The laboratory runs coating adhesion tests in order to verify the correct application of products, their quality and properties. This allows to provide an appropriate protective function for the tested object. This test is conducted for all industry sectors. Depending on the target operating conditions, various test methods are selected that allow to determine the resistance to tensile or shear forces.
In order to verify the resistance to shear stress, two test techniques are used, selected depending on the thickness of the coating system used: a crosscut and a crosscut pattern.
The X-cut in accordance with the PN-EN ISO 16276-2 standard
The coating system is cut into the substrate with a single-blade knife, making two scratches at the right angle. An adhesive tape is glued to the place where the coating is damaged, and then it is torn off after a certain time in order to remove the poorly adhering coating. The sample is assessed for damage and peeling resulting from the tests and is classified using a five-point scale. The number of measurements conducted depends on the surface of the structure. It should be specified in the design documentation.
The x-cut test is applicable to the analysis of coatings with a high degree of hardness and has no limitations depending on the thickness of the coating.
Cross-cut method in accordance with PN-EN ISO 2409
The laboratory also performs a cross-cut adhesion evaluation. It is used for coatings with a thickness of up to 250 µm, which are not markedly hard. The test principle is to make two perpendicular incisions, each consisting of 6 parallel lines. The resulting pattern should form a grid of equal squares. The intervals between the cuts (1mm, 2mm, 3mm) are adjusted depending on the thickness of the coatings and the type of substrate used (metal, plastics, wood, plaster). After making the incision, the remnants of loose coatings are removed with the use of adhesive tape, and the sample is visually inspected and classified according to a 5-point scale depending on the resulting damage and peeling.
The test of resistance to tensile forces determining the adhesion of coatings is carried out in laboratory conditions on prepared test panels in accordance with ISO 4624 or directly on the structure in accordance with ISO 16276-1.
The method of assessing the peel strength consists in sticking the measuring stamps to the painted, hardened surface with the use of an appropriate glue adjusted to the type of the tested product. After the adhesive is fully cured, the coating around the punches is cut all the way to the substrate. The punch is then placed in the centring head and a force is applied by a tearing device (at an even speed, perpendicular to the plane of the substrate) to measure the value at which the adhesive, cohesive, or adhesive-cohesive bonds break. It is recommended that the dimensions of the test plates be at least 100 mm x 100 mm x 10 mm.
The test result is given as a peel strength value and is expressed in MPa. The type of detachment is also reported. The analysis is conducted by visual assessment of fractures resulting from adhesive or cohesive tears, indicating the percentage of each of the tears and coating layers they relate to.
The result is verified with the values given in the specifications or with the requirements set by the customer.
After evaluating the adhesion of coatings, it is necessary to repair the surface due to the destructive nature of the tests.