Authors: (Irini D. Sideridou, Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki)
Abstract: Dental polymer composites were introduced commercially in the mid-1960s for the restoration of anterior teeth. Since their advent, their characteristics such as physical properties, manipulative qualities, durability and wear resistance have improved remarkably. As a result, they are widely used instead of amalgam. Composites make up for the weak points of amalgam, such as toxicity from mercury content, corrosion and low adhesive property. In addition, dental polymer composites have a better aesthetic property than amalgam. Today they are possibly the most ubiquitous materials available in dentistry as they are used in a huge variety of clinical applications, ranging from filling materials, luting agents, indirect restorations and metal facings to endodontic posts and cores. Dental polymer composites mainly have three major components: an organic polymer matrix, inorganic filler and a coupling agent. The polymer forms the matrix of the composite material binding the individual filler particles together through the coupling agent. The polymer is a rigid solid which is prepared by the free-radical polymerization of a liquid monomer or mixture of monomers. It is this ability to convert from a plastic mass into a rigid solid that allows this material to be used for the restoration of dentition. The most common monomers in modern dental polymer composites are cross-linking dimethacrylates, e.g. 2,2-bis[4-(2-hydroxy-3-methacryloxypropyl) phenyl]propane (Bis-GMA), ethoxylated Bis-GMA (Bis-EMA), 1,6-bis-[2- methacryloxyethoxycarbony- lamino]-2,2,4-trimethylhexane (UDMA), decanediol dimethacrylate (D3MA) or triethy-leneglycole dimethacrylate (TEGDMA). The radical polymerization of the matrix monomers results in a three-dimensional network, in which the filler particles are dispersed. The selection of appropriate monomers for the formulation of a composite strongly influences the reactivity, viscosity and polymerization shrinkage of the composite paste, as well as the mechanical properties, water uptake and swelling of the cured composite. To ensure an adequate long shelf life for the composite, it is essential that premature polymerization is prevented. To this end, an inhibitor such as hydroquinone (0.1%) is included. Most dental polymer composites are light curing composites, which harden by irradiation with visible light 400– 500nm.The properties of polymer composites are considerably influenced by the fillers employed. According to the nature and the particle size of the filler the dental composites have been classified into four main groups, traditional composites, microfilled composites, hybrid or blended composites and small particle hybrid composites.