Abstract
Objectives
The aim of this work was the synthesis of light-cured resin nanocomposites using nanosilica particles with different particle size and the study of some physical–mechanical properties of the composites.
Methods
Various types of silica nanoparticles (Aerosil) with average particle size of 40, 20, 16, 14, and 7 nm, used as filler were silanized with the silane 3-methacryloxypropyl-trimethoxysilane (MPS). The total amount of silane used was kept constant at 10 wt% relative to the filler weight to ensure the complete silanization of nanoparticles. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (55 wt%) were mixed with a photoactivated Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by dynamic mechanical analyzer (DMA). Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution 75 vol% for 30 days. The TGA for composites was performed in nitrogen atmosphere from 30 to 700 °C.
Results
As the average silica particle size decreases, the percentage amount of MPS attached on the silica surface increases. However, the number of MPS molecules attached on the silica surface area of 1 nm 2 is independent of filler particle size. As the average filler particles size decreases a progressive increase in the degree of conversion of composites and an increase in the amount of sorbed water is observed.
Significance
The prepared composites containing different amount of silica filler, with different particle size, but with the same amount of silanized silica and organic matrix showed similar flexural strength and flexural modulus, except composite with the lowest filler particle size, which showed lower flexural modulus.
1
Introduction
The introduction of resin-based composite technology to restorative dentistry was one of the most significant contributions to dentistry in the last century. Nowadays, esthetic tooth-shaded dental restorations are well-accepted and becoming more and more popular over metallic dental amalgams. Dental resin composites are consisted of a polymeric matrix admixed with silane reinforcing inorganic filler. Silane provides a crucial link between the matrix and the filler that can have a significant effect on the overall-performance of composite . The organic matrix is based on methacrylate chemistry, especially cross-linking dimethacrylates , while the fillers have different type (silica, ceramic, etc.), size, shape and morphology . Because of the major influence of the fillers on the physical properties, the classification of dental filling composites is based on the type and the particle size of fillers . In general, two types of composites are available on the market, the microfill and the hybrid composites. Microfill composites were formulated having fillers with an average particle size in the range of 0.01–0.05 μm and were launched in the market to overcome the problems of poor esthetic properties. However, the mechanical properties are considered low for application in regions of high occlusal forces. Hybrids, on the other hand, offer intermediate esthetic properties, but excellent mechanical properties by the incorporation of fillers with different average particle sizes (15–20 μm and 0.01–0.05 μm) .
In dentistry, posterior restorations (class I or II) require composites that show higher mechanical properties, while anterior restorations (class IV–V) need composites that have superior esthetics. The resin composite that meets all the requirements of both posterior and anterior restorations has not emerged yet. Therefore, the great interest in resin composite’s research is nanotechnology. Dental nanocomposites or nanofilled dental composites are claimed to combine the good mechanical strength of the hybrids and the superior polish of the microfills . Because of the reduced dimension of the particles (0.1–100 nm) and their wide size distribution, an increased filler load can be achieved technically resulting in reducing the polymerization shrinkage and increasing the mechanical properties, such as tensile strength, compressive strength and resistance to fracture. On the other hand, the small size of filler particles can improve the optical properties of resin composites, because their diameter is a fraction of the wavelength of visible light (0.4–0.8 μm), resulting in the human’s eye inability to detect the particles . Furthermore, the wear rate is diminished and the gloss retention is better . As a consequence, manufacturers now recommend the use of nanocomposites for both anterior and posterior restorations. Nowadays, nanocomposites are available as nanohybrid types containing milled glass fillers and discrete nanoparticles (40–50 nm); and as nanofill types containing both, nano-sized filler particles called nanomers, and agglomerations of these particles described as “nanoclusters” . The nanoclusters provide a distinct reinforcing mechanism compared with the microfill or nanohybrid systems resulting in significant improvements to the strength and clinical reliability .
This study was designed to examine the effect of nanosilica particle size, which is silane reinforced, on some selected physico-mechanical properties of light-cured composites based on Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Silica nanoparticles, used as filler, have a controlled size from 7 to 40 nm. Their surface was treated with the silane coupling agent 3-methacryloxypropyltrimethoxysilane (MPS).
2
Materials and methods
2.1
Materials
Propylamine, 99+% (Lot no 0.4419MS), cyclohexane, 99+% (Lot no S22455-155), and MPS, 98% (Lot no. 09426EC-265) used in the silanization of nanoparticles of silica were received from Sigma–Aldrich GmbH (Deisenhofen, Germany). The monomers used, i.e. 2,2-bis[4-(2-hydroxymethacryloxypropoxy) phenyl]propane (Bis-GMA) (Lot no 07210BB) and triethyleneglycol dimethacrylate (TEGDMA) (Lot no 09004BC-275) were provided also from Sigma–Aldrich. The photoinitiator systems were camphorquinone, 97% (CQ) (Lot no S12442-053) and ethyl 4-dimethylaminobenzoate, 99+% (4EDMAB) (Lot no: 90909001) from Sigma–Aldrich. The fillers (Aerosil OX50, Aerosil 90, Aerosil 130, Aerosil 150, and Aerosil 300) used were from Degussa AG (Hanau, Germany). They are fumed amorphous silica with different average specific surface area (determined by the BET gas adsorption technique; BET comes from initial letters of authors’ names who proposed it, i.e. Brunauer, Emmett, and Teller) and average particle diameter. Their specifications are listed in Table 1 . All the materials used in this study were utilized as received, without further purification.
| Aerosil OX50 | Aerosil 90 | Aerosil 130 | Aerosil 150 | Aerosil 300 | |
|---|---|---|---|---|---|
| Specific surface area (BET) [m 2 /g] | 50 ± 15 | 90 ± 15 | 130 ± 25 | 150 ± 15 | 300 ± 30 |
| Average primary particle size [nm] | 40 | 20 | 16 | 14 | 7 |
| Tapped density [g/l] | ∼130 | ∼80 | ∼50 | ∼50 | ∼50 |
| Moisture, 2 h at 105 °C [wt%] | ≤1.5 | ≤1.0 | ≤1.5 | ≤0.5 | ≤1.5 |
| Ignition loss, 2 h at 1000 °C, based on material dried for 2 h at 105 °C [wt%] | ≤1.0 | ≤1.0 | ≤1.0 | ≤1.0 | ≤2.0 |
| pH, in 4% dispersion | 3.8–4.8 | 3.7–4.7 | 3.7–4.7 | 3.7–4.7 | 3.7–4.7 |
| SiO 2 -content based on ignited material [wt%] | ≥99.8 | ≥99.8 | ≥99.8 | ≥99.8 | ≥99.8 |
| Materials no. | 23.8112-0020.99 | 23.8010-0000.99 | 23.8012-0000.99 | 23.8013-0000.99 | 23.8016-0000.99 |
| Lot No | 3155-092345 | 3155-052516 | 3155-080115 | 3155-100211 | 3155-101812 |
2.2
Silanization of silica nanoparticles
The Aerosil OX50, Aerosil 90, Aerosil 130, Aerosil 150 and Aerosil 300 silica nanoparticles were silanized with MPS, following the method of Chen and Brauer . In all cases, the amount of organosilane was kept constant at 10% wt/wt relative to silica, a ratio that was more than enough to completely cover the surface of the silicas and to provide a durable interphase . The amount of silane in grams (g) to obtain a minimum uniform coverage of the filler particles ( X ) is given by the following equation :
X = A ω f
In which A is the surface area of the filler (50, 90, 130, 150, and 300 m 2 /g), ω is the surface coverage per gram of silane MPS ( ω = 2525 m 2 /g) and f is the amount of silica (g).
The silica (5.0 ± 0.05 g), the silane (0.50 ± 0.01 g), the solvent (100 ml cyclohexane) and n-propylamine (0.1 ± 0.01 g) were stirred at room temperature for 30 min and then at 60 ± 5 °C for additional 30 min at atmospheric pressure. The mixture was then placed in a rotary evaporator at 60 °C for removing the solvent and volatile by-products. The powder was then heated at 95 ± 5 °C for 1 h in a rotary evaporator and finally was dried at 80 °C in a vacuum oven for 20 h.
The silanized silica nanoparticles were identified by FTIR spectroscopy (FTIR Spectrum One, Perkin Elmer, resolution 4 cm −1 , 32 scans, 4000–1350 cm −1 ). Spectroscopic grade potassium bromide (KBr, Sigma–Aldrich, Cas no 7758-02-3) and silica powder were pressed together into a pellet using a KBr palletizer. Thermogravimetric analysis of the silanized silica nanoparticles was performed using a Pyris 1 TGA (Perkin Elmer) thermal analyzer utilizing about 5 mg of each sample. The particles were heated to 700 °C under nitrogen atmosphere flow (20 ml/min) with a heating rate of 10 °C min −1 .
2.3
Preparation of nanocomposites
The resin matrix was consisted of Bis-GMA/TEGDMA mixture (50:50 wt/wt) which contained the photoinitiator system CQ (0.2 wt%) and 4EDMAB (0.8 wt%). Bis-GMA was first heated in an ultrasonic bath at about 40 °C for 10 min and then TEGDMA containing the photoinitiating system was added. Then the silanized silica (55 wt%) was mixed with the resin by hand spatulation, as suggested in Refs. . Once the powder was completely wetted with the resin, the composite pastes were sheared against a glass surface with a Teflon spatula until the pastes were semi-transparent to assure maximum particle dispersion in the resin. Then the composite pastes were placed into an ultrasonic bath for 3 h. This employment of high shear strength has shown to help the silanized Aerosil to form stable sol with the dimethacrylate resin . Then the pastes were placed in the vacuum oven to remove air bubbles which probably were entrapped inside the paste.
2.4
Thermogravimetric analysis
The thermal degradation of the composites and the determination of the weight percentage of fillers were studied by thermogravimetric analysis. Weight changes as a function of time and temperature were evaluated with a thermal program by heating from 30 to 700 °C at the heating rate of 10 °C min −1 in nitrogen atmosphere (flow 20 ml/min) and cooling (10 °C min −1 ) at room temperature. Thermogravimetric analysis was performed on a Pyris 1 TGA (Perkin Elmer) thermal analyzer using about 5 mg of each sample.
2.5
Degree of conversion
The FT-IR analysis was conducted in a FT-IR spectrometer, Spectrum One of Perkin-Elmer. Spectra were obtained over 4000–600 cm −1 region and were acquired with a resolution of 4 cm −1 and a total of 32 scans per spectrum. A small amount of each composite was placed between two translucent Mylar strips, which pressed to produce a very thin film. The FT-IR spectrum was recorded at zero time and immediately after exposure to visible light (for 80 s), utilizing a XL 3000 dental photocuring unit from 3M-ESPE Company (St Paul, MN, USA). This source consisted of a 75-W tungsten halogen lamp, which emits radiation between 420 and 500 nm with a maximum peak of 470 nm. For each spectrum, it was determined the height of aliphatic C C peak absorption at 1637 cm −1 , and the aromatic C C peak absorption at 1580 cm −1 , utilizing a base line technique which proved the best fit to the Beer–Lambert law . The aromatic C C vibration is used as an internal standard. The percent monomer conversion of the cured specimen, which expresses the percent amount of double carbon bond reacted, is determined according to the following equation:
Degree of conversion ( % ) = 100 1 − ( A 1637 / A 1580 ) polymer ( A 1637 / A 1580 ) monomer
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