Pull-off adhesion testing is widely used to assess the protective coating process and to determine if a coating is fit for service in new construction and for repairs to existing structures. The result is often critical to the acceptance or rejection of a coating process, as the adhesion value quoted by the paint manufacturer can be adversely affected by aspects of the coating process. Low adhesion values are indicative of premature failure of the coating and are often due to inadequate surface preparation of the substrate. ASTM D4541 and BS EN ISO 4624 describe several different test apparatus; however, the basic approach of gluing a test dolly to the coated surface and then exerting a perpendicular force to the surface in an effort to remove both the dolly and the coating from the substrate is common to all these standards. A measure of the adhesion of the coating system is the force at which the coating fails and the type of failure obtained. Trials have demonstrated that many aspects of the testing method, such as the mixing of the glue, the preparation of the coating surface and the face of the dolly and the temperature of the test, all affect the results. The tensile pull-off method for adhesion testing, as outlined in ASTM D 4541 and similarly in BS EN ISO 4624, involves gluing a test dolly to the coated surface and then pulling the dolly by exerting a force perpendicular to the surface in an effort to remove the dolly with the coating from the substrate. The force at which this occurs and the type of failure obtained is recorded as a measure of the adhesion properties of the coating. Several aspects of the test method were assessed, including the mix of the epoxy glue, different types of glue, surface preparation, the design of the dolly, temperature of the cure and the test, and cutting the coating or not. The test dolly should be glued to the surface using a suitable adhesive. Typically a two-pack epoxy adhesive is supplied with adhesion test units. The instructions for this type of adhesive state that the two components, resin and hardener, should be mixed in equal parts usually equal lengths of both parts. In order to achieve a more accurate mix, the amount of each component was measured by weight using an accurate electronic balance. The major components of a pull-off adhesion tester are a pressure source, a pressure gage and an actuator. During operation, the flat face of a pull stub (dolly) is adhered to the coating to be evaluated. After allowing for the bonding adhesive to cure, a coupling connector from the actuator is attached to the dolly. By activating the pressure source, pressure is slowly increased to the actuator within the system. When the pressure in the actuator becomes larger than the bond strength between the coating and the substrate, separation occurs and the actuator-dolly assembly lifts the coating from the substrate). The maximum pressure indicator of the system’s pressure gauge provides a direct reading of the pressure at which the pull-off occurred.
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