Dimensional accuracy of stone casts made from silicone-based impression materials and three impression techniques.
The aim of this study was to measure and compare the dimensional accuracy of stone casts of a partially edentulous mandibular arch made by two condensation silicones, two addition silicones and three impression techniques (1-step putty/light-body, 2-step putty/light-body and monophase technique). A partially edentulous standard stainless steel mandibular arch cast was obtained with reference points on the permanent mandibular right and left canines, and permanent mandibular right and left second molars. The anteroposterior distances (between the mandibular left canine and second molar and between the mandibular right canine and second molar) and transverse distances (between the mandibular left and right canines and between the mandibular left and right second molars) of stainless steel cast were measured by a microscope at 30× magnification and 0.5 µm accuracy. All impressions were made with condensation or addition silicones. The 1- and 2-step putty/light-body impressions were accomplished with putty and light-body materials and the monophase impressions with light-body material only. After the impression procedures, accuracy of each material and technique was assessed measuring the stone casts poured (n=5) from the impressions, by the same microscope. The differences between the values of stone cast and stainless steel cast were calculated, presented as percentages and analyzed statistically by two-way ANOVA and the means compared by Tukey's test (α=0.05). All distances measured on stone casts showed altered dimensions with significantly negative linear changes (shrinkage) as compared to those of the stainless steel cast. The stone casts made from the addition silicones were dimensionally more accurate. No differences were found among the impression techniques.
Factors affecting the accuracy of elastometric impression materials.
OBJECTIVES
The purpose of this study was to evaluate the effects of (1) various impression materials, (2) different storage times and (3) the proportion of inorganic filler on the accuracy and stability of elastometric impression materials.


METHODS
The impression materials studied included three alginate impression materials (Algiace Z, CAVEX and Jeltrate), five commercial silicone impression materials (Aquasil, Exaflex regular type, Express, Coltex fine and Rapid liner) and two experimental silicone impression materials designed for this study (KE106A and KE106B). Impressions were made of 10 metal dies that mimicked prepared crowns. After an impression was taken, dental stone was immediately poured into the alginate impressions, while the silicone impressions was poured 30 min later and waited for 1 h for setting. The second and third stone dies were made 1 and 24 h later, respectively. The diameters of the occlusal surfaces of the metal dies and stone casts were determined using photographs of the surfaces taken with a Kodak DC 290 digital camera. The pictures were then measured using a photomicrograph digitized integration system to calculate any discrepancy. Because each impression was used to make three rounds of stone dies, two-factor mixed factorial ANOVA was used to evaluate the effect of materials and storage time on the accuracy of the stone casts. The simple effects analysis, combined with multiple comparisons considering the per family type I error rate, was performed following confirmation that an interaction between the two factors was significant.


RESULTS
The results showed that: (1) there was a significant interaction effect between materials and storage times on the accuracy of the impressions. (2) Two addition type silicone materials, Aquasil and Exaflex, had the greatest accuracy and stability. (3) The experimental material KE106A had the least accuracy in the first and second rounds and the alginate impression material CAVEX had the least accuracy in the third round. (4) The stabilities of CAVEX and Jeltrate were the least consistent of the 10 materials and decreased significantly with storage time. (5) When the experimental material had a low proportion of filler (KE106A), there was a significantly greater dimensional discrepancy compared to the same material with a higher proportion of filler (KE106B).


CONCLUSIONS
The accuracies varied among the 10 impression materials over three rounds. Of all the materials, the addition type silicone materials, Aquasil and Exaflex, had relatively greater accuracy and stability. The discrepancy of the alginate impression materials increased with storage time. The large loading of filler showed less discrepancy.
The role of dissolved oxygen in supercritical water in the oxidation of ferritic–martensitic steel
Abstract To assess the role of dissolved oxygen in supercritical water on the growth processes of oxide scales, a marker experiment of ferritic–martensitic steel was performed by exposing to supercritical water (H 2 16 O) containing marked 18 O 2 at 600 °C under 25 MPa. The oxide scale was analysed by using Secondary Ion Mass Spectrometry (SIMS). The oxygen isotope profile shows that dissolved oxygen and supercritical water react with metals simultaneously. Dissolved oxygen changes the oxidation/reduction potential of supercritical water leading to an increase in the oxidation rate. The oxidation rate increases with increase in dissolved oxygen content in supercritical water. The mechanism of the effect of dissolved oxygen on oxidation is discussed.