ZOE cements: phase identification by thermal analysis

Z0E cements: phase identification by thermal analysis

S. C. Bayne 1, E. H. Greener 2 1Department of Operative Dentistry, University of North Carolina, 2Department of Biological Materials, Northwestern University Dental School, Chicago, USA

Bayne SC, Greener EH. Z O E cements: phase identification by thermal analysis. Dent Mater 1985: 1: 165-169. Abstract. - The greatest difficulty for interpretation of dental cement properties has been the elimination of contributions of components other than the reaction product to the set mass. The objective of this work was to determine the DTAT G A transitions of all Z O E cement phases, so that the reaction product of zinc eugenolate could be identified in clinical materials. Zinc eugenolate was found to decompose exothermically starting at 310~ without melting. Previously reported transitions for zinc eugenolate were correctly assigned to other cement phases such as zinc hydroxide, zinc acetate, or eugenol.

Key words: dental cements, eugenol, thermal properties

The reaction product of zinc oxide powder and eugenol liquid was first chemically isolated in 1955 by Copeland et al. 1 as zinc eugenolate. However, that same phase had not been observed in clinical Z O E formulations. Since that time several investigators (2, 3, 4) have attempted to identify and quantify the presence of the product in clinical cements using a variety of analytical techniques. Thus far, x-ray diffraction and infrared approaches have not revealed the contribution of the product phase to the set mass. The best technique has involved thermal analysis but there has been significant difference in opinion about thermal transitions. Copeland et al. (1) reported a dissociation temperature for the zinc eugenolate of 245~ EI-Tahawi and Craig(4) made the first concerted effort tq quantify zinc eugenolate using thermoanalytical techniques. They reported a solid-solid transition endotherm, a melting endotherm at 251~ and a 300~176 exothermic decomposition reaction. Smith (5) reported that zinc eugenolate prepared from zinc hydroxide decomposed without melting. Greener and Norling (3) reported a broad exothermic decomposition at ca. 350~ with a second exotherm near 400~ Apparently these previous analyses were complicated by the interference of the transitions of phases other than the reaction product in the set cement. The objective of the present work was to in-

pared in the laboratory by reacting zinc oxide powder and eugenol liquid using 0.75 weight % acetic acid accelerator based on eugenol. The components were mixed at a 2:1 mole ratio of eugenol to zinc oxide in a closed beaker. They were continuously agitated with a magnetic stirring bar until the reaction generated a paste-like gel. The gelled mass was stored for 24 h and then slurried with methanol F2 via spatulation to break up any aggregates. It was vacuum filtered through filter paper F6with 2 methanol washes and a final acetone wash. The filter cake was dried in air at 80~ on a hotptate for 30 rain. It was subsequently pulverized in a mortar and pestle as a smooth flowing white crystalline powder. The magnesium salt was prepared in an identical fashion with the substitution of magnesium oxide v7 for zinc oxide. Quite often the zinc oxide powder components of cements are modified with magnesium oxide. Therefore, this modifier was also considered to generate product phases.

13 Dental Materials 1:5, 1985

dividually characterize the thermal transitions of all phases found in Z O E cements. That information should form a proper foundation for applying thermal analysis to detect reaction product phases in clinical cement formulations. Identification of the reaction product phases is key to understanding and modifying the clinical properties of all zinc oxide eugenol formulations. Material and methods The phases within set-ZOE cements include both residual reactants and product phases. Reactant phases were obtained as chemically pure components. The reactants included zinc oxide powder FI and eugenol liquid vz which were accelerated with either acetic acid v3 or zinc acetate F4. Product phases, on the other hand, were isolated by special collection techniques. Zinc eugenolate crystals were preF1. Zinc Oxide, A.C.S. Grade, Allied Chemical, P.O. Box 2064R, Morristown, NJ 07980. F2. Eugenol, Pfaltz and Bauer, Inc., Division of Aceto Chemical Company, Inc., 375 Fairfield Avenue, Stamford, CN 06902. F3. Glacial Acetic Acid, Reagent Grade, Mallinckrodt, Science Products Division, P.O. Box 5439, St. Louis, MI 63147. F4. Zinc Acetate Dihydrate, Reagent Grade, Fisher Scientific, 2775 Pacific Drive, P.O. Box 829, Norcross, GA 30091.

Dr. Stephen C. Bayne, Department of Operative Dentistry, University of North Carolina at Chapel Hill, Brauer Hall, 211 H, Chapel Hill, N.C. 27514, USA. Received January 25, 1985; accepted for publication 24 June, 1985.

F5. Anhydrous Methanol, Reagent Grade, Mallinkrodt, Science Products Division, P.O. Box 5439, St. Louis, MI 63147. F6. Whatman #2 Filter Paper, Whatman Laboratory Products, Inc., 9 Bridewell Place, Clifton, NJ 07104. F7. Magnesium Oxide, Reagent Grade (USP, Heavy), J. T. Baker Chemical Chemical Company, 222 Red School Lane, Phillipsburg, NJ 08865.

166

Bayne & Greener

To distinguish zinc oxide eugenol and magnesium oxide eugenol products from other potentially confounding phases, special phases were also considered in the analysis. Zinc acetate dihydrate was included as representative of the most common method of accelerating the cement reaction. Zinc hydroxide F8 was included because it has been continually implicated (6-9) as an intermediate product phase. Differential thermal analyses (DTA) were carried out on a Fisher DTA ~ employing a Cr-A1 thermocouple for sample temperature measurement and a platinel I thermocouple for furnace temperature measurement. The Fisher DTA permitted evaluation of 10-100 mg samples supported by a quartz tube, 0.4 cm in diameter and 3.0 cm long, with the appropriately embedded thermocouple tips. The furnace arrangement included a tightly packed cluster of tubes within a steel block in the bottom of a resistance wire furnace which was inert gas purged. The reference material thermocouple was measured against the sample material thermocouple to indicate AT during the programmed temperature increase, while the actual furnace temperature was monitored by the program control thermocouple. Both the AT and T thermocouple outputs were fed to a 2-pen chart recorder Fl~ The AT and T output signals were converted to degree centigrade outputs and re-plotted against each other rectilinearly. Control runs on the Fisher DTA system indicated that zinc oxide was the most suitable reference material. A 10~ min program rate for temperature increase provided an a v e r a g e 9~ temperature rise from room temperature to 600~ Zinc oxide or titanium dioxide was used for sample dilution to minimize baseline distortion during the analysis. No substantiative differences were noted for DTA runs in air versus nitrogen. The same apparatus was modified to perform thermogravimetric analysis (TGA) measurements on the test materials. The tube holder was removed and F8. Zinc Hydroxide, Practical Grade, Sargent Welch, 7300 North Linder Avenue, Skokie, IL 60076. F9. Fisher Model 360 Linear Temperature Programmer and Model 260F Furnace, Fisher Schientific, 711 Forbes Avenue, Pittsburg, PA 15219. F10. Model 2000 x,x,t 2-Pen Recorder, Houston Instruments, 8500 Cameron Road, Austin, TX 87853.

Fig. 1. Schematic view of DTATGA apparatus and the associated thermograms.

D.T.A.

T.G.A. Thermogravirnetric Analysis

Differential Thermal Analysis

ATI - - - - - V - -

w

---i T

T

FURNACE

FURNACE

the quartz tube with the sample was suspended via a chromel hangdown wire into the center of the furnace surrounded by a parallel arrangement of other required thermocouples. The wire holding the sample tube was attached vertically to an electrobalance. The temperature increase was programmed for 10~ The weight change and the temperature were recorded on the same 2-pen recorder. A schematic view of both the DTA and T G A arrangements is provided in Fig. 1. The AT versus T and % W versus T rectilinear plots were then superimposed as a DTA-TGA composite thermogram. That combination is explained by Fig. 2. DTA curves were represented with solid lines. T G A curves were represented with dashed lines. The endothermic and exothermic deviations from the baseline in DTA must always be interpreted carefully in light of both sample geometry effects and the real meaning of such non-equilibrium testing (10). Most D T A work is considered only qualitative. Its merit lies in establishing the onset a n d type of thermal transitions within the sample. Although differences in peak heights and peak areas did reflect absolute differences in specific heat and/or enthalpy contents of the sample versus the reference material, such quantitation was inapparopriate without extremely rigorous accommodation for instrumental and heating rate effects. DTA was semi-quantitative at best if the individual DTA apparatus was wellcharacterized for its contributions to Fll. Model RTL Electrobalance, Cahn Instruments, Ventron, 16027 South Carmelita Road, Cerritos, CA 90701.

DTA-TGA THERMOGRAMS ..... ]---~- ............ §

r.eJ. DATA

\ ~ W,~h, Loss A

/

oo. SO

/~*&T,

Exotherm

oj

50

AT

~,,, D,T*A. DATA

-AT,

W rm

I ~ I , I , I , I , I00 200 300 400 500 JC

TEMPERATURE Fig. 2. Composite DTA-TGA thermogram (DTA data shown by solid lines. TGA data shown by dashed lines.) measured thermal differences. The DTA curves presented here were used exclusively to assess the transition type, onset, breadth, and curve shape. Transition temperature limits were extrapolated as corresponding to the intersection of the best straight line portion of the peak side with the baseline. The estimate error associated with this graphical method was about +3~ for all noted values. No compensation for heating rate effects, thermal transfer rates, sample geometry effects, or subtle instrumental effects was made. For T G A analysis the instrumental errors were typically less significant than for DTA. Sample packing, buoyancy, sample size, and heating rate errors were the major sources of error. The % W versus T curves were interpreted in a similar fashion when required by graphical extrapolation to the baseline via straight lines. The thermograms given in the figures are one of the 3 replications of every experiment. Although there were minor differences in superposition of curves, all transitions were accurately represented by each curve presented.

Z O E cements ZINC HYDROXIDE

Results

Zn (OH)t

The D T A - T G A composite thermograms for zinc and magnesium oxide, eugenol, zinc acetate dihydrate, zinc hydroxide, magnesium eugenolate and zinc eugenolate samples are presented in Figs. 3-8, respectively. Sample dilutions and reference materials are noted appropriately in the figure captions. Well characterized dehydration and/or decomposition reactions are ref-

9C

.......

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t

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~

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,

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3O0

,

I

400

9

500 "C

TEMPERATURE

Fig. 6. Zinc hydroxide thermogram (ZnO ZINC OXIDE or MAGNESIUM OXIDE "C

used as reference material. Sample diluted 50/50 with TiOz. )

.......................

100

-1"5

%

MAGNESIUM EUGENOLATE

80

IO0 %

',,

60

AT

o

W 40

8O

\\

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60 20

--5

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+2.5

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i

I

I

200

'

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300

,

I

400

,

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,

500 q~

0

0 20

TEMPERATURE

-2.5 I

Fig. 3. Zinc oxide and/or magnesium oxide

thermogram (TiO2 used as a reference material. No detectable changes so the baseline remained constant.)

...........

~

I 200

J

I

I

,

0

I00 %

,. 4"7.5

40

"1-5,0

6o

"5)

AT

300

400

500 "C

8o

ix

W

i

200

I

500

ZINC EUGENOLATE

80

I00

,

"C

I

-- 5.0

I

400

TEMPERATURE

80

- 2.5

,

~00

Fig. 8. Magnesium eugenolate thermogram

100 %

+2.5

I

(ZnO reference material. No sample dilution.)

EUGENOL "C

I I00

20

+2,5

0

0

TEMPERATURE

J

Fig. 4. Eugenol thermogram (ZnO refer-

ence material. Sample diluted 50/50 with TiO~.)

W 4O

32O

I I00

,

I

200

t

t

300

,

t

,

400

I

t_

0

500 "~

TEMPERATURE

Fig. 7. Zinc eugenolate thermogram (ZnO

reference material. No sample dilution.) ZINC ACETATE DIHDRATE Zn

(CH3CO0) 2 .2HtO

-c ...... ., +5

AT

~

~I\,

o -5

- ~

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l

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TEMPERATURE

Fig. 5. Zinc acetate dihydrate thermogram

(ZnO used as reference material. Sample not diluted.) 13"

erenced to the transitions whenever appropriate. Zinc oxide powder showed no detectable first or second order transitions over the range of 25~176 in either the D T A or T G A thermograms. Eugenol boiled at 259~ as indicated by the single spiked endotherm on the DTA curve. The T G A curve of eugenol showed rapid and almost complete weight loss above 265~ These values compared well with the reported value of 255~ for eugenol vaporization (11). The slight shift to higher temperatures

167

on the D T A - T G A record was apparently due to both sample packing and heat transfer difficulties within the sample tube. The higher temperatures are also caused by the high rate of heating of 9~ The thermal analysis yields non-equilibrium conditions and one would expect slightly higher temperatures on the DTA-TGA record when the experiments are conducted in this heating mode. These difficulties are normally encountered in all DTAT G A work. Zinc acetate dihydrate samples were analyzed rather than acetic acid because the latter was assumed to generate the acetate rapidly when in contact with zinc oxide. The DTA curve in Fig. 5 indicated a double spiked endotherm extending from ca. 100~176 The first peak of the endotherm was associated with absorbed water on the hydroscopic crystals. The latter and larger peak on the endotherm was caused by the dehydration of the salt. A t 225 ~ 250~ a melting endotherm occurred as a relatively shallow and broad peak, followed by a gradual baseline shift which was most marked above 300~ where salt decomposition to zinc oxide was taking place. The latter endothermic shift was more complicated than might be interpreted because it involved both the gaseous decomposition of the acetate salt and simultaneous zinc oxide formation. The difference in size of the low and high temperature endotherms reflected the greater amount of heat taken up on dehydration compared to the salt decomposition. The effect on weight change throughout the temperature range was exactly the converse of the thermal change. The dehydration represented only a small weight change while the decomposition represented the greatest weight loss. Zinc acetate dihydrate presence in zinc oxide-eugenol cements was more apparent as the dehydration endotherm in the DTA thermogram and as the salt decomposition reaction in the T G A thermogram. The zinc hydroxide DTA curve indicated an apparent dehydration onset to zinc oxide as a small broad endotherm starting at ca. 130~ The measured value compared favorably with the reported value of 125~ (12). A large broad endotherm was also evident at 210~176 with a peak maximum just above the boiling temperature for eugenol. This endotherm was hypothesized as the maximum temperature for hydroxide stability. The position and

168

Bayne & Greener

SUMMARY OF DTA TRANSITIONS A

ZiNC

i

i

,oo I , I

EUGENOL

.................

,

i

zoo I

I

i

,

,

??o, I

,

,fro,

I

I

t

i

,sp, I

None

OXIDE . . . . . . . . . . . . . . . .

MAGNESIUM OXIDE . . . . . . . . .

,

None

I

I

im

240-265 ZINC

ACETATE

DIHYDRATE"I

II

100-125 ZINC HYDROXIDE . . . . . . . . . .

7'20-250

500-580

I 125-160

II

210-280

580-405

320-420

MAGNESIUM EUGENOLATE..I

~

-

-

320-380

ZINC EUGENOLATE . . . . . . . .

I

~

-

480-520

-

~

Fig. 9. Summary of DTA transition for ZOE cement phases (temperatures indicate start and range of transitions.

size of the endotherm would make it difficult to distinguish from a eugenol vaporization endotherm if both materials were present simultaneously. The small endotherm at 380~176 was hypothesized to be related to stoichiometry changes in the residual oxide. The T G A curve indicated that sample weight losses were associated with only the first 2 transitions. The DTA-TGA thermograms for zinc eugenolate crystals are shown in Fig. 8. The first exotherm from 320 ~ 380~ was associated with marked gas evolution and was interpreted as the thermal decomposition of the zinc eugenolate crystals. The higher temperature exotherm from 4800-520~ appeared to represent the continued decomposition of products generated but not volatilized at 320L380~ This was consistent with the T G A data that showed that weight loss began slowly at ca. 300~176 was most rapid at 320 ~ 350~ but continued to temperatures in excess of 600~ Magnesium eugenolate underwent a single broad exotherm from 3200-380~ involving substantial gas evolution. This event was interpreted as salt decomposition and its temperature range overlapped with that of zinc eugenolate. All of the data from the DTA ana-

lyses were abstracted from the thermograms and summarized in Fig. 9. By comparison of the temperature ranges for the transitions of the phases it is possible to identify which ones would overlap during DTA analyses of Z O E dental cements. It is also possible to identify transitions which were interpreted improperly by previous investigators of Z O E based dental products. Discussion Composite D T A - T G A thermal analysis revealed much more discrimination than DTA analysis alone. In several instances,: the concomitant weight changes permitted confirmation of suspected dehydration or boiling endotherms. It also allowed the detection of gradual weight changes over broad temperature changes. The latter was typical for complex decomposition events such as displayed by zinc eugenolate. The assignment of well-characterized thermal transitions to each phase in Z O E cements allowed the proper interpretation of prior cement thermal analyses. Zinc eugenolate was the principal phase of interest. It did not dissociate or melt as previously reported by others (1, 4) near 250~ Endotherms in that region were properly assigned to

eugenol vaporization, zinc hydroxide dehydration, or zinc acetate changes. Of all the phases examined, only the product phases of zinc eugenolate and magnesium eugenolate displayed exothermic transitions. The oxides of zinc or magnesium displayed no transitions. Both zinc eugenolate and magnesium eugenolate began to decompose at approximately 320~ and those transitions occurred in a complex way over a wide temperature range. Therefore, it would be difficult to separate the contribution of each phase if both of these phases were present. Prior reports (3) of zinc eugenolate decomposition near 350~ were based on identifications of the exothermic peak maximum in the middle of the thermogram rather than the peak onset. It was interesting to note that the thermal decomposition of zinc eugenolate did not parallel either the proposed chemical formation reaction or chemical dissociation mechanisms which have been proposed by others (7, 8, 9). The most prevalent suspicion is that zinc eugenolate is chemically unstable in water. It is stated to release eugenol and form an intermediate species of zinc hydrozide. From the thermograms in Figs. 4 and 6 it was obvious that neither of those remnant phases would survive at 320~ Because the T G A curves in Figs. 7 and 8 demonstrated weight changes occurring up to at least 600~ the high temperature thermal reactions must be different than the chemical ones at 37~ Another related observation was the absence of melting prior to decomposition. Apparently the bonding pattern within these eugenolate crystals involved a significam degree of intermolecular coordination. The fact that the decomposition exotherm was relatively small agrees with the hypothesis that the decomposition did not occur as a simple event all at once. Because of the similarity of DTA thermograms of zinc and magnesium eugenolate, it appeared they may be analogous in chemical structure and crystalline morphology. Conclusions and summary Zinc eugenolate product crystals exothermically decomposed above 320~ with the maximum heat generation and weight loss occurring above 350~ The decomposition event extended over a broad temperature range as evidenced by continued sample weight loss to beyond 600~ Previously reported

Z O E cements

DTA endotherms for zinc oxide-eugenol materials at ca. 250~ were reassigned to unreacted eugenol, zinc acetate, and/or zinc hydroxide, but not to zinc eugenolate melting. The exothermic decomposition peak above 320~ may be used to monitor the presence of zinc eugenolate but its assignment depends on magnesium eugenolate being absent or present in trace amounts.

Thermal techniques for Z O E cement analysis do distinguish the presence of reactants and products, but the interpretation of the thermograms requires suitable precaution in light of the overlapping events. Acknowledgements - This work was sup-

ported in part by NIH-NIDR grants DEOI25 and DEO5886.

References

1. COPELANDJR. HI, BRAUERGM, SWEENEYWT, FORZIATIAF. Setting reaction of zinc oxide and eugenol. J Res N B S 1955: 55: 133-8. 2. NORLINGBK, GREENEREH. X-ray diffraction studies of the ZnO-eugenol reaction. D M G / A A D R Microfilm 1968: Paper #430. 3. GREENEREH, NORLINGBK. Thermal analysis of zinc oxide-eugenol. DMG/ A A D R Microfilm 1970: Paper #802. 4. EL-TAHAWIHM, CRAm RG. Thermal analysis of zinc oxide eugenol cements during setting. J Dent Res 1971: 50: 430-5. 5. SMITHDC. The setting of zinc oxide-eugenol mixtures. Br Dent J 1958: 105: 313-21. 6. BRAUERGM. A review of zinc oxide-eugenol type filling materials and cements. Rev Belg Med Dent Tijds Br Tandheel 1965: 20: 323-64. 7. NORLING BK. The kinetics and mechanisms of the zinc oxide-eugenol reaction. PhD Thesis, Northwestern University, 1973. 8. WILSON AD, BATCHELORR E Zinc oxide-eugenol cements: II. Study of erosion and disintegration. J Dent Res 1970: 49: 593-8. 9. WILSONAD, CLINTOND J, MILLERRP. Zinc oxide-eugenol cements: IV. Microstructure and hydrolysis. J Dent Res 1973: 52: 253-60. 10. BAYNE SC. The Detection and Characterization of Zinc Eugenolate. Ph.D. Thesis, Northwestern University, 1978. 60-90. 11. Handbook of Chemistry and Physics (49th Edition), Chemical Rubber Company, 1958. 12. Condensed Chemical Dictionary (Sth Edition), Van Nostrand Reinhold Company, 1971.

169