Boundary layer weakness concept

The first cause of weakness is the existence of weak boundary layers , a concept first introduced by Bikerman who suggested that the idea that two phases in contact were entirely isotropic and of uniform composition up to their boundaries was mistaken. The surface regions of real materials are certainly different from their bulk and form the weak layers because of ... 

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

Finally, mention should be made of the concept of a sacrificial primer which enables metals to be readily bonded underwater [210,211]. From the comments in Section 2.6.2 it is obvious that when trying to apply an adhesive to a substrate underwater it will be difficult for the adhesive to completely displace the water adsorbed on the surface and establish interfacial contact with the substrate a layer of water would remain adsorbed onto the substrate and act as a weak boundary layer. The idea of the sacrificial primer is that the substrate is firstly cleaned, typically by an abrasion pretreatment, but simultaneously a primer is applied which is formulated so that it displaces any water and is more readily adsorbed onto the substrate surface. This primer, although water repellant, is also formulated so that it is compatible with the adhesive to the extent that it, in turn, is displaced and dissolved by the adhesive when it is applied. Thus, the concept of a sacrificial primer enables the adhesive to wet the substrate completely without a weak boundary layer of water being retained at the interface. The adhesive is also especially formulated to aid these processes and contains silanes which will diffuse to the interface and establish strong and stable interfacial bonds to ensure a long service life for the joint in its underwater environment [212]. 

The types of postpolymerization kinetic curves shown can be qualitatively explained using the conception of the three reactive zones proposed earlier. Up to the moment when the polymerizing system is in the liquid monomer-polymeric phase (MPPh), namely up to the moment of conversion = 0 5, only a weak post-effect is observed. MPPh is characterized by low concentration of free radicals with a short life time. A visible post-effect is observed in the autoacceleration stage Po > Py , that is, at the beginning of polymer-monomeric phase elimination and formation of the interface layer at the phase division boundary between MPPh and the polymer-monomeric phase PMPh, which are new reactive zones. In such reactive zones the translational and segmental mobilities of the macroradicals are sharply decreased and the life times are sharply increased. This explains the essential post-effect. 

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