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Gypsum Products, Dental Waxes, and Impression Materials

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3-25. SYNTHETIC RUBBER BASE IMPRESSION MATERIALS--CHARACTERISTICS

a. General. Synthetic rubber base impression materials are flexible, rubber-like, and sufficiently elastic to return to their original shape after slight distortion. They are used for making impressions of areas containing undercuts, especially for crowns, inlays, and removable and fixed partial dentures. Figure 3-2 shows typical instruments and materials setup for rubber base impression materials.

b. Two Types. There are two types of rubber base impression materials. Type one has a synthetic rubber base of silicone. Type two has a synthetic rubber base of polysulfide. Both types are polymeric compounds. These compounds are composed of molecules of the same elements in the same proportions but differing in size. The compound containing small, simple molecules is called monomer. The compound containing large complex molecules in which the atoms are joined in chains or rings is called polymer. Because of their different molecular structure, the compounds have different physical properties. The chemical process by which the molecules of monomer are combined to form polymer is called polymerization.

c. Two Liquid Bases. Both the silicone and polysulfide bases are liquids. They are mixed with liquid chemical reactors which polymerize them. The manufacturers add inert substances or plasticizers to the bases and sometimes to the chemical reactors to make paste of a consistency that will remain in an impression tray until polymerization has taken place. The silicone type must be stored in a cool place and will deteriorate after about 6 months. The polysulfide type does not require special storage and can be stored indefinitely.

d. Usage. A thin layer of this material, uniform in thickness, is required to obtain the most accurate impression with the material. For this reason, it is used in individually designed (custom) acrylic resin trays. Synthetic rubber base materials are not adhesive; therefore, a tray adhesive is needed to prevent the impression from pulling away from the tray. If the impression material pulls away from the tray, distortion will result when the tray is removed from the patient's mouth.

Figure 3-2. Setup for rubber base impression material.

David L. Heiserman, Editor

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Revised: May 22, 2017