Adhesion Cross Hatch Test.
Photo Courtesy of BYK-Gardner.

Adhesion and Flexibility are two very different yet closely related paint / film properties. By that I mean that changing one of these two properties in a product can result in changes in the other, even though they are entirely different properties.

Adhesion

Example of a Cross Hatch Adhesion test.
Photo Courtesy of BYK-Gardner.

Adhesion can be thought of as the chemical or physical bond that the coating has to the substrate. It is measured by simple destructive tests such as a cross-hatch or scrape adhesion test, or with a more complicated pull-off test where the actual tensile stress necessary to pull the coating from the substrate is measured.

Flexibility

Bending Lacquered sheet metal over a defined radius allows an indication of the elongation and adhesion of a paint film due to bending stress.
Photo courtesy of BYK-Gardner.

Use of a conical mandrel bending tester enables testing of a large variety of bending radii at the same time.
Photo courtesy of BYK-Gardner.

Flexibility can be thought of as an internal adhesion. Do the bonds within the paint allow it to move while in good contact with the substrate or does it break apart and crack. Tests for flexibility involve distortion of the substrate and examining the coating surface for cracking.

Often a coating that fails in flexibility will also break loose from the substrate, but not always. A balance is needed to prevent a too-flexible coating from popping loose of the substrate and causing a bubble.