Physical properties of paint and coatings play an important role in the quality control. Depending on the application a coating is supposed to be extremely hard and rigid such as for example the coating on the hull of an ice-breaker. Automotive coatings however require a certain flexibility since the thin sheet metal of a car body is not fully rigid and a slamming hood should not cause the paint to break. Coil coated steel that is to be formed into products such as white goods or cladding should have excellent forming properties.
Several empirical tests
There are a number of different empirical tests that are used to deform test panels and so assess the deformation resistance or flexibility. Coatings that fail such a test will crack and/or detach from the substrate. Also there is quite some overlap between different test systems and their results. Many tests provide information about similar parameters such as elasticity / forming resistance, adhesion, hardness, brittleness, stress resistance etc..
Although some tests overlap there is no cross-reference for their results.
More information can be found in the section hardness/scratch resistance
Commonly used tests
Common tests used in the paint and coatings industry are:
- Bend test
With this system a coated test panel or lacquered sheet metal is bent over a defined mandrel. Cylindrical mandrels in different diameters or conical mandrels are used.
The smaller the diameter the sharper the forming of the test panel which leads to elongation of the panel including the coating. A sharper deforming causes more stress to the coating which may lead to cracks or adhesion failure.
Cylindrical bend test are recognised to be more precise in comparison to conical bend tests which have the advantage that a range of mandrel diameters can be checked on just one panel.
- Impact Test, or tubular impact test
With an impact test a rapid deformation of a painted test panel is generated. A specified weight with a specified punch is dropped on a test panel from different heights. The damage on the sample is observed after the test and provides information about physical properties of the coating such as elasticity, hardness and adhesion.
For the paint industry there are different tests according ISO 6272 , ASTM D2794 and others. Each standard has its own specifics such as weight, punch shape, direct or indirect impact etc..
- Cupping test
Cupping testers generate, in contradiction to impact testers, a gradual deformation of the sample. An indenter with a specified shape is slowly forced through the test panel under standardised conditions. During the deformation the panel is visually observed. The cupping value of a coating is the deformation point at which the coating starts to crack.
To perform this test correctly it is of utmost importance that the deformation is made gradually without any interruptions.
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