Coating rod definition and basic concept
A coating rod, also known as a wire rod, scraper rod, Meyer rod, dosing rod, or winding rod, is a tool used in laboratories to evenly coat coatings on the surface of a substrate, either manually or with equipment. Its basic structure is a precise stainless steel round rod with a specific process to create a microstructure that can quantify the coating material. The core function of coating rods is to prepare controllable thickness, uniform and consistent wet films on a variety of substrates such as paper, plastic film, metal foil, textiles, glass plates, etc., and provide standardized samples for subsequent performance testing, formulation screening, or process research.
Working principle and technical classification
The working principle of the coating rod is based on the scraping and quantitative effect of the measuring rod on excess paint. When the coating rod is placed on the paint-soaked substrate and dragged at a constant speed under certain pressure, the precise groove or gap between the wire winding on the surface of the rod and the substrate determines the thickness of the wet film that ends up on the substrate. Excess paint on the substrate is removed by the scratching of the rod, creating a uniform liquid film behind the rod.
According to different surface forming processes, coating rods are mainly divided into the following two types:
Wire wound coating rods are a traditional and widely used form. The manufacturing process involves precisely winding stainless steel wires of a specific diameter around a stainless steel mandrel in a tight spiral, and the gap between the wires forms grooves for storing the coating. Theoretically, there is a certain proportional relationship between the thickness of the wet film and the diameter of the steel wire, and the estimation method can be expressed as:
hwet ≈ k · dwire
Among them hwet is the thickness of the wet film,dwire is the diameter of the winding wire,k is the empirical coefficient. The advantage of wire-wound coating rods is that they can achieve a wide range of thickness options.
The extrusion one-piece coating rod is directly processed into precise grooves on the surface of the stainless steel rod through the cold extrusion process, forming a storage space equivalent to the wire winding type. This construction avoids wire-wound joints, has a smoother surface, is easier to clean, and eliminates the risk of loosening or breaking the wire due to bumps.
Definition and measurement conversion of coating thickness
When preparing samples using coating rods, three different stages of thickness concepts are involved. Theoretical wet film thickness refers to the thickness value nominated by the geometry of the coating rod itself. The actual wet film thickness is the measured thickness of the paint when it has just been coated and not yet dried. Due to the influence of the rheological behavior of the slurry, the wettability of the substrate and the operating pressure during the coating process, the actual wet film thickness is usually about 60 to 70% of the theoretical value. Dry film thickness is the final thickness of the coating after it has been dried or cured and the solvent has volatilized, and it is the direct object of material property testing.
There is a conversion relationship between the dry film thickness, the actual wet film thickness, and the theoretical wet film thickness through the coating parameters. For systems without volatile solvents, the dry film thickness should be equal to the actual wet film thickness. For solvent-based systems, the commonly used empirical conversion formula is:
hdry = hwet · (Vsolid / Vtotal)
or expressed as:hdry = hwet · (Solid content)
The solid content refers to the mass or volume percentage of non-volatile substances in the coating. Therefore, in order to obtain the target dry film thickness, it is necessary to combine the solid content of the coating and push back the specification of the desired coating rod. For example, if the target dry film thickness is 20 microns and the coating solids content is 50%, the actual wet film thickness provided by the required coating rod is about 40 microns, and then a coating rod with a nominal value of about 57 microns to 67 microns of theoretical wet film thickness needs to be selected.
In experimental studies, if the coating amount needs to be accurately known, it can be measured and verified by weighing. By weighing the quality difference of the substrate before and after coating, combined with the coating area and paint density, the actual wet film thickness can be calculated:
hwet = (mcoated - msubstrate) / (A · ρ)
Among them m For quality,A is the coating area,ρ is the density of the coating.
Key factors affecting the quality of coating film
The deviation between the actual coating effect and the nominal value of the coating rod is affected by multiple experimental conditions. Coating operation is the primary factor. The pressure and drag speed applied by the operator need to be uniform and stable, and uneven pressure or speed changes can directly lead to fluctuations in the thickness of the wet film, especially for manual operation. Using an automatic Coater with a coating rod is an effective way to reduce such human errors.
The properties of the substrate are equally important. For soft substrates with uneven surfaces or slight bulges, such as some paper, textiles, and leather, the coating rod can roll the substrate flat to a certain extent during the dragging process, so as to obtain a relatively uniform coating, which is also one of the advantages of the coating rod compared with rigid scrapers. However, for ultra-thin coating films, the roughness of the substrate itself will significantly affect the actual coating effect, and substrates with high surface flatness must be used.
The rheological nature of the slurry is another decisive factor. The viscosity, surface tension, and thixotropic properties of the coating affect its fill and outflow behavior in the rod groove, as well as its leveling on the substrate after coating. High viscosity materials require greater shear forces, which can cause the actual wet film thickness to deviate more from the theoretical value. In addition, the leveling time and environmental conditions of the wet film after coating will also affect the final uniformity, and there may be fine longitudinal streaks on the surface after coating around the wire rod, which need to be eliminated by the leveling process before drying.
The condition of the coating rod itself cannot be ignored. If not cleaned in time after use, the residual paint will dry up on the rod and block the grooves, which will seriously affect the coating accuracy of subsequent use. For wire-wound coating rods, the rods with fine wire diameter should avoid bumps to prevent wire winding damage.
The main application in the field of research and development
Coating rods occupy an important position in laboratory research and development in many industries due to their flexible operation, low sample consumption, and wide thickness coverage. In the coatings and inks industry, it is a fundamental tool for formulation development and performance testing for the preparation of standard coating samples to evaluate key metrics such as hiding power, gloss, color, adhesion and weather resistance. R&D personnel use coating sticks of different specifications to quickly compare the performance of different formulations at the same thickness, accelerating the formula screening process.
In the field of new energy and electronic materials, coating rods are widely used in laboratory-scale lithium-ion battery research to uniformly coat positive and negative electrode pastes on copper foil or aluminum foil current collectors to prepare electrode pieces for buckle battery or pouch battery testing. In the research and development of printed electronics and flexible circuit boards, it is used to coat conductive inks, insulating adhesives or optical films. Such tools are also used in the preparation of barrier coatings for photovoltaic backsheets.
In the field of packaging and functional materials, coating rods are used to prepare barrier coatings on the surface of plastic films and aluminum foils to evaluate their protective effect on packaging contents. Film sticks also play a key role in the manufacturing and development of adhesive tapes, labels, and office supplies. Additionally, in the cosmetics and pharmaceutical industries, it is used for the uniform application of creams, gels, or ointments to ensure consistent distribution of active ingredients.
Points to consider when selecting a laboratory
Choosing the right coating rod in the laboratory requires starting from experimental needs and comprehensively considering multiple technical parameters. The primary factor is the target wet film thickness. The required wet film thickness is calculated according to the final dry film thickness and the solid content of the coating according to the experiment, and then the coating rod with the corresponding specifications is selected. Different brands and types of coating rod thickness series may vary and need to be selected against the product catalog.
Matching the coating width to the size of the substrate is another fundamental consideration. The total length and effective coating width of the coating rod need to be appropriate to the size of the substrate used. Common laboratory coating sticks have an effective coating width of about 300 mm, with A4 or similar substrates and narrower specifications such as 60 mm or 250 mm. Choosing a coating rod with an effective coating width slightly greater than the width of the substrate ensures uniformity across the entire width. For expensive sample R&D, a smaller coating width coating rod can help reduce material consumption.
The choice of process type involves usage habits and ease of maintenance. The wire-wound coating rod has a long history and is rich in thickness options. Extruded one-piece coating rods have the advantages of no risk of wire breakage, relatively easy cleaning, and the ability to prepare thinner coatings. In terms of material, the mainstream coating rods are made of corrosion-resistant stainless steel, which can adapt to water-based and various solvent-based systems. The diameter of the rod also needs to be taken into account, which can affect the comfort of the grip and its suitability on some automatic Coaters.
Finally, supporting equipment and accessories should not be overlooked. For better application repeatability, it is recommended to use a coating stick with an automatic Coater. At the same time, a flat substrate platform or imprinting bed, such as a glass plate and a vacuum adsorption platform, is the prerequisite for ensuring the flatness of the substrate and the uniformity of the coating film. For coatings that need to be heated and cured, a coating platform with heating function is available.