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Pull-test / coating adhesion test ISO 4624----
In case of executing the pull-off test according the ISO 4624 standard on an existing aged multiple alkyd coat on steel, my main question in this matter is how to interpret these results in order to give a straight forward answer about the adhesion characterization. Also, what would be the minimum pull-off strength for a multiple coat system (with in mind that after sweeping it is covered with a new 3-coat alkyd system). All information is welcome!Jelle Smit
Technical University Delft, The Netherlands
The referenced ISO Standard (ISO4624) is a tensile test. This is only one facet of coating adhesion, others being shear adhesion, or some combination of the both. This test method, and the commercially available apparatus (Elcometer, HATE, PATTI, and others] are not interrelatable, and for the most part do not give statistically reliable results. Regardless, to answer your question, if the test results are valid, the type of break within the cross section of the coating system (adhesive, within primer, intermediate, topcoat, or % of each) is most relevant, along with the tensile stress applied to the coating, the testing apparatus, and the type of adhesive used (was there any interaction between the adhesive and the coating system?) Cyanoacrylates, sometimes react with alkyd coatings! I believe fast-setting two-part epoxy adhesives give most reliable results. I would expect a three coat alkyd system, depending upon its oil length, and extent of autooxidative crosslinking to exceed 200 psi tensile adhesion. Probably the best test for adhesion is a subjective knife picking, preferably with "calibration" against a control (for example, a properly applied and cured alkyd system). Good Luck
- Pittsburgh, Pennsylvania
January 12, 2012
Thanks for the information Ken.
I know of a large problem going on right now where the coating (over 1 mm thick) on a steel structure is passing the Pull off adhesion test, but it is flaking off in large sheets when a knife or fingernail is used to pry under the edge of the coating. Water washing at less than 2000psi is washing off the topcoat in large sheets.
The problem is that because the Job Specifications specify a 5 MPa pull off, the coating is considered "passed" even though the top coat practically falls off when pressure washed or picked with a knife. Can you recommend an objective test that could be used to prove the lack of shear strength for the coating? (keeping in mind the coating is over 1mm thick and very hard.)
Inspection and Instrumentation - Cardiff, NSW, Australia