d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
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Nano-porous thermally sintered nano silica as novel fillers for dental composites Mohammad Atai a,∗ , Ayoub Pahlavan b , Niloofar Moin b a b
Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran, Iran Dental School, Tehran University of Medical Sciences, Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The study evaluates properties of an experimental dental composite consisting
Received 12 January 2011
of a porous thermally sintered nano-silica as filler. The properties are compared with those
Received in revised form
of an experimental composite containing micro fillers and a commercially available nano-
14 July 2011
composite, Filtek Supreme® Translucent. Different models are used to predict the elastic
Accepted 24 October 2011
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
Keywords:
70 wt.% of the fillers into a mixture of Bis-GMA and TEGDMA as matrix phase. Having added
Dental composites
photoinitiator system the composites were inserted into the test molds and light-cured. The
Sintered nanofiller
microfiller containing composites were also prepared using micron size glass fillers. Degree
Nano-porous filler
of conversion (DC%) of the composites was measured using FTIR spectroscopy. Diametral
Mechanical properties
tensile strength (DTS), flexural strength, flexural modulus and fracture toughness were mea-
Surface topography
sured. 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. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
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 [1–3], (ii) modification of resin monomers and/or introducing new monomer systems [4–8], (iii) improvement of initiator system
Corresponding author. Tel.: +98 21 48662446; fax: +98 21 44580023. E-mail addresses:
[email protected], moh
[email protected] (M. Atai). 0109-5641/$ – see front matter © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2011.10.015
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d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
to reach higher degree of polymerization and/or controlled curing kinetics [9–12]. Developments in dental bonding agents [13,14] and composite replacement techniques [15,16] 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 [17], modulus [18], flexural strength and diametral tensile strength [19], and fracture toughness [20]. 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 [21]. 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 m2 /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 prehydrolyzed silane and the hydroxyl groups on the particle surface [22]. 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.
2.
Experimental
2.1.
Materials
2,2 -Bis-[4-(methacryloxypropoxy)-phenyl]-propane (BisGMA) and triethyleneglycol dimethacrylate (TEGDMA) were supplied by Evonik (Germany). Camphorquinone (CQ), N-N -dimethyl aminoethyl methacrylate (DMAEMA), and 3-(methacryloyloxy)propyl trimethoxy silane (␥-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 m2 /g (Aerosil® 200) was obtained from Evonik (Germany). Glass filler (micro fillers) with the average particle size of 2–5 m, (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.
2.2.
Methods
2.2.1.
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.
2.2.2.
Preparation of the composites
The micro-fillers and sintered nano-fillers were silanized with 1 and 3 wt.% ␥-MPS, respectively. ␥-MPS was prehydrolyzed for 1 h in an aqueous solution of 70 wt.% ethanol and 30 wt.% deionized 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) × 100 silane wetting surface (m2 /g)
Considering a wetting surface of 314 m2 /g for ␥-MPS and surface areas of 8.4 m2 /g for sintered particles and 3 m2 /g for microfillers (measured by BET method), the fillers were silanized using 3 wt.% and 1 wt.% ␥-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.
2.2.3.
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/cm2 (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
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Table 1 – Composition of the composites. Composites
Inorganic phase
Microfilled composite
70 wt.% glass filler (specialty glass, 2–5 m)
Sintered nanofilled composite Filtek Supreme® Translucent (3 M, ESPE, USA).
70 wt.% of sintered nanosilica (Aerosil® 200, 12 nm) 72.5 wt.% of non aggregated/non agglomerated silica nanofiller (75 nm) + agglomerates of nano silica (nanocluster, 0.6–1.4 m)a
a
Organic phase BisGMA/TEGDMA (70/30 wt./wt.) BisGMA/TEGDMA (70/30 wt./wt.) Bis-GMA, UDMA, Bis-EMA and TEGDMA
curve. The flexural modulus of the unfilled matrix resin was also measured in the same conditions.
2.2.6.
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, KIC ) was calculated according to the following equation [23]:
KIC =
3PL 2BW 3/2 −25.11
1608 cm−1 ) before and after curing of the specimen. The degree of conversion was then calculated as follows: DC% =
2.2.4.
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 =
2p DL
where p is the load at fracture (N), D (mm) and L (mm) are diameter and height of specimens, respectively.
2.2.5. 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 =
3pL 2bd2
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
W
a 3/2
− 3.07
W
a 7/2
According to the manufacturer.
a 1/2
1.93
a
+ 25.8
W
a 5/2
+ 14.53
9/2
W
W
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.
2.2.7.
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.
2.2.8.
Atomic force microscopy (AFM)
Atomic force microscopy (AFM) imaging with (DualScopeTM 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.
2.2.9.
Measurement of particles surface area
The specific-surface-area of the microfiller and silica nanoparticles before and after sintering was measured using the single point BET method (ChemBET 3000, Quantachrome, USA).
2.2.10. 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
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Table 2 – Predicted and experimental elastic modulus of the composites. Model
Equation
Experimental
–
Einstein
Ec 1+c
Mooney
Ec Em
=
Guth
Ec Em
= 1 + 2.5Vf +
Kerner
Ec Em
=
1+c 1+m)
Nielsen
Ec Em
=
Upper bound (uniform strain is assumed)
Ec = Ef Vf + Em (1 − Vf )
Lower bound (uniform stress is assumed)
Ec = 1 Ec
Counto
=
Em 1+m
=
(1 + 2.5Vf )
1+c 1+m)
exp
2.5V f 1−SVf
14.1Vf2
1+
Vf
15(1−m ) (1−Vf ) (8−10m )
1+ABVf 1−BVf
Ef Em Ef (1−Vf )+Em Vf 1/2 1−V f Em
+
1 1/2 1/2 )V Em +Ef f f
(1−V
2/3 f 2/3 1+(m−1)(V −Vf ) f
Ec = Em
Ishai and Cohen
Ec = Em 1 +
1−(m−1)V
Paul
V
1/3 m/(m−1)−V f
Results
Table 1 presents the composition of the experimental composites and Filtek Supreme® Translucent (3 M, ESPE, USA). Table 2 tabulated the predicted and experimental elastic modulus of the composites applying different models. Fig. 1 illustrates schematic representation of the sintering process of the nanosilica particles. Fig. 2 is the SEM micrographs of nano silica sintered at different temperatures. As it is seen at 1200 ◦ C no fusion of the particles/aggregates is observed, at 1400 ◦ C the particles are completely fused together forming a bulk silica glass with no nano-sized silica particles, but at 1300 ◦ C clusters of superficially sintered nanoparticles are formed due to the surface fusion of the silica
Predicted modulus for sintered nanofilled composites
5.4 (0.6)
7.6 (1.0)
3.5
3.7
46.6
120.9
9.2
10.3
4.9
5.5
7.3
9.5
34.4
37.2
3.0
3.3
4.9
5.3
6.4
7.0
4.8
5.4
measurements for degree of conversion, and 10 measurements for the mechanical tests.
3.
Predicted modulus for microfilled composites
particles. Fig. 3 illustrates degree of conversion of the composites measured by FTIR. No significant difference is observed between the DC% of the composites (p > 0.05). Fig. 4 shows diametral tensile strength of the composites. DTS of the composites is in the same range (p > 0.05). Fig. 5 depicts the flexural strength of the composites. As it shows flexural strengths of the Filtek Supreme® and the sintered nano-filled composites are not different (p > 0.05), while both presents flexural strengths higher than micro-filled composite (p < 0.05). The sintered nano filled composite shows (Fig. 6) higher flexural modulus than the other two composites (p < 0.05). Fig. 7 reveals a higher fracture toughness (KIC ) (p < 0.05), measured in single edge notch test, for the sintered nano filled composites in comparison to the other two composites. The SEM micrographs of the fractured surface of the composites in the fracture toughness test are shown in Fig. 8. The figure illustrates the details on the surface of the fractured specimens. Fig. 9 illustrates the AFM micrographs of the surface of the composites after
Fig. 1 – schematic representation of the sintering process of the nanosilica particles.
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
137
Fig. 2 – SEM micrographs of nano silica sintered at different temperatures: (a) 1200 ◦ C, (b) 1300 ◦ C, (c) 1400 ◦ C. The insets show the surface details at 100,000×.
toothbrush test which reveals smoother surfaces for the Filtek Supreme® and sintered nano filled composites. Fig. 10 shows the surface roughness (Sq) of the composites measured by AFM after toothbrush test. Lower roughness is found for both Filtek Supreme® and sintered nano filled composites.
4.
Discussion
Developing dental composites with improved physical and mechanical properties has been the goal of dental material
scientists. Introducing dental composites with nano-sized fillers is believed to overcome some shortcomings of the composites. Wear resistance, gloss retention and flexural strength are among the properties which are reported to be improved using nano-technology [17]. The main problems in the composites utilizing nano-sized particles as filler are low filler loading and lack of good dispersion of the particles. The problems arise from the very high surface area and surface charge of the particles which result in particle agglomeration. The agglomerates act as weak and local stress concentration points leading to reduced properties. As an approach to
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10
Degree of conversion (%)
80
Flexural modulus (GPa)
70 60 50 40 30 20 10 0
8
6
4
2
0 Micro-filled
Sintered nano-filled
Filtek Supreme®
Micro-filled
Fig. 3 – Degree of conversion of the composites measured by FTIR.
Diametral tensile strength (MPa)
30 20 10 0 Micro-filled
Sintered nanofilled
Filtek Supreme®
Fig. 4 – Diametral tensile strength of the composites.
Flexural strength (MPa)
1/2
140 120 100 80 60 40 20 0
Sintering of the silica nano-particles
Aerosil® is amorphous pure silicone dioxide produced by high temperature hydrolysis of silicon tetrachloride. It consists of free of pore spherical primary particles. Aerosil® 200 is one of the product grades with an average primary particle size of 12 nm and surface area of 200 m2 /g. Aerosil® particles start to sinter and turn into glass above 1200 ◦ C [24]. Therefore, in the present study, the silica particles were sintered at different temperatures of 1200 ◦ C, 1300 ◦ C, and 1400 ◦ C to find the appropriate sintering temperature. As Fig. 2 shows, at 1200 ◦ C the primary particles or aggregates are separately distinguished. At 1300 ◦ C the particles are superficially fused together forming clusters which have pores on the surface. The surface porosity provides points for penetration of the matrix resin which results in micromechanical retentions between matrix resin and the filler along with chemical bonding due to the applied silane coupling agent. At 1400 ◦ C, well above the softening point of silica, the particles completely defuse and form a glassy structure with no evidence of primary nanoparticles or aggregates. According to the aim of the study, the temperature of 1300 ◦ C was chosen to prepare particles with surface nano porosity exhibiting a texture consisting of the primary particles and aggregates. The sintered particles were then ground to break the weakly bonded clusters and passed through a 500 mesh (ASTM) sieve. BET measurement showed
Fracture toughness (MPa.m )
overcome the problems while the meritorious features of the nano-particles are kept, in this study, the silica nano particles were thermally sintered to produce secondary particles with lower surface area having the potential advantages of nanosized primary particles in the formed cluster structure. The sintering condition and properties of the composites containing the particles were then investigated.
2
1.5
1
0.5
0 Micro-filled
Micro-filled
Sintered nanofilled
Filtek Supreme®
Fig. 5 – Flexural strength of the composites.
Filtek Supreme®
Fig. 6 – Flexural modulus of the composites.
4.1. 40
Sintered nanofilled
Sintered nanofilled
Filtek Supreme®
Fig. 7 – Fracture toughness (KIC ) of the composites, measured in single edge notch test.
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
a surface area of 196 m2 /g for non sintered primary silica particles (the reported value by the manufacturer is 200 ± 25 m2 /g), 8.4 m2 /g for the sintered particles (at 1300 ◦ C), and 3 m2 /g for the micron sized glass fillers.
4.2.
Mechanical properties
Mechanical properties of dental composites directly depend on the degree of polymerization conversion (DC%). Comparison between the properties should be made at the same DC%. Fig. 3 shows that there is no significant difference among the DC% of the composites used in this study. Therefore the measured values for the properties are comparable. As Fig. 5 shows there is no significant difference between diametral tensile strength of the composites. The test is valid for the brittle materials in which plastic deformation is negligible [25]. The force–displacement curves in the test revealed a brittle behavior validating the DTS results. Flexural strength is a commonly used test for evaluation of the properties of dental composites [26–28]. The test has also been suggested by ISO 4049. Flexural strength and flexural modulus of the composites are presented in Figs. 5 and 6. Surface porosity of the sintered fillers provides good micromechanical retention between matrix resin and the sintered fillers. The strong mechanical interlocking along with the chemical bonding of the matrix resin with the methacrylate functional groups of the silane coupling (␥-MPS) prevent the crack propagation through the filler-matrix interface resulting in higher flexural strength. Flexural modulus of the sintered nano filled is also higher than the other two composites (p < 0.05). The elastic modulus of a particulate-filled composite depends on the elastic properties of its components [29]. Since the inorganic particulate fillers generally have higher stiffness, the modulus of the polymeric matrix of dental composites is improved by incorporation of the rigid fillers. Different empirical or semi-empirical equations have been proposed to predict the elastic modulus of particulate composites. We utilized some of the equations to predict and compare the elastic modulus of our experimental composites. The equations predict the modulus based on the volume fraction of the ingredients. Therefore the volume fraction of the fillers and the matrix phase of the composites were calculated according to the following equation:
Vf =
Wf Wf + (f /m )(1 − Wf )
(1)
where Vf , Wf , and f are the volume fraction, weight fraction, and density of the filler and m is density of matrix. The density of the fillers was measured using the pycnometric method of ASTM D854-10 as 2.89 g/cm3 for the microfillers and 2.4 g/cm3 for the sintered nanosilica particles. The volume fraction of the fillers in the composites equals 0.48 for microfillers and 0.52 for sintered particles. The density of matrix was also calculated as 1.13 g/cm3 considering the density of Bis-GMA (1.16 g/cm3 ) and TEGDMA (1.075 g/cm3 ) and their volume fractions according to the “rule of mixtures”.
4.2.1.
139
Prediction of the elastic modulus
A number of theories and models have been developed to describe the elastic modulus of polymeric composites based on the shape, size, aspect ratio and distribution of the reinforcing particles and the stiffness of matrix phase. According to the rigidity of the particle and matrix the models might be categorized in two classes: rigid particles in non-rigid matrix and rigid particles in rigid matrix [30]. As there is no definition distinguishing rigid and non-rigid matrixes based on their modulus, both models were assessed to predict the modulus of composites of the present study with a modulus of 1.6 GPa. The experimental date were fitted to the model equations and compared. The measured and calculated values for the modulus are tabulated in Table 2.
4.2.1.1. Models for rigid particles in non-rigid matrix. Einstein’s equation [31] is one of the earliest theories which predicts the viscosity of suspensions of rigid particles. Considering that the changes in the viscosity and modulus are analogous the Einstein’s equation is held for the prediction of shear modulus of particulate filled polymeric systems as follows: Gc = Gm (1 + 2.5Vf )
(2)
where Gc and Gm are the shear modulus of the composite and resin matrix, respectively. Young’s modulus of the matrix phase (unfilled resin), Em , was measured as 1.6 (0.1) GPa in the same conditions as the composites were tested. According to linear elasticity theory Young’s (E), shear modulus (G), and Poisson’s ratio () are inter-related via the following equation [32]: G=
E 2(1 + )
(3)
The Einstein’s equation is re-written as: Ec Em = (1 + 2.5Vf ) 1 + c 1 + m
(4)
where c and m are Poisson’s ratios of the composite and matrix, respectively. Poisson’s ratios were considered as c = 0.30 and m = 0.35 [33]. As Table 2 shows Einstein’s equation (4) predicts the modulus of the composites much lower than the experimental values. The equation is originally derived for a suspension of rigid spherical particles and only holds in extremely low concentrations [32]. Dental composites are highly filled particulate systems in which the particle–particle interactions greatly affect the properties. Guth [34] considered the interactions between the particles by adding a term to the Einstein equation. Ec = 1 + 2.5Vf + 14.1Vf2 Em
(5)
The equation can analogously be written for shear and Young’s moduli [30]. The prediction of the Guth equation is higher than the experimental values (Table 2) showing that the modified equation might not be applied in the present composite systems.
140
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
Fig. 8 – SEM micrographs (2000×) of the fractured surface of the composites in the fracture toughness test. The insets show the surface details at 60,000×.
Mooney modified Einstein’s equation in the following form which is suggested to represent the modulus of composites at high filler volume fractions [30].
Gc = exp Gm
2.5Vf 1 − SVf
(6)
where S is the crowding factor (volume occupied by the filler/true volume of the filler). For close packed spherical particles S = 1.35 [30]. Considering Eq. (3) and Poisson’s ratios of composite and matrix, Young’s moduli predicted by Mooney equation are presented in Table 2. The model predicts extremely high values for the modulus at higher filler concentrations (high Vf ). The equation assumes that the modulus
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
141
Fig. 9 – AFM micrographs of the surface of the composites after toothbrush test.
of the filler is infinitely greater than the matrix and Poisson’s ratio of the matrix is 0.5 which are not correct for a dental composite. Thus the applicability of the model is restricted to filled rigid thermo-setting polymeric matrices [30,32]. Kerner [35] equation predicts the modulus of polymeric composites containing rigid fillers (Gf > Gm ) up to moderate concentrations.
According to Table 2, the prediction of Kerner equation is close to the experimental values especially for the composite containing microfillers. Nielsen [32] generalized Halpin and Tsai [36] equation as:
Vf Gc 15(1 − m ) =1+ Gm (1 − Vf ) (8 − 10m )
1 + ABVf Ec = Em 1 − BVf
(7)
(8)
Roughness, Sq (nm)
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200
Assuming uniform strain in the individual phases (isostrain), the upper bound is given by:
150
Ec = Ef Vf + Em (1 − Vf )
when the stress is assumed to be uniform in both phased (isostress), the lower bound is obtained from:
100
50
Ec =
0 Microfiller
Sintered nano
Filtek Supreme®
Fig. 10 – Surface roughness (Sq) of the composites measured by AFM after toothbrush test.
where A is a constant which takes into account the geometry of the particulates and Poisson’s ratio of the matrix and B considers the relative modulus of the filler and the matrix: A = kE − 1
(9)
and B=
Ef /Em − 1
(10)
Ef /Em + A
kE = 2.5 for a matrix with Poisson’s ratio of 0.5. For the unfilled resin with m = 0.35 [33] the factor A was calculated as A = 1.168 for our experimental composites (kE = 2.168 as suggested in the reference [32] for a matrix with a Poisson’s ratio of 0.35). Elastic modulus of fillers was considered as Ef = 70 GPa [37]. =1+
(12)
1 − ϕm 2 ϕm
Vf
(11)
The factor ϕ depends on the maximum packing fraction of the filler. For the studied systems with the nearly spherical particles (aspect ratio of the particles is close to 1), a nonagglomerated random close packing with ϕ = 0.63 [32] was considered. The was then calculated as 1.45 for microfillers and 1.48 for sintered fillers. The Nielsen equation also predicts higher values for the both composite (Table 2). The deviation of the values predicted by the above equations is probably due to the fact that the equations initially developed for the suspensions of rigid particles in non-rigid matrix, especially in low particulate concentrations, while in dental composites the matrix can be considered as a rigid phase and the filler content is relatively high.
4.2.1.2. Models for rigid particles in rigid matrix. Considering the composites as two phase systems which undergo an average stress or strain, the simple equations of “law of mixtures” can be applied to estimate upper and lower bounds for elastic modulus.
Ef Em
(13)
Ef (1 − Vf ) + Em Vf
The upper and lower bounds often, as in our study (Table 2), are unable to predict the experimental values, implying that the assumption of either a state of uniform strain or uniform stress is not sufficient to describe the modulus of the filled systems [30]. However, the equations may serve as useful approximates for the prediction of upper and lower limits. Counto [38] proposed a simple model for two phase systems assuming perfect bonding between the particles and matrix: 1/2
1 − Vf 1 = Ec Em
+
1 1/2 1/2 (1 − Vf )/Vf
(14) Em + Ef
According to Table 2, the model predicts elastic modulus in a relatively good agreement with the experimental values especially for the composite containing microfillers. Assuming that the state of stress is macroscopically homogenous in the composite constituents, Paul [39] suggested the following equation to approximate the modulus:
Ec = Em
2/3
1 − (m − 1)Vf 2/3
1 + (m − 1)(Vf
(15)
− Vf )
in which m = Ef /Em . The prediction of Paul equation is higher than the experimental value for the microfilled composite but is still lower than the value for the composite containing sintered fillers (Table 2). Considering a uniform strain at the boundary Ishai and Cohen [40] developed the following equation:
Ec = Em 1 + (
V 1/3
m/(m − 1) − Vf
)
(16)
where m = Ef /Em . As Table 2 shows the prediction of Ishai and Cohen is lower than the experimental values. The Paul and Ishai equations are, in fact, another upper and lower bounds, respectively. Comparing the experimental and predicted values (Table 2), it can be concluded that the models which have been developed for rigid inclusions in rigid matrix approximate the modulus of the composites much closer to the experimental values. Interestingly, the modulus of the composite containing sintered particles is higher than the predictions of all models (except the upper bound of law-of-mixtures). The observed higher modulus is probably due to the strong
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
micromechanical interlocking between the porous particles and matrix resin. Penetration of the resin into the surface porosities of the filler particles provides the strong micromechanical retention upon polymerization of the resin monomers.
4.2.2.
Prediction of the strength
The strength of particulate-filled polymer composites depends on parameters such as particle size, interface adhesion and particle loading [29]. Models predict that the strength might either increase or decrease depending on the interfacial bonding between particles and matrix resin. In the case of poor interfacial interaction the applied stress cannot be transferred from the matrix to the particles and the strength of the composite is determined from the effective sectional area of load-bearing matrix in the absence of the particles [29]. A very simple expression for the strength of such a composite is given by [41]: c = m (1 − Vf )
(17)
where c and m are the strength of composite and matrix, respectively, and Vf is filler volume fraction. As it is seen the equation predicts a decrease in strength with increasing filler loading. Some other models have also been introduced which predict a decreasing trend in the strength of composite with filler loading [29]. In dental composite is assumed that a strong bonding between matrix and filler particles is provided applying silane coupling agents [22]. Taking into account the interfacial bonding the following empirical relationship has been suggested by Turcsanyi et al. [42]:
c =
1 − Vf 1 + 2.5Vf
m
exp(BVf )
(18)
where B is an empirical constant, which depends on the interfacial properties of a given system and the yield stress of the matrix. In our experimental composites the matrix phase is the same; therefore the parameter B determines the interfacial bonding between the filler and matrix. Fitting the experimental values (Table 2) in Eq. (18) the parameter B was calculated as 4.41 and 3.62 for the composites containing the sintered and microfillers, respectively. The higher value of B shows stronger interfacial adhesion between filler and matrix [42]. The result also confirms the stronger bonding between the sintered filler and the matrix resin which is attributed to the penetration of the resin into the surface porosity of the filler. It is worthwhile to indicate that according to Turcsanyi et al. [42] the B = 3.62 is in the range of values for good interfacial adhesion which confirms the hypothesis that silane coupling agents and shrinkage-induced stress provide a reliable bonding between filler and matrix in dental composites.
4.2.3.
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different directions to the dentinal tubules [44,45]. The fracture toughness of the experimental composite containing sintered nanoparticles (1.65 ± 0.10 MPa m−1/2 ) approaches the values of human dentin. Fracture is the result of initiation of cracks and their propagation through the composites. The propagation might happen through matrix, reinforcing fillers, and the matrix–filler interface. Because of the high strength and modulus of the fillers, the propagation more likely follows the other two routes. The composition and curing condition of the microfilled and sintered nanofilled experimental composites are the same, therefore, the most probable reason for the higher fracture toughness of the composite containing the porous particles is the mechanical interlocking which are formed between the matrix and filler particles due to the penetration of the matrix resin into the porosities. Utilizing porous fillers in experimental dental composites has been shown to result in the composites with improved mechanical properties [2,46]. Curtis et al. [20] using a micromanipulation technique proposed the hypothesis that the nanoclusters of nanofilled dental composites may undergo multiple fracture prior to and during the fracture which may impart damage tolerance and a modified failure mechanism providing reinforcement to the resin composites. Both hypotheses could support the higher properties of the sintered nanoparticles containing composite. SEM micrographs of the fractured surface of the composites in fracture toughness test (Fig. 8) reveal that there is a good adhesion between the matrix resin and the fillers because of using the silane coupling agent. The figure also shows more bubbles and voids on the fracture surface of the experimental composites in comparison with Filtek Supreme® . As the ingredients of the composites were hand-mixed, the formation of the voids is inevitable. It is expected that the composites would present still higher mechanical properties if they would have prepared in large scale utilizing mixing equipment. The surface topography of the sintered nanofilled and Filtek Supreme® is almost the same (insets in Fig. 8). To study the surface details of the composites, atomic force microscopy was performed on the specimens which were subjected to toothbrush test (Fig. 9). As it is expected the microfilled composite shows a rough surface due to its large filler particles. In contrast, sintered nanofiller containing composite and Filtek Supreme® show a very smooth surface. Both composites contain nano-scale filler particles. A smooth and enamel-like finish is more achievable with the nanocomposites rather than the microfilled ones. In the composites with large filler particles a rough surface is left after the matrix worn away due to the presence of the microparticles protruding from the surface or the plucked off particles. To quantify the surface smoothness, the root mean square roughness, Sq, of the brushed specimens was measured. As Fig. 10 shows, sintered nanofilled composite and Filtek Supreme® show much lower surface roughness than the microfilled one (p < 0.05). Fig. 10 confirms the SEM and AFM observations.
Fracture toughness
Fracture toughness is a determining property in the performance of the brittle materials such as dental composites. The reported values for the fracture toughness, KIC , of the human enamel is about 0.7–1.27 MPa m−1/2 measured by indentation technique [43] and about 1.13–3.13 MPa m−1/2 for dentin in
5.
Conclusions
This study describes the preparation of new dental fillers with porous structure from silica nanoparticles through
144
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 133–145
thermal sintering. It compares the properties of experimental composites containing this porous filler and conventional microfiller. The properties are also compared with those of a commercially available dental nanocomposite. The following conclusions are made according to the results. A porous structure is obtained by sintering the nano silica particles at 1300 ◦ C. The experimental composites containing the sintered nanoparticles exhibited higher flexural strength and elastic modulus and fracture toughness in comparison to the microfilled composite. The sintered nanocomposite also showed smoother surface after toothbrush abrasion test. Applying different models the elastic modulus of the experimental composite was predicted and compared with the measured values. The models which have been developed for prediction of modulus in the composites with rigid inclusion in rigid matrix presented better agreement with the experimental values. The sintered particle containing composites, however, showed higher elastic modulus than the predicted values. The higher modulus attributed to the micromechanical interlocking due to the penetration of the matrix resin into the surface porosities of the sintered particles.
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