Mechanics of Adhesion

Adhesives are substances that are used to join two or more components together through attractive forces acting across the interfaces.  The components being joined are commonly referred to as adherends or substrates.  To function as an adhesive, a polymer must be able to take on the characteristics of both a liquid and a solid and coalescing (in the case of waterborne latex adhesives)1. The ability to properly wet a given surface depends on two aspects: favorable thermodynamic surface energies that encourage intimate contact of the adhesive and substrates being bonded, and sufficient time and molecular mobility for the adhesive to conform to and wet the substrate.  Increasing pressure, temperature, or time of bonding may accelerate this latter kinetic process.  If the adhesive and adherend surfaces are incompatible thermodynamically, however, poor bonding will often result because of the difficulty of achieving intimate contact between adhesive and substrate.

Over the years, a number of mechanisms have been used to explain adhesion in its many forms.  Current understanding of the matter is that secondary bonding forces between atoms in close proximity are able to account for much of adhesion we observe.  These bonds are referred to as dispersion or van der Waal forces, and breaking these molecular attractions would require relatively small amounts of energy, typically on the order of several tens of mJ/m22.  Primary chemical bonds can exist in certain situations, and although inherently stronger and more durable, this mode of adhesion is less widespread.  Molecular inter-diffusion can also occur between two miscible polymers; a good example of this is rubber cement applied to two surfaces then brought together to allow inter-diffusion to occur.  In some situations, electrical double layers can develop across the bond plane, leading to electrostatic attraction3.  Frequently, enhanced bond strength can result from mechanically roughening the surface of substrates through abrasion or grit blasting.  While commonly thought to enhance mechanical interlocking of the adhesive within the crevices and pores of the surface, mechanical roughening can remove weakly bound surface layers and increase the available bond area as well.

Breaking the attractive forces between the atoms of the adhesive and the substrates gives rise to small energies known as the thermodynamic work of adhesion, that are often measured in tens of mJ/m2.  On the other hand, when real adhesive joints are mechanically broken, fracture energies of several kJ/m2 are often measured.  These latter energies are referred to as practical adhesion, and are typically several orders of magnitude larger than the thermodynamic surface energies.  This significant multiplication of energies is associated with the dissipation of energy in the adhesive, and sometimes the adherends, through viscoelastic or plastic deformation processes.  These practical adhesion energies are quite dependent on rate of loading and temperature, and can often depend on adhesive and adherend thickness as well.  For characterizing adhesive performance, fracture energies are often measured in such a way that plastic deformation of the adherends does not occur, as it can significantly overestimate the adhesive toughness.  On the other hand, this enhanced energy dissipation in the adherends can be an important factor in practical adhesive joints.  For example, in an automobile, passenger safety depends on substantial energy dissipation in the crush zones achieved because the adhesive bonds force the metallic components to deform in such a way that the metal dissipates large amounts of plastic energy.

The large values of practical adhesion energies for high-quality modern adhesives provide significant resistance to debond growth.  Adhesive formulators have developed a number of toughening techniques that have been successfully incorporated into commercial adhesives, improving their resistance to impact and fatigue loading.  This is one reason that properly designed adhesive joints utilizing tough adhesives can be more durable under sustained fatigue loading than joints assembled with mechanical fasteners, where stress concentrations surrounding holes or spot welds can lead to failure over repeated load cycles.

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1An apparent exception to this two stage process are pressure sensitive adhesives that remain relatively soft.  Under light pressure conditions, the soft adhesive is able to flow to fill surface asperities, wetting the substrate.  Once it has wet-out the adherend, a significant amount of energy is required to remove the adhesive.  This is especially true for relatively rapid withdrawal, where the adhesive molecules do not have sufficient time to move, making the adhesive act more like a solid.

2For comparison, the surface energy of water is 72 mJ/m2, so work of adhesion valuesinvolving secondary bond attractions are typically smaller than this.

3To see an example of this, rapidly peel a pressure sensitive adhesive tape from glass in a dark room to which your eyes have become accustomed.  You will likely see small blue sparks associated with electrical break-down of this charged double layer.