It has been reported that Ni-Ta alloys with compositions from Ni-20wt% Ta to Ni-30wt% Ta have good and sufficient mechanical properties as dental casting alloys, when comparing with cast base metal alloys, which are commercially available.The usefulness of these alloys was proved as dental casting alloys and in clinical applications.The biocompatibility of Ni-Ta alloys and human cultuyed fibloblasts were studied by tissue culture to determine the toxicity.The morphology of cultuyed fibroblast cells in the neighbourhood of metal specimens was observed and cell numbers were counted to determine the biocompatibility of the metals and the fibroblasts.The fibroblast cells grown with Ni-20wt% Ta and Ni-30wt% Ta specimens were similar to the control, revealing a good biocompatibility.These cultured fibroblast cells were arranged closely and normally around these alloy specimens.The fibroblasts grown with Ni metal showed changes in shape and disintegration, indicating cell damage.As a conclusion, alloys with compositions from Ni-20wt% Ta to Ni-30wt% Ta can be considered useful dental-and bio-materials.

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The author applied the chemical strengthening to the metal bond porcelain and studies the influence of application over mechanical properties and durability from dental material and apparatus point of view.Through the experiments, further results were acquired:1)The bending strength of metal frame with chemical strengthening treatment was improved in either case of setting on the tension side or compressive side as much as 1.5-2.1 times the untreated metal frame.In the case of the bending elastic modulus, 1.2-1.9 times, and in the case of the resilience 1.8-2.3 times improved were witnessed after chemical strengthening.2)In 45 degree breakage test, the chemical strengthened metal frame showed that the breaking load increased 2.4 times the untreated metal frame.3)In the impact test, when chemical strengthened impact strength rose 1.2-1.5 times higher than the untreated metal frame, and durability was improved.4)The dimensional change of the inside diameter and the crevice was smaller when chemically strengthened.5)There has been no significant difference in the color tone of porcelain of metal frame between the chemically strengthened metal frame and the untreated metal frame.6)The surface stress values in metal frame was over 9.0 times larger in case of chemically strengthened metal frame.

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In order to know the solidification mechanism of the centrifugal casting process for Ni-Cr alloy, several experiments were carried out.The centrifugal casting machine was altered and cooling curves during solidification of cyrindrical ingots were recorded, and the etched section of these ingots were observed.Then the grain size of ingots were determined by photomicrographs and section of ingots were analyzed by electron probe microanalyzer.And also the microstructure of wedge shaped ingot was observed.As a result, the cast structure of ingot was fully equiaxed, and nickel and chromium were distributed uniformly in ingot.Solidification time, mold temperature, supercooling and grain size were interrelated.The wedge shaped ingot has a coarse-grained structure in the tip.This result was explained that formation of equiaxed crystals was primarily due to separation of crystals from the mold wall, equiaxed crystals were carried by the convection, and ingot was packed with equiaxed grains before forming a columnar structure.A coarse-grained structure in the tip of the wedge shaped ingot result from the absence of convection.

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Grain refinement of Ni-Cr alloys containing zirconium was investigated.Nineteen ingots of alloys(ten Ni-Cr-Zr alloys, five Ni-Cr-Ti-Zr alloys, two commercial dental casting Ni-Cr alloys and two above alloys addedzirconium)were produced by melting pure nickel, chromium, titanium and zirconium, in a high-frequency induction furnace in a vacuum.Test specimens were prepared by using a standard mold preparation of phosphate-bonded investment, and using a high-frequency induction furnace with an insert atmosphere of argon.The etched surface of these specimens were photographed through the metallurgical microscope and the grain size were determined by these photomicrographs.As a result, a grain of Ni-Cr alloys containing minor zirconium was refined and the grain size was almost unchanged even by varying the amount of zirconium.0.03wt% addition of zirconium was efficient in grain refinement of Ni-Cr-Ti alloys.Relatively coarse grains of commercial dental casting Ni-Cr alloys have disappeared, depending on 0.03wt% addition of zirconium.

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Weldlng for mending technique of dental polymer have been studied.Bubbling was occurred for welding without restraint that caused insufficient welding.It has made clear that predrying was effective to prevent occurrence of bubbles about Polysulfone(PSF), Polyethersulphone(PES), Polycarbonate(PC), and predrying condition was developed.In this article, it was found that fixation of Melot moldine reduced welding distortion on PSF, PES and PC.Fixation position made no difference about bending strength.

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Binary mixtures added carnauba wax or dammar(up to 10% each)to paraffin(M.P.56-58℃)were prepared.Cylindrical samples(25mmφ, 25mmH)were compressed uniaxially up to 50% at various temperature and, after that, behavior of plastic deformation, the degree of preferred orientation and the amount of recovery were investigated.The degree of preferred orientation of above samples showed almost unchanged(about 80%).Addition of carnauba wax to the paraffin resulted in a large increase in the yield stress but a reduction of the recovery.The samples added dammer to the paraffin were similar in the plastic deformation but increased in recovery with an amount of its addition.From these results, it is confirmed that the recovery of waxes(the paraffin and binary mixtures)originates from the rubber elasticity of molecules in the amorphous region.

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The grinding wheels containing 30% TiC and the conventional grinding wheels were tested to evaluate the mechanical grinding efficiency. And in case of the TiC wheels, the electro-mechanical grinding(EMG) test was also made to compare the grinding efficiency and the surface conditions with those of the mechanical test.Findings were as follows:The mechanical grinding efficiency of the wheels containing TiC was superior to that of the conventional wheels.Especially, the wheels containing a small amount of CBN (Cubic Boron Nitride) showed a high efficiency.The EMG test showed a higher efficiency compared with the only mechanical test under the same grinding conditions.Furthermore the better grinding surface was observed in case of the EMG test.A test under the condition of 5000rpm in rotational speeds and 865mA/cm²/in electrolytic current indicated the most suitable results.

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In this study, the method of quantitative analysis of residual monomer in hardened dental resin having three dimensional structure has been investigated, employing the unfilled and filled resin based on Bis-MPEPP.The procedure of the determination established by the authors is as follows:1.The procedure for unfilled resin.The film samples of unfilled resin with less than 150μm thickness are prepared by polishing.About 30mg of the film sample is weighed exactly, then immersed in 10ml acetone.The residual monomer in the sample is extracted completely at 23℃ for 2.5 hr or at 37℃ for 2 hr.The extract obtained by evaporation of the solvent is dissolved to 10ml methanol, followed by the quantitative analysis of high performance liquid chromatography(HPLC).2.The procedure for filled resin In the case of filled resin, the procedure is similar to that for unfilled resin, except for the film thickness, the weight of sample and the extraction conditions.For filled resin, it is appropriate that the extraction is carried out at 37℃ for 3 days or under refluxing of aceton(57℃)for 10 hr using Ca.50mg film sample with less than 100μm thickness.

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We had prepared an effective monomer, 4-META, to promote adhesion to dental hard tissues and studied several adhesives based on the methacrylate.A new type adhesive resin cement was reported previously.It consisted of an adhesive liner containing 4-META, MMA as a liquid and TMPT filler, reactive microfilled composite filler, as a powder.Here, an adhesive liner was improved by addition of a silane coupling agent into the previous liner.The new liner gave good adhesion to porcelain as could be expected.The adhesive strengths were as good as 120kgf/cm² to etched enamel, 45kgf/cm² to etched dentin, 110-180kgf/cm² to sandblasted dental alloys including a type I Gold alloy and 100-160kgf/cm² to porcelains.

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In order to improve the adhesion of dental resins to the tooth structure, six kinds of alkylene glycol monomethacrylate with different lengths of alkylene group(n=3-10)were synthesized for use as diluent, and the relations between structure of these monomers and their physical properties were investigated.1)The coefficient of viscosity and contact angle of water increased but polymerization shrinkage and amounts of water sorption decreased with increasing carbon atoms in alkylene group.2)The adhesive strength to the unetched bovine enamel and dentin after standing at room temperature for 3 hrs increased but after soaking in water at 37℃ for 1 day decreased with increasing carbon atoms in alkylene group.

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The crystalline structure change of apatite at high temperature has become of interest due to dental application of laser or sprayed coating of apatite on dental metals.In the present study, fused and resolidified dental enamel by YAG laser irradiation was studied by means of micro-focus laue analyses.A certain ordering of crystalline orientation was verified by the spotty reflection rings.No significant change in lattice constant of enamel apatite was detected however, additional peaks with high intensity appeared.For instance, strong peak clearly separated at 2θ=30.81° was identified to be(441, 170)reflection of α-tricalcium phosphate by the precise analyses, and high angle peaks at 2θ=46.87° and 52.92° were determined to be(4010)and(2020)reflection of β-tricalcium phosphate respectively.Accordingly, formation of high temperature phases of apatite both α and β tricalcium phosphate was certified.

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In order to improve dental resin was synthesized EpDMA with different molecular lengths, which is alternatingly composed of rigid and water-resistant bisphenol A units and flexible alkoxide units in the main chain.Such molecular structure of EpDMA was considered to be suitable for improvement of dental resin.It was found that the longer the molecular length of EpDMA, the more favorable properties were endowed for dental use, though rather the softer it became;such properties as polymerization shrinkage, thermal expansion, water sorption, transverse strength, flexibility and abrasion resistance were improved as the molecular lengths of EpDMA increased.The copolymer of EpDMA and TEDMA was prepared at various comonomer rations.And it was found that such properties as polymerization shrinkage, thermal expansion, water sorption, transverse strength, elastic modulas and hardness were improved as the monomer ratio of EpDMA increased and that the abrasion resistance was the highest around 50mole% of EpDMA concentration.

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The mechanism of surface treatment of glass filler was investigated in the present study in order to increase the mechanical strength of composite resin.The authors synthesized ω-methacryloxy-alkyl-trimethoxysilanes(ω-MATS)with longer chain lengths than γ-methacryloxy-propyl-trimethoxysilane.Silica glass plates were treated with those silane coupling agents.It was examined how the contact angle of water and the bonding strength of polybis-GMA resin on silane treated glass surface depend upon the chain lengths of ω-MATS.It was concluded that the longer the alkylene chains of the silane agents, the more effective the treating effects.From the experiments of washing the silane treated surface by organic solvents it was found that the formation of the chemically absorbed monolayer of silane coupling agents on glass plates was essential for the adhesion of polybis-GMA resin to glass plates.Therefore, it was emphasized that the multilayer of absorbed silane coupling agents played an important role in adhesion.

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The use of the non-precious metal alloys has recently been increasing in dental casting.However, they cannot always show satisfactory fit of the casting.It may partly be attributed to the inadequate expansion of the investment.In the present study, therefore, the change in expansion of the phosphate-bonded investment was examired for the altered mixing conditions and the different batch numbers of the commercial products.When L/P ratio became smaller, the setting expansion significantly increased and the thermal one decreased.Only a slight dilution of the silica sol by water could reduce both the setting and the thermal expansions.As the spatulation was performed mechanically for longer time, the setting expansion became significantly larger with no significant changes in thermal expansion.The change in setting expansion was much larger than that in thermal expansion in any variables.On the other hand, markedly large differences were found in setting expansion among the different lots of each commercial product.They were sometimes larger than the differences caused when the mixing conditions were altered within the practically adjustable range.

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Previous studies by the authors have shown that under conditions which lead to rapid setting of gypsum bonded investments which contain small amounts of CaSO₄・2H₄O, there was a significant reversal of setting expansion after reaching an initial maximum dimension.The degree of reversal was correlated very positively with those factors which accelerate the set of the investment.These factors include CaSO₄・2H₂O content, rate of mixing, and water/powder ratio.It was the purpose of this study to evaluate the combined effects of admixed sodium citrate and CaSO₄・2H₂O on the setting expansion of a gypsum-bonded investment which initially contains no CaSO₄・2H₂O.The results of this study indicated that sodium citrate 1)retarded the initial setting time, 2)retarded the time ton achieve maximum expansion, and 3)reduced or eliminated the loss of setting expansion.Also when samples were mixed with 0.05% sodium citrate solution, crystal of CaSO₄・2H₂O and porosity of set investment grew larger compared with mixed water.There was a strong correlation between the loss of setting expansion and the rate of set of the investment.

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In the present experiments, as a part of the basic studies purposing to establish more accurate electronic method for measuring the root canal length, the author, connecting a self-made variable resistor and CR osillator(TORIO's AG-203)to an electronic apparatus on the market, equipped with the basic electric circuit, and using #15 reamer(PIERCE)as electrode, normal saline solution as electrolyte, examines the influence of resistance and frequency, the major factors, on the meter-indicating value.In the case of the fixed reamer, the meter-indicating value becomes high in accordance with the increase of resistance and slight high in accordance with the increase of frequency.In the case of the movable reamer, the meter-indicating value becomes higher in case the resistance is larger and at 400〜1000Hz in frequency becomes almost the same, but at 150Hz it becomes lower.

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