Silane coupling agents contain inorganic reactive groups on silicon and will bond well to most inorganic substrates, especially if the substrate contains silicon, aluminum, or most heavy metals in its structure. The alkoxy groups on silicon can hydrolyze to silanols, either with added water or from residual water on the inorganic surface (reaction 29.1). The silanols coordinate with metal hydroxyl groups on the inorganic surface to form an oxane bond with elimination of water (reaction 29.2). Studies of the mechanism of bond formation of silane coupling agents to inorganic substrates show covalent oxane bonds with silica and glass reinforcements [3]. Hydrolysis and reformation of siloxane bonds of silane coupling agents are in true equilibrium [4]. These bonds are hydrolyzed by water, but they reform rather easily. Acids and bases catalyze the rates of both hydrolysis and condensation. It is estimated that a trisilanol, R’Si(OH)3 (the intermediate generated by a trialkoxysilane coupling agent), has an equilibrium constant 103 more favorable toward bonding to a silica substrate than the corresponding monoalkoxysilane. Trialkoxysilanes have approximately a tenfold equilibrium constant relative to dialkoxysilanes toward bonding to the inorganic substrate. So it is fortuitous that the original silane coupling agents were based on trialkoxysilanes rather than mono- or dialkoxysilanes since the equilibrium toward retention of bonding to the inorganic substrate is much greater with trialkoxysilanes.
The condensation of the silane silanols with other silanols at the interface gives a multimolecular structure of crosslinked siloxane on the inorganic surface. The cross-linked structure exists as multiple layers of siloxane with a very tight siloxane structure existing close to the inorganic surface and a more diffuse structure away from the surface. This “grid” type of structure allows organic coatings to interpenetrate, or diffuse, into the siloxane structure to allow electrostatic forces of interpenetration, very powerful adhesive forces, to become part of the bonding mechanism. A representation of this type of IPN structure is shown in Figure.