Infrared Spectroscopic Studies on the Development of Crystallinity in Dental Zinc Phosphate Cements STEPHEN CRISP, IAN K. O'NEILL, HAVARD J. PROSSER, BRIAN STUART, and ALAN D. WILSON*
Laboratory of the Government Chemist, Department of Industry, Cornwall House, Stamford Street, London SE], England An infrared spectroscopic study has been made of the development of crystallinity (hopeite) in dental zinc phosphate cements. Crystallization in the bulk of a cement is prevented only by the incorporation, in the liquid, of aluminum which forms complexes with phosphoric acid. The development of surface crystallinity is related to the chemical composition of the cement and the speed of the reaction. No acid phosphates are to befound in the matrix which consists solely of neutral orthophosphates. J Dent Res 57(2): 245-254, February 1978.
The dental zinc phosphate cement, which originated in the last quarter of the 19th century, although irritant to tissue and not truly adhesive, has retained its popularity because of excellent working properties and flow characteristics. Until quite recently, the true nature of its microstructure remained unknown. The accepted view was that the matrix consisted of interlacing hopeite, Zn5(PO4)2 * 4H20, and perhaps ZnHPO4 * 3H20 crystallites.1"2 However, traditionally it was known that the growth of crystallites on the surface was a deleterious feature and a clause reflecting this view is incorporated in dental zinc phosphate specifications.3 Recent microstructural studies have resolved this apparent confusion of opinions. These studies show that the zinc phosphate cement sets to form an essentially amorphous matrix, and that subsequently hopeite crystals grow from Received for publication February 16, 1977. Accepted for publicationJuly 13, 1977. *for reprints.
the surface.4'5 The growth of crystallites, favored by moist conditions, reduces retention of the cement to tooth material and is thus undesirable. Infrared spectroscopy is able to distinguish between crystalline and amorphous zinc phosphate and is thus a valuable tool for monitoring the formation of crystallites. The use of infrared spectroscopy to study dental cements was introduced by Wilson and Mesley6 who found the technique of attenuated total reflectance (ATR) particularly useful in monitoring the course of the silicate cement-forming reaction. These workers found that transmission and ATR techniques yielded the same results. However, zinc phosphate cements are unusual in that the surface and bulk reactions are different. Thus, ATR and transmission measurements give different, but complementary, indications of chemical and microstructural changes taking place in this cement. The object of this study was to relate the chemical composition of zinc phosphate cements to the rate of formation of crystallites and to establish parameters which would be of importance in controlling this process. Materials and Methods
MATERIALS. - Zinc oxide was heated for 2 hours in air at 1,100 C. Two mixtures of zinc oxide and magnesium oxide (9:1 and 19:1) were treated similarly. Solutions of orthophosphoric acid of 39, 49, 59, and 65% concentrations were prepared by weighing. (Note: all percent compositions in this paper are expressed as m/m.i) A second series of solutions was pre245
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TABLE 1 COMPOSITION OF CEMENTS STUDIED Liquid Powder*
H,P04%
ZnO ZnO ZnO ZnO
39 49 59 65
VII VIII
ZnO ZnO ZnO ZnO
39 49 59 65
2.5 2.5 2.5 2.5
Ix
ZnO
49
1.25
ZnO/MgO
49 49
Cement
I II III IV V VI
X
Al%
Zn%
Powder/liquid ratio g/ml
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 6.5
2.0
2.5
-
2.0
2.5
-
2.0
19:1 XI
ZnO/MgO 9:1
*Oxides ignited
at
1,100 C for 2 hours.
pared corresponding to the previous solutions but contained in addition 2.5% of aluminum. A further solution was made by the addition of 1.25% aluminum and 6.5% zinc (as zinc oxide) to a 49% aqueous solution of orthophosphoric acid. Composition of the cements studied are given in Table 1. Zn3 (P04)2 * 4H20 was heated to 180 C to obtain Zn3 (PO4)2 * 2H20, and to 350 C to obtain anhydrous zinc orthophosphate. Thermogravimetric analysis showed that dehydration of Zn3 (PO4)2 occurred in two stages at 90 C to 180 C and at 300 C to 350 C, two water molecules being lost at each
various times up to 24 hours. Spectra were recorded on a Model 621 Spectrometer, Perkin-Elmer Corp., Norwalk, Ct. using scale expansion up to 3 X. ATR measurements were made with a Model TR5 unit (Research and Industrial Instruments, England) with a KRS -5 thallium bromideiodide hemicylinder with the angle of incidence set at 55 °. In all cases, spectra were recorded as transmittance against frequency. Interpretation of spectra was based on the work of Chapman and Thirwell7 and Ahlijah and Mooney.8 Thermogravimetric analysis plots were recorded on a Du Pont 900/950 TGA unit.
stage.
METHOD. -All cements were mixed on glass slab with a stellite dental mixing spatula at 22 C and 50% relative humidity. The required weight of powder was subdivided on the block into six equal parts. A part was mixed into the liquid every 15 seconds and the total mixing time was 1 .5 minutes. The setting time of the cement was determined at 37 C using a Gillmore needle .3 Infrared spectroscopy. - Spectra were observed both by transmission, using a KBr disk, and by attenuated total reflectance (ATR) as soon as the cement could be placed in the instrument after mixing and at
Results and Discussion
a
The infrared spectra of metal phosphates can be used to distinguish between crystalline and amorphous phases. For example, the infrared spectra of amorphous aluminum orthophosphate show but a single absorption, attributable to the P-O asymmetric stretching vibration in the tettrahedral phosphate anion, which appears as a strong band at 1,060 to 1,085 cm-' in the ATR spectrum and as a broad band between 1,060 to 1,110 cm-' in the transmission spectrum. By contrast, both the ATR and transmission spectra of crystalline Zn3
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Vol. 5 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
247
Wavelength (Microns) 6
7
8
1400
1200
9
10
12
15
FIG 1. -Infrared (ATR) spectra of cement from ZnO/39% H3PO4.
1800
(PO4)2 * 4H20 (hopeite) show a number of bands in the region 1,105 to 1,000 cm-' (Table 5, Fig 6). The appearance of these bands has been explained by a change in symmetry caused by the distortion of the tetrahedral orthophosphate anion.9 Electron diffraction studies indicate that the hopeite structure contains isolated orthophosphate tetrahedra. Two thirds of the zinc ions are tetrahedrally coordinated to four phosphate ligands, while the remainder are octahedrally coordinated to two phosphate ligands and four water molecules.'0 Formation of the structure probably distorts the phosphate anion tetrahedra with loss of symmetry, thus giving rise to a multiplicity of infrared absorption frequencies. The distortion of the phosphate anion tetrahedra is confirmed by the appearance of a band at ca.940 cm-'. This band arises from P043- breathing vibration, which is infrared inactive in a symmetrical tetrahedral phosphate anion, but is activated by distortion of the tetrahedral ion. CEMENTS I-IV. - The infrared spectra of Cement I (plain ZnO powder mixed with 39% H3PO4) show bands (Figs 1 and 2; Table 2) which correspond closely to those found in the reference spectra of hopeite, Zn3(PO4)2 * 4H20 (Table 5; Fig 6). The infrared spectra show that hopeite is rapidly
10 00
800
\' 3min 600
formed both on the surface and in the bulk of the cement. Since this behavior was found with the Cements II-IV, the orthophosphoric acid concentration does not affect the structure of the cement matrix. The formation of hopeite was too rapid for it to be possible to correlate the rate of formation with acid concentration. From the setting times (Table 6) it appears that Cements I-IV become crystalline almost as soon as they have set. Since hopeite is the thermodynamical stable form of zinc phosphate under these conditions, this result is not surprising. Comparison of the spectra of these cements with those of NaH2PO4, Na2HPO4, and zinc acid orthophosphates failed to find evidence for the formation of acid phosphates in these cements. The cement spectra do show a shoulder at about 1,140 cm-' indicating that unreacted phosphoric acid may be present in the cement. CEMENTS V-VIII. - Cements V-VIII formed from zinc oxide powder and orthophosphoric acid (39 to 65% H3PO4) containing aluminum (2.5 % Al) exhibit a profoundly different behavior from that of cements I-IV. The spectra for Cement VI, prepared from 49 % H3PO4 (Table 3; Fig 3), show that although hopeite is formed on the surface within a few minutes of mixing, crystallization does not take place
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Wavelngth (Micrs) 9
8
7
6
12
10
15
24h
FIG 2. - Infrared (transmission) spectra of cement from ZnO/39%
H,PO4. 5min
1800
1600
1200
1400
1000
800
600
Frequency (CM 1) in the bulk of the cement. Even after 24 hours the transmission spectrum (KBr disk) shows that the bulk material consists of amorphous zinc orthophosphate, as indicated by the broad featureless bands at 1,090 to 1,030 cm-' and at 960 cm-'. Cement V shows similar behavior. Cement VII prepared from a more concentrated solution of orthophosphoric acid (59% H3PO4) behaves somewhat differently. The ATR spectra (Fig 4) show that the formation of hopeite is delayed and bands attributable to the crystalline zinc orthophosphate do not become clearly defined until one hour after mixing. The bulk of the cement remains amorphous.
Ultimately, the spectra resemble those of Cements V and VI. Cement VIII prepared from 65% H3PO4 also exhibits delayed development of hopeite at the surface, amorphous zinc orthophosphate being formed initially. The ATR spectra showed that the crystalline phase developed after one hour had elapsed. Comparison of the ATR spectra for Cement I (Fig 1) and those for Cements V and VI (Figs 3 and 4) show that the relative intensities of the bands at 1,020 and 1,000 cm-1 change, the band at 1,000 cm-' becoming more intense in the aluminum-
TABLE 2 IR SPECTRA FOR CEMENT I ATR SPECTRA
TRANSMISSION SPECTRA
3 minutes
1 hour
24 hours
5 minutes
24 hours
1,620 m,b 1, 140 sh 1, 1 00 s,b 1,055 w 1,020sh 995 sh 940 sh
1,630 m,b 1,145 sh 1,100 s,b 1,065 w 1,020 m 995 sh 940 m 790 sh
1,640 m,b 1,120 sh 1,100 s,b 1,065 w 1,020 m 1,000 sh 940s 795 m
1,630 m,b 1,120 to 1,105 s,b 1,070 m 1,020 m 1,005 m 940 s
1,630 m,b 1,120 s,b 1,105 1,070 m 1,020 m 1,005 m 940 s
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CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
249
Wavelength (Microns) 6
7
8
9
10
12
15
--- KBr disc (24h)
FIG 3. - Infrared (ATR and transmission) spectra of cement from ZnO/49% H3PO4 + 2.5% Al.
1800
1600
1400
1200
1000
800
Frequency (cM-1) containing cements (Cements V and VI). This increase in intensity is not attributed to aluminum orthophosphate which has a broad band at 1,060 to 1,085 cm-', but may arise from observation of amorphous zinc orthophosphate in the bulk phase, by the ATR technique. Another difference between the ATR spectra of Cements I-IV and V-VIII is that the broad band at 940 cm-' observed in Cements I-IV becomes resolved into two bands (at about 930 cm-1 and about 940 cm-') in Cements V-VIII. The absorption band at 930 cm-' tends to increase with time. The only significant trends are that, as the phosphoric acid concentrations are
increased in the liquid, the setting time of the cements increases while the absorption band at 930 cm-' intensifies and that at 940 cm-' is attenuated. From a comparison of the cement spectra with the reference spectra of Zn3(PO4)2 * 4H20 and Zn3(PO4)2 * 2H20 (Table 5; and Fig 6), it appears that the changes in the 925 to 950 cm-' region may depend on the number of water molecules which are associated with the orthophosphate anions, i.e. the band at about 940 cm-' is associated with two water molecules, while that at about 930 cm-1 is associated with four water molecules. There is no evidence of the formation of anhydrous zinc orthophosphate.
TABLE 3 IR SPECTRA FOR CEMENT VI ATR 3 minutes
20 minutes
1,640 m,b 1,135 sh 1,100 s 1,070 s,b 1,065 s 1,030 s 1,030 m,b 1,010 s 935 to 955, w,b 950 s 935 sh
1,640 m ,b 1, 140 sh
24 hours
1,640 m,b 1,130 sh 1,095 1,060 s 1,020 sh 1,000 s 940 to 930 s,b s
TRANSMISSION 24 hours
1,640 m,b 1,090 to 1,030 s,b
960w
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250
CRISP ET AL
j Dent
Res
February
1978
TABLE 4 IR SPECTRA FOR CEMENT X ATR 5 minutes
1 hour
TRANSMISSION 24 hours
24 hours
1,630 to 1,610 m,b 1,620 m,b 1, 1 00 sh 1,040 s,b 1,060 m 1,020 sh 1,000 s,b 1,000 s 945 to 935 s,b
The setting times of Cements V-VII (Table 6) correlate with the appearance of crystallinity at the surface. The inclusion of aluminum in these cements slows down both the rate of setting and the rate of development of hopeite on the surface of cement, but the most striking effect is that aluminum prevents the formation of crystalline material in the bulk cement. The action of aluminum may be related to the ability of aluminum to form complexes with orthophosphoric acid. Evidence for the formation of these complexes comes from infrared spectroscopy6 and NMR and Raman spectroscopy." These complexes may be binuclear and it has been suggested that the dental silicate cement sets by the formation of polynuclear complexes of this type. 12 Thus, in the presence of aluminum, the zinc phosphate cementing reaction is
1,630 m,b 1,095 m 1,065 m 1,020 s 1,000 s
1,620 m,b
940 s
955 sh
1075 to 1020 s,b
between zinc and a complex aluminophosphoric acid, rather than between zinc and orthophosphoric acid. Also, the formation of amorphous aluminum orthophosphate by the reaction of aluminum with phosphoric acid may introduce a degree of disorder into the zinc phosphate cement structure, thus inhibiting crystallization. The retardation of the setting reaction with increasing phosphoric acid concentration of the liquid has been observed previously'3 and may be caused by a deficiency of water which is required both as a reaction solvent and to hydrate reaction products. The rate of development of crystallinity may be dependent on the rate of hydration of the zinc orthophosphate. CEMENT IX. - Cement IX resembles Cement VI except that the liquid contains
Wavelength (Microns) 6
FIG 4.
-
7
8
9
10
12
15
Infrared (ATR and
transmission) spectra of cement from ZnO/59% H3PO4 + 2.5% Al. KBr disc (24h)
1800
1600
1400
1200
1000
800
Frequency (CM-1) Downloaded from jdr.sagepub.com at SIMON FRASER LIBRARY on June 4, 2015 For personal use only. No other uses without permission.
600
Vol. 5 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
251
TABLE 5 REFERENCE SPECTRA AIPO4
Zn3(PO4)24H20
Zn(PO4)22H20
Zn4(PO4)2
ATR
Transmission
ATR
Transmission
Transmission
Transmission
1,640 w,b
1,630 w,b
1,640 w,b
1,640 m,b
1,640 sh
1,630 (w)
1,600 m,b
1,270 sh
1,290 sh
1,085to 1,060 s,b
1,llOto
1,200 sh 1,165 s,b
1,100
1,105 s,b 1,100 b 1,065 m 1,070 m 1,080 s,b 1,020 s 1,020 to 1,020 to 1,000 s 1,005 s,b 1,005 s,b 945 to 930 s,b 950 to 935 s,b 950 s,b
1,060 s,b
s
1,050 s,b 1,005 sh 955 s,b
870 w HSPO4 ATR
1,630 m,b 1,160 to 1,130 m,b 980 sh 880w
1.25%o Al and 6.5% Zn. The spectra of the cement show that the formation of crystalline zinc orthophosphate at the cement sur-
zinc does not form complexes with phosphoric acid, nor will it introduce disorder into the zinc orthophosphate cement struc-
face occurs within 15 minutes and is more rapid than in the case of Cement VI. The spectrum of the bulk material at 24 hours shows no evidence of crystallinity. In contrast to the effect of aluminum, the inclusion of zinc in the acid appears to have little effect on either the setting time (Table 6) of the cement, or the development of surface crystallinity. Unlike aluminum, TABLE 6
ture.
SETTING TIME AND DEVELOPMENT OF CRYSTALLINITY Cement
I-IV V
VI VII VIII Ix x XI
Setting Time (minutes) Development of 22 C 50% RH Crystallinity at surface
less than 2 2.0 2.8 3.5 6.0 2.4 5.9 7.5
few minutes* few minutes a few minutes ca 1 hour ca I hour a few minutes ca I hour ca 3 hour a a
*Crystallinity also developed in the bulk of the cement.
CEMENTS X-XI. Cement X resembles Cement VI except that the zinc oxide powder used was ignited with magnesium oxide in the ratio 19:1. The spectra of Cement X, presented in Table 4 and Figure 5, show that Cement X differs in behavior from Cement VI in that the time taken for crystallites to form at the surface is prolonged from a few minutes to one hour. The spectrum of the bulk material after 24 hours shows no evidence of crystallinity. Inclusion of the magnesium oxide also increased the setting time of the cement (Table 6). Cement XI resembles Cement X except that the powder contains an increased proportion of magnesium oxide (ratio 9:1). This difference in composition has the effect of further delaying the formation of hopeite at the surface and increasing the setting time of the cement (Table 6). Again, the bulk material remains as amorphous zinc orthophosphate. The ATR spectra of Cements X and XI -
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252
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CRISP ET AL
Vavelength (Microns) 6
8
7
9
10
12
15
5min -lh - 24h
FIG 5. - Infrared (ATR and transmission) spectra of cement from ZnO -MgO (19:1)/49% H3PO4 + 2.5% Al.
-
1800
1600
1400
1200
1000
800
600
Frequency (CM -1) exhibit a strong band at 1000 cm-'; this may be due to the presence of some magnesium orthophosphate (which has a broad band at 1,040 to 1,080 cm-' with another band at 1,000 cm-') or to amorphous zinc orthophosphate. Clearly, the incorporation of magnesium oxide in the zinc oxide powder retards the setting reaction and crystallization on the cement surface. Zinc oxide powders used in dental zinc phosphate cements have to be
deactivated by heating, which increases the particle size and density of the powder.' '4
Magnesium oxide may act as a mineralizer, magnesium oxide-zinc solid solution be formed, causing an increased setting time and slower development of surface crystallinity in the zinc phosphate ce-
or a may
ment.
The absence of acid-phosphates in the setting cements is of interest and contrary to the accepted view that they are inter-
Wavelength (Microns) 6
7
8
9
10
12
15
BC-
FIG 6.
-
Infrared (transmission)
reference spectra: (A) Zn3(PO4)
-
4H20 (B) Zn3(PO4)2, (C) Zn3 (PO4)2 2H20. A Zn3 (P04)2 4H20 B Zn3 (P4)2 2H20 c Zn (P04) 2
1800
1600
Frequency (CM-1)
1400
1200
1000
800
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Vol. 5 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
mediate in the setting reaction. " 15 Certainly, a study of the ZnO - P205 - H20 phase diagram'6'17 would indicate that as ZnO is progressively added to a concentrated phosphoric acid solution, the first substance to be deposited from solution is one of the crystalline zinc hydrogen phosphate hydrates. The composition of the acid hydrate formed is either ZnHPO4 H20 or ZnHPO4 * 3H20, depending on whether the acid concentration of the liquid is greater or less than 52% H3PO4. In the presence of excess ZnO, these acid hydrates are converted to hopeite, Zn3(PO4)2 * 4H20. However, phase diagram considerations can only be strictly applied to systems in equilibrium and, clearly, these conditions are not obtained in rapidly setting cements. Thus, in the cement reaction between plain ZnO and phosphoric acid solutions, an amorphous zinc phosphate is first formed, which after an induction period, crystallizes to hopeite, the thermodynamically stable form, throughout the matrix. Conclusions
Infrared spectroscopy has proved a valuable tool for monitoring the development of crystallinity in the bulk and at the surface of zinc phosphate cements. The spectra obtained show that no acid phosphates are formed at any stage of the cementforming reaction. The matrix consists only of neutral orthophosphates, principally those of zinc which are found both in crystalline and amorphous forms. Development of crystallinity within the bulk of the cement is prevented by the incorporation of aluminum in the cement-forming liquids. Thus, the presence of aluminum in the liquid exerts a decisive influence on the nature of the cement-matrix, an effect to be attributed to the formation of complex aluminophosphoric acids. It is these species, rather than simple phosphoric acid, which react with zinc phosphate. The presence of aluminum in the matrix introduces a disorder in the structure which inhibits
crystallization. Development of surface crystallinity is retarded by the addition of magnesium oxide to the zinc oxide powder, increasing the phosphoric acid concentration of the liq-
253
uid and incorporation of aluminum into the acid. Retardation of the rate of set appears to be a key factor in delaying the onset of surface crystallinity and in preventing crystallinity developing in the bulk cement. Crown Copyright reproduced by permission of the Controller of Her Britannic Majesty's Stationery Office.
References
1. CROWELL, W.S.: Physical Chemistry of Dental Cements, JADA 14:1030-1048, 1927. 2. DOBROWSKY, A.: Uber die Herstellung eines neuen Zinc Phosphatzementes hochster Volum konstanz, Chemische Technische, 15:159-161, 1942. 3. ISO Recommendation R 1566, Dental Zinc Phosphate Cement. 4. SERVAIS, G.E., and CARTZ, L.: Structure of Zinc Phosphate Dental Cement, J Dent Res 50:613-620, 1971. 5. CARTZ, L.; SERVAIS, G.E., and ROSSI, F.: Surface Structure of Zinc Phosphate Dental Cements,J Dent Res 51: 1668-1671, 1972. 6. WILSON, A.D., and MESLEY, R.J.: Dental Silicate Cements: VI. Infrared Studies, J Dent Res 47:644-652, 1968. 7. CHAPMAN, A.C., and THIRLWELL, L.E.: Spectra of Phosphorus Compounds: I. The Infra-Red Spectra of Orthophosphates, Spectrochim A cta 20:937-947, 1964. 8. AHLIJAH, G.E.B.Y., and MOONEY, E.F.: The Attenuated Total Reflection Spectra of Polyatomic Inorganic Anions. I. Salts of the Oxyacids of Phosphorus. Spectrochim Acta 22:547-553, 1966. 9. HAZEL, A., and Ross, S.D.: Forbidden Transitions in the Infrared Spectra of Tetrahedral Anions. III. Spectra-Structure Correlations in Perchlorates, Sulphates and Phosphates of the Formula MXO4, Spectrochim A cta 22:1949-1961, 1966. 10. LIEBAU, F.: Crystal Structure of Hopeite Zn3(PO4)2 * 4H1O, Acta Cryst 18:352-354, 1965. 11. AKITT, J.W.; GREENWOOD, N.N.; and LESTER, G.D.: Nuclear Magnetic Resonance and Raman Studies of the Aluminum Complexes Formed in Aqueous Solutions of Aluminum Salts Containing Phosphoric Acid an-d Fluoride Ions,J Chem Soc (A):2450 2457, 1971. 12. WILSON, A.D.; KENT, B.E.; CLINTON, D.
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and MILLER, R.P.: The Formation and Microstructure of Dental Silicate Cements, J Mater Sci 7:220-238, 1972. 13. WORNER, H.K., and DOCKING, A.R.: Dental Materials in the Tropics, Aust DentJ 3:215-229, 1958. 14. ZHURAVLEV, V.F.; VOLFSON, S.L.; and SHEVELEVA, B.I.: The Processes that Take Place in the Roasting of Zinc Phosphate Dental Cement, J Appl Chem, U S S R. 23:121-128, 1950 (translated from the Russian Zhur Prikiad Khim Leningr 23:118126, 1950).
F.A., and CRAIG, R.G.: Restorative Dental Materials, 4th Ed., St. Louis: C.V. Mosby Co., 1971, Chap. 13.
15. PEYTON,
16. EBERLY, N.E.; GROSS, C.V. and CROWELL,
W.S.: The System Zinc Oxide, Phosphorus Pentoxide and Water at 25°C and 370C, J Am Chem Soc42:1433-1439, 1920. 17. SALMON, J.E., and TERREY, H.: The System Zinc Oxide - Phosphoric Oxide Water at Temperatures between 250 and 100°, J Chem Soc, 1950, Part III, pp 28 13-2824.
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Laboratory of the Government Chemist, Department of Industry, Cornwall House, Stamford Street, London SE], England An infrared spectroscopic study has been made of the development of crystallinity (hopeite) in dental zinc phosphate cements. Crystallization in the bulk of a cement is prevented only by the incorporation, in the liquid, of aluminum which forms complexes with phosphoric acid. The development of surface crystallinity is related to the chemical composition of the cement and the speed of the reaction. No acid phosphates are to befound in the matrix which consists solely of neutral orthophosphates. J Dent Res 57(2): 245-254, February 1978.
The dental zinc phosphate cement, which originated in the last quarter of the 19th century, although irritant to tissue and not truly adhesive, has retained its popularity because of excellent working properties and flow characteristics. Until quite recently, the true nature of its microstructure remained unknown. The accepted view was that the matrix consisted of interlacing hopeite, Zn5(PO4)2 * 4H20, and perhaps ZnHPO4 * 3H20 crystallites.1"2 However, traditionally it was known that the growth of crystallites on the surface was a deleterious feature and a clause reflecting this view is incorporated in dental zinc phosphate specifications.3 Recent microstructural studies have resolved this apparent confusion of opinions. These studies show that the zinc phosphate cement sets to form an essentially amorphous matrix, and that subsequently hopeite crystals grow from Received for publication February 16, 1977. Accepted for publicationJuly 13, 1977. *for reprints.
the surface.4'5 The growth of crystallites, favored by moist conditions, reduces retention of the cement to tooth material and is thus undesirable. Infrared spectroscopy is able to distinguish between crystalline and amorphous zinc phosphate and is thus a valuable tool for monitoring the formation of crystallites. The use of infrared spectroscopy to study dental cements was introduced by Wilson and Mesley6 who found the technique of attenuated total reflectance (ATR) particularly useful in monitoring the course of the silicate cement-forming reaction. These workers found that transmission and ATR techniques yielded the same results. However, zinc phosphate cements are unusual in that the surface and bulk reactions are different. Thus, ATR and transmission measurements give different, but complementary, indications of chemical and microstructural changes taking place in this cement. The object of this study was to relate the chemical composition of zinc phosphate cements to the rate of formation of crystallites and to establish parameters which would be of importance in controlling this process. Materials and Methods
MATERIALS. - Zinc oxide was heated for 2 hours in air at 1,100 C. Two mixtures of zinc oxide and magnesium oxide (9:1 and 19:1) were treated similarly. Solutions of orthophosphoric acid of 39, 49, 59, and 65% concentrations were prepared by weighing. (Note: all percent compositions in this paper are expressed as m/m.i) A second series of solutions was pre245
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TABLE 1 COMPOSITION OF CEMENTS STUDIED Liquid Powder*
H,P04%
ZnO ZnO ZnO ZnO
39 49 59 65
VII VIII
ZnO ZnO ZnO ZnO
39 49 59 65
2.5 2.5 2.5 2.5
Ix
ZnO
49
1.25
ZnO/MgO
49 49
Cement
I II III IV V VI
X
Al%
Zn%
Powder/liquid ratio g/ml
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 6.5
2.0
2.5
-
2.0
2.5
-
2.0
19:1 XI
ZnO/MgO 9:1
*Oxides ignited
at
1,100 C for 2 hours.
pared corresponding to the previous solutions but contained in addition 2.5% of aluminum. A further solution was made by the addition of 1.25% aluminum and 6.5% zinc (as zinc oxide) to a 49% aqueous solution of orthophosphoric acid. Composition of the cements studied are given in Table 1. Zn3 (P04)2 * 4H20 was heated to 180 C to obtain Zn3 (PO4)2 * 2H20, and to 350 C to obtain anhydrous zinc orthophosphate. Thermogravimetric analysis showed that dehydration of Zn3 (PO4)2 occurred in two stages at 90 C to 180 C and at 300 C to 350 C, two water molecules being lost at each
various times up to 24 hours. Spectra were recorded on a Model 621 Spectrometer, Perkin-Elmer Corp., Norwalk, Ct. using scale expansion up to 3 X. ATR measurements were made with a Model TR5 unit (Research and Industrial Instruments, England) with a KRS -5 thallium bromideiodide hemicylinder with the angle of incidence set at 55 °. In all cases, spectra were recorded as transmittance against frequency. Interpretation of spectra was based on the work of Chapman and Thirwell7 and Ahlijah and Mooney.8 Thermogravimetric analysis plots were recorded on a Du Pont 900/950 TGA unit.
stage.
METHOD. -All cements were mixed on glass slab with a stellite dental mixing spatula at 22 C and 50% relative humidity. The required weight of powder was subdivided on the block into six equal parts. A part was mixed into the liquid every 15 seconds and the total mixing time was 1 .5 minutes. The setting time of the cement was determined at 37 C using a Gillmore needle .3 Infrared spectroscopy. - Spectra were observed both by transmission, using a KBr disk, and by attenuated total reflectance (ATR) as soon as the cement could be placed in the instrument after mixing and at
Results and Discussion
a
The infrared spectra of metal phosphates can be used to distinguish between crystalline and amorphous phases. For example, the infrared spectra of amorphous aluminum orthophosphate show but a single absorption, attributable to the P-O asymmetric stretching vibration in the tettrahedral phosphate anion, which appears as a strong band at 1,060 to 1,085 cm-' in the ATR spectrum and as a broad band between 1,060 to 1,110 cm-' in the transmission spectrum. By contrast, both the ATR and transmission spectra of crystalline Zn3
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Vol. 5 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
247
Wavelength (Microns) 6
7
8
1400
1200
9
10
12
15
FIG 1. -Infrared (ATR) spectra of cement from ZnO/39% H3PO4.
1800
(PO4)2 * 4H20 (hopeite) show a number of bands in the region 1,105 to 1,000 cm-' (Table 5, Fig 6). The appearance of these bands has been explained by a change in symmetry caused by the distortion of the tetrahedral orthophosphate anion.9 Electron diffraction studies indicate that the hopeite structure contains isolated orthophosphate tetrahedra. Two thirds of the zinc ions are tetrahedrally coordinated to four phosphate ligands, while the remainder are octahedrally coordinated to two phosphate ligands and four water molecules.'0 Formation of the structure probably distorts the phosphate anion tetrahedra with loss of symmetry, thus giving rise to a multiplicity of infrared absorption frequencies. The distortion of the phosphate anion tetrahedra is confirmed by the appearance of a band at ca.940 cm-'. This band arises from P043- breathing vibration, which is infrared inactive in a symmetrical tetrahedral phosphate anion, but is activated by distortion of the tetrahedral ion. CEMENTS I-IV. - The infrared spectra of Cement I (plain ZnO powder mixed with 39% H3PO4) show bands (Figs 1 and 2; Table 2) which correspond closely to those found in the reference spectra of hopeite, Zn3(PO4)2 * 4H20 (Table 5; Fig 6). The infrared spectra show that hopeite is rapidly
10 00
800
\' 3min 600
formed both on the surface and in the bulk of the cement. Since this behavior was found with the Cements II-IV, the orthophosphoric acid concentration does not affect the structure of the cement matrix. The formation of hopeite was too rapid for it to be possible to correlate the rate of formation with acid concentration. From the setting times (Table 6) it appears that Cements I-IV become crystalline almost as soon as they have set. Since hopeite is the thermodynamical stable form of zinc phosphate under these conditions, this result is not surprising. Comparison of the spectra of these cements with those of NaH2PO4, Na2HPO4, and zinc acid orthophosphates failed to find evidence for the formation of acid phosphates in these cements. The cement spectra do show a shoulder at about 1,140 cm-' indicating that unreacted phosphoric acid may be present in the cement. CEMENTS V-VIII. - Cements V-VIII formed from zinc oxide powder and orthophosphoric acid (39 to 65% H3PO4) containing aluminum (2.5 % Al) exhibit a profoundly different behavior from that of cements I-IV. The spectra for Cement VI, prepared from 49 % H3PO4 (Table 3; Fig 3), show that although hopeite is formed on the surface within a few minutes of mixing, crystallization does not take place
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248
J Dent Res February 1978
CRISP ET A L
Wavelngth (Micrs) 9
8
7
6
12
10
15
24h
FIG 2. - Infrared (transmission) spectra of cement from ZnO/39%
H,PO4. 5min
1800
1600
1200
1400
1000
800
600
Frequency (CM 1) in the bulk of the cement. Even after 24 hours the transmission spectrum (KBr disk) shows that the bulk material consists of amorphous zinc orthophosphate, as indicated by the broad featureless bands at 1,090 to 1,030 cm-' and at 960 cm-'. Cement V shows similar behavior. Cement VII prepared from a more concentrated solution of orthophosphoric acid (59% H3PO4) behaves somewhat differently. The ATR spectra (Fig 4) show that the formation of hopeite is delayed and bands attributable to the crystalline zinc orthophosphate do not become clearly defined until one hour after mixing. The bulk of the cement remains amorphous.
Ultimately, the spectra resemble those of Cements V and VI. Cement VIII prepared from 65% H3PO4 also exhibits delayed development of hopeite at the surface, amorphous zinc orthophosphate being formed initially. The ATR spectra showed that the crystalline phase developed after one hour had elapsed. Comparison of the ATR spectra for Cement I (Fig 1) and those for Cements V and VI (Figs 3 and 4) show that the relative intensities of the bands at 1,020 and 1,000 cm-1 change, the band at 1,000 cm-' becoming more intense in the aluminum-
TABLE 2 IR SPECTRA FOR CEMENT I ATR SPECTRA
TRANSMISSION SPECTRA
3 minutes
1 hour
24 hours
5 minutes
24 hours
1,620 m,b 1, 140 sh 1, 1 00 s,b 1,055 w 1,020sh 995 sh 940 sh
1,630 m,b 1,145 sh 1,100 s,b 1,065 w 1,020 m 995 sh 940 m 790 sh
1,640 m,b 1,120 sh 1,100 s,b 1,065 w 1,020 m 1,000 sh 940s 795 m
1,630 m,b 1,120 to 1,105 s,b 1,070 m 1,020 m 1,005 m 940 s
1,630 m,b 1,120 s,b 1,105 1,070 m 1,020 m 1,005 m 940 s
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Vol. S 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
249
Wavelength (Microns) 6
7
8
9
10
12
15
--- KBr disc (24h)
FIG 3. - Infrared (ATR and transmission) spectra of cement from ZnO/49% H3PO4 + 2.5% Al.
1800
1600
1400
1200
1000
800
Frequency (cM-1) containing cements (Cements V and VI). This increase in intensity is not attributed to aluminum orthophosphate which has a broad band at 1,060 to 1,085 cm-', but may arise from observation of amorphous zinc orthophosphate in the bulk phase, by the ATR technique. Another difference between the ATR spectra of Cements I-IV and V-VIII is that the broad band at 940 cm-' observed in Cements I-IV becomes resolved into two bands (at about 930 cm-1 and about 940 cm-') in Cements V-VIII. The absorption band at 930 cm-' tends to increase with time. The only significant trends are that, as the phosphoric acid concentrations are
increased in the liquid, the setting time of the cements increases while the absorption band at 930 cm-' intensifies and that at 940 cm-' is attenuated. From a comparison of the cement spectra with the reference spectra of Zn3(PO4)2 * 4H20 and Zn3(PO4)2 * 2H20 (Table 5; and Fig 6), it appears that the changes in the 925 to 950 cm-' region may depend on the number of water molecules which are associated with the orthophosphate anions, i.e. the band at about 940 cm-' is associated with two water molecules, while that at about 930 cm-1 is associated with four water molecules. There is no evidence of the formation of anhydrous zinc orthophosphate.
TABLE 3 IR SPECTRA FOR CEMENT VI ATR 3 minutes
20 minutes
1,640 m,b 1,135 sh 1,100 s 1,070 s,b 1,065 s 1,030 s 1,030 m,b 1,010 s 935 to 955, w,b 950 s 935 sh
1,640 m ,b 1, 140 sh
24 hours
1,640 m,b 1,130 sh 1,095 1,060 s 1,020 sh 1,000 s 940 to 930 s,b s
TRANSMISSION 24 hours
1,640 m,b 1,090 to 1,030 s,b
960w
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250
CRISP ET AL
j Dent
Res
February
1978
TABLE 4 IR SPECTRA FOR CEMENT X ATR 5 minutes
1 hour
TRANSMISSION 24 hours
24 hours
1,630 to 1,610 m,b 1,620 m,b 1, 1 00 sh 1,040 s,b 1,060 m 1,020 sh 1,000 s,b 1,000 s 945 to 935 s,b
The setting times of Cements V-VII (Table 6) correlate with the appearance of crystallinity at the surface. The inclusion of aluminum in these cements slows down both the rate of setting and the rate of development of hopeite on the surface of cement, but the most striking effect is that aluminum prevents the formation of crystalline material in the bulk cement. The action of aluminum may be related to the ability of aluminum to form complexes with orthophosphoric acid. Evidence for the formation of these complexes comes from infrared spectroscopy6 and NMR and Raman spectroscopy." These complexes may be binuclear and it has been suggested that the dental silicate cement sets by the formation of polynuclear complexes of this type. 12 Thus, in the presence of aluminum, the zinc phosphate cementing reaction is
1,630 m,b 1,095 m 1,065 m 1,020 s 1,000 s
1,620 m,b
940 s
955 sh
1075 to 1020 s,b
between zinc and a complex aluminophosphoric acid, rather than between zinc and orthophosphoric acid. Also, the formation of amorphous aluminum orthophosphate by the reaction of aluminum with phosphoric acid may introduce a degree of disorder into the zinc phosphate cement structure, thus inhibiting crystallization. The retardation of the setting reaction with increasing phosphoric acid concentration of the liquid has been observed previously'3 and may be caused by a deficiency of water which is required both as a reaction solvent and to hydrate reaction products. The rate of development of crystallinity may be dependent on the rate of hydration of the zinc orthophosphate. CEMENT IX. - Cement IX resembles Cement VI except that the liquid contains
Wavelength (Microns) 6
FIG 4.
-
7
8
9
10
12
15
Infrared (ATR and
transmission) spectra of cement from ZnO/59% H3PO4 + 2.5% Al. KBr disc (24h)
1800
1600
1400
1200
1000
800
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600
Vol. 5 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
251
TABLE 5 REFERENCE SPECTRA AIPO4
Zn3(PO4)24H20
Zn(PO4)22H20
Zn4(PO4)2
ATR
Transmission
ATR
Transmission
Transmission
Transmission
1,640 w,b
1,630 w,b
1,640 w,b
1,640 m,b
1,640 sh
1,630 (w)
1,600 m,b
1,270 sh
1,290 sh
1,085to 1,060 s,b
1,llOto
1,200 sh 1,165 s,b
1,100
1,105 s,b 1,100 b 1,065 m 1,070 m 1,080 s,b 1,020 s 1,020 to 1,020 to 1,000 s 1,005 s,b 1,005 s,b 945 to 930 s,b 950 to 935 s,b 950 s,b
1,060 s,b
s
1,050 s,b 1,005 sh 955 s,b
870 w HSPO4 ATR
1,630 m,b 1,160 to 1,130 m,b 980 sh 880w
1.25%o Al and 6.5% Zn. The spectra of the cement show that the formation of crystalline zinc orthophosphate at the cement sur-
zinc does not form complexes with phosphoric acid, nor will it introduce disorder into the zinc orthophosphate cement struc-
face occurs within 15 minutes and is more rapid than in the case of Cement VI. The spectrum of the bulk material at 24 hours shows no evidence of crystallinity. In contrast to the effect of aluminum, the inclusion of zinc in the acid appears to have little effect on either the setting time (Table 6) of the cement, or the development of surface crystallinity. Unlike aluminum, TABLE 6
ture.
SETTING TIME AND DEVELOPMENT OF CRYSTALLINITY Cement
I-IV V
VI VII VIII Ix x XI
Setting Time (minutes) Development of 22 C 50% RH Crystallinity at surface
less than 2 2.0 2.8 3.5 6.0 2.4 5.9 7.5
few minutes* few minutes a few minutes ca 1 hour ca I hour a few minutes ca I hour ca 3 hour a a
*Crystallinity also developed in the bulk of the cement.
CEMENTS X-XI. Cement X resembles Cement VI except that the zinc oxide powder used was ignited with magnesium oxide in the ratio 19:1. The spectra of Cement X, presented in Table 4 and Figure 5, show that Cement X differs in behavior from Cement VI in that the time taken for crystallites to form at the surface is prolonged from a few minutes to one hour. The spectrum of the bulk material after 24 hours shows no evidence of crystallinity. Inclusion of the magnesium oxide also increased the setting time of the cement (Table 6). Cement XI resembles Cement X except that the powder contains an increased proportion of magnesium oxide (ratio 9:1). This difference in composition has the effect of further delaying the formation of hopeite at the surface and increasing the setting time of the cement (Table 6). Again, the bulk material remains as amorphous zinc orthophosphate. The ATR spectra of Cements X and XI -
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252
J Dent Res February 1978
CRISP ET AL
Vavelength (Microns) 6
8
7
9
10
12
15
5min -lh - 24h
FIG 5. - Infrared (ATR and transmission) spectra of cement from ZnO -MgO (19:1)/49% H3PO4 + 2.5% Al.
-
1800
1600
1400
1200
1000
800
600
Frequency (CM -1) exhibit a strong band at 1000 cm-'; this may be due to the presence of some magnesium orthophosphate (which has a broad band at 1,040 to 1,080 cm-' with another band at 1,000 cm-') or to amorphous zinc orthophosphate. Clearly, the incorporation of magnesium oxide in the zinc oxide powder retards the setting reaction and crystallization on the cement surface. Zinc oxide powders used in dental zinc phosphate cements have to be
deactivated by heating, which increases the particle size and density of the powder.' '4
Magnesium oxide may act as a mineralizer, magnesium oxide-zinc solid solution be formed, causing an increased setting time and slower development of surface crystallinity in the zinc phosphate ce-
or a may
ment.
The absence of acid-phosphates in the setting cements is of interest and contrary to the accepted view that they are inter-
Wavelength (Microns) 6
7
8
9
10
12
15
BC-
FIG 6.
-
Infrared (transmission)
reference spectra: (A) Zn3(PO4)
-
4H20 (B) Zn3(PO4)2, (C) Zn3 (PO4)2 2H20. A Zn3 (P04)2 4H20 B Zn3 (P4)2 2H20 c Zn (P04) 2
1800
1600
Frequency (CM-1)
1400
1200
1000
800
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Vol. 5 7 No. 2
CRYSTALLINITY IN DENTAL ZINC PHOSPHATE CEMENTS
mediate in the setting reaction. " 15 Certainly, a study of the ZnO - P205 - H20 phase diagram'6'17 would indicate that as ZnO is progressively added to a concentrated phosphoric acid solution, the first substance to be deposited from solution is one of the crystalline zinc hydrogen phosphate hydrates. The composition of the acid hydrate formed is either ZnHPO4 H20 or ZnHPO4 * 3H20, depending on whether the acid concentration of the liquid is greater or less than 52% H3PO4. In the presence of excess ZnO, these acid hydrates are converted to hopeite, Zn3(PO4)2 * 4H20. However, phase diagram considerations can only be strictly applied to systems in equilibrium and, clearly, these conditions are not obtained in rapidly setting cements. Thus, in the cement reaction between plain ZnO and phosphoric acid solutions, an amorphous zinc phosphate is first formed, which after an induction period, crystallizes to hopeite, the thermodynamically stable form, throughout the matrix. Conclusions
Infrared spectroscopy has proved a valuable tool for monitoring the development of crystallinity in the bulk and at the surface of zinc phosphate cements. The spectra obtained show that no acid phosphates are formed at any stage of the cementforming reaction. The matrix consists only of neutral orthophosphates, principally those of zinc which are found both in crystalline and amorphous forms. Development of crystallinity within the bulk of the cement is prevented by the incorporation of aluminum in the cement-forming liquids. Thus, the presence of aluminum in the liquid exerts a decisive influence on the nature of the cement-matrix, an effect to be attributed to the formation of complex aluminophosphoric acids. It is these species, rather than simple phosphoric acid, which react with zinc phosphate. The presence of aluminum in the matrix introduces a disorder in the structure which inhibits
crystallization. Development of surface crystallinity is retarded by the addition of magnesium oxide to the zinc oxide powder, increasing the phosphoric acid concentration of the liq-
253
uid and incorporation of aluminum into the acid. Retardation of the rate of set appears to be a key factor in delaying the onset of surface crystallinity and in preventing crystallinity developing in the bulk cement. Crown Copyright reproduced by permission of the Controller of Her Britannic Majesty's Stationery Office.
References
1. CROWELL, W.S.: Physical Chemistry of Dental Cements, JADA 14:1030-1048, 1927. 2. DOBROWSKY, A.: Uber die Herstellung eines neuen Zinc Phosphatzementes hochster Volum konstanz, Chemische Technische, 15:159-161, 1942. 3. ISO Recommendation R 1566, Dental Zinc Phosphate Cement. 4. SERVAIS, G.E., and CARTZ, L.: Structure of Zinc Phosphate Dental Cement, J Dent Res 50:613-620, 1971. 5. CARTZ, L.; SERVAIS, G.E., and ROSSI, F.: Surface Structure of Zinc Phosphate Dental Cements,J Dent Res 51: 1668-1671, 1972. 6. WILSON, A.D., and MESLEY, R.J.: Dental Silicate Cements: VI. Infrared Studies, J Dent Res 47:644-652, 1968. 7. CHAPMAN, A.C., and THIRLWELL, L.E.: Spectra of Phosphorus Compounds: I. The Infra-Red Spectra of Orthophosphates, Spectrochim A cta 20:937-947, 1964. 8. AHLIJAH, G.E.B.Y., and MOONEY, E.F.: The Attenuated Total Reflection Spectra of Polyatomic Inorganic Anions. I. Salts of the Oxyacids of Phosphorus. Spectrochim Acta 22:547-553, 1966. 9. HAZEL, A., and Ross, S.D.: Forbidden Transitions in the Infrared Spectra of Tetrahedral Anions. III. Spectra-Structure Correlations in Perchlorates, Sulphates and Phosphates of the Formula MXO4, Spectrochim A cta 22:1949-1961, 1966. 10. LIEBAU, F.: Crystal Structure of Hopeite Zn3(PO4)2 * 4H1O, Acta Cryst 18:352-354, 1965. 11. AKITT, J.W.; GREENWOOD, N.N.; and LESTER, G.D.: Nuclear Magnetic Resonance and Raman Studies of the Aluminum Complexes Formed in Aqueous Solutions of Aluminum Salts Containing Phosphoric Acid an-d Fluoride Ions,J Chem Soc (A):2450 2457, 1971. 12. WILSON, A.D.; KENT, B.E.; CLINTON, D.
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254
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CRISP ET AL
and MILLER, R.P.: The Formation and Microstructure of Dental Silicate Cements, J Mater Sci 7:220-238, 1972. 13. WORNER, H.K., and DOCKING, A.R.: Dental Materials in the Tropics, Aust DentJ 3:215-229, 1958. 14. ZHURAVLEV, V.F.; VOLFSON, S.L.; and SHEVELEVA, B.I.: The Processes that Take Place in the Roasting of Zinc Phosphate Dental Cement, J Appl Chem, U S S R. 23:121-128, 1950 (translated from the Russian Zhur Prikiad Khim Leningr 23:118126, 1950).
F.A., and CRAIG, R.G.: Restorative Dental Materials, 4th Ed., St. Louis: C.V. Mosby Co., 1971, Chap. 13.
15. PEYTON,
16. EBERLY, N.E.; GROSS, C.V. and CROWELL,
W.S.: The System Zinc Oxide, Phosphorus Pentoxide and Water at 25°C and 370C, J Am Chem Soc42:1433-1439, 1920. 17. SALMON, J.E., and TERREY, H.: The System Zinc Oxide - Phosphoric Oxide Water at Temperatures between 250 and 100°, J Chem Soc, 1950, Part III, pp 28 13-2824.
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