The purpose of this work was the study of the effect of the amount of a urethane dimethacrylate silane (UDMS) coupling agent on physical properties of dental light-cured resin nanocomposites based on Bis-GMA/TEGDMA (50/50 wt/wt) matrix and Aerosil OX50 as filler.
Silica nanoparticles (Aerosil OX 50) used as filler were silanized with 5 different amounts of UDMS 1.0, 2.5, 5.0, 7.5 and 10 wt% relative to silica. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (60 wt%) were mixed with a Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of light cured 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 DMA analyzer. Measurements were taken in samples stored, immediately after curing, in water at 37 °C for 24 h. Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water of 75 vol% for 30 days. Thermogravimetric analysis of composites was performed in nitrogen atmosphere from 50 to 800 °C.
Almost all of used amount of silane remained chemically bounded on the surface of silica particles, forming a layer around them, which have dense accumulation of methacrylate groups. No significant statistic difference was found to exist between the degree of conversion values of composites with different silane contents. The composite with the lowest amount of UDMS (1.0 wt%) showed the lower flexural strength value, the higher static and dynamic elastic modulus values and the higher sorbed liquid value and solubility.
The optimum concentration of UDMS seems to be that of 2.5 wt%. Higher concentrations of UDMS did not improve the properties of composites.
Dental polymer composites are interconnected heterogeneous materials that generally have three discernable phases: (1) a polymeric matrix or continuous phase formed by polymerization of one or more monomer/oligomers, (2) a higher modulus dispersed phase consisting of fillers of various types (silica, ceramic, organic, etc.) sizes, shapes and morphologies and (3) an interfacial or interphasial phase that bonds to both the continuous and dispersed phases, thereby enhancing the moduli and mechanical properties of the weaker polymer phase and also facilitating stress transfer between these phases by forming a unitary material. Adhesion of lower moduli polymer matrices to higher moduli inorganic fillers can occur as a result of van der Waals forces, ionic interactions, hydrogen bonding, ionic or covalent bonding, interpenetrating polymer network formation and for certain types of fillers by micromechanical interlocking mechanisms. For most mineral reinforced dental composites the primary interphasial linkage between the polymer matrix and the filler is by chemical bond formation, mediated by a silane coupling agent .
Usually the organic matrix is based on methacrylate chemistry, especially cross-linking dimethacrylates like 2,2-bis[4-2-hydroxy-3-methacryloyloxypropyl) phenyl] propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) are used. The free-radical polymerization of the matrix monomers leads to a three-dimensional network. Most of the contemporary dental polymer composites are light curing composites, which harden by irradiation with visible light in the wavelength range 400–500 nm. Nearly all composite manufacturers are using camphorquinone as the photoinitiator. The absorption maximum of camphorquinone is at 468 nm. Amines are used as accelerators and thereby the initiation radicals are formed via proton and electron transfer . The dispersed inorganic phase may consist of several inorganic materials such as quartz, borosilicate glass, lithium aluminum silicate, strontium or zinc glass and colloidal silica. The inorganic phase is treated with an organosilane before being mixed with the unreacted dimethacrylate monomers. These organosilanes are called coupling agents because they form a bond between the inorganic and organic phases of the composite. Although adhesion is the central function of silanes, there are many factors involved in the silane modification of the organic-inorganic interface that change the properties of the composite. The silane protects the filler surface from fracture, strengthens the boundary layer of polymer matrix and thus has a positive effect on the properties of the composite even if failure is not at the interface. With the appropriate coupling agent, fracture of the composite will not occur at the interface, but will occur through the polymer matrix. Surface modification of fillers also affects the rheology of the mixture of monomers and filler by changing wetting characteristics, dispersion of particles and viscosity .
Generally the efficacy of a silane coupling agent is determined by the overall degrees of reaction of the silane with the glass filler (oxane bond formation) with itself (by siloxane formation) and with the polymer matrix (by graft copolymerization). The oxane bond (silicon–oxygen–silicon) that forms between the silane agent and the mineral filler can be especially vulnerable to hydrolysis, because this covalent bond has significant ionic character . By contrast, the carbon-carbon covalent bond that forms between the silane and the polymer matrix is considerable more stable to hydrolytic attack than the silicon–oxygen covalent bond.
The most common silane used in dental composites is γ-MPS. One approach aimed at improving the quality and durability of the filler/matrix interface involves the use of more hydrophobic and flexible silane coupling agents than γ-MPS such as the 10-(methacryloyloxy)decyltrimethoxysilane (MDTMS) . The decrease in the flexural strength upon storage of the composite sample in water can be reduced by surface modification with the more hydrophobic silane MDTMS. But the overall mechanical properties achieved with this silane bearing the flexible decyl spacer are worse than those attained with the silane possessing the propyl spacer. A further advantage of using fillers silanized with MDTMS is that this surface treatment enables higher loadings to be achieved .
Another silane which shows promising results is the adduct of glycerol dimethacrylate and 3-(isocyanato)propyltriethoxysilane, a urethane dimethacrylate silane (UDMS) ( Fig. 1 ). This has two methacrylate groups and therefore enables the preparation of a crosslinked structure. Less brittleness of the cured composites is observed after storage in water. Furthermore, the mechanical properties are better than those achieved with MDTMS . UDMS has already been used in commercial dental composites. The commercial hybrid filling material Definite ® is based on it .
In this work the effect of the amount of UDMS coupling agent used on physico-mechanical properties of dental light-cured resin nanocomposites based on Bis-GMA/TEGDMA (50/50 wt/wt) matrix and Aerosil OX50 as filler was studied. The organic monomer matrix used is hardened by irradiation with visible light in the wavelength range 400–500 nm, using camphorquinone as the photoinitiator and ethyl-4-dimethylaminobenzoate (4EDMAB) as the accelerator.
Materials and methods
3-Isocyanatopropyltriethoxysilane (IPTES) was from ABCR GmbH&Co. KG (SII 6455.0) (Karlsruhe, Germany), dibutyltin dilaurate, 95% (Metatin 812) (Lot no. 13917HC-135) and glycerol dimethacrylate (85% mixture of isomers) (GDMA) (Lot no. S32621-495) were from Sigma–Aldrich, hydroquinone, >99% (HQ) from Merck (Art 4610, 2606317) used in the synthesis of the silane UDMS. Propylamine, 99+% (Lot no. 0.4419MS) and cyclohexane, 99+% (Lot no. S22455-155) 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 system was camphorquinone, 97% (CQ) (Lot no. S12442-053) and ethyl 4-dimethylaminobenzoate, 99+% (4EDMAB) (Lot no. 90909001) both from Sigma–Aldrich. The filler Aerosil OX50 was from Degussa (Lot no. 3155092345) and it is fumed amorphous silica with average specific surface area (BET) 50 (35–65) m 2 /g and an average particle diameter of 40 nm.
All the materials used in this study were used as received without further purification.
Synthesis of the silane UDMS
1 mol of IPTES was carefully added to a mixture of 1 mol of GDMA, 150 mg of HQ and 180 mg of Metatin 812 as described in Ref. . Then the mixture was stirred at room temperature for 24 h. The prepared silane was identified by FT-IR and NMR spectroscopy as described in our previous papers .
Silanization of silica nanoparticles
The silica nanoparticles (Aerosil OX50) were silanized following the method of Chen and Brauer using as silane the UDMS. The amount of silane was at 1.0, 2.5, 5.0, 7.5, 10.0% (wt/wt) relative to silica.
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 then was placed in a rotary evaporator at 60 °C for the removing of the solvent and the volatile byproducts. The powder was then heated at 95 ± 5 °C for 1 h on the rotary evaporator and finally was dried at 80 °C in a vacuum oven for 20 h.
The silanized silica nanoparticles were identified by FT-IR spectroscopy (FTIR Spectrum One, Perkin Elmer, resolution 4 cm −1 , 32 scans, 4000–1350 cm −1 ). Spectroscopic grade KBr 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 using about 5 mg of each sample. The particles were heated to 800 °C under air or nitrogen atmosphere flow: 20 ml/min) with a heating rate of 10 °C min −1 .
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 (60 wt%) was mixed with the resin by hand spatulation, as it is 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 put into an ultrasonic bath for 3 h. This employment of high shear stress has shown to help the silanized Aerosil to form stable sol with the dimethacrylate resin . Then the pastes were placed in a vacuum oven to remove air bubbles which probably were entrapped inside the paste.
FT-IR analysis-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). For each spectrum it was determined the height of aliphatic C C peak absorption at 1637 cm −1 , as well as the aromatic C C peak absorption at either 1608 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 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 equation:
degree of conversion ( % ) = 100 × 1 − ( A 1637 / A 1608 ) polymer ( A 1637 / A 1608 ) monomer
Mechanical properties were measured in accordance with International Standard Organization (ISO) Specification No. 4049. Bar specimens were prepared by filling a Teflon mold with unpolymerized material, taking care to minimize entrapped air. The upper and lower surfaces of the mold were overlaid with glass slides covered with a Mylar sheet to avoid adhesion with the unpolymerized material. The completed assembly was held together with spring clips and irradiated by overlapping, as recommended in ISO-4049, using a XL 3000 dental photocuring unit (3M-ESPE, St. Paul, MN, USA). This source consisted of a 75-W tungsten halogen lamp, which emits radiation between 420 and 500 nm and has the maximum peak at 470 nm. This unit was used without the light guide in contact with the glass slide. Each overlap was light-cured for 80 s. The samples were irradiated on both sides. Then, the mold was dismantled and the composite was carefully removed by flexing the Teflon mold. Five specimen bars were prepared for each composite. The bar-shaped specimens had been stored in distilled water at 37 ± 1 °C in dark for 24 h, immediately after curing.
Flexural strength and flexural modulus
For flexural tests, bar-specimens were prepared by filling a Teflon mold 2 mm × 2 mm × 25 mm. The specimens were bent in a three-point transverse testing rig with 20 mm between the two supports (3-point bending). The rig was fitted to a mechanical testing machine (Instron, model 3344). All bend tests were carried out with a constant cross-head speed of 0.75 ± 0.25 mm/min until fracture occurred. The load and the corresponding deflection were recorded. The flexural modulus ( E ), in MPa, and the flexural strength ( σ ), in MPa, were calculated using the following equations:
E = F 1 l 3 4 b d h 3 and σ = 3 F l 2 b h 2