Bis-GMA/TEGDMA dental composites reinforced with nano-scaled single crystals of fibrillar silicate

Abstract

Presently, dental composites have been widely adopted by the profession as the restorative material of choice; as compared to dental amalgams, the composites possess better esthetic property, have less safety concern, and have shown reasonably satisfactory clinic results. Dental composites consist of resin matrices and inorganic fillers. The monomer of 2,20-bis-[4-(methacryloxypropoxy)-phenyl]propane (Bis-GMA) has been used as an important dental base monomer since it was invented in the 1960s [1, 2]; for example, Bis-GMA is the base monomer in the Z100 Restorative Dental Composite produced by the 3M Corporate. Bis-GMA is a very viscous, honey-like liquid. To improve handling qualities, a diluent monomer of tri(ethylene glycol) dimethacrylate (TEGDMA) is added to thin the resin. In the Bis-GMA/TEGDMA dental resin systems, Bis-GMA functions to limit the polymerization-induced volumetric shrinkage and to enhance the resin reactivity, whereas TEGDMA provides for the increased vinyl double bond conversion [3, 4]. Albeit dental resins have been reinforced with inorganic fillers of glass/ceramic powders containing surface-silanized particles, relatively low strength and durability of the composites have limited their uses [5–8]: dental composites have flexural strengths typically ranging from 80 to 120 MPa, which can fulfill the requirement of filling tooth cavities but cannot survive large stress-bearing restorations such as crowns and bridges; furthermore, the strength of dental composites decreases substantially after being used for a period of time. The average service lifetime of dental composites is considerably shorter than that of dental amalgams [9, 10]. Investigations of the failures revealed that, among numerous issues, the inorganic filler was a major contributor [11, 12]. Many inorganic filler particles currently used for dental composites are spherical or irregular in shape. Such filler particles at occlusal surfaces are susceptible to dislodgement from the resin matrix during wear with food boluses. This would cause the reinforcement effect to be lost. Chopped glass fibers and/or high strength whiskers with diameters of 5–50 lm and aspect ratios larger than 10 have also been investigated to reinforce dental resins [13–16], and the resulting composites showed higher mechanical properties. Several reinforcement mechanisms including ‘‘Bridging,’’ ‘‘Pull-out,’’ and ‘‘Load Transfer’’ were proposed for understanding the fiber/whisker reinforcement [17, 18]; in particular, ‘‘Bridging’’ is a powerful reinforcement mechanism. If a micro-crack is initiated in a resin matrix under contact wear and/or other stresses, the fillers remain intact across the crack planes supporting the applied load. Crack-opening is resisted by the bridging fillers; thus the resin matrix is reinforced. Requirements for fillers to achieve effective ‘‘Bridging’’ reinforcement include high strength and large aspect ratio values; tooth enamel rods are an example of elongated fillers, and crystalline platelets in dental glass/ceramics are another example [19]. Nonetheless, the dental composites reinforced with fibers/whiskers generally possess mechanical properties that still need further improvements; this is L. Zhang Y. Gao Q. Chen H. Fong (&) Department of Chemistry, South Dakota School of Mines and Technology, 501 East St. Joseph Street, Rapid City, SD 57701, USA e-mail: Hao.Fong@sdsmt.edu