As a part of our studies to understand the durability of an MMA-based resin cement against thermocycling in adhesion to metals, the changes of bond strength to titanium produced using various cements were examined by a thermocycling test in water with regard to the effects of bonding area, type of adherend material, cement thickness, type of initiator, powder/liquid ratio, and type of polymer powder.Cement thickness, type of adherend material, and type of initiator affected durability of the bond.The powder/liquid ratio in PMMA/MMA-TBB resin had little effect.Among the MMA-soluble polymer powders formulated in MMA-TBB resin, PMMA produced more durable bonds than a homopolymer of ethyl methacrylate or its copolymer with MMA.Measurement of glass transition temperatures of the cured resins suggested that the heterogeneity of the polymer blend may be involved in the reduced durability.

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The effects of removal time of stone models from agar impressions and agar/alginate combined impressions on the surface roughness(Ra)were evaluated between 30min and 24h after the start of stone mixing.Six agar impression materials, one alginate impression material and two dental stones were used.The Ra of stone models removed at 1, 3 and 6h was also determined on two agar impression materials which resulted large differences in Ra between 30min and 24h.The Ra of stone models removed from the impressions at 30min was larger than that of models removed at 24h.The models removed after 3h showed the minimum value of Ra.

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The effect of 4/60℃ thermal cycling test on mode I fracture toughness and the characteristics of the fatigue crack propagation were investigated to evaluate the durability of adhesion of dental adhesive resins such as Super-Bond C&B (SB)and Panavia 21(PV).Static and fatigue testing of fracture toughness were performed using the double cantilever beam(DCB)testing reported in Parts 1 and 2.Fatigue DCB testing was performed at a frequency of 1Hz.In the thermal cycling test, the value of fracture toughness( G_IC)of SB showed a tendency to suddenly drop in conjunction with changing cohesive failure to interfacial failure in the crack propagation area after passing through a certain number of thermal cycles.Therefore, SB was influenced significantly by thermal cycling.PV was observed as cohesive failure throughout the whole region, and the decrease of G_IC was a little.In the fatigue test, in the case of SB, the relation between fracture toughness( G_max)and number of cycles( N)on a log scale was approximated with a linear equation.In the case of PV, no such tendency was observed because the data of G_max were widely scattered in the G_max-N diagram.However, in the case of SB, the G_max, which might be considered as the allowed value of fracture for an adhesive, was effective for a dental prosthetic design which allowed control of the fatigue life.

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Fusion welding by infrared heating between titanium alloys(pure titanium and Ti-6 Al-4 V alloy)and two kinds of type IV gold alloys was investigated.The gold alloy spreading area and bonding strength by fusion on titanium and Ti-6 Al-4 V alloy were measured.The interfaces between gold and titanium alloys were analyzed by EPMA.The gold alloys showed reasonable spreading by fusion on both pure titanium and Ti-6 Al-4 V alloy.Bonding strength of one of the gold alloys for pure titanium was 299 MPa and that for Ti-6 Al-4 V alloy was 492 MPa. These values were the same as those of the dental gold alloy solders for pure titanium and Ti-6 Al-4 V alloy.

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The objective of this study was to evaluate weight changes and dimensional changes of glass ionomer(GI)cements in air and water.Specimens from three resin-modified glass ionomer(RMGI)cements and one acid-base glass ionomer(ABGI)cement were prepared and stored in air and water at 37℃ for 60 days to determine the weight change and the dimensional change.The weight change(%)of GI cements decreased with aging in air.The decreases of weight change of RMGI cements were much lower than those of ABGI cement.On the other hand, the weight change of GI cements increased with aging in water.The increase in weight change of ABGI cement in the early stage was higher than those of RMGI cements.Dimensional changes(%)of GI cements decreased with aging in air.Shrinkage of ABGI cement was much higher than those of RMGI cements.On the other hand, dimensional changes-of GI cements increased with aging in water.Expansion of RMGI cements was higher than that of ABGI cement.These results indicated that resin modification of GI cements influenced the stability of cements in air and water.Especially, expansion of RMGI cements in water may influence the behavior in vivo.

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To study how the shape of the cavity formed in a tooth affects the tooth's mechanical resistance, we produced a test tooth by preparing a box-like or plate-like cavity with a base or tunnel penetrating through the enamel, enamel/dentin, and dentin of the tooth, and then filling the cavity with a light curable composite resin.Next, we applied a load corresponding in force to that estimated to be present during mastication repetitively onto the test tooth to evaluate its mechanical resistance.Through this test, we examined the fatigue strength and sealing stability at the cavity margins of the tooth.In addition, we observed the boundary between the internal surface of the tooth forms and the resin by SEM.The following results were obtained.Among cavities of a given shape, those that formed in the enamel showed the largest fatigue strength, followed by those formed in the enamel/dentin, and those in the dentin.Regardless of the shape of the cavities, as the repetitively applied load increased in intensity or the number of cycles increased, the sealing stability at the margins was impaired more severely, and cracks were observed at the boundary more frequently.These factors may account for the reduced resistance of the test tooth when exposed to a repetitively applied load.The plate-shaped cavity demonstrated a sealing stability superior to that of the box-shaped cavity.I boundaries were less affected by cracks.The plate-shaped cavity's lower consumption of tooth substance for formation suggests that such cavities formed in teeth can be more effectively treated.

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The effects of Si and Nb on the castabirity and the magnetic properties of Fe-Pt magnetic alloy were investigated.The surface roughness, magnetic properties, and the attractive forces were measured, and the following results were obtained.1.Addition of 0.1Si(mole%)or more improved the surface roughness and decreased casting defects.2.Addition of 0.2Si(mole%)or less or of 0.5Nb(mole%)or more produced excellent magnetic properties.Also, the addition of Si reduced the heat treatment time.3.Addition of 0.1Si(mole%)or less or of 0.6Nb(mole%)or more made alloys with strong attractive forces.4.The optimal composition was Fe-39.5Pt-0.65Nb-0.1Si(mole%)with regard to surface roughness, magnetic properties, and attractive forces.

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The light intensities of 15 kinds of recent light curing units were measured with time.The effects of changes in light intensity on polymerization of resin were evaluated by measuring the depth of cure in light-cured composite resins.The changes in light intensity with time were classified into 3 types.The light intensities on 3 light curing units were approximately constant with time, and those on the other 12 units were reduced with time.The depth of cure on composite resins was calculated from the reduced energy for a period of 10s after 290-s continuous emission.On the units in which light intensities were greatly reduced, the depth of cure was half that of the initial 10s after switching on.This suggested that the effects of variation of light intensity with time should be considered, when using light curing units.

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Resin-based composite materials that include diacrylates such as urethane-dimethacrylate(UDMA)and triethyleneglycol-dimethacylate(Tri-EDMA)have been used as restorative materials for crowns and bridges.In this study, to improve the viscoelastic properties of such restorations, six experimental composite materials produced using ultrafine SiO₂ particles(0.007 and 0.14 μm)were made.Stress relaxation modulus(Er(20)), water absorption, thermal expansion and residual monomers for these materials were determined and discussed.The results obtained were as follows: (1)the temperature dependence of Er(20)decreased by adding the SiO₂ filler, (2)the increase of filler particle size reduced the amount of thermal expansion, (3)the increase of UDMA raised Er(20)in the temperature range below 50℃. Although the addition of filler produced an increase of water absorption or residual monomer, the marked improvemet of relaxation modulus was observed.This finding may be favorable as restorative materials for crowns and bridges.

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In this study, reduction of the dental casting procedure time only 45 minutes after the start of the mixing was achieved by mixing with an accelerator solution.Three commercial quick-heating gypsum-bonded investments and H₃PO₄ and K₂SO₄ solutions were used.The following changes were observed using the accelerator:reduced fluidity, shorter setting time, and rapid increase of setting expansion and green strength.The optimal concentrations of H₃PO₄ and K₂SO₄ were 0.5mass% and 1.0mass%, respectively.The surface roughness of castings prepared with and without the accelerator solution was not significantly different.The accuracies of casting were improved or not changed using this accelerator.Thus, the new technique of mixing a quick-heating gypsum-bonded investment with the accelerator solution reduced dental casting procedure time.

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The effects of immersion disinfection of impressions on the reproducibility of stone models obtained from agar/alginate combined impressions were examined.Two brands of agar and alginate impression materials were used.Impressions were disinfected by immersing in 2% glutaraldehyde solution for 30min and 0.5% sodium hypochrolite solution for 15min.The effects of disinfection on the reproducibility of stone models obtained from agar/alginate combined impressions were recognized.Using alginate impression materials with large hygroscopic expansion, the stone models obtained from impressions immersed in the glutaraldehyde solutions showed a large dimensional change.

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