Adhesion explanation

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

An alternative explanation has been proposed by Quesnel [62]. Assuming that the adhesion-induced deformation could be treated as a Hertzian indentor, with the load due to the force arising from the surface energy, Quesnel calculated the indentation in a self-consistent manner. That is to say, Quesnel recognized that the attractive force would vary as the particle or substrate deformed, owing to the increased circumference of the contact patch. He also recognized that, due to... 

The second observation is that the EB-curable adhesive resins show no change in adhesive properties from liquid helium temperatures up to temperatures Just below their service temperatures. This can be seen for the adhesives, EB2000 and 1 1L. At this time there is no explanation for this behavior. 

An impressive progress in the fundamental study of friction was made more than half a century ago when Bowden and Tabor proposed that friction resulted from shear of adhesive junctions at the real contact area, which took up only a tiny portion of nominal contact zone and was proportional to the load [10], as schematically shown in Fig. 11. The model presents a satisfied explanation as to why friction is proportional to the load and independent of nominal contact area. 

Other explanations of the nature of the polymer to metal bond include mechanical adhesion due to microscopic physical interlocking of the two faces, chemical bonding due to acid/base reactions occuring at the interface, hydrogen bonding at the interface, and electrostatic forces built up between the metal face and the dielectric polymer. It is reasonable to assume that all of these kinds of interactions, to one degree or another, are needed to explain the failure of adhesion in the cathodic delamination process. 

Compound 9, which Is not detectably rehydrated by water or attacked by simple alcohols, Is also converted In high yield to cyclic acetals H at 70 C, pointing again to the significant thermodynamic driving force of cyclic acetal formation. This strongly favored acetal reaction Is one explanation for the excellent adhesion performance of this system on cellulosics and glass. 

The basic science behind nucleation and forces between materials have been treated in Chapter 1. For those interested in this section, it is assumed that this basic science is (more or less, at least) understood. However, the basics treated in Chapter 1, while important to an understanding of film (as opposed to isolated crystal) formation, are not enongh by themselves to provide a phenomenological explanation of film formation. We would ideally like to be able to predict in advance, from fundamental principles, whether a particular bath formulation will result in adherent films or not. We cannot However, if we cannot reliably predict adhesion, we can at least choose conditions so that the probability of adhesion is good. 

Obviously, one requirement for an adhesive is that it flow easily to cover a surface. This is a more complex business than it first appears. One might naively think that the governing feature is whether we are dealing with a thin or a thick liquid, but this is not the case. If we put a drop of oil in an iron skillet, it spreads, but on a Teflon surface it beads up. The explanation revolves around surface energies, which are a measure of the relative strengths with which atoms on the surface of a material are attracted to atoms inside the bulk of the material. In a sense, this determines how much attraction these surface atoms can spare for other substances. In the case of Teflon, very little. Teflon is composed of long chains of carbon atoms, with each carbon also joined to two fluorine atoms. The fluorines, which stick out from the carbon skeleton, represent the exposed part of the molecules, the part that could potentially interact with other molecules. Fluorine, once it has bonded to carbon, is notoriously unreactive, and it is not interested in forging other... 

The finding that the silanes were more effective as additives in coatings than as pretreatments may at first sight seem surprising as the reverse was true of adhesives. The explanation may be in the fact that in the case of pretreatments it is likely that the silanes were used under less than optimum conditions. All the silanes were used at their natural pH, no hydrolysis catalysts were added and no attempt was made to adjust the pH of the solutions befure use. It has been shown that the structures of APES films on iron are dependent on solution pH, as is the availability of reactive groups [23],... 

FT-IR spectroscopy has been used in the investigation of welding crosslinked polyethylene pipes [23]. Three types of crosslinking systems were used namely, peroxide (PEXa), silane (PEXb) and electron beam (PEXc). Scholten and co-workers [23] observed that only PEXa pipes have a satisfactory electrofusion quality. The strength of electrofusion welds of PEXb and PEXc pipes is not acceptable. The most likely explanation for the differences in weld quality is related to the adhesion theory and more specifically to differences in composition. Figure 5.6 shows the infrared spectra of medium density polyethylene (MDPE), PEXa, PEXb and PEXc. 

Adhesion is a common daily occurrence that we recognize without difficulty, but unfortunately recognition is much easier than explaining in detail how it comes about. Why indeed should there be so strong an attraction between the molecules that it becomes difficult to separate the surfaces or bodies concerned Summarized below are three theories or explanations that have been advanced over the years, often with much compelling argument and data in substantiation. Very briefly, it is said that ... 

Three primary mechanisms have been suggested for enhanced adhesion via silane coupling agents.5 The classical explanation is that the functional group on the silane molecule reacts with the adhesive resin. Another possibility is that the polysiloxane surface layer has an open porous structure. The liquid adhesive penetrates the porosity and then hardens to form an interpenetrating interphase region. The third mechanism applies only to polymeric adherends. It is possible that the solvent used to dilute and apply the silane adhesion promoter opens the molecular structure on the substrate surface, allowing the silane to penetrate and diffuse into the adherend. 

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