure or gap. In the research and development stage, the actual coating amount of the coating machine can also be checked and calibrated by weighing method (combined with the calculation of paint density and solid content).
Epilogue
Preparing precise laboratory layers is a balancing process involving multiple factors such as equipment, process, materials, and the environment. Only by understanding the performance boundaries of the coating machine, reasonably selecting the film applicator according to the material characteristics, and systematically controlling various process parameters can the film layer that meets the test requirements can be stably obtained and provide reliable support for subsequent material characterization and evaluation.
References
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