Adhesion/Cohesion Theory

(Courtesy 3M)

Any discussion of stresses on bond performance begins with adhesion/cohesion theory. Adhesion (see figure 1) is the force that holds substrates together in opposition to stresses exerted to pull the substrates apart.

Figure 1 - Adhesion

There are three basic types of adhesion:

  1. Specific Adhesion -the molecular attraction between contacting surfaces.
  2. Mechanical Adhesion - occurs when an adhesive flows into the microstructure of the surfaces to be bonded.
  3. Effective Adhesion -combines specific and mechanical adhesion for optimum joining strength.

Cohesion (figure 2) is the attraction of particles within the adhesive (or other material) that holds the adhesive mass together. The combination of adhesion and cohesive strength determine bonding effectiveness. An adhesive bond fails if either the adhesive separates from the substrate (figure 3) or the adhesive breaks apart (figure 4). The adhesive and cohesive strengths of some adhesives are great enough that the cohesive strength of the substrate fails before the adhesive bond (figure 5).

 Figure 2 - Cohesion

 Figure 3 - Adhesive Failure Mode

 Figure 4 - Cohesive Failure Mode

 Figure 5 - Substrate Failure