Adhesion theories adsorption theory

Knowledge of interfacial tensions is of interest because of their direct theoretical and (less direct) practical relationship to adhesion (see Adsorption theory of adhesion, Peel tests, Adhesion - fundamental and practical). Three somewhat different approaches to estimating interfacial tensions by Eqn. 2 are commonly found in discussions... 

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. 

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

The mechanisms of adhesion are explained by four main theories mechanical theory, adsorption theory, diffusion theory, and electrostatic theory. 

The adsorption theory states that the bioadhesive bond formed between an adhesive substrate and tissue or mucosae is due to van der Waals interactions, hydrogen bonds, and related forces. Alternatively, when mucus or saliva are interacting with a solid dosage form, the molecules of the liquid are adsorbed on the solid surface. This is an exothermic process. The free energy of adsorption is given by Eq. (1). 

As explained under the adsorption theory of adhesion,3 an adhesive must first wet the substrate and come into intimate contact with it. (A brief description of the adsorption theory of adhesion is presented in the section below.) The result of good wetting is simply that there is greater contact area between adherend and adhesive over which the forces of adhesion (e.g., van der Waals type of forces) may act. For good wetting, the surface free energy (surface tension yLV) of the liquid adhesive must be less than that (critical surface tension yc) of the solid adherend, or... 

Adsorption Theory of Adhesion. The adsorption theory states that adhesion results from molecular contact between two materials and the surface forces that develop. Adhesion results from the adsorption of adhesive molecules on the substrate and the resulting attractive forces, usually designated as secondary or van der Walls forces. For these forces to develop, the respective surfaces must not be separated more than 5 angstroms (A) in distance. Therefore, the adhesive must make intimate, molecular contact with the substrate surface. 

Certain aspects of the adsorption theory of adhesion are developed more fully than has been done previously. The consequences of nonreciprocity of spreading are pointed out, and are used to develop a more general practical point of view with respect to the adhesive bonding of materials of low-surface free energy. The system epoxy adhesive-(nonsurface-treated) polyethylene, normally considered nonadherent, is investigated experimentally in some detail. It is shown how this system, without material modification, can be made adherent. An area of study for possible adhesives for materials of low-surface free energy is suggested. 

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. 

We develop certain aspects of the adsorption theory of adhesion more fully than has been done previously, based solely on (free) surface energetics, and show how they can be applied to real or practical systems. As a consequence, it is shown that the deBruyne adhesion rule [7] is incorrect in part. Our concern is mainly with what we believe to be the most important problem in the making of adhesive bonds—that is, the achievement of extensive and proper (no intermediate phase) interfacial contact. We discuss also the breaking strength of certain adhesive joints where this is necessary to the development of our thesis. However, the processes of making and breaking adhesive joints bear no... 

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]. 

Most historical surveys treat the work of McBain and Hopkins in 1925 as the earliest application of modern scientific investigation to the study of adhesion [9]. McBain and Hopkins considered that there were two kinds of adhesion, specific and mechanical. Specific adhesion involved interaction between the surface and the adhesive this might be chemical or adsorption or mere wetting. Specific adhesion has developed into the model we today describe in terms of the adsorption theory. 

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 title adsorption theory is a reflection of the extent to which there are very close parallels between some of the ultimate forces involved in adhesion and those involved in the adsorption of gases and vapours on to solid surfaces. It has been well known for... 

The adsorption theory of adhesion attributes adhesive strength to the action of London dispersion forces, combined in many instances with contributions from other forces (dipolar, polar or primary bonding) Calculations indicate that, in spite of their relatively low strength compared with the other types of bonding, these force can account for far greater strengths than are ever achieved experimentally. It is because all these forces are... 

The results of most studies of adhesion and composites are interpreted in terms of the Adsorption theory of adhesion the Dispersion forces and Polar forces determine the interactions between two phases, that is, the matrix and filler. 

Debye and Keesom forces together with London Dispersion forces are known coiiec-tively as van der Waals forces. See Lifshitz-van der Waals forces for a further discussion. They play a significant role in the Adsorption theory of adhesion and in surface phenomena such as Contact angles and interfacial tension. 

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 Theories of adhesion discussed in the articles Adsorption theory of adhesion, Diffusion theory of adhesion, Eiectrostatic theory of adhesion and Mechanicai theory of adhesion aim to describe the forces that cause the adhesive and substrate to adhere the rheological theory is concerned with explanation of the values obtained for adhesion measured by destructive Tests of adhesion. 

Related articles are Adsorption theory of adhesion. Contact angles and interfacial tension. Dispersion forces. Pre-treatment of metals prior to bonding. Roughness of surfaces. Surface characterization by contact angles-polymers. Surface energy and Wetting and spreading. 

The original work of Zisman was developed by several workers. Using Young s equation, two problems exist. Firstly, even in a vapour atmosphere, the term ysF (or ksv) does not correspond necessarily to the surface tension of the solid ys- Whereas ys corresponds to the surface tension of the clean solid, ysv refers to the same quantity after any potential adsorption (see Adsorption theory of adhesion) of the vapour of the Uquid. The difference, always positive or zero, is called the spreading pressure Tt, with sufOx e representing equilibrium ... 

The nature of the problem, as well as the solution to it, is represented in Fig. 1. The figure shows the circumstances in which adhesive joints are normally made with the substrate and adhesive surrounded by air. In the making of adhesive joints, the presence of air is seldom even considered, for the perfectly good reason that it does not represent a problem. However, the fact is that aU surfaces are contaminated by the permanent gases, but they are only weakly adsorbed (see Adsorption theory of Adhesion) and are readily displaced by adhesive, which then spreads freely and spontaneously over the substrate surface. In this way, intimate contact is achieved between liquid adhesive and solid substrate and adhesion results. Various Theories of adhesion are discussed elsewhere, but all require that adhesive and adherend are in intimate contact. For example, attractive intermolecular forces (van der Waals, see Dispersion forces. Polar forces) can operate only over short ranges ( 1 nm) and chemical interaction between adhesive and adherend also reqnires the two reactants to be in close contact. 

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