Adhesion diffusion mechanism

The above discussion has tacitly assumed that it is only molecular interactions which lead to adhesion, and these have been assumed to occur across relatively smooth interfaces between materials in intimate contact. As described in typical textbooks, however, there are a number of disparate mechanisms that may be responsible for adhesion [9-11,32]. The list includes (1) the adsorption mechanism (2) the diffusion mechanism (3) the mechanical interlocking mechanism and (4) the electrostatic mechanism. These are pictured schematically in Fig. 6 and described briefly below, because the various semi-empirical prediction schemes apply differently depending on which mechanisms are relevant in a given case. Any given real case often entails a combination of mechanisms. 

Fig. 6. Four mechanisms of adhesion, (a) The adsorption mechanism (contact adhesion), (b) The diffusion mechanism (diffusion interphase adhesion), (c) The mechanical interlocking mechanism. (d) The electrostatic mechanism.

Although the diffusion mechanism can be seen as mechanical but occurring at molecular dimensions, van der Waals intermolecular interactions and conformational entropic energy provide an additional mechanism that increases adhesion [62]. It is interesting to note the analogy that exists between this mechanism at the molecular level with the adherence, adhesion and viscoelastic deformations concept applied for a macroscopic adhesive. 

For drugs absorbed via passive diffusion mechanisms (paracellular or transcellular), increasing the area of the patch should increase the amount of drug absorbed. However, patch size must always be considered with respect to patient comfort and acceptability and must not be too large so that these factors are compromised. Thus the size of adhesive patches is generally in the range 2-5 cm2, with 10-15 cm2 being the upper limit. 

Several mechanisms of interaction between particles of solids are known [3]. Mechanical adhesion is achieved by flowing a metal into the support pores. The molecular mechanism of adhesion is based on the Van der Waals forces or hydrogen bonds, and the chemical mechanism on the chemical interaction of the metal particles with the support. The electric theory relates adhesion to the formation of an electric double layer (EDL) at the adhesive-substrate interface. Finally, the diffusion mechanism implies interpenetration of the molecules and atoms of the interacting phases, which results in the interface blurring. These insights into the nature of adhesion can be revealed in the papers about the interaction of transition metal... 

Physical adhesion This mechanism is controlled by diffusive bonding, where the diffusivity increases with increasing contact temperature according to Fields law. This can be maximised by substrate preheating. Because of the small diffusion depth (produced by the rapid solidification), the diffusive adhesion generally plays only a minor role as an adhesion mechanism. 

Flank wear and catastrophic failure are the main tool failure modes when cutting tempered martensitic stainless steel using coated cermet tools. Abrasive and adhesive mechanisms lead to the flank failure mode while a combination of abrasion, adhesion, diffusion, fracture, and plastic deformation results in catastrophic tool breakage. The application of coated carbide... 

Note that in all cases the initial strength is reestablished after the adhesive-bonded joint is dried. Thus, the drop in strength of adhesive-bonded joints operated in water is explained mainly by diffusion of water at the adhesive-substrate interface. Methods that increase water resistance decrease the rate of water diffusion along the interface, although it still takes place. Increase in the rate seems to be caused by both adsorption and diffusion mechanisms of the medium. 

Diffusion. This theory proposes that adhesive macromolecules diffuse into the substrate, thereby eliminating the interface, and so can only apply to compatible polymeric substrates. It requires that the chain segments of the polymers possess sufficient mobility and are mutually soluble. The solvent welding of thermoplastics such as PVC (polyvinyl chloride), softened with a chlorinated solvent, is an example of such conditions being met. Diffusion will also take place when two pieces of the same plastic are heat-sealed. The joining of plastic service pipes for carrying gas and water makes use of the diffusion mechanism. 

Historically, mechanical interlocking, electrostatic, diffusion, and adsorp-tion/surface reaction theories have been postulated to describe mechanisms of adhesion. More recently, other theories have been put forward for adhesive bonding mechanism (Table 1.1). It is often difficult to fully ascribe adhesive bonding to an individual mechanism. A combination of different mechanisms is most probably responsible for bonding within a given adhesive system. The extent of the role of each mechanism could vary for different adhesive bonding systems. An understanding of these theories will be helpful to those who plan to work with adhesives. 

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. 

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. 

Carboxymethyaltion of Gum kondagogu with monochloro-acetic acid Metformin ionotropically gelled beads Ex-vivo bio adhesion studies of CMGK beads revealed bioadhesion greater than 80% over a period of 24 h and drug release rate from the beads followed Fickian diffusion mechanism with zero-order release kinetics. [6]... 

Ethylene-propylene rubbers (EP) have low total surface energies with small polar components. As would be expected, the adhesion of paints and adhesives to untreated EP is poor. To achieve good adhesion to EP, the introduction of suitable functional groups is necessary unless a diffusion mechanism can operate. Bragole [26] found that UV treatment of EPDM coated with a thin layer of benzophenone resulted in large increases in the adhesion of acrylic, epoxy and urethane paints to the polymer. 

In this section, I will briefly discuss (1) intermolecular forces responsible for adhesion (2) mechanisms of adhesion molecular contact at the interface, molecular configuration and conformation (3) wetting and thermodynamic equilibrium (4) bond character and adhesive performance (5) the role of diffusion (6) the electrostatic contribution and (7) the locus of adhesive failures. 

Understanding adhesion implies identifying the dominant mechanism (thermodynamic adsorption, specific adsorption, diffusion, mechanical interlocking), but combinations of these theories or mechanisms are also possible. The four major mechanisms of adhesion are illustrated in Figure 6.6. 

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