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Theories of Adhesion

Edward M. Petrie – Jan 4, 2006

Theories of AdhesionEver wondered – why adhesives stick?
Unfortunately, there is no single, comprehensive theory.

Several theories attempt to describe the phenomenon of adhesion. No single theory explains adhesion in a general, comprehensive way. Some theories are more applicable for certain substrates and applications; other theories are more appropriate for different circumstances.

Each theory has been subjected to study, question, and controversy. However, each contains certain concepts and information that are useful in understanding the basic requirements for a good bond. The most common theories of adhesion are based on:

  • Adsorption
  • Mechanical interlocking
  • Electrostatics,
  • Diffusion, and
  • Weak boundary layers

Let’s understand them each of them deeply...

Adsorption Theory

The adsorption theory states that adhesion results from molecular contact between two materials and the surface forces that develop usually designated as secondary or vander Walls forces. For these forces to develop, the adhesive must make intimate, molecular contact with the substrate surface.

The process of establishing continuous contact between an adhesive and the adherend is known as "wetting". Wetting can be determined by contact angle measurements. Complete, spontaneous wetting occurs when Contact angle = 0°, or the material spreads uniformly over a substrate to form a thin sheet.

Wetting is favored when the substrate's surface tension, better known as the critical surface energy, C, is high and the surface tension of the wetting liquid, is low (i.e., C substrate < Cadhesive). Low-energy polymers, therefore easily wet high-energy substrates such as metal and glass. Conversely, substrates having low surface energies (e.g., polyethylene and fluorocarbons) will not be readily wet by other materials and are useful for applications requiring nonstick, passive surfaces.

After intimate contact is achieved between adhesive and adherend through wetting, it is believed that permanent adhesion results primarily through forces of molecular attraction. Four general types of chemical bonds are recognized as being involved in adhesion and cohesion: electrostatic, covalent, and metallic, which are referred to as primary bonds and vander Walls forces that are referred to as secondary bonds. In most practical adhesive applications, secondary bonds are the predominant element that contributes to adhesive strength.

Contact Angle between Adhesive and Adherend
Contact Angle between Adhesive and Adherend

Mechanical Theory

At one time, adhesion was thought to occur only by the adhesive flowing and filling micro-cavities on the substrate. When the adhesive then hardens, the substrates are held together mechanically. The surface of a substrate is never truly smooth but consists of a maze of peaks and valleys. According to the mechanical theory of adhesion, in order to function properly, the adhesive must penetrate the cavities on the surface; displace the trapped air at the interface, and lock-on mechanically to the substrate.

One way that surface roughness aids in adhesion is by increasing the total contact area between the adhesive or sealant and the adherend. If the interfacial or intermolecular attraction is the basis for adhesion, increasing the actual area of contact will increase the total energy of surface interaction by a proportional amount. Thus, the mechanical theory generally teaches that roughening of surfaces is beneficial because it:

  1. Gives "teeth" to the substrate (mechanical interlocking), and
  2. Increases the total effective area over which the forces of adhesion can develop. However, roughening is only effective if the adhesive wets the surface well.

Electrostatic and Diffusion Theories

The electrostatic and diffusion theories of adhesion are generally not regarded as highly as the other theories in general bonding practice. However, there are certain applications where these are very important and help explain why bonds form.

The electrostatic theory states that electrostatic forces are formed at the adhesive-adherend interface. These forces account for resistance to separation. This theory gathers support from the fact that electrical discharges have been noticed when an adhesive is peeled from a substrate. Electrostatic adhesion is regarded as a dominant factor in biological cell adhesion and particle adhesion.

The fundamental concept of the diffusion theory is that adhesion arises through the inter-diffusion of molecules in the adhesive and adherend. The diffusion theory is primarily applicable when both the adhesive and adherend are polymeric, having compatible long-chain molecules capable of movement. Solvent or heat welding of thermoplastic substrates is considered to be due to diffusion of molecules.

Weak-Boundary-Layer Theory

According to the weak-boundary-layer theory, when bond failure seems to be at the interface, usually a cohesive rupture of a weak boundary layer is the real event. This theory largely suggests that true interfacial failure seldom occurs. In most cases, joint failure results from a cohesive failure of a weak boundary layer. Weak boundary layers can originate from the adhesive, the adherend, the environment, or a combination of any of the three.

Weak boundary layers can occur on the adhesive or adherend if an impurity concentrates near the bonding surface and forms a weak attachment to the substrate. When the failure occurs, it is the weak boundary layer that fails, although failure may seem to occur at the adhesive-adherend interface. In addition to external contamination, examples of weak boundary layers are corrosion or oxide layers on metal surfaces and low molecular weight constituents (e.g., release agents, plasticizers) on polymeric surfaces. Weak boundary layers must be removed by physical or chemical means so that there is no weak link in the adhesive joint that would contribute to premature adhesive failure.

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