Abstract
Objective
To fabricate and characterize dental composites reinforced with various amounts of zirconia–silica (ZS) or zirconia–yttria–silica (ZYS) ceramic nanofibers.
Methods
Control composites (70 wt% glass particle filler, no nanofibers) and experimental composites (2.5, 5.0, and 7.5 wt% ZS or ZYS nanofibers replacing glass particle filler) were prepared by blending 29 wt% dental resin monomers, 70 wt% filler, and 1.0 wt% initiator, and polymerized by either heat or dental curing light. Flexural strength (FS), flexural modulus (FM), energy at break (EAB), and fracture toughness (FT) were tested after the specimens were stored in 37 °C deionized water for 24 h, 3 months, or 6 months. Degree of conversion (DC) of monomers in composites was measured using Fourier transformed near-infrared (FT-NIR) spectroscopy. Fractured surfaces were observed by field-emission scanning electron microscope (FE-SEM). The data were analyzed using ANOVA with Tukey’s Honestly Significant Differences test used for post hoc analysis.
Results
Reinforcement of dental composites with ZS or ZYS nanofibers (2.5% or 5.0%) can significantly increase the FS, FM and EAB of dental composites over the control. Further increase the content of ZS nanofiber (7.5%), however, decreases these properties (although they are still higher than those of the control). Addition of nanofibers did not decrease the long-term mechanical properties of these composites. All ZS reinforced composites (containing 2.5%, 5.0% and 7.5% ZS nanofibers) exhibit significantly higher fracture toughness than the control. The DC of the composites decreases with ZS nanofiber content.
Significance
Incorporation of ceramic nanofibers in dental composites can significantly improve their mechanical properties and fracture toughness and thus may extend their service life.
1
Introduction
Resin-based dental composites are widely used in dentistry for the restoration of carious teeth. They have been used to replace dental amalgam restorations because of the esthetic (tooth-colored) property of composites and the safety concern for mercury in amalgam. But resin composites may have a shorter life than amalgams due to secondary (recurrent) caries and bulk fracture . To reduce secondary caries, a number of fluoride-releasing dental composites have been developed and made commercially available. However, nearly all of the commercial fluoride-releasing dental composites have very low fluoride release and recharge capabilities, and therefore, possess minimal caries-inhibiting effects . In the past two decades, extensive research efforts have been directed toward the development of dental composites that release a higher amount of anti-caries agents (F − , Ca 2+ and PO 4 3− ions) . However, dental composites reinforced with particulate fillers, particularly composites that release anti-caries agents, still demonstrate inadequate mechanical properties and fracture toughness (FT).
To reduce bulk fracture and increase mechanical properties, various high-strength, high-modulus fibers have been used to improve flexural strength (FS) and FT of composites. Such fibrillar materials include organic polymer fibers , silica and glass fibers , ceramic (SiC and Si 3 N 4 ) whiskers and carbon nanotubes . Incorporation of those fibers can significantly increase stiffness, FS, FT and fatigue resistance of the composites, but the chemical stability, esthetics and handling properties of these materials are unsatisfactory. Fabrication of composite restorations reinforced by long polyethylene or glass fibers is technique-sensitive and time-consuming. Composites reinforced with glass fibers exhibit decreased mechanical properties after prolonged storage in water. For example, FS and flexural modulus (FM) of a commercial dental composite, DC-Tell (DCS Dental, Allschwil, Switzerland), which contains 38% short glass fibers, decreased 66% and 60% respectively after storage in water for 3 months. The flexural strength after dehydration did not recover to the same level of the dry-group . On the other hand, ceramic materials usually have excellent mechanical properties, as well as superior chemical resistance, thermal stability and good biocompatibility. It was reported that impregnation of extremely strong SiC and Si 3 N 4 ceramic whiskers could lead to a two-fold increase in strength and toughness in heat-cured composites . However, such composites cannot be light cured, which limits its application, because the mismatch of the refractive indices between the whiskers (SiC 2.65 and Si 3 N 4 2.2) and polymer resin (1.53) causes high opacity (light scattering effect) of the whisker-reinforced composites. Therefore, alternative reinforcing elements are needed for tooth-colored, light-curable fiber-reinforced dental composites.
Recently we have prepared dense zirconia–yttria (ZY), zirconia–silicia (ZS) and zirconia–yttria–silica (ZYS) ceramic nanofibers by a reactive electrospinning sol–gel method and subsequent calcinations . These dense ceramic nanofibers with diameters of 100–300 nm have a tetragonal zirconia crystalline phase. In particular, ZS and ZYS nanofibers have smooth surfaces and an amorphous silica phase. They can be good candidates for reinforcement elements of dental composites.
The objective of this research is to study the reinforcement effects of dense ceramic ZS or ZYS nanofibers on dental composites, including the effects on the mechanical properties FS, FM, energy at break (EAB), and FT, and degree of conversion (DC) of monomers in the composites. The rationale for using ZS or ZYS ceramic nanofibers to reinforce dental composites is as following: (1) zirconia-based ceramics have high toughness, good chemical stability and biocompatibility; (2) nanofibers (diameter < 200 nm) may reduce light scattering as well as improve mechanical properties and polishability.
2
Materials and methods
2.1
Materials
Camphorquinone (CQ), ethyl 4-dimethylaminobenzoate (4E), phenyl bis(2,4,6-trimethyl benzoyl)phosphine oxide (PO), benzoyl peroxide (BPO), 3-methacryloxypropyltrimethoxysilane (MPTMS) and propylamine were purchased from Aldrich. 2,2-Bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]-propane (BisGMA) was purchased from Polysciences. Ethoxylated bisphenol A dimethacrylate (EBPADMA), 1,6-hexanediol dimethacrylate (HDDMA) were provided by Esstech Inc. (Essington, PA, USA). Silanized ultrafine glass filler particles (mean diameter 0.8 μm) were provided by Caulk/Dentsply. The ceramic ZS and ZYS nanofibers used in this study (mean diameter 190 nm) were prepared by sol–gel processing and reactive electrospinning followed by calcination at 1200 °C as previously reported . The molar ratios of Zr/Si or Zr/Y/Si are shown in Table 1 .
| Nanofibers | Zr/Y/Si (molar ratios) |
|---|---|
| ZS1 | 90/0/10 |
| ZS2 | 80/0/20 |
| ZYS | 76.8/3.2/20 |
2.2
Preparation of nanofiber-reinforced dental composites
2.2.1
Silanization of nanofibers and dispersion of nanofibers in particulate glass filler
Ceramic ZS or ZYS nanofibers (1 g), silane coupling agent MPTMS (100 mg) and propylamine (50 mg, as a catalyst) were added to cyclohexane (200 ml), and the mixture was stirred at 70 °C for 2 h. Solvent was then removed with a rotary evaporator. The solid was dried overnight at 110 °C in an oven, then washed with methanol and dried again at 110 °C for 2 h. Silanized ZS or ZYS nanofibers and glass fillers (weight ratio: 2.5/67.5, 5/65 or 7.5/62.5) were added to ethanol (5 g solid in 100 ml ethanol). The mixture was sonicated for 3 min and then stirred at room temperature for 2 h. The mixtures of ZS and glass particles or ZYS and glass particles were collected by filtration and dried at 60 °C overnight.
2.2.2
Formulation and fabrication of composites
The formulations of the control and experimental composites are listed in Table 2 . All percentages listed in this paper are weight percentages. All composites were formulated with the same (29%) monomer mixture (11.6% BisGMA, 11.6% EBPADMA and 5.8% HDDMA), 70% filler mixtures, and 1% initiators (BPO for the heat-cured composites, or photoinitiator mixture for the light cured composites). Although the experimental composites with 75% or higher content of glass particles could be fabricated, the composite with 75% mixed fillers of ceramic nanofibers (2.5% or more) and glass particles had an unacceptably high viscosity and poor mechanical properties. Therefore, 70% total filler mixtures has been selected based on the formulation of several commercial products and our previous studies . The composites were fabricated by mixing the monomer mixture, filler mixture and initiators for 5 min using a SpeedMixer™ (mode DAC 150 FVZ, FlackTek, Inc.). Specimens ( n = 10 for each composite) for tests of FS, FM and EAB were prepared by either heat-cure (110 °C for 2 h in an oven) or light-cure (6× 80 s on both sides) with an Optilux 501 curing light (Kerr Corp., Orange, CA) in 2 mm × 2 mm × 25 mm stainless steel molds. Specimens ( n = 10) for FT tests were prepared by light-cure (6× 80 s on both sides) in copper molds of 2.5 mm × 5 mm × 25 mm with a 2.41 mm notch. All specimens (for both FS and FT tests) were polished with 600 grit SiC abrasive paper.
| Composites | Composition | ||||
|---|---|---|---|---|---|
| Monomer mixture a (%) | Glass filler (%) | Nanofibers (%) | Initiator (%) | ||
| BPO | CQ/4E/PO b | ||||
| H-Ctr | 29 | 70.0 | 0 | 1 | |
| H1-2.5 | 29 | 67.5 | 2.5 (ZS1) | 1 | |
| H1-5.0 | 29 | 65.0 | 5.0 (ZS1) | 1 | |
| H2-2.5 | 29 | 67.5 | 2.5 (ZS2) | 1 | |
| L-Ctr | 29 | 70.0 | 0 | 1 | |
| L1-2.5 | 29 | 67.5 | 2.5 (ZS1) | 1 | |
| L1-5.0 | 29 | 65.0 | 5.0 (ZS1) | 1 | |
| L2-2.5 | 29 | 67.5 | 2.5 (ZS2) | 1 | |
| L2-5.0 | 29 | 65.0 | 5.0 (ZS2) | 1 | |
| L2-7.5 | 29 | 62.5 | 7.5 (ZS2) | 1 | |
| L-ZYS-2.5 | 29 | 67.5 | 2.5 (ZYS) | 1 | |
2.3
Rheological characterization of the uncured resins (pastes)
The rheological properties of the uncured composite resins with different filler mixtures were measured by an ARES rheometer (TA Instruments-Waters LLC, New Castle, DE) with parallel plate configuration. The uncured composite resins were loaded onto the peltier plate at room temperature (25 °C). The viscosity was measured as a function of strain under a measuring gap of 0.5 mm and an oscillation frequency of 1 Hz.
2.4
Testing of mechanical properties
Specimens were stored in deionized water at 37 °C for 24 h, 3 months or 6 months before the tests of FS, FM and EAB. Specimens for FT tests were stored in deionized water at 37 °C for 24 h. All tests were performed by a three-point-bending method on an Instron 5566 universal testing machine. FS, FM and EAB were measured simultaneously with a crosshead speed of 1 mm/min. For the FT test, the crosshead speed was 0.1 mm/min, and the actual notch length ( a ) was measured with a microscope and digital micrometer of Micromet 5104 Hardness Tester (Buehler). FT ( K IC ) was calculated by the following equation according to ASTM E993-90:
K IC = P L f ( x ) / ( b w 1.5 )
where K IC = stress intensity factor (MN/m 3/2 ), P = load at fracture (MN), w = width of the specimen (m), b = thickness of the specimen (m), a = notch length (m), L = span (0.02 m), and f ( x ) = 3 x 0.5 {1.99 − x (1 − x )[2.15–3.93 x + 2.7 x 2 ]}/[2(1 + 2 x ) (1 − x ) 1.5 ], where x = a / w .
2.5
Measurement of degree of conversion
The degree of conversion (DC) was measured by Fourier transform near-infrared (FT-NIR) spectroscopy as described previously . Disk composite specimens (5 mm in diameter, 2 mm in thickness, n = 5) were prepared with a Teflon ring mold pressed between a pair of glass slides (0.17 mm in thickness). A specimen was placed on the testing window of a Smart NIR UpDRIFT (Thermo-Nicolet Instrument Corp., Madison, WI), a top-loading diffuse reflection accessory, and the FT-NIR spectrum of uncured specimens (monomers) was acquired by a Thermo-Nicolet Nexus 670 FT-IR spectrometer. The specimen was then light-cured in situ (without moving the specimen) through the upper glass slide for 80 s. The FT-NIR spectrum of the cured composite was acquired again. All spectra were recorded in a wavelength range of 6400–5400 cm −1 , with a resolution of 8 cm −1 , and a scan number of 120. The DC was calculated with the areas of first overtone of the vinyl absorption band around 6164 cm −1 :
DC ( % ) = 1 − A c A u × 100 %
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