Abstract
Objectives
To give an overview of aspects of silane coupling agents and surface conditioning in dentistry.
Methods
Currently, silane coupling agents are used as adhesion promoters. Silanes are effective in enhancing adhesion between resin composite and silica-based ceramics. They do not bond effectively to non-silica based dental restorative materials. Surface conditioning of non-silica based ceramics with silica coating improves the bonding. This current overview will focus on the silane coupling agents: their properties, limitations in adhesion promotion and the clinical problems with the use of silanes. It will also focus on the current surface conditioning methods as well as new surface conditioning techniques to enhance the bonding through conventional silanization approaches.
Results
Several surface conditioning methods are being used clinically to enhance the adhesion of resin composites to non-silica based restorative materials. Other approaches are under investigation. The clinical problem of using silanes in adhesion promotion is the bond degradation over time in oral environment.
Significance
The current silane coupling agents are not ideal. The current silane coupling agents can fulfill the minimum requirements in clinical practice to enhance the bonding of resin composite to dental restorative materials. Developments of novel surface conditioning methods and silane coupling agents are required to address the bond durability problem.
1
Introduction
Numerous applications in industry, medicine and dentistry rely heavily on connecting dissimilar inorganic and organic materials to achieve specific technical purposes. However, owing to difference in nature of chemical bonding in these materials, the interaction is very weak at the interfacial layer that there are no significant practical values in real situation. This problem can be resolved with the introduction of coupling agents.
In dentistry, bonding of resin composite to some dental restorative materials can be enhanced through the application of silane coupling agents. Silanes are very effective in promoting adhesion for silica-based materials such as porcelain. However, for non-silica based restorative materials such as zirconia, metals or metal alloys, the adhesion performance using silanes only is not satisfactory. Approaches to solve this problem focus on the surface conditioning. A widely used current method is tribochemical silica-coating. A silica-coated layer is anchored to the substrate surface such that silane coupling agents can form durable bond with non-silica based materials through this layer. It also enhances micro-mechanical retention due to the increased surface roughness.
Silanes are mainly used as adhesion promotions in ceramic restorations and their repairs with resin composites , glass fiber reinforced polymer composites , glassy fillers in resin composite and to form durable bond between resin composite to silica-coated metal and metal alloys . Perhaps, surprisingly, silanes do not have intrinsic toxicity
This overview will focus on silane bonding mechanism, adhesion promotion of resin composites to dental restorative materials, the weaknesses and limitations in promoting bonding. Contemporary surface conditioning methods as well as new surface conditioning techniques that are under development to enhance the chemical bonding (and micro-mechanical retention) will also be discussed.
2
Pre-activated silanes in dentistry
3-Methacryloxyproyltrimethoxysilane (MPS) is the commonly used in clinical commercial silane primers ( Table 1 ). They are applied as pre-hydrolysed in a solvent mixture consisting of ethanol and water. The silane content is usually about 1–5 vol%. However, one bottle pre-hydrolyzed silane solutions have relatively short shelf life. Over time, the solution may appear cloudy or turn milky after opening and then it cannot be used. Subsequently, the two-bottle silane systems were introduced into dentistry. These systems consist of an unhydrolyzed silane in ethanol in one bottle and an aqueous acetic acid solution in the other . The two solutions are mixed to allow hydrolysis of the silane at a low pH before use. These systems increase the shelf life of silanes in comparison to the one-bottle systems. An example is the Silicoup A and B from Heraeus Kulzer ( Table 1 ).
| Name | Manufacturer | Effective silane (%) | pH | Solution and concentration (%) | Indication | Date of information |
|---|---|---|---|---|---|---|
| Bisco Porcelain Primer | Bisco, Schaumburg, IL, USA | ‘A silane’, >1 | 5.9 | ‘Alcohol > 45, Acetone > 45’ | Porcelain, composite | April 2010 |
| Bisco Bis Silane | Bisco, Schaumburg, IL, USA | ‘A silane’, 1–10 | 4 | ‘Alcohol 30–95’ | Porcelain, composite | November 2007 |
| Cimara Silane Coupling agent | VOCO, Cuxhaven, Germany | ‘Silane’, N/A | 5.5 | 2-Propanol 50–100 | Repair of ceramics, metals | January 2011 |
| Clearfil Ceramic Primer | Kuraray, Osaka, Japan | MPS, <5, MDP N/A | 3 | Ethanol > 80 | Porcelain, ceramics, resin-based materials | October 2008 |
| Clearfil Porcelain Bond Activator | Kuraray, Osaka, Japan | MPS 40–60 | 2.3 | Hydrophobic aromatic dimethacrylate | Porcelain | October 2008 |
| ESPE Sil | ESPE Dental, Seefeld, Germany | ‘MPS’, <3 | 4.5 | Ethanol, > 97 Methyl ethyl ketone < 2 |
Metals, ceramics, composites | September 2010 |
| ESPE RelyX Ceramic Primer | 3M ESPE, St. Paul, MN, USA | ‘MPS’, <2 | 4.6 | Ethanol, 70–80 Water, 20–30 |
Ceramics, Porcelain, metals | February 2010 |
| Monobond-S | Ivoclar Vivadent, Schaan, Liechtenstein | MPS, <2.5 | 4 | Ethanol, 50–100 | Porcelain, composite | January 2011 |
| Pulpdent Silane Bond Enhancer | Pulpdent, Watertown, MN, USA | A silane, N/A | 6.3 | Ethanol, 92 Acetone, 7 |
Porcelain, composites | January 2008 |
| Silicoup A and B (a two bottle system) | Heraeus Kulzer, Hanau, Germany | N/A, MPS (Silicoup B) | N/A | Ethanol, 25–50 Ethylacetate, 25–50 Acetic acid, 5–10 |
N/A | Augest 2010 |
| Ultradent Silane | Ultradent Products, South Jordan, UT, USA | MPS, 5–15 | 5.3 | 2-Propanol, 92 | Porcelain, resin coupling agent | January 2006 |
| Vectris Wetting Agent | Ivoclar Vivadent, Schaan, Liechtenstein | MPS, 1 | N/A | Ethanol, <52 | Crowns, bridges | July 2006 |
| VITA Zahnfabrik | VITA SIL, Bad Säckingen, Germany | MPS, <2.5 | N/A | Ethanol, 25–50 | Ceramic and resin composite | April 2008 |
2
Pre-activated silanes in dentistry
3-Methacryloxyproyltrimethoxysilane (MPS) is the commonly used in clinical commercial silane primers ( Table 1 ). They are applied as pre-hydrolysed in a solvent mixture consisting of ethanol and water. The silane content is usually about 1–5 vol%. However, one bottle pre-hydrolyzed silane solutions have relatively short shelf life. Over time, the solution may appear cloudy or turn milky after opening and then it cannot be used. Subsequently, the two-bottle silane systems were introduced into dentistry. These systems consist of an unhydrolyzed silane in ethanol in one bottle and an aqueous acetic acid solution in the other . The two solutions are mixed to allow hydrolysis of the silane at a low pH before use. These systems increase the shelf life of silanes in comparison to the one-bottle systems. An example is the Silicoup A and B from Heraeus Kulzer ( Table 1 ).
| Name | Manufacturer | Effective silane (%) | pH | Solution and concentration (%) | Indication | Date of information |
|---|---|---|---|---|---|---|
| Bisco Porcelain Primer | Bisco, Schaumburg, IL, USA | ‘A silane’, >1 | 5.9 | ‘Alcohol > 45, Acetone > 45’ | Porcelain, composite | April 2010 |
| Bisco Bis Silane | Bisco, Schaumburg, IL, USA | ‘A silane’, 1–10 | 4 | ‘Alcohol 30–95’ | Porcelain, composite | November 2007 |
| Cimara Silane Coupling agent | VOCO, Cuxhaven, Germany | ‘Silane’, N/A | 5.5 | 2-Propanol 50–100 | Repair of ceramics, metals | January 2011 |
| Clearfil Ceramic Primer | Kuraray, Osaka, Japan | MPS, <5, MDP N/A | 3 | Ethanol > 80 | Porcelain, ceramics, resin-based materials | October 2008 |
| Clearfil Porcelain Bond Activator | Kuraray, Osaka, Japan | MPS 40–60 | 2.3 | Hydrophobic aromatic dimethacrylate | Porcelain | October 2008 |
| ESPE Sil | ESPE Dental, Seefeld, Germany | ‘MPS’, <3 | 4.5 | Ethanol, > 97 Methyl ethyl ketone < 2 |
Metals, ceramics, composites | September 2010 |
| ESPE RelyX Ceramic Primer | 3M ESPE, St. Paul, MN, USA | ‘MPS’, <2 | 4.6 | Ethanol, 70–80 Water, 20–30 |
Ceramics, Porcelain, metals | February 2010 |
| Monobond-S | Ivoclar Vivadent, Schaan, Liechtenstein | MPS, <2.5 | 4 | Ethanol, 50–100 | Porcelain, composite | January 2011 |
| Pulpdent Silane Bond Enhancer | Pulpdent, Watertown, MN, USA | A silane, N/A | 6.3 | Ethanol, 92 Acetone, 7 |
Porcelain, composites | January 2008 |
| Silicoup A and B (a two bottle system) | Heraeus Kulzer, Hanau, Germany | N/A, MPS (Silicoup B) | N/A | Ethanol, 25–50 Ethylacetate, 25–50 Acetic acid, 5–10 |
N/A | Augest 2010 |
| Ultradent Silane | Ultradent Products, South Jordan, UT, USA | MPS, 5–15 | 5.3 | 2-Propanol, 92 | Porcelain, resin coupling agent | January 2006 |
| Vectris Wetting Agent | Ivoclar Vivadent, Schaan, Liechtenstein | MPS, 1 | N/A | Ethanol, <52 | Crowns, bridges | July 2006 |
| VITA Zahnfabrik | VITA SIL, Bad Säckingen, Germany | MPS, <2.5 | N/A | Ethanol, 25–50 | Ceramic and resin composite | April 2008 |
3
Surface conditioning
Surface conditioning of dental materials is a treatment of a surface that increases the surface roughness, i.e. , the surface energy. Surface treatments also create micropores for infiltration of silanes and resin cements . Increase in the surface energy results in better wetting for bonding. We will discuss in the following sections some of the most important surface conditioning methods for zirconia, metals, base and noble metal alloys and porcelain (including repair of fractured porcelain), used in dentistry.
3.1
Grit blasting
In dental laboratories, the typical procedure is blasting the surfaces with alumina particles of an average size of, e.g. 50 μm under an air pressure of 380 kPa for around 10–15 s at a perpendicular short distance ( ca. 10 mm) from the nozzle to the surface . However, some alumina particles may be embedded into the surfaces during grit blasting. Given this, a layer of alumina-coating formed onto the substrate surfaces after such a grit-blasting. The amount of alumina increased with increasing blasting pressure . After silanization, Al O Si bonds may be formed. However, they are hydrolytically unstable .
3.2
Pyrochemical silica-coating
This is the process of applying pyrochemically and thermally silica coating onto substrate surface to form durable covalent Si O Si bonds . The thermal silica-coating systems that were previously used in dental laboratories were Silicoater Classical, Silicoater ® MD and Siloc ® (Heraeus-Kulzer, Wehrheim, Germany). The substrates that were silica-coated by this method included all base metal alloys, noble metal alloys and dental porcelain .
The Silicoater ® system (Heraeus-Kulzer, Wehrheim, Germany) was composed of a carousel where the sandblasted substrate passes through a flame. A silane solution, consisting of tetraethoxysilane, Si(OC 2 H 5 ) 4 , was injected into the flame and a series of pyrochemical reactions took place. The main chemical net reaction is :
Si ( OCH 2 CH 3 ) 4 ( g ) flame SiO 2 ( s ) + x CO 2 ( g ) + y H 2 O ( g )
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