Role of Ingredients in Alginate Impression Compounds

¹ F. H. Wright Dental Manufacturing Co., Ltd., Dundee, Scotland

The effects of some compounding ingredients on the handling characteristics and physical properties of a commercial alginate have been studied and the results indicated.

High proportions of the alginate salt promoted improved resilience as measured by rebound resilience and by permanent deformation tests.

Trisodium phosphate, when added in excess, lengthened the setting time, detracted from the elastic properties, and reduced the hardness of the impression.

Calcium sulphate dehydrate, with or without calcium sulphate hemihydrate, produced a more acceptable impression material than the hemihydrate alone.

Individual fillers exerted a profound effect on the handling characteristics of the powder-water paste and on some of the properties of the gel which were of clinical importance; e.g., setting time, elasticity, hardness, and dimensional stability.

Certain special additives, such as sodium silicofluoride and lead silicate, used individually or together, gave improved gels of high elasticity and good dimensional stability.

The changes which occurred in the pH values of these impression materials were examined over the first 6 minutes from the start of mixing. The decrease in pH was more marked with the color-change alginate than with the other two. At their setting points all three gels were very slightly alkaline, but one of them became slightly acid about 25 seconds after setting. The effect of pH on the properties of the alginate gel was not studied.