Introduction
In the process of R&D and performance evaluation of coating materials, film making is a key pretreatment step. Film applicators and spray guns are two mainstream laboratory film making tools, and their working principles and operating characteristics are significantly different. These differences directly affect the thickness uniformity, surface morphology and internal structure of the coating samples, which in turn have a systematic impact on the subsequent adhesion, hardness, corrosion resistance and other performance test results. The purpose of this paper is to compare and analyze the technical characteristics of the two film preparation methods, and to explore the possible impact of these methods on coating performance evaluation, so as to provide a reference for the standardization and data comparison of experimental protocols.
Comparison of working principles
Coating machines, typically referring to automatic coating machines, apply coatings evenly to the surface of the substrate at a constant speed and pressure through precision-controlled scrapers or winding rods. Its core advantage is the ability to prepare highly uniform thickness, accurately repeatable wet films, and wet film thickness (WFT) can be formulatedh = k × GMake estimates, whichhis the thickness of the wet film,kis the constant related to the rheological properties of the coating,GIt is a gap in the coating. This approach minimizes human manipulation variables.
The spray gun uses compressed air to atomize the paint and spray it onto the surface of the substrate. Its film-forming process involves the formation of droplets, flight, impacting the substrate, spreading, and fusion. The final film thickness and uniformity are affected by multiple factors such as gun caliber, air pressure, spraying distance, moving speed and paint viscosity, and the variable control is more complex. Film thickness is usually formed by multiple sprays cumulatively, and its uniformity may be macroscopically inferior to that of film coating machines, but closer to some actual construction processes.
Different film preparation methods can physically alter the coating, which can affect the performance test results. Here's a breakdown of the key impacts:
Film thickness uniformity and test consistency
The film thickness of the sample prepared by the coating machine fluctuates less in the lateral and longitudinal directions. When performing hardness tests (e.g., pencil hardness) or wear tests, the data dispersion is usually low, which facilitates accurate formulation comparisons. However, the film thickness of the sample prepared by the spray gun may have gradients or local differences, and the multi-point measurement value on the same sample may fluctuate greatly, requiring an increase in the number of sampling points to ensure the representativeness of the results.
Surface topography and optical/protective properties
The coating surface formed by the coating machine is relatively smooth, depending on the squeegee accuracy and the leveling of the coating. The coating surface formed by the spray gun may exhibit a slight "orange peel" appearance due to the atomization droplet fusion process. This difference in microscopic morphology directly affects the gloss measurement of the coating and has a potential impact on the initial adhesion and penetration path of the corrosive medium in salt spray resistance, water resistance, and other tests.
Coating internal structure and mechanical properties
The atomization spraying process of the spray gun may bring more air into the coating or create a different internal pore structure under the rapid volatilization of the solvent. In contrast, the scraping of the coating machine has less disturbance to the paint structure. This difference in internal structure can lead to different trends in adhesion (pull-apart method), flexibility (shaft bending test), and electrochemical impedance tests prepared by both methods.
Applicable scenarios
The selection of film production method should be closely focused on the experimental purpose. Film coaters are suitable for R&D phases where high repeatability is required for precise control of formulation variables, or for specific industrial coating processes such as simulated roll coating. The spray gun is more suitable for evaluating the adaptability of coating application, simulating the actual spraying process effect, or studying the influence of spraying parameters on coating performance. In standard tests, the membrane preparation method used and its specific parameters should be clarified and documented.
Summary
As different film-making methods, film coating machines and spray guns have their own technical characteristics and applicable fields. The coatings prepared by them differ in uniformity, surface morphology and internal structure, which are systematically transmitted to subsequent performance evaluation results. In laboratory research, clarifying the film-making process is a prerequisite for ensuring data accuracy and comparability. It is recommended to record the type of film-making equipment, key process parameters, and environmental conditions in the experimental report or research paper in detail to build a more rigorous and reproducible coating performance database.