Adhesion models

There currently exists no uniquely valid model of adhesion but rather a series of different models each having its own field of application. The chemists task is to combine 

Dipole forces are effective when polarity is present and can be generated from permanent or induced dipoles. Polar molecular groups of adhesives must contact the substrate surface in the distance of 10 cm or less. An example for adhesion between polar binder and polar substrate is pictured in Fig. 43. 

Dispersion forces. The long chain of a polymeric macromolecule diffuses very close to the substrate and non-polar interactions are generated. Such Van der Waals interaction mechanism are here responsible for bonding forces. This diffusion procedure is more significant for bonding of plastics than of metals (Fig. 45). 

The calculated cohesion forces per square meter range as follows [1,2]  

Adhesion promoter in adhesive Reaction of two silane groups 

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Wear is a complex phenomenon, involving a wide range of possible processes (11) but invariably two of the most important governing factors are the normal load and sliding distance. At a constant normal load we have observed that both the non-stationarity and microslip increase with velocity. In terms of the adhesion model of friction, which provides a quantitative description of the frictional behaviour of PET fibres (2-3) a frictional event is the formation and subsequent rupture of an adhesive junction. It is reasonable to suppose that these events are the underlying cause of wear. Micro-... 

The nitrile-epoxy structural adhesive systems have moved from an aircraft orientation to include industrial, automotive and electronics areas. These include a host of film adhesive products (21, 22) and thixotropic pastes (23-26). Representative adhesive models have also been studied from a fracture toughness polnt-of-view permitting comparisons of bulk fracture to that of fracture in the adhesive joint (27, 28). 

IV. ADHESION MODELS FOR WELL-DEFINED POLYMERS TO WELL-DEFINED SURFACES... 

V. DYNAMIC ADHESION MODELING OF MOLECULARLY WELL-DEFINED SYSTEMS... 

The vast majority of fibrillar adhesive materials have been prepared to (1) test adhesion model prediaions or (2) explore and optimize attachment to dry surfaces. Nevertheless, in order to expand the range of applications for synthetic fibrillar adhesives, it is desirable to improve performance under humid and wet conditions. For example, compared to flat and unstmc-tured polyvinylsiloxane, generating microscale mushroom fibrillar architectures with this material improved adhesion to both hydrophilic and hydrophobic surfaces under water. ... 

Figm 11 The Phragmatopoma califomica adhesive model. Reprinted with permission from Stevens, M. J. Steren, R. E. Hlady, V. Stewart, R. J. Langmuir 2007,23,5045. Copyright 2007 American Chemical... 

Six PU adhesives, modelled on practical formulations, have been studied. They are coded PUl - PU6. All were two-part compounds, denoted as A and B in Table 7.5. In all cases, part B contained a tin catalyst. 

As more sophisticated adhesion models become available, it is obvious that they will greatly influence the applications of adhesion, from colloids, pastes and gels to aerospace, electronics and cells. It seems likely that, just as photocopying, food, and aerospace drove adhesion science forward in the last 50 years, electronics and bioadhesion will pnsh back the bonndaries in the new milleninm. We will move increasingly towards molecnlar applications. 

Durability (63,64) of bonded assemblies continues to receive the prime attention it deserves in the performance regime of structural adhesive testing. And aging studies of selected nitrile-epoxy structural adhesive models are being reported (65,66). 

Tp=plastic adhesive shear stress in the elastic-plastic adhesive model Ye=elastic adhesive shear strain in the elastic-plastic adhesive model Yp=plastic adhesive shear strain in the elastic-plastic adhesive model. 

Figure 5.32 Adhesive shear stress distribution as a function of time in a single-lap joint using a viscoelastic-viscoplastic adhesive model, based on Groth (reference 5.36).

Russell (reference 5.35) has presented a method to analyse adhesively bonded joints under generalised in-plane loading. His approach is based on the Hart-Smith method using a non-linear adhesive model. 

The consequences of the mechanisms of diffusion, discussed above, for the strength of the interfaces formed are further considered in Polymer-polymer adhesion models. 

When M < Me, the role of entanglements is no longer present and Eqns. 2 and 3 cannot be used since Gie = 0 at Me. However, the Nail solution applies for weak interfaces (see Eqn. 3 in Polymer-polymer adhesion models) and the chain segments simply pullout at fracture such that (Gic-Gq) M, where Gq is the surface energy term, and we obtain. 

The self-adhesion (autohesion) of polymers is of considerable practical importance as many moulding operations involve the need for streams of merging polymer melt to self-adhere to form a strong bond necessary to the integrity of the moulded article. Mechanisms of polymer diffusion are discussed in Polymer diffusion reptation and interdigitation and theories that relate interfacial structure to strength in Polymer-polymer adhesion models. In the latter article, the vector percolation (VP) model was described, which is here applied to welding, and the practical consequences of its predictions are drawn out. - ... 

Polymer-polymer adhesion models R P WOOL Nail and vector percolation models... 

In the Globals subsection, the contact models for particle-to-particle and particle-to-geometry interactions are defined (see C in Figure 7.15). EDEM has a number of built-in contact models such as Hertz-Mindlin no-slip model (i.e., Hertz model is used for normal contact force calculations [see Section 7.1.4.1.2] and Mindlin no-slip model is used for tangential contact force calculations [Section 7.1.4.1.4]), linear-spring model (see Section 7.1.4.1.1), and JKR adhesive model (see Section 7.1.4.2.1). 

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