Studies on the water-settable phosphate cement. 2. Further examination of the setting mechanism of currently available zinc phosphate cement.

Abstract

Zinc phosphate cement, which has a variety of uses for the crown, bridge, inlay, lining material, pulp capping, impression model preparation, abutment tooth preparation and temporary cavity filling, is widely accepted as one of the indispensable materials in clinical dentistry. There is available a great deal of research literature and many other studies are being carried out dealing with this very important dental material.According to the literature [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], the setting mechanism of this cement is established to be due to the generation and subsequent hardening of crystals of secondary zinc phosphate salt (ZnHPO4·3H2O) or crystals of tertiary zinc phosphate salt [Zn3(PO4)2·4H2O] through the chemical reaction of zinc oxide and phosphoric acid. However, as this inorganic chemical reaction takes place instantaneously, it interferes with a mixing procedure to obtain uniform product and, by way of eliminating this drawback, the following method of manufacture is currently in force.Zinc phosphate cement which is commercially available today is made from zinc oxide with the addition of divalent and trivalent metal oxides from ten to twenty per cent for the adjustment of setting rate. The mix is burnt at 1000-1400°C to produce clinkers which will be screened for adequate particles after they have been finely crushed. On the other hand, a liquid is manufactured from phosphoric acid of fairly high concentration (60%) and aluminum or zinc powders of about ten per cent which are solved through heating.The usual practice of a dentist is to mix the powder and liquid by the use of glass plate and cement spatula in a small installment at a time, completing each operation in 1 minute or 1 minute and half. Soft cement thus prepared starts to harden gradually by giving off heat and though it reaches apparent setting in about 15 minutes, an increase in its strength is observable till after 3 or 4 days.Although this cement has a feature of attaining to over 400kg/cm2 compressive strength only after 30 minutes, there are known such drawbacks as the possibility of reduction in strength because of free phosphoric acid and free zinc oxide which remain unreacted, irritation to the pulp and excessive heat more than 80°Cat the time of setting. These are the problems yet to be solved.In the hope that further light might be thrown on this subject, the authors have undertaken to examine the setting mechanism of zinc phosphate cement and, consequently, to develop a new cement which would serve better practical purposes.