Nano-porous thermally sintered nano silica as novel fillers for dental composites

Abstract

Objectives

The study evaluates properties of an experimental dental composite consisting of a porous thermally sintered nano-silica as filler. The properties are compared with those of an experimental composite containing micro fillers and a commercially available nano-composite, Filtek Supreme ® Translucent. Different models are used to predict the elastic modulus and strength of the composites.

Methods

Nano-silica with primary particles of 12 nm was thermally sintered to form nanoporous filer particles. The experimental composites were prepared by incorporating 70 wt.% of the fillers into a mixture of Bis-GMA and TEGDMA as matrix phase. Having added photoinitiator system the composites were inserted into the test molds and light-cured. The microfiller containing composites were also prepared using micron size glass fillers. Degree of conversion (DC%) of the composites was measured using FTIR spectroscopy. Diametral tensile strength (DTS), flexural strength, flexural modulus and fracture toughness were measured. SEM was utilized to study the cross section of the fractured specimens. The surface topography of the specimens was investigated using atomic force microscopy (AFM). The specific surface area of the sintered nano silica was measured using BET method. The data were analyzed and compared by ANOVA and Tukey HSD tests (significance level = 0.05).

Results

The results showed improvements in flexural modulus and fracture toughness of the composites containing sintered filler. AFM revealed a lower surface roughness for sintered silica containing composites. No significant difference was observed between DTS, DC%, and flexural strength of the sintered nanofiller composite and the Filtek Supreme ® . The results also showed that the modulus of the composite with sintered filler was higher than the model prediction.

Significance

The thermally sintered nano-porous silica fillers significantly enhanced the mechanical properties of dental composites introducing a new approach to develop materials with improved properties.

Introduction

Since the introduction of dental composites to dentistry, their properties have greatly been improved to overcome the shortcomings of the esthetically interesting materials. The developments in material point of view can be summarized in three categories: (i) improvement of filler phase , (ii) modification of resin monomers and/or introducing new monomer systems , (iii) improvement of initiator system to reach higher degree of polymerization and/or controlled curing kinetics .

Developments in dental bonding agents and composite replacement techniques should also be added to the aforementioned attempts for achieved higher efficacy of the modern dental composites. Although, one may also consider some other aspects of new composites such as fluoride release capacity, radiopacity and translucency as influencing factors for clinical choices, they have little impact on the mechanical properties of the composites.

The particulate fillers which are incorporated into the resin matrix of dental composites cover a wide range of hard glassy particles from the ground quartz with the particle size of several microns to nanosized silica particles.

The incorporation of nanoparticles into the dental composites may improve some properties such as wear resistance, gloss retention , modulus , flexural strength and diametral tensile strength , and fracture toughness . On the other hand, the large surface to volume ratio in the nanoparticles may result in the higher water uptake and resultant degradation of resin–matrix interface . Other problems in the incorporation of nanoparticles into the high viscosity resin monomers are lack of good wetting of the particles and low filler loading. The problems arise from the high surface area of the nanoparticles which is in the range of several hundred m 2 /g. The surface charge of the nano-sized particles results in agglomerated structures which makes them very difficult to be thoroughly dispersed in the matrix phase. Lack of good dispersion of the particles leaves lots of weak points in the composite which may cause local stress concentration resulting in the failure of the restoration.

The main interaction mechanism between matrix resin and filler surface in the composites is suggested to be chemical bonding of the matrix monomers and methacrylate group of the silane coupling agent bonded onto the filler surface through condensation of the silanol functional groups of pre-hydrolyzed silane and the hydroxyl groups on the particle surface .

In this study, silica nanoparticles were thermally sintered in order to provide porous particles with lower surface area to increase loading capacity of the nano fillers. The surface porosity of the sintered particles also provides mechanical interlocking between the cured matrix and the filler particles. Physical and mechanical properties of the experimental composites containing the sintered nanoparticles were then compared with those of the composites prepared using conventional micron-sized glass fillers.

Experimental

Materials

2,2′-Bis-[4-(methacryloxypropoxy)-phenyl]-propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) were supplied by Evonik (Germany). Camphorquinone (CQ), N-N′-dimethyl aminoethyl methacrylate (DMAEMA), and 3-(methacryloyloxy)propyl trimethoxy silane (γ-MPS) were obtained from Sigma–Aldrich (Germany). Amorphous fumed silica with the primary particle size of 12 nm in diameter and surface area of 200 m 2 /g (Aerosil ® 200) was obtained from Evonik (Germany). Glass filler (micro fillers) with the average particle size of 2–5 μm, (SP345; aluminum fluoride: 5–10%, barium fluoride: 5–10%, calcium fluoride: 5–10%, silicon dioxide: 40–70%, zinc oxide: 5–10%) was kindly supplied by Specialty Glass (USA). Filtek Supreme ® Translucent (3 M, ESPE, USA) was used as a commercially available dental composite.

Methods

Sintering of nano-silica

The fumed silica was sintered at different temperatures of 1200 °C, 1300 °C, and 1400 °C using an electric furnace (Carbolite, UK) for 15 min and a heating rate of 20 °C/min to reach the sintering temperature. The sintered silica was then evaluated using SEM to find the optimum sintering temperature in order for the nano particles to be sintered on the surface without being completely melted and diffused. The sintered clusters were then ground by a ball mill (MP100, Retsch, Germany), passed through a 500 mesh (ASTM) sieve and used as the filler phase.

Preparation of the composites

The micro-fillers and sintered nano-fillers were silanized with 1 and 3 wt.% γ-MPS, respectively. γ-MPS was prehydrolyzed for 1 h in an aqueous solution of 70 wt.% ethanol and 30 wt.% de-ionized water (pH adjusted to 3–4 adding a few droplets of acetic acid). The treated fillers were dried for over 1 week at room temperature.

The weight percent of silane used for the silanization of the fillers was calculated according to the following equation:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Silane(wt.%)=filler surface area(m2/g)silane wetting surface(m2/g)×100′>Silane(wt.%)=filler surface area(m2/g)silane wetting surface(m2/g)×100Silane(wt.%)=filler surface area(m2/g)silane wetting surface(m2/g)×100
Silane ( wt. % ) = filler surface area ( m 2 / g ) silane wetting surface ( m 2 / g ) × 100

Considering a wetting surface of 314 m 2 /g for γ-MPS and surface areas of 8.4 m 2 /g for sintered particles and 3 m 2 /g for microfillers (measured by BET method), the fillers were silanized using 3 wt.% and 1 wt.% γ-MPS, respectively.

0.5 wt.% camphorquinone and 0.5 wt.% N,N′-dimethyl aminoethyl methacrylate, as photo-initiator system, were dissolved in the matrix resins (Bis-GMA/TEGDMA, 70/30 wt./wt.) under sub-ambient light. The silanized fillers were then incorporated into the matrix phase and thoroughly mixed to obtain a homogenous paste. The compositions of the experimental and the Filtek Supreme ® are shown in Table 1 .

Table 1
Composition of the composites.
Composites Inorganic phase Organic phase
Microfilled composite 70 wt.% glass filler (specialty glass, 2–5 μm) Bis-GMA/TEGDMA (70/30 wt./wt.)
Sintered nanofilled composite 70 wt.% of sintered nanosilica (Aerosil ® 200, 12 nm) Bis-GMA/TEGDMA (70/30 wt./wt.)
Filtek Supreme ® Translucent (3 M, ESPE, USA). 72.5 wt.% of non aggregated/non agglomerated silica nanofiller (75 nm) + agglomerates of nano silica (nanocluster, 0.6–1.4 μm) a Bis-GMA, UDMA, Bis-EMA and TEGDMA

a According to the manufacturer.

Measurement of degree of conversion

The degree of photopolymerization conversion of samples was measured using FTIR spectroscopy (EQUINOX 55, Bruker, Germany) at a resolution of 4 cm −1 and 32 scans in the range of 4000–400 cm −1 . The samples were placed between two polyethylene films, pressed to form a very thin film and the absorbance spectrum of the un-cured samples were obtained. The samples were then light-cured for 40 s using a QTH dental light source with an irradiance of circa 600 mW/cm 2 (Optilux 501, Kerr, USA) and the spectrum was then collected for the cured samples. Degree of conversion (DC%) was determined from the ratio of absorbance intensities of aliphatic carbon-carbon double bond (peak at 1638 cm −1 ) against internal reference of aromatic carbon–carbon double bond (peak at 1608 cm −1 ) before and after curing of the specimen. The degree of conversion was then calculated as follows:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='DC%=1−(1638cm−1/1608cm−1)peak area after curing(1638cm−1/1608cm−1)peak area before curing×100′>DC%=(1(1638cm1/1608cm1)peak area after curing(1638cm1/1608cm1)peak area before curing)×100DC%=1−(1638cm−1/1608cm−1)peak area after curing(1638cm−1/1608cm−1)peak area before curing×100
DC % = 1 − ( 1638 cm − 1 / 1608 cm − 1 ) peak area after curing ( 1638 cm − 1 / 1608 cm − 1 ) peak area before curing × 100

Measurement of diametral tensile strength (DTS)

Diametral tensile strength (DTS) test was performed adopting the procedure of ANSI/ADA specification No. 27 for light cure resins. The composite pastes were inserted into a cylindrical stainless-steel mold with the internal diameter of 6 mm and height of 3 mm and cured for 40 s from both sides using the light-curing unit. The specimens were removed from the mold and stored in deionized water for one day at 37 °C prior to the test. A universal testing machine (SMT-20, Santam, Iran) was utilized for the test at a cross-head speed of 10 mm/min. The DTS (MPa) was then calculated according to the following equation:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='DTS=2pπDL’>DTS=2pπDLDTS=2pπDL
DTS = 2 p π D L

where p is the load at fracture (N), D (mm) and L (mm) are diameter and height of specimens, respectively.

Measurement of flexural strength and flexural modulus

Flexural strength of the composites was conducted according to the 3-point bending method suggested in ISO 4049. The bar specimens (2 mm × 2 mm × 25 mm) were prepared in stainless-steel rectangular mold utilizing the light curing unit. An overlapping regime was applied to irradiate the whole specimens on both sides (40 s for each irradiation). Having stored in deionized water at 37 °C for one day, the three-point bending test was performed on the specimens using the universal testing machine at a cross-head speed of 1 mm/min. The flexural strength (FS) in MPa was calculated as:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='FS=3pL2bd2′>FS=3pL2bd2FS=3pL2bd2
FS = 3 p L 2 b d 2

where p stands for load at fracture (N), L is the span length (20 mm), and b and d are, respectively, the width and thickness of the specimens in mm. The elastic modulus was also determined from the slope of the initial linear part of stress–strain curve. The flexural modulus of the unfilled matrix resin was also measured in the same conditions.

Measurement of fracture toughness

To determine the fracture toughness, single-edge notch beam (SENB) specimens were fabricated in a 5 mm × 2 mm × 25 mm split steel mold with a razor blade providing a 2.5 mm notch in the middle of the specimens. The bending fracture test was performed at a cross-head speed of 0.1 mm/min using the universal testing machine and the fracture toughness (critical stress intensity factor, K IC ) was calculated according to the following equation :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='KIC=3PL2BW3/21.93aW1/2−3.07aW3/2+14.53aW5/2−25.11aW7/2+25.8aW9/2′>KIC=3PL2BW3/2(1.93(aW)1/23.07(aW)3/2+14.53(aW)5/225.11(aW)7/2+25.8(aW)9/2)KIC=3PL2BW3/21.93aW1/2−3.07aW3/2+14.53aW5/2−25.11aW7/2+25.8aW9/2
K IC = 3 P L 2 B W 3 / 2 1.93 a W 1 / 2 − 3.07 a W 3 / 2 + 14.53 a W 5 / 2 − 25.11 a W 7 / 2 + 25.8 a W 9 / 2

where P is load at fracture (N), L , W , B , and a are length, width, thickness, and notch length (in mm), respectively.

The mode of failure of specimens was also observed under scanning electron microscope.

Scanning electron microscopy (SEM)

The sintered nano-silica for determining the optimum sintering temperature was observed using XL30 (Philips, USA) SEM and fracture surfaces of the specimens in the fracture toughness test for evaluating the mode of fracture were observed by TESCAN (VEGAII, XMU, Czech Republic) SEM. The samples were gold coated by a sputter coater before SEM observations.

Atomic force microscopy (AFM)

Atomic force microscopy (AFM) imaging with (DualScope™ DS95-50, DME, Denmark) was carried out, using non-contact mode and silicon tips, under ambient conditions, to study the surface topography and roughness of the specimens being abraded by a toothbrush testing machine (V8 cross brushing machine, Sabri, USA). For the toothbrush test, the specimens were cured for 40 s (Optilux 501, Kerr, USA) under mylar strip, mounted in an acrylic mold, fixed in the sample holder and inserted into the machine reservoirs. The reservoirs filled with a mixture of 20 g toothpaste (Crest ® , tartar control) and 40 ml distilled water as the abrasive media. The apparatus was set at 75 strokes/min with back and forth motion resulting in approximately 16,000 toothbrush strokes per three and half hours. The force applied from the brushes (Butler GUM, Classic, soft 411) was set at 400 gf.

Measurement of particles surface area

The specific-surface-area of the microfiller and silica nano-particles before and after sintering was measured using the single point BET method (ChemBET 3000, Quantachrome, USA).

Statistical analysis

The results were analyzed and compared using one-way ANOVA and the Tukey test at the significance level of 0.05. The reported values are at least the average of 3 measurements for degree of conversion, and 10 measurements for the mechanical tests.

Experimental

Materials

2,2′-Bis-[4-(methacryloxypropoxy)-phenyl]-propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) were supplied by Evonik (Germany). Camphorquinone (CQ), N-N′-dimethyl aminoethyl methacrylate (DMAEMA), and 3-(methacryloyloxy)propyl trimethoxy silane (γ-MPS) were obtained from Sigma–Aldrich (Germany). Amorphous fumed silica with the primary particle size of 12 nm in diameter and surface area of 200 m 2 /g (Aerosil ® 200) was obtained from Evonik (Germany). Glass filler (micro fillers) with the average particle size of 2–5 μm, (SP345; aluminum fluoride: 5–10%, barium fluoride: 5–10%, calcium fluoride: 5–10%, silicon dioxide: 40–70%, zinc oxide: 5–10%) was kindly supplied by Specialty Glass (USA). Filtek Supreme ® Translucent (3 M, ESPE, USA) was used as a commercially available dental composite.

Methods

Sintering of nano-silica

The fumed silica was sintered at different temperatures of 1200 °C, 1300 °C, and 1400 °C using an electric furnace (Carbolite, UK) for 15 min and a heating rate of 20 °C/min to reach the sintering temperature. The sintered silica was then evaluated using SEM to find the optimum sintering temperature in order for the nano particles to be sintered on the surface without being completely melted and diffused. The sintered clusters were then ground by a ball mill (MP100, Retsch, Germany), passed through a 500 mesh (ASTM) sieve and used as the filler phase.

Preparation of the composites

The micro-fillers and sintered nano-fillers were silanized with 1 and 3 wt.% γ-MPS, respectively. γ-MPS was prehydrolyzed for 1 h in an aqueous solution of 70 wt.% ethanol and 30 wt.% de-ionized water (pH adjusted to 3–4 adding a few droplets of acetic acid). The treated fillers were dried for over 1 week at room temperature.

The weight percent of silane used for the silanization of the fillers was calculated according to the following equation:

Silane(wt.%)=filler surface area(m2/g)silane wetting surface(m2/g)×100
Silane ( wt. % ) = filler surface area ( m 2 / g ) silane wetting surface ( m 2 / g ) × 100

Considering a wetting surface of 314 m 2 /g for γ-MPS and surface areas of 8.4 m 2 /g for sintered particles and 3 m 2 /g for microfillers (measured by BET method), the fillers were silanized using 3 wt.% and 1 wt.% γ-MPS, respectively.

0.5 wt.% camphorquinone and 0.5 wt.% N,N′-dimethyl aminoethyl methacrylate, as photo-initiator system, were dissolved in the matrix resins (Bis-GMA/TEGDMA, 70/30 wt./wt.) under sub-ambient light. The silanized fillers were then incorporated into the matrix phase and thoroughly mixed to obtain a homogenous paste. The compositions of the experimental and the Filtek Supreme ® are shown in Table 1 .

Table 1
Composition of the composites.
Composites Inorganic phase Organic phase
Microfilled composite 70 wt.% glass filler (specialty glass, 2–5 μm) Bis-GMA/TEGDMA (70/30 wt./wt.)
Sintered nanofilled composite 70 wt.% of sintered nanosilica (Aerosil ® 200, 12 nm) Bis-GMA/TEGDMA (70/30 wt./wt.)
Filtek Supreme ® Translucent (3 M, ESPE, USA). 72.5 wt.% of non aggregated/non agglomerated silica nanofiller (75 nm) + agglomerates of nano silica (nanocluster, 0.6–1.4 μm) a Bis-GMA, UDMA, Bis-EMA and TEGDMA

a According to the manufacturer.

Measurement of degree of conversion

The degree of photopolymerization conversion of samples was measured using FTIR spectroscopy (EQUINOX 55, Bruker, Germany) at a resolution of 4 cm −1 and 32 scans in the range of 4000–400 cm −1 . The samples were placed between two polyethylene films, pressed to form a very thin film and the absorbance spectrum of the un-cured samples were obtained. The samples were then light-cured for 40 s using a QTH dental light source with an irradiance of circa 600 mW/cm 2 (Optilux 501, Kerr, USA) and the spectrum was then collected for the cured samples. Degree of conversion (DC%) was determined from the ratio of absorbance intensities of aliphatic carbon-carbon double bond (peak at 1638 cm −1 ) against internal reference of aromatic carbon–carbon double bond (peak at 1608 cm −1 ) before and after curing of the specimen. The degree of conversion was then calculated as follows:

DC%=(1(1638cm1/1608cm1)peak area after curing(1638cm1/1608cm1)peak area before curing)×100
DC % = 1 − ( 1638 cm − 1 / 1608 cm − 1 ) peak area after curing ( 1638 cm − 1 / 1608 cm − 1 ) peak area before curing × 100

Measurement of diametral tensile strength (DTS)

Diametral tensile strength (DTS) test was performed adopting the procedure of ANSI/ADA specification No. 27 for light cure resins. The composite pastes were inserted into a cylindrical stainless-steel mold with the internal diameter of 6 mm and height of 3 mm and cured for 40 s from both sides using the light-curing unit. The specimens were removed from the mold and stored in deionized water for one day at 37 °C prior to the test. A universal testing machine (SMT-20, Santam, Iran) was utilized for the test at a cross-head speed of 10 mm/min. The DTS (MPa) was then calculated according to the following equation:

DTS=2pπDL
DTS = 2 p π D L

where p is the load at fracture (N), D (mm) and L (mm) are diameter and height of specimens, respectively.

Measurement of flexural strength and flexural modulus

Flexural strength of the composites was conducted according to the 3-point bending method suggested in ISO 4049. The bar specimens (2 mm × 2 mm × 25 mm) were prepared in stainless-steel rectangular mold utilizing the light curing unit. An overlapping regime was applied to irradiate the whole specimens on both sides (40 s for each irradiation). Having stored in deionized water at 37 °C for one day, the three-point bending test was performed on the specimens using the universal testing machine at a cross-head speed of 1 mm/min. The flexural strength (FS) in MPa was calculated as:

FS=3pL2bd2
FS = 3 p L 2 b d 2

where p stands for load at fracture (N), L is the span length (20 mm), and b and d are, respectively, the width and thickness of the specimens in mm. The elastic modulus was also determined from the slope of the initial linear part of stress–strain curve. The flexural modulus of the unfilled matrix resin was also measured in the same conditions.

Measurement of fracture toughness

To determine the fracture toughness, single-edge notch beam (SENB) specimens were fabricated in a 5 mm × 2 mm × 25 mm split steel mold with a razor blade providing a 2.5 mm notch in the middle of the specimens. The bending fracture test was performed at a cross-head speed of 0.1 mm/min using the universal testing machine and the fracture toughness (critical stress intensity factor, K IC ) was calculated according to the following equation :

KIC=3PL2BW3/2(1.93(aW)1/23.07(aW)3/2+14.53(aW)5/225.11(aW)7/2+25.8(aW)9/2)
K IC = 3 P L 2 B W 3 / 2 1.93 a W 1 / 2 − 3.07 a W 3 / 2 + 14.53 a W 5 / 2 − 25.11 a W 7 / 2 + 25.8 a W 9 / 2
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Nano-porous thermally sintered nano silica as novel fillers for dental composites
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