The Electrostatic Theory of Adhesion

The electrostatic theory originated in the proposal that if two metals are placed in contact, electrons will be transferred from one to the other so forming an electrical double layer, which gives a force of attraction. As polymers are insulators, it seems difficult to apply this theory to adhesives. 

---

Electrostatic Theory. The basis of the electrostatic theory of adhesion is the differences ia the electroaegativities of adheriag materials (1,2). 

Aside from contact electrization, local accumulation of charges takes place as a result of mechanical separation and formation of a double electrical layer (DEL). Mechanical separation of charges is brought about by exfoliation of adhesive films from the metal or semiconducting samples. DEL may arise in response to chemical interactions of two phases or as a result of selective adsorption of similar ions, e.g. in the presence of oriented dipoles on the contact surface of one of the phases. Electrization can also be induced by the donor-acceptor (DA) interaction, since in agreement with the electrostatic theory of adhesion DEL are formed at the interface of two substances at the expense of DA links and govern the efficiency of adhesive interactions [41]. 

Electrostatic and Diffusion Theories. The electrostatic theory states that electrostatic forces in the form of an electrical double layer are present at the adhesive-adher-end interface. These forces account for resistance to separation. The electrostatic theory of adhesion is not generally applicable for common production assembly, but it does apply to the adhesion of particulates (e.g., dust) on plastic film. 

Horn and Smith have used the SFA to examine the contact between dissimilar high modulus materials (sapphire and mica). " Their results were drastically different from that produced by other workers in that a force of adhesion significantly in excess of Wa was measured. The pull-off force also exhibited an odd response. The results of the measurements indicated that charge had transferred between the two materials in contact. This work, using the SFA, provides one of the few conclusive measurements indicating the validity of the electrostatic theory of adhesion. 

Many different measures may be used to specify this fundamental adhesion. It may be expressed in terms of forces or in terms of energies. Again, depending on the context, these may be forces or energies of attachment or else of detachment. Sometimes values of fundamental adhesion can be calculated from a theoretical model (see Electrostatic theory of adhesion, Good-Girifalco interaction parameter) occasionally, they may be deduced from experimental measurement (see Adhesion-fundamental and practical) for many practical adhesive bonds, they are not available by either route. 

Thus, when investigating the nature and mechanism of adhesion between an adhesive, coating or polymer matrix and the substrate, it is important to consider the possibility of primary bond formation in addition to the interactions that may occur as a result of Dispersion forces and Poiar forces. In addition to the Adsorption theory of adhesion, adhesion interactions can sometimes be described by the Diffusion theory of adhesion, Electrostatic theory of adhesion, or Mechanical theory of adhesion. Recent work has addressed the formation of primary bonding at the interface as a feature that is desirable from a durability point of view and a phenomenon that one should aim to design into an interface. The concept of engineering the interface in such a way is relatively new, but as adhesives become more widely used in evermore demanding applications, and the performance XPS and ToF-SIMS systems continues to increase, it is anticipated that such investigations can only become more popular. 

This article, and related ones giving a more detailed explanation of individnal theories (Adsorption theory of adhesion. Diffusion theory of adhesion. Electrostatic theory of adhesion and Mechanical theory of adhesion), exponnd what could be termed classical theories of adhesion. In cross-referenced articles, more recent ideas are explored. As emphasized above, the concepts of the classical theories overlap and merge seamlessly in providing a model of the empirical observations. The tendency of reducing the interpretation of adhesion phenomena to narrowly conceived theories of adhesion should be avoided, and a broader view should be adopted, using whichever blend of concepts best suits the purpose. 

Electrostatic Theory of Adhesion. According to the electrostatic theory [9], the adhesion forces between adherent and adhesive layer are applied by contact or transfer potentials. These transfer potentials cause the buildup of an electric double layer at the adhesive-adherent boundary and corresponding Coulomb attraction forces between the two components. In principle, the occurrence of transfer potentials is unquestionable, as shown by the electrostatic discharges that can be detected in the destruction of adhesive joints. However, the practical significance of the attraction forces associated with these discharges is still being debated [4, pp. 150-153], [10]. 

We will not explain here why a given adhesive should stick to a given material, because this is explained in the chapter Theory of adhesion in Volume 2 in the light of wetting, surface energy, adsorption, work of adhesion, electrostatic, diffusion, covalent bonds and van der Waals forces, and it is also discussed in all the chapters dealing with the various chemical families of adhesives. 

If the adhesive and substrate have different electronic band structures there is likely to be some electron transfer bn contact to balance Fermi levels which will result in the formation of a double layer of electrical charge at the interface. The electronic theory of adhesion is due primarily to Deryaguin and coworkers [55-57] and they have suggested that the electrostatic forces arising from such contact or junction potentials may contribute significantly to the intrinsic adhesion. The controversy this theory has caused is due to this final statement that such electrostatic forces are an important cause, rather than merely a result, of high joint strength. 

The presence of polymers or polyelectrolytes have important effects on the Van der Waal interaction and on the electrostatic interaction. Bacterial adhesion, as discussed in Chapter 7.9 may be interpreted in terms of DLVO theory. Since the interaction in bacterial adhesion occurs at larger distances, this interaction may be looked at as occurring in the secondary minimum of the net interaction energy (Fig. 7.4). Particle Size. The DLVO theory predicts an increase of the total interaction energy with an increase in particle size. This effect cannot be verified in coagulation studies. 

At present three theories of adhesion exist—the adsorption theory [9], the diffusion theory [24,25, 26], and the electrostatic theory [10, 23]. The majority of those who have concerned themselves with the subject of adhesion lean toward the adsorption theory, if for no other reason than a recognition that the phenomenon of wetting is intimately related to adhesion. The word "adhesion (unmodified) here means strictly an interfacial phenomenon, while "practical adhesion" means the strength with which two materials stick together—that is, the force required to separate them, or joint strength. 

While the adsorption theory is the most accepted one, mechanical interlocking comes into play in case of substrates with a special kind of roughness such as galvannealed steel where the liquid can spread into cavities and thereby interlock with the substrate. The diffusion theory does not play an important role for polymer-metal interfaces. The contribution of the electrostatic theory is not easy to estimate. However, the electrical component of the adhesive force between the planar surfaces of solids becomes important if the charge exchange density corresponds to 10 electronic charges, meaning about 1% of the surface atoms [71]. 

A number of adhesion theories have been proposed to identify the formation of adhesive forces. The contributed adhesion mechanisms are (1) chemical bonding such as chemisorption theory (2) physical interaction such as polarization, electrostatic, and diffusion theory (3) thermodynamical interpretation such as adsorption theory and (4) mechanical interlocking. No single theory exists to explain the entire property of adhesion oti various substrates and adhesives. However, those theories may provide a guideline to understand the principle of the adhesion as the following details (Fig. 2). 

The first general theory of adhesion was Newton s universal law of gravity, which was extremely successful in describing the motions of planets. No correction was needed to this law until Einstdn s theory of relativity emerged two centuries later. Newton also had noted adhesion due to electrostatic and magnetic forces, which are well desoibed in the theory of electromagnetism. The problem, as Newton recognized, was that none of these well-understood forces could account for the adhesion observed between bodies in close contact. 

We will review each of the four general theories of adhesion which have been advanced mechanical, adsorption, diffusion and electrostatic. 

No two surfaces are absolutely identical and there will be some contact electrification. The electrostatic theory considers the two surfaces to be bonded as the two plates of an electrostatic condenser, and is due to Deryaguin [30]. According to this theory adhesion occurs due to the electrostatic forces formed by interaction between the substrates. This theory explains the pressure dependence of tack/autohesion very well but it does not explain why raw and compounded rubbers lose most tack/autohesion as they are cured and brought into molecular contact under pressure. Further this theory is also not successful in explaining the time and temperature dependence of the tack/autohesion. By using potential contrast scanning electron microscopy the existence of an electric double layer at the polymer interface has been demonstrated [31]. 

Adhesion is the thermodynamic work of adhesion , i.e. intrinsic interaction across the interface. Several theories of adhesion have been su ested and these may be classified into three categories (1) Adsorption theories, (2) diffusion theories and (3) electrostatic theories. 

There are two other explanations which are relevant to some situations though neither is important in connection with structural adhesives. The first is the electrostatic theory which, as its title indicates, involves adhesive and substrate as the plates of an electric condenser, work being necessary to separate the plates. The second is the mechanical theory in which mechanical interlocking of adhesive and substrate involves fracture of parts of one or other before separation can be achieved. The former theory has some part to play in explanations of the adhesion of pressure sensitive tapes to substrates and the adhesion of sputtered metal in the surface decoration of plastics. The latter is important in the adhesion of polymers to textiles and papers and, to a much lesser extent, to wood. 

---