Paint adhesion testing
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. 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 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. An uncalibrated pressure sensitive tape is applied to the sample and pulled off. This testing method is usually used for 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.[1]
Scrape Adhesion
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.[2]
Pull-Off Test
The adhesion of a coating or several coated sample 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 securing a dolly perpendicular to the surface of a coating with an adhesive. Then the testing apparatus is attached to the loading fixture and is then 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.[3]