Problem Solving Forum
August 27 - August 31, 2018
How do you adequately assess the adhesion of a newly applied coating?
Zenith Czora of Durotech Industries, Inc. on
September 3, 2018:
Adhesion testing of coating is a destructive metho ...read more Adhesion testing of coating is a destructive method. ASTM D3359 ( Tape Test) and ASTM D6677 (Knife Test) are most widely used test methods in evaluating the adhesion properties of a coating or coating system to the substrate. Both test methods apply shear force to the coating, are qualitative and subjective. On the Knife Test, using the knife and cutting guide, two cuts are made into the coating to form an "X" pattern. At the vertex, the point of the knife is used to attempt to lift up the coating from the substrate. Performance is based on both the degree of difficulty to remove the coating from the substrate and the size of the removed coating. Another way to test is to apply a tape on the "X"cut and pull it off. This test is highly subjective and its value is dependent on the inspector's experience. Cross-hatch Tape Test method provides a laboratory or field procedure for adhesion assessment of a coating to substrate by applying and removing the pressure-sensitive adhesive tape on the cuts made in the coatings. It uses a cross-hatch pattern rather than the "X" pattern. A cutting guide or special cross-hatch cutter with multiple preset blade is needed to make sure the incisions are properly spaced and parallel. Using 3M Filament /Tape No. 898, semi-transparent pressure-sensitive tape with a minimum adhesion strength of 6N and a maximum adhesion strength of 7.5N, apply the tape with sufficient pressure on the cross-hatch cut and then pull off. Then the cut area is inspected and rated according to ASTM D3359. This test method is not applicable to soft and friable substrates and not suitable for coatings of total thickness greater then 250 um or for textured coatings. For quantitative test for adhesion, Pull -Off Test (ASTM D4541) is used with a loading fixture commonly known as a dolly, which is affixed with 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 psi or MPa, Failure will occur along the weakest plane within the system comprised of the dolly, adhesive, coating system, and substrate, and will be exposed by the fractured surface. Pull-off strength measurements depend upon the instrument used in the test. This test method can be used for all coatings on rigid materials but is particularly suitable for adhesion testing of coatings on concrete and masonry surfaces. In multi-coating system, if the adhesion failure occurs between coats, the adhesion properties of the coating system to the substrate cannot be determined.