Hot air drying is a kind of heating and drying method which takes hot air of 40-60℃ as the heat carrier to transfer the heat energy to the coating on the surface of the workpiece, and the coating absorbs the energy and cures into a film. Generally, electricity or steam is used as the heat source to heat the air first, and the hot air is transferred to the surface of the coating through convection to make the coating dry quickly.
When drying with hot air, the hot air around the coating is the heating medium. The coating always has a certain thickness. The heat transfer from the surface of the coating to the inner boundary takes a certain amount of time. The heat transfer rate depends on the thickness of the coating and its thermal conductivity. Therefore, when convection heating occurs, the surface of the coating is always heated first. In the early stage of drying, the surface layer evaporates the most moisture and the curing of the coating starts from the surface layer and then gradually expands to the bottom layer, leading to the final drying of the bottom layer.
At present, the commonly used film-forming materials are acrylate, polyester, epoxy resin, polyurethane, amino alkyd resin and so on. The formed coating film has a high gloss, and many applications do not need a high-gloss coating film. It is an effective method to add a matting agent to reduce the surface gloss of the coating film. For example, the gloss of furniture and decorative coating film is generally 50%~60%. It has good decorative properties, soft gloss and full hand feeling. It is loved by users. From the perspective of environmental protection, this is more in line with the requirements of human physiological hygiene and is not easy to cause human eyes. Fatigue. Hospitals and large computer rooms and other places with strict requirements for dust use matte floor coatings, and the light is soft, which provides favorable conditions for providing a quiet and comfortable environment. Matte paint is generally used for the surface of large-scale signboards, traffic signs and other coating films that need anti-flicker for safety purposes. Military equipment and facilities, such as military vehicles, tanks, armored vehicles, radars, and large artillery, use matte coatings for concealment and confidentiality, wax emulsion paint and are not easy to be discovered by the other party.
In order to increase the adhesion of the coating, matt coatings are sometimes used, such as the primer and intermediate coating of automobiles. In addition, the coil coatings used in construction are generally matte, mainly to prevent light pollution and conceal some defects of the coating film, which is also one of the purposes.
At present, the largest amount of paint matting agent is micron-sized synthetic silica. The earliest synthetic silica matting agent was produced by Davison in the United States in 1947, with the brand name Syl oid 308. It quickly replaced aluminum stearate, the main matting agent at the time, in the market. So far, micron-level synthetic silica is still the main variety of coating matting agents, with better quality and more varieties. Micronized wax is also another important type of matting agent for coatings, and aluminum, calcium, magnesium, and zinc stearates are used in small amounts. In addition, adding human body pigments, increasing pigment content, adding incompletely compatible resin components, and mechanically making the coating surface rough or surface embossing can also reduce the gloss of the coating film.
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