6. The Pull-Off Test
When testing coating adhesion using a pull-off adhesion gauge; once a properly prepared dolly has been glued to the prepared coating, and the glue has fully cured, you are ready to perform the pull-off test.
Reset or rewind the gauge or actuator head to its start position and attach the dolly.
When performing a pull off test on a coating applied to a thin substrate, it’s essential that a thin substrate base ring or thin substrate skirt is used, otherwise the force applied could deform the substrate and cause the coating to fail prematurely.
If testing adhesion on a vertical surface, to ensure that the gauge or actuator doesn’t fallwhen the dolly ‘pops’ off, attach a lanyard from the test unit; either to your wrist or part of the structure;or use a magnetic anchor clamp.
To begin the adhesion test, apply a uniform increase in force in accordance with your test method or standard - typically a rate of pull of 1MPa/s or 150psi/s is used for a 20mm dolly.
Whilst the rate of pull is controlled manually for most pull off adhesion testers, the Elcometer 510 automatic pull off adhesion tester allows the rate of pull to be selected, and when the start button is pressed, the gauge applies the pre-selected rate of pull automatically.
Typically pull off adhesion tests require the dolly to be completely pulled off, or pop off the surface. Some gauges such as the Elcometer 510 allow the user to set a maximum pull value, at which point the gauge automatically returns the pull force to zero, and records the result as “greater than the maximum pull value” – a so-called “non-destructive pull”.
In fact, research showed that pull-off tests undertaken at an ambient temperature of between 20 - 24°C provide the highest pull off values, resulting in a 32% increase in values compared to tests undertaken outside this temperature range.
To achieve comparable results when testing on the same structure therefore, it’s worth undertaking the pull off tests at a consistent temperature, and recording the ambient temperature at the time of the pull; as tests on the same structure could yield different pull off forces depending on the time of day and the location each pull off test was completed.
This video is part of a series on Pull-Off Adhesion Testing, to see the next video please click here.