In order for coatings to perform well, they need to adhere to the substrate to which they are applied. Various accepted methods can be used to determine the extent to which the coating binds to the substrate. Commonly used measurement techniques are performed with a knife or a pull adhesion Tester. After any testing, it is important to document if the bond failure is either adhesive (failure at the coating/substrate interface) or bond (failure in the coating or substrate).
Try the knife
This simple test requires the use of a practical knife to select the coating. It determines whether the degree of adhesion of the coating to the substrate or other coatings (multi-coat systems) is average. Performance is based on how difficult it is to remove the coating and the size of the coating removed.
Using a knife and a cutting guide, make a 30 to 45 degree angle between the two incisions, intersecting between the legs and the bottom to form an "X". At the apex, use the tip of the knife to try to lift the coating from the substrate or from the coating underneath.
This is a highly subjective test, and its value depends on the inspector's experience. Coatings have high cohesive strength, and their adhesion can be worse than brittle coatings, so they are prone to breakage when probed. There is no known correlation with other adhesion test methods (peeling, tape, etc.).
The application and performance of standard methods in this test are available in ASTM D6677.
Tape test
On a metal substrate, a more formal version of the knife test is the tape test. Pressure-sensitive tape is applied and cuts in the coating are removed. There are two variants of cutting tape tested; Test and cross-test tapes.
Cutting tape testing is mainly used in the workplace. Using a sharp razor blade, scalpel, knife, or other cutting device, the two incisions are made at an angle of 30 to 45 degrees, intersecting between the legs and at the bottom to form an "X". Steel or other metal straightedges are used to ensure straight cuts. The tape is placed at the intersection of the cutting center and then quickly removed. The cutting area is then inspected to remove the coating from the substrate or previous coating and rating.
The main purpose of the cross-incubation zone test is to use in the laboratory on coatings less than 5 mils (125 microns) thick. It uses the crosshatch pattern instead of the X pattern. It is necessary to have a cutting guide or a special cross-shaped cutter with multiple preset blades to ensure that the cuts are properly spaced and parallel. Once the tape has been applied and unplugged, the cut area will be inspected and rated.
The application and performance standard methods of these tests are available in ASTM D3359.
Pull-out test
A more quantitative adhesion test is a tensile test, in which a loading fixture, often referred to as a trolley or stub, is adhered to the coating by an adhesive. By using a portable peel adhesion Tester, the load is increasingly applied to the surface until the trolley is pulled out. The force required to pull the trolley or force the trolley to hold up, yielding tensile strength (PSI) or megapascals (MPa) per square inch-pound. Failure will occur on the weakest plane of a system consisting of a trolley, adhesive, coating system, and matrix, and will be exposed by fractured surfaces.
This test method maximizes tensile stresses and may not be comparable in results compared to the shear stresses applied by other methods, such as scraping or knife adhesion. In addition, the measurement of pull-off strength depends on the instrument used in the test. The results obtained by different devices or results using the same coating on substrates of different hardness may not be comparable.
The Tester operates mechanically (manual twisting), hydraulics (oil), or pneumatic (pneumatic). They are classified as fixed alignment or self-alignment, depending on their ability to ensure a vertical pull-off force. When the pull-off force is perpendicular to the surface to be measured, very good repeatability is obtained.
A full set of peel adhesion Testers (Pull-out adhesIon MeterThe availability of models helps to measure coating adhesion on almost all rigid substrates. For example, a 20mm doll may be a typical metal coating with a good bond strength for plastic and wood, while a 50mm cart masonry substrate such as a concrete coating is more satisfying. Many manufacturers also offer custom Dolly sizes to meet special measurement needs. For example, reducing the trolley size from 20 mm to 10 mm increases the gauge by a factor of 4, allowing some manufacturers to accurately measure bond strength in excess of 12,000 psi. Keeping the cart cheap does not require the use of the cart, greatly simplifying the preparation process to keep the physical sample alive for future reference or evidence of pull-out test results.
A standard method for application and performance in this test ASTM D4541 and ISO 4624 is available.
Scratch test
This test is usually performed in a laboratory and is limited to testing on a smooth flat surface. Adhesion is achieved by pushing the coated panel onto a circular pen ring or ring until the increased coating is removed from the surface of the substrate. A device called a balance beam scraping Tester.
The application and performance standard methods in this test are available in ASTM d2197.
Other tests
There are many other adhesion tests. Some of these involve the use of a Tensile Testing Machine, where a layer of fabric embedded in the paint is applied to the substrate (the tensile machine pulls the substrate and cloth), or paint is applied between two sheets of substrate (the tensile machine pulls on the two substrates). ASTM d2370 describes such a test for elongation, tensile strength, stiffness, and organic films when tested for free movies. Organic coating adhesion to plastic substrates mounted on aluminum studs and disassembly with the tensile Tester is covered with ASTM D5179.
ASTM d4145 describes flexure testing to determine the flexibility of pre-coated metal substrates and coating adhesion. These organic coatings are subjected to stress when they are made into products through roll forming, bending, or other texturing processes. These stresses can exceed the flexibility or bond strength of the coating, causing the coating to break, expose the substrate, or lose adhesion to the substrate. This test is a way to evaluate the ability of a coating system to withstand manufacturing pressures.
Formability and adhesion tests for factory zinc-rich primers/chromate composite coatings are described in ASTM 4146. In this test, the coated specimen is stretched at a specified distance at a suitable machine, the tape is applied to the deformation zone (dome) and then removed, compared to the photographic standard that determines the adhesion level of the coating.
Adhesion is also measured with pencil hardness, hardness test gravelometer with measurable results, impact (dart, etc.) or shaft bending. Coating detachment should be recorded during the test. Finally, in some chemical resistance tests, adhesion is lost when the coating blisters, blisters, or even peels off.
