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Dental Materials

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Gypsum Products, Dental Waxes, and Impression Materials

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3-15. AGAR-TYPE HYDROCOLLOID--CHARACTERISTICS

a. General. The agar-type hydrocolloids are thermoplastic, elastic materials. They are called reversible hydrocolloids because they are softened by heating, hardened by cooling, and used repeatedly. In the hardened state, they are flexible and elastic.

b. Composition. The basic component of these hydrocolloids is agar-agar, a product extracted from certain types of seaweed. The exact composition of the material varies with different manufacturers. Most preparations contain about 80 percent water, 15 percent agar-agar, and 5 percent chemicals and inert substances.

c. Inert Substances. Inert substances in the material are fillers that increase its strength and stiffness. Small amounts of borax and potassium sulfate are usually included. These chemicals do not affect agar-type hydrocolloids the same way they affect gypsum products. Borax is used to increase the strength of the hydrocolloid. However, both borax and hydrocolloid retard the setting time of gypsum products and may prevent casts from hardening. Consequently, potassium sulfate is added to the impression material to partly counteract the action of the borax.

d. Instruments and Materials. Figure 3-1 shows typical instruments and materials for agar-type hydrocolloid impressions. Manufacturers furnish agar-type hydrocolloids either in tubes for making impressions or in bulk form for duplicating casts. Both forms must be stored in 100 percent relative humidity in airtight containers in a cool place. A relative humidity of 100 percent indicates that the air contains as much water vapor as it can take up at a given temperature, usually room temperature. Some of the properties of the agar-type hydrocolloid impression materials are discussed below.

David L. Heiserman, Editor

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Revised: October 24, 2010