Studies on the Water-Settable Phosphate Cement Part 1

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Studies on the Water-settable Phosphate Cement Part 1. Introduction and Examination of the Setting Mechanism of Currently Available Zinc Phosphate Cement

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 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 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 setting. 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 phosphate salts and unreacted zinc oxide and it is impossible to detect the secondary zinc phosphate 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 composition 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 structure 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 measurement 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 setting 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 powder and liquid in the usual manner. A durometer-A was used to measure changes in the hardness of the cement material in the setting process.

ii. Experimental conditions.

Four mixing ratios were postulated : 40/100, 60/100, 80/100 and 100/100 (cc/g).

The mixing operation was performed at 6 different environmental temperatures : 21•Ž,

23•Ž, 25•Ž, 27•Ž, 30•Ž and 35•Ž. With each of sample prepared, changes in the hardness were measured from the initiation of trituration to final setting.

iii. Preparation of cement samples.

The powder and liquid were separately placed on the glass plate and the powder was added to the liquid in small increments and effort was made to obtain a homoge- nous mix as uniformly as possible in 30 seconds. Immediately after the trituration, the mix was inserted in a polyethylene tubiform cup. One sample was measured by the durometer-A for three times, the mean being used for data. 86

Experimental Results

When the setting of a cement sample is observed, 1) the hardr_.Qsssuddenly in- creases in the neighborhood of 10 durometer reading, 2) the hardness continues and gradually attains to a point where apparent setting has finished, but 3) an increase in the hardness still continues though in slow measures and finally reaches 100. Respec- tive hardnesses at these points are here to be referred to as the primary, secondary and tertiary hardness. Figs. 3 to 8 represent the relationship between the hardness and time passage at a definite temperature. Generally speaking, it requires a longer time in proportion to an increase in the powder-liquid ratio. With a sample prepared in 40 cc/100 g ratio, for instance, the primary hardness takes place in about 3 minutes since the start of trituration. The secondary hardness occurs in about 4 minutes with 90 durometer reading and the tertiary hardness sets in after about 5 minutes. A similar tendency is observable with other mixing ratios as follows.

Mixting ratios Primary Secondary Tertiary 60/100 3.5 min. 4.5 min. 7.0 min. 80/100 4.0 min. 5.3 min. 8.5 min. 100/100 5.7 min. 7.0 min. 10.5 min.

These findings enable us to know that though the primary and tertiary hardnesses are retarded by an increase in the powder-liquid ratio, the secondary hardness seems to be more or less unchanged. When all the secondary hardnesses are plotted on a sheet of paper, they align themselves in a straight line. The durometer reading of this hardness is lowered by an increase in the powder-liquid ratio. When the X-axis of this straight line has a wide angle , the setting time is short and, inversely, when the angle is small it will become correspondingly longer.

Fig. 3. Diagram of hardness vs. time (21•Ž)

Fig. 4. Diagram of hardness vs. time (23•Ž) 87

Fig. 6. Diagram of hardness vs. time (27•Ž)

Fig. 5. Diagram of hardness vs. time (25•Ž)

Fig. 7. Diagram of hardness vs. time (30•Ž)

Fig. 8. Diagram of hardness vs. time (35•Ž)

Discussion

1. Influence of power-to-liquid ratio.

As has been made clear by the previous experiments conducted so far, the setting

of this kind of cement more or less completes itself by the time it reaches secondary

hardness. In order to examine possible influence of the hardness owing to a powderto

liquid ratio, we prepared Fig. 9, where there is observed a definite tendency thatchange

in the hardness becomes nearly straight in proportion to an increase in a powder to

liquid ratio with the single exception at a temperature of 27•Ž.

2. Influence of environmental temperature.

When various powder to liquid ratios were maintained at a fixed level, changes

in the hardness were examined as dependent on changes in the environmental tempera-

ture (Fig. 10, 11. 12, 13). Here again a peculiar deviation out of the usual patterns

was observed at an environmental temperature of 27•Ž. 88

Fig. 9. Relation of hardness to powder-liquid ratio (secondary setting)

Fig. 10. Relation of hardness to Fig. 11. Relation of hardness to powder-liquid ratio (40/100) powder-liquid ratio (60/100)

Fig. 12. Relation of hardness to Fig. 13. Relation of hardness to powder-liquid ratio (80/100) powder-liquid ratio (100/100) 89

On the strength of these two experiments, we feel quite justified to assume that some peculiar boundary line of the setting mechanism is associated with this temperature. However, HIGASHI in his published reports [18, 19, 20, 21] concerning the setting mechanism of alginate impression material has pointed out the fact that the setting of this impression material takes place, regardless of changes in an environmental temperature, at a uniform speed and used the gelation equation profounded by P. P. von Weimarn:

where, V; the velocity of gelation Q; total concentration of colloidal solution S; normal solubility of the substance, and K; constant. With this equation, this interpretation is to the effect that when Q happens to be too large as compared with S, V will be controlled almost only by Q. In this connec- tion, as regards a peculiar boundary line which seems to appear around 26℃ or 27℃ HIGASHI is of the opinion that since the gelation of a colloidal substance is intimately connected with the evaporation of water, this peculiar phenomenon is brought about by an influence of the vapor pressure on temperature curves.

Conclusions

The following can be given as conclusions arrived at as the result of a series of experimental tests relative to the setting mechanism of zinc phosphate cement. 1. It has been made quite clear that, as long as an environmental temperature is maintained at a constant level, the velocity of setting of zinc phosphate cement tends to become slower in proportion to an increase of a phosphoric acid liquid. Therefore, when a powder to liquid ratio is rendered small, we can obtain a correspondingly rapid degree of cement setting. With due consideration of a temperature, however, a clinical 30℃ procedure beyond a temperature of about will be quite undesirable because of too rapid setting of the cement material. 2. When changes take place in an environmental temperature, there will appear curves of irregular nature that correspond to each hardness line. In particular, peculiar phenomena can be observed in the neighborhood of 27℃ when a powder to liquid ratio is either 40/100 or 100/100 and also in the neighborhood of 30℃ when a ratio happens to be 80/100. 3. The above theory has been well accepted as regards alginate impression material which is of a definitely colloidal nature. Therefore, we should be inclined to the view that the previously accepted notion concerning the setting mechanism of zinc phosphate cement will have to be re-examined profitably by the measurement of various physical changes that are undergone by this material in the setting process.

References

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[3] MJYAZU, H. and ASAINE: A Comparative Report of Silicate Cement and Zinc Phosphate Cement, 1934. [4] PAFFENBARGER,G. C.: J. A. D. A., 22, Jan., 1934. [5] HARVEY, W., LEBROCQ, L. F., and RAKOWSKI, L.: B. D. J., 7761-7769, 7789-7799, 1944. [6] SOUDER, W. and PAFFENBARGER, G. C.: Physical Properties of Dental Materials , p. 105 (1942). [7] PAFFENBARGER, G. C. and CANL, H. J.: Dental Cements , pp. 232-237 (1940). [8] HARVEY, W. and PETCK, N. J.: B. D. J., 35-42, 1946. [9] MOLNAR, E. J. and SKINNER, E. W.: J. A. D. A., 29: 744-751, 1942. [10] DRUM, W.: Zahnarztlichen Rundschau, 59, 1950. [11] MIYAZU, H.: Dental Technology, Nagasue Co., Kyoto (1952). [12] SKINNER, E. W. : The Science of Dental Materials, 3rd ed., (1946). [13] Instruction Guide prepared by G-C Dental Mfg. Co., Tokyo (1951). [14] MIYAZU, H.: On the Trituration of Zinc Phosphate Cement, Shofu-Echo, Vol. 6, Kyoto. [15] GROWELL, W. S. : J. A. D. A., 14: 1034, 1927. [16] IIYAMA, T.: Report of Dental Materials Laboratory, Vol. 5, p. 13 (1959). [17] EBERLY, N. E., GROSS, C. V. and CROWELL, W. S.: J. A. D. A., 42: 1434, 1920. [18] MORIMOTO, T. : Researches on Zinc Phosphate Cement, J. Japan Oral Dis., 28: 2, 56- 68, 1961. [19] HIGASHI, S. : The Bulletin of Tokyo Medical and Dent. Univ., 7: 4, 541-548 , 1960. [ 20] HIGASHI, S.: B. T. M. D. U., 7 : 3, 549-555, 1960. [21] HIGASHI, S.: B. T. M. D. U., 7 : 4, 587-595, 1960. [22] HIGASHI, S.: J. Dent. Tech., 1: 1, 1960.