Different types of pull-off adhesion-testers can produce a substantialy different adhesion-value
Pull-off adhesion-tests as per ISO 4624 or ASTM D4541 are a classic test through which the adhesion of a paint or coating can be determined quite easily.
The test-results not only supply a quantitative adhesion-value (expressed in MPa, kg/m², N/mm², PSI, ...), but also provides for a visual image of the "weak link" in a coating-system, making it the perfect test-procedure for comparing different coating-systems when it comes to their adhesive properties.
There are currently different models in the market available to execute such testing:
- Mechanical pull-off adhesion-testers (fixed allignment)
- Manual hydraulic pull-off adhesion-testers (self-aligning)
- Automatic hydraulic pull-off adhesion-testers (self-aligning + automatic & constant increase of pull-off load)
Pay attention however to the fact that the test-results obtained through different types of adhesion-testers CANNOT be compared with one another! Notwithstanding the fact that they all operate according to the same (above-mentioned) standards, they can produce a substantially different test-result. See also the attached graph showing the test-results of a comparative study where the pull-off adhesion of a coating in its curing-phase was tested using 3 different pull-off adhesion-testers, clearly showing the difference in pull-off adhesion-values obtained with different types of instruments (on the same coating, on the same panel, at the same time).
So, for true comparison of the adhesion of multiple coating-systems, a comparative test should be run using the SAME test-instrument.
And whenever specifying a minimum pull-off adhesion-value in a coating-specification, one should also specify the type of instrument that needs to be used for the required test.
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Comparitive graph illustrating the different adhesion-strength values as obtained when using different adhesion-testers