This chapter presents the most common tests to evaluate the physical resistance of coating films. Such mechanical tests are usually conducted in parallel with corrosion tests. The mechanical resistance provides an idea of: paint reticulation, paint adhesion to the substrate, anomaly relating to the pretreatment.
• Paint adhesion test by cross-cutting (ISO 2409 or ASTM D3359)
Using a certified cross hatch cutter, the painted surface is engraved to create a grid of squares. After rapid brushing, a normalized adhesive tape is applied with following pull-of adhesion step. The results are noted on the basis of the estimated percentage of detached paint. The ISO evaluation provides a classification between 0 (no detachment) and 5 (total detachment), the ASTM evaluation foresees an inverse classification, from 5B (no detachment) and 0B (total detachment).
• Rapid deformation test (impact resistance) of the painting film using a falling weight (ISO 6272-2)
A spherical weight is dropped by means of a column support, under which the specimen (a metal sheet that reflects the regulations) is inserted. The height, the weight of the sphere and the impact side on the sheet are defined by the operator based on the analytical requests. After the impact, detachment or damage of the coating is observed and the results are noted, even with the help of a magnifying glass.
• Progressive deformation test of the painting film by cupping tester (ISO 1520)
A specimen (metal sheet) is inserted inside an apparatus capable of pushing a steel ball on the back of the sheet, forcing it to a slow and methodical deformation, with a rate between 0,1 mm/s and 0,3 mm/s. At the end, the cracking is evaluated with a magnifying glass.
• Measurement of the hardness of the coating using pencils, according to Wolff Wilborn (ISO 15184)
Pencils with graphite leads of varying hardness are used, inserting them in a small tool with wheels that act as a guide. With the tip of the selected pencil you will have to try to engrave or damage the pretreatment or the paint; if this does not happen, the hardness of the pencil must be increased. The test ends when the maximum resistance of the coating to the pencil is determined (for example 7H). The test finds greater application on pre-treatments or on "soft" paints.
• Paint bending resistance test using cylindrical mandrel (ISO 1519)
The specimens (generally 5 cm wide) are inserted into the grip of a mandrel equipped with cylinders of various diameters. Slowly bend the metal sheet using the tool lever, starting with a standard cylinder. If the paint does not break, you can proceed to use thinner cylinders in order to tighten the bending angle.
• Determination of the degree of paint reticulation using Methylethylketone (MEK)
A gauze or non-colored fabric is soaked with methylethylketone and rubbed manually at least 20 times on the painted surface (10 times forward and 10 times back) observing damage or changes to the film. The operation of this method can be customized according to need, it represents a quick index of paint reticulation. Ideally, the layer thickness should be measured before and after the test.