The requirements of the spraying process on the dispersion performance of the coating include two aspects: the coating is in a highly uniform dispersion state before use, and it is easy to form a highly dispersed atomization state during use. The two can be judged by two indicators of fineness and atomization performance.
1. Fineness of paint
Pigments, extender pigments and other solid components used in color paints should be evenly dispersed in the paint with tiny particles, and no granular objects should appear in the formed paint film. The index of fineness is commonly used in the paint industry to detect the degree of dispersion of pigments and other components in paint. Fineness is an important performance index of color paint, which has a great influence on the quality, gloss, durability and storage stability of the formed coating film. Pigment particles with fine particles and good dispersion can be well wetted, and the spaces between the particles are less filled by the paint. Medium pigments are not easy to precipitate and agglomerate, which improves storage stability. The simple and practical way to measure the fineness is to pinch a small amount of uniformly stirred paint with your index finger and thumb and twist it, try to feel the particle size in the paint, if you can feel the particle size, it means the fineness is poor. If there are no particles when you twist it, but there are still traces of pigments between the fingerprints, it means that the fineness is relatively high. It can also be measured with a Fineness Meter, which is to spread the paint into films of different thicknesses, and observe to what extent pigment particles can be observed when the thickness reaches. The fineness is expressed by the measured thickness value (µm), and the larger the thickness value, the higher the fineness.
At present, all countries in the world basically use the scraper Fineness Meter to measure the fineness. The scraper fineness gauge is a polished flat plate with a wedge-shaped groove from zero to several microns deep, and there are scale marks on the edge of the groove to indicate the depth of the groove. When measuring, drop the sample into the deepest part of the groove, then use a scraper with both edges polished to touch the flat plate vertically, pull the paint across the entire length of the groove at an appropriate speed, and then observe along the direction of the incident light angle of 3o° The depth at which the particles in the groove are uniformly exposed, and this thickness represents the fineness of the sample.
2. Atomization ability
The ease with which a liquid substance is dispersed into fine particles in a gas is called the atomization ability. Coatings used in the spraying process require good atomization capabilities. Studies have shown that the atomization ability of the coating is directly related to the intermolecular force of the film-forming substances that make it up, that is, it is related to the viscosity of the coating. If there is a strong cohesion between them, they will not be easily dispersed. Therefore, Trying to reduce their intermolecular force can improve their atomization ability. A commonly used method is to add a diluent to dilute the coating solution and increase the distance between the molecular groups of the film-formers to reduce the mutual attraction between them and become easy to be separated. Nitrocellulose paints and enamels are often used in this way to improve their atomization ability. However, for coatings with relatively strong intermolecular forces, satisfactory atomization ability cannot be obtained only by diluting with solvents, and excessive use of solvents will reduce the adhesion of the coating to the substrate after drying. At this time, the high pressure of compressed air or hydraulic pressure is also used to destroy the cohesion between the paint particles, which is the principle of air spraying and pressure spraying. Studies have also shown that the atomization ability of liquids is not only related to viscosity, but also related to their original degree of dispersion. Although grease is a highly viscous liquid, it is easily atomized, while cement slurry has a low viscosity, but it is difficult to be atomized. This is because the particles in the oil have been dispersed to the molecular level, so they can be easily dispersed and atomized under the action of external force, while the cement particles in the cement slurry are aggregates composed of many cement particles, and the force between these aggregates Although it is not large (expressed as low viscosity), there is a strong binding force inside the aggregate, so even under the action of external force, it is difficult to disperse it into fine particles required by the atomized state.
