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The effect of five silane coupling agents on the bond strength of a luting cement to a silica-coated titanium
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1173–1180
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The effect of five silane coupling agents on the bond strength of a luting cement to a silica-coated titanium Jukka P. Matinlinna ∗ , Lippo V.J. Lassila, Pekka K. Vallittu University of Turku, Institute of Dentistry, Department of Prosthetic Dentistry and Biomaterials Science, ¨ Lemminkaisenkatu 2, FI-20520 Turku, Finland
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The adhesive performance of five silane coupling agents in adhering resin
Received 2 November 2005
composite cement (3 M ESPE) to silica-coated titanium was evaluated. Titanium was tri-
Received in revised form
bochemically silica-coated by using the RocatecTM system.
2 March 2006
Methods. Two volume percent solutions of 3-acryloyloxypropyltrimethoxysilane (Toray
Accepted 22 June 2006
Dow Corning Silicone), N-[3-(trimethoxysilyl)propylethylenediamine] (Dow Corning), 3mercaptopropyltrimethoxysilane (Toray Dow Corning Silicone) and bis-[3-(triethoxysilyl) propyl]polysulfide (Dow Corning) were prepared in 95 vol.% acidified ethanol and allowed to
Keywords:
activate (hydrolyze). A pre-activated ca. 2 vol.% 3-methacryloyloxypropyltrimethoxysilane
Silica-coating
(ESPETM Sil) was used as a control. The silanes were applied onto silica-coated titanium
Titanium
slides. Chemical activation reactions of the silanes were monitored by Fourier transform
3-Methacryloyloxypropyltrimetho-
infrared spectrometry (Perkin-Elmer Spectrum One). RelyXTM ARC (3 M ESPE) resin com-
xysilane
posite cement stubs were applied and photo-polymerized onto silica-coated titanium. The
3-Acryloyloxypropyltrimethoxy-
specimens were thermo-cycled (6000 cycles, 5–55 ◦ C).
silane
Results. Statistical analysis (ANOVA) showed that the highest shear bond strength
3-Mercaptopropyltrimethoxy-
(n = 8 per group) value after thermocycling, 14.8 MPa (S.D. 3.8 MPa), was obtained
silane
with 2.0 vol.% 3-acryloyloxypropyltrimethoxysilane. Silanization and results with 3-
Bis-[3-(triethoxysilyl)propyl]poly-
methacryloyloxypropyltrimethoxysilane (control, ESPETM Sil) did not statistically differ from
sulfide
3-acryloyloxypropyltrimethoxysilane, 14.2 MPa (S.D. 5.8). The lowest shear bond strength
N-[3-(Trimethoxysilyl)propylethy-
was 7.5 (S.D. 1.9) MPa for N-[3-(trimethoxysilyl)propylethylenediamine] and 7.5 (S.D. 2.5) MPa
lenediamine]
for bis-[3-(triethoxysilyl)propyl]polysulfide. Both the type of silane (p < 0.001) and storage
Silanization
conditions affected significantly the shear bond strength values (p < 0.001). All silanes
Silane coupling agent
became activated according to the infrared spectroscopic analysis.
Luting cements
Significance. Silanization with 3-acryloyloxypropyltrimethoxysilane or 3-mercaptopropyltrimethoxysilane might offer an alternative for bonding a luting cement to silica-coated titanium. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In the oral cavity, adhesion of particulate or fiber fillers and prosthetic materials of multiphase constructions is always subject to humidity, temperature and thermo-mechanical
∗
stresses. Adhesion promotion between dissimilar materials is successfully achieved with silane coupling agents, not only in dentistry [1] but also in numerous technical applications [2]. Silane coupling agents are widely accepted as adhesion promoters and applied in practical clinical work
Corresponding author. Tel.: +358 2 333 8357; fax: +358 2 333 8390. E-mail address: jumatin@utu.fi (J.P. Matinlinna). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.052
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for silica-coated surfaces [3,4]. Those silanes employed as coupling agents are usually chemically organofunctional trialkoxysilane esters, with three hydrolyzable alkoxy groups directly on the silicon atom, i.e., they have direct –Si–C– bonds. To-date, the only silane actually used in dentistry is 3-methacryloyloxypropyltrimethoxysilane [1,5]. Typical dental resin composites are structured on a reaction product of bis-phenol A and glycidylmethacrylate [6]. Bis-GMA can undergo polymerization when exposed to ultraviolet (or blue) light or also when mixed with a catalyst, and bis-GMA is used as the resin component of dental sealants and restorative materials [7]. The resin composite systems typically contain diluting co-monomers, such as triethyleneglycoldimethacrylate, TEGDMA, and also silanized glassy micro-fillers [8]. On the other hand, fillers such as silica and glass in resin composites are always silanized to stabilize the cured resin composite system against rupture and wear [9]. Chemically, silane coupling agents have an organofunctional group that can co-polymerize with the non-reacted carbon–carbon double bonds in monomers of a resin composite. Before application, silane coupling agents must be chemically activated: their hydrolyzable alkoxy groups are allowed to react in aqueous alcohol solution, at a pH of 4–5, to form reactive silanols, ≡Si–OH, which condense and deposit and thus form a siloxane film with the siloxane bonds, –Si–O–Si–O–, as presented in Fig. 1 [2]. The siloxane film thickness depends mainly on the silane concentration and it is usually more than a simple monolayer [10]. To improve adhesion of the material phases in restorations or devices, specific surface conditioning methods have been developed. For instance tribochemical silica-coating with Rocatec® is a conventional method at dental laboratories [11]: the alumina particles that are chemically silica-coated are blasted onto surfaces, such as base metal or noble metal alloy, or titanium [4], but also on polymeric resin composite or ceramic surfaces [12]. The particles form a reactive sil-
ica layer onto the substrate and thereafter, silane must be immediately applied to provide covalent bonds between the silica-coating and the resin composite cement [11]. In this evaluation RocatecTM treated titanium has been used mainly as a model material for evaluating the function of silane coupling agents. Today, 3-methacryloyloxypropyltrimethoxysilane is the dominant silane in clinical dentistry [1]. It has been shown to enhance adhesive properties of porcelain teeth when attaching to an acrylic denture resin [13]. Recently, some other silanes of the same chemical type as 3-methacryloyloxypropyltrimethoxysilane, such as 3isocyanatopropyltriethoxysilane [14], a vinylsilane and an isocyanuratesilane [15] have been evaluated in vitro. In this study, four non-common silanes, have been evaluated and compared with a commercially available, pre-activated 3-methacryloyloxypropyltrimethoxysilane in promoting a luting cement adhesion to silica-coated titanium. 3-Acryloyloxypropyltrimethoxysilane with its acrylate end-group has already given some promising results as a coupling agent in dentistry [16]. The terminal acrylate group is more reactive than the methacrylate group. 3-Mercaptopropyltrimethoxysilane has in the backbone a functional, slightly acidic thiol (mercapto) group, –SH, with its free electron pairs, and bis-[3(triethoxysilyl)propyl]polysulfide might be able to bond chemically also utilizing the empty 3d atomic orbitals of sulphur [17]. N-[3-(Trimethoxysilyl)propylethylenediamine] might be able to bond coordinatively through the nitrogen atoms. Structurally, all the silanes mentioned above share some common key properties, e.g., their organofunctional groups are connected with a propylene link to the silicon atom, and they have all three alkoxy groups (Fig. 2a–d). Bis[3-(triethoxysilyl)propyl]sulfide silane is a blend where two triethoxysilyl groups are connected via two propylene links to the sulphur chain, –Sn –, where n = 2–4 or poly (‘several’) as presented in Fig. 2e.
Fig. 1 – Trialkoxysilanes form a particular siloxane film between inorganic and organic matrices. There is a gradient zone between silica-coated surface and between silane and polymer (by courtesy of Prof. Wim van Ooij, University of Cincinnati, OH, USA, 2001).
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1173–1180
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than 3-methacryloyloxypropyltrimethoxysilane due to their structural and chemical properties. The shear bond strength values after thermo-cycling were expected to be lower than for samples that were tested without thermo-cycling.
2.
Materials and methods
2.1.
Silane coupling agents
The silanes of this evaluation are presented in Table 1 and other study materials in Table 2. A 95.0 vol.% ethanol-based solution diluted with de-ionized water (milli-Q water) was prepared [18]. The pH was adjusted with 1 M acetic acid at 4.5. It was allowed to stabilize for 24 h at room temperature. Then, 2.0 vol.% solutions of 3-acryloyloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N-[3-(trimethoxysilyl) propylethylenediamine] and bis-[3-(triethoxysilyl)propyl] polysulfide were separately prepared in the acidified ethanolwater solution, in 50 mL polyethylene bottles. The silanes were allowed to hydrolyze for 1 h at room temperature as described in the literature [5,18]. Un-opened ESPETM Sil, which contains ca. 2 vol.% 3-methacryloyloxypropyltrimethoxysilane was used as a control.
2.2.
Silicatization (silica-coating)
Commercially pure grade 2 titanium (thickness 1 mm) was cut into 40 mm × 20 mm slides (n = 20). The upper half of the titanium slide surface, an area of approximately 40 mm × 120 mm, was silica-coated with tribochemical RocatecTM grit-blasting system (3 M ESPE, Seefeld, Germany) using RocatecTM sand (Table 2). The sand was blasted with a slow rotating motion, using a jet of 280 kPa from a distance of 10 mm for 15 s, perpendicular to the titanium surface. The silica-coated Ti slides were then randomly divided into study groups (two slides/group). Each silane was applied by brushing onto the silica-coated Ti slides, and allowed to react with the surface for 3 min, before being gently air-blasted.
2.3.
Fig. 2 – (a) 3-Methacryloyloxypropyltrimethoxysilane, (b) 3-acryloyloxypropyltrimethoxysilane, (c) N-[3-(trimethoxysilyl)propylethylenediamine], (d) 3-mercaptopropyltrimethoxysilane and (e) bis-[3-(triethoxysilyl)propyl]polysulfide (n = 2–4, poly).
The hypothesis was that any of the activated four silane coupling agents might perform better than a commercial preactivated 3-methacryloyloxypropyltrimethoxysilane in bonding resin composite cement onto silica-coated titanium. The four silanes might provide more hydrolytically stable bonding
Resin composite cement stubs
A constant squeezed amount of RelyXTM ARC cement was dispended onto a mixing pad, as per manufacturer’s instructions. It was spread and rapidly mixed with a plastic spatula for 10 s. The cement was applied in polyethylene molds as stubs, with a 4 mm height and a 2 mm diameter. Four stubs were evenly placed on the upper horizontal borders of the titanium slide. The resin stubs were photo-polymerized for 40 s (Optilux 501, SDS Kerr, Danbury, USA), with an intensity of 490 mW cm−2 . The mold was removed gently by pressing the cured resin stub to the substrate. The stub preparation, silanization and bonding were performed by the same operator, and the cement treatment was according to the manufacturer’s instructions. There were eight resin stubs in a study group.
2.4.
Thermocycling and shear bond
Titanium slides with bonded cement stubs were subjected to thermocycling for 6000 cycles at temperatures alternating
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Table 1 – Silanes used in this evaluation study Name
Silane code in testing
ESPETM Sil (3-methacryloyloxypropyltrimethoxy-silane) 3-Mercaptopropyltrimethoxysilane Sulfur silane mixture Bis-(3-(triethoxysilyl)propyl)trisulfide Bis-(3-(triethoxysilyl)propyl)polysulfide Bis-(3-(triethoxysilyl)propyl)tetrasulfide Bis-(3-(triethoxysilyl)ropyl)disulfide 3-Acryloyloxypropyltrimethoxysilane N-(3-(Trimethoxysilyl)propyl)ethylenediamine
MPS MER PSS
Manufacturer
ESPE Dental, Seefeld, Germany Toray Dow Corning Silicone, Tokyo, Japan Dow Corning, Cardiff, UK
Manufacturer’s code
Solution and concentration (%) or purity
Batch number
–
Ethanol, >90
199624
SH 6062
N/A
0002027204
Z-6940
01446732350 30.0 27.0 26.0 15.0
ACR TPEA
Toray Dow Corning Silicone, Tokyo, Japan Dow Corning, Seneffe, Belgium
AY 43-310M
98.0
VN02011454
Z-6020
81.0
0002153427
The information given below is based on the available manufacturers’ product safety data sheets. Key—N/A: not available.
between 5 ± 2 and 55 ± 2 ◦ C, with a standard immersion time of 30 s (Heto CBN 18–30 baths, Allerø, Denmark). The samples were then kept a short while in a water bath (37 ◦ C) prior to testing that took place immediately. The dry samples were tested immediately after the sample preparation and photopolymerization. Shear bond strength tests were carried out with a universal material testing machine (LRX® , Lloyd Instrument, Fareham, UK), at room temperature (22 ± 2) ◦ C. The specimens were mounted in a jig with the shearing rod against and parallel to the silica-coated titanium level. The bonded resin composite stub was then loaded at a cross-head speed of 1.0 mm min−1 until fracture, and shear bond strengths were calculated by dividing the highest fracture force (in N) with the area of the stub (with a diameter of 2 mm), and recorded (in MPa) using Nexygen® PC software (LRX® , Lloyd Instrument, Fareham, UK).
2.5.
by the independent variables, type of silane and storage. Multiple comparisons were performed using Tukey’s HSD test. Statistical significance was set at ˛ = 0.05. A t-test was used for shear bond values that were numerically close to each other.
2.6.
Infrared spectroscopy
A Fourier transform infrared (FTIR) spectrophotometer (Perkin-Elmer Spectrum One, Perkin-Elmer, Beaconsfield, UK) was used by taking 32 scans at a resolution of 2 cm−1 to observe silane activation (hydrolysis). An inert, attenuated total reflectance (ATR) device (Perkin-Elmer, Beaconsfield, UK), equipped with a ZnSe crystal, was utilized by applying a few drops of fresh silane onto the ATR device and left to evaporate. The observation points for hydrolysis reaction were time = 0, 15, 30 and 60 min.
Statistical analysis 2.7.
The shear bond strength data for all the test groups were analyzed statistically with SPSS 11.0 (Statistical Package for Statistical Science, SPSS, Chicago, IL, USA). Three-way factorial analysis of variance (ANOVA) was used for investigations. The dependent variable, shear bond strength, was explained
Light microscopy analysis
A light microscopy analysis (WILD M38, Leica, Heerbrugg, Switzerland) was carried out after the shear bond testing to visually examine the resin stub contact areas (thermo-cycled samples) to the silica-coated titanium. A magnification of 40
Table 2 – Other materials used in this study Description Ethanol anhydricumTM Acetic acid Titanium RelyXTM ARC RocatecTM sand
Ethanol Metallic planar Ti commercially pure, grade 2 Adhesive resin cement paste, shade A3 Silica-coated alumina sand, 110 m
Manufacturer Primalco, Helsinki, Finland Merck, Darmstadt, Germany Permascand, Ljungaverk, Sweden 3M ESPE, St. Paul, MN, USA 3M ESPE, Seefeld, Germany
Purity (%)
Batch number
99 100 99.70 N/A N/A
030305 202K12716063 AS TMB26589 20041012 305
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was utilized to conclude the failure modes (adhesive, cohesive or mixed).
3.
Results
3.1.
Mechanical properties
The highest shear bond strength values for silica-coated silanized, thermo-cycled specimens, was 14.8 (S.D. 3.8) MPa obtained by using 3-acryloyloxypropyltrimethoxysilane. However, it did not differ statistically from the value obtained with 3-methacryloyloxypropyltrimethoxysilane, 14.2 (S.D. 5.8) MPa. The lowest shear bond strengths were 7.5 (S.D. 1.9) MPa for N-[3-(trimethoxysilyl)propylethylenediamine] and 7.5 (S.D. 2.5) MPa bis-[3-(triethoxysilyl)propyl]polysulfide. According to the t-test, the two lowest shear bond strength values did not differ statistically. For samples kept in dry conditions, the highest shear bond value was 22.5 (S.D. 2.5) MPa for 3-acryloyloxypropyltrimethoxysilane, and the lowest 10.0 (S.D. 1.2) MPa for bis-[3(triethoxysilyl)propyl]polysulfide, which did not statistically differ from N-[3-(trimethoxysilyl)propylethylenediamine] at 11.3 (S.D. 2.2) MPa. Statistical analysis (ANOVA) showed that the means differed significantly for the silane (p < 0.001, F = 31.416) and for the storage conditions (p < 0.001, F = 43.285). There was no interaction between silane and storage type (p = 0.392, F = 1.042). Spontaneous de-bonding occurred (25%) for samples silanized with bis-[3(triethoxysilyl)propyl]polysulfide during thermocycling. The results are shown in Fig. 3 and Table 3.
3.2.
FTIR analysis
All the spectra revealed that the silanes had been activated in 60 min. This can been observed when a symmetric C–H stretch of the Si–O–CH3 (or Si–O–CH2 CH3 ) group signals became smaller at ca. 2840 cm−1 and the silanol, Si–OH stretching mode had increased at 910–830 cm−1 . Finally, strong siloxane, –Si–O–Si–, signals, appeared at 1070–1040 cm−1 . There were differences as to what extent hydrolysis had occurred, but this had no significant effect to the adhesive properties of the silane solution.
Fig. 3 – The shear bond strength results obtained in two storage type. Key—‘dry’: testing without thermo-cycling; ‘thermocycling’: thermo-cycled between 5 and 55 ◦ C (6000 cycles). Key—MPS: 3-methacryloyloxypropyltrimethoxysilane (commercial ESPETM Sil), TPEA: N-[3-(trimethoxysilyl)propylethylenediamine], ACR: 3-acryloyloxypropyltrimethoxysilane, MER: 3-mercaptopropyltrimethoxysilane, and PSS: bis-[3-(triethoxysilyl)propyl]polysulfide. According to the t-test, for thermo-cycled samples, groups MPS + ACR and TPEA + PSS did not differ significantly.
Three FTIR spectra of some rare silanes are shown as examples. Signals due to –S–C– bonds can be seen at 1070–1100 cm−1 , so they overlap partially with siloxane signals. The –S–S– conjugation could be seen as weak signals at 550–450 cm−1 but this was beyond the recording scale (Fig. 4a). The mercapto group, SH–, showed weak signals at 2600–2550 cm−1 and would be seen better using Raman spectroscopy (Fig. 4b). Aminosilanes, such as N-[3-(trimethoxysilyl)propylethylenediamine], reveal typical complex NH– signals at 1580–1490 cm−1 which overlap with –CH2 , –CH3 signals (Fig. 4c). The hydrogen-bonded OH-group band at 3600–3200 cm−1 derived from water and ethanol. The signals represented were for free and bonded –OH groups.
Table 3 – Percentage of spontaneous de-bonding of RelyXTM ARC resin composite cement stubs during thermo-cycling and measured shear bond strengths Group code
MPS TPEA ACR MER PSS
n per group
De-bonded cement stubs
De-bonding (%)
Shear bond strength (MPa)
8 8 8 8 8
0 0 0 0 2
0 0 0 0 25
14.2a 7.5b 14.8a 10.5 7.5b
Standard deviation (MPa)
5.8 1.9 3.8 1.3 2.5
Cement stub failure mode
Shear bond strength value compared to non-thermo-cycled value (%)
Cohesive Mixed Cohesive Mixed Mixed
75 66 66 75 75
Key—MPS: 3-methacryloyloxypropyltrimethoxysilane (ESPETM Sil), TPEA: N-[3-(trimethoxysilyl)propylethylenediamine], ACR: 3acryloyloxypropyltrimethoxysilane, MER: 3-mercaptopropyltrimethoxysilane, and PSS: bis-[3-(triethoxysilyl)propyl]polysulfide. According to t-test, groups labeled with letters a, b do not differ significantly.
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polysulfide the failure mode was mainly mixed failure for both storage types.
4.
Fig. 4 – The FTIR spectra of some activated 2.0 vol.% silane solutions. Measurement curves presented at the points where t (time) = 0 min (start for hydrolysis), t = 15 min, t = 30 min, and t = 60 min. (a) Bis-[3-(triethoxysilyl)propyl]polysulfide (PSS), (b) 3-mercaptopropyltrimethoxysilane (MER) and (c) N-[3-(trimethoxysilyl)propylethylenediamine] (TPEA).
All the spectra showed some noise due to the atmospheric carbon dioxide, at 2300–1950 cm−1 . This is typical when the FTIR spectra are recorded in open air conditions. Nevertheless, noise does not overlap with any interesting signals at these measurements. Qualitative light microscopy analysis suggested that the failure mode was cohesive for all the cement samples silanized with 3-methacryloyloxypropyltrimethoxysilane and 3-acryloyloxypropyltrimethoxysilane, for both storage types. For N-[3-(trimethoxysilyl)propylethylenediamine], 3-mercaptopropyltrimethoxysilane and bis-[3-(triethoxysilyl)propyl]
Discussion
Titanium was selected as the study material, since it is widely used as a dental biomaterial for crowns, milled and implant fixed partial denture substructures, but also in implantology [19]. There are also several studies on the use of titanium as a model material for evaluating the function of coupling agents [14–16,18,20,21]. Resin composite cements, such as RelyXTM ARC are usually dual-cured resin cements: they are cured both chemically and light-induced. Such cements are used for cementing bridges, metal crowns, onlays and inlays, and resin-bonded fixed partial dentures. Clinically they are also suitable for the adhesive bonding of amalgam restorations. Modern cements contain acrylic resins with carbon–carbon double bonds which can polymerize with the organofunctional group in activated silane that has reacted with a silica-coated surface. Efforts enhancing adhesion interface chemistry are continuously carried out in dental materials research [22]. Silane coupling agent based adhesion research requires extensive experimental in vitro laboratory work, because the problems are particularly sensitive to the selected materials [2,23]. Methacrylate as a functional group is the most often favored due to its copolymerization properties [24] and another characterization study that undertaken with composites supported this [25]. A relatively low silane concentration, such as below 5 vol.%, maintains the silane–solvent system in such a balance that the silane monomers do not autopolymerize [5]. Commercial dental silanes are typically of the relatively same concentration level which is ca. 1–5 vol.% and their shelf-life is typically a couple of years. ESPETM Sil contains about 2 vol.% 3-methacryloyloxypropyltrimethoxysilane. It is also noticeable that other solvents, such as isopropanol and acetone, diluted with water, are used [1]. The bonding of veneering resin composites, or luting cements to base metal or noble alloys requires a tribochemical silicacoating followed by an immediate silanization, typically with 3-methacryloyloxypropyltrimethoxysilane [11]. Silica-coated Ti slides had been ultrasonically cleaned before silanization as suggested in the literature [21]. According to some reported studies, bonding might be performed directly to a titanium surface [20], and some rare silanes might promote it to some extent [15]. Bonding resin composites to silanized surfaces is generally through interdiffusion of the oligomeric siloxanes at the interface with possible cross-linking to the interpenetrating polymer network in the interphase region. Such structures are supposed to be stable and water resistant across the interfaces (cf. Fig. 1). It could be concluded according to the FTIR spectra (Fig. 4) that all the silanes were hydrolyzed before the bonding procedure. The time for complete hydrolysis varies, and it is not even mandatory [26]. FTIR is a suitable and reliable laboratory method for observing the activation of trialkoxysilanes and for characterization of chemical bonds [5]. It must be emphasized that all the silanes as such are sensitive to humidity but not for dry air, and oxygen [1]. Shear bond
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1173–1180
strength testing is a relatively fast and widely used method for evaluations of adhesive interfaces. On the other hand, it has met some criticism and other methods, such as tensile bond testing, have been proposed [27]. However, shear bond strength measurement was selected as the main tool for comparing the four silane performances to the control silane since the most results in literature are obtained by employing this test set-up. All the shear bond values became lower, as expected in the hypotheses. For the control silane (ESPETM Sil), 3-acryloyloxypropyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane, the values exceeded a standard 5 MPa, set by ISO [28]. Statistically, there were no significant differences between the control silane and 3-acryloyloxypropyltrimethoxysilane. Silanization with 3-mercaptopropyltrimethoxysilane gave somewhat promising results, though not better than the two best silanes. What might be interesting is that the thermocycled value was 75% of the non-thermocycled value (Table 3). Thus, there might be vistas for optimizing the use of 3-mercaptopropyltrimethoxysilane with some novel dental cements, which however requires more studies in thiol chemistry that is underway in the authors’ laboratory. The lowest shear bond strength values, obtained with N-[3-(trimethoxysilyl)propylethylenediamine] and bis[3-(triethoxysilyl)propyl]polysulfide, worsened by 66% and 75%, respectively after thermocycling. It can also be noted that bis-[3-(triethoxysilyl)propyl]polysulfide silane was in fact a blend of four different sulfide silane monomers (Table 1). Differences in bond strength values after thermo-cycling might be contributed to the fatigue of the siloxane film interface due to differences in thermal expansion coefficients. It might be that silane coupling agents that promoted adhesive siloxane films have a lower modulus of elasticity than the luting cement or silica-coated titanium matrix. Nevertheless, this could not be measured. The conclusions were also supported by the results of the light microscopy analysis. Thus, the siloxane film might have provided a stressbreaking interface. This should be studied closer in the near future. 3-Acryloyloxypropyltrimethoxysilane has proved to be a promising coupling agent as also recently reported [16]. Optimization work and stability studies should be carried out in the near future. On the other hand, the long-term hydrolytic stability needs to be evaluated.
5.
Conclusions
In the hypotheses of this study, four rare silane coupling agents were thought to yield stronger shear bond strength values after thermo-cycling than 3-methacryloyloxypropyltrimethoxysilane. Only one silane, 3-acryloyloxypropyltrimethoxysilane, agreed with it. 3-mercaptopropyltrimethoxysilane might be kept in mind for some novel adhesion systems in dentistry before long. All the shear bond strengths were lowered during the thermocycling procedure. Two remaining silane coupling agents gave inferior shear bond strength results. The hypothesis of this study was not completely met with the results obtained, but further silane optimization studies, might be useful.
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Acknowledgements Tekes (National Technology Agency of Finland) is acknowledged for grants. This research is a part of ‘the Bio- and Nanopolymers Research Group’ activity of the Centre of Excellence of the Academy of Finland. It also belongs to the ‘NIOM Biomaterials Network Cooperation’. Dow Corning GmbH, Germany (Mr. Manfred Goeggler) and 3M ESPE, Finland (Mr. Artti Juusela) are thanked for donating silanes and cements to these experiments. Mr. Jack Wright (London, UK) is thanked for the proofreading.
references
¨ [1] Matinlinna J, Lassila LVJ, Ozcan M, Yli-Urpo A, Vallittu PK. An introduction to silanes and their clinical applications in dentistry. Int J Prosthodont 2004;17:155–64. [2] Rosen MR. From treating solution to filler surface and beyond: the life history of a silane coupling agent. J Coat Technol 1978;50:70–82. [3] Barghi N. To silanate or not to silanate: making a clinical decision. Compend Contin Educ Dent 2000;21:659–64. [4] Matinlinna JP, Vallittu PK. Silane based concepts on bonding resin composite to metals. J Contemp Dent Practice 2007;8:1–8. [5] Hooshmand T, van Noort R, Keshwad A. Storage effect of a pre-activated silane on the resin to ceramic bond. Dent Mater 2004;20:635–42. [6] Bowen RL. Properties of a silica-reinforced polymer for dental restorations. J Am Dent Assoc 1963;66:57–64. [7] Bowen RL. Use of epoxy resins in restorative materials. J Dent Res 1956;35:361–9. [8] Roulet JF. Degradation of dental polymers. Karger: Basel; 1987. [9] Plueddemann EP. Composites having ionomer bonds with silanes at the interfaces. J Adhes Sci Technol 1989;3: 131–9. [10] van Ooij W, Child TF. Protecting metals with silane coupling agents. Chemtech 1998;28:26–35. [11] Guggenberger R. Das rocatec system—haftung durch tribochemische beschichtung. Dtsch Zahnarztl Z 1989;44:874–6. [12] Cobb DS, Vargas MA, Fridrich TA, Bouschlicher MR. Metal surface treatment: characterization and effect on composite-to-metal bond strength. Oper Dent 2000;25:427–33. [13] Paffenbarger GC, Sweeney WT, Bowen RL. Bonding porcelain to acrylic resin denture bases. J Am Dent Assoc 1967;74:1018–23. [14] Matinlinna JP, Lassila LVJ, Kangasniemi I, Vallittu PK. Isocyanato- and Methacryloxysilanes Promote Bis-GMA adhesion to titanium. J Dent Res 2005;84:360–4. ¨ [15] Matinlinna J, Ozcan M, Lassila LVJ, Vallittu PK. The effect of a 3-methacryloxypropyltrimethoxysilane and vinyltriisopropoxysilane blend and tris(3-trimethoxysilylpropyl)isocyanurate on the shear bond strength of composite resin to titanium metal. Dent Mater 2004;20:804–13. [16] Matinlinna JP, Lassila LVJ, Vallittu PK. The effect of a novel silane blend system on resin bond strength to silica-coated Ti substrate. J Dent 2006;34:436–43. [17] Vollhardt KPC, Schore NE. Organic chemistry—structure and function. New York: W.H. Freeman; 2002.
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[18] Matinlinna JP, Lassila LVJ, Kangasniemi I, Yli-Urpo A, Vallittu PK. Shear bond strength of bis-GMA and methacrylated dendrimer resins on silanized titanium substrate. Dent Mater 2005;21:287–96. [19] Anusavice KJ. Dental casting alloys. In: Anusavice KJ, editor. Phillips’ science of dental materials. Philadelphia, PA: W.B. Saunders Co.; 1996. pp. 455–458. [20] Kern M, Thompson VP. Durability of resin bonds to pure titanium. J Prosthodont 1995;4:16–22. [21] Kern M, Thompson VP. Effects of sandblasting and silica-coating procedures on pure titanium. J Dent 1994;22:300–6. [22] Rammelsberg P, Pospiech P, Gernet W. Clinical factors affecting adhesive fixed partial dentures: a 6-year study. J Prosthet Dent 1993;70:300–7. [23] Arkles B. Silane coupling agents: connecting across boundaries. Morrisville: Gelest; 2003. p. 48.
[24] Johansson OK, Stark FO, Vogel GE, Fleischmann RM. Evidence for chemical bond formation at silane coupling agent interfaces. J Compos Mater 1967;1:278–92. [25] Mohsen NM, Craig RG. Effect of silanation of fillers on their dispersability by monomer systems. J Oral Rehabil 1995;22:183–9. [26] Arkles B, Steinmetz JR, Zazyczny J, Mehta P. Factors contributing to the stability of alkoxysilanes in aqueous solution. In: Mittal KL, editor. Silanes and other coupling agents. Utrecht: VSP B.V.; 1992. p. 91–104. [27] Della Bona A, van Noort R. Shear versus tensile bond strength of resin composite bonded to ceramic. J Dent Res 1995;74:1591–6. [28] International Standardization Organization (ISO), Dentistry-polymer-based crown and bridge materials, Amendment 1996; ISO 10477.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
The effect of five silane coupling agents on the bond strength of a luting cement to a silica-coated titanium Jukka P. Matinlinna ∗ , Lippo V.J. Lassila, Pekka K. Vallittu University of Turku, Institute of Dentistry, Department of Prosthetic Dentistry and Biomaterials Science, ¨ Lemminkaisenkatu 2, FI-20520 Turku, Finland
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The adhesive performance of five silane coupling agents in adhering resin
Received 2 November 2005
composite cement (3 M ESPE) to silica-coated titanium was evaluated. Titanium was tri-
Received in revised form
bochemically silica-coated by using the RocatecTM system.
2 March 2006
Methods. Two volume percent solutions of 3-acryloyloxypropyltrimethoxysilane (Toray
Accepted 22 June 2006
Dow Corning Silicone), N-[3-(trimethoxysilyl)propylethylenediamine] (Dow Corning), 3mercaptopropyltrimethoxysilane (Toray Dow Corning Silicone) and bis-[3-(triethoxysilyl) propyl]polysulfide (Dow Corning) were prepared in 95 vol.% acidified ethanol and allowed to
Keywords:
activate (hydrolyze). A pre-activated ca. 2 vol.% 3-methacryloyloxypropyltrimethoxysilane
Silica-coating
(ESPETM Sil) was used as a control. The silanes were applied onto silica-coated titanium
Titanium
slides. Chemical activation reactions of the silanes were monitored by Fourier transform
3-Methacryloyloxypropyltrimetho-
infrared spectrometry (Perkin-Elmer Spectrum One). RelyXTM ARC (3 M ESPE) resin com-
xysilane
posite cement stubs were applied and photo-polymerized onto silica-coated titanium. The
3-Acryloyloxypropyltrimethoxy-
specimens were thermo-cycled (6000 cycles, 5–55 ◦ C).
silane
Results. Statistical analysis (ANOVA) showed that the highest shear bond strength
3-Mercaptopropyltrimethoxy-
(n = 8 per group) value after thermocycling, 14.8 MPa (S.D. 3.8 MPa), was obtained
silane
with 2.0 vol.% 3-acryloyloxypropyltrimethoxysilane. Silanization and results with 3-
Bis-[3-(triethoxysilyl)propyl]poly-
methacryloyloxypropyltrimethoxysilane (control, ESPETM Sil) did not statistically differ from
sulfide
3-acryloyloxypropyltrimethoxysilane, 14.2 MPa (S.D. 5.8). The lowest shear bond strength
N-[3-(Trimethoxysilyl)propylethy-
was 7.5 (S.D. 1.9) MPa for N-[3-(trimethoxysilyl)propylethylenediamine] and 7.5 (S.D. 2.5) MPa
lenediamine]
for bis-[3-(triethoxysilyl)propyl]polysulfide. Both the type of silane (p < 0.001) and storage
Silanization
conditions affected significantly the shear bond strength values (p < 0.001). All silanes
Silane coupling agent
became activated according to the infrared spectroscopic analysis.
Luting cements
Significance. Silanization with 3-acryloyloxypropyltrimethoxysilane or 3-mercaptopropyltrimethoxysilane might offer an alternative for bonding a luting cement to silica-coated titanium. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In the oral cavity, adhesion of particulate or fiber fillers and prosthetic materials of multiphase constructions is always subject to humidity, temperature and thermo-mechanical
∗
stresses. Adhesion promotion between dissimilar materials is successfully achieved with silane coupling agents, not only in dentistry [1] but also in numerous technical applications [2]. Silane coupling agents are widely accepted as adhesion promoters and applied in practical clinical work
Corresponding author. Tel.: +358 2 333 8357; fax: +358 2 333 8390. E-mail address: jumatin@utu.fi (J.P. Matinlinna). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.052
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for silica-coated surfaces [3,4]. Those silanes employed as coupling agents are usually chemically organofunctional trialkoxysilane esters, with three hydrolyzable alkoxy groups directly on the silicon atom, i.e., they have direct –Si–C– bonds. To-date, the only silane actually used in dentistry is 3-methacryloyloxypropyltrimethoxysilane [1,5]. Typical dental resin composites are structured on a reaction product of bis-phenol A and glycidylmethacrylate [6]. Bis-GMA can undergo polymerization when exposed to ultraviolet (or blue) light or also when mixed with a catalyst, and bis-GMA is used as the resin component of dental sealants and restorative materials [7]. The resin composite systems typically contain diluting co-monomers, such as triethyleneglycoldimethacrylate, TEGDMA, and also silanized glassy micro-fillers [8]. On the other hand, fillers such as silica and glass in resin composites are always silanized to stabilize the cured resin composite system against rupture and wear [9]. Chemically, silane coupling agents have an organofunctional group that can co-polymerize with the non-reacted carbon–carbon double bonds in monomers of a resin composite. Before application, silane coupling agents must be chemically activated: their hydrolyzable alkoxy groups are allowed to react in aqueous alcohol solution, at a pH of 4–5, to form reactive silanols, ≡Si–OH, which condense and deposit and thus form a siloxane film with the siloxane bonds, –Si–O–Si–O–, as presented in Fig. 1 [2]. The siloxane film thickness depends mainly on the silane concentration and it is usually more than a simple monolayer [10]. To improve adhesion of the material phases in restorations or devices, specific surface conditioning methods have been developed. For instance tribochemical silica-coating with Rocatec® is a conventional method at dental laboratories [11]: the alumina particles that are chemically silica-coated are blasted onto surfaces, such as base metal or noble metal alloy, or titanium [4], but also on polymeric resin composite or ceramic surfaces [12]. The particles form a reactive sil-
ica layer onto the substrate and thereafter, silane must be immediately applied to provide covalent bonds between the silica-coating and the resin composite cement [11]. In this evaluation RocatecTM treated titanium has been used mainly as a model material for evaluating the function of silane coupling agents. Today, 3-methacryloyloxypropyltrimethoxysilane is the dominant silane in clinical dentistry [1]. It has been shown to enhance adhesive properties of porcelain teeth when attaching to an acrylic denture resin [13]. Recently, some other silanes of the same chemical type as 3-methacryloyloxypropyltrimethoxysilane, such as 3isocyanatopropyltriethoxysilane [14], a vinylsilane and an isocyanuratesilane [15] have been evaluated in vitro. In this study, four non-common silanes, have been evaluated and compared with a commercially available, pre-activated 3-methacryloyloxypropyltrimethoxysilane in promoting a luting cement adhesion to silica-coated titanium. 3-Acryloyloxypropyltrimethoxysilane with its acrylate end-group has already given some promising results as a coupling agent in dentistry [16]. The terminal acrylate group is more reactive than the methacrylate group. 3-Mercaptopropyltrimethoxysilane has in the backbone a functional, slightly acidic thiol (mercapto) group, –SH, with its free electron pairs, and bis-[3(triethoxysilyl)propyl]polysulfide might be able to bond chemically also utilizing the empty 3d atomic orbitals of sulphur [17]. N-[3-(Trimethoxysilyl)propylethylenediamine] might be able to bond coordinatively through the nitrogen atoms. Structurally, all the silanes mentioned above share some common key properties, e.g., their organofunctional groups are connected with a propylene link to the silicon atom, and they have all three alkoxy groups (Fig. 2a–d). Bis[3-(triethoxysilyl)propyl]sulfide silane is a blend where two triethoxysilyl groups are connected via two propylene links to the sulphur chain, –Sn –, where n = 2–4 or poly (‘several’) as presented in Fig. 2e.
Fig. 1 – Trialkoxysilanes form a particular siloxane film between inorganic and organic matrices. There is a gradient zone between silica-coated surface and between silane and polymer (by courtesy of Prof. Wim van Ooij, University of Cincinnati, OH, USA, 2001).
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1173–1180
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than 3-methacryloyloxypropyltrimethoxysilane due to their structural and chemical properties. The shear bond strength values after thermo-cycling were expected to be lower than for samples that were tested without thermo-cycling.
2.
Materials and methods
2.1.
Silane coupling agents
The silanes of this evaluation are presented in Table 1 and other study materials in Table 2. A 95.0 vol.% ethanol-based solution diluted with de-ionized water (milli-Q water) was prepared [18]. The pH was adjusted with 1 M acetic acid at 4.5. It was allowed to stabilize for 24 h at room temperature. Then, 2.0 vol.% solutions of 3-acryloyloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N-[3-(trimethoxysilyl) propylethylenediamine] and bis-[3-(triethoxysilyl)propyl] polysulfide were separately prepared in the acidified ethanolwater solution, in 50 mL polyethylene bottles. The silanes were allowed to hydrolyze for 1 h at room temperature as described in the literature [5,18]. Un-opened ESPETM Sil, which contains ca. 2 vol.% 3-methacryloyloxypropyltrimethoxysilane was used as a control.
2.2.
Silicatization (silica-coating)
Commercially pure grade 2 titanium (thickness 1 mm) was cut into 40 mm × 20 mm slides (n = 20). The upper half of the titanium slide surface, an area of approximately 40 mm × 120 mm, was silica-coated with tribochemical RocatecTM grit-blasting system (3 M ESPE, Seefeld, Germany) using RocatecTM sand (Table 2). The sand was blasted with a slow rotating motion, using a jet of 280 kPa from a distance of 10 mm for 15 s, perpendicular to the titanium surface. The silica-coated Ti slides were then randomly divided into study groups (two slides/group). Each silane was applied by brushing onto the silica-coated Ti slides, and allowed to react with the surface for 3 min, before being gently air-blasted.
2.3.
Fig. 2 – (a) 3-Methacryloyloxypropyltrimethoxysilane, (b) 3-acryloyloxypropyltrimethoxysilane, (c) N-[3-(trimethoxysilyl)propylethylenediamine], (d) 3-mercaptopropyltrimethoxysilane and (e) bis-[3-(triethoxysilyl)propyl]polysulfide (n = 2–4, poly).
The hypothesis was that any of the activated four silane coupling agents might perform better than a commercial preactivated 3-methacryloyloxypropyltrimethoxysilane in bonding resin composite cement onto silica-coated titanium. The four silanes might provide more hydrolytically stable bonding
Resin composite cement stubs
A constant squeezed amount of RelyXTM ARC cement was dispended onto a mixing pad, as per manufacturer’s instructions. It was spread and rapidly mixed with a plastic spatula for 10 s. The cement was applied in polyethylene molds as stubs, with a 4 mm height and a 2 mm diameter. Four stubs were evenly placed on the upper horizontal borders of the titanium slide. The resin stubs were photo-polymerized for 40 s (Optilux 501, SDS Kerr, Danbury, USA), with an intensity of 490 mW cm−2 . The mold was removed gently by pressing the cured resin stub to the substrate. The stub preparation, silanization and bonding were performed by the same operator, and the cement treatment was according to the manufacturer’s instructions. There were eight resin stubs in a study group.
2.4.
Thermocycling and shear bond
Titanium slides with bonded cement stubs were subjected to thermocycling for 6000 cycles at temperatures alternating
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Table 1 – Silanes used in this evaluation study Name
Silane code in testing
ESPETM Sil (3-methacryloyloxypropyltrimethoxy-silane) 3-Mercaptopropyltrimethoxysilane Sulfur silane mixture Bis-(3-(triethoxysilyl)propyl)trisulfide Bis-(3-(triethoxysilyl)propyl)polysulfide Bis-(3-(triethoxysilyl)propyl)tetrasulfide Bis-(3-(triethoxysilyl)ropyl)disulfide 3-Acryloyloxypropyltrimethoxysilane N-(3-(Trimethoxysilyl)propyl)ethylenediamine
MPS MER PSS
Manufacturer
ESPE Dental, Seefeld, Germany Toray Dow Corning Silicone, Tokyo, Japan Dow Corning, Cardiff, UK
Manufacturer’s code
Solution and concentration (%) or purity
Batch number
–
Ethanol, >90
199624
SH 6062
N/A
0002027204
Z-6940
01446732350 30.0 27.0 26.0 15.0
ACR TPEA
Toray Dow Corning Silicone, Tokyo, Japan Dow Corning, Seneffe, Belgium
AY 43-310M
98.0
VN02011454
Z-6020
81.0
0002153427
The information given below is based on the available manufacturers’ product safety data sheets. Key—N/A: not available.
between 5 ± 2 and 55 ± 2 ◦ C, with a standard immersion time of 30 s (Heto CBN 18–30 baths, Allerø, Denmark). The samples were then kept a short while in a water bath (37 ◦ C) prior to testing that took place immediately. The dry samples were tested immediately after the sample preparation and photopolymerization. Shear bond strength tests were carried out with a universal material testing machine (LRX® , Lloyd Instrument, Fareham, UK), at room temperature (22 ± 2) ◦ C. The specimens were mounted in a jig with the shearing rod against and parallel to the silica-coated titanium level. The bonded resin composite stub was then loaded at a cross-head speed of 1.0 mm min−1 until fracture, and shear bond strengths were calculated by dividing the highest fracture force (in N) with the area of the stub (with a diameter of 2 mm), and recorded (in MPa) using Nexygen® PC software (LRX® , Lloyd Instrument, Fareham, UK).
2.5.
by the independent variables, type of silane and storage. Multiple comparisons were performed using Tukey’s HSD test. Statistical significance was set at ˛ = 0.05. A t-test was used for shear bond values that were numerically close to each other.
2.6.
Infrared spectroscopy
A Fourier transform infrared (FTIR) spectrophotometer (Perkin-Elmer Spectrum One, Perkin-Elmer, Beaconsfield, UK) was used by taking 32 scans at a resolution of 2 cm−1 to observe silane activation (hydrolysis). An inert, attenuated total reflectance (ATR) device (Perkin-Elmer, Beaconsfield, UK), equipped with a ZnSe crystal, was utilized by applying a few drops of fresh silane onto the ATR device and left to evaporate. The observation points for hydrolysis reaction were time = 0, 15, 30 and 60 min.
Statistical analysis 2.7.
The shear bond strength data for all the test groups were analyzed statistically with SPSS 11.0 (Statistical Package for Statistical Science, SPSS, Chicago, IL, USA). Three-way factorial analysis of variance (ANOVA) was used for investigations. The dependent variable, shear bond strength, was explained
Light microscopy analysis
A light microscopy analysis (WILD M38, Leica, Heerbrugg, Switzerland) was carried out after the shear bond testing to visually examine the resin stub contact areas (thermo-cycled samples) to the silica-coated titanium. A magnification of 40
Table 2 – Other materials used in this study Description Ethanol anhydricumTM Acetic acid Titanium RelyXTM ARC RocatecTM sand
Ethanol Metallic planar Ti commercially pure, grade 2 Adhesive resin cement paste, shade A3 Silica-coated alumina sand, 110 m
Manufacturer Primalco, Helsinki, Finland Merck, Darmstadt, Germany Permascand, Ljungaverk, Sweden 3M ESPE, St. Paul, MN, USA 3M ESPE, Seefeld, Germany
Purity (%)
Batch number
99 100 99.70 N/A N/A
030305 202K12716063 AS TMB26589 20041012 305
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1173–1180
was utilized to conclude the failure modes (adhesive, cohesive or mixed).
3.
Results
3.1.
Mechanical properties
The highest shear bond strength values for silica-coated silanized, thermo-cycled specimens, was 14.8 (S.D. 3.8) MPa obtained by using 3-acryloyloxypropyltrimethoxysilane. However, it did not differ statistically from the value obtained with 3-methacryloyloxypropyltrimethoxysilane, 14.2 (S.D. 5.8) MPa. The lowest shear bond strengths were 7.5 (S.D. 1.9) MPa for N-[3-(trimethoxysilyl)propylethylenediamine] and 7.5 (S.D. 2.5) MPa bis-[3-(triethoxysilyl)propyl]polysulfide. According to the t-test, the two lowest shear bond strength values did not differ statistically. For samples kept in dry conditions, the highest shear bond value was 22.5 (S.D. 2.5) MPa for 3-acryloyloxypropyltrimethoxysilane, and the lowest 10.0 (S.D. 1.2) MPa for bis-[3(triethoxysilyl)propyl]polysulfide, which did not statistically differ from N-[3-(trimethoxysilyl)propylethylenediamine] at 11.3 (S.D. 2.2) MPa. Statistical analysis (ANOVA) showed that the means differed significantly for the silane (p < 0.001, F = 31.416) and for the storage conditions (p < 0.001, F = 43.285). There was no interaction between silane and storage type (p = 0.392, F = 1.042). Spontaneous de-bonding occurred (25%) for samples silanized with bis-[3(triethoxysilyl)propyl]polysulfide during thermocycling. The results are shown in Fig. 3 and Table 3.
3.2.
FTIR analysis
All the spectra revealed that the silanes had been activated in 60 min. This can been observed when a symmetric C–H stretch of the Si–O–CH3 (or Si–O–CH2 CH3 ) group signals became smaller at ca. 2840 cm−1 and the silanol, Si–OH stretching mode had increased at 910–830 cm−1 . Finally, strong siloxane, –Si–O–Si–, signals, appeared at 1070–1040 cm−1 . There were differences as to what extent hydrolysis had occurred, but this had no significant effect to the adhesive properties of the silane solution.
Fig. 3 – The shear bond strength results obtained in two storage type. Key—‘dry’: testing without thermo-cycling; ‘thermocycling’: thermo-cycled between 5 and 55 ◦ C (6000 cycles). Key—MPS: 3-methacryloyloxypropyltrimethoxysilane (commercial ESPETM Sil), TPEA: N-[3-(trimethoxysilyl)propylethylenediamine], ACR: 3-acryloyloxypropyltrimethoxysilane, MER: 3-mercaptopropyltrimethoxysilane, and PSS: bis-[3-(triethoxysilyl)propyl]polysulfide. According to the t-test, for thermo-cycled samples, groups MPS + ACR and TPEA + PSS did not differ significantly.
Three FTIR spectra of some rare silanes are shown as examples. Signals due to –S–C– bonds can be seen at 1070–1100 cm−1 , so they overlap partially with siloxane signals. The –S–S– conjugation could be seen as weak signals at 550–450 cm−1 but this was beyond the recording scale (Fig. 4a). The mercapto group, SH–, showed weak signals at 2600–2550 cm−1 and would be seen better using Raman spectroscopy (Fig. 4b). Aminosilanes, such as N-[3-(trimethoxysilyl)propylethylenediamine], reveal typical complex NH– signals at 1580–1490 cm−1 which overlap with –CH2 , –CH3 signals (Fig. 4c). The hydrogen-bonded OH-group band at 3600–3200 cm−1 derived from water and ethanol. The signals represented were for free and bonded –OH groups.
Table 3 – Percentage of spontaneous de-bonding of RelyXTM ARC resin composite cement stubs during thermo-cycling and measured shear bond strengths Group code
MPS TPEA ACR MER PSS
n per group
De-bonded cement stubs
De-bonding (%)
Shear bond strength (MPa)
8 8 8 8 8
0 0 0 0 2
0 0 0 0 25
14.2a 7.5b 14.8a 10.5 7.5b
Standard deviation (MPa)
5.8 1.9 3.8 1.3 2.5
Cement stub failure mode
Shear bond strength value compared to non-thermo-cycled value (%)
Cohesive Mixed Cohesive Mixed Mixed
75 66 66 75 75
Key—MPS: 3-methacryloyloxypropyltrimethoxysilane (ESPETM Sil), TPEA: N-[3-(trimethoxysilyl)propylethylenediamine], ACR: 3acryloyloxypropyltrimethoxysilane, MER: 3-mercaptopropyltrimethoxysilane, and PSS: bis-[3-(triethoxysilyl)propyl]polysulfide. According to t-test, groups labeled with letters a, b do not differ significantly.
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polysulfide the failure mode was mainly mixed failure for both storage types.
4.
Fig. 4 – The FTIR spectra of some activated 2.0 vol.% silane solutions. Measurement curves presented at the points where t (time) = 0 min (start for hydrolysis), t = 15 min, t = 30 min, and t = 60 min. (a) Bis-[3-(triethoxysilyl)propyl]polysulfide (PSS), (b) 3-mercaptopropyltrimethoxysilane (MER) and (c) N-[3-(trimethoxysilyl)propylethylenediamine] (TPEA).
All the spectra showed some noise due to the atmospheric carbon dioxide, at 2300–1950 cm−1 . This is typical when the FTIR spectra are recorded in open air conditions. Nevertheless, noise does not overlap with any interesting signals at these measurements. Qualitative light microscopy analysis suggested that the failure mode was cohesive for all the cement samples silanized with 3-methacryloyloxypropyltrimethoxysilane and 3-acryloyloxypropyltrimethoxysilane, for both storage types. For N-[3-(trimethoxysilyl)propylethylenediamine], 3-mercaptopropyltrimethoxysilane and bis-[3-(triethoxysilyl)propyl]
Discussion
Titanium was selected as the study material, since it is widely used as a dental biomaterial for crowns, milled and implant fixed partial denture substructures, but also in implantology [19]. There are also several studies on the use of titanium as a model material for evaluating the function of coupling agents [14–16,18,20,21]. Resin composite cements, such as RelyXTM ARC are usually dual-cured resin cements: they are cured both chemically and light-induced. Such cements are used for cementing bridges, metal crowns, onlays and inlays, and resin-bonded fixed partial dentures. Clinically they are also suitable for the adhesive bonding of amalgam restorations. Modern cements contain acrylic resins with carbon–carbon double bonds which can polymerize with the organofunctional group in activated silane that has reacted with a silica-coated surface. Efforts enhancing adhesion interface chemistry are continuously carried out in dental materials research [22]. Silane coupling agent based adhesion research requires extensive experimental in vitro laboratory work, because the problems are particularly sensitive to the selected materials [2,23]. Methacrylate as a functional group is the most often favored due to its copolymerization properties [24] and another characterization study that undertaken with composites supported this [25]. A relatively low silane concentration, such as below 5 vol.%, maintains the silane–solvent system in such a balance that the silane monomers do not autopolymerize [5]. Commercial dental silanes are typically of the relatively same concentration level which is ca. 1–5 vol.% and their shelf-life is typically a couple of years. ESPETM Sil contains about 2 vol.% 3-methacryloyloxypropyltrimethoxysilane. It is also noticeable that other solvents, such as isopropanol and acetone, diluted with water, are used [1]. The bonding of veneering resin composites, or luting cements to base metal or noble alloys requires a tribochemical silicacoating followed by an immediate silanization, typically with 3-methacryloyloxypropyltrimethoxysilane [11]. Silica-coated Ti slides had been ultrasonically cleaned before silanization as suggested in the literature [21]. According to some reported studies, bonding might be performed directly to a titanium surface [20], and some rare silanes might promote it to some extent [15]. Bonding resin composites to silanized surfaces is generally through interdiffusion of the oligomeric siloxanes at the interface with possible cross-linking to the interpenetrating polymer network in the interphase region. Such structures are supposed to be stable and water resistant across the interfaces (cf. Fig. 1). It could be concluded according to the FTIR spectra (Fig. 4) that all the silanes were hydrolyzed before the bonding procedure. The time for complete hydrolysis varies, and it is not even mandatory [26]. FTIR is a suitable and reliable laboratory method for observing the activation of trialkoxysilanes and for characterization of chemical bonds [5]. It must be emphasized that all the silanes as such are sensitive to humidity but not for dry air, and oxygen [1]. Shear bond
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strength testing is a relatively fast and widely used method for evaluations of adhesive interfaces. On the other hand, it has met some criticism and other methods, such as tensile bond testing, have been proposed [27]. However, shear bond strength measurement was selected as the main tool for comparing the four silane performances to the control silane since the most results in literature are obtained by employing this test set-up. All the shear bond values became lower, as expected in the hypotheses. For the control silane (ESPETM Sil), 3-acryloyloxypropyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane, the values exceeded a standard 5 MPa, set by ISO [28]. Statistically, there were no significant differences between the control silane and 3-acryloyloxypropyltrimethoxysilane. Silanization with 3-mercaptopropyltrimethoxysilane gave somewhat promising results, though not better than the two best silanes. What might be interesting is that the thermocycled value was 75% of the non-thermocycled value (Table 3). Thus, there might be vistas for optimizing the use of 3-mercaptopropyltrimethoxysilane with some novel dental cements, which however requires more studies in thiol chemistry that is underway in the authors’ laboratory. The lowest shear bond strength values, obtained with N-[3-(trimethoxysilyl)propylethylenediamine] and bis[3-(triethoxysilyl)propyl]polysulfide, worsened by 66% and 75%, respectively after thermocycling. It can also be noted that bis-[3-(triethoxysilyl)propyl]polysulfide silane was in fact a blend of four different sulfide silane monomers (Table 1). Differences in bond strength values after thermo-cycling might be contributed to the fatigue of the siloxane film interface due to differences in thermal expansion coefficients. It might be that silane coupling agents that promoted adhesive siloxane films have a lower modulus of elasticity than the luting cement or silica-coated titanium matrix. Nevertheless, this could not be measured. The conclusions were also supported by the results of the light microscopy analysis. Thus, the siloxane film might have provided a stressbreaking interface. This should be studied closer in the near future. 3-Acryloyloxypropyltrimethoxysilane has proved to be a promising coupling agent as also recently reported [16]. Optimization work and stability studies should be carried out in the near future. On the other hand, the long-term hydrolytic stability needs to be evaluated.
5.
Conclusions
In the hypotheses of this study, four rare silane coupling agents were thought to yield stronger shear bond strength values after thermo-cycling than 3-methacryloyloxypropyltrimethoxysilane. Only one silane, 3-acryloyloxypropyltrimethoxysilane, agreed with it. 3-mercaptopropyltrimethoxysilane might be kept in mind for some novel adhesion systems in dentistry before long. All the shear bond strengths were lowered during the thermocycling procedure. Two remaining silane coupling agents gave inferior shear bond strength results. The hypothesis of this study was not completely met with the results obtained, but further silane optimization studies, might be useful.
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Acknowledgements Tekes (National Technology Agency of Finland) is acknowledged for grants. This research is a part of ‘the Bio- and Nanopolymers Research Group’ activity of the Centre of Excellence of the Academy of Finland. It also belongs to the ‘NIOM Biomaterials Network Cooperation’. Dow Corning GmbH, Germany (Mr. Manfred Goeggler) and 3M ESPE, Finland (Mr. Artti Juusela) are thanked for donating silanes and cements to these experiments. Mr. Jack Wright (London, UK) is thanked for the proofreading.
references
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