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Standard Test Method For Pull-Off Strength Of Coatings Using Portable Adhesion Testers Part 1
- Nov 15, 2017 -

Summary of Test Method

The general pull-off test is performed by securing a loading fixture (dolly, stud) normal (perpendicular) to the surface of the coating with an adhesive. After the adhesive is cured, a testing apparatus is attached to the loading fixture and aligned to apply tension normal to the test surface. The force applied to the loading fixture is then gradually increased and monitored until either a plug of material is detached, or a specified value is reached. When a plug of material is detached,the exposed surface represents the plane of limiting strength within the system. The nature of the failure is qualified in accordance with the percent of adhesive and cohesive failures, and the actual interfaces and layers involved. The pull-off strength is computed based on the maximum indicated load, the instrument calibration data, and the original surface area stressed. Pull-off strength results obtained using different devices may be different because the results depend on instrumental parameters.

Test Preparation

The method for selecting the coating sites to be prepared for testing depends upon the objectives of the test and agreements between the contracting parties. There are, however,a few physical restrictions imposed by the general method and apparatus. The following requirements apply to all sites:

1.1 The selected test area must be a flat surface large enough to accommodate the specified number of replicate tests.The surface may have any orientation with reference to gravitational pull. Each test site must be separated by at least the distance needed to accommodate the detaching apparatus.

The size of a test site is essentially that of the secured loading fixture. At least three replications are usually required in order to statistically characterize the test area.

1.2 The selected test areas must also have enough perpendicular and radial clearance to accommodate the apparatus, be flat enough to permit alignment, and be rigid enough to support the counter force. It should be noted that measurements close to an edge may not be representative of the coating as a whole.

1.3 Since the rigidity of the substrate affects pull-off strength results and is not a controllable test variable in field measurements, some knowledge of the substrate thickness and composition should be reported for subsequent analysis or laboratory comparisons. For example, steel substrate of less than 3.2 mm (1⁄8 in.) thickness usually reduces pull-off strength results compared to 6.4 mm (1⁄4-in.) thick steel substrates.

1.4 Subject to the requirements of 1.1, select representativetest areas and clean the surfaces in a manner that will not affect integrity of the coating or leave a residue. To reduce the risk of glue failures, the surface of the coating can be lightly abraded to promote adhesion of the adhesive to the surface. If the surface is abraded, care must be taken to prevent damage to the coating or significant loss of coating thickness. Solvent clean the area to remove particulates after abrading. Select a solvent that does not compromise the integrity of the coating.

1.5 Clean the loading fixture surface as indicated by the apparatus manufacturer. Failures at the fixture-adhesive interface can often be avoided by treating the fixture surfaces in accordance with an appropriate ASTM standard practice for preparing metal surfaces for adhesive bonding.

1.6 Prepare the adhesive in accordance with the adhesive manufacturer’s recommendations. Apply the adhesive to the fixture or the surface to be tested, or both, using a method recommended by the adhesive manufacturer. Be certain to apply the adhesive across the entire surface. Position fixture on the surface to be tested. Carefully remove the excess adhesive from around the fixture. (Warning—Movement, especially twisting, can cause tiny bubbles to coalesce into large holidays that constitute stress discontinuities during testing.)

1.7 Based on the adhesive manufacturer’s recommendations and the anticipated environmental conditions, allow enough time for the adhesive to set up and reach the recommendedcure. During the adhesive set and early cure stage, a constant contact pressure should be maintained on the fixture. Magnetic or mechanical clamping systems work well, but systems relying on tack, such as masking tape, should be used with care to ensure that they do not relax with time and allow air tointrude between the fixture and the test area.

1.8 Scoring around the fixture violates the fundamental in situ test criterion that an unaltered coating be tested. If scoring around the test surface is employed, extreme care is required to prevent micro-cracking in the coating, since such cracks may cause reduced adhesion values. Scored samples constitute a different test, and this procedure should be clearly reported with the results. Scoring is only recommended for thicker-film coatings, that is, thicknesses greater than 500 μm (20 mils),reinforced coatings and elastomeric coatings. Scoring, if performed,shall be done in a manner that ensures the cut is made normal to the coating surface and in a manner that does not twist or torque the test area and minimizes heat generated and edge damage or microcracks to the coating and the substrate.For thick coatings it is recommended to cool the coating and substrate during the cutting process with water lubrication.

1.9 Note the approximate temperature and relative humidity during the time of test.