ADHESION

The bond between a coating and a surface, or a bond between two coatings. The means of adhesion are primarily mechanical, which requires that the coating comes into intimate contact with the substrate. (Substrate = a more fancy term meaning surface). If the surface is dirty, waxy, oily or slick, the coating will be prevented from contacting substrate and poor adhesion will result.. To gain excellent adhesion it may be necessary to clean, degloss, or roughen the surface.

A Tape AdhesionTest (ASTM D3359) is the most common method used to evaluate the adhesion of a coating to a surface. In simplified terms the test requires cutting an X-pattern or # crosshatch pattern through the coating down to the substrate with a razor blade. A strip of tape is firmly applied over the cuts and then removed. The adhesion is rated based on the amount of coating removed with the tape.

COHESION

A coating forms a film by changing from a liquid to a solid, or in the case of powder coatings from a dry powder to a solid film. Cohesion refers to the internal strength of the film. A Pull Of Adhesion tester can be used to determine the relative cohesive strength. The test requires gluing dollies to the surface of the coating. After the glue has cured a device is attached to the top of each dolly any applies upward pressure to pull the dolly free. The point that separation occurs indicates the cohesive strength of that layer and the cohesive strength of other layers is considered greater.

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