Interfacial contact and intrinsic adhesion

The purpose of surface preparation is to remove contamination and weak surface layers, to change the substrate surface geometry, and/or introduce new chemical groups to provide, at least in the case of metals, an oxide layer more receptive to the adhesive. An appreciation of the effects of pretreatments may be gained from surface analytical or mechanical test techniques. Experimental assessments of the effects of surface pretreatment, even when using appropriate mechanical tests, are of limited value unless environmental exposure is included. Self-stressed fracture mechanical cleavage specimens, as discussed in Chapter 4 and in the texts edited by Kinloch(2,5) for example, are therefore referred to wherever possible. 

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Interfacial contact and intrinsic adhesion Table 3.2. Interactions which may contribute to adhesion (Refs. 2, 22)... 

We have so far considered the establishment of interfacial contact between the adhesive and substrate, the nature of the intrinsic adhesion forces which hold the interface together, and the role of surface pretreatments to assist in the above processes. The next stage in the formation of an adhesive joint is usually, but not always, the hardening of the adhesive so that it is capable of... 

The previous chapter considered the various aspects involved in the attainment of intimate molecular contact at the adhesive/substrate interface. As discussed, the attainment of such interfacial contact is invariably a necessary first stage in the formation of strong and stable adhesive joints. The next stage is the generation of intrinsic adhesion forces across the interface, and the nature and magnitude of such forces are extremely important. They must be sufficiently strong and stable to ensure that the interface does not act as the weak link in the joint, either when the joint is initially made or throughout its subsequent service life. The various types of intrinsic forces which may operate across the adhesive (or primer)/substrate interface are commonly referred to as the mechanisms of adhesion, and they are discussed in this chapter. 

The previous two chapters have emphasized the importance of the chemistry, topography and morphology of the substrate s surface in ensuring both that intimate interfacial contact is achieved and that adequately strong and stable intrinsic adhesion forces are forged across the interface. Therefore, the next chapter will consider the various surface pretreatments that are employed to attain these aims. 

As discussed in Chapter 2, for the formation of intrinsic adhesion forces across the adhesive/substrate interface it is a necessary, though sometimes insufficient, requirement that intimate interfacial contact occurs between the adhesive and substrate. This means that at some time in the bonding operation the adhesive must be in a liquid form, ideally with a relatively low viscosity. However, in service most adhesive joints carry some mechanical loads, even if only those resulting from the weight of the substrates, and to be able to transfer mechanical loads efficiently from one substrate to the other requires that the adhesive is a solid with an adequate stiffness and strength. These conflicting requirements mean that once the adhesive has established interfacial contact, i.e. completely wet the substrate, it must then harden or set . In this chapter the methods by which the adhesive may be transformed from a liquid state to a solid state will be considered and this classification will also be used to introduce briefly some of the main chemical types of adhesive to those unfamiliar with this aspect of adhesives technology. 

The van der Waals and other non-covalent interactions are universally present in any adhesive bond, and the contribution of these forces is quantified in terms of two material properties, namely, the surface and interfacial energies. The surface and interfacial energies are macroscopic intrinsic material properties. The surface energy of a material, y, is the energy required to create a unit area of the surface of a material in a thermodynamically reversible manner. As per the definition of Dupre [14], the surface and interfacial properties determine the intrinsic or thermodynamic work of adhesion, W, of an interface. For two identical surfaces in contact ... 

As is true for macroscopic adhesion and mechanical testing experiments, nanoscale measurements do not a priori sense the intrinsic properties of surfaces or adhesive junctions. Instead, the measurements reflect a combination of interfacial chemistry (surface energy, covalent bonding), mechanics (elastic modulus, Poisson s ratio), and contact geometry (probe shape, radius). Furthermore, the probe/sample interaction may not only consist of elastic deformations, but may also include energy dissipation at the surface and/or in the bulk of the sample (or even within the measurement apparatus). Study of rate-dependent adhesion and mechanical properties is possible with both nanoindentation and... 

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