Graphical abstract
Highlights
- •
Time-of-flight secondary ion mass spectrometry functions in analyzing organic coating.
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The covalent bond between MDP and zirconia is verified by zirconium phosphate-related ions.
- •
A sequential application of MDP and MPS primers does not improve zirconia–resin bonding.
Abstract
Objective
To evaluate the 3-methacryloyloxypropyltrimethoxysilane (MPS)- and 10-methacryloyloxydecyl-dihydrogen-phosphate (MDP)-base primers, in their single or sequential applications, with regard to modifying zirconia surfaces and improving resin–zirconia adhesion.
Methods
Zirconia disks received different treatments: without primer (Zr), MPS-base primer (S), MDP-base primer (M), MPS/MDP mixture (SMmix), MPS followed by MDP (SM), and MDP followed by MPS (MS). The compositions and chemical interactions of the coatings to zirconia were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and reconstructed 3D ion images. Surface wettability of these coatings to water and resin adhesive was assessed. The shear bond strength (SBS) between resin and the treated zirconia was also examined before and after thermocycling.
Results
Groups S and MS presented substantial OH − ions in the coatings and zirconia substrate. PO 2 − and PO 3 − fragments existed in all MDP-treatment groups with various proportions and distributions, while groups M and SM showed higher proportions of PO 3 − and the zirconium phosphate related ions. In 3D ion images, PO 3 − in groups M and SM was denser and segregated to the interface, but was dispersed or overlaid above PO 2 − in SMmix and MS. All the primers increased the surface wettability to water and resin, with M and SM presenting superhydrophilic surfaces. All MDP-treatment groups showed improved SBS before thermocycling, while M and SM retained higher SBS after this.
Significance
The MDP-base primer shows a relevant function in facilitating P O Zr bonding and enhancing resin–zirconia bonding. The co-treated MPS impairs the chemical activity of MDP, especially if it is the final coat.
1
Introduction
Yttria-partially-stabilized tetragonal zirconia (YTZ) has been widely used in prosthodontic reconstructions due to its stable chemical properties, high mechanical strength, and aesthetic appearance. With these properties, it can be applied in both anterior and posterior tooth regions to support physiological chewing functions. However, the chemical stability and toughness of YTZ also indicates its low susceptibility to surface preparation for resin adhesion. YTZ is resistant to most acids, except strong hazardous hydrofluoric acids . Mechanical grinding and sandblasting increase the retention of zirconia restorations, but may cause phase changes that accelerate hydrothermal and fatigue degradation . In terms of chemical modification, both silane and 10-methacryloxydecyl dihydrogen phosphate (MDP) have been recommended to improve resin–zirconia adhesion.
The application of silane coupling agents is a standard protocol to establish the bonding between resin and silica-base ceramics . Bifunctional silane molecules react with ceramics through the formation of siloxane bonds and copolymerization with organic resin . The commonly used silane agents are trialkoxysilanes, such as 3-methacryloyloxypropyltrimethoxysilane (MPS), 3-acryloyloxypropyl-trimethoxysilane (ACPS), and 3-isocyanatopropyl-triethoxysilane (ICS). Due to the lack of silica in YTZ, effective chemical bonds with resin cannot be generated. Some investigators have thus developed novel silane systems , or combined silane and a tribochemical coating as alternative approaches. A previous meta-analysis reported that silanization to silica-coated YTZ rendered the bonding resistant to hydrolysis . However, the results have not achieved significant improvements in bond strength, and the effects of silane when used on zirconia need to be further examined.
Organo-phosphate monomers such as MDP, 6-methacryloyloxy phosphonacetate (6-MHPA), or 4-methacryloyloxyethyl trimellitate anhydride (4-META) have been introduced for the pre-cementation preparation of YTZ restorations. MDP exhibits a phosphate ester group that reacts with zirconia and forms the P O Zr covalent bond, a hydrophobic alkylene group that prevents water penetration, and a terminal double bond group to copolymerize with resin . MDP has been incorporated in luting cements to improve resin adhesion . MDP primers, in simple application or combined with silicalization, also show good performance . In the evolution of dentin adhesives, new universal systems incorporate MDP as an acidic monomer to treat both dentin and ceramic restorations, thus forming a zirconia–resin–dentin complex .
Regardless of the improvements achieved with MDP primer, the bonding is still susceptible to hydrolytic degradation . MDP is often supplied as a ceramic primer which also contains a polymerizable monomer, suitable solvent, and silane. Primers comprising of both MDP and silane show enhanced resin–zirconia bonding , but there is no information on whether they function well without the silane agents. In particular, some investigators advocate the use of serial treatments, including silica coating, silane application, and MDP-containing cement . The stability of functional siloxane may be impaired when MDP causes pH changes, and the effectiveness of MDP could also be compromised due to the presence of silane. The co-existence or sequential placement of silane and MDP primers may yield different adhesion results, although few studies have examined this issue.
The chemical reactions at the zirconia–resin interface have been examined by X-ray photoelectron spectroscopy (XPS) , revealing the formation of functional groups or covalent bonds, but not clearly indicating the fundamentals of the adhesion. High-resolution time-of-flight secondary ion mass spectrometry (ToF-SIMS) offers an almost ideal solution to the analysis of organic and inorganic substances. In SIMS analysis, a high energy primary ion beam scans the specimen, and then secondary ions desorb from the surface and become fragment representatives of the substrates . ToF-SIMS collects the sputtered atomic and molecular secondary ions in a sequence of ion attack and detection events without time lapses, and thus the detailed surface constituents can be more accurately analyzed. The built-in depth profiling technology involves the controlled removal of surface material in the mono- or submonolayer regimes, and characterization of the acquired ions and their spatial distributions with a lateral submicron resolution. The reconstructed 3D ion or molecular images then facilitate the realization of the detailed chemical compositions.
As a precise and versatile surface analytical technique, the SIMS methodology has been adapted in previous studies of developing surface coatings or regenerative mineralization of biomedical implants to characterize low levels of calcium phosphate compounds . SIMS can examine the apatite minerals and their crystallinity on the surface, even if only microscopic amounts of depositions are present on the surface. The introduction of ToF-SIMS analysis into this field further enhances the sensitivity and ability of nano-structure characterization with or without the aid of conventional XPS methods . The SIMS methodology has also been applied to examine the zirconium phosphate (ZrP) compounds , in which PO 3 − was recognized as the key negative ion and its related polyatomic ions were fragment representatives of the crystalline α-ZrP. Based on the results of ToF-SIMS, the existence of P O Zr covalent bonds between MDP and zirconia has also been reported . However, information with regard to the detailed chemical analysis has been not revealed yet.
There is no agreement on the sequential application of silane and MDP primers. Regarding the case of multiple primer coatings on zirconia, ToF-SIMS enables users to resolve the complicated chemical and spatial chemical information in the interfacial layer. The aim of this study was thus to investigate whether changing MDP and silane primers and their placement orders altered the resulting coating, the chemical bonding to YTZ, and bond durability. The constituents of the surface coating layers were analyzed through ToF-SIMS to reveal the molecular distributions. The null hypothesis was that the primer application order neither affected the compositions of the surface coatings on zirconia, nor the resin–zirconia adhesion.
2
Materials and methods
2.1
Preparation of the specimens
Cercon base pre-sintered blanks (DeguDent, Hanau, Germany) were used in the study. The zirconia blanks were cut with a low-speed saw, then sintered in the Cercon heat furnace (DeguDent, Hanau, Germany) to generate thirty full density disks (Ø 19.5 × 2.5 mm). Each disk was further cut into six equal pieces, yielding a total of 180 pieces. These zirconia slabs were then embedded in acrylic resin with the test surfaces exposed. The ceramic surfaces were polished using wet 320-, 400-, and 600-grit silicon carbide papers, and then cleaned ultrasonically for 5 min.
Clearfil SE Bond Primer and Clearfil Porcelain Bond Activator (Kuraray, Okayama, Japan) were used as the MDP and MPS primers ( Table 1 ). Six groups with different primers and application orders were designed: group Zr, the untreated zirconia; group S, application of the MPS primer for 5 s, and then blow dried; group M, application of MDP primer for 5 s, then blow dried; group SMmix, for which mixture of MPS and MDP primers in equal volumes was applied; group SM, for which MPS primer was applied first, as in group S, followed by a second coat of MDP primer; and group MS, for which MDP primer was applied first, as in group M, followed by a second coat of MPS primer.
| Material | Trade name | Manufacturer | Composition |
|---|---|---|---|
| Zirconia | Cercon base | DeguDent, Hanau, Germany | Zirconium dioxide: 92 wt% |
| Yttrium trioxide: 5.0 wt% | |||
| Hafnium dioxide: <2.0 wt% | |||
| Aluminum oxide: <1.0 wt% | |||
| MDP primer | Clearfil SE bond primer | Kuraray Medical, Okayama, Japan | Bis-GMA, HEMA, 10-MDP, hydrophobic aliphatic methacrylate, colloidal silica, dl-camphorquinone, initiators, accelerators |
| MPS primer | Clearfil porcelain bond activator | Kuraray Medical, Okayama, Japan | MPS, hydrophobic aromatic dimethacrylates, others |
| Resin cement | Variolink II | Ivoclar Vivadent, Schaan, Liechtenstein | Base paste : Bis-GMA, UDMA, TEGDMA, DMA, barium, sulfate, Ba-Al-F-Si-glass, silica cont. composite resin. |
| Catalyst paste : Benzoylperoxide | |||
2.2
ToF-SIMS analysis
To prepare for the ToF-SIMS analysis, the ceramic disks underwent a series of fine polishing processes using 9-, 6-, and 1-μm diamond suspensions (Metadi Supreme, Buehler, Lakebluff, IL, USA), followed by 0.06 μm silica colloidal suspension (MasterMet, Buehler). The specimens were serially cleaned in water and ethanol for 5 min, and blow-dried with a nitrogen gun. These disks were treated according to their assigned treatments.
A ToF-SIMS spectrometer (PHI TRIFT IV, ULVAC-PHI, Kanagawa, Japan) equipped with a C60 sputter ion source and a bismuth liquid metal ion gun (Bi-LMIG) was used for the depth profiling analysis, which involved repeated ToF cycles of C60 sputtering and mass data acquisition by Bi-LMIG. For each cycle, the high-energy C60 gun (beam energy: 20 keV, DC current 1.5 nA) created a 120 × 120 μm 2 crater on the coated layer in a sputter time of 1 s. A primary Bi 3 + beam (beam energy: 30 keV; DC current: 1.5 nA; frequency: 8400 Hz; pulse width: 16 ns) rastered the center of the C60 crater to obtain 30 × 30 μm 2 mass data in a sputter time of 7 s. The ion masses were acquired simultaneously at an ion influence of 1.1 × 10 12 ion cm −2 . To achieve the highest mass accuracy for the organic peaks, the negative spectra were calibrated with CH − , OH − , C 2 H − , and C 16 H 31 O 2 − , while positive spectra were calibrated using CH 3 + , C 2 H 3 + , C 3 H 5 + , C 3 H 3 O + , C 4 H 7 + , and C 5 H 14 NO + .
As the ToF cycles proceeded, the signals of the organics decreased but those of the substrate increased. The ToF cycle finalized 20 cycles after the sputter passed through the zirconia substrate. The total number of ToF cycles varied in each group, with a maximum of 130. Positive ions Zr + (90) and ZrO + (106), and negative ions ZrO 2 − (121.9) and ZrO 2 (OH) − (138.9), are identified as the characteristic ions of zirconia substrate. A positive ion of trimethoxysilyl group SiO 3 C 3 H 9 − (121) is the characteristic peak of MPS, while MDP is identified by PO 2 − (63) and PO 3 − (79).
2.3
ZrP related molecule analysis
PO 3 − and PO 2 − secondary ions have been registered as the characteristic negative ions in all phosphate containing compounds, in which their ratios could be adapted to deduce the chemical reaction and specific minerals . In order to better understand the action mechanism of these primers, a further analysis of negative ions PO 3 − and PO 2 − was performed. Furthermore, the presence of fragmental ions related to ZrP was also inspected.
In an SIMS examination, the desorbed polyatomic secondary ions could be either the direct emissions of the intact portion of the surface, or the recombination of sputtered atoms and ions in the gas phase. According to previous studies , several PO 3 − related polyatomic ions from m/z 97 to 589 could be traced back to the ZrP compound ( Fig. 1 ). Since the ToF cycles used in each specimen varied, the amounts of these ions were normalized as percentages of total phosphate-related ions using a modified formula :
In this case, N(x) is the amount of a specific ion x of interest; N(P) is the amount of total phosphate-related ions (listed in Fig. 1 ). Accordingly, the relative percentage ratios of these specific ions were computed.
2.4
Depth profiling TOF-SIMS analysis and 3D image reconstruction
After completion of ToF cycles, the configuration software was applied for stacking the mass data. A total of 28 ToF cycle data at the vicinity of interfaces, with half above the zirconia surface and half below, was used to reconstruct the 3D ion images. Only the ions with the greatest mass or significance were shown. For the positive ion images, three ions were plotted: Zr + (90), ZrO + (106), and C 3 H 5 + (41). For the negative ion images, the ions shown were O − (16), OH − (17), C 2 H − (25), PO 2 − (63), PO 3 − (79), and C 4 H 5 O 2 − (85).
After the 3D ion images were reconstructed, a cross-sectional view of each negative ion stack was extracted to facilitate the depth profiling analysis of the ions or molecules in the coating layer. Distributions of O − , OH − , C 2 H − , PO 3 − , and PO 2 − at the interface were also mapped to reveal their infiltration depths below the zirconia surface.
2.5
Wettability test
The hydrophilicity of the treated zirconia was examined using the sessile drop method, by dropping 2 μl deionized water on the specimens. Instant photography was performed with a CCD camera to capture images of the droplet. The surface wettability of zirconia to resin was also examined by replacing water with the resin adhesive Scotchbond Mutipurpose Adhesive (3M/ESPE, St. Paul., MN). The static contact angles between the liquid drop and zirconia were measured by the Drop Analysis plug-in for ImageJ software.
2.6
Shear bond strength (SBS) test
The adhesion between resin cements and zirconia was evaluated by means of an SBS test. The zirconia disks (n = 24) underwent air abrasion by 45-μm Al 2 O 3 at 0.25 MPa for 15 s, and were then ultrasonically cleaned in ethanol and distilled water for 5 min and kept dry.
Pre-cured composite resin cylinders were fabricated by filling polyethylene tubes of 3-mm inner diameter and 2-mm height with the resin composite Filtek Z250 (3M ESPE, St. Paul, USA), and then light curing these for 20 s. After the primer treatments, an adhesive tape with a 3-mm diameter circular hole was attached to the zirconia surface. The pre-cured resin cylinder adhered to zirconia in the hole using a resin cement (Variolink II, Ivoclar/Vivadent, Schaan, Liechtenstein) under 4.6-N loading. After removal of excess, material the cement was light cured for 40 s. The bonded specimens were allowed to set for 30 min at room temperature, and then stored in a 37 °C water bath for 24 h. From each group, half of the specimens (n = 12) were subjected to an SBS test after 1-day storage, and half were tested after thermocycling. The thermocycling test consisted of 6000 thermocycles alternately in water at 5 °C and 55 °C, each with a dwelling time of 30 s.
The SBS test was performed on a universal material test machine (AGI, Shimadzu Kyoto, Japan). Zirconia disks were secured vertically on a jig. A shear force was applied by loading a chisel on the composite cylinder at a crosshead speed of 1 mm/min. SBS was obtained through dividing the failure load by the bond area.
2.7
Statistical analysis
One-way analysis of variance (ANOVA) tests were conducted to examine the statistical differences in contact angles and SBS among the experimental groups, followed by post-hoc Tukey tests. The effect of thermocycling on the SBSs in the same group was evaluated by a Student’s t test. Statistical analysis was conducted using SPSS 17.0 software (SPSS, Chicago, IL, USA), and a p -value <0.05 indicates statistical significance.
2
Materials and methods
2.1
Preparation of the specimens
Cercon base pre-sintered blanks (DeguDent, Hanau, Germany) were used in the study. The zirconia blanks were cut with a low-speed saw, then sintered in the Cercon heat furnace (DeguDent, Hanau, Germany) to generate thirty full density disks (Ø 19.5 × 2.5 mm). Each disk was further cut into six equal pieces, yielding a total of 180 pieces. These zirconia slabs were then embedded in acrylic resin with the test surfaces exposed. The ceramic surfaces were polished using wet 320-, 400-, and 600-grit silicon carbide papers, and then cleaned ultrasonically for 5 min.
Clearfil SE Bond Primer and Clearfil Porcelain Bond Activator (Kuraray, Okayama, Japan) were used as the MDP and MPS primers ( Table 1 ). Six groups with different primers and application orders were designed: group Zr, the untreated zirconia; group S, application of the MPS primer for 5 s, and then blow dried; group M, application of MDP primer for 5 s, then blow dried; group SMmix, for which mixture of MPS and MDP primers in equal volumes was applied; group SM, for which MPS primer was applied first, as in group S, followed by a second coat of MDP primer; and group MS, for which MDP primer was applied first, as in group M, followed by a second coat of MPS primer.
| Material | Trade name | Manufacturer | Composition |
|---|---|---|---|
| Zirconia | Cercon base | DeguDent, Hanau, Germany | Zirconium dioxide: 92 wt% |
| Yttrium trioxide: 5.0 wt% | |||
| Hafnium dioxide: <2.0 wt% | |||
| Aluminum oxide: <1.0 wt% | |||
| MDP primer | Clearfil SE bond primer | Kuraray Medical, Okayama, Japan | Bis-GMA, HEMA, 10-MDP, hydrophobic aliphatic methacrylate, colloidal silica, dl-camphorquinone, initiators, accelerators |
| MPS primer | Clearfil porcelain bond activator | Kuraray Medical, Okayama, Japan | MPS, hydrophobic aromatic dimethacrylates, others |
| Resin cement | Variolink II | Ivoclar Vivadent, Schaan, Liechtenstein | Base paste : Bis-GMA, UDMA, TEGDMA, DMA, barium, sulfate, Ba-Al-F-Si-glass, silica cont. composite resin. |
| Catalyst paste : Benzoylperoxide | |||
2.2
ToF-SIMS analysis
To prepare for the ToF-SIMS analysis, the ceramic disks underwent a series of fine polishing processes using 9-, 6-, and 1-μm diamond suspensions (Metadi Supreme, Buehler, Lakebluff, IL, USA), followed by 0.06 μm silica colloidal suspension (MasterMet, Buehler). The specimens were serially cleaned in water and ethanol for 5 min, and blow-dried with a nitrogen gun. These disks were treated according to their assigned treatments.
A ToF-SIMS spectrometer (PHI TRIFT IV, ULVAC-PHI, Kanagawa, Japan) equipped with a C60 sputter ion source and a bismuth liquid metal ion gun (Bi-LMIG) was used for the depth profiling analysis, which involved repeated ToF cycles of C60 sputtering and mass data acquisition by Bi-LMIG. For each cycle, the high-energy C60 gun (beam energy: 20 keV, DC current 1.5 nA) created a 120 × 120 μm 2 crater on the coated layer in a sputter time of 1 s. A primary Bi 3 + beam (beam energy: 30 keV; DC current: 1.5 nA; frequency: 8400 Hz; pulse width: 16 ns) rastered the center of the C60 crater to obtain 30 × 30 μm 2 mass data in a sputter time of 7 s. The ion masses were acquired simultaneously at an ion influence of 1.1 × 10 12 ion cm −2 . To achieve the highest mass accuracy for the organic peaks, the negative spectra were calibrated with CH − , OH − , C 2 H − , and C 16 H 31 O 2 − , while positive spectra were calibrated using CH 3 + , C 2 H 3 + , C 3 H 5 + , C 3 H 3 O + , C 4 H 7 + , and C 5 H 14 NO + .
As the ToF cycles proceeded, the signals of the organics decreased but those of the substrate increased. The ToF cycle finalized 20 cycles after the sputter passed through the zirconia substrate. The total number of ToF cycles varied in each group, with a maximum of 130. Positive ions Zr + (90) and ZrO + (106), and negative ions ZrO 2 − (121.9) and ZrO 2 (OH) − (138.9), are identified as the characteristic ions of zirconia substrate. A positive ion of trimethoxysilyl group SiO 3 C 3 H 9 − (121) is the characteristic peak of MPS, while MDP is identified by PO 2 − (63) and PO 3 − (79).
2.3
ZrP related molecule analysis
PO 3 − and PO 2 − secondary ions have been registered as the characteristic negative ions in all phosphate containing compounds, in which their ratios could be adapted to deduce the chemical reaction and specific minerals . In order to better understand the action mechanism of these primers, a further analysis of negative ions PO 3 − and PO 2 − was performed. Furthermore, the presence of fragmental ions related to ZrP was also inspected.
In an SIMS examination, the desorbed polyatomic secondary ions could be either the direct emissions of the intact portion of the surface, or the recombination of sputtered atoms and ions in the gas phase. According to previous studies , several PO 3 − related polyatomic ions from m/z 97 to 589 could be traced back to the ZrP compound ( Fig. 1 ). Since the ToF cycles used in each specimen varied, the amounts of these ions were normalized as percentages of total phosphate-related ions using a modified formula :
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