Journal of Dental Research, Vol 65, 877-884, Copyright © 1986 by International & American Associations for Dental Research Online Journals
Thermal behavior of the gypsum binder in dental casting investments
T. Mori
This study examined the thermal behavior of cast gypsum specimens, with and
without additives, by means of simultaneous differential thermal
analysis-thermogravimetry (DTA-TG) and dilatometry. Specimens were prepared
from wet-calcined hemihydrates (Hydrocal and Densite). The additives
studied were boric acid (H3BO3) and sodium chloride (NaCl), and these were
added to the hemihydrate powders in concentrations of 2 wt% (in the case of
H3BO3) and 0.5 wt% (in the case of NaCl). A large shrinkage was observed in
the range of 300 to 500 degrees C, and this was greatly reduced when either
H3BO3 or NaCl was present. The dehydration of gypsum (calcium sulfate
dihydrate) was not completed until the initial stage of this large
shrinkage was reached, but the phase transition of calcium sulfate
anhydrite (III-CaSO4 to II-CaSO4) was the major cause for the large
shrinkage. This phase transition occurred over a much wider temperature
range than that suggested by the DTA-TG results. Dehydration conditions
similar to those employed in wet calcination of gypsum appeared to be
produced under atmospheric pressure when NaCl was present.
