Dental Silicate Cements. I. The Chemistry of Erosion

¹ Dental Materials Section, Laboratory of the Government Chemist, Ministry of Technology, Cornwall House, London, England

A considerable amount of material is eluted from dental silicate cements in the 24 hours after contact between the cement and solution. The chemical composition of this eluate bears little relationship to that of the parent cement and may be regarded as a solution of the mixed sodium salts of dihydrogen phosphate and complex fluorides. Sodium dihydrogen phosphate is the soluble product of a neutralization reaction between powder and liquid. For one cement, two thirds of the total phosphate going into solution (measured over 3 months) was eluted in this initial period. The leaching pattern of phosphate from cements, in which the amount eluted is initially considerable and eventually drops to trace proportions, suggests that the process is a simple washing out of the soluble salts from the cement gel. The amount of phosphate salt varies considerably between cements; it probably depends on the degree of attack by the liquid on the powder and is greatest where the relative amount of liquid used in cement preparation is high and its phosphate concentration is high. Since the amount of phosphate salt eluted is related to clinical performance, initial conditions must be of considerable importance. The difference between cements becomes less as the age increases.

The leaching pattern of silica is entirely different from that of phosphate; decrease in the amount eluted as the cement ages is much less and suggests that the process of solution is from slow peptization of silica from the cement gel.

The amounts of aluminum, zinc, and calcium eluted are small and elution rapidly decreases to negligible proportions in the cements.

The elution pattern of sodium and fluoride is generally similar to that of phosphate and may also be ascribed to the simple washing out of soluble salts. However, even when the cement has aged 3 months, these ions are still being eluted and may come from unreacted powder rather than the gel matrix. The continuing elution of fluoride no doubt accounts for the anticariogenic properties of these cements. However, the solution of material from 3-month-old cements is small compared with the initial erosion; if failure is from this cause, it would take many years to effect.

This work indicates that, in media that are adequately represented by water, the erosion of cement in the first 2 to 3 weeks after preparation is alone of significance. Dental specification tests of "solubility and disintegration" only give a limited, although important, account of the erosion of dental cements.