Dental Materials Journal 2011; 30(4): 493–500

INTRODUCTION

The field of adhesive dentistry has made a significant progress over the past decade. A large part of this success is attributed to the major advances in a bonding technology1). Notably, the major advances are ascribed to an advanced adhesion-promoting monomer (adhesive monomer). It was developed to enhance the bond strength for a multitude of adherend materials that coexist in the oral environment, ranging from dental hard tissues (e.g., enamel and dentin) to dental precious metal alloys (e.g., gold and gold alloys), dental non-precious metals, and dental ceramics [e.g., silica-based ceramics (dental porcelain), zirconium oxide (zirconia)-based ceramics, and aluminum oxide (alumina)-based ceramics]2). The current trend in the development of dental adhesive systems seeks to simplify bonding steps for convenience, with a constant quest to optimize both speed and efficiency. Therefore, there emerges a clinical demand for the development of single-bottle, multi-purpose primers or adhesives which are able to provide strong and durable adhesion indiscriminately to a variety of adherend materials.

The typical structure of an adhesive monomer consists of three parts: a polymerizable functional group, a connecting group, and an adhesion-promoting group3). Acidic adhesive monomers and sulfur-containing

monomers are classified as two different categories of adhesive monomers: the former contains acidic groups such as carboxylic acid, phosphoric acid or phosphonic acid, while the latter contains a sulfur atom in their structures. For acidic adhesive monomers, they play a key role in the action of recently developed self-etching primers and adhesives in that they can interact chemically as ligand monomers not only with hydroxyapatite in dental hard tissues4), but also with metal oxides on the surfaces of non-precious metals5) and with alumina-based and zirconia-based ceramics6). For sulfur-containing monomers, they are able to chemically interact with metal atoms on the surfaces of precious metals7-9). Chemical adsorption of organic sulfur compounds on precious metal surfaces, followed by a spontaneous assembly of organic thiol-like molecules, eventually results in the formation of monolayer films on precious metal surfaces10,11). Silane coupling agents possess alkyloxysilane group as an adhesion-promoting group in their structures. For this reason, silane coupling agents are considered as a category of adhesive monomers for silica-based ceramics because of strong adhesion achieved via a silane coupling reaction.

Multi-purpose primers, which contained silane coupling agents, acidic adhesive monomers, and sulfur-containing monomers for bonding to porcelain, zirconia, dental precious and non-precious metal alloys, were

Design of a new, multi-purpose, light-curing adhesive comprising a silane coupling agent, acidic adhesive monomers and dithiooctanoate monomers for bonding to varied metal and dental ceramic materialsKunio IKEMURA1, Hisaki TANAKA1, Toshihide FUJII1, Mikito DEGUCHI1, Noriyuki NEGORO1, Takeshi ENDO2 and Yoshinori KADOMA3

1Department of Research and Development, Shofu Inc., 11 Kamitakamatsu-cho, Fukuine, Higashiyama-ku, Kyoto 605-0983, Japan2Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan3Department of Applied Functional Molecules, Division of Biofunctional Molecules, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, JapanCorresponding author, Kunio IKEMURA; E-mail: k-ikemura@shofu.co.jp

A newly designed, light-curing adhesive was investigated for its bonding effectiveness to porcelain, alumina, zirconia, Au, Au alloy, Ag alloy, Au-Ag-Pd alloy, and Ni-Cr alloy. Four experimental adhesives were prepared using varying contents of the following: a silane coupling agent [3-methacryloyloxypropyltriethoxysilane (3-MPTES)], acidic adhesive monomers [6-methacryloyloxyhexyl phosphonoacetate (6-MHPA), 6-methacryloyloxyhexyl 3-phosphonopropionate (6-MHPP) and 4-methacryloyloxyethoxycarbonylphthalic acid (4-MET)], and dithiooctanoate monomers [6-methacryloyloxyhexyl 6,8-dithiooctanoate (6-MHDT) and 10-methacryloyloxydecyl 6,8-dithiooctanoate (10-MDDT)]. After all adherend surfaces were sandblasted and applied with an experimental adhesive, shear bond strengths (SBSs) of a light-curing resin composite (Beautifil II, Shofu Inc., Kyoto, Japan) to the adherend materials after 2,000 times of thermal cycling were measured. For the experimental adhesive which contained 3-MPTES (30.0 wt%), 6-MHPA (1.0 wt%), 6-MHPP (1.0 wt%), 4-MET (1.0 wt%), 6-MHDT (0.5 wt%) and 10-MDDT (0.5 wt%), it consistently yielded the highest SBS for all adherend surfaces in the range of 20.8 (4.8)–30.3 (7.9) MPa, with no significant differences among all the adherend materials (p>0.05). Therefore, the newly designed, multi-purpose, light-curing adhesive was able to deliver high SBS to all the adherend materials tested.

Keywords: Multi-purpose light-curing adhesive, Silane coupling agent, Dithiooctanoate monomer, Dental ceramic, Precious metal alloy

Received Jan 20, 2011: Accepted Mar 31, 2011doi:10.4012/dmj.2011-012 JOI JST.JSTAGE/dmj/2011-012

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developed recently12-14). However, multi-purpose, light-curing adhesives or bonding resins which contain the same ingredients listed above are rarely reported —let alone single-bottle adhesives. In our previous studies, acidic adhesive monomers bearing carboxylic acid or its anhydride group15,16) and phosphonic acid group5,17) were developed. Besides, eight kinds of acryloyloxyalkyl and methacryloyloxyalkyl 6,8-dithiooctanoates (dithiooctanoate monomers) as novel sulfur-containing monomers were also developed18).

The focus of our new research strategy is a single-bottle, multi-purpose, light-curing adhesive which bonds light-curing resin composites to dental metal alloys for fixed prosthodontics repair. To up the ante, this new, multi-purpose adhesive should also display excellent handling properties and which enables reductions in both operation time and technical errors.

In the present study, we designed a multi-purpose, light-curing adhesive which contained a silane coupling agent, acidic adhesive monomers, and dithiooctanoate monomers. The aim of the present study was to investigate the effect of this newly designed, multi-purpose adhesive on the bonding of a light-curing resin composite to varied metal and dental ceramic materials: porcelain, alumina, zirconia, pure gold, Au alloy, Ag alloy, Au-Ag-Pd alloy, and Ni-Cr alloy. The null hypothesis was that this ternary combination of functional monomers (silane coupling agent, acidic adhesive monomers, and dithiooctanoate monomers) would not result in high bond strength between the resin composite and the all adherend materials tested after 2,000 thermal cycles.

MATERIALS AND METHODS

Preparation of reagentsFigure 1 depicts the chemical structures of the dithiooctanoate monomers and phosphonic acid monomers employed in this study. For the dithiooctanoate monomers —namely 6- methacryloyloxyhexyl 6,8-dithiooctanoate (6-MHDT) and 10-methacryloyloxydecyl 6,8-dithiooctanoate (10-MDDT), they were synthesized via an esterification reaction between 6,8-dithiooctanoic acid and 6-hydroxyhexyl methacrylate (6-HHMA) or 10-hydroxydecyl methacrylate (10-HDMA) respectively

according to the method described in our previous study18). For the phosphonic acid monomers —namely, 6-methacryloyloxyhexyl phosphonoacetate (6-MHPA)17) and 6-methacryloyloxyhexyl 3-phosphonopropionate (6-MHPP)5), they were synthesized via an esterification reaction between 6-HHMA and phosphonoacetic acid or 3-phosphonopropionic acid respectively according to the method described in our previous studies5,17). As for 4-methacryloyloxyethoxycarbonylphthalic acid (4-MET) in the form of white needle crystals, it was synthesized according to the method described in our previous study16).

2,2-Bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane (=bisphenol A diglycidyl methacrylate) (Bis-GMA) was synthesized by adding bisphenol A to glycidyl methacrylate in 1:2 molar ratio as previously reported19). 1,6-Bis (2-methacryloyloxyethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA) was synthesized by adding 2,4,4-trimethylhexane-1,6-diisocyanate to 2-hydroxyethyl methacrylate in 1:2 molar ratio as previously described15). Triethylene glycol dimethacrylate (TEGDMA; Mitsubishi Rayon Co. Ltd., Tokyo, Japan), D,L-camphorquinone (CQ; Tokyo Chemical Industry Co., Ltd., Tokyo, Japan), ethyl p-dimethylaminobenzoate (EDAB; Wako Pure Chemical Industries, Ltd., Osaka, Japan), 3-methacryloyloxypropyltriethoxysilane (3-MPTES; Shin-Etsu Chemical Co., Ltd., Tokyo, Japan), and hydrophobic fumed silica fillers (Aerosil® R 972, Nippon Aerosil Co., Ltd., Tokyo, Japan) were purchased and used without further purification.

Preparation of experimental light-curing adhesivesTable 1 presents the compositions of four experimental, single-bottle, light-curing adhesives (code names: Si-AA-SS-1, Si-AA-SS-2, AA, and SS) to be investigated in this study. According to the percent compositions listed in Table 1, silane coupling agent, acidic adhesive monomers, dithiooctanoate monomers, polymerizable monomers, photoinitiator system, filler, and solvent were added and uniformly mixed to produce the respective experimental adhesives. All prepared, single-bottle adhesives (5.0 g) were placed in black plastic containers. The code name of “Si-AA-SS” indicated a combination of a silane coupling agent (Si), acidic adhesive monomers (AA), and cyclic disulfides (SS) as

Fig. 1 Schematic illustration of the chemical structures of dithiooctanoate monomers (6-MHDT18) and 10-MDDT18)) and phosphonic acid monomers (6-MHPA17) and 6-MHPP5)) employed in this study.

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dithiooctanoate monomers.

Preparation of adherendsEight kinds of adherend materials were examined in this study. Their composition and manufacturer details are given in Table 2. Three ceramic adherends used were namely porcelain (Vintage Halo, Shofu Inc., Kyoto, Japan), aluminum oxide (alumina) plate (Al2O3; Japan Fine Ceramics Co., Ltd., Sendai, Japan), and zirconium oxide (zirconia) plate (ZrO2 containing 3 mol% Y2O3; Japan Fine Ceramics Co., Ltd., Sendai, Japan). Five metal adherends used were pure gold (Au; Ishifuku Metal Industry Co., Ltd., Tokyo, Japan), three types of

precious metal alloys [Au alloy (Super Gold Type 4, Shofu Inc., Kyoto, Japan), Au-Ag-Pd alloy (Castwell M.C., GC Dental Products Corp., Tokyo, Japan), and Ag alloy (Sunsilver, Sankin Kogyo Co. Ltd., Tokyo, Japan)], and one non-precious Ni-Cr alloy (Dent Nickel, Shofu Inc., Kyoto, Japan). All dental alloys were cast using a casting machine (Argon Caster, Shofu Inc., Kyoto, Japan).

Shear bond strength measurementDisk-shaped rods (6.0±0.1 mm in diameter, 4.0±0.1 mm in height) of both ceramic and metal adherends were embedded in an epoxy resin. Using porcelain for firing

Ingredient (wt%) AbbreviationCode names of experimental adhesives

Si-AA-SS-1 Si-AA-SS-2 AA SS

Silane coupling agent 3-MPTES 30.0 30.0 – –

Acidic adhesive monomer 6-MHPA6-MHPP4-MET

2.0––

1.0 1.0 1.0

2.0––

–––

Dithiooctanoate monomer 10-MDDT6-MHDT

0.5–

0.5 0.5

––

0.5–

Polymerizable monomer Bis-GMAUDMATEGDMA

30.0–

15.0

15.010.010.0

15.010.010.0

15.010.010.0

Photoinitiator system CQEDAB

0.2 0.3

0.2 0.3

0.2 0.3

0.2 0.3

Filler R-972 3.0 3.0 3.0 3.0

Solvent EthanolAcetone

19.0–

–27.5

–59.5

–61.0

The code name of “Si-AA-SS” indicates a combination of a silane coupling agent (Si), acidic adhesive monomers (AA), and dithiooctanoate monomers (SS).

Table 1 Compositions (wt%) of experimental multi-purpose light-curing adhesives

Adherend Code Composition (mass%) Manufacturer

Vintage Halo Porcelain SiO2, etc. Shofu Inc.

Aluminum oxide Alumina Al2O3 Japan Fine Ceramics Co., Ltd.

Zirconium oxide Zirconia ZrO2 containing 3 mol% Y2O3 Japan Fine Ceramics Co., Ltd.

Pure gold Au Au (99.99) Ishifuku Metal Industry Co., Ltd.

Super Gold Type 4 Au alloy Au (70), Cu (13), Ag (10), Pt (1), Pd (4), others (2)

Shofu Inc.

Sunsilver Ag alloy Ag (79), Zn (7), In (7), Cu (5), others (2)

Sankin Kogyo Co., Ltd.

Castwell M. C. Au-Ag-Pd alloy Ag (45), Pd (20), Cu (18), Au (12), others (5)

GC Dental Products Corp.

Dent Nickel Ni-Cr alloy Ni (63.5), Cr (15), Nb (5), Mn (5), others (11.5)

Shofu Inc.

Table 2 Eight kinds of ceramic and metal adherend materials used in this study

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dental materials, the disk-shaped rods were made using a vacuum electric furnace for firing porcelain (Twin Mat, Shofu Inc., Kyoto, Japan). To prepare the bond surfaces of adherends, the exposed flat ends of the rods were polished —under running water— using #240 and #600 silicon carbide papers to obtain smooth and flat bonding surfaces. The polished flat surfaces were subjected to air abrasion treatment (50-µm alumina beads; Shofu Hi-alumina, Shofu Inc., Kyoto, Japan) under a pressure of 0.25 MPa, ultrasonic-cleaned, and then air-dried.

After drying, a double-sided adhesive tape with a 4.0-mm-diameter hole was fixed on each adherend surface to define the bonding area. An experimental light-curing adhesive was applied on the bonding area with a microbrush, air-blown, and then light-cured with a visible light apparatus (Grip Light II, Shofu Inc., Kyoto, Japan) for 20 seconds. A cylindrical Teflon mold (4.0 mm inner diameter, 2.0 mm height) was fixed on the bonding area, and a light-curing resin composite (Beautifil II, Shofu Inc., Kyoto, Japan) was packed into the mold and light-cured for 30 seconds. After the mold was removed, the resin-ceramic or resin-metal bonded specimen was stored in 37°C distilled water for 24 hours. After water storage, the specimens (n=6 in each group) were subjected to 2,000 times of thermal cycling whereby they were alternately immersed in 4 and 60°C water for one minute each. Shear bond strength (SBS) measurements were made using a universal testing machine (Model 5543, Instron Corp., Norwood, MA, USA) at a crosshead speed of 1.0 mm/min.

Failure mode analysisAfter SBS testing, the fracture surfaces of debonded specimens were examined using a stereomicroscope (Leica DM IL, Leica Microsystems Japan, Tokyo, Japan) at ×15 magnification. The failure modes were classified as follows: interfacial failure, cohesive failure of adhesive resin, or mixed failure. Interfacial failure, conventionally known as adhesive failure, was defined as the fracture

which occurred at the bonded interface between adhesive and adherend. Cohesive failure was defined as cohesive fracture of the adhesive resin on the debonded adherend surface. Additionally, cohesive failure also included the cohesive failure of adherend, such as fractured porcelain. Mixed failure was defined as the failure mode in which both interfacial and cohesive failures coexisted on the debonded adherend surface.

After examining all the debonded specimens, the failure modes of specimens in each group were identified as I/M/C (=Interfacial failure/Mixed failure/Cohesive failure) and numbered accordingly.

Statistical analysisStatistical analysis was performed for the SBS data of all the four experimental, multi-purpose, light-curing adhesives. One-way analysis of variance (ANOVA) was performed to determine the existence of significant differences in SBS among the different types of experimental adhesives for each adherend. Statistical significance was set in advance at 0.05 probability level. Multiple comparisons were performed using Student-Newman-Keuls test at =0.05.

RESULTS

Table 3 presents the effects of four experimental light-curing adhesives (coded Si-AA-SS-1, Si-AA-SS-2, AA, and SS) on the SBS of a light-curing resin composite (Beautifil II, Shofu Inc., Kyoto, Japan) to the ceramic (porcelain, alumina, and zirconia) and metal (Au, Au alloy, Ag alloy, Au-Ag-Pd alloy, and Ni-Cr alloy) adherends after 2,000 thermal cycles.

With Si-AA-SS-1 (which contained 30.0 wt% 3-MPTES, 2.0 wt% 6-MHPA, and 0.5 wt% 10-MDDT), there were no statistically significant differences in SBS (p>0.05) among all the adherend materials. However, higher SBS values were seen for the metal adherends than for the ceramics. For the three ceramic adherends,

AdherendShear bond strength [mean (standard deviation, SD), MPa]

Si-AA-SS-1 Si-AA-SS-2 AA SS

Porcelain 17.7 (4.2)a 26.4 (5.6)a 5.3 (1.1)a fell off

Alumina 16.8 (4.7)a 23.3 (4.9)a 16.3 (3.0)b fell off

Zirconia 17.5 (5.0)a 20.8 (4.8)a 13.5 (3.4)b fell off

Au 19.5 (5.2)a 30.3 (7.9)a fell off 6.7 (1.5)a

Au alloy 23.2 (5.3)a 28.4 (7.7)a fell off 6.3 (1.2)a

Ag alloy 27.3 (6.5)a 27.8 (6.6)a 5.5 (0.8)a 5.7 (1.8)a

Au-Ag-Pd alloy 24.3 (5.6)a 28.2 (6.9)a 4.3 (1.3)a 5.8 (2.0)a

Ni-Cr alloy 26.9 (6.8)a 29.7 (7.1)a 14.0 (3.8)b fell off

Note: n=6. One thermal cycle: 4°C water for 1 min and 60°C water for 1 min. Groups from the same column (adhesive) that are identified with the same superscript letter are not significantly different (p>0.05). Fell off: Composite specimen fell off from adherend during thermal cycling.

Table 3 Shear bond strengths of experimental multi-purpose, light-curing adhesives after 2,000 thermal cycles

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mean SBS ranged between 16.8 (4.7) MPa and 17.7 (4.2) MPa. For the five metal adherends, the range was 19.5 (5.2)–27.3 (6.5) MPa. With Si-AA-SS-2 (which contained 30.0 wt% 3-MPTES, 1.0 wt% 6-MHPA, 1.0 wt% 6-MHPP, 1.0 wt% 4-MET, 0.5 wt% 6-MHDT, and 0.5 wt% 10-MDDT), there were no statistically significant differences in SBS (p>0.05) among all the adherend materials. For the three ceramic adherends, mean SBS ranged between 20.8 (4.8) MPa and 26.4 (5.6) MPa. For the five metal adherends, the range was 27.8 (6.6)–30.3 (7.9) MPa.

With AA (which contained only 2.0 wt% of acidic adhesive monomer 6-MHPA), there were no statistically significant differences in SBS (p>0.05) among alumina, zirconia, and Ni-Cr alloy. In contrast, the mean SBS values of AA to porcelain, Au, Au alloy, Ag alloy, and Au-Ag-Pd alloy were significantly lower than those to alumina, zirconia, and Ni-Cr alloy (p<0.05). In particular, composite specimens fell off (debonded) from both Au and Au alloy adherends during thermal cycling. With SS (which contained only 0.5 wt% of dithiooctanoate monomer 10-MDDT), the composite specimens fell off/debonded from all the three ceramic and Ni-Cr alloy adherends during thermal cycling. For Au, Au alloy, Ag alloy, and Au-Ag-Pd alloy, there were no statistically significant differences in SBS which ranged between 5.7 (1.8) MPa and 6.7 (1.5) MPa.

Table 4 presents the failure mode distribution of fractured specimens after SBS testing. With Si-AA-SS-2, it was predominantly mixed failures and cohesive failures in adhesive, but no interfacial failures between adhesive and adherend. With AA and SS, the predominant failure mode was contrastingly that of interfacial failure. There were a few incidents of mixed failure but no cohesive failures, except between AA and alumina. With Si-AA-SS-1 and Si-AA-SS-2 —which contained silane coupling agent, acidic adhesive monomers, and dithiooctanoate monomers, they yielded

high SBS to all the adherend materials tested even after 2,000 thermal cycles. Predominant failure modes with these two adhesives were namely mixed failure and cohesive failure in adhesive. With AA, very low SBS values were exhibited for porcelain, Au, Au alloy, Ag alloy, and Au-Ag-Pd alloy. With SS, similarly low SBS values were exhibited for all the adherend materials.

DISCUSSION

In the present study, we tried to design a new, single-bottle, multi-purpose, light-curing adhesive which contained a silane coupling agent (3-MPTES), acidic adhesive monomers (6-MHPA, 6-MHPP and 4- MET)5,15-17), and dithiooctanoate monomers (6-MHDT and 10-MDDT)18). With silane coupling agents, the mechanism of their silane coupling reaction is that the trialkyloxysilyl group [-Si-(OR)3] of silane coupling agents, such as 3-MPTES, is hydrolyzed in acid solutions to form reactive silanols [-Si-(OH)3]. Partial condensation reaction follows and oligomers are formed, which are adsorbed on silica-based ceramic surfaces by hydrogen bonding. Dehydration condensation, which is applied using heat treatment, then causes covalent chemical bonds to be formed between silanes and silica-based ceramics20).

With acidic adhesive monomers, the hypothetical bonding mechanism to ceramics and non-precious metal alloys can be understood as follows. 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) and 6-MHPA, both phosphorus-containing monomers, chemically bond to metal oxides of ceramics and non-precious metal alloys5,21). It was thought that the phosphoric acid group [-O-P(=O)(OH)2] of MDP and phosphonic acid group [-P(=O)(OH)2] of 6-MHPA promote good interaction with metal oxides on the surfaces of both aluminum oxide-based (Al2O3) and zirconium oxide-based (ZrO2) ceramics as well as non-precious

Adherend

Failure mode of fractured surfaces after debonding

Si-AA-SS-1 Si-AA-SS-2 AA SS

I/M/C§ I/M/C§ I/M/C§ I/M/C§

Porcelain 2/4/0 0/3/3 6/0/0 6/0/0#

Alumina 2/3/1 0/3/3 3/2/1 6/0/0#

Zirconia 2/4/0 0/4/2 4/2/0 6/0/0#

Au 2/3/1 0/1/5 6/0/0# 6/0/0

Au alloy 1/4/1 0/1/5 6/0/0# 6/0/0

Ag alloy 0/4/2 0/1/5 6/0/0 6/0/0

Au-Ag-Pd alloy 0/4/2 0/0/6 6/0/0 6/0/0

Ni-Cr alloy 0/3/3 0/0/6 5/1/0 6/0/0#

Note: n=6. §Failure mode: Number of specimens of Interfacial failure/Mixed failure/Cohesive failure in adhesive resin. #: Composite specimens fell off from adherends during thermal cycling.

Table 4 Failure modes of fractured specimens after SBS testing

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metal alloys. Moreover, phosphonic acid monomers (6-MHPA and 6-MHPP) are water-soluble monomers, whereas MDP is a water-insoluble monomer. This means that phosphonic acid monomers can be partially or completely ionized with a small amount of water on the metal oxide surfaces of ceramics and non-precious metal alloys. The ionized phosphonic acid monomer selectively and immediately penetrates the metal oxide structures and chemically interacts with the metal cations in the metal oxide structures to form metal salts. Hydrogen bonds are also formed between undissociated phosphonic acid groups and metal oxides. With subsequent application of light irradiation, the phosphonic acid monomers of light-curing adhesive penetrate the metal oxide structures and photopolymerize in situ at the resin-adherend interface, such as the resin-alumina, resin-zirconia, and resin-Ni-Cr alloy bonding interfaces.

Sulfur-containing monomers are able to chemically interact with precious metal atoms7-9). It has been reported that the cyclic -S-S- group of dithiooctanoate monomers provided strong adhesion to Au, Au alloy, Ag alloy, and Au-Ag-Pd alloy18). In the present study, the experimental adhesive Si-AA-SS-2 which contained 3-MPTES, 6-MHDT, 10-MDDT, 6-MHPA, 6-MHPP, and 4-MET exhibited high SBSs to both precious and non-precious metals alike: 30.3 (7.9) MPa for Au, 28.4 (7.7) MPa for Au alloy, 27.8 (6.6) MPa for Ag alloy, 28.2 (6.9) MPa for Au-Ag-Pd alloy, and 29.7 (7.1) MPa for Ni-Cr alloy (Table 3).

Apart from precious and non-precious metals, Si-AA-SS-2 also exhibited strong adhesion to porcelain, alumina, and zirconia with no incidents of interfacial failure (Table 4). Contrary to the effective bonding performance of Si-AA-SS-2, AA and SS —which did not contain silane coupling agent but only either 6-MHPA or 10-MDDT— showed insufficient bond strengths not only to porcelain, but to the other adherend materials too (Table 3). Their weak bond strengths were evidenced by a high number of interfacial failures between adhesive and adherends (Table 4). The outstanding bonding performance of Si-AA-SS-2 to ceramic adherends was attributed to the ternary combination of a silane coupling agent, acidic adhesive monomers, and dithiooctanoate monomers in its formulation.

It was reported that a combined application of a hydrophobic phosphate ester monomer (MDP) with a silane coupling agent on the silica-coated yttrium-oxide-partially-stabilized zirconia (YPSZ) ceramic yielded stable shear bond strength, thus rendering this system as a promising method for ceramic restorations in clinical settings22). Apart from MDP monomer, it was reported in another study that a commercial metal primer (AZ Primer; Shofu Inc., Kyoto, Japan) which contained the phosphonic acid monomer, 6-MHPA, delivered strong bonding to alumina- and zirconia-based all-ceramic prostheses23). In the present study, the bond strength between Si-AA-SS-2 and zirconia was 20.8 (4.8) MPa. It should be pointed out that bond strength values are affected by the zirconia systems in use. In a recent

study24), the highest SBS values reported for four types of commercially utilized zirconia systems and veneering ceramics using an acrylic resin were in the range of 20.2 (5.1)–40.5 (8.4) MPa.

The bonding ability as a multi-purpose, light-curing adhesive should be validated by comparing with bonding data previously reported for the combined effect of sulfur-containing monomers and acidic adhesive monomers on adhesion to precious and non-precious metals8,25,26). In a study by Suzuki et al.7), the effect of adding MDP (0.2 wt%) to a primer which contained 6-(4-vinylbenzyl-n-propyl)amino-1,3,5-triazine-2,4-dithione (VBATDT) (0.5 wt%) on the tensile bond strength (TBS) of MMA-PMMA/TBBO resin to metals, after 2,000 thermal cycles, was investigated. It was reported that the TBSs to Au, Ag, and chromium (Cr) were 25.2 (3.6) MPa, 39.7 (2.6) MPa, and 34.2 (9.7) MPa respectively7). In a study by Okuya et al.27), the effects of three commercial metal primers (including Metal Link Primer) on bonding of an MMA-PMMA/TBBO resin to four pure metals and two dental alloys after 2,000 thermal cycles were investigated. Metal Link Primer (M.L. Primer; Shofu Inc., Kyoto, Japan), which was commercially launched in 2003, contained both 10-MDDT and 6-MHPA to promote strong adhesion of adhesive resins to both precious and non-precious metals and their alloys. It was reported that the highest SBS values achieved for pure Au and high-gold-content alloy were 33.5 MPa and 33.3 MPa with M.L. Primer27). Despite the differences in bond testing methodology, Si-AA-SS-2 exhibited comparable bonding ability in this study for Au (30.3 MPa) when pitted against the published bonding data (namely, 33.5 MPa27) and 25.2 MPa7)).

When sulfur-containing monomers are combined with acidic adhesive monomers, an intriguing issue is the adsorption behaviors of these monomers on the adherends and the impact thereof on bonding efficacy. In a study by Suzuki et al.8), surface-enhanced Raman scattering (SERS) spectroscopy and infrared reflection absorption (IRA) spectroscopy were used to investigate the adsorption behaviors of VBATDT and MDP on Au, Ag, Cu, and Cr surfaces. Results showed that VBATDT was chemisorbed mainly on Au, Ag, and Cu surfaces with respect to thickness, whereas MDP was adsorbed only on Cr. In a study by Koizumi et al.25), it was found that the combined application of phosphonic acid monomers (6-MHPA and 6-MHPP) and dithiooctanoate monomers (6-MHDT and 10-MDDT) did not interfere with the bonding of phosphonic acid monomers to base metals nor dithiooctanoate monomers to precious metals.

Applying the findings of previous studies8,25) to the present study, it could be suggested that 6-MHDT-precious metal and 10-MDDT-precious metal interactions were independent from 6-MHPA-base Ni-Cr alloy and 6-MHPP-base Ni-Cr alloy interactions. Moreover, the adhesion-promoting groups of sulfur-containing groups and acid groups indubitably contributed to improving the bond strengths to precious and non-precious metals respectively. As for the cooperation of an exceedingly large amount of silane coupling agent (30 wt% of

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3-MPTES) together with both acidic adhesive monomers and dithiooctanoate monomers on improving bonding to all the adherend materials tested, 3-MPTES is a surface-active agent, and therefore, it was probable that the exceedingly large amount of 3-MPTES in Si-AA-SS-1 and Si-AA-SS-2 promoted active mobility of both phosphonic acid monomers and dithiooctanoate monomers in the hydrophobic light-curing adhesives. As a result, both phosphonic acid monomers and dithiooctanoate monomers were able to chemically interact with all the ceramic and metal adherends — unlike the adhesives AA and SS, which contained an inactive solvent (acetone) for adhesion.

To design a new, single-bottle, multi-purpose, light-curing adhesive, the effects of a ternary combination of a silane coupling agent (3-MPTES), acidic adhesive monomers (6-MHPA, 6-MHPP, and 4-MET), and dithiooctanoate monomers (6-MHDT and 10-MDDT) on adhesion to varied dental ceramic and metal materials were investigated. Results of this study showed that the experimental adhesive Si-AA-SS-2 was able to deliver high SBSs to all the adherend materials tested. Based on these results, the null hypothesis was rejected, which stated that the ternary combination of the above-mentioned functional monomers would not result in high bond strength between resin composite and the all adherend materials tested after 2,000 thermal cycles. In this series of studies on multi-purpose adhesives, our next target is to design a multi-purpose, self-etching adhesive which contains dithiooctanoate monomers and acidic adhesive monomers. The bonding performances of this novel, multi-purpose, self-etching adhesive to dental hard tissues, dental ceramics, and dental precious alloys shall also be investigated and assessed.

CONCLUSIONS

Based on the findings in the present study, the following conclusions were drawn:

1. With the experimental adhesive Si-AA-SS-2, which contained 3-MPTES (30.0 wt%), 6-MHPA (1.0 wt%), 6-MHPP (1.0 wt%), 4-MET (1.0 wt%), 6-MHDT (0.5 wt%), and 10-MDDT (0.5 wt%), high shear bond strengths ranging between 20.8 (4.8) and 30.3 (7.9) MPa were exhibited for dental porcelain, alumina, zirconia, Au, Au alloy, Ag alloy, Au-Ag-Pd alloy, and Ni-Cr alloy adherends.

2. The high shear bond strengths of Si-AA-SS-2 to all the adherend materials tested after 2,000 thermal cycles was attributed to the ternary combination of a silane coupling agent (3-MPTES), acidic adhesive monomers (6-MHPA, 6-MHPP, and 4-MET), and dithiooctanoate monomers (6-MHDT and 10-MDDT) in its formulation.

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