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Gypsum Products, Dental Waxes, and Impression Materials

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3-19. ALGINATE-TYPE HYDROCOLLOID--CHARACTERISTICS

a. General. The alginate-type hydrocolloids are an elastic type impression material. An alginate is a salt of alginic acid (an extract from seaweed). Alginate-type hydrocolloids gel by chemical action. Once the gelation process begins, it is irreversible.

b. Composition. The composition of the alginate-type hydrocolloids varies with different manufacturers. The basic components are a soluble alginate (either potassium alginate or sodium alginate) and a reactor (calcium sulfate), which causes the alginate to gel. The product also contains a retarder (sodium or potassium sulfate, oxalate, or carbonate) to prevent gelation from occurring too rapidly. A fluoride is usually added to prevent retardation of the setting time of the casts. The remainder of the material is composed of fillers that increase the strength and stiffness of the gel.

c. Usage. Alginate-type hydrocolloids are supplied in powder form, either in bulk or in measured portions packaged in foil envelopes. The powder must be stored in a cool place. The bulk form must be kept in a tightly closed container to protect it from contamination and to prevent it from absorbing moisture from the air. The containers are agitated to loosen the bulk powders before they are measured, thus preventing use of an excessive proportion of the powder. The powder is mixed with a measured amount of water. Further discussion follows below.

David L. Heiserman, Editor

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Revised: May 22, 2017