Two physical forces form the basis of any adhesive bond: adhesion and cohesion.
Adhesion refers to the tendency of different substances or surfaces to cling to one another. It is caused by intermolecular attraction forces that work between surfaces up to a distance of 10-5mm. The smaller the distance between the surfaces, the greater the adhesion. Under a microscope smooth appearing surfaces actually show jagged surfaces. When these gaps are filled with adhesive (glue) the contact area between the two surfaces is optimised thus leading to maximum adhesion forces.
Cohesion can be described as the tendency of similar substances or surfaces to cling to one another, or in simple terms: the “inner strength” of an adhesive. It is caused by cohesive forces and determines the strength of the cured adhesive when under load. Cohesion and adhesion are interrelated: As a general rule of thumb: The harder an adhesive, the higher the cohesion and lower the adhesion. And vice versa. Various cohesive and adhesive properties can be achieved by modifying the formulation of the respective glue, also resulting in harder and softer adhesive products.
What forces impact the adhesive bond?
When it comes to adhesive tape, peel- and shear- forces have the greatest impact. Peel forces attack the bond at an angle > 0°. A practical example to explain peel forces is: Peeling adhesive tape off a cardboard box by hand. Shear force describes the resistance of a bond to forces parallel to its application direction. This force ensures that a packaging tape safely fixes the flaps of a cardboard box under static or dynamic load.
What are the failure modes of an adhesive bond?
Adhesive residue on the substrate surface generally indicates a cohesive failure, and can be interpreted as insufficient “inner strength” of the adhesive. With adhesion failure the „inner strength“ of the adhesive is greater than the adhesive bond with the surface, resulting in a clean surface without adhesive residue. Depending on the application, cohesive or adhesion failure are in fact technical product features required for the specific task: For surface protection clean removal (adhesion failure) is desirable, contrary to a “security tape” used for temper evident carton closure, where adhesive residue on the surface (cohesive failure) points to unauthorised access. Either failure mode may also indicate that the chosen tape is unsuited for the application, or might be attributed to a product defect. Choosing the right adhesive tape that’s suitable to the required task is vital.
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