Adhesion is the tendency of dissimilar particles and/or surfaces to cling to one another. The adhesive strength of paint and coatings are of crucial importance to enable the material to meet the basic functions of protection and decoration.
Adhesion testing in the paint and coating industries is necessary to ensure the paint or coating will adhere properly to the substrates to which they are applied. Adhesion testing after the coating process will quantify the strength of the bond between substrate and coating, or between different coating layers or the cohesive strength of materials.
The adhesive strength of applied coatings depends heavily on the quality of the pre-treatment process prior to the coating application. Also compatibility between coatings or coating and substrate is a key factor.
Adhesion testing is used as part of inspection and maintenance procedures to help detecting potential coating failures, control the quality of a coating job or to define if an existing coating system has to be removed before applying new paint.
There are three different tests to measure the resistance of paints and coatings from substrates: cross-cut test, scrape adhesion, and pull-off test.
Cross-Cut or Cross Hatch Test
A right angle lattice pattern is used to measure the resistance of paints and coatings to separation from substrates. The pattern is cut into the coating and penetrates through the substrate. A specified pressure sensitive tape is applied to the sample and pulled off. This testing method is often used as a fast pass or fail test. If this test is used on a multi-coated sample, assessment of the resistance to separation of individual layers of the coating from each other can be made.
The scrape adhesion test measures the determination of the adhesion of organic coatings when applied to smooth, flat panel surfaces. It is helpful in giving relative ratings for a number of coated panels showing significant differences in adhesion. The materials being tested are applied at uniform thickness to flat panels, mainly some sort of sheet metal. When the materials have dried the adhesion is determined by pressing panels under a rounded stylus that is loaded with increasing amounts of weight until the coating is removed from the substrate surface.
The adhesion of a coating or several coated samples of any paint product is measured by assessing the minimum tensile stress needed to detach or rupture the coating perpendicular to the substrate. Unlike the other methods, this method maximizes the tensile stress, therefore results may not be comparable to the others. The test is done by adhering a dolly perpendicular to the surface of a coating with an adhesive. Then the testing apparatus is attached to the loading fixture and is aligned to apply tension perpendicular to the test surface. The force that is applied gradually increases and is monitored until a plug of coating is detached, or a previously specified value is reached.
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