In recent years, with the rapid rise of the new energy industry and the continuous development of the precision electronics industry, including lithium-ion battery electrodes, hydrogen fuel cell electrodes, flexible solar cells, optical films of various flat-panel displays, flexible printed circuit boards, etc., higher and higher requirements have been put forward for coating technology and coating processes, and the application field of precision coating technology has been continuously expanded.
Coating equipment
The technical advancement of coating equipment mainly examines four aspects: coating technology, tension technology, deviation correction technology, and drying technology.
Coating technology: It needs to meet the production requirements of different thicknesses. Now the thickness of positive lithium battery aluminum foil has been as thin as 6-8 microns, and the thickness of negative lithium battery copper foil has been as thin as 4.5-6 microns. The diaphragm coating is only a few microns, and the graphene coating is even thinner. Different thicknesses also require different coating methods to be developed for customers to ensure that the coating thickness of the slurry is controlled below 2 microns.
Tension technology: As the web moves along the coating direction, it is inevitable that the tension is uneven, resulting in a lack of consistency in coating quality. Therefore, it is necessary to ensure that each section of the sheet path has good tension control during operation.
Correction technology: As the length of the coating equipment is usually tens of meters, position deviation will occur during the operation of the sheet path. In order to ensure that copper film, aluminum film or very thin diaphragm can run smoothly and effectively on the sheet path and achieve precision coating, different drive forms need to be selected with a responsive control system for correction.
Drying technology: The speed bottleneck of coating production lies in drying. The most direct means is to lengthen the bellows, but it will increase costs and space. After strengthening, it is also necessary to enhance the deviation correction and tension control. To further improve the drying efficiency, it is necessary to improve the control of the wind field, the control of the temperature field, and the layout form, and try to reduce the length of the bellows while ensuring the coating speed.
Factors affecting coating quality
There are many factors affecting coating quality, including people, machines, materials, methods, and environment, but the basic factors are several conditions directly related to the coating process: coating substrate, adhesive, coating steel roller/rubber roller and laminating machine, etc.
1) Coating substrate: mainly material, surface characteristics, thickness and uniformity, etc.
2) Adhesive: mainly its working viscosity, affinity and adhesion to the substrate surface, etc.
3) Coating steel roller: It is both the direct carrier of the adhesive and the supporting reference of the coating substrate and rubber roller, so it is the core of the entire coating mechanism. Its geometric tolerance, rigidity, dynamic and static balance quality, surface quality, temperature uniformity and thermal deformation conditions all affect the uniformity of coating.
4) Coating rubber roller: The rubber roller is an important variable of coating quality. Its material (such as the life of the rubber layer), hardness, geometric tolerance, rigidity, dynamic and static balance quality, surface quality, thermal deformation condition, etc. also affect the uniformity of coating.
5) Composite machine: It is the basic platform of coating. In addition to the accuracy and sensitivity of the coating steel roller and the rubber roller pressing mechanism, it also includes the maximum design speed and the overall stability of the machine.
The factors affecting the uniformity of coating in the horizontal and vertical directions are different, so the control measures are also different accordingly. In the specific implementation, it is related to the design and manufacture of the machine, as well as the operation and process control.
Coating process selection
Comma scraper coating process: suitable for high viscosity glue, thicker coating, smooth coating surface, coating thickness (wet glue) 20~450μm, glue viscosity 1000~50000cps. It is mostly used in protective film, functional tape, multifunctional coating, etc.
Pictured: comma scraper coating operation principle
Gravure coating process: Gravure coating has precise measurement, simple operation, smooth coating surface, fast coating speed, coating thickness (wet glue) 10~25μm, and glue viscosity 10~2000cps.
Pictured: Gravure coating process operation principle
Slit coating process: Its working principle is that the coating liquid is squeezed and sprayed along the gap of the coating mold under a certain pressure and flow rate and transferred to the substrate. Compared with other coating methods, it has the characteristics of fast coating speed, high precision, and uniform wet thickness; the coating system is closed, which can prevent pollutants from entering during the coating process, high slurry utilization rate, can maintain the stability of slurry properties, and can carry out multi-layer coating at the same time.
Pictured: Sheet coating
Pictured: Curtain coating
Extrusion coating process: Extrusion coating is a process in which a fluid is extruded through a coating head with a special flow channel and coated on a moving substrate. According to the distance between the coating head and the substrate and the fluid form formed between the coating head and the substrate, it is divided into slot coating and extrusion coating, as shown in the figure below.
The main difference between the two is whether the fluid wets the lip. The former wets the lip, while the latter does not. The coating process of lithium-ion power batteries adopts slit extrusion coating (solt coating).
Introduction to the principle of slit extrusion coating: The two-dimensional schematic diagram of the flow distribution of slit extrusion coating is as follows:
The factors that determine the coating quality mainly include: the gap between the coating head and the base belt, flow rate, substrate speed, coating head structure, fluid characteristics, etc.