Weak boundary layer theory

Mechanical Theory Electrostatic (Electronic) Theory Diffusion Theory Wetting Theory Chemical Bonding 1.6.5.1 Acid-Base Theory Weak Boundary Layer Theory... 

Weak Boundary Layer Theory. According to the weak boundary layer theory, when bond failure seems to be at the interface, usually a cohesive break of a weak boundary layer is the real event. Weak boundary layers can originate from the adhesive, the adherend, the environment, or a combination of any of the three. When bond failure occurs, it is the weak boundary layer that fails, although failure seems to occur at the adhesive-ad-herend interface. 

There are two further theories namely electrostatic theory and weak boundary layer theory. All are discussed in detail below. 

The weak boundary layer theory proposes that clean surfaces can give strong bonds to adhesives, but some contaminants such as rust and oils or greases give a layer which is cohesively weak. Not all contaminants will form weak boundary layers, as in some circumstances they will be dissolved by the adhesive. However, in some cases, contaminants such as oils and greases can actually be removed by the adhesive dissolving them [101]. 

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. 

However, 50 years ago, when there was considerably less basic understanding, and more confusion, in the study of adhesion than there is now, weak boundary layer theory would often be discussed alongside theories such as the adsorption theory and the electrostatic theory. The tendency to group theories of adhesion in this way has persisted, and so it may now be useful to discuss the significance of weak boundary layers. 

Weak boundary layer. WBL theory proposes that a cohesively weak region is present at the adhesive-substrate interface, which leads to poor adhesion. This layer can prevent the formation of adhesive bonds, or the adhesive can preferentially form bonds with the boundary layer rather that the surface it was intended for. Typically, the locus of failure is interfacial or in close proximity to the silicone-substrate interface. One of the most common causes of a WBL being formed is the presence of contaminants on the surface of the substrate. The formation of a WBL can also result from migration of additives from the bulk of the substrate, to the silicone-substrate interface. Alternatively, molecular... 

The removal of an electron from an acceptor level or a hole from a donor level denotes, as we have seen, not the desorption of the chemisorbed particle but merely its transition from a state of strong to a state of weak bonding with the surface. The neglect of this weak form of chemisorption (i.e., electrically neutral form) which is characteristic of all papers on the boundary-layer theory of adsorption makes it quite impossible to depict the chemisorbed particle in terms of an energy level, i.e., to apply the energy band scheme depicted in Fig. 10 and used in these papers. ... 

When there is no weak bonding at all, one returns within the frame of the boundary-layer theory. In this case, however, the chemisorbed particles do not produce any levels in the crystal energy spectrum. 

An apparent weakness of the film model is that it suggests that the mass transfer coefficient is directly proportional to the diffusion coefficient raised to the first power. This result is in conflict with most experimental data, as well as with more elaborate models of mass transfer [surface renewal theory considered in the next chapter, e.g., or boundary layer theory (Bird et al., I960)]. However, if we substitute the film theory expression for the mass transfer coefficient (Eq. 8.2.12) into Eq. 8.8.1 for the Sherwood number we find... 

Fourth, the criterion that we have derived does not override other well-known criteria, such as that the presence of weak boundary layer material is a sufficient, though not a necessary, condition for poor adhesion. Indeed, our theory may be regarded as showing just when boundary-layer material will qualify as "weak", in a particular system. 

C. Theory of Weak Boundary Layers Concept of Interphase... 

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. 

All the studies conducted on fracture of bulk polymers are certainly relevant to the adherence of polymers, the mechanisms of losses at a crack tip being the same viscoelastic losses due to moving stresses, work to extract chains or fibrils, and viscous drag in the presence of a liquid. It is probable that the various theories of adhesion, namely, theory of wetting, theory of the rheological factor, theory of the chemical bond, theory of the weak boundary layer, and theory of interdiffusion, are all valid, each corresponding to an emphasis on a dominant mechanism. 

The idea of a weak boundary layer is a theory of non-adhesion rather than one of adhesion. However, weak boundary layers certainly occur on some substrates. Examples include lubricating oil on metals and rust on iron, but a less obvious case lies in the tendency of foreign material in plastics to diffuse to the surface such materials might be the various additives discussed in Section 2.4 or simply low molar mass polymer. In either case the remedy is to remove the weak boundary layer with a solvent or by abrasion. 

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