Methacrylate bonding to zirconia by in situ silica nanoparticle surface deposition

Highlights

  • A novel method to deposit nano-silica onto yttria-stabilized zirconia ceramic and improve bonding of methacrylate-based materials is described.

  • The zirconia and silica precursors used in this study are safe, non-toxic, and reasonably inexpensive.

  • The application technique could use the sintering time and temperature used to process or glaze dental ceramics.

  • Primers with silica/zirconia content of 25:75 can favour the establishment of a stable bond more resistant to hydrolytic degradation.

Abstract

Objective

This study introduces an innovative method to enhance adhesion of methacrylate-based cements to yttria-stabilized zirconia (Y-TZP) by means of a silica-nanoparticle deposition process.

Methods

Two alkoxide organic precursors, tetraethyl-orthosilicate (TEOS) and zirconium tert -butoxide (ZTB) were diluted in hexane at different concentrations in order to obtain several experimental materials to enhance deposition of a SiO x reactive layer to Y-TZP. This deposition was attained via sintering alkoxide precursors directly on pre-sintered zirconia (infiltration method—INF) or application on the surface of fully sintered zirconia (coating method—COA). Untreated specimens and a commercial tribochemical silica coating were also tested as controls and all the treated Y-TZP specimens were analyzed using SEM-EDX. Specimens were bonded using silane, adhesive and dual-cure luting cement and submitted to shear bond strength test after different water storage periods (24 h, 3-, 6- and 12-months).

Results

SEM-EDX revealed Y-TZP surface covered by silica nanoclusters. The morphology of silica-covered Y-TZP surfaces was influenced by sintering method, employed to deposit nanoclusters. High bond strength (MPa) was observed when using COA method; highest TEOS percentage achieved the greatest bond strengths to Y-TZP surface (36.7 ± 6.3 at 24 h). However, bonds stability was dependent on ZTB presence (32.9 ± 9.7 at 3 months; 32.3 ± 7.1 at 6 months). Regarding INF method, the highest and more stable resin–zirconia bond strength was attained when using experimental solutions containing TEOS and no ZTB. Both sintering methods tested in this study were able to achieve a bonding performance similar to that of classic tribochemical strategies.

Significance

This study demonstrates that it is possible to achieve a reliable and long-lasting bonding between yttria-stabilized zirconia ceramic and methacrylate-based cements when using this novel, simple, and cost-effective bonding approach.

Introduction

The use of yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) ceramics increases every year in biomedical fields. In dentistry, Y-TZP bioceramics can be employed as bone implants due to their excellent osseointegration and biocompatibility properties. Moreover, Y-TZP ceramics are commonly used for indirect dental restorations as they possess excellent esthetics and mechanical properties (i.e. high fracture toughness) as well as chemical inertia. The introduction of novel CAD-CAM technologies has simplified laboratory procedures of Y-TZP for dental prosthesis with fewer occurrences of pre-fails during processing.

A significant shortcoming of Y-TZP ceramics may be attributed to a lack of bonding performance when using methacrylate-based resin cements. This limits the use of Y-TZP ceramics in dental preparations with reduced frictional retention (e.g. teeth with short or conical abutments) such as inlays, onlays and full dental crowns . Two main factors are responsible: (1) The homogeneous single-phase structure of Y-TZP is highly dense; hence impeding formation of selective micro-retentions ; (2) the absence of silica (glass-phase) in Y-TZP structure which avoids formation of chemical bonds when using organo-silanes.

Although alternative methods have been advocated to enhance adhesion to Y-TZP ceramics, a reliable, practical, and cost-effective method is still needed to overcome this issue entirely. Undeniably, one of the most recent methods to improve bonding between Y-TZP ceramics and resin-based materials is based on inclusion of functional phosphoric-acid ester monomers within resin cements composition which have some chemical interaction with zirconia. Nevertheless, despite good initial adhesion results reported, bonding longevity is still unreliable and requires further improvements . A further approach based on tribochemical deposition of silica on zirconia surface by means of air-blasting devices has shown good results, but occasionally inconsistent as a significant drop in bond strength has been reported after aging . Alternative methods such as vapor deposition of SiCl 4 and SiO 2 fusion by plasma treatment have been proposed; however, these are still expensive, complex, and require specific equipment and a high level of proficiency.

It has been shown that reactivity of Y-TZP ceramics with resin cements may be addressed by depositing silica layer onto Y-TZP surface as well as deposition of a silica-containing layer through application and calcination of a silane-based solution making it effective in providing early bonding to Y-TZP ceramics . Therefore, the aim of the present study was to generate an innovative strategy to provide a long-lasting bonding to Y-TZP ceramics. The method evaluated in this study consisted of a simplified direct and indirect nano-silica-coating method using organic silica (Si) and zirconia (Zr) alkoxy precursors at different concentrations. The hypothesis tested was that the use of organic Si/Zr precursors would induce silica deposition on zirconia surface and enhance bonding performance of resin cements to Y-TZP ceramic.

Materials and methods

Preparation of experimental solutions and sintering methods

Four experimental solutions were prepared using tetraethyl orthosilicate (TEOS) and zirconium tert -butoxide (ZTB) diluted in hexane. The use of zirconia precursors mixed with silica precursors was tested in order to increase the compatibility of surface coating with zirconia substrate. All tested compositions are shown in detail in Table 1 , while molecular structures of metallic precursors are depicted in Fig. 1 .

Table 1
Formulation of the experimental solutions (% mass).
Solution Alkoxide precursor Hexane
TEOS ZTB
100:00 5 0 95
75:25 3.75 1.25 95
50:50 2.5 2.5 95
25:75 1.25 3.75 95
TEOS: tetraethyl orthosilicate; ZTB: zirconium tert -butoxide.

Fig. 1
Molecular structure of the organic-metallic precursors tested. TEOS: tetraethyl-orthosilicate; ZTB: zirconium tert -butoxide.

Pre-sintered blocks (40 × 19 × 19 mm) of a Y-TZP dental ceramic (Zircon-CAD; Angelus, Londrina, PR, Brazil) were used in this study. They were initially cut into smaller blocks (10 × 9 × 9 mm dimensions) using a diamond saw, and then polished with no. 320, 400, 600, and 1200 SiC-papers under water-cooling (Aropol E; Arotec, Cotia, SP, Brazil). The following silica-coating processes were performed before or after Y-TZP ceramic specimens sintering.

Silica coating before zirconia sintering (INF method)

Pre-sintered zirconia blocks were immersed in the experimental solutions for 5 min in order to allow maximum infiltration of organic precursors into the Y-TZP, as observed in a pilot study. Subsequent to infiltration period, the zirconia-infiltrated specimens were fully sintered in a computer-controlled furnace (FEZ-1600/4; INTI, São Carlos, SP, Brazil) using the protocol recommended by the manufacturer: heating rate of 100 °C/h until reaching 1350 °C and maintained constant for 2 h.

Silica coating after zirconia sintering (COA method)

Zirconia blocks were fully sintered as previously described. The surface of blocks was coated with ∼100 μL of prepared solutions with organic precursors, followed by a heat treatment consisting of heating rate of 10 °C/min until 800 °C and maintained constant for 2 h in order to condensate the SiO x network and evaporate the solvent.

A negative control group (untreated sintered zirconia specimens) as well as sintered zirconia specimens treated with a commercial tribochemical silica deposition method (Rocatec Plus; 3M ESPE, St. Paul, MN, USA) were also tested; the latter was applied according to the manufacturer’s instructions.

SEM-EDX analysis

Specimens treated with different solutions and infiltration/sintering methods were non-sputter coated or sputter-coated with carbon 10 s without significantly affect the surface morphology of specimens and compositional analysis. The microstructural evaluation was performed using a scanning electron microscopy (SEM-SSX-550; Shimadzu, Tokyo, Japan) along with an x-ray energy dispersive spectroscopy (EDX) for elemental analysis.

Bond strength test and failure analysis

Specimens of all groups tested in this study received a layer of organo-silane (Silano; Angelus), which was applied onto the entire surface and solvent evaporated following manufacturer’s instructions. Subsequently, a layer of solvent-free adhesive (Scotchbond Multipurpose; 3M ESPE) was applied with a microbrush and polyvinylsiloxane molds (thickness 0.5 mm, diameter 1.5 mm) were placed onto the surface of zirconia blocks. The adhesive was light-cured for 20 s using a light-emitting diode curing unit (Radii-Cal; SDI, Bayswater, Australia) with 1200 mW/cm2 irradiance, and the molds were filled with a regular, dual-cure resin cement (RelyX ARC; 3M ESPE). A polyester strip and glass slide were placed onto the filled molds, and the cement was light-cured for 40 s. For each group, 10 resin–cement cylinders were built up on ceramic surfaces.

The samples were stored in distilled water at 37 °C for 24 h, 3 months, 6 months and 1 year; water was replaced every month. A stainless steel wire (diameter 0.2 mm) was looped around each cement cylinder, aligned with bonding interface and shear test performed using a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. The fractured specimens were observed using a stereomicroscope (40×). Failures were classified as mixed failure (remnants of cement left on ceramic) or adhesive failure (interfacial debonding). Bond strength values were calculated in MPa and data analyzed using two-way ANOVA (surface treatment × storage time). All pairwise multiple comparison procedures were performed by the Student–Newman–Keuls’ method ( α = 0.05).

Materials and methods

Preparation of experimental solutions and sintering methods

Four experimental solutions were prepared using tetraethyl orthosilicate (TEOS) and zirconium tert -butoxide (ZTB) diluted in hexane. The use of zirconia precursors mixed with silica precursors was tested in order to increase the compatibility of surface coating with zirconia substrate. All tested compositions are shown in detail in Table 1 , while molecular structures of metallic precursors are depicted in Fig. 1 .

Table 1
Formulation of the experimental solutions (% mass).
Solution Alkoxide precursor Hexane
TEOS ZTB
100:00 5 0 95
75:25 3.75 1.25 95
50:50 2.5 2.5 95
25:75 1.25 3.75 95
TEOS: tetraethyl orthosilicate; ZTB: zirconium tert -butoxide.

Fig. 1
Molecular structure of the organic-metallic precursors tested. TEOS: tetraethyl-orthosilicate; ZTB: zirconium tert -butoxide.

Pre-sintered blocks (40 × 19 × 19 mm) of a Y-TZP dental ceramic (Zircon-CAD; Angelus, Londrina, PR, Brazil) were used in this study. They were initially cut into smaller blocks (10 × 9 × 9 mm dimensions) using a diamond saw, and then polished with no. 320, 400, 600, and 1200 SiC-papers under water-cooling (Aropol E; Arotec, Cotia, SP, Brazil). The following silica-coating processes were performed before or after Y-TZP ceramic specimens sintering.

Silica coating before zirconia sintering (INF method)

Pre-sintered zirconia blocks were immersed in the experimental solutions for 5 min in order to allow maximum infiltration of organic precursors into the Y-TZP, as observed in a pilot study. Subsequent to infiltration period, the zirconia-infiltrated specimens were fully sintered in a computer-controlled furnace (FEZ-1600/4; INTI, São Carlos, SP, Brazil) using the protocol recommended by the manufacturer: heating rate of 100 °C/h until reaching 1350 °C and maintained constant for 2 h.

Silica coating after zirconia sintering (COA method)

Zirconia blocks were fully sintered as previously described. The surface of blocks was coated with ∼100 μL of prepared solutions with organic precursors, followed by a heat treatment consisting of heating rate of 10 °C/min until 800 °C and maintained constant for 2 h in order to condensate the SiO x network and evaporate the solvent.

A negative control group (untreated sintered zirconia specimens) as well as sintered zirconia specimens treated with a commercial tribochemical silica deposition method (Rocatec Plus; 3M ESPE, St. Paul, MN, USA) were also tested; the latter was applied according to the manufacturer’s instructions.

SEM-EDX analysis

Specimens treated with different solutions and infiltration/sintering methods were non-sputter coated or sputter-coated with carbon 10 s without significantly affect the surface morphology of specimens and compositional analysis. The microstructural evaluation was performed using a scanning electron microscopy (SEM-SSX-550; Shimadzu, Tokyo, Japan) along with an x-ray energy dispersive spectroscopy (EDX) for elemental analysis.

Bond strength test and failure analysis

Specimens of all groups tested in this study received a layer of organo-silane (Silano; Angelus), which was applied onto the entire surface and solvent evaporated following manufacturer’s instructions. Subsequently, a layer of solvent-free adhesive (Scotchbond Multipurpose; 3M ESPE) was applied with a microbrush and polyvinylsiloxane molds (thickness 0.5 mm, diameter 1.5 mm) were placed onto the surface of zirconia blocks. The adhesive was light-cured for 20 s using a light-emitting diode curing unit (Radii-Cal; SDI, Bayswater, Australia) with 1200 mW/cm2 irradiance, and the molds were filled with a regular, dual-cure resin cement (RelyX ARC; 3M ESPE). A polyester strip and glass slide were placed onto the filled molds, and the cement was light-cured for 40 s. For each group, 10 resin–cement cylinders were built up on ceramic surfaces.

The samples were stored in distilled water at 37 °C for 24 h, 3 months, 6 months and 1 year; water was replaced every month. A stainless steel wire (diameter 0.2 mm) was looped around each cement cylinder, aligned with bonding interface and shear test performed using a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. The fractured specimens were observed using a stereomicroscope (40×). Failures were classified as mixed failure (remnants of cement left on ceramic) or adhesive failure (interfacial debonding). Bond strength values were calculated in MPa and data analyzed using two-way ANOVA (surface treatment × storage time). All pairwise multiple comparison procedures were performed by the Student–Newman–Keuls’ method ( α = 0.05).

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Methacrylate bonding to zirconia by in situ silica nanoparticle surface deposition
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