Aging Characteristics of Elastic Duplicating Compounds

One of the principal differences between the use of agar duplicating compounds compared with agar impression materials is that the duplicating compounds are subjected to repeated heating and gelation cycles and also to extended storage at temperatures from 1250 to 1500 F. The change in physical properties of duplicating compounds with use or aging is, therefore, of particular interest in an evaluation of these materials. The changes on heating and storage of one commercial agar-type duplication compound and an experimental 5 per cent agar material were determined by Margetis and Hansen.' The properties selected for this evaluation were changes in pH, viscosity, and crushing time. The pH and crushing time decreased during heating and storage of the agar compounds for 32 days. The viscosity of the agar sols decreased slightly after repeated heating and cooling cycles. On storage of the agar compounds at elevated temperatures, the viscosity tended either gradually to increase or to increase after an initial decrease, probably as a result of slow partial gelation of the agar. The properties of duplicating and impression compounds, as influenced by storage for 3 days at elevated temperatures and by 12 boiling and cooling cycles were recently reported by Duke and Ryge.2 The tests used in this study were those described in the A.D.A. Specification No. 11 for Hydrocolloid Impression Material-Agar Type.3 The limited control of the water content of the agar duplicating compounds during the repeated heating and gelation cycles appeared to have a pronounced effect on the per cent strain in compression and the per cent permanent set values. Little change was observed in the per cent strain in compression and the per cent permanent set after storage for 3 days at 1550 F. Duplicating compounds, however, are often stored for 30 days or more at elevated temperatures, and the lack of change in physical properties after 3 days' storage represents a limited evaluation of the stability of the duplicating compound. The first purpose of this study was to evaluate the changes in physical properties as a result of aging changes in a number of commercial duplicating compounds after storage at elevated temperatures for periods up to 60 days and after repeated boiling and cooling cycles under carefully controlled conditions to prevent changes in the water content of the duplicating compounds. The second purpose of the study was to examine the effect of secondary components and manipulative variables on the aging changes of agar duplicating compounds. The components normally present in agar compounds and possible impurities present as a