eugenol 3

Thermal Analysis of Zinc Oxide-Eugenol CementsDuring Setting

H. M. EL-TAHAWI and R. G. CRAIGSchool of Dentistry, University of Michigan, Ann Arbor, Michigan 48104, USA

Characteristic thermal transitions of zincoxide and eugenol materials were comparedwith those of zinc eugenolate.It has been accepted traditionally that thesetting of zinc oxide-eugenol (ZOE) ce-ments is the result of the formation of zinceugenolate crystals.1 A recent study pointedout that at least seven hours elapsed be-fore zinc eugenolate crystals precipitatedfrom a clear solution of zinc acetate ineugenol.2 Accelerated ZOE, however, setin as short a time as three minutes.3

Electron microscopy, electron diffrac-tion,2 and X-ray diffraction4 studies indi-cated that a set ZOE mix may contain lessthan 2% zinc eugenolate. Even that lowconcentration is doubtful because the stud-ies did not identify clearly whether thediffraction patterns were due to a crystallineor amorphous phase. Differential thermalanalysis of set commercial ZOE did notshow the characteristic transition of zinceugenolate on melting.5 These studies sug-gest that the setting of ZOE cements maynot be because of the formation of a zinceugenolate phase.

Various studies on the same system indi-cated that the addition of o-ethoxybenzoicacid,6 large amounts of zinc acetate dihy-drate (accelerator),3 or zinc eugenolatepowder to standard ZOE mixes improvedthe mechanical properties of the hardenedmass.The purpose of this investigation was to

establish the characteristic thermal transi-tions of ZOE materials, to compare themto those of zinc eugenolate, and to studythe effect of adding o-ethoxybenzoic acid,large amounts of accelerator, and rosin onthe formation of zinc eugenolate.This investigation was presented at the 48th General

Meeting of the TADR, New York, NY, March 19, 1970.Received for publication April 13, 1970.430

Materials and MethodsOne commercial* cement dispensed as

powder and liquid and several laboratoryprepared mixes were used in this investi-gation. The materials used to prepare vari-ous cement compositions were as follows:zinc oxide (ZnO),t eugenol (E),: zincacetate dehydrate (ZnAc), rosin,ll ando-ethoxybenzoic acid (EBA).#

Zinc eugenolate was prepared by dissolv-ing zinc acetate in eugenol to form a clearsolution. After centrifuging, the clear liquidwas left to stand and zinc eugenolate wasprecipitated from the solution. The precipi-tate was washed from the eugenol severaltimes with methanol and dried at 45 C.

Differential thermal analysis was doneat from 25 to 500 C for the various mixes.The analysis was conducted in a nitrogenatmosphere and a heating rate of 25 C/minute was used. Glass beads were used asa reference material in all instances, ex-cept for zinc eugenolate in zinc oxide mix-tures. Zinc oxide was used as a referencematerial in these instances. chromel-alumel thermocouples were used. The curvesshowed uncorrected temperatures but thosereported in the text are corrected values.The relative humidity of the room duringmixing was about 25%.

ResultsThermograms of zinc oxide, 100% zinc

eugenolate, and 8% zinc eugenolate in zincoxide are shown in Figure 1. It can be seen

* S.S. White Co., Philadelphia, Pa.t Analytical reagent, Mallinckrodt, St. Louis, Mo.t U.S.P., Bowler Chem. Co., Detroit, Mich. Reagent grade, J.T. Baker Chem. Co., Phillipsburg,

NJ.Supplied by Kerr Mfg. Co., Romulus, Mich.

# City Chem. Corp., New York, NY. DuPont 900 differential thermal analyzer. E.I. Du-Pont de Nemours & Co. (Inc.), Instrument Product

Division, Wilmington, Del.

THERMAL ANALYSIS OF ZOE 431

_Zn0 or Commerical pdr r0z

w-j0zw

zwa:

0oIAT C

FIG 2.-Differential temperature (AT) vsconcentration of zinc eugenolate.used in preparing the laboratory mixes areshown in Figure 3. Eugenol was charac-terized by one transition, an endothermindicating boiling at 253 C. Zinc acetatedehydrate showed endotherms at 105, 111,

100 200 e 300 400 500

FIG 1. Zinc oxide, zinc eugenolate, and 8%zinc eugenolate in zinc oxide.

that within the range of temperature usedzinc oxide had no transitions. Zinc eugeno-late was characterized by two endothermicand one exothermic transitions; the endo-therms were at 226 C* and at 251 C, andthe exothermic transition was at 348 C.The same transitions were detected easilyat concentrations of 2% zinc eugenolate inzinc oxide. Below this concentration onlythe endotherm characteristic of zinc eu-genolate melting (251 C) was detectable.The differential temperature (AT) vs the

concentration of zinc eugenolate in zincoxide is shown in Figure 2. The same sam-ple weight (0.05 gm) was used for all theindicated concentrations, and the 251 Cendotherm was used to obtain the curve.

Differential thermograms of the reactants* All temperature values are corrected temperatures.

0EdA

0_zEdJ

Rosin

I~~~~

FIG 3. Components used to prepare ZOEand EBA cements.

w0xo

00zw

0.5

,1 (E)usp

Ic/div

ic/div

100 200 300 400 500-

Vol 50 No. 2

a _

432 EL-TAHAWI AND CRAIG251, and 355 C. The o-ethoxybenzoic acidhad an endotherm at 311 C. The thermo-gram of rosin showed endotherms at 84and 393 C.

Differential thermograms of unacceleratedmixes of zinc oxide and eugenol containingno accelerators were obtained at variousperiods of time from the start of mixing.Up to 96 hours the only transition obtainedwas that characteristic of the boiling ofeugenol, and the mix had not set.Thermograms of commercial ZOE mixes

during and after setting are shown in Figure4. During setting the mixture showed twoendothermic transitions characteristic of theboiling points of water and eugenol. Ther-mograms of older mixtures indicated adecrease in both endotherms and the simul-taneous appearance of two exotherms. Athermogram of a 30-minute-old* mixtureshowed no endotherms, but exotherms wereobtained at 353 and 373 C.Thermograms of laboratory prepared

ZOE that contained 0.6% zinc acetate areshown in Figure 5. It can be seen that thiscompound had transitions typical of thecommercial product used.The effect of zinc acetate concentration

on thermograms of ZOE is shown in Figure6. As the concentration of zinc acetate wasincreased an endotherm starts to appear in

Commercial23.6 Liq76.4 Pdr3 Mins

Commercial23.6 Liq76.4 Pdr15 Miss

2c/div.

lI -l-100 200 300Tc

* Time recorded when run started.

I-

FIG 5.-Laboratory

0x

0zw

400 500

Tcprepared ZOE mixes.

Lob Prep30 Min23.6 E

- 0.6 Zn Ac75.8 Zn 0

Lab Prep30 Min23.6 E 5c di_ 2.4 Zn Ac 5c/div74 Zn 0

1c/div

100 200 300 400 500TC1FIG 6.-Effect of zinc acetate concentration

on thermograms of ZOE laboratory mixes.FIG 4.-Commercial ZOE cement.

J Dent Res March-April 1971

0xw

I0iozw

THERMAL ANALYSIS OF ZOE 433

the region of the melting of zinc eugenolate.The two exotherms at higher temperaturesalso were replaced by one exotherm. Whenmix containing a 5.6% zinc acetate wasanalyzed after 24 hours, an endotherm ap-peared at the temperature characteristic ofthe melting of zinc eugenolate.The effect of rosin on the reaction be-

tween zinc acetate, zinc oxide, and eugenolis shown in Figure 7. The endotherms typi-cal of ZOE mixes with high acceleratorconcentration were flattened.

Differential thermograms of mixes con-taining o-ethoxybenzoic acid are shown inFigure 8. A liquid of 62.5% o-ethoxyben-zoic acid and 37.5% eugenol, by volume,had an endotherm at 285 C, which washigher than the boiling point of eugenoland lower than that of o-ethoxybenzoic acid.Thermograms of the product of the reac-tion between o-ethoxybenzoic acid and zincoxide had three endotherms at 90, 305, and425 C. In addition to these transitions, athermogram of the reaction product be-tween zinc oxide, eugenol, and o-ethoxy-benzoic acid showed an endotherm at thetemperature characteristic of the zinc eu-genolate melting point; the lower endo-therm found for o-ethoxybenzoic acid andzinc oxide was shifted to 125 C.

DiscussionThermograms of zinc oxide showed no

transitions within the temperature range ofinterest and therefore could be used as astandard with mixes of zinc oxide and zinc

FIG 7.-Effect of rosin on the thermogramsof ZOE laboratory mixes.

x0r0zw

FIG 8.-ZOE cements containing EBA.

eugenolate. The thermograms of zinc eu-genolate in Figure 1 had two endothermsand one exotherm; the first endotherm wasa solid-solid transition and the second at251 C was the melting transition. The melt-ing point of zinc eugenolate is reported as245 C.' The exotherm for zinc eugenolateis interpreted to be a result of decompo-sition.The sensitivity of the thermal analysis in

detecting zinc eugenolate was estimated tobe 0.1 % from Figure 2 and using theassumption that a peak one-eighth inchfrom the base line could be detected at thehighest sensitivity.

Eugenol had one endothermic transitionat 253 C, which is in agreement with theboiling point of 255 C. The endothermscharacteristic of zinc acetate dehydratewere due to loss of moisture, loss of waterof hydration, melting, and degradation,respectively. The o-ethoxybenzoic acid hadone endotherm, which indicated boiling.The two endotherms in the thermograms

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434 EL-TAHAWI AND CRAIGof rosin were because of melting and vapor-ization.When differential thermograms of a ZOE

formulation containing no accelerator weredetermined at times of up to 96 hours afterthe start of mixing, the only transition ob-tained was that typical of the boiling pointof eugenol. This indicates that the reactionbetween zinc oxide and eugenol alone pro-duced neither zinc eugenolate nor any by-product that had transitions within thetemperature range used.Thermograms of the commercial cement

used during setting had two endotherms dueto the loss of water and eugenol. These twoendotherms disappeared after 15 minutesfrom the start of mixing and two exothermswere obtained. The exotherms were atsomewhat higher temperatures (5 and 25C) than the temperature for zinc eugenolatedecomposition. The thermograms of the setcement were in general agreement withthose reported by Brauer, Termini, andBruns5; however, the curves reported hadlarge increases in AT at temperatures justabove room temperature. This increase wasprobably a result of setting the heat pro-grammer at too high an initial value withrespect to starting temperature of the sam-ple and is an artifact rather than a realpart of the thermogram.A laboratory cement that contained 0.6%

zinc acetate had transitions typical of thecommercial product. Thermograms of boththe commercial and laboratory preparedcements were obtained at periods of up totwo weeks and the curves were the same.The data presented in Figures 4 and 5 indi-cate that zinc eugenolate crystals do notform during or after setting of conven-tional ZOE cements; however, it does indi-cate the formation of an amorphous phasecharacterized by an exothermic transitionnear the decomposition temperature of zinceugenolate. It should be emphasized thatthis phase is not characterized by either thesolid-solid transition or the melting of crys-talline zinc eugenolate.

Increase in the concentration of zinc ace-tate had two effects on the thermograms ofconventional ZOE mixes. The two exo-therms of set conventional ZOE were re-placed by one exotherm characteristic ofthe zinc eugenolate, and an endotherm char-acteristic of the melting of crystalline zinceugenolate was obtained. The endotherm

J Dent Res March-April 1971

was more prominent in the thermogram ofthe mixes 24 hours after setting. Thesechanges suggest the formation of crystallinezinc eugenolate in ZOE mixes that containhigh concentrations of zinc acetate, andmay explain the finding that the compres-sive strength of ZOE was increased as adirect function of zinc acetate concentra-tion.2-3 The observations that crystallinezinc eugenolate forms as a result of thereaction between zinc acetate and eugenolin a ZOE mix and does not form as a re-sult of the reaction between zinc oxide andeugenol of the same clinical consistency,may be based on the different solubility ofzinc acetate dehydrate and zinc oxide ineugenol.The effect of rosin on the endotherms

of ZOE mixes with high concentrations ofzinc acetate showed that rosin interferedwith the formation of crystalline zinc eu-genolate. This may be because of the solu-tion of rosin in eugenol, or the effect ofrosin on the precipitation of crystalline zinceugenolate, or both.

Differential thermograms of o-ethoxy-benzoic acid-ZOE cements had endothermscharacteristic of the melting of crystallinezinc eugenolate. Thus the addition ofo-ethoxybenzoic acid to the system encour-aged the formation of crystalline zinceugenolate. The formation of crystalline zinceugenolate in o-ethoxybenzoic acid-ZOEmixes may be because of the higher solu-bility of zinc oxide in o-ethoxybenzoic acid.The solution of a zinc salt in a liquid bringszinc ions in contact with eugenol to formzinc eugenolate. The observation that crys-talline zinc eugenolate improves the com-pressive strength of ZOE2 cements may ex-plain the efficiency of o-ethoxybenzoic acidin improving the strength.

ConclusionsThe experimental evidence furnished by

this investigation leads to the followingconclusions: (1) The setting of unaccel-erated ZOE mixes, or those acceleratedwith concentrations of less than 1 % zincacetate, did not result in the formation ofmore than trace amounts, if any, of crys-talline zinc eugenolate. Hence, the settingof these ZOE cements was not due to theformation of such a crystalline phase.(2) Set ZOE mixes had two exotherms inthe region of the exotherm for zinc eu-

THERMAL ANALYSIS OF ZOE

genolate, which may indicate the formationof an amorphous zinc eugenolate. (3) Thepresence of large amounts of zinc acetatein ZOE mixes caused the formation of anappreciable amount of crystalline zinc eu-genolate. (4) The addition of o-ethoxy-benzoic acid to the system resulted in theformation of crystalline zinc eugenolateand hence improved the mechanical proper-ties. (5) The addition of rosin to o-ethoxy-benzoic acid-ZOE cements interfered withthe formation of crystalline zinc eugenolate.

References1. COPELAND, H.I.; BRAUER, G.M.; SWEENEY,

W.T.; and FORZIATI, A.F.: Setting Reactionof Zinc Oxide and Eugenol, J Res Nat BurStand 55:133-38, 1955.

2. EL-TAHAWI, HTM.: Relationship of Eugeno-late Formation to the Setting Mechanism of

Zinc Oxide Eugenol Cements, IADR Den-tal Materials Group Microfilm, Houston,Texas, March 1969.

3. MOHAMMED, H.M.: Effect of Some Acceler-ators on the Setting Time of Zinc OxideEugenol Reaction Product, thesis, North-western University, April 1967.

4. NORLING, B.K., and GREENER, E.H.: X-rayDiffraction Studies of the ZnO-Eugenol Re-action, IADR Dental Materials Group Mi-crofilm, San Francisco, California, March1968.

5. BRAUER, G.M.; TERMINI, D.J.; and BRuNs,C.L.: Characterization of Components ofDental Materials and Components of ToothStructure by Differential Thermal Analysis,J Dent Res 49:100-110, 1970.

6. BRAUER, G.M.; WHITE, E.E.; and MOSHONAS,M.G.: The Reaction of Metal Oxides withO-ethoxybenzoic Acid and Other ChelatingAgents, J Dent Res 37:547-560, 1958.

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