Test method for drying time of coating film (surface dry and actual dry)

The drying process can be divided into different stages according to the physical properties of the coating film, mainly the viscosity change process. It is customarily divided into three stages: surface drying, actual drying and complete drying. As for the drying time, the construction department requires that the shorter the better, so as to prevent the coated workpiece from being stained with rain, dew and dust, and the construction period can be greatly shortened. For paint manufacturing, a certain drying time is often required to ensure the quality of the film after formation. It takes a long time for the paint to completely dry, so generally only the surface dry (surface dry) and actual dry (hard dry) are measured. The drying time of the paint film is affected by factors such as the temperature, humidity, ventilation, and light of the surrounding environment. Therefore, certain environments and equipment are required for the measurement, and it is carried out in a constant temperature and humidity room.

(1) Commonly used methods for measuring surface drying time include cotton ball blowing method and finger touch method (GR/T 1728-79(88)). The cotton ball blowing method is to put a degreased cotton ball on the surface of the paint film, and gently blow the cotton ball along the horizontal direction with your mouth. If it can be blown away and no cotton filaments are left on the film surface, the surface is considered dry. The finger touch method is to lightly touch the surface of the paint film with your fingers. If you feel a little sticky, but no paint sticks to your fingers, the surface is considered dry or called finger dry.

(2) The commonly used methods for determining the actual drying time are the filter paper method, the cotton ball method, the blade method and the thick layer drying method. my country's national standard CB/T 1728-79 (88) has detailed regulations, and the filter paper method and cotton ball method are usually used. The filter paper method is to press a piece of qualitative filter paper on the paint film with a drying Tester, remove the Tester after 30 seconds, turn the sample over and the filter paper can fall freely, which is considered to be actually dry. Similarly, remove the Tester and absorbent cotton balls after 30 seconds in the cotton ball pressing method. If there are no traces of cotton balls and loss of light on the paint film, it is considered to be actually dry.

According to the curing process of the coating film, it can be seen that the drying time of the coating film is not only affected by environmental factors, but also mainly affected by the curing crosslinking reaction properties of the film-forming material, the type of grouping agent, the solvent volatilization rate, and the polymer chain segment. effect on mobility. If the environmental factors are not matched, too fast or too slow drying time will bring disadvantages and defects to the coating film. If it is too fast, it will easily cause problems such as wrinkling, low gloss, low adhesion or even loss of coating film.