Aluminum orthophosphate (AlPO₄) has three polymorphic forms similar to those of silica, which has been used as a refractory material for dental investments. Studies on the characteristics of AlPO₄ have suggested that AlPO₄ could be used for casting investments instead of SiO₂. Heat behavior, thermal expansion and thermostability of AlPO₄ were examined. The transitions between AlPO₄ polymorphic forms occurred more readily than those between SiO₂ polymorphic forms. The reconstructive transformation of β-berlinite to β-cristalphite occurred at about 900〜1,000℃, and was accompanied by volume expansion of 20%. Aluminum orthophosphate neither decomposed by heat treatment (1,400℃, 1hour) nor reacted with silica, alumina, magnesia, graphite and dental cast alloy. Aluminum orthophosphate was found to be suitable for dental investments.

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Two types of new dental investments were developed for low-and high-temperature castings by using AlPO₄ as a substitute for SiO₂, which has been used as a refractory material in many commercially available investments.Heating of AlPO₄ at higher temperatures resulted in the formation of berlinite and cristalphite with the ratio of their phases depending on the heating temperature. The low temperature investments prepared by mixing 20wt% of gypsum with 80wt% of the mixture of berlinite and crystalphite, i.e. gypsum bonded investment, showed 2% greater heating expansion at 900℃ as the amount of crystalphite increased.Heating expansion of the high temperature investment prepared by mixing 20wt% of NH₄H₂PO₄ and MgO with 80wt% of berlinite was more than 7% at 1,000℃. Using this investment with no appreciable setting expansion, it is possible to compensate for casting shrinkage by heating expansion alone, thus avoiding possible deformation of wax petterns during setting expansion.

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The dimensional accuracy and deformation of stone cast with an undercut obtained from condensation-type and addition-type silicone rubber impression materials, was measured by a three-dimensional measuring system. The influences of temperature during impression-taking, impression-removing speed and pouring-time of die material were investigated. The increase in diameter of stone cast was obtained in larger undercut area at the oral temperature, but no apparent influence of the temperature was observed in dimensional accuracy of height or in deformation. The faster impression-removing speed resulted in better reproductivity of the undercut. A delay of pouring-time of die material influenced the dimension of the stone cast obtained from the condensation-type silicone rubber impression. No definite influence was observed on the stone cast obtained from the addition-type silicone impression. The stone cast obtained from the condensation-type silicone impression showed a good reproductivity to the original dimension, except when the die material was poured 24hours after removal of the impression.

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The hygroscopic expansion and the water sorption of a series of composite resins consisting of several binary copolymers were investigated. The hygroscopic expansion was about one third of the water sorption of each composite resin.In the composite resins containing a binary copolymer system, linear regression analysis was applied to both the hygroscopic expansion and the water sorption against the mol fraction of the components. The water sorption of each component was as follows; 2.36±0.23wt% for UDMA, 0.16±0.05wt% for bis MPEPP, 3.52±0.55wt% for TEGDMA, 2.48±0.45wt% for bis GMA. The hygroscopic expansion measurement of the composites was less affected by the elution from the composites than the water sorption measurement, thus the hygroscopic expansion measurement was suitable for the estimating the water sorption of the composite.

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A method for the quantitative evaluation of transmissivity of light-cured composite resins was developed. Transmissivity could be evaluated by the dependence of depth of cure on radiant emittance or exposure time. Light permeability of the commercial composite resins are affected by several factors. In this study, the influence of pigments on the light permeability of composite resin was examined using the proposed method. It was demonstrated that the pigments which absorb the light with wavelength from 450 to 500nm greatly decreased the transmissivity of the composite resin. Based on the Lambert law, the relative emittance of polymerization sources was also calculated from depth of cure data.

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Reciprocation of wheels in electro-mechanical grinding was investigated. Three kinds of dental alloys (Co-Cr, Ni-Cr, Ag-In) were ground and the amount of grinding and the grinding surface were examined. The electro-mechanical grinding with reciprocation of wheels showed higher amounts of grinding and better surface textures than the mechanical grinding with reciprocation of wheels for each alloy. The electro-mechanical grinding with reciprocation of wheels enhanced electrolysis even with a small amount of electrolyte, especially in Co-Cr alloy and Ni-Cr alloy. The electro-mechanical grindig with reciprocation of wheels was found to be a more practical method than the electro-mechanical grinding with conventional revolution of wheels with respect to safety and manipulation.

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Polarization resistance method was employed to determine the corrosion rate of dental alloys without disturbing the corrosion system. Where corrosion rates were small, the measurement of polarization resistance using The Square Wave Current Method became very time-consuming, in view of the relatively long period required to establish steady-state conditions. For such conditions, polarization resistance was determined by the analysis of transient response, on the assumption that the electrode/solution interface behaved like a simple RC-parallel circuit, the resistance corresponding to the polarization resistance. Polarization resistance data of eleven dental alloys correlated well with weight loss data. The proportionality constant K values, which convert the polarization resistance to the corrosion rate, determined in Ringer's solution at 37℃ were whithin the range of 14〜35mV.

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We evaluated the adhesiveness to human enamel of a new enamel adhesive coating agent and its effect on remineralization of white spots. Adhesiveness was measured using impacted bovine teeth that were bonded with coating agent surface to surface. Then bond strength was determined.In experiments on remineralization, sound human premolar were extracted, and artificial white spots were produced by acid gelatin. Then the teeth were treated with flouride containing the coating agent and then immersed in the calcifying solution. The degree of remineralization of the white spots were evaluated qualitatively. The bond strength of the new coating agent to the white spots was considered to be adequate. The remineralization of the white spots was enhanced by the application of the flouride containing coating agent. An F^- level of 10ppm effectively increased the remineralization of white spots.

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Effects of interval and frequency of applying a new fluoride-containing enamel adhesive coating agent on remineralization were investigated. The coating agent containing 10ppm F was applied to the artificial white spots on the enamels of permanent bovine incisors at various intervals (1, 3, 7 and 14 days) and frequencies (0, 1, 2, 3, 4 and 5 times). The teeth were immersed in the calcifying solution (pH7.0) for 35 days at 37℃. The degree of remineralization of the white spots was evaluated by the solubility reducing effect in acetate buffer (pH4.0). Solubility reducing of the white spots increased with increases in the intervals between applying the coating agent, but was constant at intervals of more than 7 days. As to the frequency of coating, the white spots in which the coating agent was applied 1〜3 times were superior to untreated white spots with respect to reducing effects. These results suggest that the most suitable conditions of application with the coating agent are too coat 1〜3 times at an interval of more than 7 days.

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Addition of calcium hydrogen phosphatedihydrate crystals to supersaturated calcium sulfate dihydrate solution remarkably accelerates crystallization of calcium sulfate dihydrate. In this study, addition of calcium hydrogen phosphate (DHP) crystals to improved dental stone (Fujirock) was examined to improve mechanical properties of gypsum model materials. Bending strength of the hardened mixture, which contained 8wt% DHP, was 33 percent higher than that of the dental stone. Based on SEM observation of the fractured surface of the hardened mixture, the surface roughness markedly decreased.

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Ti-Ni shape memory alloys sometimes dissolute at low electric potential when they are anodically polarized in 1% NaCl solution. The origin of this phenomenon was investigated with respects to crevice corrosion, strain by working and inclusions. To examine the effect of crevice corrosion, a small crevice was made between a part of insulator coating film and a specimen, and anodic polarization was measured. In this specimen, dissolution was observed, but pitting was formed on the central part of the specimen but not on the prepared crevice. The effect of strain was examined in the specimens tensile strained from 0.5% to 4.0%, but no correlation was observed between dissolution at low potential and tensile strain that is, dissolution did not always occur in a specimen that was strongly tensile strained. Observations by means of EPMA and SEM were carried out on the specimens that showed dissolution at low potential, and many precipitates with high Ti and low Ni concentration in comparison with the matrix were found in the pitting hole and whole specimen. Direct correlation between these precipitates and dissolution at low potential was not confirmed yet, but they could be a defect to the corrosion resistance in specimen. The oxide film coating to suppress the dissolution at low potential was carried out, and after shape memory procedure and anodic polarization, no dissolution at low potential was observed and a stable surface was preserved. Oxide film coating is considered to improve corrosion resistance in Ti-Ni shape memory alloy.

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The relationship between the bond strength and rigidity of adherends was examined using commercial dental resin adhesives. The adherends were seven alloys with Vickers hardness of 10〜220, and human and bovine teeth. The alloys were plated with Tin to make the bonding activity uniform. In case of alloy, there exists an obvious relation between hardness (x) of adherends and bond strength (y). The regression equations were y=1.01x+140 for resin cement of 4-META-TBB-O system, and y=1.04x+111 for that containing phosphate ester as a promotive monomer. In regard to teeth, although the strongest correlation was observed between the distance (μm) from enamel surface and bond strength, there was no correlation between bond strength and hardness. In addition, increasing gradient of bond strenght for hardness of enamel is rela tively small compared to that of the alloys. As the bovine teeth were similar to human teeth in bond strength, bovine teeth were considered to be suitable substitutes of human teeth in these adhesion testing.

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Hydration of CaO has been an obstacle to apply CaO to dental investments for higher temperature casting. In this study mixing media as a substitute for H₂O were investigated and the characteristics for the dental investments were examined. Fused calcia-methanol system was found to have a good possibility for dental use. In this system the green strength was 3.5〜5.0MPa, the fired strength was 2.0〜5.0MPa and the thermal expansion was 0.5% at 1,000℃.

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The effect of current conditions on some electric discharge machining properties of Co-Cr alloy was investigated. The increase of pulse magnitude and pulse on time caused an increase of electric discharge energy, and had effects on machining properties. The machining speed, the surface roughness, and the clearance between the electrode and the metal increased with the increase of pulse magnitude and pulse on time when stable machining was performed. The electrode elimination ratio decreased with the increase of pulse on time. To perform the electric discharge machining stably, it was very important to set pulse off time suitably.

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Electric discharge machining properties of some biometals, namely, Co-Cr alloy, two kinds of stainless steel (SUS 304, SUS 316L), and pure titanium were investigated. Co-Cr alloy and two kinds of stainless steel had similar machining properties. The machining speed, the surface roughness, and the clearance between the electrode and the metal increased with the increase of pulse magnitude from 4A to 16A. Pulse on time also had an effect on the machining speed, and there was a suitable pulse on time for each pulse magnitude. The electrode elimination ratio increased with the increase of pulse magnitude, and decreased with the increase of pulse on time. Pure titanium showed very low machining speed and a high electrode elimination ratio compared with the other alloys. The machining speed increased with the increase of pulse magnitude and pulse on time. The effect of pulse magnitude and pulse on time on other machining properties was similar to that on other alloys.

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