Theory of adhesives

Diffusion Theory. The diffusion theory of adhesion is mosdy appHed to polymers. It assumes mutual solubiUty of the adherend and adhesive to form a tme iaterphase. The solubiUty parameter, the square root of the cohesive eaergy deasity of a material, provides a measure of the iatermolecular iateractioas occurring within the material. ThermodyaamicaHy, solutioas of two materials are most likely to occur whea the solubiUty parameter of oae material is equal to that of the other. Thus, the observatioa that "like dissolves like." Ia other words, the adhesioa betweea two polymeric materials, oae an adherend, the other an adhesive, is maximized when the solubiUty parameters of the two are matched ie, the best practical adhesion is obtained when there is mutual solubiUty between adhesive and adherend. The diffusion theory is not appHcable to substantially dissimilar materials, such as polymers on metals, and is normally not appHcable to adhesion between substantially dissimilar polymers. 

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

The wettabiHty theory of adhesion is iaextricably related to the study of contact angles of Hquids on soHd surfaces. A force balance at the poiat of contact between the Hquid and the soHd can be written (3)... 

Another manifestation of a time dependence to particle adhesion involves the phenomenon of total engulfment of the particle by the substrate. It is recognized that both the JKR and MP theories of adhesion assume that the contact radius a is small compared to the particle radius R. Realistically, however, that may not be the case. Rather, the contact radius depends on the work of adhesion between the two materials, as well as their mechanical properties such as the Young s modulus E or yield strength Y. Accordingly, there is no fundamental reason why the contact radius cannot be the same size as the particle radius. For the sake of the present discussion, let us ignore some mathematical complexities and simply assume that both the JKR and MP theories can be simply expanded to include large contact radii. Let us further assume that, under conditions of no externally applied load, the contact and particle radii are equal, that is a(0) = R. Under these conditions, Eq. 29 reduces to... 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

The chemical bonding theory of adhesion applied to silicones involves the formation of covalent bonds across an interface. This mechanism strongly depends on both the reactivity of the selected silicone cure system and the presence of reactive groups on the surface of the substrate. Some of the reactive groups that can be present in a silicone system have been discussed in Section 3.1. The silicone adhesive can be formulated so that there is an excess of these reactive groups, which can react with the substrate to form covalent bonds. It is also possible to enhance chemical bonding through the use of adhesion promoters or chemical modification of the substrate surface. 

TABLE 1 Single Chemical Bond Forces (in pN) for Every Tip-Substrate Combination, Calculated on the Basis of the JKR Theory of Adhesion Mechanics ... 

Wake, W.C. (1978). Theories of adhesion and uses of adhesives A review. Polymer 19, 291-308,... 

There is no unify ing theory of adhesion describing the relationship between practical adhesion and the basic intermolecular and interatomic interactions which take place between the adhesive and die adherend either at die interface or within the interphase. The existing adhesion theories are, for the most part, rationalizations of observed phenomena, although in some cases, predictions regarding the relative ranking of practical adhesion can actually be made. 

Diffusion Theory. The diffusion theory of adhesion is mostly applied to polymers. It assumes mutual solubility of the adherend and adhesive to form an interphase. 

Surface Energetics and Wettability Theory. The surface energetics and wettability theory of adhesion is concerned with the effect of intermolecular and interatomic forces on the surface energies of the adhesive and the adherend and the interfacial energy between the two. 

The existence of various theories of adhesion and the enormous number of publications dealing with the adhesion of polymers to solid surfaces indicates that the problems of adhesion are very complicated. 

The problems in bonding polymers to hydrophilic mineral surfaces were discussed and a theory of adhesion in which silane coupling agents provide a bond at the interphase was proposed by Plueddemann in 1974 [3]. However, silane coupling agents specifically designed to impart hydrophilicity to mineral surfaces have been proposed only recently [34], A trimethoxysilyl-terminated poly( /V-... 

As the science of adhesion has developed, various theories of adhesion have been advocated for one material or another. With wood as a substrate, mechanical interlocking, interdiffusion of polymers, intermolecular attractive forces, and covalent chemical bonding all have been proposed, either individually or collectively, to explain adhesion. In reality, no experiments reported to date have been able to disprove the existence of any one of these mechanisms, or to quantify their relative importance. A most exasperating feature of research on adhesion to wood is that factors presumed to be independent in experiments are never totally independent. 

H. Schonhorn, Theory of Adhesive Joints, in Adhesion and Bonding, N. M. Bikales, Ed., Wiley, New York, 1971. 

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. 

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