Shika rikogaku zasshi. Journal of the Japan Society for Dental Apparatus and Materials最新文献

The authors tried to manufacture a casting machine in dentistry by application of the infrared image furnace with a high heating speed and an easy control of a heating temperature. This machine melts an alloy in a carbon crucible set in the furnace, held in the horizontal position. Then, the furnace is turned to the vertical position to drop a melted alloy on the casting mold, and the alloy is cast in the mold by the pressure of Argon gas. The functions of trial casting machine were follows. 1. The trial casting machine was capable of heating to 1250 degrees C within one minute under 4 kW electric power. 2. The castability of the 20% Au-Pd-Ag commercial alloy cast in all casting conditions by the trial casting machine was higher than that of Thermotrol D-2 automatic centrifugal casting machine. 3. Castings of the trial casting machine showed higher tensile strength and elongation than those of the centrifugal casting machine, and the deviation of values got by the trial casting machine was small. In particular, some casting of the trial casting machine showed three times or over elongation values as compared with those of the centrifugal casting machine. 4. When casting conditions (casting temperature, casting pressure) of the trial casting machine changed, the physical properties of castings did not change so much. However, when the mold was not prevented from heating by the furnace in casting, the elongation of castings increased.

The thermal and viscoelastic properties of commercial inlay waxes were investigated by measurements of JIS (Japanese Industrial Standard), differential thermal analysis, dilatometry and dynamic viscoelasticity. The crystal transition from orthorhombic to hexagonal caused by paraffin was observed in inlay waxes. The thermal expansion of waxes was influenced by crystal transition temperature. The cooling shrinkage of internal wax pattern is smaller in wax having larger expansion from 25 degrees C to 37 degrees C. The viscoelastic properties of inlay waxes shifted to low temperature region with decreasing in crystal transition temperature. The various properties of inlay waxes greatly changed at temperature of peak for loss tangent. There were some problems on measurement methods of JIS and the reexamination of thermal expansion and flow was suggested. Commercial inlay waxes, respectively, have various different properties, but from these results the qualities of waxes can not be determined so it's necessary to choose a suitable wax according to various conditions.

Based on the fracture energy form the compression test of composite resins under the repeated impact loads in compression after stored for a period in water at 37 degrees C, the dynamic durability of commercially used composite resins was evaluated. The results obtained were summarized as follows. (1) The fracture energy of composite resins under the repeated impact compressive loads after stored in water at 37 degrees C is clearly lower than the initial fracture energy. Particularly the decrement of the initial fracture energy for Concise, Epolite 100 and Superlux after stored for 30 days in water at 37 degrees C is influenced considerably both by the physicochemical degradation caused by water and the mechanical deterioration caused by the repeated impact compressive loads, but Clearfil is superior to the mechanical deterioration. (2) In each composite resins, there is a close correlation between fracture energy and compressive strain. In composite resins without Superlux, also a correlation is in fracture energy and compressive strength. (3) In clearfil, concise and Epolite 100, the morphology of fracture in compression test of composite resins which are subjected to and are subjected to the repeated impact loads in compression is classified in a mode of shear fracture, but in Superlux, a mode of crack fracture.

Commercial Ag-Pd-Cu-Au alloys of which Au content was changed form 0 to 50 wt. % were quenched from 800 degrees designated by JIS as solution treatment temperature and aged isochronically. The maximum hardness of the alloys by ageing decreased with increasing the Au content. Shape of age-hardening curve of the alloys was divided into two groups, one was of the lower Au content alloys containing below 30 wt. / Au and the other was of the higher Au content alloys containing more than 40 wt. % Au. As quenched structure of the lower Au content alloys was mixture of f.c.c alpha 2 phase and L20 type ordered phase (beta phase), on the other hand, the higher Au content alloys was composed simply by f.c.c alpha phase only. The maximum hardness of the lower Au content alloys given by ageing was caused by precipitates appeared at grain boundary, and of the higher Au content alloys was caused by precipitates in the grain were found. These facts may suggest that the mechanism of age hardening for the alloys change with increasing the Au content.

Commercial Ag-Pd-Cu-Au alloys of which Au content was changed from 0 to 30 wt. % were quenched from 800 degrees C designated by JIS as solution treatment temperature. The structure of the alloys were composed by f. c. c. alpha 2 phase and L20 type ordered phase (beta phase). When solution treatment temperature was elevated up to 940 degrees C-980 degrees C, as quenched structure shows only one phase named alpha phase. A new hardening stage which was not appeared in the case solution treated at 800 degrees C was found at the temperature near 250 degree C in age-hardening curve, and the maximum hardness given by ageing increased. The maximum hardness given by ageing of the alloys which were solution treated at 940 degrees-980 degrees C was not caused by precipitates appeared at grain boundary but by precipitates in the grain.

Short term biocompatibility of biomaterials including various dental materials has been elucidated so far. When considering the practical cases of utilization of the biomaterials, it is known that they stay for quite a long period in situ once after brought into the living system. It seems, therefore, essential that long term biocompatibility of the materials should be clarified in order to provide better knowledge on various aspects of biocompatibility. The present paper deals with long term biocompatibility in vitro of common dental materials, dental amalgams.

The liquid of zinc phosphate dental cement contains Al3+ and Zn2+, and these ions effect greatly on the setting reaction of the cement. When zinc oxide powder is mixed with this cement liquid, reaction products in the set cement are amorphous and not made sure by X-ray diffraction. Therefore the setting reaction of zinc phosphate cement is not certain. In this paper the effects of Al3+ on the setting reaction of cement are examined by X-ray diffraction, thermal analysis and infrared spectrum, and set cements are observed by SEM and microradiography. Judging from these results, it is conclude that Al3+ in the cement liquid precipitates amorphous aluminium phosphate due to change of pH of liquid when it is mixed with zinc oxide powder.

Several raw material waxes used in the inlay waxes, such as paraffins, carnauba wax, beeswax and dammar were investigated by measurements of X-ray diffraction, dilatometry, differential thermal analysis and dynamic viscoelasticity. The relationships between the viscoelastic behaviour and the physical properties of these waxes were discussed. The solid-solid phase transition from orthorhombic to hexagonal occurred in paraffins and the difference of the crystal transition temperature was observed between the kinds of paraffins. The viscoelastic properties and the thermal expansion of paraffins changed considerably in the crystal transition temperature region. Carnauba wax was an orthorhombic material, but such a crystal transition as paraffins did not appear. The dynamic modules of carnauba wax was greater than that of paraffin and decreased slowly to 70 degrees C and the loss tangent was small in this region. It was found, therefore, that carnauba wax had an elastic property in the crystal transition region of paraffin. Beeswax was also an orthorhombic material. The dynamic modulus of this was smallest in these waxes and the loss tangent increased relatively slowly with increases in temperature. Dammar was an amorphous solid. The dynamic modulus and the loss tangent were approximately constant in the low temperature region and changed greatly in the glass transition region. The thermal expansion of dammar was smallest in these waxes.