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 1 stressed. Pull-off strength results obtained using different devices may be different because the results depend on instrumental parameters.
Significance and Use
The pull-off strength of a coating is an important performance property that has been used in specifications. This test method serves as a means for uniformly preparing and testing coated surfaces, and evaluating and reporting the results. This test method is applicable to any portable apparatus meeting the basic requirements for determining the pull-off strength of a coating.
Variations in results obtained using different devices or different substrates with the same coating are possible. Therefore, it is recommended that the type of apparatus and the substrate be mutually agreed upon between the interested parties.
Test Preparation and Procedure
The method for evaluating adhesion is the pull-off test. With this method, a loading fixture commonly called a dolly or stud is glued to the surface. A special device is then used to apply an increasing force until the coating disbonds or the glue fails. The pull is perpendicular to the surface, so tensile strength is being measured. This is different from the tape tests where shear is being measured. Therefore, the results obtained from the two different types of tests are not comparable.
A portable adhesion tester, loading fixtures, and adhesive are needed for this test. The first step is to prepare the loading fixtures. They are supplied as smooth steel and must be cleaned so the glue will stick. This usually involves solvent cleaning. It is a good idea to roughen the bonding surface of the loading fixture either with sandpaper or light abrasive blasting. This will minimise the number of glue failures that occur.
The coating surface must also be cleaned. Surface abrasions can induce flaws, so only fine sandpaper (400 grit or finer) should be used, if needed, to remove loose or weakly adherent contaminants such as chalking or dirt that cannot be washed off.
Epoxy or acrylic adhesives are used to glue the loading fixture in place. The adhesive must cure for the amount of time recommended by the manufacturer. This can be several hours to a day, depending on the adhesive and the temperature. Using other adhesive types such as cyanoacrylates that cure in a few minutes may not be acceptable because these materials may contain solvents that can attack the coating. Mix the adhesive and apply it to the base of the loading fixture, making sure a continuous film covers the entire surface. A critical step in the test procedure is positioning the fixture onto the surface. Avoid any movement, especially twisting, that can result in stress discontinuities during the pull testing. Wipe away any excess adhesive with a cotton swab once the test fixture is in place.
It is important that constant contact pressure be maintained as the adhesive sets and begins to cure. Magnetic or mechanical clamps work best. For pull-off adhesion testing, at least three test fixtures per test area are normally required. The number of areas being tested will determine how many fixtures and clamps are needed. The fixtures can be taped in place with masking tape or duct tape if enough clamps are not available. However, tape can relax with time and allow air to get between the fixture and the test area. Thus, at least three pieces of tape across the top of the fixture in different directions should be used. Also, it is important to space the fixtures far enough apart so the loading device can be properly positioned when the pull is performed.
There are two main types of commonly used adhesion testers. One is a fixedalignment, mechanical adhesion tester; the other is a fixed-alignment, pneumatic adhesion tester. These instruments come in different force ranges, so the proper range instrument must be selected.
Now comes the next critical step in the procedure—connecting the loading fixture to the central grip of the tester (sometimes called the detaching assembly). Remove the clamping device or tape. Then follow the manufacturer’s instructions for attaching the tester to the loading fixture, making sure not to bump, hit, bend, or otherwise apply any load to the fixture that will knock it off the vertical.
If the test is being performed on any other surface than a horizontal one (e.g., a vertical web or the underside of a flange), the tester must be supported so its weight does not contribute to the force exerted.
Once the fixture is attached to the instrument, it is time to run the test. Again, review the manufacturer’s instructions on how this should be done. No matter what instrument is being used, the load to the fixture should be increased continuously and smoothly at a rate not to exceed 1 MPa/s (145 psi/s). Keep applying the load until failure occurs (or until the maximum force has been applied). Any test fixtures that do not detach with the maximum load can be easily removed by tapping them on their side. In fact, this is a good demonstration of how easily the test fixtures can be removed in shear compared to the tensile failure mode of the actual test. Also, make sure you have a firm grip on the loading device, because coatings with high tensile strength can cause the loading device to rebound when the break occurs.
Some instruments come supplied with a circular hole cutter to score through the coating to the substrate around the loading fixture before running the test. Scoring around the fixture violates the fundamental criterion of the test that an unaltered coating be tested. However, the practice is a subject of debate among some adhesion test experts. Therefore, if scoring is required for some reason, be very careful to prevent micro-cracks in the coating because they will give lower pull-off strengths. The hole cutter must be held perpendicular to the surface so that no sideways pressure is placed on the test fixture.
- ASTM International, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, data of access: 29 July 2016, http://marron-co.com/marron/PAINT%20STANDARD/ASTM%20%20PAINTS/0602/D4541.pdf
- JPCL, Adhesion Testing on Steel, January 2001
- Oxford Brookes University, On Site Pul-Off Method, data of access: 29 July 2016, http://mems.brookes.ac.uk/industry/resources/pull-off.pdf