Adhesion, the connection between adhesive and substrate may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of numerous chemical mechanisms. The strength of adhesion depends on many factors, including the means by which it occurs.

 

In some cases an actual chemical bond occurs between adhesive and substrate. In others electrostatic forces, as in static electricity, hold the surfaces together. A third means involves the van der Waals forces that arise between molecules. A fourth way involves the moisture-aided diffusion of the glue into the substrate, followed by hardening.

 

 

 

 

 Breakdown of the adhesive bond can happen in dissimilar locations

 

There are quite a few factors that could add to the breakdown of two adhered surfaces. Sunlight and heat may weaken the adhesive. Solvents can deteriorate or dissolve adhesive. And physical stresses might cause the separation of surfaces. When subjected to loading, debonding may take place at different locations in the adhesive joint. The main fracture types are the following:

 

Cohesive fracture is obtained if a fracture propagates in the bulk polymer which constitutes the adhesive. In this case the substances of both adherents after debonding will be covered by fractured adhesive. The crack may propagate in the centre of the layer or near an interface. For this last case, the cohesive fracture can be said to be “cohesive near the interface”. Most quality control standards consider a good adhesive bond to be cohesive.

 

The fracture is adhesive or interfacial when debonding occurs among the adhesive and the adherent. In the majority cases, the occurrence of interfacial fracture for a given adhesive goes along with a smaller fracture toughness. The interfacial nature of a fracture surface is usually to identify the precise location of the crack path in the inter-phase.

 

 

Other types of fracture include:

•           The mixed type, which occurs if the crack propagates at some spots in a cohesive and in others in an interfacial manner. Mixed fracture substances can be characterised by a certain percentage of adhesive and cohesive areas.

 

•           The alternating crack path type which takes place if the cracks jumps from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.

 

•           Fracture can also happen in the adherent if the adhesive is stronger than the adherent. In this instance the adhesive remains intact and is still bonded to one substrate and remnants of the other. For example, when one removes a price label, adhesive typically continues on the label and the surface. This is cohesive breakdown. If, however, a layer of paper remains fixed to the surface, the adhesive has not failed. Another example is when someone tries to pull apart Oreo cookies and all the filling remains on one side; this is an adhesive failure, rather than a cohesive failure.