There are methods available for creating stronger interfacial bonds. Silane coupling agents or other adhesion promoters coupled with structural adhesive formulations tailored for specific applications have vastly improved the performance of bonded assemblies, including those involving ceramics. Chapter 12 offers an in-depth review of adhesion promoters including silane chemistry and its mechanism of adhesion promotion.
Silane groups usually have two different reactive groups, as shown in Figure. One group is reactive to the substrate and the other to the adhesive. An example can be seen in Figure. Most metals and inorganic materials have hydroxyl groups in their structure which are capable of hydrogen bonding to other materials and giving tightly bound water on their surface. In the bonding process with silanes, silane coupling agents form silicon-hydroxyl (silanol) groups which then form bonds with these inorganic surface hydroxyl groups by covalent or hydrogen bonding. The usual inorganic-reactive group(s), single bondO-R, on the silanes which promote adhesion, are hydrolysable organic groups, like alkoxy or acetoxy silanes. The X, or organic, group on the silane is typically a reactive group with which the adhesive will react or interact. Figureshows the reaction mechanism of an ideal silane.
Silanes come with a variety of organic-reactive end groups including epoxy, amine, isocyanate, and alkene-containing. The type of reaction and the required catalyst depend on the end groups. Figure illustrates the reaction mechanism of the silane shown in Fig.
Inorganic surfaces are pretreated with a silane either by a dry method or a wet method and at room temperature or at elevated temperature. If a filler surface which contains residual moisture is being treated, the moisture will hydrolyze the silane. Water and/or an alcohol may be added to enhance hydrolysis. For fillers a high-intensity mixer, such as a Henschel or Littleford Mixer, can be used for the treatment process. Filler treatment may also benefit from the addition of a trace of amine or titanate to catalyze the reaction with the filler surface. At the least a monolayer of silane must form on the inorganic surface to enhance adhesion. In the wet method, a low concentration of silane is dissolved in water and/or alcohol, and applied to the substrate, and the solvent is evaporated to give a thin coating of silane on the surface. In dental restorations there is a problem with using silanes in adhesion promotion because of the bond degradation over time in the oral environment, but, without silanes, the bond degradation is even more pronounced.