A dental composition contains a polymerizable monomer, a polymerization initiator, and an inorganic filler, and is most widely used today as a restorative material for repairing fractures of teeth and dental caries. Such a dental composition is required to have the following properties. Specifically, as a cured product obtained after polymerization, the dental composition is required to have sufficient mechanical strength and hardness to serve as a substitute for natural teeth, wear resistance against occlusion of teeth in an oral cavity, surface smoothness and gloss, color matching with natural teeth, transparency, etc. Furthermore, from the viewpoint of handling properties, the dental composition, as a paste which has not yet been polymerized, is desired to have ease of handling for dental clinicians, for example, proper fluidity and high forming property. Particularly, a type of composition to be filled directly in a cavity must have a viscosity low enough to be filled directly from a syringe. If a specific component is added or the blend ratio of the components is adjusted, the resulting composition can produce favorable effects as a dental material.
These properties of the dental composition are greatly influenced by the component materials, shape, particle size of an inorganic filler used therein, and further by the content of that filler. For example, when an inorganic filler having a large average particle size of 1 μm or more is used, the filling rate of the filler in the polymerizable monomer can be increased easily and therefore sufficient mechanical strength as a cured product and high handling properties as a paste can be obtained. The use of such an inorganic filler has, however, a drawback in that it is difficult to obtain satisfactory gloss even after final polishing. On the other hand, when an inorganic ultrafine particle filler having an average particle size of less than 1 μm is used, the surface smoothness and gloss after polishing of the cured product and the gloss durability in the oral cavity are improved. The use of such an inorganic ultrafine particle filler has, however, a drawback in that when the inorganic filler is mixed and kneaded with the polymerizable monomer, the viscosity of the resulting paste increases significantly, which makes it difficult to increase the content of the filler. As a result, the mechanical strength of the cured product decreases, and the unpolymerized pasty composition becomes sticky, which reduces the handling properties. When a conventional dental composition, particularly a type of composition to be filled directly in a tooth is used, a reduction in the content of an inorganic filler seems to be a good solution to meet the handling properties requirements. However, when the content of the inorganic filler is reduced, the physical properties such as flexural strength of the resulting composition are low in value. When the content of the inorganic filler is increased to increase the strength, the viscosity of the resulting composition also increases. Such a composition cannot be used for direct filling during dental treatment. Furthermore, a spherical filler can be used to increase the surface smoothness and gloss, but it is difficult to increase the strength while maintaining the handling properties. Under these circumstances, it is difficult to increase the mechanical strength and the surface smoothness and gloss after polishing of the cured product and the handling properties of the paste in a balanced manner.
Patent Literature 1 discloses a dental restorative material in which an inorganic filler treated with a specific silane coupling agent and a polymerizable monomer having high hydrophobicity are used in combination to achieve high density filling, high strength, high aesthetic quality, and durability. In a dental restorative material disclosed in Patent Literature 2, a mixed filler composed of irregular-shaped inorganic particles, spherical inorganic particles, and inorganic ultrafine particles are used. The irregular-shaped particles having a small average particle size are used in this mixed filler, and acylphosphine oxide is used as a photopolymerization catalyst, so that the resulting dental restorative material has increased surface smoothness and gloss while maintaining high fracture toughness and strength. Patent Literature 3 discloses a dental composition in which a spherical filler having a specific particle size is used to achieve a good balance between the paste properties and the polishability.