In order to make the test results of the coating film accurate and reliable, it is necessary to prepare a standard coating film that meets the requirements. According to the provisions , preparing a uniform coating film with a specified thickness on a specified substrate is the basis for coating film testing. The prepared coating film should be able to truly reflect the essence of the coating film, even if there are defects, it must be reflected, but the essence of the coating film cannot be changed due to external reasons, such as the environment .
In order to obtain a uniform coating film model, attention should be paid to the selection and treatment of the bottom plate, the preparation method and conditions. The material of the bottom plate is selected according to the product standard, and the surface treatment must meet the requirements. When preparing a coating film, the viscosity of the coating, preparation method, ambient temperature and humidity, drying conditions and time, etc. must strictly abide by the specified requirements.
All countries have established standard methods for coating film preparation. CHINA's national standard GB/T 1727-1992 "General Preparation Method for Paint Films" stipulates the materials for preparing general coating films, surface treatment of bottom plates, plate making methods, drying and drying of coating films. State adjustment, constant temperature and humidity conditions, and coating film thickness, etc. The plate making method lists the brush coating method, spray coating method, dip coating method, scrape coating method, uniform paint film preparation method and pouring method. Among them, paint film preparers (blade coater and wire Bar Coater) are commonly used preparation instruments.
The instrument used in the doctorblade method is called a doctorblade or a paint Film Applicator, as shown in Figure 4-3-26. During operation, the sample is poured on the bottom plate, and the sample is flattened with a scraper. Because there is a certain gap between the scraper blade and the plane, a wet film with a certain thickness can be obtained. According to the needs of the test, the gap between the blade and the plane can be adjusted to make paint films of various thicknesses. Generally speaking , the wet film thickness of scrape coating is only half of the gap distance between the scraper and the bottom plate, and the success of the scrape coating method depends on the flatness of the bottom plate and the quality of the scraper, otherwise there will be rippled coating film or other Irregularities .
Later, a wire-rod scraper was developed, which has two forms. One is a metal rod, on which a metal wire is tightly wound, and the paint flows on the sample plate through the space formed by the metal wire. The thicker the metal wire, the larger the space, and the thicker the paint film . This drawdown applicator is especially suitable for flexible substrates such as paper or thin metal sheets. Another form is a nylon rod that is threaded directly onto the rod. You can use a black rod for a light paint or a white rod for a dark cleaning.
In order to make the operation of the squeegee coater smooth and uniform, and to eliminate man-made operating errors, an automatic paint Film Applicator driven by a motor has been developed to make the squeegee-coated paint film more uniform. The one shown in Figure 4-3-27 is one of them, see ASTM D 823-1995.
