The effects of several additives(NaCl, KCl, CaCl₂・2H₂O, Na₂SO₄, K₂SO₄, H₃BO₃ and Na₂B₄O₇・10H₂O)on the reaction between II-CaSO₄ and quartz (1:1 in molar ratio) were investigated by TG-DTA and X-ray diffraction.All of the additives promoted the reaction though CaCl₂ initially suppressed it.The reaction proceeded in three stages in the sample with non additive:the first stage-α-and β-CaSiO₃ formation by the reaction between II-CaSO₄ and quartz;the second and the third stages-Ca₂SiC₄ formation by the reaction between II-CaSO₄ and CaSiO₃ and by that between I-CaSO₄ and CaSiO₃, respectively.The reaction process did not change essentially in the samples containing the additives.While the Ca₂SiO₄ was α'-form in the sample containing Na₂B₄O₇, β-Ca₂SiO₄ was formed in the other samples.Na₂O・2CaO・3SiO₂ was also formed above 1200℃ in the sample containing Na₂B₄O₇.The additives used did not promote the decomposition of CaSO₄ itself below 1300℃.It was suggested that their promoting effect in the reaction of CaSO₄ and quartz was due to the increase in reactivity of the quartz by their addition.

View full abstract

The effects of LiCl, NaCl and KCl on the reaction between CaSO₄ and quartz were investigated between 1000 and 1300℃ by X-ray diffraction.The fraction of CaSO₄ reacted was increased by the addition of 2〜5mol% NaCl.But when more NaCl was added, the fraction began to decrease and reached a minimum at 7〜10mol% addition and then increased again.The added NaCl and CaSO₄ melted simultaneously at about 725℃ and reacted with each other to form sodium sulphate.The sulphate reacted with quartz to form Na₂O, which dissolved in the quartz.The dissolved Na₂O resulted in some non-bridging oxygens in the quartz in a same way as in alkali silicate glasses.Therefore it is supposed that the reactivity of quartz will be increased and react easily with CaSO₄.It is also supposed that Na₂O dissolved in a product phase(CaSiO₃)and the rate of diffusion for Ca²⁺ ion in the phase was increased.But in case of 7〜10mol% NaCl addition the transformation of quartz to cristobalite preferentially occurred and the reaction of CaSO₄ and SiO₂ was retarded.When more NaCl(20mol%)was added, the amount of CaSO₄ reacted increased again because the NaCl acted as a mineralizer, that is, CaSO₄ dissolved in the liquid NaCl reacted with the quartz.The effects of LiCl and KCl additions were similar to that of NaCl addition.

View full abstract

The physicochemical properties of synthetic fluoridated hydroxyapatites were studied.The crystallinity increased initially, then decreased, and finally increased again with increasing fluoride content.In contrast to the unexpected crystallinity phenomena, the solubility in acetate buffer solution at pH4.0 and 37℃ decreased monotonically with increasing fluoride content, and scarcely changed above 2mmol/g of fluoride content, in which CaF₂ was detected with fluorapatite.

View full abstract

With regard to the present electronic method for measuring the root canal length, various studies are being carried out especially on variable factors having effects on measured values, purposing to establish more accurate measuring method.But, it still involves many problems to be solved.The author, in the present study purposing to get fundamental data showing the phases of change in meter-indicating values that appear in case the reamer is inserted into a root canal in the process of measuring its length by the present electronic method, using glass tubes in place of root canals to be erected in the normal saline solution used as electrolytic solution, into which inserts uncovered or covered electrode made of stainless steel wire the same material as of reamer, to measure and record the change in impedance by the electronic apparatus for measuring the root canal length on the market, and examined the effects of the surface area of electrode contacted with the solution and the inner diameter of glass tube.The following results were obtained.The glass tube functioned as impedance and its effect became larger, in case it was smaller in inside diameter.The covered electrode, different from the uncovered one, had a fixed surface area to be in contact with the solution.Consequently, when it was inserted into the glass tube in the solution, the meter-indicating value became low, and constant without fail once the electrode came out of the glass tube.

View full abstract

A new crystalline dimethacrylate monomer, 2, 2-bis(4-methacryloxy cyclohexyl)propane(MCP), was prepared from methacryloyl chloride and 2, 2-bis(4-hydroxy cyclohexyl)propane.And the effect of variation of the concentration of the synthesized monomer-MMA copolymer on the physical properties(water sorption, compressive strength and Vickers hardness)were investigated.In addition, SDMA(9mol%)-MMA and BPDMA(9mol%)-MMA copolymers were prepared and their physical properties were measured.Amounts of water sorption were independent on the concentration of MCP, while compressive strength and Vickers hardness increased with increasing the concentration of MCP in the copolymers.Compressive strength and Vickers hardness of MCP(9mol%)-MMA copolymers were equal or higher than SDMA(9mol%)-MMA and BPDMA(9mol%)-MMA copolymers.

View full abstract

In the processing of upper and lower complete dentures, pressures exerted to resin dough in the plester mold at trial closure were measured with the pressure measuring sheets.In according to the increase of the applied load in the range of 400kg to 1750kg, pressures at the portions of residual ridges in upper and lower dentures and of palate in upper denture were increased from 10〜20kg/cm² to 35〜45kg/cm².But at the portions of occlusal rims, little change of pressures was subjected by the applied load and those pressures in upper and lower dentures were 10〜14kg/cm² and 15〜20kg/cm², respectively.Since, while relatively larger load, such as 1750kg, is employed in much case of trial closure, it indicates that the achievement of one of the purposes of trial closure, complete packing of dough in the mold, is satisfied even by smaller load.When 850kg of load is employed at trial closure, it may be sure that the another purpose of trial closure, removing out of the excess dough as the flush, is also achieved.

View full abstract

In the preceeding paper, influence of YAG laser irradiation on the dental hard tissues was studied and it was reported that cavities were formed more easily in carious enamel than in sound one.In the present paper, the effect of surface glossiness of dental hard tissues on cavity formation by laser was considered;the effect of black staining on the surface has already been established by several authors.The importance of surface glossiness became confirmed for the volume of cavities formed in sound enamel increased greatly when the glossiness was less than a critical value.Furthermore, degree of ease of cavity formation in artificial carious bovine tissues by laser was studied using demineralization period from one to six days.An increasing volume of cavities with time was observed in the enamel.Inversely, in the dentine, the volume of cavities was reduced to about 30% that of sound dentine, presumably due to the fractional increase in organic components in dentine resulting from demineralization.Natural caries in human dental enamel were also irradiated with a defocused beam, and cavities with glassy surface were easily formed.Microradiographic and electron microprobe analyses revealed both a decrease in transmittance and an increase in Ca-K_α and P-K_α X-ray intensities on the cavity surface probably due to the high density of the glassy surface.

View full abstract

Considering that the appearance of the post-sintering stress localized in the surface layer of the sinter depends greatly on the methods of cooling, investigations were made on thermal changes in the sinter at cooling and thermal expansion curves by experiments with varying conditions:(a)cooling in the furnace, (b)cooling in the cup, (c)natural air cooling and(d)forced air cooling.Also, quantitative analysis of the surface stress induced by them was made, with results as follows:1)In the results of investigation on thermal changes, there was hardly any noticeable difference in the rates of depression between the interior and the surface layer of the sample by cooling in the furnace, a slight difference was observed in the cases by cooling in the cup and natural air cooling, and in the cases by forced air cooling, there was a great difference between the two parts, the thermal difference surpassing 100℃.2)Average coefficients of thermal expansion at 200〜720℃ showed a gradual diminution in the following order cooling in the furnace 1.21%, natural air cooling 1.08%, cooling in the cup 1.05% and forced air cooling 0.95%.3)The velocity of thermal depression and the thermal difference between the interior and the surface layer of the sample gave influence on the generation of stress.4)Whereas the surface stress measurement in the cases of forced air cooling and natural air cooling resulted in 8.5kg/mm² and 2.5kg/mm², no value was obtained from the cases by cooling in the cup and cooling in the furnace probably because of the generation of extremely little stress.

View full abstract

A newly designed cutting test apparatus for regulating the pressure was developed in order to evaluate the machinability of the two experimental typodont tooth materials, as compared to natural teeth.By using the apparatus, the machinability of the materials was determined by magnetically monitering the change of the bur revolution and time spent.Surface charateristics of the bur blades were observed under SEM before and after use.The findings were as follows:1.The machinability of the experimental materials could be precisely evaluated by determining both decrease in turbine revolution during cutting and time required for cutting a given length of the materials.2.Two kinds of the experimental materials(made from composite resin and melamine resin respectively)were not suitable as substitute for enamel because of either extremely lower or higher machinability.3.Wear of cutting edge and surface roughening of the burs were observed under SEM.These facts would support the results of the cutting test.4.The experimental typodont tooth materials are requested to be further improved.

View full abstract

The curing contraction of addition-type silicone impression materials was measured by the mercury bath method.The results point to a classification of this group of materials into the following two types:1)Materials with maximum 0.08% polymerization contraction at 22℃ 15 min after the start of mixing and maximum 0.03% contraction at 22℃ during storage from 15 min to 24 hours(Permagum, President, Reflect and Reprosil).2)Materials with more than 0.08% polymerization contraction at 22℃ 15 min after the start of mixing and significantly more then 0.03% contraction during storage at 22℃ from 15 min to 24 hours(Exaflex and Xantogum).

View full abstract

The study on the relationship between free curing contraction and effective curing contraction of the impression materials have not been so far reported.The purpose of this study was 1)to study the relationship between free curing contraction and effective curing contraction and 2)to evaluate the relationship between rheological properties and effective curing contraction.Three kinds of addition-type impression material;P, E and X, having characteristics of their contraction pattern were examined according to the following methods.Effective curing contraction was determined by measuring dimensional deviation of the stone dies and free curing contraction was measured by means of mercury bath method.Rheology of the impression materials during setting was studied by the oscillating rheometer and cone-plate rheometer.The findings were as follows:1)There was no correlation between on the one hand the free curing contracion of the three selected addition-type silicone impression materials and on the other hand the effective curing contraction.2)The coefficient of correlation between on the one hand free curing contraction of the impression materials during the time interval from 15 min to 24hrs after the start of mixing, and on the other hand differences in diameter of stone dies poured 24hrs and 10 min after removal of the impression was very high(r=0.83).3)Depending on specific rheological properties having a substantially high plasticity during setting process, free curing contraction of an impression material in the impression tray can be partly compensated for by flow which will be induced by an underpressure in the impression paste between the tray and teeth.

View full abstract