Y. Araki

Socioeconomic factors and dental caries in developing countries: A cross-national study

The purpose of the present study was to analyze statistically correlations between socioeconomic factors and the prevalence of dental caries in developing countries. The DMFT index, which evaluates the incidence of dental caries, showed a positive correlation (P < 0.01) with several socioeconomic factors, such as life expectancy, adult literacy rate, school attendance rate, population employed in the service sector, population aged 15-64 years, and urban population. According to multiple regression analysis, population aged 15-64 years, population employed in the service sector, and urban population were the most influential independent socioeconomic variables, in descending order, with a regression coefficient of 0.635 and a coefficient of determination of 0.404 (P < 0.001). This finding suggests that the prevalence of dental caries in developing countries increases with the degree of urbanization.

Destruction of metal-resin adhesion due to water penetrating through the resin

Polymethyl methacrylate (PMMA) film bonded by 4-META dental adhesive resin to mild steel was used as a specimen to study the mechanism of water permeation into the adhesive interface. Degradation of the adhesion interface after immersion in water was investigated by measurement of peeling area after imposing thermal stress using liquid nitrogen. The amount of water penetrated at the interface was calculated from the solution to Ficks second equation. The degradation at the interface is discussed with respect to the water content. Water enters the interface by diffusion through the resin rather than by passage along the interface. The interface is broken by water when the water content at the interface reaches 48% of the equilibrium water concentration of PMMA. Observation through the PMMA film shows no change in the mild steel surface at 48% equilibrium water concentration; at 95% water content many small white spots appear on the surface. The surface colour gradually changes to black due to the formation of corrosion products.

Effect on the grain interior and grain boundary reactions by Cr addition to low gold content alloys

The effect on grain interior and grain boundary reactions by Cr addition to low gold content alloys were investigated by electric resistivity measurements, hardness tests, optical microscopic observations, and TEM observations. The grain interior reactions were accelerated by the chromium addition while grain boundary reactions were retarded. The formation of AuCu l type ordered phase in the grain interior was accelerated, and the T-T-T curve of AuCu l type ordered phase shifted to higher temperatures and shorter times with increasing chromium content.

ESCA study on the destruction mechanism of metal-resin adhesion due to water penetrating through the resin

The mechanism destroying the adhesion between PMMA and mild steel, and the action of water molecules reaching the adhesion interface was investigated. Mild steel and a thin PMMA film were bonded with dental adhesive (4-META resin). After immersion in water, the adhesive layer was peeled from the metal surface. The surface was analysed by ESCA, reflection electron diffraction, and X-ray diffraction to elucidate changes in surface structure. After 2 weeks immersion, there was a very thin brown product and after 2 months a thick black corrosion product composed of Fe3O4 formed at the adhesion interface. The thin brown layer showed no X-ray diffraction pattern but reflection electron diffraction showed it to be FeO(OH). After 3 days immersion, the mild steel surface appeared unchanged through the clear resin layer, and ESCA analysis showed the same chemical state as the surface of the specimen immersed for 2 weeks. Destruction of adhesion between metal and adhesive is caused by destruction of the chemical bond by the water molecules permeating through the resin layer. It is followed by corrosion of mild steel which produced iron oxide hydrate.

Effects of tin addition on the ordering process of PdCu ordered alloys

The effects of tin additions on the ordering process of PdCu ordered alloys were investigated by electric resistivity measurements, optical microscopic observations, hardness tests, X-ray diffraction, and electron probe micro-analysis. The ordering process was found to be acccelerated by addition of tin, 1.0% addition being the most effective. This acceleration of ordering by the addition of tin may be explained by assuming that the formation of ordered nuclei becomes easier. The β-phase is stable up to 900 °C with 1.5% Sn addition.

Effects of pre-ageing and cold working on grain-interior and grain-boundary reactions of Ag-Pd-Cu alloys

The effects of pre-ageing and cold working on grain-interior and grain-boundary reactions of Ag-Pd-Cu alloys were investigated by electrical resistivity measurements, hardness tests, and optical microscopic observations. Hardness values of the grain interior after the final thermomechanical treatment increased 20%–30% above the values after conventional ageing. The hardness values of alloys preaged at 400°C for 1 min and then cold worked at 30% are the highest (Hv=320).