Definition and basic functions
A heated film Coater is a type of special equipment used in a laboratory environment to evenly coat liquid or molten slurry on the surface of a substrate, and simultaneously or subsequently heat and dry the wet film. At its core, it integrates two key processes, "coating molding" and "heat treatment", aiming to produce solid-state films or coatings with controllable thickness and uniform structure through precise control of the coating process and thermal history. Such equipment is widely used in the research and development of new materials, energy device preparation, functional coating research and other fields, and is an important bridge connecting material formulation design and small-scale sample preparation.
Principle
The working principle of heated coating machine varies according to different coating methods, which can be mainly summarized into the following typical types:
In the scrape heating coating machine, the principle is to fix the substrate on a flat plate with a heating function, and by precisely adjusting the gap between the scraper and the substrate, the slurry accumulated in front of the scraper is spread out by using the uniform linear movement of the scraper relative to the substrate, forming a wet film of a certain thickness. A heating module under the substrate transfers heat to the coating through heat conduction, accelerating solvent volatilization or promoting chemical reactions, allowing the film to be quickly shaped. The thickness of the wet film can theoretically be determined by the scraper gap, but the actual thickness of the dry film is related to the solids content of the slurry, the volatilization rate of the solvent and other factors.
The working principle of rotary heated coating machine (spin Coater) is different. It fixes the specimen on a high-speed rotating rotary table through vacuum adsorption, and uses centrifugal force to evenly shake off the slurry dripping on the surface of the sample to form a thin film. Its heating function is usually integrated into the upper cover, and uses radiant heating methods such as infrared lamps to bake the film during the rotation process to achieve rapid curing. In addition, there is also a slit coating heated film coating machine, which extrudes the slurry from the slit die by a precision metering pump for contact or non-contact coating on the moving substrate, and the substrate platform also has a heating function. For the coating film of hot melt adhesives, the equipment melts the solid glue by heating the scraper and storage tank, and then directly coats it, without the need for subsequent drying.
Measurement method of coating thickness and uniformity
The measurement of films prepared by heated coating machines is usually divided into in-situ monitoring during the wet film process and offline characterization after drying the film. In the coating process, for scraper or slit coating, the thickness is mainly controlled indirectly by controlling parameters such as scraper clearance, coating speed, and feeding flow. Some advanced equipment can be equipped with in-situ thickness measurement systems, such as lasers or spectral sensors, to monitor the wet film thickness in real time during the coating process and feed the data back to the control system for dynamic adjustments.
After the coating is completed, measuring the thickness of the dry film is a key step in evaluating the coating effect. Commonly used measurement methods include: contact measurement using high-precision thickness gauges; Use a step meter to create a step between the film layer and the substrate and scan its height difference. Or measure the thickness of transparent films by optical methods such as ellipsometer or spectrophotometry. Film uniformity is typically assessed by measuring the thickness of multiple points at different points of the substrate, such as the center, edges, and diagonal, and calculating their standard deviation or extreme deviation.
Key factors affecting the quality of coating film
The final film formation quality of a heated film coating machine is influenced by the interaction of multiple process parameters, and understanding these factors is fundamental to achieving high-precision film coating.
The first is the coating process parameters. For the scraping method, the accuracy of the scraper gap directly affects the initial setting of the wet film thickness, and the resolution and stability of its adjustment mechanism are crucial. The coating speed will affect the leveling and shear behavior of the slurry, and too fast may lead to streaks or lack of material on the coating surface, and too slow may cause the coating to be too thick or edge effect. In the spin coating process, the rotation speed and rotation time directly determine the magnitude and duration of the centrifugal force, which in turn controls the final thickness of the film.
Secondly, the rheological properties of the slurry, such as viscosity, solid content and surface tension, are also intrinsic factors that determine the quality of the coating film. High viscosity slurry is easier to maintain shape when scraped, but the leveling is poor; Low-viscosity slurries spread easily but are more sensitive to coating gaps and edge effects. The thixotropy of the slurry also affects its flow behavior under shear forces.
Finally, the heating temperature is the core element that distinguishes the heating coating machine from ordinary film coating machines. The heating temperature and uniformity of the substrate directly affect the volatilization rate of the solvent and the drying kinetics of the film. Too high temperature may cause the coating surface to quickly crust, and the internal solvent cannot escape and form bubbles or pinholes. Too low a temperature can lead to sagging or incomplete drying, affecting the microstructure and adhesion of the film. Different heating methods (such as bottom contact heating, infrared radiation heating, hot air drying) will also bring different heat transfer efficiency and temperature distribution.
Typical application scenarios in laboratory research
Heated film coating machines play a key role in several cutting-edge scientific research fields due to their flexible processing capabilities and integrated heat treatment capabilities. In the field of new energy, it is a common tool for the research of perovskite solar cells, which is used to scrape or slit the coating of perovskite precursor solutions on glass or flexible substrates, and to rapidly crystallize the thin film by heating, so as to prepare high-quality perovskite light-absorbing layers. Similarly, in lithium-ion battery research, it is used to evenly coat positive and negative electrode pastes on aluminum foil or copper foil current collectors, and to prepare electrode pieces for buckle or pouch battery testing by heating and drying.
In the field of functional materials and electronic devices, the device can be used to prepare semiconductor or dielectric layers with airfield-effect transistors, as well as conductive polymer films. In the field of optical films, it is used to prepare anti-reflective films or optical filter coatings. In addition, ceramic films for specific catalytic reactions can be prepared by heating the applicators, as well as diagnostic test strips and skin patches for the biomedical field. For the study of hot melt adhesives, the equipment is able to simulate the coating process in industrial production and is used to evaluate the spreadability and bond strength of adhesives on different substrates.
Points to consider when selecting a laboratory
Choosing the right heated coating machine in the laboratory requires comprehensive consideration of the match between experimental needs and equipment performance. The first is the choice of coating method. If you are primarily studying low-viscosity solutions and need to prepare nanoscale ultra-thin films, a spin-on heated film Coater may be a suitable choice. If you need to prepare functional coatings with thicknesses ranging from microns to tens of microns, and you want the process to be closer to roll-to-roll production, scrape or slit coating heated coating machines are more versatile.
The second is the compatibility of the substrate with the coating. The size and material of the substrate used in the experiment need to be clarified. The maximum substrate size that the device can accommodate (e.g., 4-inch wafer, A4 paper size, or glass of a specific length and width) determines the sample throughput for a single experiment. At the same time, for flexible substrates, whether the equipment has a vacuum adsorption function to keep the substrate flat is a detail that needs to be paid attention to. For rigid substrates, whether their thickness is within the allowable range of the equipment also needs to be considered.
The performance of the heating system is another core factor in the selection. It is necessary to pay attention to the temperature range of the heating platform (such as room temperature to 200°C), temperature control accuracy, temperature uniformity and heating rate. If the experiment involves temperature-sensitive solvents or materials, it is necessary to evaluate whether the heating method is gentle and controllable, and whether segmented or programmed temperature control functions are required. In addition, the coating accuracy of the equipment (such as scraper adjustment resolution, speed control stability), ease of operation, and adaptability of the equipment in environments such as fume hoods are also factors that cannot be ignored.