An adhesive wets and spreads well only when its surface tension is lower than the surface energy of the part. Clean metals and glass have high surface energy; polyethylene sits around 31 mN/m, polypropylene around 29 and PTFE below 20. Many bonding and printing specs ask for at least 38 to 42 dynes per centimeter on plastics, reached by treating the surface.
Surface energy on the PP housing is about 30, nothing will stick until we flame or plasma it.
Some powder coatings and paints are as hard to bond as polyolefins, so dyne-test coated parts too.
Also heard as
- surface free energy
- dyne level
Related terms
Wetting
How well a liquid adhesive spreads over and makes intimate contact with a surface.
Dyne pen
A marker filled with test fluid of known surface tension, used to check a surface's treatment level.
LSE
Low surface energy, describing plastics and coatings that most adhesives cannot wet or bond without help.
Plasma treatment
Exposing a surface to ionized gas to clean it and add reactive groups that raise its surface energy.