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Paints and coatings are only worth selling and using if they stick to the surfaces where they are applied. An adhesion test is usually how products are assessed to ensure that they do what they are supposed to. There are several recognized methods of testing.
The knife test is a simple method that can usually be performed with a basic utility knife. Two cuts that form an "X" are normally made into the coating. At the vertex of the cut, the person performing the test will use the point of the knife and try to lift the coating.
This adhesion test bases performance on how difficult it is to remove the coating and the amount of coating that is removed. The knife test can be tricky because it is subjective. If the person conducting the test is experienced, he can quickly determine whether a coating is satisfactory. For a person with less experience, this test may prove problematic because a coating may have good cohesion but be brittle.
The tape test can be effective when measuring several coated layers. This adhesion test assesses the pressure needed to detach the coating from a surface or an underlying layer. There are two popular versions of the tape test.
First, there is the X-cut tape test, which is often used on job sites. To perform this adhesion test, it is also usually necessary to cut an "X" into the surface. Pressure sensitive tape is then applied at the intersection of the cuts and quickly removed. The quality of the coating is based on amount that is removed from the cut area.
The second tape test is the cross hatch test. This adhesion test is usually conducted in laboratories. It is usually limited to coatings that are less than 125 microns thick. The "X" is replaced with a cross hatch pattern, and the incisions are usually carefully spaced and proper care is used to ensure that they are parallel. Then, tape is applied, removed, and the test area is inspected.
A scrape test is a method that is also usually conducted in a laboratory. This adhesion test is limited to the testing of paint or coating adhesion on smooth, flat panel surfaces. The test substances are usually applied at a measured thickness to numerous identical surfaces. Once the paint or coating has dried, the surfaces are pressed under a stylus. Using a device called a balanced-beam scrape adhesion tester, the weight is increased until the coating is removed.
There are many other types and methods of adhesion testing. These include bend testing, pull-off testing, and chemical resistance testing. Which adhesion test is used depends on factors such as the type of paint or coating in question, the materials available for the test, and the experience of the person performing the test.