Definition
A coating specimen is a specialized device used to prepare uniform and controlled coatings under laboratory conditions. Its core function is to apply liquid or semi-solid coating material to the surface of the substrate at a preset thickness, width, and speed to simulate the basic operation in the actual coating process. These instruments are widely used in the preparation of coating specimens for substrates such as polymer films, paper, metal foils, textiles, and composites, and provide standardized samples for subsequent performance evaluations such as adhesion, barrier properties, smoothness, and durability.
Principle
The basic working principle of a coating specimen relies on the relative motion between the coating head and the substrate, and regulates the thickness of the coating layer with the help of precise gap control or metering devices. Common modes include: scraper coating, which determines the thickness of the wet film by adjusting the gap width between the scraper and the surface of the substrate; Roller coating uses the gap between the metering roller and the coating roller to transfer the coating material; and slit coating, which extrudes the coating through a precision slit onto a moving substrate. During the working process, the rheological properties of the coating material, the coating speed and the stability of the gap jointly determine the uniformity and repeatability of the final specimen. The relationship between the thickness of the coated wet film and the thickness of the target dry film can be expressed as follows:
ddry = dWet × (CSolid / 100)
where ddry is the thickness of the dry film, dWet is the thickness of the wet film, CSolid It is the mass fraction of the solid components in the coating material.
Measurement method
The coating specimen itself is mainly used for coating operations, but its performance verification and calibration usually rely on the following measurement methods. First, the thickness of the wet film is measured directly by a wheel gauge or step gauge to evaluate the accuracy and consistency of the coating gap. Second, dry film thickness uses a thickness gauge based on the principle of magnetic or eddy current to non-destructively measure the coating on the metal substrate, or the cross-section is measured by optical microscopy. Third, the uniformity of the coating layer is assessed by multi-point thickness measurement standard deviation or spectral reflectometry to ensure that there are no streaks, bubbles, or flow breaks during the coating process. For high-viscosity materials, the shear characteristics can also be measured in conjunction with a rheometer to pre-set the optimal coating parameters.
Influencing factors
The preparation quality of the coating specimen was affected by the interaction of multiple variables. The surface tension and roughness of the substrate determine the spreadability of the coating solution, and low-energy surfaces can easily lead to shrinkage holes or orange peel defects. The viscosity and thixotropy of the coating material directly affect the stability of the coating thickness when the shear rate changes, and the equilibration time of highly thixotropic materials is longer. Increased coating speed can exacerbate shear thinning effects, which can cause coating thinning or edge buildup. Ambient temperature and humidity affect the coating drying path by changing the volatilization rate of the solvent, causing pinholes or cracks. The mechanical stiffness of the instrument itself, the parallelism of the guide rail and the wear state of the scraper edge will all introduce systematic deviations in long-term use.
Application:
Coated specimens play a key role in sample preparation in a variety of industries and research. In the packaging materials industry, it is used to prepare barrier film samples and test water vapor and oxygen transmission. In the electronics industry, flexible circuit board substrates are coated with insulation or conductive pastes to evaluate their electrical properties. In the field of coatings and inks, standard color plates or adhesion test plates are applied to verify the weathering resistance and bond strength of the formulation. In the field of new energy, functional coatings are applied to battery separators and electrode substrates to optimize ion transport and mechanical properties. In addition, in textile and paper processing, specific additives are applied to improve feel or printability, providing reliable process parameters for mass production.
Selection
When selecting a coating specimen, it is necessary to comprehensively consider the characteristics of the coating material, the morphology of the substrate and the intended use environment. First, the coating method is determined according to the viscosity range of the coating solution: low-viscosity solution is suitable for slit or gravure coating, and high-viscosity paste is suitable for scraper or roller coating system. Secondly, the width of the substrate and the shape of the coil or sheet determine the feeding mechanism of the equipment, the coil needs to be equipped with a rewinding unit, and the sheet adopts a vacuum adsorption platform. The coating speed range should match the laboratory flux, from low-speed manual to high-speed automatic. The clearance control accuracy and repeatability index need to meet the experimental requirements, and it is recommended to choose a model with digital display or servo adjustment. Solvent compatibility and ease of cleaning should also be considered, and corrosion-resistant stainless steel or polymer materials should be used for contact coating parts. Finally, the footprint, noise level and user-friendliness of the equipment all affect the daily use experience, and the solution with easy maintenance and easy availability of spare parts should be preferred.