Adhesion Theories

Adhesion is an event in which allows the transference of load from the adherent to the adhesive joint [7]. There are numerous theories of adhesion which have been proposed over a long period of time. Some of the classical adhesion theories are summarized in the figure 1.

Figure 1 Adhesion Theories [1]

The theories of mechanical interlocking, electrostatic, diffusion and adsorption/surface reaction theories propose the mechanisms of adhesion. More recently there are other theories under the surface reaction theory such as the wettability, chemical bonding and weak boundary layer theory which describes the mechanism of adhesion in atomic and molecular levels. Andrew stated “It is often difficult to fully ascribe adhesive bonding to an individual mechanism. A combination of different mechanisms is most probably responsible for bonding within a given adhesive system” [8]. An important

between the adhesive and adherent. Some interactions scale from the atomic level to the molecular level. For instance, for mechanical interlocking the contact surface of the adhesive and the adherent plays as an important parameter, in the case of electrostatic mechanism, surface charge is the macroscopic factor and so on.

2.2.1 Mechanical Theory

According to this theory, adhesion occurs by penetration of adhesives into pores, cavities and other surface irregularities on the surface of the substrate [8]. Air is displaced from the interface between the contact surface and the adhesive and thus the adhesive penetrating into the surface roughness of two adherents can bond themselves. According to researches, bonding can be enhanced by surface modification of the adherents, however the attainment of good adhesion between smooth surfaces exposes the mechanical interlocking theory. Brewis studied the adhesion between two perfectly smooth mica surfaces and examined the adhesion to optically smooth surfaces, clearly demonstrated that adhesion may be attained with smooth surfaces [9].

The mechanical interlocking model, proposed by McBain and Hopkins as early as 1925 [8], considers that mechanical interlocking, of the adhesive into cavities, pores and surface irregularities is the major factor in determining the adhesion strength. To overcome the difficulty of good adhesion between smooth surfaces, the following approach primarily suggested by Gent and Schultz, Wake proposed that the effect of both mechanical and thermodynamic interfacial interactions could be taken into account as multiplying factors for estimating the joint strength G [8].

G = (constant) x (mechanical interlocking component) x (interfacial interaction component) (Eq.1)

According to the equation (1), a high level of adhesion should be achieved by improving both the surface morphology and physiochemical surface properties of both the substrate and the adhesive [8]. Apart from the roughness and porosity of the substrate surface, to generate adhesion anchor points, it is necessary that the adhesive has a good filling capacity, the adhesive should be able to penetrate into the valleys, pores and ridges of the substrate surface and this property is directly related to the viscosity of the adhesive.

2.2.2 Polarization Theory

This theory is also known as the “Electronic theory” [1], which describes that the adhesive bond between the substrate and the adhesive strengthens as the intermolecular interaction (forces) between the atoms increases. These intermolecular interactions have recognized the existence of permanent or oscillating dipoles, which interact one with another in chemically saturated systems, but which are also able to induce dipoles in other materials. These interactions are generally lower in strength or have lower binding energies when compared to those of the chemical interactions, they neither change the nature of materials. The interaction can be termed as ‘physical bonds’ and can be divided into three categories;

Permanent dipoles: This kind of dipoles are formed when an atom with a higher atomic number is bound to another atom with a lower atomic number, thus producing a permanent dipole. Theses dipoles are able to build electrostatic attraction forces in the form of a dipole interaction with another permanent dipole. For example, hydrogen has a permanent dipole-dipole bond.

Induced dipoles: When a non-polar atom interacts with a permanent dipole atom, the permanent dipole can induce counter dipoles with which they build up static attraction forces. The binding

energy is lower when compared to the permanent dipole-dipole bonds.

Dispersion forces: These are the forces which may exist between two non-permanent dipoles. A weak oscillating dipole can be found between the involved atoms, but the statistical probability distribution of the binding electrons is not completely uniform. Their binding energy is generally lower than the permanent dipole-dipole bonds.

These dipoles play a role in the adhesion of adhesive to substrates and vice versa. Generally, an adhesive containing a polar group can be shown to adhere better to a polar substrate or a solid-state material than to a non-polar material [1].

2.2.3 Diffusion Theory

In simple terms, diffusion theory explains the concept of adhesion by the compatibility between polymers and the movement that occur in the anchorage areas and adhesion points take place. The mobility and degree of

penetration of the polymers is determined directly by their molecular weight. Short polymer chains have higher mobility and penetrate into other material before the long polymer chain diffuse completely.

Figure 2 Rate of diffusion of polymeric chain between components A] Partial

2.2.4 Chemical Reactions Theory

In the research by McBain and Lee [10] focused on the adhesion of polished steel and aluminium parts with organic adhesives (mainly shellac); the authors referred to ‘cohesive failure’ in the adhesive, with the adhesion forces having proved to be stronger. It was assumed during their research that the binder had a chain structure that was influenced by the surface and reached deep into the structure of the adhesive layer. There was no clear mention of any chemical interactions with the metal as being responsible for this high degree of adherence. These were the earliest result which are used until today for explaining the adhesive strengths of adhesive and the chemical interactions involved which could prove the impact on properties of the bonds.

2.2.5 Adsorption Theory

This theory is also referred to the Thermodynamic theory or Wettability or the Acid-Base theory. The thermodynamic model of adhesion, generally attributed to Sharpe and Schonhorn [11], is certainly the most widely used approach in adhesion science at present. This theory considers that the adhesive will adhere to the substrate because of the interatomic and intermolecular forces established at the interface, provided that an intimate contact is achieved. The most common forces found are the Van der Waals and Lewis acid-base interactions. Fundamental thermodynamic quantities such as surface free energies of both the adherent and the adhesive play an important role in the magnitude of forces in the adhesive bond.

Wettability of the surfaces can be calculated by the simple test of surface tension by dropping a drop of water on the substrate and calculating the contact angle between the drop and the surface using an optical microscope, thus calculating the surface free energy by the law of equation of states. When the adhesive has a

it is capable of wetting the surface, generating a contact angle less than 90ᵒ, thus achieving adhesion between the adhesive and the substrate. This angle is known as the ‘critical angle for adhesion’.

Against the mechanical theories and the diffusion theory, adsorption theory explains the phenomenon of adhesion without penetration by the adhesive to the substrate; the adhesion is generated by the contact between the adhesive and the substrates [9].