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ABSTRACT: Resin composites are widely used in esthetic restorative dentistry. Since their introduction in the mid-1960s, these composites made steady gains in popularity. Their increased use is attributed to their excellent biocompatibility, absence of taste, odour, tissue irritation and toxicity, insolubility in body fluids, easy operation, excellent aesthetic properties, stable colures, optical properties, easy pigmentation, low cost and repairability. The composite resins in current use still suffer from several shortcomings such as poor mechanical properties. In order to improve these properties, microparticles have been used as fillers for a long time. However, the inadequate mechanical properties of resin composites remain problematic. Recently, researchers have utilised nanoparticles as dental composite fillers. This article reviews the relevant literature on the mechanical properties of polymer dental composites filled with micro- and nano-scale particles. The effects of particle size on fracture toughness, flexural strength, and hardness were examined with emphasis on other important factors for improvement. The second section focused on the toughening mechanisms of particulate-polymer composites.
Keywords: Polymer dental composites, particle size, nanoparticles, mechanical properties, toughening mechanisms
1.INTRODUCTION
Polymer-based composites have been widely used in dentistry since their introduction in the late 1950s. Recently, nano-composites were introduced as dental polymers.1-2 Resin composites are widely used in dentistry as restorative materials, cavity liners, pit and fissure sealants, cores and buildups, inlays, onlays, crowns, veneers, denture teeth, provisional restorations, cements for single or multiple tooth prostheses and orthodontic devices, endodontic sealers, root canal posts, structured scaffolds, and plates and screws.3-5 Excellent biocompatibility, superior aesthetic qualities as fillings, moderate cost compared with ceramics, and strong bonding ability to the tooth structure, make resin composite the preferred material in dental restorative applications.1-6-7 Most dental composites consist of an organic matrix (polymer phase), inorganic fillers (dispersed phase) and interphase (coupling agent).8 Usually, the organic matrix is based on methacrylates, epoxy and polyethylene.9-11 The filler is added to enhance polymer properties and usually consists of different compositions, sizes and size distributions of glass or ceramic particles, nanotubes, whiskers, fibres and nanoclusters.12-13
The coupling agent such as silane is designed to strongly bond the matrix to the filler, thus improving composite performance. The most commonly used silane in dental restorative composites is 3-methacryloxypropyltrimethoxysilane (y-MPS). Filler content, type, shape, size and morphology are important factors...