Adhesion Testing Methods
For coatings to perform satisfactorily, they must adhere to the substrates on which they are applied. A variety of recognized methods can be used to determine how well a coating is bonded to the substrate.
This simple test requires the use of a utility knife to pick at the coating. It establishes whether the adhesion of a coating to a substrate or to another coating (in multi-coat systems) is at a generally adequate level. Performance is based on both the degree of difficulty to remove the coating from the substrate and the size of removed coating.
Using the knife and cutting guide, two cuts are made into the coating with a 30 – 45 degree angle between legs and down to the substrate which intersects to form an "X". At the vertex, the point of the knife is used to attempt to lift up the coating from the substrate or from the coating below.
This is a highly subjective test and its value depends upon the inspector's experience. A coating which has a high degree of cohesive strength may appear to have worse adhesion than one which is brittle and hence fractures easily when probed. There is no known correlation to other adhesion test methods (pull-off, tape, etc.).
A standard method for the application and performance of this test is available in ASTM D6677.
On metal substrates, a more formal version of the knife test is the tape test. Pressure sensitive tape is applied and removed over cuts made in the coating. There are two variants of this test; the X-cut tape test and the cross hatch tape test.
The X-cut tape test is primarily intended for use at job sites. Using a sharp razor blade, scalpel, knife or other cutting device, two cuts are made into the coating with a 30 – 45 degree angle between legs and down to the substrate which intersects to form an "X". A steel or other hard metal straightedge is used to ensure straight cuts. Tape is placed on the center of the intersection of the cuts and then removed rapidly. The X-cut area is then inspected for removal of coating from the substrate or previous coating and rated.
The cross hatch tape test is primarily intended for use in the laboratory on coatings less than 5 mils (125 microns) thick. It uses a cross-hatch pattern rather than the X pattern. A cutting guide or a special cross-hatch cutter with multiple preset blades is needed to make sure the incisions are properly spaced and parallel.
A standard method for the application and performance of these tests is available in ASTM D3359.
A more quantitative test for adhesion is the pull-off test where a loading fixture, commonly called a dolly or stub, is affixed by an adhesive to a coating. By use of a portable pull-off adhesion tester, a load is increasingly applied to the surface until the dolly is pulled off. The force required to pull the dolly off or the force the dolly withstood, yields the tensile strength in pounds per square inch (psi) or mega Pascals (MPa). Failure will occur along the weakest plane .
The availability of a full range of pull off adhesion tester models facilitates the measurement of coating adhesion on almost any rigid substrate. For example, 20 mm dollies may be ideal for typical coating bond strengths on metal, plastic and wood, where as 50 mm dollies are more ideal for coatings on masonry substrates such as concrete.
A standard method for the application and performance of this test is available in ASTM D4541 and ISO 4624.
This test is typically performed in a laboratory and is limited to testing on smooth, flat panel surfaces. Adhesion is determined by pushing the coated panels beneath a rounded stylus or loop that is loaded in increasing amounts until the coating is removed from the substrate surface. A device called a balanced-beam scrape-adhesion tester is used.A standard method for the application and performance of this test is available in ASTM D2197.