Effect of storage conditions on calcium sulphate hemihydrate-containing products

Dental gypsum products are liable to deterioration on exposure to the air because of reaction between calcium sulphate hemihydrate and water vapour. The purpose of this investigation was to refine the knowledge of the conditions and rates at which this occurred for a representative selection of such products with a view to improved storage recommendations.

Absorption of water from controlled atmospheres (25-40 degrees C, 75-99% RH) was monitored gravimetrically for a dental artificial Stone, Plaster of Paris, an Impression plaster and a gypsum-bonded Investment.

An initial steady absorption stage was followed by a transition to a rapid absorption; this led to a completion stage as the presumed hydration reaction approached completion. The logarithm of time of the transition showed approximately linear dependence on the relative humidity (and not on water vapour pressure as might have been expected) and a slight but significant (P<0.005) negative dependence on temperature in the range 25-35 degrees C for Stone, Plaster of Paris and the Impression plaster, but not for the Investment (P>0.5). The behaviour for the first three materials at 40 degrees C departed significantly from the temperature-dependent trend (P<0.05) while the Investment showed no such effect. The absorption by the Investment was substantially faster than for the other materials.

There is no evidence to suggest that there is a 'safe' humidity for exposure to the atmosphere although for the Stone and Plasters at RH <70% in excess of 1000 h is predicted to be necessary before rapid reaction commences. For the Investment, the equivalent time at 70% RH is 1d.