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.

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.