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Why do adhesives stick?


Frequently Asked Questions

There are four main theories that can be used to explain adhesion:

  • Adsorption theory - adhesion occurs as a result of inter-atomic and intermolecular forces that are established when the adhesive wets out the surface of the adherend. This theory is generally acknowledged to have the widest applicability to adhesive bonding. Although primary (e.g. covalent or ionic) and secondary (e.g. hydrogen or van der Waals) bonds can be formed through direct chemical reactions, the main concept that explains adhesion is the thermodynamic work of adhesion, which is related to the surface free energies (surface tensions) of the adhesive and adherend.
  • Mechanical interlocking - the adhesive penetrates irregularities on the surface of the adherend and 'locks' itself into the material. This theory is the simplest to comprehend, but has the drawback that adhesion can occur on smooth surfaces, which means that interlocking is not the sole answer.
  • Diffusion theory - molecules diffuse from one material to another, across the interface between the materials. This theory is mainly applicable to welding of thermoplastics, and sputtering or evaporation processes used to form polymer-metal interfaces.
  • Electronic theory - proposes that there is electronic charge transfer between two materials that have different electronic band structures. This charge transfer creates an electrostatic force, although there is still debate as to whether these electrostatic forces are the cause of adhesion rather than an effect due to the intimate contact that has been established. Experiments indicate that the electrostatic component is always significantly less than the van der Waals contribution, and is at best 10% of the total bond energy.

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