Adhesion diffusion theory

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. 

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. 

C.3 Mutual Dijfusion of Polymers in Contact (Adhesion). Diffusion theory is one of the theories of adhesion. According to this theory, the molecules of two polymers in mutual contact diffuse across the interface (Fig. 4.27), so that after some time the interface does not exist. The strength of the joint will then depend on the distance the macromolecules have interpenetrated each other. Crack healing is also based on the theory of diffusion. 

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 involves the interdiffusion of macromolecules between the adhesive and the substrate across the interface. The original interface becomes an interphase composed of mixtures of the two polymer materials. The chemical composition of the interphase becomes complex due to the development of concentration gradients. Such a macromolecular interdiffusion process is only... 

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

The diffusion theory states that interpenetration and entanglement of polymer chains are additionally responsible for bioadhesion. The intimate contact of the two substrates is essential for diffusion to occur, that is, the driving force for the interdiffusion is the concentration gradient across the interface. The penetration of polymer chains into the mucus network, and vice versa, is dependent on concentration gradients and diffusion coefficients. It is believed that for an effective adhesion bond the interpenetration of the polymer chain should be in the range of 0.2-0.5 pm. It is possible to estimate the penetration depth (/) by Eq. (5),... 

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. 

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. 

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]. 

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... 

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). 

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. 

In diffusion theory, the major driving force for adhesion is the mutual diffusion of polymer molecules across the interfaces with resulting... 

This theory suggests that adhesion is developed through the interdiffusion of molecules in between the adhesive and the adherend. The diffusion theory is primarily applicable when both the adhesive and the adherend are polymers with relatively long-chain molecules capable of movement. The nature of materials and bonding conditions will influence whether and to what extent diffusion takes place. The diffuse interfacial (interphase) layer typically has a thickness in the range of 10-1,000 A (1-100 nm). Solvent cementing or heat welding of thermoplastics is considered to be due to diffusion of molecules. ... 

Electrostatic and Diffusion Theories. The electrostatic theory states that electrostatic forces in the form of an electrical double layer are present at the adhesive-adher-end interface. These forces account for resistance to separation. The electrostatic theory of adhesion is not generally applicable for common production assembly, but it does apply to the adhesion of particulates (e.g., dust) on plastic film. 

What follows is a brief review of the theories of "adhesicm", readly "interfacial adhesion", although one of them, the diffusion theory, involves more than a two-dimensional view of the interface. 

Despite these criticisms, however, we are not saying that diffusion-created interfaces or interphases do not exist or do not influence mechanical behavior of joints. They do. But Voyutskii s diffusion theory simply is not, and cannot be, the basis for a general understanding of the mechanical behavior of, e.g., adhesive joints or other adhering systems. 

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. 

Some rubber-based adhesives need vulcanization to produce adequate ultimate strength, and the adhesion is mainly due to chemical interactions at the interface. Other rubber-based adhesives (see Contact adhesives) do not necessarily need vulcanization but need adequate compounding to produce the adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. See Diffusion theory of adhesion and Polymer diffusion reptation and interdigitation. 

Autohesive tack differs from the other types in that it involves mutual diffusion of polymer molecules across the interface it is discussed elsewhere (see the articles on Diffusion theory of adhesion and Polymer diffusion replation and interdigitation). Adhesive tack between elastomeric materials and other, usually rigid, surfaces is considered in this article. 

Diffusion theory of adhesion K W ALLEN Polymer/polymer autohesion, molecular weight dependence... 

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