Improving performance of dental resins by adding titanium dioxide nanoparticles

Abstract

Objective

The objective of this study is to improve the performance of dental resins by adding a small amount of titanium dioxide nanoparticles (TiO 2 NPs), which have outstanding mechanical properties and unique photoactivities.

Methods

Acrylic acid modified TiO 2 NPs (AP25) were prepared and added to a mixture of bis-phenol-A-dimethacrylate and triethylene glycol dimethacrylate (mass ratio 1:1) at seven mass fractions. Disks made of these resins were subjected to FTIR microspectroscopy, nanoindentation, microindentation, and 3-point bending to determine the degree of vinyl conversion (DC) modulus and hardness. The shear bond strengths (SBS) of dentin adhesives containing various amount of AP25 were also examined.

Results

The DC increased as a function of mass fraction of AP25 and reached a plateau at 0.1%. The DC of the resin mixture was improved by ≈7% up to 91.7 ± 0.8%. The elastic modulus and hardness of the composites increased initially as more AP25 were added, and decreased after reached the maximum value at approximately 0.06% mass fraction of AP25. The maximum elastic modulus was ≈48% higher than that of the NP-free resin, and the maximum hardness was more than twice higher than that of the NP-free resin. Using these resin composites as dental adhesives, the mean SBS using resins with 0.1% mass fraction of AP25 was ≈30% higher than those using NP-free resin.

Significance

By adding a small amount of AP25 to the resin, the DC and the mechanical properties of resins were improved dramatically. These findings could lead to better performing dental adhesives.

Introduction

Titanium dioxide (TiO 2 ) nanoparticles (NPs) have been demonstrated to be an effective multifunctional material . As the particle size decreases, especially <50 nm, they exhibit photo-induced activities that originate from the semiconductor band gap of TiO 2 NPs . With their energy higher than the band gap, photons can generate electron–hole pairs whose energy may be applied (1) electrically (solar cells) ; (2) chemically (photocatalytic activities including killing bacteria and viruses) ; or (3) by changing the hydrophilicity of the particle surface and resulting in superhydrophilicity . This energy is expressed chemically via free radicals (HO ) following a redox reaction mechanism . The photoactivities mentioned above can be efficiently activated over time by easily accessible resources (such as light) . TiO 2 NPs also have excellent mechanical properties, for example, the elastic modulus of TiO 2 NPs is approximately 230 GPa, and it is inexpensive with titanium being the fourth most abundant metal on earth, following aluminum, iron and magnesium . The unique photoactivities of TiO 2 NPs and their superior mechanical properties make them one of the ideal additives to enhance the performance of polymeric materials .

In dentistry, strong and durable dental adhesives are vital for the success of long-term resin composites used to restore tooth cavities . Dental adhesives bond resin composite to tooth structure and must withstand a harsh oral environment and endure occlusal loads from biting, grinding and chewing. The major components in dental adhesives are dimethacrylate derivatives (dental resins) with/without fillers. Besides high bond strength to tooth and composites, good mechanical properties of the dental resins including modulus and hardness are also essential for success of the dental restoration .

TiO 2 NPs have been used as an additive in dental materials to match the opaque properties of teeth , but using TiO 2 NPs as additives to enhance the mechanical properties of dental resins, has not been successful due to the inconsistent agglomeration of TiO 2 NPs . Many methods have been attempted to prevent the aggregation by modifying the TiO 2 surface with different reagents including surfactants/ligands , proteins and acids . However, adding TiO 2 NPs to substantially improve mechanical properties of the resins was still a concept rather than a fact.

In this study, we developed a convenient protocol to improve the performance of dental resins by adding a small amount (less than 0.1% mass fraction) of TiO 2 NPs with controlled agglomeration. A mixture (mass fraction 1:1) of bis-phenol-A-dimethacrylate (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) was used as base resin, and the TiO 2 NPs were treated with acrylic acid. The agglomeration and dispersion of particles was controlled by centrifugation and solvent evaporation. The successful attachment of acrylic acid to the surface of TiO 2 NPs was confirmed using Fourier transform infrared (FTIR), X-ray powder diffraction (XRD) and thermal gravimetric analysis (TGA). The dispersion of TiO 2 NPs agglomerates in ethanol was evaluated using dynamic light scattering (DLS) and transmission electron microscopy (TEM). Significant improvement of the performance of the resins were achieved: first, the addition of the TiO 2 NPs increased degree of vinyl conversion (DC) of the resin; second, the mechanical properties including elastic modulus and hardness were improved dramatically; third, using the TiO 2 -containing resin as bonding agents of dentin adhesives, the shear bond strength was enhanced.

Materials and methods

Materials and sample preparation

Resins, including 2,2-bis(4-(2-hydroxy-3-methacryloxypropoxy)phenyl)propane (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), 1

1 Certain equipment, instruments or materials are identified in this paper in order to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology or the ADAF, nor does it imply the materials are necessarily the best available for the purpose.

and pyromellitic glycerol dimethacrylate (PMGDM), were obtained from Esstech Inc. Mg(N-tolylglycine glycidyl methacrylate) 2 was also from Esstech Inc. The initiators, camphorquinone (CQ), and ethyl 4- N , N -dimethylaminobenzoate (4E) and benzoyl peroxide, were purchased from Aldrich Corp. Acrylic acid was obtained from Sigma Corp. Titanium dioxide nanoparticles (P25, AEROXIDE TiO 2 ), a known photo-catalytically active material composed with both anatase and rutile phases, were provided by Evonik. All reagents were used as received.

The P25 TiO 2 NPs were modified with acrylic acid, and the product was labeled as AP25. A mixture of acrylic acid (7.2 mL) and water (0.8 mL) was first stirred in a 25 mL vial. Then 8 mL ethyl acetate solution of P25 (0.2 g P25) was added dropwise in 30 min, and then the mixture was agitated at 37 °C for 24 h. A milky mixture was formed and transferred into a 50 mL centrifuge tube. The AP25 precipitated in the bottom and was collected after centrifuging at 3000 rpm for 6 min. This precipitate was then redistributed in 25 mL of ethanol and centrifuged at 3000 rpm for 3 min to remove the remaining acrylic acid. The same step was also used to prepare AP25 organosols in ethanol. The AP25 organosols (≈0.12% by mass or ≈0.02% by volume) in ethanol did not form precipitate for several days. The mass fraction of AP25 in these organosols was determined as the solid percentage after the ethanol was removed via vacuum (1–2 mm Hg) at 22 °C for 24 h.

BisGMA and TEGDMA were mixed at mass ratios 1:1. The resin mixture was activated for blue light photopolymerization by adding 0.2% CQ and 0.8% 4E (by mass) and stored in the dark until use. The AP25 ethanol organosols were mixed into the activated resins by 10 min sonication. The ethanol was then removed via vacuum (1–2 mm Hg) at room temperature for 24 h. Resin monomers with seven mass fractions (0%, 0.02%, 0.03%, 0.05%, 0.06%, 0.08% and 0.5%) of AP25 were prepared.

Preparation of resin disks . A mixture of monomers and AP25 was pipetted into a TEFLON cylinder mold (8 mm in diameter and 1 mm thick) that was placed on top of a piece of Mylar film and a glass slide. Another piece of Mylar film covered the top of the filled mold. The monomer was cured using a Dentsply Triad 2000 visible light curing unit with a tungsten halogen light bulb (250 W and 120 V) for 1 min each from both open sides of the mold. Five disks of each AP25 mass fraction were prepared.

Characterization of AP25 TiO 2 NPs

The AP25 powder was characterized via X-ray diffraction (XRD) for crystalline forms of TiO 2 . Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to determine particle size and morphology, while Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA) were used to evaluate the attachment of acrylic acid on the surface of TiO 2 NPs. XRD patterns of P25 and AP25 powders were recorded with Cu Kα radiation ( λ = 0.154 nm) using a Rigaku 2200 D-Max X-ray diffractometer (Rigaku/USA Inc., Danvers, MA, USA) operating at 40 kV and 40 mA at 10–80° 2 range with intervals of 0.010° 2 . The same divergence and anti-scatter slits (1°) and receiving slit (0.6 mm) were used for all samples. The TEM images were obtained using a Philips EM400T operating at 200 kV. Samples for TEM measurements were prepared by dropping a 0.5 μL AP25 solution onto a carbon-coated copper grid followed by evaporation at room temperature over night. The DLS was performed on an instrument using classical geometric optics i.e., the detected volume was defined by a series of lens–aperture–pinhole combinations. A Coherent Innova 90 laser with a wavelength of 532 nm was used. The DLS measurements were carried out at 11 different angles from 30° to 130° with 10° intervals at 25 °C. The hydrodynamic radius ( R h ) of particles was calculated using the Brookhaven Instruments-DLS software by the method of cumulants using one- or two-exponential fits . The R h distribution was evaluated by both cumulants and CONTIN . The FTIR measurements of AP25 NPs or P25 NPs were carried out in the Nexus 670 FTIR spectrophotometer (Thermo Scientific, Madison, WI). AP25 NPs or P25 NPs were mixed with KBr powder (1.5 mg AP25 in 150 mg KBr) and pressed into pellets. A total of 64 scans were collected from 650 cm −1 to 4000 cm −1 at 4 cm −1 resolution. FTIR of acrylic acid was measured using the same methods except that the acrylic acid was sandwiched between two KBr pellets. TGA was carried out using a TGA Q500 (TA Instruments). Samples (≈5 mg) were placed on a platinum weighing pan and heated from room temperature to 800 °C at a rate of 20 °C/min under flowing N 2 .

Degree of vinyl conversion (DC)

The degree of vinyl conversion for the resins in the disks after photopolymerization was determined using FTIR reflectance microspectroscopy (FTIR-RM) . The Nicolet Continuum FT-IR microscope (Thermo Scientific, Madison, WI) operated in reflectance mode and interfaced with a Nicolet 6700 FT-IR spectrophotometer is equipped with two liquid nitrogen-cooled mercury cadmium telluride detectors (MCT-A: 11,700–650 cm −1 ; MCT-B: 11,700–400 cm −1 ), a video camera, and a computer-controlled x y translation stage. Spectra were collected with 64 scans from 650 cm −1 to 4000 cm −1 at 8 cm −1 spectral resolution with a beam spot size of 90 μm × 90 μm. Ten spectra each of three disks (8 mm in diameter and 1 mm in thickness) of every combination of resin and AP25 were obtained from the flat top and bottom of the disks. Each spot was manually focused before data collection. The reflectance spectra were proportioned against a background of a gold coated slide and transformed to absorbance spectra using the Kramers–Kronig transform algorithm for dispersion correction, which converts the reflectance spectra to absorbance-like spectra. The degree of vinyl conversion (DC) was calculated as the reduction in the methacrylate peak (1634 cm −1 ) height using the phenyl absorbance peak (1610 cm −1 ) as an internal standard . The peak heights were determined using the ISys software (Spectral Dimensions, Olney, MD, USA) . The DC was the average of 30 spectra of three disks of each mass fraction of AP25. The DC of all of the resin disks that were subjected to nanoindentation, Knoop hardness and microindentation were determined. The standard uncertainty associated with the FTIR-RM measurements is <1%.

Nanoindentation

Nanoindentation measurements were performed using an Agilent NanoXP instrument equipped with a 10 μm radius, 90° diamond cone indenter. Samples were indented to a maximum depth of 500 nm or 1500 nm using a single loading and the continuous stiffness method. The contact stiffness between the sample and tip was measured by superposing a small oscillation (45 Hz, 5 nm) over the load profile. The loading time was approximately 120 s with a 30 s hold at the maximum load before unloading. This stiffness was used to calculate the elastic modulus of the sample assuming a constant Poisson’s ratio of 0.45, a representative value for dental composites . The elastic modulus and hardness were determined as the average value obtained over a depth ranging from 250 nm to 450 nm at a maximum depth of 500 nm and 950 nm to 1450 nm at a maximum depth of 1500 nm for each indent and the average of 15 measurements are reported. All indentation experiments were conducted using a constant indentation strain rate of 0.05 s −1 . The standard uncertainty associated with the nanoindentation measurement is 5%.

Knoop hardness

The Knoop hardness in this study was measured in accordance with ASTM standard E 384. A Leitz Miniload 2 microhardness machine was used with indentation loads of 0.25–5 N. Indentation sizes were measured with the same machine using a 10× or 50× objective depending on the length of the indentation on the samples. The loading time for an indentation was of the order of 15 s with a dwell at peak load of 15 s. Knoop hardness was the result of test force divided by the indentation projected surface area: HK = 14.229 P / d 2 , where P is the indenter force and d is the long diagonal length . The Knoop hardness test was performed on resin disks that have been evaluated by FTIR-RM first. The hardness was an average of five measurements of one resin disk of each mass fraction of AP25. The standard uncertainty associated with the microindentation measurement is 5%.

Microindentation to determine elastic modulus

A microindentation technique using static load indenters was also used to measure the elastic modulus of the AP25 nanocomposites . The gravity load from stainless steel spheres (radius = 6.35 mm) indent the substrate, forming a contact area dictated by the substrate modulus and indenter geometry. Contact areas were measured using an inverted optical microscope images (Leica DMIRE II), and image analysis was performed to find contact radii. From Hertzian contact mechanics, the Young’s modulus, E , of the substrate could be calculated from indenter geometry and the indentation load, using the previously set Poisson’s ratio of 0.45. Indentations were performed at five different positions, and images were immediately taken after the indenter was placed on the polymer substrate. For the 0.5% by mass nanoparticle sample, a thinner sample was required to visualize the indentation due to particle light scattering obscuring the contact area. For this sample, a modified version of the Hertzian indentation model was used to correct for the non-infinite substrate thickness and calculate the elastic modulus .

Flexural modulus

Flexural modulus was determined according to ISO4049: 2009. Six rectangular specimens of each material for each test were made by inserting the composite material into a brass mold (25 mm × 2 mm × 2 mm), covering the surfaces with a Mylar film to prevent air-inhibited layers. In this test, all of the bars were cured using a Dentsply Triad 2000 visible light curing unit with a tungsten halogen light bulb (250 W and 120 V) for 2 min each from both open sides of the mold. After curing, the specimens were stored at room temperature for 24 h. Flexural modulus of the resins was determined using Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA) at a cross-head speed of 1 mm/min. The specimens were placed on a 3-point bending test device, which was constructed with 20 mm distance between supports and ensuring an equally distributed load. The flexural modulus of each resin was calculated according to ISO4049: 2009.

Shear bond strength (SBS) test

The SBS test followed a previously established protocol in our center . Briefly, teeth were embedded with Fastray composite (Harry J. Bosworth Company, Skokie, IL, USA) in cylindrical holders and ground perpendicular to their long axis with 320-grit SiC paper until the occlusal enamel was completely removed. A three-step adhesive procedure was used: (1) the dentin surface was etched with a 37% (by mass) phosphoric acid gel (Etch-Rite; Pulpdent Corp., Watertown, MA, USA) for 15 s and rinsed with distilled water. After rinsing, the dentin surface was kept hydrated with a moist blotting paper; (2) a mixture of 20 μL 5% (by mass) Mg(N-tolylglycine glycidyl methacrylate) 2 acetone solution and 40 μL of 20% (by mass) PMGDM acetone solution that contained 2% benzoyl peroxide (by mass based on resin) was applied as a primer and brushed on the dentin surface accumulating 5 layers, air drying between layers to evaporate the solvent; and (3) mixtures of resin and AP25 were applied as bonding agents and were brushed once on the coated dentin surface. The entire dentin surface was then light cured for 10 s with the use of an 8 mm tip on a quartz halogen light source having 450 mW/cm 2 intensity (Max 100, Caulk/Dentsply, Milford, DE, USA). A poly(tetrafluoroethylene)-covered stainless steel ring with an opening, 4 mm in diameter and 1.5 mm in depth, defined the bonding area through which the composite was applied on to the coated dentin. The ring was held down with the assistance of a polycarbonate holder and the iris was filled with TPH composite (Caulk/Dentsply, Milford, DE, USA), which was then irradiated for 1 min with the same light source. The entire assembly was placed in distilled water 5 min after light irradiation and stored for 24 h at approximately 22 °C before conducting a bond test in the shear mode. A commercially available bonding solution (Scotchbond, 3M ESPE, St. Paul, MN, USA) was used as the control. Six mass fractions (0%, 0.02%, 0.08%, 0.10% 0.12%, and 0.50%) of AP25 were examined, and five measurements of each mass fraction were carried out.

A holding device was used to evaluate the SBS. The brass ring holding the dentin-bonded composite was placed against a vertical surface of a nylon block. The ring and the composite were sheared off, at a crosshead speed of 0.5 mm/min, with a flat chisel pressing against the edge of the steel iris. The flat chisel was connected to the platen of a Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA). The maximum debond load was converted into the SBS of the specimen. The mean values of SBS were the average of five measurements for each adhesive.

Statistical analysis

The DC, modulus and hardness were analyzed using one-way analysis of variance (ANOVA) with a 95% confidence interval to indicate significant differences.

Materials and methods

Materials and sample preparation

Resins, including 2,2-bis(4-(2-hydroxy-3-methacryloxypropoxy)phenyl)propane (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), 1

1 Certain equipment, instruments or materials are identified in this paper in order to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology or the ADAF, nor does it imply the materials are necessarily the best available for the purpose.

and pyromellitic glycerol dimethacrylate (PMGDM), were obtained from Esstech Inc. Mg(N-tolylglycine glycidyl methacrylate) 2 was also from Esstech Inc. The initiators, camphorquinone (CQ), and ethyl 4- N , N -dimethylaminobenzoate (4E) and benzoyl peroxide, were purchased from Aldrich Corp. Acrylic acid was obtained from Sigma Corp. Titanium dioxide nanoparticles (P25, AEROXIDE TiO 2 ), a known photo-catalytically active material composed with both anatase and rutile phases, were provided by Evonik. All reagents were used as received.

The P25 TiO 2 NPs were modified with acrylic acid, and the product was labeled as AP25. A mixture of acrylic acid (7.2 mL) and water (0.8 mL) was first stirred in a 25 mL vial. Then 8 mL ethyl acetate solution of P25 (0.2 g P25) was added dropwise in 30 min, and then the mixture was agitated at 37 °C for 24 h. A milky mixture was formed and transferred into a 50 mL centrifuge tube. The AP25 precipitated in the bottom and was collected after centrifuging at 3000 rpm for 6 min. This precipitate was then redistributed in 25 mL of ethanol and centrifuged at 3000 rpm for 3 min to remove the remaining acrylic acid. The same step was also used to prepare AP25 organosols in ethanol. The AP25 organosols (≈0.12% by mass or ≈0.02% by volume) in ethanol did not form precipitate for several days. The mass fraction of AP25 in these organosols was determined as the solid percentage after the ethanol was removed via vacuum (1–2 mm Hg) at 22 °C for 24 h.

BisGMA and TEGDMA were mixed at mass ratios 1:1. The resin mixture was activated for blue light photopolymerization by adding 0.2% CQ and 0.8% 4E (by mass) and stored in the dark until use. The AP25 ethanol organosols were mixed into the activated resins by 10 min sonication. The ethanol was then removed via vacuum (1–2 mm Hg) at room temperature for 24 h. Resin monomers with seven mass fractions (0%, 0.02%, 0.03%, 0.05%, 0.06%, 0.08% and 0.5%) of AP25 were prepared.

Preparation of resin disks . A mixture of monomers and AP25 was pipetted into a TEFLON cylinder mold (8 mm in diameter and 1 mm thick) that was placed on top of a piece of Mylar film and a glass slide. Another piece of Mylar film covered the top of the filled mold. The monomer was cured using a Dentsply Triad 2000 visible light curing unit with a tungsten halogen light bulb (250 W and 120 V) for 1 min each from both open sides of the mold. Five disks of each AP25 mass fraction were prepared.

Characterization of AP25 TiO 2 NPs

The AP25 powder was characterized via X-ray diffraction (XRD) for crystalline forms of TiO 2 . Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to determine particle size and morphology, while Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA) were used to evaluate the attachment of acrylic acid on the surface of TiO 2 NPs. XRD patterns of P25 and AP25 powders were recorded with Cu Kα radiation ( λ = 0.154 nm) using a Rigaku 2200 D-Max X-ray diffractometer (Rigaku/USA Inc., Danvers, MA, USA) operating at 40 kV and 40 mA at 10–80° 2 range with intervals of 0.010° 2 . The same divergence and anti-scatter slits (1°) and receiving slit (0.6 mm) were used for all samples. The TEM images were obtained using a Philips EM400T operating at 200 kV. Samples for TEM measurements were prepared by dropping a 0.5 μL AP25 solution onto a carbon-coated copper grid followed by evaporation at room temperature over night. The DLS was performed on an instrument using classical geometric optics i.e., the detected volume was defined by a series of lens–aperture–pinhole combinations. A Coherent Innova 90 laser with a wavelength of 532 nm was used. The DLS measurements were carried out at 11 different angles from 30° to 130° with 10° intervals at 25 °C. The hydrodynamic radius ( R h ) of particles was calculated using the Brookhaven Instruments-DLS software by the method of cumulants using one- or two-exponential fits . The R h distribution was evaluated by both cumulants and CONTIN . The FTIR measurements of AP25 NPs or P25 NPs were carried out in the Nexus 670 FTIR spectrophotometer (Thermo Scientific, Madison, WI). AP25 NPs or P25 NPs were mixed with KBr powder (1.5 mg AP25 in 150 mg KBr) and pressed into pellets. A total of 64 scans were collected from 650 cm −1 to 4000 cm −1 at 4 cm −1 resolution. FTIR of acrylic acid was measured using the same methods except that the acrylic acid was sandwiched between two KBr pellets. TGA was carried out using a TGA Q500 (TA Instruments). Samples (≈5 mg) were placed on a platinum weighing pan and heated from room temperature to 800 °C at a rate of 20 °C/min under flowing N 2 .

Degree of vinyl conversion (DC)

The degree of vinyl conversion for the resins in the disks after photopolymerization was determined using FTIR reflectance microspectroscopy (FTIR-RM) . The Nicolet Continuum FT-IR microscope (Thermo Scientific, Madison, WI) operated in reflectance mode and interfaced with a Nicolet 6700 FT-IR spectrophotometer is equipped with two liquid nitrogen-cooled mercury cadmium telluride detectors (MCT-A: 11,700–650 cm −1 ; MCT-B: 11,700–400 cm −1 ), a video camera, and a computer-controlled x y translation stage. Spectra were collected with 64 scans from 650 cm −1 to 4000 cm −1 at 8 cm −1 spectral resolution with a beam spot size of 90 μm × 90 μm. Ten spectra each of three disks (8 mm in diameter and 1 mm in thickness) of every combination of resin and AP25 were obtained from the flat top and bottom of the disks. Each spot was manually focused before data collection. The reflectance spectra were proportioned against a background of a gold coated slide and transformed to absorbance spectra using the Kramers–Kronig transform algorithm for dispersion correction, which converts the reflectance spectra to absorbance-like spectra. The degree of vinyl conversion (DC) was calculated as the reduction in the methacrylate peak (1634 cm −1 ) height using the phenyl absorbance peak (1610 cm −1 ) as an internal standard . The peak heights were determined using the ISys software (Spectral Dimensions, Olney, MD, USA) . The DC was the average of 30 spectra of three disks of each mass fraction of AP25. The DC of all of the resin disks that were subjected to nanoindentation, Knoop hardness and microindentation were determined. The standard uncertainty associated with the FTIR-RM measurements is <1%.

Nanoindentation

Nanoindentation measurements were performed using an Agilent NanoXP instrument equipped with a 10 μm radius, 90° diamond cone indenter. Samples were indented to a maximum depth of 500 nm or 1500 nm using a single loading and the continuous stiffness method. The contact stiffness between the sample and tip was measured by superposing a small oscillation (45 Hz, 5 nm) over the load profile. The loading time was approximately 120 s with a 30 s hold at the maximum load before unloading. This stiffness was used to calculate the elastic modulus of the sample assuming a constant Poisson’s ratio of 0.45, a representative value for dental composites . The elastic modulus and hardness were determined as the average value obtained over a depth ranging from 250 nm to 450 nm at a maximum depth of 500 nm and 950 nm to 1450 nm at a maximum depth of 1500 nm for each indent and the average of 15 measurements are reported. All indentation experiments were conducted using a constant indentation strain rate of 0.05 s −1 . The standard uncertainty associated with the nanoindentation measurement is 5%.

Knoop hardness

The Knoop hardness in this study was measured in accordance with ASTM standard E 384. A Leitz Miniload 2 microhardness machine was used with indentation loads of 0.25–5 N. Indentation sizes were measured with the same machine using a 10× or 50× objective depending on the length of the indentation on the samples. The loading time for an indentation was of the order of 15 s with a dwell at peak load of 15 s. Knoop hardness was the result of test force divided by the indentation projected surface area: HK = 14.229 P / d 2 , where P is the indenter force and d is the long diagonal length . The Knoop hardness test was performed on resin disks that have been evaluated by FTIR-RM first. The hardness was an average of five measurements of one resin disk of each mass fraction of AP25. The standard uncertainty associated with the microindentation measurement is 5%.

Microindentation to determine elastic modulus

A microindentation technique using static load indenters was also used to measure the elastic modulus of the AP25 nanocomposites . The gravity load from stainless steel spheres (radius = 6.35 mm) indent the substrate, forming a contact area dictated by the substrate modulus and indenter geometry. Contact areas were measured using an inverted optical microscope images (Leica DMIRE II), and image analysis was performed to find contact radii. From Hertzian contact mechanics, the Young’s modulus, E , of the substrate could be calculated from indenter geometry and the indentation load, using the previously set Poisson’s ratio of 0.45. Indentations were performed at five different positions, and images were immediately taken after the indenter was placed on the polymer substrate. For the 0.5% by mass nanoparticle sample, a thinner sample was required to visualize the indentation due to particle light scattering obscuring the contact area. For this sample, a modified version of the Hertzian indentation model was used to correct for the non-infinite substrate thickness and calculate the elastic modulus .

Flexural modulus

Flexural modulus was determined according to ISO4049: 2009. Six rectangular specimens of each material for each test were made by inserting the composite material into a brass mold (25 mm × 2 mm × 2 mm), covering the surfaces with a Mylar film to prevent air-inhibited layers. In this test, all of the bars were cured using a Dentsply Triad 2000 visible light curing unit with a tungsten halogen light bulb (250 W and 120 V) for 2 min each from both open sides of the mold. After curing, the specimens were stored at room temperature for 24 h. Flexural modulus of the resins was determined using Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA) at a cross-head speed of 1 mm/min. The specimens were placed on a 3-point bending test device, which was constructed with 20 mm distance between supports and ensuring an equally distributed load. The flexural modulus of each resin was calculated according to ISO4049: 2009.

Shear bond strength (SBS) test

The SBS test followed a previously established protocol in our center . Briefly, teeth were embedded with Fastray composite (Harry J. Bosworth Company, Skokie, IL, USA) in cylindrical holders and ground perpendicular to their long axis with 320-grit SiC paper until the occlusal enamel was completely removed. A three-step adhesive procedure was used: (1) the dentin surface was etched with a 37% (by mass) phosphoric acid gel (Etch-Rite; Pulpdent Corp., Watertown, MA, USA) for 15 s and rinsed with distilled water. After rinsing, the dentin surface was kept hydrated with a moist blotting paper; (2) a mixture of 20 μL 5% (by mass) Mg(N-tolylglycine glycidyl methacrylate) 2 acetone solution and 40 μL of 20% (by mass) PMGDM acetone solution that contained 2% benzoyl peroxide (by mass based on resin) was applied as a primer and brushed on the dentin surface accumulating 5 layers, air drying between layers to evaporate the solvent; and (3) mixtures of resin and AP25 were applied as bonding agents and were brushed once on the coated dentin surface. The entire dentin surface was then light cured for 10 s with the use of an 8 mm tip on a quartz halogen light source having 450 mW/cm 2 intensity (Max 100, Caulk/Dentsply, Milford, DE, USA). A poly(tetrafluoroethylene)-covered stainless steel ring with an opening, 4 mm in diameter and 1.5 mm in depth, defined the bonding area through which the composite was applied on to the coated dentin. The ring was held down with the assistance of a polycarbonate holder and the iris was filled with TPH composite (Caulk/Dentsply, Milford, DE, USA), which was then irradiated for 1 min with the same light source. The entire assembly was placed in distilled water 5 min after light irradiation and stored for 24 h at approximately 22 °C before conducting a bond test in the shear mode. A commercially available bonding solution (Scotchbond, 3M ESPE, St. Paul, MN, USA) was used as the control. Six mass fractions (0%, 0.02%, 0.08%, 0.10% 0.12%, and 0.50%) of AP25 were examined, and five measurements of each mass fraction were carried out.

A holding device was used to evaluate the SBS. The brass ring holding the dentin-bonded composite was placed against a vertical surface of a nylon block. The ring and the composite were sheared off, at a crosshead speed of 0.5 mm/min, with a flat chisel pressing against the edge of the steel iris. The flat chisel was connected to the platen of a Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA). The maximum debond load was converted into the SBS of the specimen. The mean values of SBS were the average of five measurements for each adhesive.

Statistical analysis

The DC, modulus and hardness were analyzed using one-way analysis of variance (ANOVA) with a 95% confidence interval to indicate significant differences.

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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Improving performance of dental resins by adding titanium dioxide nanoparticles
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