Hardness is one of the important mechanical properties of coatings, which is related to the service life of coated parts in the use process. The hardness of the coating is related to the type of coating and the curing degree of the coating. The coating hardness of oil paint and alkyd resin paint is low, while that of synthetic resin paint is high. In most cases, increasing the proportion of curing agent will increase the hardness of the coating, while the flexibility, impact resistance and other properties of the coating will decrease. The curing degree of the coating also directly affects its hardness value, and the completely dry coating has its specific highest hardness. In the coating industry, a series of pencils with different hardness are often used to test the hardness of the coating film. The test results obtained are called the pencil hardness of the coating film.

Pencil hardness, a simple test, can also be used to measure the hardness of the coating, or to measure the scratch resistance of the coating. Although some people may think it is a low technology, it is still the mainstream use method for many coating manufacturers and some finishing workshops to determine the hardness by scraping the coating with special pencils of different hardness. In addition, this method is also used to determine the degree of curing of these coatings, especially when heat is used for intensive drying.

According to industrial standards, the hardness of pencil refills is divided into 13 grades. Gradually decrease from the hardest 6H to 5H, 4H, 3H, 2H, H, then HB with moderate hardness, and then from B, 2B to the softest 6B. H stands for hardness and B stands for blackness. The hardness decreases from 6H to 6B, and the pencil color becomes darker in turn. The color depth is related to the graphite content. The darker the color, the higher the graphite content, and the softer the pencil.

Sharpen the pencil to expose 4-6 mm, so that its end face is flat and the edge is sharp. Place the coating sample on a horizontal surface. Hold the pencil so that it is 45 degrees away from the sample surface (the tip of the pencil is far away from the operator), and then draw a 6.5 mm line away from the operator. Start with the hardest pencil and reduce the hardness of the pencil in turn until any of the following occurs. First, the pencil will not cut into the coating, or scratch the coating (pencil hardness); Second, the pencil will not scratch the coating (scratch hardness). Then, the pencil hardness is the pencil hardness of the coating to be tested.

In order to measure the hardness of coated pencils more conveniently and quickly, the coated pencil hardness tester, which is composed of pencil clamp and film template moving table, also came into being. Fix the film sample plate to be tested on the mobile platform with the front side facing up, and the pencil is clamped on the pencil clamp, which forms a 45 ° angle with the plane of the film. The upper end of the clamp is equipped with a heavy hammer, so that the pen tip is pressed against the film. Shake the handle, and the mobile table will move forward with the template, so that the pencil can move on the film in a push plow style. Every time, change a pencil, starting from the hardest pencil to the soft one, and test one by one until you find out the pencil whose film is not broken. The hardness of this pencil is the hardness of the film to be tested.

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