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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 , (ii) modification of resin monomers and/or introducing new monomer systems , (iii) improvement of initiator system to reach higher degree of polymerization and/or controlled curing kinetics .

Developments in dental bonding agents and composite replacement techniques 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 , modulus , flexural strength and diametral tensile strength , and fracture toughness . 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 . 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 m ² /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 .

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.