THEORIES OF ADHESION

Many theories of adhesion have been proposed which can describe some features of the formation of adhesive joints and their properties. The thermodynamic analysis of adhesion, discussed above, is sometimes identified with the thermo-dynamic theory of adhesion.³⁶However, it cannot be recognized as an independ-ent theory, as it is an application of fundamindepend-ental thermodynamic principles to the description of interfacial interactions. In adsorption theory,^3,37adhesion is considered as a result of molecular interaction forces acting between molecules of adhesive and substrate in physical contact. Molecular interactions proceed by a formation of contact between heterogeneous surfaces. However, Berlin also was correct³to emphasize that the statement indicating that molecular interac-tion is the origin of adhesion hardly means anything.

Unfortunately, problems of the adsorption or molecular theory of adhesion are in most instances solved exclusively at the qualitative level and are limited to consideration of a role of the polarity of components in adhesion (the so-called polarity rule: high adhesion cannot be achieved between a polar substrate and apolar adhesive, and vice versa). It is very unfortunate that in many books on ad-hesion the description of adad-hesion is not given at the molecular level, which is now accessible for the description of intermolecular interactions in liquids and solids. At the same time, it is obvious that from a physical point of view the ad-sorption theory presents a rather correct concept of interfacial phenomena and agrees with thermodynamics. Within this context, adhesion can be regarded as a particular case of adsorption, inasmuch as the formation of molecular bonds at

the interface and the transfer of polymer molecules from the bulk to the inter-face are phenomena typical of adsorption.

The existing theories of adsorption (see Chapter 1) and ideas about the structure of adsorption layers provide us with the concept of the character of the interactions which occur during adhesion. If we consider adsorption as an excess concentration of the component at the interface, this definition is applicable to adhesion because the study of the structure of polymer surface layers on solid surfaces shows that redistribution of different components of adhesive resulting from selective adsorption is taking place. With a prolonged adhesive contact un-der conditions ensuring the required molecular mobility (above the glass transi-tion temperature) molecules redistribute, in the course of time, according to their molecular mass and surface activity.³⁸However, one cannot draw a paral-lel between the adsorptivity of polymers from solutions and adhesion, as there is none, and a direct connection between them cannot exist. The absence of this connection is explained by difference in conditions of interaction and conforma-tion of molecules in the polymer soluconforma-tion and in the solventless polymeric adhe-sive.³⁹

Adsorption theory also considers chemisorption and formation of primary valent bonds between the surface and adhesive if some specific groups are pres-ent at the surface. Evidpres-ently, there are several bases on which the adsorption theory can be criticized⁴⁰- among them, it cannot explain the mechanical re-sponse of a system of materials in which volume deformations occur.⁴⁰The elec-tric theory of adhesion, developed by Deryagin and associates,¹postulates that all adhesion phenomena can be explained in terms of the transfer of electrons through the interfacial border, leading to the formation of an electrical double layer. This theory treats the adhesion joint as though it were a capacitor, charged due to the contact of the two materials composing the joint. In contrast to other theories of adhesion, this theory interprets phenomena taking place during the separation of the adhesive from the substrate on the basis of the con-cepts of the electrical double layer. The latter interferes with destruction of the adhesion joint at dynamic loads and increases the work of separation. Electrical phenomena not only accompany film separation, but also act as an important factor determining the resistance of film to failure even when it regards adhe-sive of mixed character (adheadhe-sive and coheadhe-sive failure simultaneously).

Without diminishing the role of electrostatic phenomena in adhesion, it should be noted that their influence is greatest in the course of adhesion joint failure and depends on the conditions of separation (especially its rate). In most instances, PCM are used under conditions, far from failure and separation, where the electric theory can be applied.

In evaluation of the significance of the electrical double layer at the bound-ary of two amorphous bodies due to the donor-acceptor bond, the authors of the theory proceeded from the fact that the near-surface layer of one of the contact-ing bodies is saturated with donor molecules and the other with acceptor mole-cules or functional groups. A double layer is formed only when the free energy of the system decreases in this process. These provisions bring the electrical the-ory of adhesion and adsorption (molecular) thethe-ory closer to each other. The elec-trical theory has often been criticized. Authors usually mention it but almost nobody accepts it. Wake³⁶criticized the theory and particularly emphasized that the nature of the origin of charges on the surface of polymer remains unclear.

Crushing arguments against this theory were given by Bikerman.⁴¹

Recently, the theory was criticized again by Sharpe,⁴⁰one of whose argu-ments considers that electrically conductive materials should not form adhesion joints, because they could not support separation of charge (however, they do).

The generalized thermodynamic discussion on the surface energy of solids, in-cluding polymer materials, as suggested by Fowkes, and accounting for polar, donor-acceptor, acid-base interactions, includes, among possible interfacial in-teractions, the donor-acceptor interactions to which is attributed (by the Deryagin theory and to our mind unjustifiably) the underlying role in adhesion.

In fact, various forces act at the interface, depending on the nature of contacting bodies, including purely ionic and chemical interactions. It is thus more reason-able to describe them within the framework of a unified molecular approach, i.e., in terms of molecular theory of adhesion.

The diffusion theory of adhesion of two heterogeneous polymer bodies was proposed by Voyutsky.⁴²The theory is based on the concept of a mutual diffusion of segments to the interface with the formation of a transition layer responsible for the strength of the adhesion joint. When applied to polymer pairs, this theory has been well corroborated experimentally, largely with regards to the influence of the conditions of the formation of adhesive contacts on adhesive strength.

This theory is mainly applicable when both polymers are at a higher tempera-ture than their glass transition temperatempera-ture, and when there is a possibility to achieve a high molecular mobility of the macromolecules. The main condition for the formation of an adhesion joint on contact of two polymers is their thermody-namic compatibility. However, the most important question of what this com-patibility should be in order that diffusion processes can ensure a sizable contribution to adhesion remains open. Because most polymeric pairs are in-compatible and many of them have the lower critical solution temperature (LCST), other processes leading to the formation of the transition layer should be responsible for adhesion (for example, pure rheological factors - see Chapter 7). Therefore, it would be valid to say that it is unreasonable to refer to a special “diffusion theory of adhesion”. One can only refer to the role of diffusion processes in the adhesion of some polymeric systems, since theoretical consider-ations for the time being have only a qualitative character.

The first attempt to theoretically describe mutual diffusion at the interface was made by Helfand^43-45who also took into account the thermodynamic affinity of polymers. As a result of theoretical calculations, it was established that the thickness of the transition layer formed on diffusion is negligibly small and does not correspond to the concept of the diffusion theory.

In considering the above theories, one has to admit that the most useful concept of adhesion stays within the boundaries of the molecular theory and the thermodynamics of interfacial phenomena. At the same time, no one theory of adhesion can predict the real adhesion between solid and polymer or adhesion joint strength. A large number of theoretical ideas on adhesion do not refer to the phenomena of adhesion but rather to the processes of failure of adhesion joints and their description. A clear distinction between the processes of adhesion and the formation of adhesion contact and failure is a key to understanding this com-plex set of phenomena, referred to for convenience as adhesion.