Effect of silane incorporated adhesive or adhesive using separate silane on bonding of silica-based ceramics

Introduction

3-Methacryloxyproyltrimethoxysilane (MPS) is the commonly used clinical commercial silane primers (1). There are two types of silane, one is a pre-hydrolyzed one-bottle system and the other one is a two-bottle system. Pre-hydrolyzed silane results in a self-condensation reaction over time, and thus cannot be used for the clinical purpose. This also results in a shorter shelf life compared to the two-bottled one. The two-bottle system contains un-hydrolyzed silane in ethanol in one bottle and an aqueous acetic acid solution in the other. Keeping them in separate bottles increases the shelf life of silane (2). The combined use of silane with a resin adhesive improves the bond strength of the composite to the glass ceramic (3).

Condensation reaction, referred to as dehydration synthesis, is a chemical reaction in which two molecules combine to form a larger molecule with the loss of a small molecule. The most common small molecule is water. The condensation reaction of silane also produces a by-product, which is water. The larger molecule formed as a result of this condensation reaction loses the water molecule and has a greater density than the reactants (4). The condensation reaction between two hydroxyl groups results in a covalent bond (chemical bond), where the silane molecule firmly attached to the porcelain surface. A by-product is the loss of a water molecule (5).

Le Chatelier’s principle, also called “The Equilibrium Law”, stated that “When a system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to counteract the effect of the applied change and a new equilibrium is established” (6). The condensation reaction of silane generates water, and this water changes the equilibrium in the system. The water must be eliminated in order to promote condensation reaction. This is the reason why warm air-drying is recommended after you apply silane (7).

The chemical reaction between silane and a silica-based porcelain surface produces water as a by-product. This chemical reaction is called condensation reaction, and in order for this reaction to be continuous, the resulting by-product of water must be continuously removed. Equilibrium, according to “Le Chatelier’s” principle, can be achieved by water removal for further chemical reaction. The dental adhesive has to contain a hydrophobic resin monomer in order to increase bond strength and prevent any hydrolytic degradation. If a hydrophobic resin monomer and silane are combined into a one-bottle adhesive system, then the hydrophobic resin monomer disturbs the process of by-product elimination, which is water evaporation. The trend these days is to use a hydrophobic resin with higher hydrophobic properties in order to increase the longevity of the bonding layer. Recently marketed Single Bond Universal adhesive from 3M ESPE (3M ESPE, St. Paul, MN, USA) is one adhesive system which contains this type of hydrophobic monomer, MDP.

There are several adhesives which include silane in the same bottle such as Single Bond Universal mentioned above. It is convenient to use the same bottle for the adhesive as well as to treat the surface of the silica-based ceramic. However, it is possible that the effectiveness of the resin and silane may be reduced because of the change in properties when they are combined in one bottle.

Therefore, the null hypothesis of this study was that there would be no significant difference between a) the adhesive is applied after silane group and 2) silane and adhesive combined in one bottle group on silica-based porcelain in terms of surface hydrophobicity and shear bond strength.

Materials and Methods

1. Material

A silane monomer, MPS, which is the active ingredient for most dental silane primers, was added into a universal adhesive (All-Bond Universal, ABU, Bisco, Schaumburg, IL, USA), resulting in an experimental silane-containing adhesive (Sil-ABU) which contains 4 wt% of the silane monomer.

The materials used in this study are as follows: ABU only, silane mixed with ABU (Sil-ABU), porcelain primer (which is a regular silane primer) only, and Scotch Bond Universal (SBU, 3M ESPE, St. Paul, MN, USA) which is a commercial silane-containing universal adhesive. IPS e-max press (Ivoclar Vivadent, Schaan, Liechtenstein), a lithium disilicate, was used as the silica-based ceramic. In terms of shade matching, IPS e-max press has more aesthetic characteristics compared to any other CAD-CAM ceramic block. Duolink resin cement (Bisco, Schaumburg, IL, USA) was used as a composite applied to the surface to test for shear bond strength (Table 1).

Table 1.

Materials used in this study

2. Contact angle

For the contact angle test, lithium disilicate, IPS e.max, disc was etched with hydrofluoric acid (Bisco Porcelain Etchant, 4% HF, Lot # 1200010466, Bisco, Schaumburg, IL, USA) for 25 sec., rinsed with water and dried (Group 1). The etched lithium disilicate disc was then treated with ABU (Group 2), ABU and Silane (Group 3), SBU (Group 4) or porcelain primer (Group 5) (Table 2). All samples were then left undisturbed for 5 min and cleaned by ultrasonication for 2 min in ethanol. Samples were then dried and contact angles were measured with contact angle meter (NRL-CA Goniometer, Rame-Hart Inc, Randolph, NJ, USA).

Table 2.

Control and experimental group for measuring contact angle

3. Shear bond strength

Lithium disilicate (IPS e.max, Ivoclar Vivadent) was wet-polished with 320-grit SiC sandpaper, rinsed with water and dried. The dual cured resin cement Duolink (Bisco; Lot #1300004782), was applied on polished lithium disilicate disc followed by light curing with 40 sec/500 mW/cm² (Group 1). The polished lithium disilicate was treated with either ABU (Group 2), ABU-Sil (Group 3) or SBU (Group 4) (Table 3). Samples were then left undisturbed (1 min), and light light-cured for 10 sec. Polished disc that were left undisturbed for 1 min after treated with porcelain primer was applied with ABU separately, and light cured for 10 sec (Group 5) (Table 3). Duolink was used to fabricate a composite post (bonding area = 4.5 mm²). Duolink was light cured for 40 sec using light curing unit at energy density of 500 mW/cm² from the top. The polymerized specimens were stored in de-ionized water for 24 hr at 37 ℃, then tested until failure using universal testing machine (Instron 4466) at a cross head speed of 1 mm/min.

Table 3.

Control and experimental group for shear bond strength test

Results

1. Contact angle

The results of the contact angle test showed that the experimental groups (Groups 2~4) and the control group (Group 1) were not statistically different (p>0.05), except for Group 5. Group 5, the group that used silane only with porcelain primer, showed the highest contact angle compared to any other groups (p<0.05) (Figure 1).

Figure 1.

Contact angle on primed etched lithium disilicate. Horizontal bar means that there are no significantly different (p>0.05).

2. Shear bond strength

In the shear bond strength test, all experimental groups (Group 2~5) were statistically different from the control group (Group 1) (p<0.05) (Figure 2). However, there was a significant difference between Group 5 and the other experimental groups (Groups 2~4) and the control group (Group 1) (p<0.05).

Figure 2.

Shear bond strength on polished lithium disilicate. Horizontal bar means that there are no significantly different (p>0.05).

Discussion

Silica-based ceramics, such as feldspathic porcelain and glass ceramics, are frequently used to veneer metal frameworks or high-strength ceramic coping for all-ceramic restorations. Availability of improved dental ceramic materials such as lithium disilicate, alumina, and zirconia as core materials has to led to a widespread use of all-ceramic restorations over the past decade (8).

Due to the development of new restorative material in today’s esthetic dentistry, there emerges a clinical demand for the development of multi-purpose primers or adhesives that can deliver strong and durable adhesion to surfaces such as dental hard tissues, dental ceramics, dental precious metal alloys and dental non-precious metals (9).

Universal bonding adhesives are the most recent bonding agent in dental area. Universal bonding adhesive refers to a dental adhesive that can be used for both total etch and self etch techniques. These universal bonding adhesives are not only technically universal, but also functionally universal. They can be used as a multi-purpose surface treatment for silica-based ceramic, zirconia and dentin. Before this generation of one-bottle adhesives, MDP was and still is used separately to chemically treat the surface of zirconia (10) and silane was and still is commonly used for treatment of silica-based ceramic surfaces. Now, the so-called universal bonding adhesives incorporate these ingredients into one bottle to treat different surfaces as well as to act as an adhesive.

Even though mechanical strength is an important factor that controls the clinical success of dental restoration (11), the surfaces of the specimens were polished in order to eliminate any bias from micro mechanical retention due to an uneven surface. Two major factors of porcelain bonding strength are how to treat the porcelain surface both physically and chemically. Hydrofluoric acid etching helps to expand the surface (12) and enhances bonding strength(13). Silane primer increases chemical bonding strength through condensation reaction with the ceramic surface. Thus, in this study, all of the surfaces of the specimens were polished by sand paper to evaluate only the chemical function of silane.

Adhesion requires intimate contact of the materials to be joined. Surface has to be more hydrophobic for better adhesion, such as universal adhesive which hydrophobic resin as a ingredient (14). Most manufacturers use Bis GMA or MDP as the hydrophobic monomer in their universal adhesives. Some of these adhesives combine both silane and a hydrophobic functional monomer, such as BisGMA or MDP.

However, this hydrophobic resin interferes with the condensation reaction of the pre-hydrolyzed silane inside the one-bottle universal adhesives (15). This resin interferes with the evaporation process of the water by-product of the condensation reaction and prevents equilibrium for further chemical reaction. There is a study that shows that warm air drying significantly increases the coupling potential of silane-based primers used as intermediate agents (7).

The result of this study shows that combining silane with ABU or SBU produces a relatively low contact angle. This means the surface is still hydrophilic and resin-unfriendly after applying the material. Silane does not work on a porcelain surface if it is mixed with a hydrophobic resin monomer. Applying silane to the porcelain surface generates water as a by-product. Water must be removed in order to accelerate the chemical reaction, but due to the hydrophobic resin inside the one bottle adhesive, water will be captured inside the resin matrix, and the chemical reaction cannot continue.

In terms of shear bond strengths, silane mixed with a hydrophobic resin monomer (Bis-GMA or MDP) showed relatively lower data than applying only silane. Silane in a hydrophobic resin monomer cannot effectively react with the porcelain surface, and results in a relatively low contact angle and poor bond strength.

Combining silane with the hydrophobic resin monomer hinders the silane from properly treating the porcelain surface. This is proven by the results of the contact angle tests and shear bond strength tests comparing one-bottle adhesives that include silane to using silane separately from the adhesive.

Conclusions

Silane should be used separately from a hydrophobic resin monomer in order to maximize its porcelain surface treatment function. Using silane separately and using silane incorporated into the universal adhesive (contains hydrophobic resin) do not perform equally on silica-based ceramic.

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