Adhesion and flexibility are two very different but closely related coating/film properties. Changing one of these two attributes in a product can cause the other to change, even though they are completely different attributes.
Coating Adhesion
Adhesion can be thought of as the chemical or physical bond between the coating and the substrate. It is made more complex from simple destructive test determinations such as cross-hatch or scrape adhesion tests, or with a pull-off test in which the actual tensile stress necessary to pull the coating from the substrate is measured.
Coating flexibility
Flexibility can be thought of as internal adhesion. Does the adhesion in the paint make it move while making good contact with the substrate or does it crack and crack. Flexibility testing involves deformation of the substrate and inspection of the coated surface for cracking.
Often, but not always, coatings that are not very flexible will also come off the substrate. A balance is needed to prevent coatings that are too flexible from peeling off the substrate and causing air pockets.
