The surface energy is one decisive criterion for the adhesion of printing ink, glue, varnish, etc. on many plastic and metal surfaces.

It is given in mN/m (milliNewton per meter, currently valid unit) or in dyne/cm (dyne per centimeter, old unit, same numerical value).

Apart from some exceptions the general rule is: The higher the surface energy of a material, the better is the adhesion of anything meant to be fixed to its surface.

Reasons for a low surface energy on the one hand might be material specific. For example, many plastics show a very low surface energy and therefore need a chemical or physical treatment to achieve a good adhesion on them.

On the other hand, contaminations of the surface (grease, oil, or finger prints) might be a reason for a low surface energy.