The Diffusion Theory of Adhesion

The theories of adhesion that have been described in this chapter pertain to the adhesion of any two substrates. There is, however, one further theory which should be discussed that pertains in particular to the adhesive bonding of polymers to polymers. This is the diffusion theory of adhesion. 

The theory of adhesion by diffusion was espoused by various Russian scientists, the most prominent of whom was Voyutskii, who wrote a monograph on the subject. The theory was given a quantitative theoretical basis by the work of Vasenin, who derived the relationship for bond strength in autohesion shown in Eq. (108). 

The above discussion deals mostly with rubber materials and not with structural adhesives it serves, though, to introduce the subject. From the work of Iyengar and Erickson,it appears that adhesive strength is related to the solubility parameter match of the adhesive to the substrate. In this work the substrate was Mylar (a polyester), which gives some further credence to the diffusion theory of adhesion for materials which can be considered structural members. 

---

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. 

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. 

X 2 and consequently the adhesive strength will increase. The diffusion theory of adhesion also involves kinetic aspects. Obviously, if there is diffusion of polymeric chains across the interface, then the adhesive strength will increase with increasing contact time. Thus, the time-dependence of adhesive strength can be expressed by a power law equation... 

It should be pointed out that, interestingly enough, in the diffusion theory of adhesion (Voyutskii and Vakula, 1969), polymer molecular diffusion is regarded as the mechanism behind the creation of an adhesive bond, and the strength of the bond is attributed to intermolecular forces. 

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 most widely used and successful theory which explains tack or autohesion is the diffusion theory of adhesion of Voyutskii [23]. He suggested that if two rubber surfaces were in sufficiently close contact, part of the long chain molecules on the surface would diffuse across the interface. The surface molecules will interpenetrate and eventually the interface will disappear and the two parts will have become one. For such an interdiffusion to take place, the molecules must be relatively mobile, which further requires that the rubbers must be above their glass transition temperature and that there should not be any appreciable degree of crosslinking in either of them. There is some experimental evidence that such diffusion takes place across the interface during a relatively short time of contact [24]. However, the diffusion theory does not explain the variation of autohesion with the temperature of measurement, or the increase in autohesion with increase in contact pressure. 

R. M. Vasenin, Problems in the diffusion theory of adhesion of polymers, in Adhesion, Fundamentals and Practice, p. 29, Ministry of Technology, U.K., Gordon and Breach, London (1969). 

The limits of the diffusion theory of adhesion show up in the adhesion of polymers, for example, to metal or glass. In this case, this theory does not make any useful contribution to the understanding of adhesion. 

The diffusion theory of adhesion of two heterogeneous polymer bodies was proposed by Voyutsky. The 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. 

Voyutskii [30-32] is the chief advocate of the diffusion theory of adhesion which states that the intrinsic adhesion of polymers to themselves (autohesion), and to each other, is due to mutual diffusion of polymer molecules across the interface. This requires that the macromolecules, or chain segments of the polymers (adhesive and substrate) possess sufficient mobility and are mutually soluble. This latter requirement may be restated by the condition that they... 

---