Adhesion fundamental

Hayes, R.A. and Ralston, J., Application of atomic force microscopy in fundamental adhesion studies. In Mittal, K.L. and Pizzi, A. (Eds.), Adhesion Promotion Techniques — Technological Applications. Dekker, New York, 1999, pp. 121-138. 

Qualitatively speaking, the term adhesion designates the cohesion between two media, whether they be identical or not. However, it is more difficult to give a quantitative definition. This concept may be understood from three complementary angles fundamental adhesion, thermodynamic adhesion and experimental adhesion. 

Fundamental adhesion is connected with the nature of the bonds producing cohesion between two media. These bonds may be classified into two categories, namely strong bonds (polar, covalent and metallic bonds) and secondary bonds (hydrogenous and Van der Waals bonds). Different atomic or molecular models have been proposed to describe the electronic structure of interfaces. None however, is sufficient for calculating the intensity of adhesion forces for systems of practical interest. 

The introduction to the concept of static and kinetic friction in Chapter 7, Section 2 is admittedly simplistic. Familiarity with the experimental details of measuring friction leads to a more realistic view in behavioristic terms and also to some theoretical questions. In particular, the theory of stick-slip friction requires that be greater than and distinct from and indeed Fig. 7-5a shows a discontinuity between static and kinetic friction. But the model for the fundamental adhesive mechanism of friction does not predict such a discontinuity. 

The literature of archaeology and conservation seems to yield little in the way of systematic studies, information, recommendations, or experiences related to the gluing of archaeological wood. Individual experiences are referenced (e.g., 7, 9) and fundamental adhesion theory is presented in several conservation articles and book chapters (12, 16), but little is to be... 

This current state has to do with the discovery that the action of fundamental adhesion forces is not restricted to the interface. They not only fix some layer of adhesive molecules on the surface of the adherend but they can exert strong influence on the formation of chemical and morphological structure as well as on molecular mobility in the adjacent region of the adhesive during solidification. Hence an interphase is formed. These interphases depend on the combination of adhesive and adherend surface and on the process of contact formation as well. Due to its distinct structure, the interphase possesses properties that can be much different from the behavior of the bulk adhesive. Compared to the complexity of the problem, we just start understanding what is behind interphases. 

TABLE 62.1 Overview of Some Fundamental Adhesion Kinetic Models... 

Part 1 Fundamental Adhesion Aspects i.g. Wood Bonding Part 2 Synthetic Adhesives Part 3 Environment friendly adhesives and Part 4 Wood Welding and General Paper. Many different ramifications of wood adhesives are accorded due coverage in this book. The bonding (welding) of wood components without using any adhesives is a relatively recent development which should prove very useful in times to come. 

Self-fastening joint designs generally produce very high stresses in the plastic part during the assembly operation. With brittle plastics, such as thermosets, press-fit assembly may cause the plastic to crack if conditions are not carefully controlled. In addition, certain plastics, especially thermoplastics, are subject to cold flow under stress. Under continued stress, which is the fundamental adhesive that holds self-fastened parts together, the plastic may relax, causing the joint to fad. 

J. P. Wightman, "Surface Analysis Examines Fundamental Adhesion Questions," Adhesives Aae. pp. 30-32, August, 1987. 

Fundamental adhesion In one use, adhesion refers to the forces between atoms at the interface. This is sometimes called true adhesion or fundamental adhesion. Here, of course, the concept is necessarily tied to one of the Theories of adhesion and to a particular model for the interface concerned. 

Many different measures may be used to specify this fundamental adhesion. It may be expressed in terms of forces or in terms of energies. Again, depending on the context, these may be forces or energies of attachment or else of detachment. Sometimes values of fundamental adhesion can be calculated from a theoretical model (see Electrostatic theory of adhesion, Good-Girifalco interaction parameter) occasionally, they may be deduced from experimental measurement (see Adhesion-fundamental and practical) for many practical adhesive bonds, they are not available by either route. 

The adhesion recorded reflects not only the fundamental adhesion at the interface but also the mechanical response of the adhesive, substrate and interfacial region (see Rheological theory). Equation 7 in the article on Peel tests shows how the practical adhesion (peel strength) may be related to the fundamental adhesion (work of adhesion) for that type of test. 

The mode of failure in a test may be adhesive, cohesive or mixed (see Stress distribution mode of failure). Sometimes authors talk as if a test only measured adhesion in the first of these cases. Such a distinction does not seem helpful as none directly measures fundamental adhesion and all give a number that reflects indirectly the interfacial forces. 

The ability to control the interface is central to successful application of adhesion in a composite. By modifying or tailoring the interface, the improved fundamental adhesion might result in the improved practical adhesion for example, various surface treatments of ultra-high modulus polyethylene fibres used as reinforcemenf in polyester and epoxy composites that changed the values of both monofilament pull-out adhesion and interlaminar shear strength. ... 

In early work on the test, it was argued that the critical load was determined only by the properties of the interface, the radius of the stylus and the hardness of the substrate. On this basis, the scratch test would give a measure of fundamental adhesion (see Adhesion - fundamental and practical). However, there are many reports of dependency on the thickness and mechanical properties of the thin film, even several of those upon which supposedly similar styluses were used. 

The study of fundamental adhesion has been hampered because standard Tests of adhesion provide a result that is a complicated combination of fundamental adhesion, the physical properties of the adherend and the viscoelastic/plastic character of the adhesive (see Adhesion - fundamental and practical, Peel tests). Our understanding of adhesion has been significantly improved with the advent of mechanical devices that are able to probe the forces of adhesion under conditions that minimize all of the confounding effects of adherend, viscoelasticity, and so on. The Surface Forces Apparatus (SFA) as developed by Israelachvili and Tabor is a mechanical device that has allowed adhesion scientists to directly measure the forces of adhesion under very low rate, light loading, almost equilibrium conditions. Attention is also drawn to Atomic force microscopy. 

MMA based adhesives are broadly used as fundamental adhesives because of their excellent strength, fatigue resistance, thermal shock,... 

As stated above, peel tests provide practical adhesion values that include polymer and substrate mechanical properties, stored stresses, plastic deformation, and other parameters. It has been demonstrated that an analysis of the peel test mechanics allows to extract the fundamental adhesion from the experimental data [71]. A method to calculate the interfacial fracture energy of a polymer bonded to a rigid substrate by using peel tests has also been presented [72]. 

As Newton realized, there are two different types of questions to be asked about the phenomenon of adhesion. One type is concerned with the forces and so on, which have to be apphed to separate the bonded components, the other concerns what holds the components together in the first place. These two types of questions are usually referred to as concerning, respectively, practical and fundamental adhesion. Some authors have written as if they thought that fundamental and practical adhesions were the same. This tendency was more prevalent in the past, but is stiU occasionally encountered. 

Serious scientific concern with fundamental adhesion is usually dated back to the classic work of McBain and Hopkins in the 1920s, and various theories were advanced during the middle part of the last century. There was a strong tendency during much of this period to regard different theories as rival explanations of the same phenomena, rather than as complementary aspects of a broader rationalization. Much of the literature of the that time, and even some of the literature today, reflect this approach, and therefore it is appropriate in this chapter first to consider different theories individually. A synthesis will be given in the conclusions. 

From a simple standpoint, Eq. 2.3 can be taken as representing the relationship between practical adhesion (represented by the measured G) and fundamental adhesion associated with molecular forces at the interface, Gq. To repeat, usually ij/ is very much larger than Go, and so practical fracture energies for adhesive joints are almost always orders of magnitude greater than work of adhesion or work of cohesion. 

The weak-boundary-layer theory is essentially a theory of fracture, rather than of fundamental adhesion. However, it is frequently discussed along with these other theories, so consideration is given to it here in Sect. 2.6. 

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