The application of hot air drying technology can significantly accelerate the drying speed of the coating and has the characteristic of being highly adaptable. It is a widely used form of drying. In large furniture factories, the hot air drying line is currently the main form of forced drying for solvent-based coatings. It is used to dry water-based paints. Due to the long drying time of water-based coatings, it is necessary to study the drying equipment on the one hand and the drying process on the other. The temperature, humidity, operating speed (drying time) and ventilation conditions of the various parts of the drying line will affect the drying quality and the final coating film formation as an important key.
The extinction principle of the surface of the coating film
The decrease in gloss on the surface of the coating film is caused by the formation micronized ptfe resin of tiny irregularities on the surface of the dried coating film, which diffuse reflection of light from people.
During the coating process, the surface of the newly applied coating film is not very smooth. Due to the effect of surface tension, it tries to keep the minimum surface area and quickly becomes a smooth wet film. The use of leveling agent accelerates this process. Before the surface of the coating film is dry, the gloss is often very high, and the surface of the dried coating film forms tiny irregularities and becomes a rough solid surface. Han-sen proposed the definition of surface roughness,
A——The real area of the zigzag surface;
A.—The projected area of A;, the geometric shell area. For liquids, I=1.00, and for solids, I≥1.00, and the surface of the matting coating is slightly uneven (that is, rough).
The solvent evaporates and the paint film shrinks to form a micro-coarse surface
T film coating-substrate
The shrinkage of the coating film and the microscopic surface morphology of the dry film
There are generally two conditions for the formation of small unevenness on the coating surface: a sufficient amount of matting agent particles with appropriate particle size exist in the wet film of the first product, and the volume shrinkage of the second film during the drying or curing process. If one condition cannot be fully satisfied, the extinction effect is unsatisfactory. Traditional solvent-based coatings have a volatile content between 30% and 80%. Therefore, during the drying or curing process, as the volatiles volatilize, the coating film shrinks significantly and the matting is easier. Water-based paint uses water as a diluent. During the drying or curing process, due to the evaporation of water, the coating film shrinks significantly, so the matting is not difficult. With strict environmental protection requirements, coating technology has been gradually reducing the content of volatile organic compounds (VOC). For example, UV-curable coatings contain almost no volatile solvents and have a curing shrinkage of less than 10%, while powder coatings have no organic volatiles. In this way, during the drying or curing process, the coating film shrinks little or not. The matting of this kind of coating must use a special matting agent, and still need to form tiny bumps on the surface of the coating.
The matting agent is used to form tiny bumps on the surface of the coating. This is only an optical unevenness, which is invisible to the eyes. The average roughness R. is used for quantitative characterization.
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