The hardness of the cured film is a key index to evaluate the quality of the paint film, and an important index to investigate whether the coating of the coil material can be formed through processing. The methods for testing the hardness of fixed-line coatings include pendulum hardness, pencil hardness, and Shore hardness. Because the pencil hardness is simple and easy to operate, it is widely used in the color-coated board industry.
Coating hardness is usually related to coating formulation components and curing conditions. In addition, the thickness of the coating also has a more obvious effect on the hardness of the fixed film. When the coating is thin, the temperature can be absorbed more evenly, the curing is uniform and effective, and the overall hardness of the film layer is higher; the heat absorption effect of the thick coating has a gradient effect, the curing is uneven, and the overall hardness is relatively low.
The coating hardness test of color steel plate is usually measured by pencil scratch Hardness Tester, that is, the pen core with known hardness from 14 grades in the range of 6H-6B is used to plow through the coating, among which 6H is the hardest and 6B is the hardest. soft. There are two methods for determination: manual pencil method and instrument pencil method.
A manual pencil method
Use a pencil sharpener to sharpen the pencil until the 4~6mm cylindrical core is exposed, hold the pencil so that it is perpendicular to the surface of No. 400 sandpaper, and grind on the sandpaper until the end surface is flat and the edge is sharp. Then hold the pencil at a 45° angle to the coating and apply enough even pressure downward and forward, just enough to break the pencil edge or plow through the coating. The formation of the pencil advance is 6.5mm. The test shall begin with the pencil of the hardest grade, and each grade shall be tested in turn. Stroke 5 times with a pencil at each level until at least 4 of the 5 times the pencil cannot plow through the coating. The hardness of this pencil is the pencil hardness of the coating to be tested.
B instrument pencil method
After handling the pencil according to the manual pencil method, put it into the instrument trolley and fix it so that the pencil tip can freely contact the surface of the sample. Push the trolley so that the stroke it advances on the sample is 6.5mm. As with the manual method, test each grade of pencil in turn. Each grade of pencil is drawn 5 times until at least 4 of the 5 times the pencil cannot pierce the coating. The hardness of this pencil is the pencil hardness of the coating to be measured.
So far, there have been pencil Hardness Testers of QJQ type (desktop) and QHQ-A type (portable) in CHINA (see Figure 4-14). Pencil scratch Hardness Testers produced by many manufacturers have come into the market and can be used for this test. supporting instruments.
