Effectiveness and stability of silane coupling agent incorporated in ‘universal’ adhesives

Abstract

Objective

For bonding indirect restorations, some ‘universal’ adhesives incorporate a silane coupling agent to chemically bond to glass-rich ceramics so that a separate ceramic primer is claimed to be no longer needed. With this work, we investigated the effectiveness/stability of the silane coupling function of the silanecontaining experimentally prepared adhesives and Scotchbond Universal (3MESPE).

Methods and materials

Experimental adhesives consisted of Scotchbond Universal and the silane-free Clearfil S3 ND Quick (Kuraray Noritake) mixed with Clearfil Porcelain Bond Activator (Kuraray Noritake) and the two adhesives to which γ-methacryloxypropyltrimethoxysilane (γ-MPTS) was added. Shear bond strength was measured onto silica-glass plates; the adhesive formulations were analyzed using fourier transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance (NMR). In addition, shear bond strength onto CAD-CAM composite blocks was measured without and after thermo-cycling ageing.

Results

A significantly higher bond strength was recorded when Clearfil Porcelain Bond Activator was freshly mixed with the adhesive. Likewise, the experimental adhesives, to which γ-MPTS was added, revealed a significantly higher bond strength, but only when the adhesive was applied immediately after mixing; delayed application resulted in a significantly lower bond strength. FTIR and 13 C NMR revealed hydrolysis and dehydration condensation to progress with the time after γ-MPTS was mixed with the two adhesives. After thermo-cycling, the bond strength onto CAD-CAM composite blocks remained stable only for the two adhesives with which Clearfil Porcelain Bond Activator was mixed.

Significance

Only the silane coupling effect of freshly prepared silanecontaining adhesives was effective. Clinically, the use of a separate silane primer or silane freshly mixed with the adhesive remains recommended to bond glass-rich ceramics.

Introduction

In restorative dentistry, ceramics meet best the patient’s demand for aesthetics. Glass-rich dental ceramics, such as feldspar ceramics, the diverse kinds of glass-ceramics (including monolithic lithium disilicate ceramics) and some new polymer-infiltrated ceramics, are least invasive to restore teeth, as they can be fully bonded to the remaining sound tooth tissue using composite cements. At the restoration site, the bonding protocol of preference to adhesively lute these ‘etchable’ ceramics consists of hydrofluoric acid (HF) etching to provide micro-retention to the cement that by silanization also chemically bonds to the ceramic. A silane functional monomer, like the most commonly in dentistry used methacrylate silane monomer (γ-methacryloyloxypropyltrimethoxysilane or γ-MTPS), basically possesses a methacrylate end to co-polymerize with the adhesive and/or composite cement and the actual silane group to covalently bond to the ceramic glass phase. Besides needed for bonding to ceramic, silane coupling agents are also often used as part of a restoration-repair protocol to intra-orally repair both ceramic and composite restorations . Silane primers are mostly water-free solutions ; one-bottle and two-bottle silane primers exist. Silane primers that contain non-hydrolyzed silane, are most often dissolved in ethanol in one bottle that needs to be activated and hydrolyzed by mixing it with an aqueous acetic acid solution or an acidic adhesive in the other bottle . Both water and lower pH cause silane to hydrolyse. The latest generation of silane primers contain silane mostly dissolved in a water-free and only mildly acidic solution , so that it has a relatively long shelf life. A kind of multi-purpose ceramic/metal primers are also water-free solutions and contain besides a silane functional monomer other functional monomers in order not only to bond ‘glass-rich’ (requiring silane) and ‘glass-poor’ ceramics like zirconia (requiring 10-methacryloyloxydecyl dihydrogen phosphate or 10-MDP), but also to bond precious metal alloys (requiring a monomer with one end carrying a thiol ( SH) group). Most recently, so-called ‘universal’ adhesives enable the dentist not only to opt for an ‘etch-and-rinse’ or ‘self-etch’ bonding protocol, but they can also be employed for both direct and indirect indications . Among such universal adhesives, some adhesives also incorporate a silane coupling agent, having been claimed to provide the adhesive direct chemical bonding potential to glass-rich ceramics without the need of a separate ceramic primer .

It is generally well known that water-containing and acidic, single-bottle, pre-hydrolyzed silane coupling agents have a relatively short shelf life . In light of this knowledge and because independent research data are insufficiently available, we hereby investigated the silanization potential of a universal adhesive incorporating a silane coupling agent. Bond strength to glass plates and CAD-CAM composite blocks was measured, as well as the adhesive formulations were chemically characterized using Fourier transform infrared spectrometry (FTIR) and nuclear magnetic resonance (NMR). Besides the commercially available ‘universal’ and silane-containing adhesive Scotchbond Universal (3M ESPE), we also investigated the commercially available silane-free adhesive Clearfil S3 Bond ND Quick (Kuraray Noritake Dental, Tokyo, Japan), as well as experimental adhesives to which γ-methacryloxyproyltrimethoxysilane (γ-MPTS) was added. The null hypotheses tested were that an adhesive incorporating a silane coupling agent was more effective in terms of bonding effectiveness than an adhesive that does not contain silane (1) and that the addition of silane to the adhesive formulation improved bond strength (2) and remained effective with time (3).

Materials and methods

Shear bond strength onto glass plates

Silica-glass plates (Shin-Etsu Quartz Products, Tokyo, Japan) with a smooth surface and thus low potential for micro-mechanical interlocking were employed to represent glass-rich ceramics; the plates were 10 mm × 10 mm wide and 3 mm thick. Six different adhesive protocols using the universal and silane-containing adhesive Scotchbond Universal (3M ESPE) and the silane-free adhesive Clearfil S3 Bond ND Quick (Kuraray Noritake) were tested as follows: (1) application of the adhesive ‘as such’; (2) immediate application of a 1:1 drop mixture of the adhesive with Clearfil Porcelain Bond Activator (Kuraray Noritake); (3) ‘immediate’ application of the adhesive to which 2 wt% γ-MPTS (Sigma–Aldrich, St. Louis, MO, USA) was added; (4) ‘1 day’ delayed application of the 2 wt% γ-MTPS-containing adhesive; (5) ‘3 days’ delayed application of the 2 wt% γ-MTPS-containing adhesive; (6) ‘7 days’ delayed application of the 2 wt% γ-MTPS-containing adhesive. Clearfil Porcelain Bond Activator (Kuraray Noritake) is a water-free silane coupling agent. We used 2 wt% γ-MTPS as this concentration corresponded to the silane concentration most common within commercial silane primers, as they were listed by Lung and Matinlinna . To achieve a homogeneous mixture, the 2 wt% γ-MTPS was added to the adhesives using a magnetic stirrer. The silica-glass plates were treated following the abovementioned experimental protocols, upon which they were air-dried. Zirconia cylinder blocks (Tosoh, Tokyo, Japan) with a 3.6-mm diameter were sandblasted using Shofu High Blaster (Shofu, Kyoto, Japan) with 50-μm alumina particles (Shofu), followed by silanization using Clearfil Ceramic primer (Kuraray Noritake); they were eventually luted onto the glass plates using the composite cement Clearfil Esthetic Cement (Kuraray Noritake). The cement was light-cured for 40 s from two opposing directions (totaling to a 80-s curing time) using G-Light Prima II Plus (light irradiance of 2800 mW/cm 2 ; GC, Tokyo, Japan); the specimens were next stored in 37 °C water for 24 h prior to being subjected to a shear bond-strength test. Per experimental group, ten specimens were prepared. The specimens were mounted into a testing machine (AG-X, Shimadzu, Kyoto, Japan) and subjected to shear stress at a crosshead speed of 0.5 mm/min. All fractured specimens were analyzed utilizing a light microscope (SMZ-10, Nikon, Tokyo, Japan) at 4× magnification to determine the mode of fracture. For statistical comparisons of the data, the Scheffé’s test was applied with p < 0.05 considered statistically significant.

FTIR chemical analysis of the adhesive formulations

The commercial adhesives Scotchbond Universal and Clearfil S3 Bond ND Quick, and the experimental adhesive formulations consisting of the commercial adhesives to which 2 wt% γ-MTPS was mixed, were analyzed using a Shimadzu IRAffinity-1 FTIR Spectrophotometer (Shimadzu, Kyoto, Japan) with a KBr plate (Jasco, Tokyo, Japan) in transmission mode. As control, we used a 2 wt% γ-MTPS in 98% ethanol solution and the same solution mixed with a solution of 2 wt% acetic acid (Sigma–Aldrich) in 70 wt% ethanol and 28 wt% water. The FTIR spectra were recorded upon 256 successive scans and a spectral resolution of 4 cm −1 . Each sample was measured three times.

NMR chemical analysis of the adhesive formulations

Half of the samples of Scotchbond Universal and Clearfil S3 Bond ND Quick, to which 4 wt% γ-MTPS was added, were analyzed immediately after mixing, while the others were kept for 1 day before being analyzed. As control, Scotchbond Universal and Clearfil S3 Bond ND Quick were also measured. Before measurement, the same amount of d-ethanol was added to the samples, after which they were poured into NMR test glass tubes with a 5-mm diameter and an 8-inch length (Wilmad, Buena, New Jersey). An NMR spectrometer (UNITY INOVA400NB, Varian Japan, Tokyo, Japan) was employed to acquire 13 C NMR spectra at 100.58 MHz in CD 3 CD 2 OD. 13 C NMR spectra were referenced at the internal CD 3 peak ( δ = 17 ppm) of d -ethanol. Two samples of each adhesive formulation were analyzed by NMR.

Shear bond strength onto CAD-CAM composite blocks

CAD-CAM composite blocks (shade A3-LT, size 14L; Lava Ultimate, 3M ESPE) were cut to discs with a 1.5-mm thickness. The surface was polished using 15-μm diamond lapping film (Struers, Ballerup Denmark) in order to reduce the potential for mechanical micro-retention; this was followed by either one of the following surface treatments: (1) Scotchbond Universal (incorporating silane) applied ‘as such’; (2) Clearfil S3 Bond ND Quick (silane-free) applied ‘as such’; (3) 1:1 drop mixture of Scotchbond Universal with Clearfil Porcelain Bond Activator; (4) 1:1 drop mixture of Clearfil S3 Bond ND Quick with Clearfil Porcelain Bond Activator. Zirconia cylinders, prepared in the same manner as for the bond-strength measurements onto glass plates, were luted onto CAD-CAM composite blocks using Clearfil Esthetic Cement and light-cured for 40 s from two opposing directions (totaling to a 80-s curing time) using G-Light Prima II Plus. Per experimental group, 20 specimens were prepared; all specimens were subjected to a shear bond-strength testing protocol, likewise as described above, this for half of the specimens after 24-h storage in water at 37 °C, while the other half were artificially aged through thermo-cycling (60 s of immersion, alternatively, in a 5 and 55 °C water bath) during 15,000 times prior to bond-strength measurement. For statistical comparisons of data, two-way ANOVA followed by Tukey’s post-hoc tests ( α < 0.05) was used with p < 0.05 considered statistically significant.

Materials and methods

Shear bond strength onto glass plates

Silica-glass plates (Shin-Etsu Quartz Products, Tokyo, Japan) with a smooth surface and thus low potential for micro-mechanical interlocking were employed to represent glass-rich ceramics; the plates were 10 mm × 10 mm wide and 3 mm thick. Six different adhesive protocols using the universal and silane-containing adhesive Scotchbond Universal (3M ESPE) and the silane-free adhesive Clearfil S3 Bond ND Quick (Kuraray Noritake) were tested as follows: (1) application of the adhesive ‘as such’; (2) immediate application of a 1:1 drop mixture of the adhesive with Clearfil Porcelain Bond Activator (Kuraray Noritake); (3) ‘immediate’ application of the adhesive to which 2 wt% γ-MPTS (Sigma–Aldrich, St. Louis, MO, USA) was added; (4) ‘1 day’ delayed application of the 2 wt% γ-MTPS-containing adhesive; (5) ‘3 days’ delayed application of the 2 wt% γ-MTPS-containing adhesive; (6) ‘7 days’ delayed application of the 2 wt% γ-MTPS-containing adhesive. Clearfil Porcelain Bond Activator (Kuraray Noritake) is a water-free silane coupling agent. We used 2 wt% γ-MTPS as this concentration corresponded to the silane concentration most common within commercial silane primers, as they were listed by Lung and Matinlinna . To achieve a homogeneous mixture, the 2 wt% γ-MTPS was added to the adhesives using a magnetic stirrer. The silica-glass plates were treated following the abovementioned experimental protocols, upon which they were air-dried. Zirconia cylinder blocks (Tosoh, Tokyo, Japan) with a 3.6-mm diameter were sandblasted using Shofu High Blaster (Shofu, Kyoto, Japan) with 50-μm alumina particles (Shofu), followed by silanization using Clearfil Ceramic primer (Kuraray Noritake); they were eventually luted onto the glass plates using the composite cement Clearfil Esthetic Cement (Kuraray Noritake). The cement was light-cured for 40 s from two opposing directions (totaling to a 80-s curing time) using G-Light Prima II Plus (light irradiance of 2800 mW/cm 2 ; GC, Tokyo, Japan); the specimens were next stored in 37 °C water for 24 h prior to being subjected to a shear bond-strength test. Per experimental group, ten specimens were prepared. The specimens were mounted into a testing machine (AG-X, Shimadzu, Kyoto, Japan) and subjected to shear stress at a crosshead speed of 0.5 mm/min. All fractured specimens were analyzed utilizing a light microscope (SMZ-10, Nikon, Tokyo, Japan) at 4× magnification to determine the mode of fracture. For statistical comparisons of the data, the Scheffé’s test was applied with p < 0.05 considered statistically significant.

FTIR chemical analysis of the adhesive formulations

The commercial adhesives Scotchbond Universal and Clearfil S3 Bond ND Quick, and the experimental adhesive formulations consisting of the commercial adhesives to which 2 wt% γ-MTPS was mixed, were analyzed using a Shimadzu IRAffinity-1 FTIR Spectrophotometer (Shimadzu, Kyoto, Japan) with a KBr plate (Jasco, Tokyo, Japan) in transmission mode. As control, we used a 2 wt% γ-MTPS in 98% ethanol solution and the same solution mixed with a solution of 2 wt% acetic acid (Sigma–Aldrich) in 70 wt% ethanol and 28 wt% water. The FTIR spectra were recorded upon 256 successive scans and a spectral resolution of 4 cm −1 . Each sample was measured three times.

NMR chemical analysis of the adhesive formulations

Half of the samples of Scotchbond Universal and Clearfil S3 Bond ND Quick, to which 4 wt% γ-MTPS was added, were analyzed immediately after mixing, while the others were kept for 1 day before being analyzed. As control, Scotchbond Universal and Clearfil S3 Bond ND Quick were also measured. Before measurement, the same amount of d-ethanol was added to the samples, after which they were poured into NMR test glass tubes with a 5-mm diameter and an 8-inch length (Wilmad, Buena, New Jersey). An NMR spectrometer (UNITY INOVA400NB, Varian Japan, Tokyo, Japan) was employed to acquire 13 C NMR spectra at 100.58 MHz in CD 3 CD 2 OD. 13 C NMR spectra were referenced at the internal CD 3 peak ( δ = 17 ppm) of d -ethanol. Two samples of each adhesive formulation were analyzed by NMR.

Shear bond strength onto CAD-CAM composite blocks

CAD-CAM composite blocks (shade A3-LT, size 14L; Lava Ultimate, 3M ESPE) were cut to discs with a 1.5-mm thickness. The surface was polished using 15-μm diamond lapping film (Struers, Ballerup Denmark) in order to reduce the potential for mechanical micro-retention; this was followed by either one of the following surface treatments: (1) Scotchbond Universal (incorporating silane) applied ‘as such’; (2) Clearfil S3 Bond ND Quick (silane-free) applied ‘as such’; (3) 1:1 drop mixture of Scotchbond Universal with Clearfil Porcelain Bond Activator; (4) 1:1 drop mixture of Clearfil S3 Bond ND Quick with Clearfil Porcelain Bond Activator. Zirconia cylinders, prepared in the same manner as for the bond-strength measurements onto glass plates, were luted onto CAD-CAM composite blocks using Clearfil Esthetic Cement and light-cured for 40 s from two opposing directions (totaling to a 80-s curing time) using G-Light Prima II Plus. Per experimental group, 20 specimens were prepared; all specimens were subjected to a shear bond-strength testing protocol, likewise as described above, this for half of the specimens after 24-h storage in water at 37 °C, while the other half were artificially aged through thermo-cycling (60 s of immersion, alternatively, in a 5 and 55 °C water bath) during 15,000 times prior to bond-strength measurement. For statistical comparisons of data, two-way ANOVA followed by Tukey’s post-hoc tests ( α < 0.05) was used with p < 0.05 considered statistically significant.

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Effectiveness and stability of silane coupling agent incorporated in ‘universal’ adhesives
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