Theory of adhesion mechanical

A classic instance of the mechanical theory of adhesion is where one phase is keyed into the other. Here the adhesion is enhanced above the increase proportional... 

Packham, D.E., The mechanical theory of adhesion — a seventy year perspective and its current status. In van Ooij, W.J. and Anderson, H.R., Jr. (Eds.), 1st International Congress on Adhesion Science and Technology Invited Papers. VSP Publishers, Utrecht, 1998, pp. 81-108. 

The anodic film formed on aluminium in aggressive electrolytes such as phosphoric, sulphuric and chromic acids is porous. The classical description of the film is of a regular hexagonal array of cylindrical pores penetrating normal to the surface almost to the base metal (Fig. 1). The actual structure revealed by Electron microscopy varies greatly with electrolyte and anodizing conditions. Some details are given in Mechanical theory of adhesion see also Fig. 2 in Electron microscopy. 

Venables to argue for the importance of mechanical interlocking (see Mechanical theory of adhesion) in adhesion to porous anodized surfaces. 

The morphology of the surface has been studied by transmission Electron microscopy Under optimum conditions, it consists of a cell structure with oxide whiskers protruding fi om the surface." It has been suggested that microscopic interlocking (see Mechanical theory of adhesion) appears to be a crucial factor in the adhesion." It may be that the sensitivity to processing conditions, mentioned above, is a result of the production of a morphology with less potential for interlocking. 

Some form of substrate pre-treatment is almost always essential before adhesive bonding (see many articles herein entitled Pre-treatment of... ). The extent to which pretreatments increase the Roughness of surfaces and the connection between this and adhesion are complex issues that have interested scientists for more than 50 years (see Mechanical theory of adhesion). The advent of Scanning electron microscopy (SEM) in the late 1960s made much more detail of surface topography available. Some surfaces of metals can be seen in SEM to be covered with acicular projections with heights of the order of 1 p.m (Fig. 1). Such surfaces have been termed microfibrous . 

The adhesion of paint films is influenced primarily by the chemical composition of the binder. Films may adhere by a mix of all possible mechanisms, for example. Dispersion forces, Acid-base interactions. Hydrogen bonding, covalent bond formation and other specific chemical interactions, such as chelation. In addition to these molecular interactions across an interface, the micro-roughness of the substrate can contribute by increasing the surface area and points of contact (see Mechanical theory of adhesion). In addition, if the substrate is itself another paint film (or a plastic), chain ends of the overcoating polymer can diffuse into the substrate, provided that the binder types are mutually compatible (see Theories of adhesion). 

Garnish and Haskins found that exposure of polypropylene to trichloroethylene vapour for 10 s resulted in a sixfold increase in joint strength using an Epoxide adhesives. The authors concluded that the improved adhesion was due to the removal of a weak boundary layer. However, the treatment causes the formation of a very porous surface, and an alternative explanation for the improved adhesion is the mechanical keying of the adhesive into the porous surface (see Mechanical theory of adhesion). Garnish and Haskins found that the optimum treatment time was about 10 s and that after 25 s the adhesion level was similar to that of the untreated polymer. This reduction is probably due to weakening of the surface region of the polypropylene. 

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. 

An unprimed silicone adhesive implies that it is free of any adhesion promoter. The substrate on the other hand, may still need to be activated or primed. Adhesion relies mainly on chemical and/or mechanical mechanisms (see Mechanical theory of adhesion and Primary bonding at the interface). The chemical adhesion depends on both the reactivity of the selected silicone cure system and on the natural presence of reactive groups on the surface of the substrates. 

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. 

Mechanical theory of adhesion K W ALLEN Macro- and micro-interlocking influence on adhesion... 

The mechanical theory of adhesion predicts mechanical interlocking as the driving force for all adhesion, and is especially useful for materials with rough surfaces, and/or porous... 

Mechanical Theory of Adhesion. The oldest theory of adhesion is definitely the mechanical theory. It is based on mechanical anchorage of the adhesive in pores and irregularities in the adherent and is discussed primarily in reference to wood and similar porous materials [8]. 

Mechanical theory. The surface of a sohd material is never truly smooth but consists of a maze of microscopic peaks and valleys. According to the mechanical theory of adhesion, the adhesive must penetrate the cavities on the surface and displace the trapped air at the interface. 

Rough surfaces are central to the concept of the mechanical theory of adhesion, discussed in O Sect. 2.3 below. However, practical surfaces are always rough to some degree, and the discussion of the adsorption theory so far has tacitly assumed that the surface to be bonded was smooth. 

Many of the features of the adsorption theory of adhesion, just discussed, can be traced back to McBain and Hopkins classical work in the 1920s, where they are referred to it as specific adhesion (McBain and Hopkins 1925 Packham 1998, 2002 2003). They also described mechanical adhesion to porous substrates, such as wood, unglazed porcelain, pumice, and charcoal. They regarded it as obvious that a good joint must result whenever a strong continuous film of partly embedded adhesive is formed in situ. This is, in essence, the mechanical theory of adhesion. 

Despite the obvious nature of the mechanical theory of adhesion, by the 1950s, the validity of the mechanism was largely denied by scientists working on adhesion. This denial was based on the results of studies in which adhesion to similar surfaces with different degrees of roughness was reported. These generally showed an inverse relationship between roughness and practical adhesion. 

Control experiments have shown scientifically that very rough surfaces, such as those illustrated in O Fig. 2.5, give high adhesion under circumstances where adhesion to a corresponding smooth surface is low. So it could reasonably be said that the mechanical theory of adhesion applies in these cases. Of course, this does not necessarily mean that the high adhesion is the only technological reason for using the pretreatment. For example, the anodization of aluminum enhances durability of an adhesive bond in humid environments in the aviation context, this is more important than the actual level of practical adhesion achieved. 

On the phenomenological level, it is clear that there are many rough surfaces known to adhesion science and technology where the surface roughness plays an essential role in adhesion the mechanical theory of adhesion applies. Rather than simply ascribing adhesion to mechanical effects, it is useful to explore why roughness can lead to good adhesion. 

Thus, the basic ideas, related by McBain and Hopkins in the 1920s, underlying both the adsorption and mechanical theories of adhesion still provide useful models for rationalizing observations in adhesion science. Two further adhesion theories, the electrostatic and diffusion theories, emerged from the Soviet Union in the middle of the twentieth century. We now turn to consider them, and how they are regarded today. 

Koh SK, Park SC, Kim SR, Choi WK, Jung HJ, Pae KD (1997) J Appl Polym Sci 64 1913 Leadley SR, Watts JF (1997) J Adhes 60(1 ) 175-196 McBain JW, Hopkins DG (1925) J Phys Chem 29 188 Packham DE (1998) The mechanical theory of adhesion-a seventy year perspective and its current status. 

Packham DE (2003) Mechanical theory of adhesion. In Pizzi A, Mittal KL (eds) Handbook of adhesive technology, 2nd edn. Marcel Dekker, New York Popper KR (1959) The logic of scientific discovery. Hutchinson, London... 

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