Nano-porous thermally sintered nano silica as novel fillers for dental composites
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], [2], [3], (ii) modification of resin monomers and/or introducing new monomer systems [4], [5], [6], [7], [8], (iii) improvement of initiator system to reach higher degree of polymerization and/or controlled curing kinetics [9], [10], [11], [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 pre-hydrolyzed 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.
Section snippets
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 m2/g (Aerosil® 200) was obtained from Evonik (Germany). Glass filler (micro fillers) with
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
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
Conclusions
This study describes the preparation of new dental fillers with porous structure from silica nanoparticles through 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
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2022, Journal of the Mechanical Behavior of Biomedical MaterialsCitation Excerpt :This shortcoming of fillers with a high specific surface area has been addressed to be overcome by the utilization of more advanced mixing equipment (Atai et al., 2012). Furthermore, the presence of dead pores in the structure of the filler particles can be associated with the FS decline at high filler loadings (Atai et al., 2012). It is worth mentioning, EDAX map of the dental composite fracture surface shows that excellent dispersion of the filler particles in the resin matrix is maintained at the intermediate filler content range (Fig. 1S).