Adhesive bond electrostatic theory

Among these models, one usually distinguishes rather arbitrarily between mechanical and specific adhesion, the latter being based on the various types of bonds (electrostatic, secondary, chemical) that can develop between two solids. Actually, each of these theories is valid to some extent, depending on the nature of the solids in contact and the conditions of formation of the bonded system. Therefore, they do not negate each other and their respective importance depends largely on the system chosen. 

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). 

Historically, mechanical interlocking, electrostatic, diffusion, and adsorp-tion/surface reaction theories have been postulated to describe mechanisms of adhesion. More recently, other theories have been put forward for adhesive bonding mechanism (Table 1.1). 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. The extent of the role of each mechanism could vary for different adhesive bonding systems. An understanding of these theories will be helpful to those who plan to work with adhesives. 

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. 

The existence of an electrical double layer at the interface between a metal and a polymer adhering to it has been satisfactorily demonstrated. Undoubtedly, the electrostatic forces developed from this interaction could contribute to the total adhesive bond strength. However, the theories that have been advanced are less than rigorous and have been subject to severe criticism. Nevertheless, there are some phenomena that cannot be explained without recourse to this explanation in some form (see Electrical adhesion). 

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 refers to the state in which two dissimilar bodies are held together by intimate interfacial contact such that mechanical force or work can be transferred across the interface. There is unifying theory of adhesion that relates the physical-chemical properties of materials to the actual physical strength of an adhesive bond. The interfacial forces holding the two phases together may arise from van der Waals forces, chemical bonding, or electrostatic attraction. The mechanical strength of the system is determined not only by the interfacial forces, but also by the... 

The adhesion between two solid particles has been treated. In addition to van der Waals forces, there can be an important electrostatic contribution due to charging of the particles on separation [76]. The adhesion of hematite particles to stainless steel in aqueous media increased with increasing ionic strength, contrary to intuition for like-charged surfaces, but explainable in terms of electrical double-layer theory [77,78]. Hematite particles appear to form physical bonds with glass surfaces and chemical bonds when adhering to gelatin [79]. 

In this theory, the adhesion is due to electrostatic forces arising from the transfer of electrons from one material of an adhesive joint to another. Evidence in support of this theory includes the observation that the parts of a broken adhesive joint are sometimes charged [48]. It has been shown that peeling forces are often much greater than can be accounted for by van der Waals forces or chemical bonds. 

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. 

---