The Astm Testing Procedure

The ASTM testing procedure provides a method for evaluating the pull-off strength (adhesion) of a coating by applying the maximum perpendicular force that the coated surface can withstand until the weakest plane causes a break to occur within the coating system or causes the loading fixture to become detached. The tensile value is stated in pounds per square inch (psi), kilopascals (KPa), or megapascals (MPa). The break is defined according to the weakest plane, and is identified as either an adhesion, cohesion or adhesive (glue) break.

The testing procedure employs pull-off adhesion testers that are capable of applying a simultaneous concentric and counter load to the surface, even though only one side of the surface is accessible. A loading fixture (e.g., pull stub or dolly) is secured to the coated surface using an adhesive. The contact surface of the loading fixture is cleaned and sometimes abraded prior to application of the adhesive to enhance the bond of the adhesive to the loading fixture. Additionally, the coated surface can also be gently cleaned using a fine grit paper (e.g., 400) to enhance the adhesion of the adhesive to the coated surface (this is particularly important when tests are conducted on glossy coatings). Care must be taken however to avoid introducing flaws in the surface of the coating which could cause a reduction in adhesion strength. In the past, it was common practice by some to score the coating around the perimeter of the pull stub. However, scoring the coating is not recommended by the standard as it may cause microcracking of the coating and generate lower adhesion values. After the adhesive cures, a test apparatus is attached to the loading fixture and the perpendicular force is gradually increased until a break occurs, or the maximum adhesion value is obtained without generating a break. If a break occurs, the maximum load value and the location of the break are recorded.

EXPERIMENTAL