Test principle of paint film indentation hardness

Indentation hardness is a static test of viscoelasticity. The shape of the indentation hardness-temperature curve and the tensile modulus-temperature curve are very similar, and represented by T. Also the same. This fully demonstrates that the indentation hardness test is actually the modulus -temperature behavior of the paint film.

The test method of indentation hardness uses an indentation head to press the paint film under a certain weight for a certain period of time. Raise the indentation head and measure the length or area left on the film with a calibrated microscope. The indentation length of colored paint is easy to judge. There are various types of test instruments.

CHINA's national standard GB 9275-88 "Buchholz indentation test for paints and varnishes" stipulates that Buchholz test the hardness of the coating film. The measured indentation length represents the resistance of the coating to the indenter , and the result is expressed in indentation resistance. The calculation formula is:

In the formula, H is the indentation resistance, L is the indentation length, mm. American ASTM D 1474-68 (79) stipulates that Knoop indenter and Pfund indenter can be used. The Knoop indenter is a diamond pyramid; the Pfund indenter is a transparent colorless quartz hemispherical body. The test result with the Knoop indenter is called the Knoop hardness number, or KHN for short, which is the mass divided by the indentation area. Calculated according to the following formula:

In the formula, L is the load mass on the indenter, kg; l is the indentation length, mm; c _p is the constant of the indenter, 7.028×10 ^-2 .

The test result with the Pfund indenter is called the Pfund hardness value, or PHN for short, and its formula is:

In the formula, L is the load mass of the indenter, which is specified as 1.kg; A is the indentation area, mm ² ; d is the average indentation diameter, mm. Indentation hardness For the same coating, the indentation hardness value of thin film is higher than that of thick film. Indentation test for elastic such as rubber

Coating results are inaccurate. Only the test results obtained for high T _{g films are meaningful. The intermediate} T _g , films partially recover from indentation during the experimental operation , and the rubbery (low T _g ) films may not leave indentations. This does not indicate that the rubbery paint film is hard, but actually soft. Indentation hardness is most suitable for baking lacquers, because the Tg of baked lacquer films _is generally higher than the test temperature.