Susumu Hirano

Materials Science Initial Dimensional Change of Composites in Dry and Wet Conditions

A study of the expansion of restorative composite resins associated with water sorption was conducted. The results show that this expansion depends upon the mass of water absorbed by the composite restorative. Although polymerization shrinkage can be compensated by expansion due to water sorption, the length of time involved in achieving this compensation is approximately seven d.

Diffusion coefficient of water through dental composite resin

Water sorption of polymer filling materials affects dimensional stability, mechanical properties and bonding strength with tooth structures. To clarify the effect of the degradation on service life and micro-leakage, the diffusion coefficient of water through the resin should be identified. Distributions of time-dependent water concentrations in the resin were computed. Water sorption of composite resin discs with different thicknesses was measured and compared with the solution of Ficks second law. The diffusion coefficient of water through the resin discs was computed to be D=3.9-5.0 x 10(-13)m(2)/s from the measurements of specimens with different thicknesses. Results of water sorption measurements for the discs with different thicknesses were in good agreement with the theoretical results. The relationship among the thickness of the disc, the diffusion coefficient and the water sorption ratio was shown clearly. The testing method for water sorption by International Standard ISO 4049 for resin-based filling materials was discussed.

Casting of dental alloys: mold and alloy temperature effects

Abstract A casting monitor consisting of a square polyester grid pattern was used to evaluate the casting behavior of 6 non-precious dental casting alloys. A castability value, Cv, was determined and is defined as the fraction or percentage of cast grid segments. The dependence of Cv on the alloy superheat casting temperature, Ta, and mold temperature, Tm, was evaluated. It was found that the standard deviation (SD) of Cv was dependent on Cv. For curve fitting, a transformation, Cv,t, of Cv was used such that the SD(Cv,t)≠f (Cv,t). The transformed castability value is given by C v , t = ln ⁡ 2 / 3 + C v 2 / 3 + 1 / C v It was found that a single equation was capable of representing Cv,t for all 6 dental alloys at the 95% confidence level: Cv,t=a+bTa1/2Tm2. This expression has many potential uses for alloy design, product quality assurance and dental laboratory casting control.

Relative efficiency of radiation sources for photopolymerization

The aim of this study was to evaluate the characteristics of new-generation light-emitting diode (LED) units in comparison with the conventional tungsten-halogen, plasma arc, and first-generation LED units reported in our previous study. The irradiance of light from new-generation LED units, the temperature rise of the bovine enamel surface, and the depth of cure of composites exposed to each unit were investigated. The irradiances in the range 400–515 nm emitted from the new-generation LED units were greater than those from the first-generation LED units. The temperature increase was 15–25°C for new-generation LED units compared with a typical value of 5°C for the first-generation LED units at 10 s of irradiation. The relationship between the depth of cure and the logarithm of total exposure energy suggested that LED units can cure light-cured composite resins more efficiently than tungsten-halogen or plasma arc units.