The Properties and Manipulation of Mercaptan Base and Silicone Base Impression Materials

Abstract

The presently available rubber impression materials can be classified as mercaptan base and silicone base materials, according to their composition. The sub-classes of each type can be designated as “heavy type,” “regular type” and “syringe type” according to their viscosity after mixing and their general specialized uses. “Thiokol Polysulfide Rubber,” a polyfunctional mercaptan, is the chief constituent of the mercaptan base materials. A polymerization to a synthetic rubber is effected by means of lead peroxide and sulfur as accelerators. Both the base and the accelerator are supplied in paste form containing plasticizers and fillers. One of the products of the reaction is water. The principal ingredient of the silicone rubber impression materials is thought to be polydimethyl siloxane, which is formed into a paste with a carefully compounded filler. The accelerator (an organic tin compound) is usually supplied as a liquid. Hydrogen and other gases may be evolved during polymerization and may cause pitting of the stone model unless eliminated by a hydrogen acceptor or by the use of a more complex dimethyl siloxane. Volatiles of some nature are always evolved during the vulcanization reactions. It has been shown that the working time of various products varies between 3 and 9 minutes, and the setting time varies between 6 and 13 minutes; the silicone base materials generally set faster than the mercaptan base materials. An increase in temperature accelerates the setting time, particularly in the case of the mercaptan base impression materials. The setting time of the silicone base materials can be controlled by the amount of accelerator employed, but the relation is not in direct proportion. The control of the setting time of the mercaptan base materials can be effected by the mixing temperature, or by the addition of a drop of water for acceleration or a small amount of fatty acid for retardation. The better rubber impression materials may possess elastic properties equal or superior to those of the hydrocolloid impression materials. In general, the strength of these materials is comparatively high. All of the rubber impression materials shrink after setting; the present silicone base materials tend to shrink more than the mercaptan base materials. Although the materials cannot be considered to be dimensionally stable, in some instances the change may be within the allowed tolerance of error. In all instances they are superior in this respect to the hydrocolloid impression materials. Rubber impressions can be electroplated successfully with silver. In general, the electroplated die may not be quite as accurate as a stone die constructed within 30 minutes after the removal of the impression from the mouth. Techniques were suggested for the proper procedures based upon the research findings. The importance of a uniform mix of the base and accelerator was emphasized, as well as the necessity for complete retention of the material in the tray during withdrawal from the parts impressed. The rubber impression pastes are virtually universal impression materials for any dental procedure. The silicone materials at present generally set too rapidly and exhibit poor dimensional stability in comparison to the mercaptan base materials. On the other hand, the silicone materials present the advantages over the mercaptan materials of cleanliness in handling, less tendency to stain, less sensitivity to temperature change, and less toxicity when in continuous contact with the tissues. Once these important disadvantages are eliminated, it is predicted that the silicone base impression materials will supplant the mercaptan base materials.