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Bonding metals to poly(methyl methacrylate) using aryldiazonium salts Omar Alageel a,b , Mohamed-Nur Abdallah a , Zhong Yuan Luo c , Jaime Del-Rio-Highsmith d , Marta Cerruti c,∗ , Faleh Tamimi a,∗∗ a
Faculty of Dentistry, McGill University, Montreal, QC, Canada College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia c Department of Mining and Materials Engineering, McGill University, Montreal, QC, Canada d Faculty of Dentistry, Complutense University of Madrid, Madrid, Spain b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Many dental devices, such as partial dentures, combine acrylic and metallic parts
Received 24 January 2014
that are bonded together. These devices often present catastrophic mechanical failures due
Received in revised form
to weak bonding between their acrylic and metallic components. The bonding between
15 July 2014
alloys and polymers (e.g. poly(methyl methacrylate), PMMA) usually is just a mechanical
Accepted 3 November 2014
interlock, since they do not chemically bond spontaneously. The aim of this study was to
Available online xxx
develop a new method to make a strong chemical bond between alloys and polymers for dental prostheses based on diazonium chemistry.
Keywords:
Methods. The method was based on two steps. In the first step (primer), aryldiazonium salts
Dental prosthesis
were grafted onto the metallic surfaces. The second step (adhesive) was optimized to achieve
Bonding diazonium
covalent binding between the grafted layer and PMMA. The chemical composition of the
Poly(methyl methacrylate)
treated surfaces was analyzed with X-ray photoelectron spectroscopy (XPS), and the tensile
Titanium
or shear bonding strength between metals and poly(methyl methacrylate) was measured.
Stainless steel
Results. XPS and contact angle measurements confirmed the presence of a polymer coating on the treated metallic surfaces. Mechanical tests showed a significant increase in bond strength between PMMA and treated titanium or stainless steel wire by 5.2 and 2.5 folds, respectively, compared to the untreated control group (p < 0.05). Significance. Diazonium chemistry is an effective technique for achieving a strong chemical bond between alloys and PMMA, which can help improve the mechanical properties of dental devices. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Correspondence to: Department of Mining and Materials Engineering, McGill University, Room 2M020, 3610 University Street, Montreal, QC H3A 0C5, Canada. Tel.: +1 514 398 5496; fax: +1 514 398 4492. ∗∗ Correspondence to: Faculty of Dentistry, McGill University, Room M64, 3640 University Street, Montreal, QC, H3A 0C7, Canada. Tel.: +1 514 398 7203x09654; fax: +1 514 398 8900. E-mail addresses: [email protected] (M. Cerruti), [email protected] (F. Tamimi).
http://dx.doi.org/10.1016/j.dental.2014.11.002 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Alageel O, et al. Bonding metals to poly(methyl methacrylate) using aryldiazonium salts. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.11.002
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1.
Introduction
Chemical bonding between alloys and polymers does not occur spontaneously; indeed, composite materials that combine polymers with alloys often suffer from mechanical failure at the interface between them. One example of this challenge is dental devices, which often present catastrophic mechanical failures due to weak bonding between their metallic and polymeric components [1–3]. These devices include dental prostheses, combining metallic frameworks and wrought wires with acrylic resin; and orthodontic appliances, combining acrylic resin with stainless steel wrought wires. Poly(methyl methacrylate) (PMMA) is extensively used in denture materials for dental prostheses and orthodontic devices because of its biocompatibility, excellent esthetic, and mechanical properties [4]. Titanium (Ti) is increasingly used in dental implants, implant abutments, and milled prostheses because of its excellent mechanical properties (i.e. strength to weight ratio) and biocompatibility [5]. PMMA and Ti in dental prostheses are usually bonded by mechanical interlocking the PMMA into the irregularities of the Ti surface [2,6,7]. Further improvement in the bonding strength between Ti and PMMA is still needed to prevent debonding, which are otherwise common in clinical practice, and reduce microleaks at the Ti/PMMA interface that causes accumulation of oral debris and discoloration of denture base materials [1,2,8]. Ti–PMMA bond can be strengthened by adding a chemical link between PMMA and Ti, since Ti and PMMA do not chemically bind together spontaneously [1,7,9–13]. Several methods have been tested to increase the bond strength between polymers and alloys in dental prostheses [1,9,12,14–24]. Table 1 summarizes the strengths obtained by binding PMMA and Ti with different methods, measured with shear bond, four-point bending, and tensile strength tests (it should be noted that the reported values are not comparable with each other as the specimen size, test methods and practice vary between different studies). In general, higher bond strengths are reported for the four-point bending and the shear bond tests (values ranging between 25.5–42.5 MPa and 7.0–46.6 MPa, respectively), while the lowest values are obtained with the tensile strength test (0–23.5 MPa). Indeed, the latter test is the most challenging one; however, it is also the most accurate technique to measure bond strength because it applies a direct and uniform force to the surface [25]. On the contrary, the shear bond and four-point bend tests do not distribute stress uniformly on the surfaces being tested [26]. Most of the methods reported in Table 1 require sandblasting the metallic surface, and all of them use either silane or phosphonate groups to create a chemical bond between the two surfaces [1,9,12,14–24]. Silanes and phosphonates covalently bind to Ti, while sandblasting increases the surface area of the exposed Ti, thus increasing the overall bonding strength [27]. The highest bond strengths reported were achieved using phosphonate-based adhesives (MHPA, MDP, and VDT; see Table 1) in combination with sandblasting. Specifically, the highest tensile strength reported without sandblasting was 7.4 MPa [12], while using a combination of sandblasting and bonding agents the tensile strength went up to 23.5 MPa
[12,15,17,19–21,24]. These values are still too low for dental applications. Overdentures, for example, have to resist biting forces of up to 662.2 N, and pressures of up to 51.1 MPa [28]. This implies that masticatory forces can exceed the strength of the Ti–PMMA bond and lead to prosthesis failure. An ideal goal would be to have a metal/PMMA interface that is at least as strong as PMMA alone, which has a tensile strength of 65 MPa [29]. Another example of metal–acrylic interface found in dental applications is that between wrought wires and acrylic-based dental devices such as dental prostheses and orthodontic appliances [30]. Wrought wires are usually made of stainless steel or cobalt–chromium alloys, which both lack the ability to bind chemically to acrylic resins [31]. Surprisingly, improving the adhesion between wrought wires and acrylic has hardly been investigated. Dental devices combining wrought wires with acrylic such as acrylic removable partial dentures usually cannot be made when not enough volume of PMMA is available to support the wire [30,31]. By increasing bond strength between stainless steel wrought wire and PMMA through this treatment, more leverage is possible for fabricating acrylicbased dental devices when not enough volume of acrylic is available to support the wire. In this paper we will show a technique to improve the binding between PMMA and alloys used in dental applications based on diazonium chemistry. Aryldiazonium salts have been used to modify material surfaces for many applications [32,33]. Diazonium ions can be produced from aromatic amines and grafted onto almost any surface, including metals, glass, and carbon [34–37]. Initially, diazonium grafting was performed using electrochemical reduction, but recently this has been achieved using chemical reducing agents in acidic solutions [38]. The reducing agents transform the aryldiazonium salts into aryl radicals, which can covalently bind to the surface of interest [39,40]. If an extra amino group is present on the aryldiazonium precursor, a polyaminophenylene (PAP) layer is formed on the metallic surface. The amino groups sticking out from the PAP layer can be further activated in a second step, and used to bind a second layer onto the original surface [39,41]. In this work, we optimize such second step to bind PMMA and metals for dental applications. The aim of this study was to develop a new method of creating a strong chemical bond between alloys and polymers for dental prostheses based on diazonium chemistry.
2.
Materials and methods
2.1.
Materials
Poly-methyl methacrylate (PMMA) and methyl methacrylate (MMA) were obtained from Great Lakes Orthodontics (Tonawanda, NY), and were used without any further purification. The rest of the reagents were obtained from Sigma Aldrich (St. Louis, MO). P-phenylenediamine (PPD), sodium nitrite (NaNO2 ), sodium dodecyl sulfate (SDS), benzoyl peroxide (BP), and iron powder (Fe) were used as received. Concentrated hydrochloric acid (HCl) was diluted in distilled water (DW) to a concentration of 0.5 M.
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Table 1 – The bond strengths between titanium and PMMA (MPa) using different bonding methods. Bonding agent (commercial name)
Surface topography
Type of PMMA used (commercial name)
None
Sandblasted
Self-cured with EGDMA and TBB (super-bond C&B) Heat-cured
Shear bond
38.1 ± 2.3
[1]
Tensile strength
Self-cured with BP (multi-bond) Self-cured Self-cured Self-cured Self-cured with EGDMA and TBB (super-bond C&B) Self-cured with BP (multi-bond) Heat-cured
Shear bond
20.0 16.1 ± 1.6 3.2 ± 0.4 13.6 ± 1.6
[2] [3] [4] [1]
9.9 46.6 45.7 39.8 ± 2.0
[5] [5] [5] [1]
MHPA (AZ Primer) MDP and VTD (alloy primer)
Sandblasted Sandblasted
Testing technique
Shear bond Shear bond
Bond strength (MPa)a
Ref.
22.0 ± 6.6
Tensile strength Shear bond
27.5 ± 4.0 16.0 ± 3.6 45.4
[6] [4] [5]
39.6 ± 2.5
[1]
Sandblasted
Self-cured
MATP (Espe-Sil) MATP (Espe-Sil) MATP MATP (silicoater M D) MATP then Silica-coating (Espe-Sil; Rocatec system) Silica-coating (Rocatec system)
Polished Sandblasted Sandblasted Sandblasted Sandblasted
Self-cured with EGDMA and TBB (super-bond C&B) Self-cured with BP (multi-bond) Heat-cured Heat-cured Self-cured Self-cured Heat-cured
Shear bond Shear bond Tensile strength Shear bond Shear bond
0.0 5.9 ± 2.1 14.3 21.9± 1.7 16.2 ± 2.3
[7] [7] [8] [9] [7]
Sandblasted
Self-cured
Shear bond
38.7
[5]
META
Sandblasted
Four-point bend
23.8 ± 1.7 31.9 ± 1.5
[3] [10]
MDDT and MHPA (metal link primer)
META (super bond) META (new metacolor) MDP (Estenia opaque primer)
Sandblasted Sandblasted Sandblasted
Heat-cured Heat-cured (Trevalon) Heat-cured (Metadent) Heat-cured Heat-cured Self-cured Self-cured
16.5 ± 2.3
42.5 ± 2.2 Tensile strength Shear bond Shear bond Shear bond
Heat-cured MDP
Sandblasted
Heat-cured Self-cured with EGDMA and TBB (super-bond C&B)
Tensile strength
MDP (Cesead) MEPS (thermoresin) MPS and n-propylamine MAC (MR bond) DOPA
Sandblasted Sandblasted Polished Polished Polished
Self-cured Self-cured Heat-cured Heat-cured Heat-cured
Shear bond Shear bond Four-point bend Tensile strength Tensile strength
21.0 19.1 ± 8.9 21.5 ± 2.2 42.7
[2] [6] [9] [5]
7.0 ± 3.0
[6]
23.5 21.2 ± 4.7
[2] [11]
16.2 ± 5.9 19.0 ± 2.2 14.0 ± 0.6 25.5 ± 6.4 7.4 ± 2.1 1.8
[12] [9] [9] [13] [4] [14]
Abbreviations: BP: benzoyl peroxide; DOPA: 3,4-dihydroxyL-phenylalanine; MAC:11-metacryloyloxyundecan 1,1-dicarboxylic; MATP: Methacryloxypropyl-trimethoxysilane; MDDA:10-methacyloyloxydecyl 6,8-dithioctanoate; MDP:10-Methacryloyloxydecyl dihydrogen phosphate; MDDT:10-methacryloxydecly 6,8-dithiooctanoate; MEPS: methacryloxydecly thiophosphate derivative; META: Methacryloxy ethyl trimellitate anhydride; MHPA:6-Methacryloxyethexy phosphonacetate; MPS:3-Methacryloxypropyl trimethoxysilane; TBB:tribuylborane; VTD:10-Methacryloyloxydecyl dihydrogen phosphate. a It should be noted that the bond strength values are not comparable with each other as the specimen size, test methods and practice vary between various studies.
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Table 2 – Conditions tested in the second step. The overall solution volume was 12 ml, and was water-based. Groups
Metal
Solution
1 2 3
Ti Ti Ti
4
Ti
5
Ti
6
SS
None None MMA emulsion without SDS MMA emulsion with SDS MMA emulsion with SDS and initiator MMA emulsion with SDS and initiator
Abbreviation
HCl 0.5 M; NaNO2 0.05 M; Fe 0.25 g; MMA 2.0 ml (ml)
SDS (M)
[BP] (mg/ml)
Control D D+M
0 0 12
0 0 0
0 0 0
D+M+E
12
9 × 10−3
0
D+M+E+I
12
9 × 10−3
8–48
D+M+E+I
12
9 × 10−3
40
D: diazonium grafting (step 1); M: monomer (MMA); E: emulsifier (SDS); I: initiator (BP); Ti: titanium; SS: stainless steel.
The metallic samples used in the experiments were either orthodontic wrought wires (stainless steel) or polished rectangular bars (Ti). The wrought wires (Tur-Chrome S.S, Rocky Mountain Orthodontic, Denver, CO) had a diameter of 0.6 mm and were cut into 200.0 mm long sections. The Ti samples (Ti alloy grade 2, McMaster-Carr, Cleveland, OH) were obtained as rectangular bars (6.4, 12.7 and 305.0 mm) and cut into smaller sections (12.7, 6.4, and 6.4 mm) using an abrasive cutter (Delta AbrasiMet, Buchler, Whitby, ON).
2.2.
Preparation of the metallic samples
The Ti samples were polished using a six step polishing method to obtain a flat surface. First, they were polished by means of a water-cooled trimmer and 240–600 grit silicon carbide papers (Paper-c wt, AA Abrasives, Philadelphia, PA). Then, they were further polished on a polishing wheel (LapoPol5, Struers, Rodovre, Denmark) using two types of polishing cloths; rough-to-intermediate polishing cloth (15–0.02 m; TexMet C) and final polishing cloth (1–0.02 m; ChemoMet), with Colloidal Silica Suspension (≤0.06 m; MasterMet; Buchler, Whitby, ON). The orthodontic wrought wires did not undergo any specific preparation prior to surface treatment besides being cleaned. All metallic samples were cleaned in an ultrasonic bath (FS20D Ultrasonic, Fisher Scientific, Montreal, Canada) with DW, ethanol, and acetone for 5 min in each solution at 37 ◦ C.
2.3.
Surface treatment of the metallic samples
The surface treatment was performed in a two steps protocol based on p-phenylenediamine diazotization (primer and adhesive). Both steps were carried out in acidic DW solution at pH ≤ 2, since diazonium cations are stable at pH ≤ 2.5, at room temperature in a simple glass beaker [39,41]. The first step (primer) was conducted as follows: PPD (0.054 g; 0.05 M) and NaNO2 (0.034 g; 0.05 M) were dissolved in a glass beaker containing 10 ml of 0.5 M HCl. After ultrasonicating the solution for 5 min, all metallic samples except control group were immersed in the solution and Fe powder (0.250 g) was added as a reducing agent. The samples were left to react for 15 min before ultrasonicating them in DW and acetone for 5 min. This first step leads to spontaneous grafting of a
polyaminophenylene (PAP) layer on the metallic samples (i.e. titanium and stainless steel wrought wire). These samples are referred to as metal–PAP from here onwards. Different approaches were investigated in the second (adhesive) step in order to optimize the adhesion of MMA to metal–PAP samples. These approaches can be summarized in four groups (Table 2). All groups share the following process: NaNO2 (0.034 g; 0.05 M) was dissolved in 10 ml of 0.5 M HCl. Then, the metal–PAP samples were introduced in the solution before adding Fe powder (0.250 g). In the first group, only the monomer (MMA) was added to the solution. In groups 4, 5, and 6, a surfactant (SDS, 0.026 g) was added along with MMA to help emulsify the hydrophobic monomer [42,43]. The reaction was allowed to continue for 15 min in the ultrasonic bath and for another 30 min on the bench top; during this period the monomer polymerized and formed a layer of PMMA on the metallic surface. In groups 5 and 6, an initiator (benzoyl peroxide, BP) was added after the fifteen minute sonication stage to accelerate the polymerization reaction on the metallic surface. Finally, the samples were thoroughly rinsed with acetone, and then ultrasonicated in DW and acetone for 5 min in order to discard any ungrafted matter.
2.4.
Spectroscopic analysis
A monochromatic X-ray photoelectron spectrometer K Alpha (Thermo Fischer Scientific Inc, East Grinstead, UK) was used for determining the relative quantities and chemical environments of the elements on the Ti surfaces. The setup was equipped with an Al K␣ X-Ray radiation source (1486.6 eV, 0.834 nm), a micro-focused monochromator and an ultrahigh vacuum chamber (10−9 torr). For all the groups (control; D; D + M; D + M + E; D + M + E + I), survey scans were obtained over the range of 0–1350 eV with a pass energy of 200 eV at a step of 1.0 eV, while high resolution scans were collected with a pass energy of 50 eV at a step of 0.1 eV. Energies were calibrated by setting the binding energy of the carbon bonded to hydrogen or carbon (C (H, C)) at 285.0 eV on all samples. In addition, the samples were ion sputtered using an Argon gun operating at medium current and 500 eV to conduct depth profiling analysis. The sputtering time was 2 s per cycle and each spot was sputtered for seven cycles. These conditions correspond to a typical sputter rate of 0.28 nm/s on a Ta oxide (Ta2 O5 ) surface.
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speed of 10 mm/min. The tensile force was applied to the specimen until fracture occurred at the PMMA–Ti interface, and the strength of the bond between Ti and PMMA was calculated in megapascals (MPa). The specimens used to test the bond between wrought wire and PMMA were prepared using a different custom-made silicone mold. The resulting specimens consisted of 20 mm of wrought wire embedded vertically into a plate of PMMA (3 mm thick; 20 mm wide; 130 mm long). The acrylic plate and the wire were secured into a universal testing machine (as described earlier) in order to measure the shear strength of the bond between the wire and the PMMA (Fig. 1b).
2.7.
Statistical analysis
Statistical analysis on all XPS, contact angle, and mechanical test results was performed using Origin 8.0 (Origin lab, Northampton, MA). All the data were analyzed using nonparametric tests, Kruskal–Wallis test and the significance level was set at p < 0.05.
3. Fig. 1 – Schematic showing the testing specimens for (a) the tensile bond strength between Ti and PMMA; and (b) the shear bond strength between wrought wire and PMMA.
Data analysis and peak fitting were performed using Avantage (5.41v, Thermo Fischer Scientific Inc., East Grinstead, UK) chemical surface analysis software.
2.5.
Contact angle measurement
Hydrophobicity of the Ti surfaces in all groups was evaluated by the contact angle measurement that was recorded and analyzed at room temperature on contact angle meter (OAC 15, Data Physics, Germany). The static contact angle was automatically calculated by measuring the angle produced by a drop of DW (2 ml) placed on the Ti surface of each samples, and the side view images were captured.
2.6.
Mechanical tests
Tensile or shear strength tests were used to measure the bond strength between PMMA and the metallic surface. To test the Ti-PMAA bond strength, a custom-made mold was fabricated from a silicone (ExaktosilN 21, Bredent, Germany), and a piece of Ti was fixed in the middle of the mold. Then, a mix of PMMA powder and MMA liquid monomer (Biocryl Resin Acrylic, Great Leakes, NY) with ratio of 2:1 was poured to fill the sides of the mold. The PMMA was left to set for 3 h at room temperature and humidity. This procedure generated a final specimen that was 130 mm long, 13 mm wide, and 3 mm thick with two grips of bulk PMMA polymerized at the sides of the Ti samples (Fig. 1a). After complete setting of the acrylic resin, the tensile bond strength between PMMA and Ti was measured using a universal testing machine (H25K-S, Tinius Olsen Testing Machine Co., Inc Willow Grove, PA) set up at a constant
Results
XPS confirmed the grafting of a PAP layer on the Ti surface after the first step of the diazonium treatment (group D) by showing an increase of C and N contents from 17.9% and 0.5% in control samples up to 64.2% and 4.3%, respectively, as well as a decrease of Ti content from 20.5% in control samples to 6.2% in group D (Fig. 2a and b). The high resolution C 1s spectra showed that the components relative to hydrocarbon (C C/C H, centered at 285.0 eV), C O and C N groups (centered at 286.4 eV), and carboxyl groups (O C O, centered at 288.8 eV) changed from 61.2%, 26.7%, and 12.1%, respectively, in control samples to 79.6%, 18.3%, and 2.1%, respectively, in group D (Fig. 2c and d). In addition, XPS confirmed the presence of PMMA on the PAP grafted layer after the second step of the treatment in groups D + M + E and D + M + E + I by showing a decrease in N content down to 4.4% and 3.2%, respectively, and an increase of O content up to 31.7% and 42.6%, respectively, on the surface as well as an increase in the O C O component up to 9.1% and 17.6%, respectively, in the high resolution C 1s spectrum (Fig. 2). Contact angle measurements (Fig. 3) confirmed the presence of the organic, hydrophobic PAP layer on Ti samples in group D (p < 0.5) and the presence of the hydrophobic PMMA layer on Ti samples in groups D + M, D + M + E and D + M + E + I (p < 0.5). A DW contact angle of 53.5 ± 14.8◦ was measured for polished titanium before treatment; the contact angle changed to 84.7 ± 3.5◦ , 84.1 ± 1.9◦ , 83.1 ± 0.5◦ , and 82.4 ± 0.9◦ , respectively, for the samples of groups D, D + M, D + M + E and D + M + E + I after treatment. The mechanical test results (Fig. 4a) showed that the bond strength between Ti and PMMA was increased after grafting PAP layer (group D) on the Ti surface, from 1.54 ± 1.02 (control) to 2.33 ± 0.66 MPa. The bond strength was further increased after adding the MMA alone (2.6 ± 0.35 in group D + M) and with SDS (3.4 ± 1.2 MPa in group D + M + E) to the second step of the treatment. Adding the initiator benzoyl peroxide (BP) into the second step increased the bond strength even further, up to
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Fig. 2 – (a) XPS general surveys and (b) elemental compositions of Ti surface for different groups. (c) High resolution C 1s spectra on Ti surfaces for different groups; and (d) peaks concentrations from high resolution C 1s spectra. See Table 2 for what the groups are. * indicates significant difference between the different groups (p < 0.05).
8.14 ± 1.10 MPa in group D + M + E + I (p < 0.05). Similarly to what was obtained with Ti, the bonding strength between stainless steel wrought wire and PMMA (4.34 ± 0.68 MPa) was significantly higher when the wire was treated with the formulation D + M + E + I than when it was left untreated (1.71 ± 0.23 MPa) (Fig. 5).
4.
Discussion
The diazonium chemistry method that we used to bind PMMA and dental alloys consisted of two steps. In the first step, PPD was first transformed into an amino diazonium cation by adding one equivalent of NaNO2 (Fig. 6a). The diazonium
Fig. 3 – Photographs of water droplets placed on different Ti groups. * indicates significant difference between the different groups (p < 0.05).
cation was then reduced with Fe to achieve an aminophenyl radical (Fig. 6b). This radical spontaneously grafted onto the titanium surface (Fig. 6c), and kept reacting with itself forming multilayers (PAP, Fig. 6d) [39]. The second step (adhesive) of the reaction was optimized in order to achieve covalent binding between metals and PMMA for applications in dental prostheses. The amino groups of the PAP layer were reduced to radicals using again NaNO2 and Fe in an acidic environment, as in the first step (Fig. 6e). The radicals reacted with MMA, and partially polymerized MMA into metal–PAP layer (Fig. 6f). The addition of an emulsifier, such as SDS, seemed to have improved this outcome of the reaction probably by increasing the availability of MMA to the treated surface [42–45]. The use of BP in the reaction seemed to have increased the reactivity of MMA with PAP layer [46–49] (Fig. 6g). XPS results (Fig. 2a and b) showed that the Ti surface for the control (untreated) samples was covered by TiO2 and carbon; the carbon is most probably related to the unavoidable contamination upon exposition to air prior to XPS analysis [50]. The bond strength between the untreated polished Ti and PMMA was very low (1.54 ± 1.02 MPa), indicating that the mechanical and chemical bonds between Ti and PMMA were minimal (Fig. 4a). It is hard to find a comparison between this value and what is reported in the literature (Table 1), since the reported data for PMMA-Ti bonds on untreated samples
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Fig. 4 – (a) Tensile strength of the bond between PMMA and treated Ti surfaces. See Table 2 for what the groups are. (b) Tensile strength of the bond between PMMA and Ti as a function of BP concentration in the aqueous phase of MMA emulsion. * indicates significant difference between the different groups (p < 0.05).
refer to sandblasted Ti surfaces, which provides an increased mechanical bond. After sandblasting, the reported tensile strengths varied between 3.2 and 20.0 MPa [1,9,12,14–24,27]. In fact it is extremely difficult to test the bond between a polished Ti surface and PMMA, since the samples tend to fail very easily [25]. XPS confirmed the grafting of a PAP layer on the Ti surface after the first step of the diazonium treatment (Scheme 6d) by showing an increase of N and C contents and a decrease of Ti content in group D confirming the presence of the PAP layer covering the Ti surface (Fig. 2b). The high resolution C 1s spectra (Fig. 2c) showed that the components relative to C C/C H, C O/C N and O C O changed in group D. The drastic decrease of the carboxyl groups is especially indicative of the formation of the PAP layer, since no carboxyls are present in this layer. Contact angle measurements gave more evidence of the grafting of the PAP layer on Ti surfaces (Fig. 3). A DW contact angle of 53.5 ± 14.8◦ was measured for polished titanium before treatment; the contact angle changed
Fig. 5 – Shear bond strength of PMMA and stainless steel wires for the control group and stainless steel wrought wire that were treated with diazonium in MMA emulsion using the surfactant SDS and the initiator. * indicates significant difference between the different groups (p < 0.05).
to 84.7 ± 3.5◦ for the samples of group D, thus confirming the formation of the organic, hydrophobic PAP layer on these samples. The presence of the PAP layer increased the bond strength with PMMA, from 1.54 (control) to 2.33 MPa (Fig. 4a). This increase might be due to some entanglement between the PMMA chains and the PAP layer. These results indicate grafting of the PAP on the metallic samples was achieved successfully; however, the mechanical performance of this coating was limited. The second step of the treatment (adhesive) was designed to change the amino ends of the metal–PAP layer ( C6 H4 NH2 ) into diazonium radicals ( C6 H4 N2 ) and then grow a few layers of PMMA on it. As a first attempt, together with the reactants used to achieve the reduction of the NH2 group into the diazonium radical, we added the MMA monomer alone (group D + M). MMA reactivity with PAP layer appeared to be very limited in the aqueous environment assessed in this study even though MMA has some solubility in water phase [51]. In fact, XPS and contact angle results for the group D + M are quite similar to those of group D (Figs. 2 and 3), and the mechanical tests showed almost identical bond strengths for the groups D + M and D (Fig. 4a). The reason for the failure to grow PMMA in this condition is that MMA is a hydrophobic monomer, slightly soluble in the aqueous solution used to modify the Ti surface. XPS and mechanical tests indicate that the polymerization of PMMA in group D + M + E, which includes the addition of SDS to emulsify MMA in the second step, was better than in group D + M [43,45]. We added SDS with a concentration of 9 × 10−3 M, which is above SDS critical micelle concentration (8.2 × 10−3 M) [42]. This allowed us to polymerize MMA using emulsion instead of suspension polymerization [43,44]. The droplet size of such micelles is reported to be in the range of 30–100 nm [52]. In this group, some of the pure MMA (soluble in water) and MMA micelles could polymerize on the metal–PAP layer increasing the binding strength to the PMMA [42,53]. XPS confirmed the presence of PMMA on the grafted layer in group D + M + E by showing an increase in oxygen as well as a decrease in the concentration of Ti down to 3.9%,
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Fig. 6 – Schematic showing the reaction sequence performed in first (primer) and second (adhesive) steps. (a) diazotisation of p-phenylenediamine (PPD) in an acidic solution; (b) formation of aryl radicals by reducing aryldiazonium ions using iron powder; (c) attachment of aryl radicals to the metallic surfaces; (d) growing of polyaminophenylene layer (PAP) multilayer; (e) reduction of amino groups of the PAP layer to radicals; (f) binding of MMA to the activated PAP layer; (g) polymerizing PMMA with benzoyl peroxide.
indicating that PMMA was polymerized on the metal–PAP layer covering the Ti surface (Fig. 2b). Despite the slight decrease in overall C, the high resolution C 1s spectra for group D + M + E showed an increase in O C O group concentration compared to the previous groups, thus confirming PMMA polymerization [54–56] (Fig. 2c and d). The contact angle in group D + M + E was 83.1 ± 0.5◦ confirming the presence of hydrophobic layer on these samples (Fig. 3). The formation of a PMMA adhesive layer increased the tensile bond strength between PMMA and the treated Ti in group D + M + E up to 3.4 ± 1.2 MPa (p < 0.05) (Fig. 4a). Most likely this was due to the entanglement achieved between the PMMA chains in solution and those grown on the metal–PAP surface thanks to the better solubilization of the MMA monomer. To further increase the bond strength between PMMA and Ti, we added BP to help polymerization (group D +M + E + I).
Accelerating the PMMA Polymerization that grows on the metal–PAP layer was critical to increase the strength of the bond between Ti and PMMA [57]. PMMA polymerizes by free radical addition polymerization, which requires the presence of an initiator such as BP to start. BP is a relatively unstable compound, which forms radicals simply upon heating or irradiation [49]. BP radicals react with MMA and create MMA radicals, which then propagate and polymerize layers of PMMA [49,51]. BP is the most commonly used initiator for PMMA polymerization [46–48]. BP seems to increase the reactivity of MMA and its polymerization rate on the PAP layer [46–49]. However, excess amounts of BP can destroy part of the PAP layer or inhibit PMMA polymerization [49,58]. Thus, different concentrations of BP were added to the MMA emulsion, in order to investigate their effect on the Ti–PMMA bond strength. Our results demonstrated that Ti–PMMA bonding
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strength increased remarkably by adding BP of concentrations up to 40 mg/ml in the MMA emulsion. However, increasing the BP concentration beyond this point decreased the bonding strength (Fig. 4b). In group D + M + E + I, XPS showed the N concentration was negligible after PMMA polymerization with BP. This may indicate that BP reacted with the few aromatic NH2 still present on the metal–PAP surface, giving rise to a further polymeric initiator that are able to increase PMMA polymerization rate [58]. The formation of a PMMA layer is confirmed by the observed increase in O content on the surface of this sample and the increase in the O C O component in the high resolution C 1s spectrum (Fig. 2) [54–56]. Ti content was increased in this group due to the reactivity of BP that might destroyed part of the PAP layer [49]. Contact angle measurements in group D + M + E + I confirmed the formation of the hydrophobic PMMA layer on these samples. This contact angle was significantly higher than control samples and was similar to the other surfaces (D + M, D + M + E, and D + M + E + I), probably because they have similar hydrophobic PMMA groups that have hydrophobic layers on their surfaces probably because groups have similar hydrophobic PMMA layer on their surface (Fig. 3). The addition of BP leads to the strongest tensile bond between PMMA and Ti (8.14 ± 1.10 MPa). This bond strength was significantly higher than that achieved in any other group (p < 0.05) (Fig. 4). The resulting high bond strength indicates that formation of PMMA layer containing large number of PMMA chains on the top of metal–PAP layer that can entangle very strongly with the PMMA chains that are polymerized in the bulk PMMA casted on the sample. However, XPS depth profiling result (supplementary Fig. 1) revealed that the thickness of the coating layers was similar in all groups without any significant differences between them.
5.
Conclusion
The treatment of metallic surfaces (titanium and stainless steel) with diazonium ions in a two-step procedure where the second step includes an emulsion containing monomer (MMA), an emulsifier (SDS), and an initiator (BP) increase the bond strength of PMMA to Ti, and PMMA to stainless steel wrought wire by 5.2 and 2.5 folds respectively. Diazonium ions covalently bind onto metallic surface while the SDS and the BP help the polymerization of PMMA with diazonium layer to the metallic surface. Further improvements may be obtained by combining this technique with mechanical interlocking.
Acknowledgments The authors would like to acknowledge King Saud University in Riyadh, Saudi Arabia; Natural Sciences and Engineering Research Council (NSERC) of Canada–Discovery grant (F.T. and M.C.); Canada Research Chair Foundation (M.C); Canada Foundation for Innovation (CFI); and the Fondation de l’Ordre des dentists du Québec (FODQ), Le Réseau de recherche en santé Buccodentaire et osseuse (RSBO) for their financial support. Thanks to Enrique Lopez Cabarcos and Xuan Tuan Le for their technical support.
9
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.dental. 2014.11.002.
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Bonding metals to poly(methyl methacrylate) using aryldiazonium salts Omar Alageel a,b , Mohamed-Nur Abdallah a , Zhong Yuan Luo c , Jaime Del-Rio-Highsmith d , Marta Cerruti c,∗ , Faleh Tamimi a,∗∗ a
Faculty of Dentistry, McGill University, Montreal, QC, Canada College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia c Department of Mining and Materials Engineering, McGill University, Montreal, QC, Canada d Faculty of Dentistry, Complutense University of Madrid, Madrid, Spain b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Many dental devices, such as partial dentures, combine acrylic and metallic parts
Received 24 January 2014
that are bonded together. These devices often present catastrophic mechanical failures due
Received in revised form
to weak bonding between their acrylic and metallic components. The bonding between
15 July 2014
alloys and polymers (e.g. poly(methyl methacrylate), PMMA) usually is just a mechanical
Accepted 3 November 2014
interlock, since they do not chemically bond spontaneously. The aim of this study was to
Available online xxx
develop a new method to make a strong chemical bond between alloys and polymers for dental prostheses based on diazonium chemistry.
Keywords:
Methods. The method was based on two steps. In the first step (primer), aryldiazonium salts
Dental prosthesis
were grafted onto the metallic surfaces. The second step (adhesive) was optimized to achieve
Bonding diazonium
covalent binding between the grafted layer and PMMA. The chemical composition of the
Poly(methyl methacrylate)
treated surfaces was analyzed with X-ray photoelectron spectroscopy (XPS), and the tensile
Titanium
or shear bonding strength between metals and poly(methyl methacrylate) was measured.
Stainless steel
Results. XPS and contact angle measurements confirmed the presence of a polymer coating on the treated metallic surfaces. Mechanical tests showed a significant increase in bond strength between PMMA and treated titanium or stainless steel wire by 5.2 and 2.5 folds, respectively, compared to the untreated control group (p < 0.05). Significance. Diazonium chemistry is an effective technique for achieving a strong chemical bond between alloys and PMMA, which can help improve the mechanical properties of dental devices. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Correspondence to: Department of Mining and Materials Engineering, McGill University, Room 2M020, 3610 University Street, Montreal, QC H3A 0C5, Canada. Tel.: +1 514 398 5496; fax: +1 514 398 4492. ∗∗ Correspondence to: Faculty of Dentistry, McGill University, Room M64, 3640 University Street, Montreal, QC, H3A 0C7, Canada. Tel.: +1 514 398 7203x09654; fax: +1 514 398 8900. E-mail addresses: [email protected] (M. Cerruti), [email protected] (F. Tamimi).
http://dx.doi.org/10.1016/j.dental.2014.11.002 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
Introduction
Chemical bonding between alloys and polymers does not occur spontaneously; indeed, composite materials that combine polymers with alloys often suffer from mechanical failure at the interface between them. One example of this challenge is dental devices, which often present catastrophic mechanical failures due to weak bonding between their metallic and polymeric components [1–3]. These devices include dental prostheses, combining metallic frameworks and wrought wires with acrylic resin; and orthodontic appliances, combining acrylic resin with stainless steel wrought wires. Poly(methyl methacrylate) (PMMA) is extensively used in denture materials for dental prostheses and orthodontic devices because of its biocompatibility, excellent esthetic, and mechanical properties [4]. Titanium (Ti) is increasingly used in dental implants, implant abutments, and milled prostheses because of its excellent mechanical properties (i.e. strength to weight ratio) and biocompatibility [5]. PMMA and Ti in dental prostheses are usually bonded by mechanical interlocking the PMMA into the irregularities of the Ti surface [2,6,7]. Further improvement in the bonding strength between Ti and PMMA is still needed to prevent debonding, which are otherwise common in clinical practice, and reduce microleaks at the Ti/PMMA interface that causes accumulation of oral debris and discoloration of denture base materials [1,2,8]. Ti–PMMA bond can be strengthened by adding a chemical link between PMMA and Ti, since Ti and PMMA do not chemically bind together spontaneously [1,7,9–13]. Several methods have been tested to increase the bond strength between polymers and alloys in dental prostheses [1,9,12,14–24]. Table 1 summarizes the strengths obtained by binding PMMA and Ti with different methods, measured with shear bond, four-point bending, and tensile strength tests (it should be noted that the reported values are not comparable with each other as the specimen size, test methods and practice vary between different studies). In general, higher bond strengths are reported for the four-point bending and the shear bond tests (values ranging between 25.5–42.5 MPa and 7.0–46.6 MPa, respectively), while the lowest values are obtained with the tensile strength test (0–23.5 MPa). Indeed, the latter test is the most challenging one; however, it is also the most accurate technique to measure bond strength because it applies a direct and uniform force to the surface [25]. On the contrary, the shear bond and four-point bend tests do not distribute stress uniformly on the surfaces being tested [26]. Most of the methods reported in Table 1 require sandblasting the metallic surface, and all of them use either silane or phosphonate groups to create a chemical bond between the two surfaces [1,9,12,14–24]. Silanes and phosphonates covalently bind to Ti, while sandblasting increases the surface area of the exposed Ti, thus increasing the overall bonding strength [27]. The highest bond strengths reported were achieved using phosphonate-based adhesives (MHPA, MDP, and VDT; see Table 1) in combination with sandblasting. Specifically, the highest tensile strength reported without sandblasting was 7.4 MPa [12], while using a combination of sandblasting and bonding agents the tensile strength went up to 23.5 MPa
[12,15,17,19–21,24]. These values are still too low for dental applications. Overdentures, for example, have to resist biting forces of up to 662.2 N, and pressures of up to 51.1 MPa [28]. This implies that masticatory forces can exceed the strength of the Ti–PMMA bond and lead to prosthesis failure. An ideal goal would be to have a metal/PMMA interface that is at least as strong as PMMA alone, which has a tensile strength of 65 MPa [29]. Another example of metal–acrylic interface found in dental applications is that between wrought wires and acrylic-based dental devices such as dental prostheses and orthodontic appliances [30]. Wrought wires are usually made of stainless steel or cobalt–chromium alloys, which both lack the ability to bind chemically to acrylic resins [31]. Surprisingly, improving the adhesion between wrought wires and acrylic has hardly been investigated. Dental devices combining wrought wires with acrylic such as acrylic removable partial dentures usually cannot be made when not enough volume of PMMA is available to support the wire [30,31]. By increasing bond strength between stainless steel wrought wire and PMMA through this treatment, more leverage is possible for fabricating acrylicbased dental devices when not enough volume of acrylic is available to support the wire. In this paper we will show a technique to improve the binding between PMMA and alloys used in dental applications based on diazonium chemistry. Aryldiazonium salts have been used to modify material surfaces for many applications [32,33]. Diazonium ions can be produced from aromatic amines and grafted onto almost any surface, including metals, glass, and carbon [34–37]. Initially, diazonium grafting was performed using electrochemical reduction, but recently this has been achieved using chemical reducing agents in acidic solutions [38]. The reducing agents transform the aryldiazonium salts into aryl radicals, which can covalently bind to the surface of interest [39,40]. If an extra amino group is present on the aryldiazonium precursor, a polyaminophenylene (PAP) layer is formed on the metallic surface. The amino groups sticking out from the PAP layer can be further activated in a second step, and used to bind a second layer onto the original surface [39,41]. In this work, we optimize such second step to bind PMMA and metals for dental applications. The aim of this study was to develop a new method of creating a strong chemical bond between alloys and polymers for dental prostheses based on diazonium chemistry.
2.
Materials and methods
2.1.
Materials
Poly-methyl methacrylate (PMMA) and methyl methacrylate (MMA) were obtained from Great Lakes Orthodontics (Tonawanda, NY), and were used without any further purification. The rest of the reagents were obtained from Sigma Aldrich (St. Louis, MO). P-phenylenediamine (PPD), sodium nitrite (NaNO2 ), sodium dodecyl sulfate (SDS), benzoyl peroxide (BP), and iron powder (Fe) were used as received. Concentrated hydrochloric acid (HCl) was diluted in distilled water (DW) to a concentration of 0.5 M.
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Table 1 – The bond strengths between titanium and PMMA (MPa) using different bonding methods. Bonding agent (commercial name)
Surface topography
Type of PMMA used (commercial name)
None
Sandblasted
Self-cured with EGDMA and TBB (super-bond C&B) Heat-cured
Shear bond
38.1 ± 2.3
[1]
Tensile strength
Self-cured with BP (multi-bond) Self-cured Self-cured Self-cured Self-cured with EGDMA and TBB (super-bond C&B) Self-cured with BP (multi-bond) Heat-cured
Shear bond
20.0 16.1 ± 1.6 3.2 ± 0.4 13.6 ± 1.6
[2] [3] [4] [1]
9.9 46.6 45.7 39.8 ± 2.0
[5] [5] [5] [1]
MHPA (AZ Primer) MDP and VTD (alloy primer)
Sandblasted Sandblasted
Testing technique
Shear bond Shear bond
Bond strength (MPa)a
Ref.
22.0 ± 6.6
Tensile strength Shear bond
27.5 ± 4.0 16.0 ± 3.6 45.4
[6] [4] [5]
39.6 ± 2.5
[1]
Sandblasted
Self-cured
MATP (Espe-Sil) MATP (Espe-Sil) MATP MATP (silicoater M D) MATP then Silica-coating (Espe-Sil; Rocatec system) Silica-coating (Rocatec system)
Polished Sandblasted Sandblasted Sandblasted Sandblasted
Self-cured with EGDMA and TBB (super-bond C&B) Self-cured with BP (multi-bond) Heat-cured Heat-cured Self-cured Self-cured Heat-cured
Shear bond Shear bond Tensile strength Shear bond Shear bond
0.0 5.9 ± 2.1 14.3 21.9± 1.7 16.2 ± 2.3
[7] [7] [8] [9] [7]
Sandblasted
Self-cured
Shear bond
38.7
[5]
META
Sandblasted
Four-point bend
23.8 ± 1.7 31.9 ± 1.5
[3] [10]
MDDT and MHPA (metal link primer)
META (super bond) META (new metacolor) MDP (Estenia opaque primer)
Sandblasted Sandblasted Sandblasted
Heat-cured Heat-cured (Trevalon) Heat-cured (Metadent) Heat-cured Heat-cured Self-cured Self-cured
16.5 ± 2.3
42.5 ± 2.2 Tensile strength Shear bond Shear bond Shear bond
Heat-cured MDP
Sandblasted
Heat-cured Self-cured with EGDMA and TBB (super-bond C&B)
Tensile strength
MDP (Cesead) MEPS (thermoresin) MPS and n-propylamine MAC (MR bond) DOPA
Sandblasted Sandblasted Polished Polished Polished
Self-cured Self-cured Heat-cured Heat-cured Heat-cured
Shear bond Shear bond Four-point bend Tensile strength Tensile strength
21.0 19.1 ± 8.9 21.5 ± 2.2 42.7
[2] [6] [9] [5]
7.0 ± 3.0
[6]
23.5 21.2 ± 4.7
[2] [11]
16.2 ± 5.9 19.0 ± 2.2 14.0 ± 0.6 25.5 ± 6.4 7.4 ± 2.1 1.8
[12] [9] [9] [13] [4] [14]
Abbreviations: BP: benzoyl peroxide; DOPA: 3,4-dihydroxyL-phenylalanine; MAC:11-metacryloyloxyundecan 1,1-dicarboxylic; MATP: Methacryloxypropyl-trimethoxysilane; MDDA:10-methacyloyloxydecyl 6,8-dithioctanoate; MDP:10-Methacryloyloxydecyl dihydrogen phosphate; MDDT:10-methacryloxydecly 6,8-dithiooctanoate; MEPS: methacryloxydecly thiophosphate derivative; META: Methacryloxy ethyl trimellitate anhydride; MHPA:6-Methacryloxyethexy phosphonacetate; MPS:3-Methacryloxypropyl trimethoxysilane; TBB:tribuylborane; VTD:10-Methacryloyloxydecyl dihydrogen phosphate. a It should be noted that the bond strength values are not comparable with each other as the specimen size, test methods and practice vary between various studies.
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Table 2 – Conditions tested in the second step. The overall solution volume was 12 ml, and was water-based. Groups
Metal
Solution
1 2 3
Ti Ti Ti
4
Ti
5
Ti
6
SS
None None MMA emulsion without SDS MMA emulsion with SDS MMA emulsion with SDS and initiator MMA emulsion with SDS and initiator
Abbreviation
HCl 0.5 M; NaNO2 0.05 M; Fe 0.25 g; MMA 2.0 ml (ml)
SDS (M)
[BP] (mg/ml)
Control D D+M
0 0 12
0 0 0
0 0 0
D+M+E
12
9 × 10−3
0
D+M+E+I
12
9 × 10−3
8–48
D+M+E+I
12
9 × 10−3
40
D: diazonium grafting (step 1); M: monomer (MMA); E: emulsifier (SDS); I: initiator (BP); Ti: titanium; SS: stainless steel.
The metallic samples used in the experiments were either orthodontic wrought wires (stainless steel) or polished rectangular bars (Ti). The wrought wires (Tur-Chrome S.S, Rocky Mountain Orthodontic, Denver, CO) had a diameter of 0.6 mm and were cut into 200.0 mm long sections. The Ti samples (Ti alloy grade 2, McMaster-Carr, Cleveland, OH) were obtained as rectangular bars (6.4, 12.7 and 305.0 mm) and cut into smaller sections (12.7, 6.4, and 6.4 mm) using an abrasive cutter (Delta AbrasiMet, Buchler, Whitby, ON).
2.2.
Preparation of the metallic samples
The Ti samples were polished using a six step polishing method to obtain a flat surface. First, they were polished by means of a water-cooled trimmer and 240–600 grit silicon carbide papers (Paper-c wt, AA Abrasives, Philadelphia, PA). Then, they were further polished on a polishing wheel (LapoPol5, Struers, Rodovre, Denmark) using two types of polishing cloths; rough-to-intermediate polishing cloth (15–0.02 m; TexMet C) and final polishing cloth (1–0.02 m; ChemoMet), with Colloidal Silica Suspension (≤0.06 m; MasterMet; Buchler, Whitby, ON). The orthodontic wrought wires did not undergo any specific preparation prior to surface treatment besides being cleaned. All metallic samples were cleaned in an ultrasonic bath (FS20D Ultrasonic, Fisher Scientific, Montreal, Canada) with DW, ethanol, and acetone for 5 min in each solution at 37 ◦ C.
2.3.
Surface treatment of the metallic samples
The surface treatment was performed in a two steps protocol based on p-phenylenediamine diazotization (primer and adhesive). Both steps were carried out in acidic DW solution at pH ≤ 2, since diazonium cations are stable at pH ≤ 2.5, at room temperature in a simple glass beaker [39,41]. The first step (primer) was conducted as follows: PPD (0.054 g; 0.05 M) and NaNO2 (0.034 g; 0.05 M) were dissolved in a glass beaker containing 10 ml of 0.5 M HCl. After ultrasonicating the solution for 5 min, all metallic samples except control group were immersed in the solution and Fe powder (0.250 g) was added as a reducing agent. The samples were left to react for 15 min before ultrasonicating them in DW and acetone for 5 min. This first step leads to spontaneous grafting of a
polyaminophenylene (PAP) layer on the metallic samples (i.e. titanium and stainless steel wrought wire). These samples are referred to as metal–PAP from here onwards. Different approaches were investigated in the second (adhesive) step in order to optimize the adhesion of MMA to metal–PAP samples. These approaches can be summarized in four groups (Table 2). All groups share the following process: NaNO2 (0.034 g; 0.05 M) was dissolved in 10 ml of 0.5 M HCl. Then, the metal–PAP samples were introduced in the solution before adding Fe powder (0.250 g). In the first group, only the monomer (MMA) was added to the solution. In groups 4, 5, and 6, a surfactant (SDS, 0.026 g) was added along with MMA to help emulsify the hydrophobic monomer [42,43]. The reaction was allowed to continue for 15 min in the ultrasonic bath and for another 30 min on the bench top; during this period the monomer polymerized and formed a layer of PMMA on the metallic surface. In groups 5 and 6, an initiator (benzoyl peroxide, BP) was added after the fifteen minute sonication stage to accelerate the polymerization reaction on the metallic surface. Finally, the samples were thoroughly rinsed with acetone, and then ultrasonicated in DW and acetone for 5 min in order to discard any ungrafted matter.
2.4.
Spectroscopic analysis
A monochromatic X-ray photoelectron spectrometer K Alpha (Thermo Fischer Scientific Inc, East Grinstead, UK) was used for determining the relative quantities and chemical environments of the elements on the Ti surfaces. The setup was equipped with an Al K␣ X-Ray radiation source (1486.6 eV, 0.834 nm), a micro-focused monochromator and an ultrahigh vacuum chamber (10−9 torr). For all the groups (control; D; D + M; D + M + E; D + M + E + I), survey scans were obtained over the range of 0–1350 eV with a pass energy of 200 eV at a step of 1.0 eV, while high resolution scans were collected with a pass energy of 50 eV at a step of 0.1 eV. Energies were calibrated by setting the binding energy of the carbon bonded to hydrogen or carbon (C (H, C)) at 285.0 eV on all samples. In addition, the samples were ion sputtered using an Argon gun operating at medium current and 500 eV to conduct depth profiling analysis. The sputtering time was 2 s per cycle and each spot was sputtered for seven cycles. These conditions correspond to a typical sputter rate of 0.28 nm/s on a Ta oxide (Ta2 O5 ) surface.
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speed of 10 mm/min. The tensile force was applied to the specimen until fracture occurred at the PMMA–Ti interface, and the strength of the bond between Ti and PMMA was calculated in megapascals (MPa). The specimens used to test the bond between wrought wire and PMMA were prepared using a different custom-made silicone mold. The resulting specimens consisted of 20 mm of wrought wire embedded vertically into a plate of PMMA (3 mm thick; 20 mm wide; 130 mm long). The acrylic plate and the wire were secured into a universal testing machine (as described earlier) in order to measure the shear strength of the bond between the wire and the PMMA (Fig. 1b).
2.7.
Statistical analysis
Statistical analysis on all XPS, contact angle, and mechanical test results was performed using Origin 8.0 (Origin lab, Northampton, MA). All the data were analyzed using nonparametric tests, Kruskal–Wallis test and the significance level was set at p < 0.05.
3. Fig. 1 – Schematic showing the testing specimens for (a) the tensile bond strength between Ti and PMMA; and (b) the shear bond strength between wrought wire and PMMA.
Data analysis and peak fitting were performed using Avantage (5.41v, Thermo Fischer Scientific Inc., East Grinstead, UK) chemical surface analysis software.
2.5.
Contact angle measurement
Hydrophobicity of the Ti surfaces in all groups was evaluated by the contact angle measurement that was recorded and analyzed at room temperature on contact angle meter (OAC 15, Data Physics, Germany). The static contact angle was automatically calculated by measuring the angle produced by a drop of DW (2 ml) placed on the Ti surface of each samples, and the side view images were captured.
2.6.
Mechanical tests
Tensile or shear strength tests were used to measure the bond strength between PMMA and the metallic surface. To test the Ti-PMAA bond strength, a custom-made mold was fabricated from a silicone (ExaktosilN 21, Bredent, Germany), and a piece of Ti was fixed in the middle of the mold. Then, a mix of PMMA powder and MMA liquid monomer (Biocryl Resin Acrylic, Great Leakes, NY) with ratio of 2:1 was poured to fill the sides of the mold. The PMMA was left to set for 3 h at room temperature and humidity. This procedure generated a final specimen that was 130 mm long, 13 mm wide, and 3 mm thick with two grips of bulk PMMA polymerized at the sides of the Ti samples (Fig. 1a). After complete setting of the acrylic resin, the tensile bond strength between PMMA and Ti was measured using a universal testing machine (H25K-S, Tinius Olsen Testing Machine Co., Inc Willow Grove, PA) set up at a constant
Results
XPS confirmed the grafting of a PAP layer on the Ti surface after the first step of the diazonium treatment (group D) by showing an increase of C and N contents from 17.9% and 0.5% in control samples up to 64.2% and 4.3%, respectively, as well as a decrease of Ti content from 20.5% in control samples to 6.2% in group D (Fig. 2a and b). The high resolution C 1s spectra showed that the components relative to hydrocarbon (C C/C H, centered at 285.0 eV), C O and C N groups (centered at 286.4 eV), and carboxyl groups (O C O, centered at 288.8 eV) changed from 61.2%, 26.7%, and 12.1%, respectively, in control samples to 79.6%, 18.3%, and 2.1%, respectively, in group D (Fig. 2c and d). In addition, XPS confirmed the presence of PMMA on the PAP grafted layer after the second step of the treatment in groups D + M + E and D + M + E + I by showing a decrease in N content down to 4.4% and 3.2%, respectively, and an increase of O content up to 31.7% and 42.6%, respectively, on the surface as well as an increase in the O C O component up to 9.1% and 17.6%, respectively, in the high resolution C 1s spectrum (Fig. 2). Contact angle measurements (Fig. 3) confirmed the presence of the organic, hydrophobic PAP layer on Ti samples in group D (p < 0.5) and the presence of the hydrophobic PMMA layer on Ti samples in groups D + M, D + M + E and D + M + E + I (p < 0.5). A DW contact angle of 53.5 ± 14.8◦ was measured for polished titanium before treatment; the contact angle changed to 84.7 ± 3.5◦ , 84.1 ± 1.9◦ , 83.1 ± 0.5◦ , and 82.4 ± 0.9◦ , respectively, for the samples of groups D, D + M, D + M + E and D + M + E + I after treatment. The mechanical test results (Fig. 4a) showed that the bond strength between Ti and PMMA was increased after grafting PAP layer (group D) on the Ti surface, from 1.54 ± 1.02 (control) to 2.33 ± 0.66 MPa. The bond strength was further increased after adding the MMA alone (2.6 ± 0.35 in group D + M) and with SDS (3.4 ± 1.2 MPa in group D + M + E) to the second step of the treatment. Adding the initiator benzoyl peroxide (BP) into the second step increased the bond strength even further, up to
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Fig. 2 – (a) XPS general surveys and (b) elemental compositions of Ti surface for different groups. (c) High resolution C 1s spectra on Ti surfaces for different groups; and (d) peaks concentrations from high resolution C 1s spectra. See Table 2 for what the groups are. * indicates significant difference between the different groups (p < 0.05).
8.14 ± 1.10 MPa in group D + M + E + I (p < 0.05). Similarly to what was obtained with Ti, the bonding strength between stainless steel wrought wire and PMMA (4.34 ± 0.68 MPa) was significantly higher when the wire was treated with the formulation D + M + E + I than when it was left untreated (1.71 ± 0.23 MPa) (Fig. 5).
4.
Discussion
The diazonium chemistry method that we used to bind PMMA and dental alloys consisted of two steps. In the first step, PPD was first transformed into an amino diazonium cation by adding one equivalent of NaNO2 (Fig. 6a). The diazonium
Fig. 3 – Photographs of water droplets placed on different Ti groups. * indicates significant difference between the different groups (p < 0.05).
cation was then reduced with Fe to achieve an aminophenyl radical (Fig. 6b). This radical spontaneously grafted onto the titanium surface (Fig. 6c), and kept reacting with itself forming multilayers (PAP, Fig. 6d) [39]. The second step (adhesive) of the reaction was optimized in order to achieve covalent binding between metals and PMMA for applications in dental prostheses. The amino groups of the PAP layer were reduced to radicals using again NaNO2 and Fe in an acidic environment, as in the first step (Fig. 6e). The radicals reacted with MMA, and partially polymerized MMA into metal–PAP layer (Fig. 6f). The addition of an emulsifier, such as SDS, seemed to have improved this outcome of the reaction probably by increasing the availability of MMA to the treated surface [42–45]. The use of BP in the reaction seemed to have increased the reactivity of MMA with PAP layer [46–49] (Fig. 6g). XPS results (Fig. 2a and b) showed that the Ti surface for the control (untreated) samples was covered by TiO2 and carbon; the carbon is most probably related to the unavoidable contamination upon exposition to air prior to XPS analysis [50]. The bond strength between the untreated polished Ti and PMMA was very low (1.54 ± 1.02 MPa), indicating that the mechanical and chemical bonds between Ti and PMMA were minimal (Fig. 4a). It is hard to find a comparison between this value and what is reported in the literature (Table 1), since the reported data for PMMA-Ti bonds on untreated samples
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Fig. 4 – (a) Tensile strength of the bond between PMMA and treated Ti surfaces. See Table 2 for what the groups are. (b) Tensile strength of the bond between PMMA and Ti as a function of BP concentration in the aqueous phase of MMA emulsion. * indicates significant difference between the different groups (p < 0.05).
refer to sandblasted Ti surfaces, which provides an increased mechanical bond. After sandblasting, the reported tensile strengths varied between 3.2 and 20.0 MPa [1,9,12,14–24,27]. In fact it is extremely difficult to test the bond between a polished Ti surface and PMMA, since the samples tend to fail very easily [25]. XPS confirmed the grafting of a PAP layer on the Ti surface after the first step of the diazonium treatment (Scheme 6d) by showing an increase of N and C contents and a decrease of Ti content in group D confirming the presence of the PAP layer covering the Ti surface (Fig. 2b). The high resolution C 1s spectra (Fig. 2c) showed that the components relative to C C/C H, C O/C N and O C O changed in group D. The drastic decrease of the carboxyl groups is especially indicative of the formation of the PAP layer, since no carboxyls are present in this layer. Contact angle measurements gave more evidence of the grafting of the PAP layer on Ti surfaces (Fig. 3). A DW contact angle of 53.5 ± 14.8◦ was measured for polished titanium before treatment; the contact angle changed
Fig. 5 – Shear bond strength of PMMA and stainless steel wires for the control group and stainless steel wrought wire that were treated with diazonium in MMA emulsion using the surfactant SDS and the initiator. * indicates significant difference between the different groups (p < 0.05).
to 84.7 ± 3.5◦ for the samples of group D, thus confirming the formation of the organic, hydrophobic PAP layer on these samples. The presence of the PAP layer increased the bond strength with PMMA, from 1.54 (control) to 2.33 MPa (Fig. 4a). This increase might be due to some entanglement between the PMMA chains and the PAP layer. These results indicate grafting of the PAP on the metallic samples was achieved successfully; however, the mechanical performance of this coating was limited. The second step of the treatment (adhesive) was designed to change the amino ends of the metal–PAP layer ( C6 H4 NH2 ) into diazonium radicals ( C6 H4 N2 ) and then grow a few layers of PMMA on it. As a first attempt, together with the reactants used to achieve the reduction of the NH2 group into the diazonium radical, we added the MMA monomer alone (group D + M). MMA reactivity with PAP layer appeared to be very limited in the aqueous environment assessed in this study even though MMA has some solubility in water phase [51]. In fact, XPS and contact angle results for the group D + M are quite similar to those of group D (Figs. 2 and 3), and the mechanical tests showed almost identical bond strengths for the groups D + M and D (Fig. 4a). The reason for the failure to grow PMMA in this condition is that MMA is a hydrophobic monomer, slightly soluble in the aqueous solution used to modify the Ti surface. XPS and mechanical tests indicate that the polymerization of PMMA in group D + M + E, which includes the addition of SDS to emulsify MMA in the second step, was better than in group D + M [43,45]. We added SDS with a concentration of 9 × 10−3 M, which is above SDS critical micelle concentration (8.2 × 10−3 M) [42]. This allowed us to polymerize MMA using emulsion instead of suspension polymerization [43,44]. The droplet size of such micelles is reported to be in the range of 30–100 nm [52]. In this group, some of the pure MMA (soluble in water) and MMA micelles could polymerize on the metal–PAP layer increasing the binding strength to the PMMA [42,53]. XPS confirmed the presence of PMMA on the grafted layer in group D + M + E by showing an increase in oxygen as well as a decrease in the concentration of Ti down to 3.9%,
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Fig. 6 – Schematic showing the reaction sequence performed in first (primer) and second (adhesive) steps. (a) diazotisation of p-phenylenediamine (PPD) in an acidic solution; (b) formation of aryl radicals by reducing aryldiazonium ions using iron powder; (c) attachment of aryl radicals to the metallic surfaces; (d) growing of polyaminophenylene layer (PAP) multilayer; (e) reduction of amino groups of the PAP layer to radicals; (f) binding of MMA to the activated PAP layer; (g) polymerizing PMMA with benzoyl peroxide.
indicating that PMMA was polymerized on the metal–PAP layer covering the Ti surface (Fig. 2b). Despite the slight decrease in overall C, the high resolution C 1s spectra for group D + M + E showed an increase in O C O group concentration compared to the previous groups, thus confirming PMMA polymerization [54–56] (Fig. 2c and d). The contact angle in group D + M + E was 83.1 ± 0.5◦ confirming the presence of hydrophobic layer on these samples (Fig. 3). The formation of a PMMA adhesive layer increased the tensile bond strength between PMMA and the treated Ti in group D + M + E up to 3.4 ± 1.2 MPa (p < 0.05) (Fig. 4a). Most likely this was due to the entanglement achieved between the PMMA chains in solution and those grown on the metal–PAP surface thanks to the better solubilization of the MMA monomer. To further increase the bond strength between PMMA and Ti, we added BP to help polymerization (group D +M + E + I).
Accelerating the PMMA Polymerization that grows on the metal–PAP layer was critical to increase the strength of the bond between Ti and PMMA [57]. PMMA polymerizes by free radical addition polymerization, which requires the presence of an initiator such as BP to start. BP is a relatively unstable compound, which forms radicals simply upon heating or irradiation [49]. BP radicals react with MMA and create MMA radicals, which then propagate and polymerize layers of PMMA [49,51]. BP is the most commonly used initiator for PMMA polymerization [46–48]. BP seems to increase the reactivity of MMA and its polymerization rate on the PAP layer [46–49]. However, excess amounts of BP can destroy part of the PAP layer or inhibit PMMA polymerization [49,58]. Thus, different concentrations of BP were added to the MMA emulsion, in order to investigate their effect on the Ti–PMMA bond strength. Our results demonstrated that Ti–PMMA bonding
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strength increased remarkably by adding BP of concentrations up to 40 mg/ml in the MMA emulsion. However, increasing the BP concentration beyond this point decreased the bonding strength (Fig. 4b). In group D + M + E + I, XPS showed the N concentration was negligible after PMMA polymerization with BP. This may indicate that BP reacted with the few aromatic NH2 still present on the metal–PAP surface, giving rise to a further polymeric initiator that are able to increase PMMA polymerization rate [58]. The formation of a PMMA layer is confirmed by the observed increase in O content on the surface of this sample and the increase in the O C O component in the high resolution C 1s spectrum (Fig. 2) [54–56]. Ti content was increased in this group due to the reactivity of BP that might destroyed part of the PAP layer [49]. Contact angle measurements in group D + M + E + I confirmed the formation of the hydrophobic PMMA layer on these samples. This contact angle was significantly higher than control samples and was similar to the other surfaces (D + M, D + M + E, and D + M + E + I), probably because they have similar hydrophobic PMMA groups that have hydrophobic layers on their surfaces probably because groups have similar hydrophobic PMMA layer on their surface (Fig. 3). The addition of BP leads to the strongest tensile bond between PMMA and Ti (8.14 ± 1.10 MPa). This bond strength was significantly higher than that achieved in any other group (p < 0.05) (Fig. 4). The resulting high bond strength indicates that formation of PMMA layer containing large number of PMMA chains on the top of metal–PAP layer that can entangle very strongly with the PMMA chains that are polymerized in the bulk PMMA casted on the sample. However, XPS depth profiling result (supplementary Fig. 1) revealed that the thickness of the coating layers was similar in all groups without any significant differences between them.
5.
Conclusion
The treatment of metallic surfaces (titanium and stainless steel) with diazonium ions in a two-step procedure where the second step includes an emulsion containing monomer (MMA), an emulsifier (SDS), and an initiator (BP) increase the bond strength of PMMA to Ti, and PMMA to stainless steel wrought wire by 5.2 and 2.5 folds respectively. Diazonium ions covalently bind onto metallic surface while the SDS and the BP help the polymerization of PMMA with diazonium layer to the metallic surface. Further improvements may be obtained by combining this technique with mechanical interlocking.
Acknowledgments The authors would like to acknowledge King Saud University in Riyadh, Saudi Arabia; Natural Sciences and Engineering Research Council (NSERC) of Canada–Discovery grant (F.T. and M.C.); Canada Research Chair Foundation (M.C); Canada Foundation for Innovation (CFI); and the Fondation de l’Ordre des dentists du Québec (FODQ), Le Réseau de recherche en santé Buccodentaire et osseuse (RSBO) for their financial support. Thanks to Enrique Lopez Cabarcos and Xuan Tuan Le for their technical support.
9
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.dental. 2014.11.002.
references
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