What is an adhesion test?
In the paint and roof coatings industries, paint adhesion testing is often used to determine if the paint or coating will adhere properly to the substrates to which they are applied. This is a very important test to perform since no matter how good the coating may be if it does not “stick” it will not work. Adhesive failure is not acceptable and needs to be avoided 100% of the time. Although paint or coating failure can be caused by many factors; some beyond the control of the contractor, testing can help eliminate a lot of potential issues. For example when coating an EPDM (black rubber) roof the type of coating that may be specified for that surface may have worked well in every other application but due to age, chemical make-up of the rubber, or other factors, you could have a failure. However if you field apply some test patches and complete testing with a couple of products you are more able to discern which product will adhere best and thus provide better results for your client and your project. There are three different tests to measure the adhesive strength of paints and coatings from substrates: cross-cut test, scrape adhesion, and pull-off test. When painting a substrate such as wood or metal crosscut works well. However on roof substrates I typically do a pull-off test since cutting into a roof surface such as EPDM rubber or other single ply will basically cut a hole in the roof and allow water to intrude. With any test be sure your surface is clean and dry as it would be when you actually coat it. Dirty, chalking surfaces or ones covered in mildew are not good candidates to test adhesion. Basically you need to prepare your test area(s) just like you would when actually doing the job. Otherwise your results may not be valid.
A right angle lattice pattern or X-cut (contingent on paint thickness) is used to measure the resistance of paints and coatings to separate from substrates. The pattern is cut into the coating and penetrates through to the substrate. Pressure sensitive tape or duct tape is applied to the sample and pulled off. This testing method is usually used to determine whether the adhesion of a coating to a substrate is adequate. If the current substrate is already coated, you can also establish the strength of the current coating or of the top layer of paint. You can see if the tape pulls off the top layer and leaves underlying layers still on the surface. This “intercoat” failure lets you know that the base adhesion is good and allows you to understand if the top layers are exhibiting strong enough adhesion to not be pulled off by another coat of paint.
The scrape adhesion test measures the tenacity of the adhesion of organic coatings when applied to smooth, flat panel surfaces. It is helpful in giving relative ratings for a number of coated panels showing significant differences in adhesion. The materials being tested are applied at uniform thickness to flat panels, mainly some sort of sheet metal. When the materials have dried the adhesion is determined by pressing panels under a rounded stylus that is loaded with increasing amounts of weight until the coating is removed from the substrate surface. This is a bit more scientific and usually if performed in a lab.
This can also be performed in a lab setting but the simple version of this test, and one we use mainly when testing adhesion of roof coatings is completed by applying our test materials to the actual substrate we intend to coat. The test area needs to be clean and dry. Apply some of the base coat or primer if specified. Immediately embed a small piece of polyester fabric (6”X2”) into the coating and then completely coat over the fabric with the coating. You want to leave about a 2” tab of the fabric out of the coating so you can pull it after the coatings have cured. Typically it takes 48-72 hours before the coating is cured enough and needs to be pulled. What you are looking for is that once you pull on the fabric tab the coating pulls apart, or has inner coat failure. If the coating is stuck so good that you pull it apart, it should obviously be stuck well to the substrate. If you pull the coating off clean to the substrate then adhesion has failed and other testing with other products is required.
View this video that properly illustrates how to install a pull test:
President – Custom Coatings Inc