The adhesion pull off test, also called the paint pull off test, is a process used to determine how well a coating performs and bonds to a particular substrate, such as concrete. This test is performed by gluing a loading fixture, which is known as a dolly or stub, to the coating with an epoxy. Consistent measured pressure is then applied to this loading fixture in an effort to break the loading fixture's bond with the surface coating, break the coating's bond with the substrate or break the surface of the substrate itself.
As a near-to-surface test, the pull off test determines the tensile strength of the bond, rather than the sheer strength of the coating, which is tested by additional procedures such as the knife test, scrape test and tape test. The pull off test should require a significant amount of pressure to be able to break the bond of suitable substrate coatings. In the case of the strongest coatings, it is actually possible to break the substrate before the bond with the coating is breached.
The pull off test is accomplished using a machine called an adhesion pull off tester. Pull off testers are available in three styles: mechanical, pneumatic and hydraulic. This testing equipment is designed to deliver the vertical pull off force that is required to accomplish an accurate test. For the most accurate result, the pull off test equipment should deliver this force on a plane that is perpendicular to the test surface.
The adhesion pull off tester works by exerting a controlled, measured force on the dolly. These loading fixtures are available in a number of sizes to allow for the physical properties of various types of substrate and to provide the ability to accurately determine a wider range of tensile force measurements. The pull off test ends when the weakest component of the system loses its bond with the other components. This system includes: the dolly, the epoxy adhesive, the surface coating and the substrate.
If the dolly is pulled off the surface coating without damaging the coating's adhesion to the substrate, the measurement of the pressure required to accomplish the task is recorded as the tensile strength of the surface coating. When the bond between the coating and the substrate is broken, this measurement is recorded as the adhesive failure point. If the surface coating or the substrate experience failure, this measurement is recorded as the cohesive failure point.