Dental Tribune Indian Edition

5Dental Tribune Indian Edition - January 2013 Silane coupling agents and surface conditioning in dentistry Dr Christie Ying Kei Lung, Jukka Pekka Matinlinna Hong Kong & Finland In dental restorations, it is desira- ble to have durable and strong bon- ding between resin composite and dental restorative materials. Weak bonding at the interface can be dra- matically enhanced with a coupling agent. Silane coupling agents, which are synthetic hybrid inorganic-organic compounds, are used to promote adhesion between dissimilar mate- rials. They are good at promoting adhesion in silica-based materials such as porcelain. However, adhesion in non-silica-based restorative mate- rials such as zirconia, metals and me- tal alloys is not satisfactory. A solution to this problem may be surface conditioning of the restorati- ve materials. Currently, a widely used surface-conditioning method in den- tistry is tribochemical silica coating. After this treatment, a silica layer is formed on the surface so that the silane coupling agent can react che- mically to form a durable bond with non-silica-based materials. Moreover, this treatment increases surface rou- ghness, which will enhance microme- chanical interlocking for bonding. This review will discuss surface- conditioning methods and some new surface-conditioning techniques, si- lane chemistry, silane application in dentistry, and the limitations of sila- nes in adhesion promotion. The silane monomer most com- monly used in clinical commercial products is 3-methacryloxypropyltri- methoxysilane. This is pre-hydrolysed in a solvent mixture usually consi- sting of ethanol and water that is aci- dified with acetic acid. The shelf life for a single-bottle si- lane solution is relatively short. The solution will turn cloudy over time and cannot be used for adhesion. Two- bottle silane systems have been de- veloped to offer a more stable system. One bottle contains an unhydrolysed silane in ethanol and the other one contains an aqueous acetic acid solu- tion.1 The two solutions are mixed for silane hydrolysis before use. Surface-conditioning me- thods The surface conditioning of resto- rative materials is an important pre- liminary step in clinical practice to modify surface properties for durable and hydrolytically stable adhesion. The surface pretreatment methods widely used in dental technology are grit blasting, tribochemical silica co- ating and hydrofluoric acid etching, which will be discussed briefly in the following section. Grit blasting The surface of materials such as metals, alloys and some ceramics is sand-blasted with alumina particles of 110 µm in size at a perpendicular distance of 10 mm under an air pres- sure of 380 kPa for ten to 15 seconds.2 This process is intended to increase the surface roughness of the mate- rials. It also enhances micromechani- cal retention for bonding.3 Pyrochemical silica coating Over the years, several silica- coating systems have been used in dental laboratories. Briefly, they are Silicoater Classical, Silicoater MD and Siloc (all Heraeus Kulzer) and PyrosilPen (SURA Instruments).2 In these systems, a tetraethoxysilane solution is injected into a flame and burned with butane in oxygen. The silane decomposes and forms reactive SiOx-C fragments, which are deposi- ted on the substrate surface. A glass- like silica layer is thereby formed on the surface.4 The use of this surface treatment is not popular in clinical practice. Tribochemical silica coating The tribochemical Rocatec system (3M ESPE) that uses silica-coated alumina particles was introduced in 1989. It is indicated for silica coating of ceramic and metal surfaces.5 It en- hances the adhesion of a silane cou- pling agent to a silica-coated material by forming a durable siloxane Si-O-Si bond. This surface treatment also increases the surface roughness that provides micromechanical retention for resin bonding, that is, for the resin to penetrate pores on the surface.6, 7 Hydrofluoric acid etching Hydrofluoric acid is normally used to etch porcelain veneers and for in- tra-oral repair of fractured porcelain restorations before cementation.1 Low concentrations of 4 to 10 % hydrofluo- ric acid are used in clinical practice. When a porcelain surface is etched with hydrofluoric acid etching gel, the acid dissolves the glassy matrix of the porcelain. A microscopically porous and micro-retentive surface is thus produced and micromechanical interlocking for resin bonding is en- hanced.9 New surface-conditioning methods The quest for enhanced and du- rable bonding continues. Several new surface-conditioning methods are currently under investigation globally. These include laser surfa- ce treatment9,10 selective infiltration etching11 nanostructured alumina coating12 internal coating14 chemical vapour deposition14 and plasma fluo- rination.16 Laser surface treatment Laser stands for light amplification by stimulated emission of radiation and the technology was introduced in the 1950s. Er:YAG, Nd:YAG, and CO2 lasers are used in dentistry for soft-tissue surgery and hard-tissue treatment and surface treatment.10 Laser irradiation of a ceramic surface produces irregularities on the surface, which increase the surface roughness for mechanical retention.16 The main problem, however, of this surface treatment method is the formation of surface cracks owing to thermal ef- fects of laser irradiation at high power settings.10,16 Therefore, appropriate laser settings for different ceramic surfaces is important to prevent for- mation of surface cracks. Selective infiltration etching In this method, a thin layer of a glass conditioning agent is coated on to the zirconia surface and is then heated to above the glass tran- sition temperature. The molten glass particles may infiltrate between the surface grains. After this process, the specimens are allowed to cool at room temperature. The conditioning agent is then removed by applying hydrofluoric acid and rinsing it off. This creates a new retentive surface for resin-zirconia bonding.17 Nanostructured alumina coating In this coating method, the zirconia is immersed in a suspension of alumi- nium nitride. Aluminium nitride un- dergoes hydrolysis to form boehmite, which is deposited on to the zirconia surface. A heat treatment at 900°C is carried out. Boehmite undergoes a phase transition to d-alumina. Throu- gh this treatment, a micro-retentive surface area is created that may incre- ase mechanical interlocking for resin bonding.13 Internal coating with porcelain The zirconia surface is sand-bla- sted with alumina particles of 70 µm in size. Then, the surface is coated with high fusing porcelain, which is prepared by stirring the porcelain powder into an excess amount of di- stilled water. The porcelain is fired at a high temperature in a vacuum. After the firing process, the surface is sand blasted again. A silica-containing lay- er forms on the zirconia surface. This enhances adhesion with a silane cou- pling agent, that is, siloxane linkage formation.14 Chemical vapour deposition In a chemical vapour deposition system, the zirconia surface is ex- posed 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 con- trolled by deposition time. This silica seed layer provides the reactive sites for the silane coupling agent.15 Plasma fluorination In a plasma reactor, the zirconia surface is exposed to sulphur hexa- fluoride plasma. An oxyfluoride layer is formed on the surface. This layer may increase the reactivity of zirco- nia towards a silane coupling agent. However, the exact mechanism of the bonding formation between the zirco- nium oxyfluoride layer with silane is still unclear.15 Silane chemistry Functional and non-functional silanes Functional silanes contain two dif- ferent functional groups that can react with inorganic matrices, 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 cal- led the non-functional silanes. They contain one reactive functional group that can react with inorganic mate- rials. They are widely used for some specific surface modification of ma- terials. In addition, there are bis-fun- ctional/cross-linking/dipodal silanes that possess two silicon atoms with three hydrolysable alkoxy groups. Cross-linking silanes are used in the steel and tyre industries.18 Such silane is also incorporated with functional silane to increase the bonding and hydrolytic stability of resin composite to titanium.19 Silane activation mechanism Silanes can create a bond betwe- en inorganic and organic materials. A general formula for a functional 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 Fig. 1: The silane hydrolysis mechanism.—Fig. 2: The steric effect of alkoxy groups on silane hydrolysis using ball-and-stick models between butoxysilane and methoxysilane. Trends & Applications