Adhesive failure

Adhesive and cohesive failures are the two most common forms of adhesive bond failure.

One speaks of an adhesive failure when the adhesive separates from the substrate, as opposed to cohesive failure, which occurs when the bonding forces inside the adhesive fails. If, after the test or the fracture, the entire adhesive is to be found on one substrate and there is no adhesive at all on the other, this is referred to as a pure adhesive failure.  

Pure adhesive failures are mostly due to:

• insufficient surface preparation
• surface contamination
• an insufficiently cured adhesive film

If the adhesive and the component to be bonded show very different thermal behaviour, this can also lead to adhesive fractures, as strong physical forces occur.   
The coefficient of thermal expansion of the adhesive and the substrate should be very close to avoid the stresses that can occur during temperature changes or following cooling after an  elevated temperature cure. Another possibility to equalise these tensions is to use elastic adhesives. Examples of this would be pressure-sensitive adhesives, but also so-called APAO Amorphous Poly-Alpha-Olefins (PO with a longer open time).