(1) The drying process of the coating
The drying process of the coating generally changes from liquid to solid, the viscosity gradually increases, and the performance gradually meets the specified requirements (except for powder coatings). The change process goes through three stages: surface dry , semi-dry, and completely dry.
(1) Surface drying refers to the stage when the coating film dries from a flowable state to when the finger touches the coating film lightly, the finger does not stick to the paint but the coating film feels sticky and there are finger marks on the coating film.
(2) Semi-dry is a dry state in which no fingerprints can be left when the film is lightly pressed with fingers, and it is a semi-hard dry state.
(3) Completely dry is a dry state in which no fingerprints are left on the coating film when pressed with fingers, and no scratches are left on the coating film . At this time, the coating film can resist pressure and abrasion. However, there are often specific requirements for the complete drying of different coating films . The standard method is to test the mechanical properties such as the hardness of the coating film to judge the drying degree of the coating film.
(2) The drying method of the coating
According to the specific conditions of drying, it can be divided into three types: natural drying, heating drying and special drying.
1. Natural drying
Natural drying is a common drying method, which can be applied both indoors and outdoors. The advantage is that no drying equipment is needed and energy is saved. Put the coated workpiece with the coated film at room temperature , and the wet film will gradually dry. Oxidative polymerization conversion coatings can be dried by natural drying.
This method is suitable for large-area painting outdoors. The disadvantage is that it takes a long , takes up a large area, and is greatly affected by natural conditions. Air temperature, humidity all have a significant impact on coating performance. The natural drying rate is not only greatly affected by the composition of the coating, but also related to the thickness of the coating. During the process of solvent volatilization and film formation, the drying rate gradually slows down, and the greater , the faster the drying rate decreases. As the viscosity increases, it becomes , and it takes a long time to achieve complete drying.
2. Heating and drying
Heating drying or drying is the main drying method of coating film in modern industrial coating. The coating of some conversion coatings (such as polycondensation reaction type and hydrogen transfer polymerization type) can only undergo polymerization reaction and become dry under the action of external heat. dry. Some coating films can also be dried by heating to shorten the drying time. The advantage of the heating drying method is that it can speed up the drying speed, save time, improve efficiency, shorten the operation and maintenance time, reduce the drying stacking area, and can be carried out in a closed environment, thus reducing the chance of the coating being polluted by the environment. , It also reduces the pollution to the environment caused by solvent volatilization; it can enhance the physical and mechanical properties of the coating, and can realize assembly line production. But it has the disadvantages of relatively large energy consumption and large investment in equipment. At present, two heating methods are mainly used: hot air and infrared radiation generated by steam, gas or electric energy . Heating drying is divided into three . Low temperature drying is suitable for the surface coating of wood and plastic that are easily deformed by heat.
It can also be forced to dry the coating film that can be dried naturally to improve the drying speed.
Moderate and high temperatures are suitable for drying thermoset coatings and metal surface coatings. To save energy, bake at a lower temperature as much as possible.
3. Special drying method
This is a drying method that uses external energy to promote the polymerization reaction of the coating and cure it, such as ultraviolet (UV) irradiation.
