Studies on the Water-settable Phosphate Cement Part 1. Introduction and Examination of the Setting Mechanism of Currently Available Zinc Phosphate Cement by Setsuo HIGASHI *, Takaharu MORIMOTO *, Akiya SATOMURA **, Kazuo IDA **, Akira KAMIO *, Tamotsu SHIMOJO * and Isao MAKI * 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•Ž to produce clinkers which will be screened for adequate particles after they have been finely cru- shed. 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 opera- tion 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 400 kg/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 ℃ at the time of set- ting. These are the problems yet to be solved. In the hope that further light might be thrown on this subject, the authors have * 東 節 男 ,森 本 孝 治,神 尾 辮,下 条 有,牧 功: Dept. of Dental Technology, Nihon Univ. School of Dentistry ** 里 村 明 弥 ,井 田 一 夫: Dept. of Prosthetic Dentistry, Tokyo Medical and Dental University 82 83 undertaken to examine the setting mechanism of zinc phosphate cement and, consequently, to develop a new cement which would serve better practical purposes. Critique of the Previous Literature As stated above, although it is accepted that zinc phosphate cement is produced through the chemical reaction of phosphoric acid and zinc oxide, we can not achieve the purpose by merely combining one zinc oxide with another phosphoric acid. The- refore, the primary concern in the outset of these studies had been placed on the dis- covery of an adequ ate composition of powders and liquid. SKINNER [12] states that " When a zinc oxide powder is mixed with phosphoric acid, a solid substance is formed very rapidly with a considerable evolution of heat...... The reaction does not go to completion, since all of the powder is not attacked by the liquid. The surface layer of the powder particles is first dissolved by the liquid, and then the above reactions take place. As the crystals of the final product precipitate, the density of the crystalline deposition is greatest at the surface of the particle." MIYAZU [11] and CROWELL[15] respectively investigated the reaction of ZnO-P2O5-H2O and found the insoluble crystals to be ZnHPO4.3H2O and Zn3 (PO4)2 4H2O, stating the fact that although this distributional pattern is uncertain the initial crystal structure of cement is chiefly composed of the former and it gradually changes to the latter in proportion to the length of exposure in the oral cavity. In a similar vein, Fig. 1. Structure of ZnO-P2O5-H2O IIYAMA[16] gave his findings to the effect that, when the hardened zinc phosphate cement is examined by the X-ray diffraction, the majority of crystals are the tertiary zinc phos- phate salts and unreacted zinc oxide and it is impossible to detect the secondary zinc phos- phate salts by the diffraction method, with an assumption that even if the secondary zinc phosphate salt existed the amount would be quite smaller than either. 84 Fig. 2. Structure of ZnO-P205-H20 As a result of his examination of researches by EBERLY, GROSSand CROWELL[17], IIYAMA translated the crystalline structure into a triangle diagram (Fig. 2). His main contention is as follows. When the powder is gradually added to the angle A., a chemical composition expresses itself as a straight line linking A with the tip of ZnO and all the powder will have been dissolved by the time it reaches the angle D. It is only after B is reached when crystals of ZnHPO4.3H2O are formed and their composi- tion will change into the direction of the tip of ZnO by the addition of ZnO. ZnHPO4 3H2O to be represented by P3 will co-exist with the liquid at C' formed by a line con- necting C and P3 and a straight line ST. When D is reached by further addition of ZnO, ZnHPO4.3H2O and Zn3(PO4)2 4H2O will coexist with the liquid represented by T. Thus the chemical composition of the liquid remains unchanged between D and E . On the other hand, when E is reached, ZnHPO4.3H20 will disappear with only Zn3- (PO4)2.4H2O and the liquid at T remaining behind. In proportion to the change from EF to E in the direction of ZnO, the liquid will go along a curve TF'U and the volume of tertiary zinc phosphate salts gradually increase. When a change from G to H along GH, ZnO will no longer react with the liquid and the nearer a change is to H, the larger will become the volume of unreacted ZnO. As is known from these configura- tions, when the liquid reacts by maintaining its chemical equilibrium the principal struc- ture of hardened cement will consist of ZnO and Zn3(PO4)2.4H2O. As regards the opinion of CROWELL, it is our belief that while he examines the reaction of ZnO-P2O5-H2O nearly in the excessive state of P2O5 and gives the chemical product at the time of cement setting to be the secondary zinc phosphate, as a matter of fact the production of crystals and subsequent setting take place at a point far to the left of A in Fig. 1. On the other hand, whereas IIYAMA gives his assumption of setting mechanism based on the structure make-up of CROWELL at 25•Ž, there must be far more com- plicated thermal conditions in the actual situations because of heat evolution on the part of a cement mix. As is made clear from these examinations, there is yet room for further improvement on this very important dental material. 85 Objective of the Present Studies The majority of previous researches connected with zinc phosphate cement are made in the direction of physical effects of its composition, X-ray diffraction of the cement product, elucidation of physical properti.es, etc. On the other hand, the measu- rement of physical changes in the setting process of zinc phosphate cement has been largely neglected. Therefore in order to contribute to this aspect of zinc phosphate cement study the authors have conducted a series of experiments and, as a result, discovered the fact that although changes in environmental temperature will not regularly affect the retardation of setting time a peculiar phenomenon is observable around a temperature of 27•Ž [18]. This phenomenon resembles another phenomenon that occurs in the setting mechanism of alginate impression material investigated by HIGASHI [19, 20, 21, 22]. On the strength of these findings, a conclusion is reached that the setting of this kind of cement begins by solution of zinc phosphate with water at the same time when the former is chemically formed by zinc oxide and phosphoric acid and the sol turns into the gel when super- saturation is attained and, finally, it results in crystals of zinc phosphate after going through the process of crystallization. If this conclusion is to be accepted as a right one, it will be possible to manufacture a water-settable cement which is far advantageous over the previous zinc phosphate cement. The present part of our studies is concerned with the detailed investigation of set- ting behavior of zinc phosphate cement. 1. Materials. Since the chemical reaction of pure ZnO and H3PO4 takes place very rapidly, it is quite inconvenient for practical purposes. Therefore, a usual commercially available zinc phosphate cement contains 10-20% of magnesia, silica or bismuth trioxide as a retard- ing agent of the chemical reaction. For our experimental purposes, use was made of zinc phosphate cement commercially sold by the G-C Chemical Manufacturing Company, Tokyo. 2. Method. i. Instruments and apparatus. Ordinary dental triturating glass plate and spatula were used for mixing the pow- der and liquid in the usual manner.