The paint is coated on the surface of the object with a certain thickness, and forms a solid film through physical volatilization or chemical oxidative polymerization, or by adding a curing agent, drying or light curing. The time required for such a process is called Drying time, expressed in hours or minutes. It depends on the chemical and physical properties of the coating itself, and can usually be divided into two stages. ①Surface drying: the drying time of a wet paint film of a certain thickness, the surface changes from liquid to solid but the bottom is still liquid, which is called surface dry time. ②Actual drying: The drying time of a solid paint film formed from a liquid paint film of a certain thickness after application is called the actual drying time.
The national standard GB 1728-89 (79) stipulates the determination method of the drying time of paint film and putty film. Among them, the determination of surface drying time mainly includes blowing cotton ball method and finger touch method, and this kind of test mainly depends on the experience of the Tester. Gently put a cotton ball on the surface of the paint film, and gently blow the cotton ball with your mouth. If it can be blown away and there is no cotton thread left on the paint film surface, it is considered to be surface dry. This method is the cotton ball blowing method. The finger touch method is to lightly touch the surface of the paint film with your fingers. If you feel a little sticky, but not sticky to your fingers, and no paint sticks to your fingers, it is considered dry.
The methods for determining the dryness include the filter paper method, the cotton ball method, and the blade method. The method of pressing filter paper and cotton ball is to put a filter paper or cotton ball on the surface of the paint film, and then press it on the dry Tester (200g weight type), and remove the weight after 30 seconds. Turn the test panel over, the filter paper (or cotton ball) can fall freely, and the fibers do not stick to the coating film; if there are 1~2 fibers that can also fall off lightly, it is considered that the paint film is dry. The time is set as the actual working time. According to experience, when the paint film is pressed with fingers, it is hard, not sticky, and there is no fingerprint, so it can be estimated that it is hard. The paint film can be scraped off on the sample board with a safety blade, and the bottom layer of the paint film is cut without sticking, so the paint film is considered to be actually dry.
Since the drying of the paint and the formation of the paint film are a slow and continuous process, in order to observe the overall changes in the drying process, the accurate method is to use an automatic paint film drying time Tester. Such as the drying time Recorder of BYK and other companies, the QGZ automatic paint film drying time Tester of Tianjin Weida Testing Machine Factory, etc. One (such as the BYK company instrument) is to use the motor to drive the gear through the reduction box, and move linearly on the paint film at a slow speed of 30mm/h, and the whole process takes 24 hours. As the paint film gradually dries, the paint film becomes harder and harder, and the traces of the gear test needle gradually change from deep to shallow until they disappear completely. The hard drying time is when the needle mark completely disappears. Another automatic measurement is to use the motor to drive the funnel equipped with spun yarn to move slowly on the sample plate coated with paint film, and the sand will continuously fall on the paint film to form linear traces of sand grains. When the sand grains are blown, there will be no When it sticks, it is the dry time. In addition, use the tip of a needle to slowly draw a circle with a radius of 5cm on the paint film. It takes 24 hours to draw a circle, so that you can observe the drying degree of the paint film over time on a small test panel area, and the time required for hard drying. time. However, these measurement methods are also related to the type of paint and the level of solid content of the paint.
Factors that affect the drying time of paint films are temperature, humidity and the properties of the painted substrate. Generally speaking, as the temperature rises, the paint film dries faster, but in addition to drying the paint, too high a temperature will cause the paint film to be brittle and the solvent to volatilize too quickly. At the same time, the type of coating has a great influence on the drying time. Physically volatile non-conversion coatings such as nitro, vinyl, chlorinated rubber, etc. generally dry in about half an hour, and dry in about 4 hours; , Polyurethane and other coatings usually take more than 4 hours to dry on the surface, and it takes about 24 hours to dry. Please refer to the relevant content in Chapter 1 and Chapter 4 for the surface-drying time, hard-drying time and suitable painting time interval of specific paint varieties. It should be noted that the paint coating after hard drying cannot be put into use immediately, because this kind of drying is only about 95% dry, and it takes more than 7 days to be completely dry before it can be put into normal use.
