M. Yamaki

Analysis of Photo-initiators in Visible-light-cured Dental Composite Resins

Seven commercial visible-light-cured (VL) dental composite resins were analytically studied for identification of the photo-initiator consisting of photo-sensitizer and reducing agent. Gas-liquid chromatography (GC) was used for the determination of the dilute components extracted from the composite resin. Mass spectroscopy (GC-MS) was used for confirmation of the qualitative data obtained by GC. The results showed that all composite resins examined included camphorquinone (CQ) as a photo-sensitizer. The concentration of CQ in the resin phase, however, ranged from 0.17 to 1.03% w/w. The composite resin with hybrid-sized filler tended to have a higher concentration of CQ than did the micro-filled composite resin. As for the reducing agent, two out of seven brands contained dimethylaminoethyl methacrylate (DMAEMA), and one included dimethyl-p-toluidine (DMPTI). The mixing ratio between CQ and the amine in these three composite resins also varied. Another four brands did not contain either DMAEMA or DMPTI, and would utilize different reducing agents.

Refractive-index-adjustable Fillers for Visible-light-cured Dental Resin Composites: Preparation of TiO 2-SiO2 Glass Powder by the Sol-gel Process

New fillers have been prepared for visible-light-cured (VL) dental resin composites with the refractive index adjustable to that of the resin phase. These SiO2 glass powders containing TiO2 up to 20 wt% were formed by heating to 1000°C ground gels made from a mixture of Ti[OCH(CH3)2] 4 and Si(OC2H5)4· With increasing TiO2 content, the refractive index of the prepared powder increased linearly, while the optical transmittance at 467 nm decreased linearly. The experimentally formulated VL-cured resin composites, consisting of (TEGDMA and Bis-GMA) monomer mixture and TiO2-SiO2 glass filler, had greater transmittance when the refractive index of the filler matched that of the monomer mixture, resulting in a greater degree of monomer conversion upon irradiation with VL.

Studies on radiopaque composites containing ZrO2SiO2 fillers prepared by the sol-gel process

Radiopacity can facilitate diagnostic observations adjacent to dental composites. To produce radiopaque SiO2-based fillers, ZrO2-SiO2 powders containing up to 45wt% ZrO2 were manufactured by the sol-gel process. The radiopacity of experimental composites containing Bis-GMA+TEGDMA+CQ+DMAEMA monomer mixture (25 wt%) and ZrO2-SiO2 fillers (75 wt%) was evaluated and compared to that of human enamel and two commercial composites with different radiopacity. It became evident that the radiopacity of the experimental composites increased linearly with zirconium content and could match that of the control materials, while the composite containing 15wt% ZrO2-85wt% SiO2 filler possessed radiopacity analogous to that of human enamel. It was demonstrated here that the radiopacity of the composites could be precisely controlled by adjusting the ZrO2 content in ZrO2-SiO2 fillers. These radiopacity-adjustable composites have the potential to be very useful filling materials by enabling dentists to accurately distinguish secondary caries adjacent to the composite restorations.

Cutting effectiveness and wear of carbide burs on eight machinable ceramics and bovine dentin.

As a first approach in evaluating the feasibility of industrial machinable ceramics in dentistry, we performed weight-load-cutting tests on eight machinable ceramics and bovine dentin, using #1557 carbide burs driven by an air-turbine handpiece. While the transverse load applied to the bur was cyclically varied between 20 and 80 g, we measured the cutting speed (i.e., the steady-state handpiece speed during cutting) and the cutting volume. The greater the applied load, the more the cutting speed decreased and the cutting volume increased. The degree of this trend, however, differed among the workpieces. When dentin and mica-based glass ceramics were being cut, the cutting speed was moderately reduced, the cutting effectiveness of the bur remained high, and the wear of the bur was small. When other ceramics--such as AIN-based, Si3N4-based, and CaO.SiO2-based ceramics--were being cut, however, the cutting speed was less diminished, and the cutting efficiency of the bur was smaller and decreased rapidly, along with extensive wear of the bur. We speculate that mica-based glass ceramics could be used as the substitute for dentin in the pre-clinical cutting exercise, and that another potential use of machinable ceramics examined might be in the production of future machined dental prostheses.

Curing performances of four experimental bis-GMA based binary monomer mixtures for dental visible-light-cured composite resin inlays

Visible-light-cured (VLC) composite resins have been widely accepted in dental clinics because of their aesthetic merit and ease in handling [1]. Visible light (VL) irradiation of composite resins at intraoral temperature is, however, said to be insufficient for maximum polymerization, leading to wear and discoloration problems [2-4]. To offset these drawbacks, a method for post-curing by heat has been introduced. These composites are now used as V L C inlay composite resins [5]. For the purpose of providing VLC inlay composite resins with improved clinical performances, it seems necessary to study the monomer mixture suitable for both VL and heat curing. Bisphenol A glycidyl dimethacrylate (bis-GMA) has been proven to be a useful base monomer for various dental composite resins [6]. Due to its high viscosity, however, bis-GMA has to be added with diluent monomers to render the handling and incorporation of filler easier [7]. Although some attempts have been made to examine the influence of diluent monomers on the curing behaviour of bis-GMA based binary monomer mixtures [8], such effects are still not well clarified. Therefore, the objectives of this study were, firstly, to prepare four bis-GMA based binary monomer mixtures and evaluate their curing performance by VL and by VL plus heat with residual monomer measurements and mechanical strength (diametral tensile strength) measurements and, secondly, to assess the effects of post-curing by heat on the properties of the once photo-cured copolymers. Table I shows the formulation of four bis-GMA (D-GMA, Shin Nakamura Co., Japan; lot no. 1105K) based binary monomer mixtures, in which one of the following four monomers was used as the diluent monomer: triethyleneglycol dimethacrylate

Studies on dental high-speed cutting with carbide burs used on bovine dentin

Bovine dentin was cut by hand with carbide burs and two cutting procedures while the rotational handpiece speed and the applied load were simultaneously monitored. It became evident that the maximum applied load during intermittent cutting varied from approximately 30 to 40 gm, whereas that during successive cutting ranged from approximately 20 to 60 gm. It showed that intermittent cutting produced a greater cutting effectiveness (for example, cutting volume per cut contact time) when carbide burs were used on dentin, compared with continuous cutting. It is recommended that the intermittent cutting procedure be used during work on patients.

CURE PERFORMANCE OF MULTIFUNCTIONAL MONOMERS TO PHOTO-INITIATORS : A THERMOANALYTICAL STUDY ON BIS-GMA-BASED RESINS

In order to assess cure performance in experimental monomer systems containingbis-GMA and 3G, differential scanning calorimetry was used to obtain heat for curing, and residual monomers present within the resin systems were measured by high-performance liquid chromatography. Visible light-activated polymerization of these monomer systems was studied using camphorquinone (CQ) and reducing agents of dimethylaminoethyl methacrylate (DMAEMA) and dimethyl-para-toluidine (DMPT). For unfilledbis-GMA-based resin (60 wt%bis-GMA and 40 wt% 3 G) containing CQ (0.5 wt%) and DMAEMA (0.5 wt%) indicating approximately 100% polymerization, less residual monomer and larger hardness than the other monomer systems were observed.

Preparation of SiO2-Al2O3 glass powders by the sol-gel process for dental applications

SiO2-Al2O3 binary glass powders with Al2O3 contents up to 50 wt% have been synthesized by the sol-gel process. The starting solution (sol) consisted of 1 mole of tetraethoxysilane, 50 moles of water containing aluminum nitrate, 10 moles of ethyl alcohol and 0.02 moles of HCl catalyst. The sol was converted into gel by heating at 85°C and suction by running water. The sintering process was analysed by DTG thermal analysis. The fired powders were examined by XRD. It was confirmed that the gels were converted into the glass structure without crystallization. The measurements of refractive index and powder size of fired powders revealed that these powders could be used as refractive-index-adjustable fillers for dental composite resins. The other potential usage of the SiO2-Al2O3 powders obtained might be in the production of powders for dental cements.

Application of multifunctional base monomer to dental composite resins

Experimental composite resin systems were prepared with visible-light-cured multifunctional matrices to which various amounts of organic composite filler were added. In comparison with the unfilled resins, the filled resin systems were tested for Knoop hardness, compressive and diametral tensile tests, thermal properties, and water sorption. Analyses of the results obtained for the unfilled resins indicate that the increased hardness and mechanical strength were dependent on the multifunctional base monomer. The effective composite resin system was the one that was filled with organic composite filler in the multifunctional comonomer having triethylene glycol dimethacrylate as a diluent monomer.

High-temperature oxidation behaviour of base metal elements in nickel-base alloys

Oxidation of non-precious metals nickel, chromium and copper, nickel-base alloys in an air atmosphere was studied under differential thermal analysis. The results suggest that adding chromium and copper to high-purity nickel metal reduces the amount of oxidation and gives a slower oxidation than in untreated high-purity metals. It is then shown that the activation energy for oxidation in nickel-base alloys is almost the same as that in high-purity nickel metal when alloying elements are added to nickel.