X-ray diffraction study of crystalline phases of calcium sulphate in alginate impression materials.

In the present study the crystalline phases of calcium sulphate present in various commercially available alginate impression materials were identified by means of x-ray diffraction. The diffractograms obtained indicated that the predominant crystalline phase in most materials was calcium sulphate dihydrate and confirmed the presence of calcium sulphate hemihydrate in some products.

Relationship between pH changes and dimensional stability in irreversible hydrocolloid impression material during setting.

This study investigated the relationship between pH changes and dimensional stability during setting of irreversible hydrocolloid impression materials. Twelve specimens of seven irreversible hydrocolloid materials were used; pH readings for 20 minutes and dimensional (mass and linear) measurements for a 24-hour period were taken after mixing. The study of the curves and the comparison between them in range and in variance, together with the application of the t test, showed that pH changes were strongly related to the linear and mass changes derived from the variety and the chemistry of the included components and their amounts.

The effect of pH changes at the impression-stone cast interface during setting.

It is of great clinical significance to obtain a cast that reflects the optimal reproducibility of the irreversible hydrocolloid materials. This study investigated whether pH changes that occur during setting (as a single sequence of the chemical reactions that lead to gelation and the formation of the impression) are related to the reproducibility of irreversible hydrocolloids. The results indicated that as the pH range was increased for materials with alkaline pH values, the reproducibility improved. The opposite effect was noticed in materials with acidic pH values. It was also demonstrated that when the pH curve was in the acidic range, it was closer to the neutral pH values, and the reproducibility of the irreversible hydrocolloid was better.

Retentive ability of various denture adhesive materials: an in vitro study.

This in vitro investigation of the retentive ability of various commercially available denture adhesives was performed by measuring the adhesion and cohesion that developed between a glass surface and an acrylic resin disk surface when some liquids, adhesive materials, or both were placed between them. Student's paired t-test, repeated measures, and two-factor design analysis of variance were performed to determine the significance among mean values. The adhesive materials studied exhibited a significantly greater retentive ability than that of saliva, and their retentive ability increased significantly when they were used in combination with artificial saliva. Highly significant differences of the retentive ability among the different brands of denture adhesives were observed.

The relation between thermal and pH changes in alginate impression materials.

OBJECTIVES: The purpose of this study was to determine if pH changes are related to thermal changes during setting in alginate impression materials, and to investigate the possibility of establishing a mathematical model between thermal and pH changes. METHODS: Thermal and pH changes were recorded during the 20 min gelation period following the mixing of five commercial alginate materials. The data obtained were plotted and statistically analyzed using a student's T-test, multiplicative, linear and reciprocal analyses at a significance level of p<0.05. These analyses were used to determine the differences between various materials and to establish regression models between thermal and pH changes for each of the materials tested. RESULTS: For all samples, the pH values fell within the first minutes from the alkaline range into the neutral and even into the acidic range, while the initial sharp fall in temperature was followed by a subsequent gradual increase. Differences in the final gel structures of the alginate materials were shown to be related to the differences in thermal and pH changes during setting. This could be expected from their various chemical compositions such as the variety of components, the chemical forms of principal components and the proportion of each component. SIGNIFICANCE: This work contributes to the development of a mathematical model relating pH changes and thermal changes to form a basis for the correlation between chemical properties and characteristics in practical use.

Dimensional stability and reproduction of surface detail of tissue conditioning materials.

Four commercial tissue conditioners have been assessed in terms of their dimensional stability and ability to reproduce surface detail according to ADA specification #19 for impression materials. Results have been compared with those of a number of elastomeric impression materials. The tissue conditioners performed adequately and as such could be used as impression materials within 24 hours of their application. While they did not compare well with the elastomeric impression materials they comply with the limitations of the ADA specifications. It is suggested that standards specific to these materials should be developed.

Crown pattern waxes: a study of their behavior on heating and cooling.

The values of the critical physical properties, flow and linear thermal expansion, of crown pattern waxes were determined and compared with those reported by previous investigators. The method is one proposed by the American Dental Association, and measurements were conducted in the temperature range of 27 to 52 degrees C. At temperatures higher than 37 degrees C most of the materials tested exhibited flow values that although rather low seem to be suitable for laboratory use. As expected, the linear thermal expansion was considerable, ranging from 1% to 1.5% or more as the temperature was increased from 27 degrees to 47 degrees C. The arrest points of the cooling curves, which are indicative of the composition of the materials, were recorded and studied. The possibility of correlating the arrest points with the transition points observed on the flow and expansion curves was investigated. Although attempts have been made to improve them, the commonly used dental waxes are not yet entirely free of their known drawback of large and perhaps uncontrollable thermal expansion.

Influence of tissue conditioners upon the surface hardness of dental stone and improved dental stone.

The surface hardness of dental stone and improved dental stone was investigated when crystallised in contact with tissue conditioners and compared with that observed when the two types of gypsum are crystallised in contact with impression materials used clinically. Vickers hardness tests were performed 45 minutes, 2 hours and 24 hours after the beginning of stone mixing. The tissue conditioners did not have adverse effects on the surface hardness of gypsum products. On the contrary, they gave casts with excellent surface hardness, as did the elastomeric impression materials.

Dental stone and improved dental stone surface hardness: the effect of the addition of potassium salts.

Changes in dental stone and improved dental stone surface hardness observed when prepared with aqueous solutions of potassium salts (K2SO4, KCl, KF, KBr, KNO4, KMnO4, K2Cr2O7) in concentrations of 0.5-5.0% were investigated. Parallel studies were carried out on dental stone and improved dental stone prepared with distilled water and crystallised in contact with alginate impression material, previously immersed in the above solutions for 2 minutes. Vicker's hardness tests were performed 3 hours and 24 hours after the initial set. The surface hardness of dental stone and improved dental stone is increased significantly by either method. The most effective hardening solution is potassium sulphate in concentrations of 0.5-5.0%. The degree of increase in surface hardness is directly proportional to the concentration of the solutions and proportionately greater at a shorter time (3 hours after the initial set).