Adhesion general theories

The number of these factors and their complicated relationship is clearly shown by the numerous theories which attempt to describe the phenomenon of adhesion. These theories represent different approaches to the problem, describe one interaction well, but usually do not offer a general solution [27,28]. 

This theory is also applicable to polymer wear. Indeed, the theory that we will present could be considered to cover the "wear aspect of adhesion and the adhesional aspect of wear, simultaneously. See the recent reviews by Briscoe, for earlier discussions of the adhesional aspects of polymer wear (7,8). We will discuss wear explicitly, after developing the general theory. 

The importance of the surface polarity and the surface characteristics for polymer adhesion has been considerably discussed in scientific literature [87]. A useful generalized theory of adhesion, however, can be built upon the basis of electrical attractions. The electrical attractions, resulting from uneven surfaces, which are not normally considered to be electrical, participate easily in attractive interactions if adhesives can be found that will wet them. Thus the reason that polyethylene and poly(tetrafluoroethylene) are difficult to bond is simply that available adhesives are thermodynamically more stable if their molecules attract one another than if they interact with low energy surfaces. The solution to this problem would... 

The first general theory of adhesion was Newton s universal law of gravity, which was extremely successful in describing the motions of planets. No correction was needed to this law until Einstdn s theory of relativity emerged two centuries later. Newton also had noted adhesion due to electrostatic and magnetic forces, which are well desoibed in the theory of electromagnetism. The problem, as Newton recognized, was that none of these well-understood forces could account for the adhesion observed between bodies in close contact. 

The thermodynamic model of adhesion, generally attributed to Sharpe and Schonhom [1], is certainly the most widely used approach in adhesion science nowadays. This theory considers that the adhesive will adhere to the substrate because of interatomic and intermolecular forces established at the interface, provided that an intimate contact between both materials is achieved. The most common interfacial forces result from van der Waals (London, Debye and Keesom) and Lewis acid-base interactions. The magnitude of these forces can generally be related to fundamental thermodynamic surface characteristics, such as surface free energies y, of both materials in contact. 

We will review each of the four general theories of adhesion which have been advanced mechanical, adsorption, diffusion and electrostatic. 

The section on design of structural adhesive joints will describe and cite advantages and disadvantages of joint geometries, such as butt, lap, scarf, strap, and combined versions of these. A general design criterion will also be included. Another section of the chapter will pertain to fracture mechanics. General theories on fracture mechanics and test techniques used to characterize structural adhesives fracture behavior will be discussed. 

This chapter provides a summary of the generalized theory and a discussion of the parameters which appear in its equations. The use of the theory is illustrated for several different adhesion situations, including the peeling of rubberlike and pressure-sensitive adhesives, adhesion to skin, and the moisture resistance of structural adhesives bonded to metals and glass. 

The processes for forming adhesive bonds between materials have been developed empirically. Current theories of adhesion remain controversial (2-4). The development of a general theory of adhesion has been deterred in part due to the experimental inaccessibility of interfacial interactions between solids and the difficulty in establishing the nature of the interface (3). 

Thermodynamic adsorption is the most general theory and the only explanation in the absence of, for example, chemical promoters, high mobility of polymeric chain and porosity. It should be emphasized that the van der Waals (dispersion) forces between particles/surfaces, which are involved, are not as short range as those between molecules they decrease with the third power of the distance (not the seventh power). Adhesive forces are discussed next (Section 6.3.2). 

Contemporary discussion of the fundamentals of adhesion generally reflects the historical development of the subject. For convenience, this approach will be adopted here. Early work in the 1920s discussed two kinds of adhesion. Where surfaces were smooth, what would now be described as adsorption theory applied, with rough or porous surfaces mechanical theory. The next two sections, Sects. 2.2 and 2.3, of this chapter discuss these in turn. 

The rule of thumb ( ... the reinforcing effect is directly related to the viscosity decrease of the blends compared to the neat thermoplastics ) stated by Fekete et alP is intriguing but of course too simple and limited. There is a need for a more general theory or predictive model of LCP blends which would allow input of rheological parameters, parameters related to adhesion, orientability, degradation temperature, etc. and of course the cost of the components, and allow one to compare the expected material to commercially available grades of glass-filled thermoplastics, etc. 

The most-often cited theoretical underpinning for a relationship between practical adhesion energy and the work of adhesion is the generalized fracture mechanics theory of Gent and coworkers [23-25] and contributed to by Andrews and Kinloch [26-29]. This defines a linear relationship between the mechanical work of separation, kj, , and the thermodynamic work of adhesion ... 

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. 

In the limit of large, softer solids in vapor pressure closer to the value marking the onset of capillary condensation, the generalized Hertz and the original JKR theories are found to be qualitatively identical. However, the contact area for zero applied load will in general be different, since it is dependent upon the nature of the source of adhesion ... 

Improve adhesion of dissimilar materials such as polymers to inorganic substrates. Also called primers. Primers generally contain a multifunctional chemically reactive species capable of acting as a chemical bridge. In theory, any polar functional group in a compound may contribute to improved bonding to mineral surfaces. However, only a few organofunc-tional silanes have the balance of characteristics required... 

Sustained adhesion of the dosage form (tablet, patch) to the mucosa is an important first step to successful buccal delivery. The mucus plays an important role during this mucoadhe-sive process by buccal drug delivery systems. The interaction between the mucus and mucoadhesive polymers generally used in most dosage forms can be explained by theories summarized in Table 9.1. 

Although the mechanisms of polyimide/metal adhesion remain to be fundamentally elucidated, it is generally accepted that the interfacial diffusion of metallic entities into the polyamic acid plays a key role at the interface [156-158]. Two main theories have been reported explaining the adhesion of the Pl/metal bond chemical and mechanical bonding [159]. Initial work emphasized mechanical bonding and most efforts were dedicated to the physical roughening of the substrate by different abrasive methods as well as chemical treatments in order to improve metal to polyimide adhesion by increasing the metal surface area [156,160-164]. 

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