Silanes mechanism with adhesion promoters

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

Three primary mechanisms have been suggested for enhanced adhesion via silane coupling agents.5 The classical explanation is that the functional group on the silane molecule reacts with the adhesive resin. Another possibility is that the polysiloxane surface layer has an open porous structure. The liquid adhesive penetrates the porosity and then hardens to form an interpenetrating interphase region. The third mechanism applies only to polymeric adherends. It is possible that the solvent used to dilute and apply the silane adhesion promoter opens the molecular structure on the substrate surface, allowing the silane to penetrate and diffuse into the adherend. 

Mechanistic theories of adhesion promotion have been described in detail by Rosen [11] with sole reference to silanes, but it is likely that many of the proposed theories apply equally well to coupling agents in general. The proposed mechanisms are described below. 

Ceramic brackets are used for aesthetic reasons, and they are commonly treated with Silane adhesion promoters, either a vinyl silane, typically methacryloxypropy-Itrimethoxysilane, coupling agent or a micro-crystalline roughened base to enhance mechanical retention. 

The chemical bonding mechanism strongly depends on both the reactivity of the selected silicone cure system and the presence of reactive groups on the surface of the substrate. The silicone adhesive can be formulated so that there is an excess of these reactive groups, which can react with the substrate to form covalent bonds. It is also possible to enhance chemical bonding through the use of adhesion promoters or chemical modification of the substrate surface. The mechanism of chemical adhesion is probably best studied and demonstrated by the use of Silane adhesion promoters. 

Although silane adhesion promoters are probably the most known, used, and versatile, there are many others. Some are based on titanium, chromium, or zirconium, while others, still based on silicon chemistry totally differ in their structure from regular organosilanes. This section will highlight briefly a few examples with their chemistry, mechanisms, and fields of applications. 

---