Aging Characteristics of Elastic Duplicating Compounds

¹ University of Michigan, School of Dentistry, Ann Arbor, Michigan

The changes in physical properties, or aging, of agar duplicating materials when subjected to reboiling cycles and to extended storage at temperatures of 135° and 150° F. were determined.

The compressive strength, tear strength, and per cent set values generally were stable for 15-20 reboiling cycles or for storage at 150° F. of 4-6 weeks.

The crushing-time test was more sensitive to small changes in the viscoelastic properties of the agar gels, and aging changes could be detected with this test after only 1 reboiling cycle.

The aging of agar gels appeared to take place in two stages, an initial rapid change followed by or accompanied by a much more gradual change in viscoelastic properties.

Storage at 135° F. affected the viscoelastic behavior of the agar gels less than any other condition studied.

Aging of agar duplicating materials by being reboiled daily appeared to be comparable with aging by storage at 150° F.

The presence of potassium sulfate or n-methyl or n-propyl p-hydroxybenzoate in concentrations used in agar duplicating materials did not affect the aging characteristics.

The presence of borax in agar duplicating compounds increased the original strength, and the strength remained higher after aging than for agar compounds containing no borax.

Calcium sulfate, as an impurity in agar duplicating compounds, accelerated the degradation of the viscoelastic properties of the gels.

Subdividing agar gels by grinding procedures did not alter the properties unless liquid was lost during the operation.

Ultrasonic treatment of agar sols did not, in general, improve the resistance of the materials to aging as a result of recoiling or storage at elevated temperatures.