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 where 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 when rubbed with fingers. At this time, the coating film can resist pressure and abrasion. However, there are often specific requirements for the complete drying standards 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.
According to the specific conditions of drying, it can be divided into three situations: natural drying, heating drying and special 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 coating film at room temperature, and the wet film will dry gradually. Oxidative polymerization, initiator polymerization, partial polycondensation reaction and hydrogen conversion polymerization in non-conversion coatings and conversion coatings can all be dried by natural drying.
This method is suitable for large-area painting outdoors. The disadvantage is that it takes a long time to dry, takes up a large space, and is greatly affected by natural conditions. Air temperature, humidity, ventilation and light during drying all have a significant impact on coating performance.
The rate of natural drying 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 slows down gradually, and the greater the thickness, the faster the drying rate decreases. As the viscosity increases, it becomes more and more difficult for the solvent to volatilize from the inside of the coating, and it takes a long time to achieve complete drying.
Heating drying or drying is the main method of drying coatings in modern industrial coatings. The coatings of some conversion coatings (such as polycondensation reaction type, hydrogen transfer polymerization type) can only undergo polymerization reaction under the action of external heat. dry. Some coating films that can be dried naturally can also be dried by heating in order 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 drying 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 dirt in the environment. , It also reduces the environmental pollution 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 and drying are divided into three forms according to temperature: low temperature (below 100°C), medium temperature (100~150°C) and high temperature (150°C). Low-temperature drying is suitable for drying wood and plastic surface coatings that are easily deformed by heat. It can also be forced to dry natural drying coatings to increase the drying speed. Moderate and high temperatures are suitable for drying thermoset coatings and metal surface coatings. In order to save energy, it should be baked at a lower temperature as much as possible.
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.
张颖;姚同杰;崔占臣;杨柏 - 《石油化工高等学校学报》
张颖;谢斌;姚同杰;杨柏 - 《热固性树脂》