Physical properties of elastic duplicating materials.

Agar has been used for a number of years in the preparation of elastic duplicating materials. These duplicating materials have been used mainly in partial denture prosthesis for the preparation of a duplicate refectory cast of the original stone cast of the patient's mouth. Since moderately large quantities of duplicating materials are used in these procedures, the low cost and reversibility of the agar materials are a definite advantage. The mercaptan and silicone rubber elastic impression materials have not been used extensively in duplicating procedures because of their high cost and the fact that these materials, once set, are irreversible. Recently, a plastic duplicating material has been developed which is a reversible polyvinyl chloride plastic gel. Because of its chemical and physical nature, this material has elastic properties somewhat different from the agar compounds. The agar duplicating compounds are composed primarily of agar and water but may also contain borax to increase the strength, salts to accelerate the setting of the investment, and other ingredients. Cellulose fibers, waxes, and fillers, which may be found in impression compounds, generally are omitted from duplicating compounds, since the duplicating materials are often stored in the sol state for extended periods and these ingredients tend to separate under this condition of storage. Since the duplicating compounds may be stored at temperatures of 130°-150° F., their resistance to thermal degradation is an important property. The basic composition of the agar impression compounds is much the same as for the agar duplicating compounds. The principal difference is that the impression compounds may contain 10-15 per cent agar, while the duplicating materials may contain only 5 per cent agar and, as just indicated, need not contain waxes, fillers, and other modifiers common to impression materials. In fact, agar impression materials have been used as duplicating compounds by simply diluting the impression material with an equal amount of water. The properties of the agar impression compounds have been studied by numerous investigators. Skinner and Kern' determined the linear-dimensional changes in air and subsequently in water of various agar impression compounds. They also measured the compressive properties of proportional limit, maximum flexibility, and crushing strength, as well as the plastic flow properties of these agar materials. Paffenbarger2 reported the compressive properties of strength, deformation, set, and stiffness of a number of agar compounds. The temperature of gel formation and the impression-