Every bonded joint relies on two different forces. Understanding them is the key both to choosing the right product and to finding the cause of a failure.
What is adhesion?
Adhesion is the force with which the adhesive holds onto the surface. It is the sum of physical (van der Waals forces, hydrogen bonds), chemical (covalent bonds) and mechanical (interlocking in pores) interactions at the interface. Good adhesion requires the adhesive to wet the surface completely.
What is cohesion?
Cohesion is the internal strength of the cured adhesive film. Polymer chain entanglement and crosslinking determine it. In reactive systems, more crosslinking means higher cohesion and heat resistance.
What failure modes tell you
Looking at the surfaces of a failed joint tells you a lot:
| Failure mode | Appearance | Likely cause |
|---|---|---|
| Adhesive failure | Adhesive stays entirely on one surface | Poor wetting, contamination, low surface energy, no primer |
| Cohesive failure | Adhesive remains on both surfaces | Adhesive strength exceeded; a stronger chemistry may be needed |
| Substrate failure | The material itself tears | Ideal: the bond is stronger than the material |
What to do in practice
For adhesion problems, focus on surface preparation: cleaning, sanding, corona or plasma treatment and a suitable primer. For cohesion problems, consider a stronger or crosslinking system, for example PUR instead of EVA.
The goal is always substrate failure, where the weakest link in the joint is the material rather than the adhesive.
