cosmetic dentistry_ beauty & science

32 I I review _ surface-conditioning a high temperature in a vacuum. After the firing process, the surface is sand blasted again. A silica- containing layer forms on the zirconia surface. This enhances adhesion with a silane coupling agent, that is, siloxane linkage formation.14 Chemicalvapourdeposition Inachemicalvapourdepositionsystem,thezirco- nia surface is exposed to a vapour mixture of tetra- chlorosilane and water. The silane hydrolyses and a SixOy seed layer is deposited as a coating on the surface. The thickness of the seed layer is controlled by deposition time. This silica seed layer provides the reactive sites for the silane coupling agent.15 Plasmafluorination In a plasma reactor, the zirconia surface is ex- posed to sulphur hexafluoride plasma. An oxyfluo- ride layer is formed on the surface. This layer may increase the reactivity of zirconia towards a silane coupling agent. However, the exact mechanism of the bonding formation between the zirconium oxyfluoride layer with silane is still unclear.15 Silane chemistry Functionalandnon-functionalsilanes Functional silanes contain two different func- tional groups that can react with inorganic ma- trices, for example ceramics, and organic materials, for example resins. Therefore, they can be used as coupling agents to connect dissimilar materials. There is also a group of silanes called the non- functional silanes. They contain one reactive func- tionalgroupthatcanreactwithinorganicmaterials. They are widely used for some specific surface modification of materials. In addition, there are bis- functional/cross-linking/dipodalsilanesthatpossess two silicon atoms with three hydrolysable alkoxy groups. Cross-linking silanes are used in the steel andtyreindustries.18 Suchsilaneisalsoincorporated with functional silane to increase the bonding and hydrolytic stability of resin composite to titanium.19 Silaneactivationmechanism Silanescancreateabondbetweeninorganicand organic materials. A general formula for a func- tional silane coupling agent is Z-(CH2)n-Si-(OR)3. Z is an organo-functional group that reacts with organic resin, (CH2)n is a linker group, and OR is an alkoxy group. The alkoxy groups are activated by hydrolysis ([SiOR➝ [SiOH) before they react with the surface hydroxyl groups of the substrate.20 The first step of silane hydrolysis is the fast and reversible protonation of the alkoxy group at a low pH (3–5). Next, a bimolecular nucleophilic substi- tution(SN2)reactionatthesiliconatomtakesplace. A nucleophile, a water molecule, attacks backside tothesiliconatom,anelectrophile,toformapenta- coordinate transition state. A new bond is formed betweenthesiliconandthenucleophile,andabond iscleavedbetweenthesiliconandtheleavinggroup, alcohol. This yields the product with an inversion of the configuration.21 A suggested mechanism for silane hydrolysis is shown in Figure 1. The silane hydrolysis rate depends on the steric (size)andinductive(electronic)effectsofalkoxygroups on the silane. The steric effect is the dominant factor that affects the silane hydrolysis rate.22 This effect is bestillustratedusingaball-and-stickmodel(Fig.2). As shown in Figure 2, the steric repulsion in- creaseswhenthesizeofthealkoxygroupischanged from a methoxy to butoxy group. The approach of a water molecule, a nucleophile, to the silicon atom is more difficult for the bulky butoxy groups. This may explain why ethoxysilanes are employed in commercialdentalproductsbecauseoffasthydrol- ysis of small ethoxy groups. Methoxysilanes are not used, as the by-product methanol is very toxic. Theorgano-functionalgroupsofsilanecoupling agentsconsistof,forexample,>C=C<,avinyldou- ble bond that can react with the functional groups of resin composite consisting of >C=C< bonds. The reaction is initiated by the initiators in the resin composite, which are decomposed by visible blue light to form free radicals. These free radicals re- act with the >C=C< bond in the resin composite monomer or in the silane molecule to generate an- other free radical species. The reaction of these free radicals with resin composite monomers and silane molecules forms new C-C single bonds. Therefore, the silane coupling agents connect the resin com- posite and the inorganic substrate surface.23 _Application of silanes in dentistry Ceramicrestorationsandrepairs Silane coupling agents are used in dental restoration,suchasceramicrepairsofonlays,inlays, crownsandbridges.Formostpatients,repairismore economicalandtime-savingthanthefabricationof newrestorations,unlessdamageduetoafractureis beyond repair. The clinical procedure for repairing ceramic restoration usually involves the following steps: roughening the surface with diamond burs, sand-blastingthesurface,acidetching,silanisation and finally bonding to resin composite.24 cosmeticdentistry 2_2013