There are two methods to measure the cleanliness: the laboratory instrument method and the production site simple method. Instrument measurement methods include electron microscope method, laser scattering method, surface analysis instrument method and so on. These methods are mainly used when the substrate cleanliness and coating mechanical properties are particularly high, but these methods are subject to many objective limitations. At present, most of them stay in the scope of laboratory research. In the production and application of color coated sheet, the measurement of substrate cleanliness is mainly carried out by the following several on-site measurement methods.
(1) Wipe method. Wipe the surface of the steel strip with clean, white and non fluffy cloth (such as gauze), cotton and filter paper, and then observe the degree of dirt on the cloth, cotton and filter paper to judge the cleanliness of the steel strip surface. This method is simple but not accurate enough.
(2) Water drop method. It is an application of contact angle to evaluate cleanliness. Under certain conditions, the larger the diameter of the water drop (a certain volume) on the surface, the smaller the contact angle and the higher the cleanliness. That is to say, the diameter and shape of the water droplets formed on the surface are taken as the basis for comparison of cleanliness.
(3) Infiltration method. The clean metal surface is hydrophilic, so it can be completely wetted by water. When the surface of the steel strip is stained with dirt or oil, there will be a "water loss area" that is not soaked by water. However, because the metal oxide film is also hydrophilic, the effect of removing the oxide film can not be detected by infiltration method.
(4) Exhalation. Exhale to the surface of the degreased and baked steel strip sample. If it is a clean surface, the surface will form a uniform continuous mist. When the cleanliness is high, the mist spot is uniform, while there is no mist in the place with oil stain. When the surface is very smooth and clean, the mist spot will disappear in a very short time.
(5) Water cut-off method. Immerse the cleaned sample in water, make the sample perpendicular to the water surface, and then move out of the water surface. Observe whether there is water drop or no water area on the surface of the sample after leaving the water surface from the angle of 45 degrees. If there is, there is dirt on the surface of the steel strip. If the water film formed on the surface of the sample can cover all the surface evenly, it means that the cleanliness is high.