Morphology and properties of denture acrylic resinscured by microwave energy and conventionalwater bathC.-P. Laia, M.-H. Tsaia, M. Chena,*, H.-S. Changb, H.-H. TaybaInstitute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424,Taiwan, ROCbDental Department, Veterans General Hospital-Kaohsiung, Kaohsiung 813 Taiwan, ROCReceived 7 March 2002; received in revised form 1 January 2003; accepted 12 March 2003KEYWORDSDenture; Poly(methylmethacrylate);Microwave; Morphology;Transmission electronmicroscopySummary Objectives. This study examined the inuence of microwave energy levelson the morphology and properties of an impact resistant denture material poly(methylmethacrylate) with a thickness of 10 mm.Methods. A microwave ask containing two resin blocks was processed at 80, 160,240, and 560 W for 15, 10, 7, and 2 min, separately. Each Flask was then turned over,and cured for an additional 2 min at 560 W. The process using conventionalmethodswas carried out at 70 8C for 9 h. The blocks were tested for hardness, porosity, exuralproperties, solubility, and molecular weight. The morphology of the specimens afterstaining with osmium tetroxide was examined by transmission electron microscope.Results.Thechangesintemperaturewithtimewererecordedduringmicrowaveheatingat80,160,and240 W,respectively.Asignicantlylargedifferenceinthecuringtemperaturewasobservedwhencomparingthesetwoprocessingmethods.Therewaslittledifferenceinthemeanvaluesofsurfacehardnessandtheweightpercents of the insoluble parts. The mean domain size and the volume fraction of therubberphasefavorofthewater-bathmethod.However,theporosityinthewater-bath-curedspecimenswasmuchlessthanthatinthemicrowave-curedspecimens.Thus, the conventionally cured specimens showed better exural strength and exuralmodulus than the microwave-cured specimens.Signicance. This study has shown that microwave energy can efciently polymerizedenture base polymer. Highly statistical differences in morphology and exuralproperties favor of the water-bath method. Choice of a suitable microwave power andpolymerization time is important in order to reduce porosity to a minimum level andincrease the domain size and volume of the rubber phase.Q 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.IntroductionAcrylic resins have been used for denture fabricationforover60years.Themostpopulardenturebasematerial is heat-curedpoly(methyl-methacrylate)0109-5641/$ - see front matter Q 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.doi:10.1016/S0109-5641(03)00084-8Dental Materials (2004) 20, 133141http://www.intl.elsevierhealth.com/journals/dema*Corresponding author. Tel.: 886-7-5252000x4062; fax:886-7-525-4099.E-mail address: mingchen@mail.nsysu.edu.tw(PMMA). In additiontohomo-PMMA, impact resistantresins have been developed.1Virtually all denturesare constructed fromthese materials using theconventional polymer/monomer dough moldingprocess and cured using a water-bath system. Sincethe introduction of acrylic resins for dentureconstruction, there has been a continual search formodiedprocedures toprocess theresin. Nishii2rstreported the use of microwave energy to polymerizedenture base materials in 1968. Kimura et al.3reportedthatitwaspossibletocureacrylicresinin a very short time using this technique. In19844and1985,5thelightber-reinforcedplastic(FRP)askwas substituted for the heavy brass ask andcompress, and the water-bath curing tank gaveway to a microwave oven.Methyl methacrylate(MMA) isapolarliquidatroom temperature. The microwaves cause the MMAmoleculeswithintheacrylicresintoorientthem-selves in the electromagnetic eld at a frequency of2450 MHz, andnumerouspolarizedmolecules areipped over rapidly and generate heat due tomolecular friction. Initiating radicals are thenabletoreact withmonomers tostart polymeriz-ation. Microwave heating is independent of thermalconductivity, therefore, curing cycles involving theapplicationofrapidheatmaybeusedwithoutthedevelopment of a highexothermic temperature.Themajoradvantagesofmicrowaveheatingoverconventional heating are: (1) the inside and outsideof substancearealmost equallyheated, and(2)temperature rises rapidly. Peyton6reviewedthecuring methods and concluded, that when allfactorswereconsidereditwasdoubtful that anyofthesemethodshadanyrealadvantageoverthewater-bathmethodandtheimportant factor forconsideration was the careful control of thetemperature during processing.Reitzetal.5comparedporosity, hardness, andtransversestrengthofmicrowaveandwater-bathcuredspecimens andfoundnosignicant differ-ences.Thefrequencyandsizeofporosityinthickspecimens couldbereducedto30%by alongerpolymerizationtimeatalowerwattage.Hayden7compared the exural strength of three acrylicresins cured by water-bath and microwave energy.The microwave-cured acrylic did not absorb asmuch energy before fracture as the water-bathcuredacrylic. Shlosbergetal.8alsotestedtrans-versestrengthandhardness.Nostatistical differ-enceswerenotedwhenmicrowaveorwater-bathcuringwereused.Smithetal.9investigatedhard-ness, transverse strength, modulus of elasticity,and Izod impact strength of seven resins cured usinga water bath, microwave energy, or by visiblelight. Microwavecuringimprovedthemodulusofelasticity of two resins, decreased the impactstrength of one, and had little effect on theproperties of the other two resins.In the literature, some data on the morphology ofacrylic resins was found. The purpose of this studywas to establish the time and temperature requiredforthecompleteconversionofmonomertopoly-mer and to determine if denture bases processed bymicrowave energy varied signicantly in mor-phologyfromthoseprocessedbytheconventionalwater-bath curing method. Finally, the size and thedistributionofrubberphasewerecorrelatedwiththe heating methods, microwave power, the physi-cal properties and mechanical properties.Materials and methodsExperimental materials. The Optilon-399 (Coltene/WhaledentInc.,Ohio,USA)usedinthisstudyisaPMMA denture base polymer of Hygenic brandmarketed as two parts, powder and liquid. ThepowdercontainsmainlyPMMAmodiedwithgraftcopolymer and a small percentage of benzoylperoxideas initiator, titaniumdioxide, andcad-miumpigments.PrincipleingredientsofliquidareMMA monomerandethyleneglycoldimethacrylateas the crosslinking agent. The proportion of powderto liquid was 3010 ml.Specimen preparation. The domestic microwaveovenusedinthisstudywasaSamsungmodelMW-3190T (Korea) with a full power of 800 W. Forproduction of specimens using the microwavetechnique, a special dental FRPask (AcronMCmicrowaveask,GCAmericaInc.)wasused.Moldseparation(alginatesolution),packingandclamp-ingproceduresfollowedstandardpractice. Whilethe resin was in dough stage, it was packed into twomolds (65 mm 15 mm 10 mm). These two moldswere 52 mmapart which were geometricallyequivalentwithrespecttotheask.The halvesofthe ask were pressed together in a pneumaticpress (Kavo EWL 5414, Germany). The pressure wasincreased up to 40,000 N in several steps in order toremoveexcessresin.ThepackedFRPaskswereusedfortemperaturemeasurementandspecimenpreparation.Forthewater-bathcuredspecimens,theprep-aration procedures were the same as for themicrowave-specimens except that conventionalmetal dental asks (Brass ask, Kavo EWL,Germany) were used. Acrylic resins were processedin a water-bath curing tank at 70 8C for 9 h, and thenthe dental asks were cooled to room temperature.C.-P. Lai et al. 134Measurement of temperature changeand microwave curingTwothermocoupleswentthrough the holes on theupper part of theFRPask, andlocatedat thecenterpositionofeachdenturebaseresinsmold.The turntable of the microwave oven was removedduring the temperature measurement (models 3711and 3712, LR4100 recorders, Yokogawa ElectricCo.). Ideally, the FRP ask was placed in the centeroftheovencavity,andatthesameheightastheturntable by putting the ask on two blocks of curedresin.Thechangesintemperaturewithtimewererecorded during the microwave heating at 80, 160,and240 W, respectively. Themeasurement of 10blocks of resinmolds was madefor eachpowercondition. It was found that the temperaturetendedtowardanapproximately constant value,andthetimerequiredtoreachthis temperaturewasselectedtocureacrylicresinsforeachpowercondition.Microwave curing was carried out in a microwaveovenequippedwithaturntable. Themicrowaveaskcontainingtwoofthesamesizeresinblockswas processed at 80, 160, 240, and 560 W for 15, 10,7, and 2 min, separately. Then each ask wasturnedover,andcuredforanadditional 2 minat560 W.Tenblockswerepreparedforuseineachconditionandcooledtoroomtemperaturebeforedeasking. A total of 50 blocks of cured acrylic resinwerethusprepared,40intheFRPasksand10inthe brass asks. These blocks were tested forhardness, porosity, exural properties, solubility,and molecular weight in sequence.Transmission electron microscopySlices of 1 mm 15 mm 10 mmwere cut fromthecenter part of thecuredblocks usingaslowcutter (Minitom, Struers). Thecenter sections ofthe slices were sharpened into a cross-sectionofabout0.01 mm2acrossthetransversedirectionof the thickness. Specimens of 3050 nmthickwere cut fromthe sharpened section using anultramicrotome(Ultracutter, Reichert-Jung) andmounted on formvar-carbon 200 mesh coppergrids (TedPella, Inc.). Theultrathinspecimenswere stained with 2% OsO4(Osmiumtetroxide,Ted Pella, Inc.) for 48 h. The morphology wasobservedunder aJEOL(JEM-200CX) transmissionelectronmicroscope(TEM) using anacceleratingvoltage of 160 kV. To quantitatively determinethe domain size, at least 360 rubber phasedomains (or 10 TEM pictures) were examinedforeachcondition.Thesize,thedistribution,andthe volume fraction of rubber phase domainsweremeasuredusingOptimas software.HardnessThe specimens were polished on both surfaces using0.5 mm aluminum oxide grit. The nal thickness wasless than 10 mm. Their hardness was determined ina Shimadzu hardness tester (HMV-2000) with a loadof300 gloadingtimeof30 s.Measurementsof15areas along uniformly selected points of bothsurfaces were made for each water-bath curedresin. A total of 150 VHN (Vickers hardness) valueswere obtained for 10 specimens. In each conditionof microwave method, at least six areas weremeasured per specimen and a total of 46 VHN valueswereobtainedforeightspecimens.Anaverageof150 or 46 VHN values was performed in each groupto determine if the variables of curing methods hadany statistically signicant effect on the hardness.PorosityThepulseechoC-scanswereperformedwiththeTestech ultrasonic tester (model VI-100). Thepolished blocks were inspected at a test frequencyof 10 MHz using a 25.4 mm (1 in.) element transdu-cer withapproximatelya38.1 mm(1.5in.) focallengthinthecouplingmedium(water),andwerescanned along the top surface at a speed of0.2 mm/s.Theinterfacedelaywasmeasuredandconverted into a gray scale value on a C-scan image.Flexural testThe mechanical test used was the three-point-bendingtestwithaspan-to-thicknessratio s=tof4:1. Six to eight blocks of each condition weretested at roomtemperature on a MTS dynamictensile testing machine (Model 810) at a crossheadspeed of 1.25 mm/min. The deection reading as afunctionoftheappliedloadwasrecordedduringthe entire test until the applied load droppedsubstantially,as a result of internal damage in thespecimens.SolubilityAsamplefromthefracturesurfacewascutintoapiece of about 0.2 g. It was then reuxed with150 ml of tetrahydrofuran (THF) in a round-bot-tomed ask at a cycle of about 45 min. Thisprocedure continued for 2526 h to ensure that allsolublePMMAinthecuttings was extractedintosolution. The insoluble part was dried and thenweighed to determine its percentage. ThreeMorphology and properties of denture acrylic resins 135samples were taken fromeach group for thesolubilityanalysis.Thesolpartswereusedforthemolecular weight determinations.Gel permeation chromatography (GPC)The analysis was performed on a Waters 150CV GPC.THF was used as the mobile phase at a ow rate of1.0 ml/min with two StyragelwHT columns in seriesat 40 8C. The systemwas calibrated with eightstandard samples of polystyrene (PS) of knownmolecular weight. APSmoleculehas anaverageweight of 41 for each A of length in THF at ambienttemperature.TheQ-factorforPSistherefore41.PMMA has an approximate Q-factor of 23 under thesame condition. Q-factor method was used toestimatethemolecularweightsofthesolpartsofcured PMMA resins.StatisticsMeans and standard deviations were calculated foreach measurement or property. A one-way analysisof variance (ANOVA) was conducted on the data ofdomain size, hardness, and exural propertiesseeking statistically signicantdifferencebetweenonewater-bathandfour microwavecuredspeci-mens. Means werecomparedat the95%level ofcondence, and difference between means wassignicant if Pvalue was less than 0.05.ResultsFig. 1 shows the average temperature changes of 10measurements duringPMMApolymerizationwhenheated by microwave power at 80, 160 and 240 W,respectively. The temperature of the specimenscuredat highpower increasedmorerapidlyandreached a constant value more quickly than that atlowerones.Inthecaseof240 W(opentriangles),thetemperatureof theresins begantoincreaserapidly to 97 ^ 16 8C after irradiation for 5 min. Thetemperature tended towards 120 8C with a standarddeviation of 5.6 8C at 7, 4.3 8C at 8 min, and nally3 8C from 9 to 15 min. The variation of temperaturewith time for acrylic resin cured at 160 W is plottedas open circles. In the rst 1 min, the temperaturerose at almost the same rate as that at240 W.Beyond1 min,thetemperatureincrementsbecame slower and it was 75 ^ 8 8C at 6 min. Afterthe specimens were irradiated for 6 min, thetemperature rose more rapidly to 107 ^ 7 8C at9 min, thentoanal temperatureof 111 ^ 3 8Cfrom10 to15 min.Thesametrendwas seen whentheoutput power was 80 W(lledsquares). Theplateauof temperaturewas 109 ^ 4 8Cwhenthespecimens had been irradiated at 80 Wfor 1420 min.Thisplotrevealstheeffectofdecreasingtheoutput power, i.e. lower theeldstrength,lower the heating rate, and lower the level offtemperature.Fig. 2a and b depicts the typical morphology of aspecimen cured in a water-bath. This cured acrylicresinmaybeconsideredtocomprisethreecom-ponents: a plastic phase that is continuous, adispersed graft copolymer, and an effective rubberphase that is also dispersed. The gray or whitephase represents PMMA (circle C). Circle Billustrates the dispersed stained phase with adiameter of about 0.1 mm. Circle A in Fig. 2arepresents the effective rubber phase (domain) withadiameterbetween0.24and0.67 mm.Aclose-upFigure1 Specimen temperature versus time for micro-wave power at 80, 160 and 240 W, respectively.Figure2 TEM micrographs of denture base resins cured in conventional water-bath (a) overall view and denition ofdomains, (b) close view.C.-P. Lai et al. 136of atypical circleA is showninFig. 2bwherePMMA was trapped in a large rubber phase and somesmallstainedrubberphaseswerealso dispersedinit. The number of domains, means, standarddeviations, and the bounds of 95% condencelevel for thestatistical analysis of theeffectiverubber phase are listed in Table 1. As shown in therst row, the mean domain size was0.395 ^ 0.068 mm, and 95% condence level rangedfrom0.388to0.402 mmforthewater-bathcuredspecimens. The domain size distribution was 1.042(see Fig. 4a), and the volume fraction of theeffective rubber phase was 10.3%.The TEMmicrographs in Fig. 3ad showthespecimens cured at a microwave power of 80, 160,240, or 560 W, respectively. The domain size rangesfrom 0.18 to 0.62 mm, and their means are0.379 ^ 0.060, 0.378 ^ 0.070,0.370 ^ 0.072,and0.354 ^ 0.062 mm, respectively(seeFig. 4b, androws 36 of Table 1). The corresponding distri-butions are 1.038, 1.048, 1.051, and 1.044.The occupied area (or the volume fraction) oftheeffectiverubber phasedecreasedfrom8.44,7.99, 7.80 to 7.50%, i.e. the effective rubber phasedecreasedwithincreasingmicrowavepower. Forthe overall microwave-cured specimens, the distri-butionisplottedinFig.4a,andiscomparedwiththewater-bathcuredspecimens.Asshowninthelast two columns of Table 1, ANOVA revealedsignicant difference P , 0:05 of the meandomainsizebetweentheoverallmicrowavecuredspecimens, orthemicrowavespecimenscuredateach energy level, and the water-bath curedspecimens.Table2comparesthesolubility,porosity,mol-ecular weights, andpolydispersity for thespeci-mens cured in the water-bath and in the microwaveoven at four different powers. The insoluble wt% ofall the specimens is shown in Table 2, along with thestandard deviations. Mean values were found to be60.6 ^ 2.2 wt% for the water-bath method and60.068.0 wt% for the specimens cured in a micro-wave oven. The resin blocks showed the leastamount of porosity, 010%, compared to whenTable 1 Statistical analysis of the domain size of the effective rubber phase.Number ofdomainsMean domainsize (mm)Std. dev.(mm)95% Condenceinterval (mm)Water-bath vs. microwaveLowerboundUpperboundP-value P , 0:05Water-bath curing 383 0.395 0.068 0.388 0.402Microwave power Overall 2485 0.373 0.067 0.370 0.375 1.346 1027Yes80 W 685 0.379 0.060 0.375 0.384 8.926 1025Yes160 W 756 0.378 0.070 0.373 0.383 1.101 1024Yes240 W 675 0.370 0.072 0.365 0.375 2.985 1028Yes560 W 369 0.354 0.062 0.347 0.360 1.576 10217YesFigure3 TEM micrographs of denture base resins cured by microwave power at (a) 80, (b) 160, (c) 240, and (d) 560 W,respectively.Morphology and properties of denture acrylic resins 137cured in the water-bath. The blocks cured at 80 and160 W had a similar amount of porosity, 60 and 65%,respectively.Theblocksshowedlargeamountsofporosity, 80%, when cured at 560 W. Compared withall the blocks for the microwave method, the blockscuredat240 Wwereconsideredtohavemarkedlydecreased porosity (40%). The molecular weightdistribution Mw= Mn; togetherwiththepeak Mp;number Mn and weight Mw averages, are listed inTable 2. All of the sol parts of the cured resins hadaveragemolecularweightsintheorderof6 104for

Mn; of 105for

Mwand

Mp: The molecular weightdistributions were between 2.37 and 2.75.The mean Vickers hardness is shown in Table 3,along with the standard deviations, and 95%condenceinterval.Theoverall microwavecuredspecimensshowedasmall but signicantlylower(17.38 ^ 0.33 VHN) value than the water-bathcured specimens (17.62 ^ 0.78 VHN) for meanhardness P , 0:05: Specimens 160 and 560 W alsoshowed a small but signicantly lower value,17.20 ^ 0.27and17.25 ^ 0.32VHN,respectively.However, specimens 80and240 Whadvalues of17.41 ^ 0.28 and 17.65 ^ 0.27 VHN, and showed nostatistical differenceP . 0:05 when comparedwith the water-bath cured specimens.From the load/displacement curves, the exuralstrengthandexural moduluswerecalculatedbyusingtheequationsofhomogeneousbeamtheory.Fig. 5 shows the exural strength and the displace-ment for the specimens cured by differentmethods. The mean exural strength was found tobe 144 ^ 17 MPa and the mean displacement valuewas 0.83 ^ 0.17 for the specimens cured in water-bath. The results of the statistical analysis ofexural strength and displacement are listed inTables 4 and 5, respectively. ANOVA exhibitedhighly signicant differences between the twoprocessingmethods. Theexural strengthof thespecimenscuredbymicrowaveshowedastrengthof 8992%of water-bathcuredones. Discernibledifferences wereevident inacomparisonof theexural displacement and the exural modulus.Theexuraldisplacementwas2.32.4 mmforthemicrowave cured specimens, which was at least 2.8times that cured in the water-bath. In contrast, theexural modulus was one-third. Examination of Fig.5, Tables 4 and 5 revealed there was littledifferenceamongthemicrowavecuredspecimensFigure4 Domain size distribution for denture baseresins cured by (a) water-bath versus microwavemethods, (b) Microwave power at 80, 160, 240, and560 W, respectively.Table 2 Solubility, porosity, molecular weights, and polydispersity.Water-bath curing Microwave power80 W 160 W 240 W 560 WInsoluble wt% 60.6 (2.2)a65.2 (2.8) 60.0 (7.9) 68.0 (1.8) 62.7 (1.8)Porosity 010% 60% 65% 40% 80%Molecular weights

Mn64400 (1350) 68600 (4600) 59500 (3140) 65300 (2690) 66900 (1230)

Mw171500 (1630) 170300 (4260) 163000 (7120) 154900 (6400) 167200 (8470)

Mw= Mn2.75 2.48 2.74 2.37 2.50

Mp153,500 141,800 140,700 129,400 130,500aThe number inside the parentheses is the standard deviation.C.-P. Lai et al. 138for the mean values of exural strength anddisplacement. The statistical analysis for signi-cance conrms this observation (P . 0:05; as shownin the last two columns of Tables 4 and 5).DiscussionMicrowavepolymerizationisaffectedbytheran-domavailability of benzoyl peroxide, and thevariation in duration andmagnitudeof exothermicreaction, etc.10,11Thehalf-lives of benzoyl per-oxideinitiatorare7.3,1.4 h,and19.8 minat70,85, and 100 8C, respectively.12The higher thepower appliedtothesystem, themoretheresintemperaturerises,13thefaster thebenzoyl per-oxide decomposes, and the more rapid is thepolymerization reaction. In these experiments,thetemperaturesroserapidlyinthecenteroftheresin blocks. Because a microwave oven has almostnothermal inertia, theresponseis effectiveandalmostinstantaneous.Fromthehalf-livesofben-zoyl peroxide, it is clear that the time required forsubstantial conversion of monomer to polymerwould be impractically long attemperatures muchbelow100 8C.14Toavoidporosityespeciallyinthethick parts of dentures, polymerization must beconductedatarelativelylowtemperature(70 8C)and slowly (9 h) in a hot-water bath. Whencomparing both heating methods, quite a largedifference in the curing temperature was observed.Thisstudyhasshownthat microwaveenergycanefciently polymerize denture base polymer. Micro-waveprocessinghasapotentialforsavingagreatamount of time in processing dentures.Microwaves acted initially on the MMA monomersinsidetheresinblocks, thereforeMMAabsorbedmuchenergyinthebeginningof polymerization.Afterinitiationofpolymerizationandaperiodofconversionof MMAmonomers, theresultingexo-thermraised the heating rate.10The standarddeviation of the nal equilibrium temperaturewasless than 4 8C in all cases, as shown in Fig. 1.However, the standard deviation was 13 16 8Cwhen the resin blocks were irradiated at 240 W for26 min, and it was 1117 8C at 80 W for 612 min.Itmeansthatatthepositionofthethermocouplesome resin blocks have absorbed more than averageenergy inthe beginning andthe initial stageofpolymerizationisveryefcient. Theotherblocksmay absorb more energy at a later stage, and thenthere is a rapid rise in the resin temperature and therate of polymerization that continues until sub-stantial conversion of monomer is achieved.Onewouldnormallyexpectthepolymerizationrate and the temperature to fall rapidly with time inthenal stagesduetoadecreaseinthethermalmotion of polar monomers as a result of polymeriz-ation and crosslinking. In the De Clercks studies,10thepolymerdidnotbecomehotinthemicrowaveoven because PMMA has a glass transition tempera-ture Tg at 105 8C. Since the polar ester side groupwas not involved in curing, the dipolar relaxation ofthe cured PMMA did not disappear completely. Therelaxation time of the orientation movement ofsuch groups (b relaxation) is shorter than therelaxationtimeof main-chainsegments(arelax-ation), allowing them to retain their mobility belowTg; sothat theresinblocks couldbeheatedandTable 3 Statistical analysis of Vickers hardness (VHN).Data points(# of specimens)Mean Std. dev. 95% Condence interval Water-bath vs.microwaveLower bound Upper bound P-value P , 0:05Water-bath curing 150 (10) 17.62 0.78 17.49 17.75Microwave power Overall 184 (32) 17.38 0.33 17.33 17.43 1.820 1024Yes80 W 46 (8) 17.41 0.28 17.33 17.50 0.0825 No160 W 46 (8) 17.20 0.27 17.12 17.29 5.070 1024Yes240 W 46 (8) 17.65 0.27 17.57 17.73 0.815 No560 W 46 (8) 17.25 0.32 17.15 17.34 1.961 1023YesFigure5 Flexural strength and exural displacementfor specimens cured in water-bath and microwave oven.Morphology and properties of denture acrylic resins 139maintained at a high temperature that depended onthemicrowavepower.Becauseofthemergingofthe a and the b relaxations in PMMA at frequencies.10 kHz, the critical temperature of PMMA at2.45 GHz is 185 8C.15In these experiments, thenal temperaturewas below185 8C. It suggestedthatthemicrowaveenergyabsorbedbytheresinblocks was in equilibriumwith the energy con-ductedawayfromtheresinblocks.Thisisimport-ant because a form of self-regulation of the curingprogramtakes place. This temperature must becontrolled accurately and the timing must becorrect. Therefore, 7, 10, and 15 min were selectedto cure acrylic resins at 240, 160, and 80 W,respectively. Then the microwave ask was turnedover, and cured for an additional 2 min at 560 W inorder to make the curing complete.Since the initial size of the dispersed graftcopolymer was very small, 0.1 mm, the possibleeffect of the initial size (circle B) on the toughnessof denture resins can be neglected. For the water-bathspecimens,thevolumefractionoftheeffec-tive rubber phaseincreasedupto 10.3%with amean domain size of 0.395 mm. For the microwavecured specimens, the volume fraction of theeffective rubber phase decreased from8.44 to7.50% with an increasing microwave power from 80to560 W. Themeandomainsizealsodecreasedfrom 0.379 to 0.354 mm with a P value of2.65 1029(,0.05) among these four powerlevels. The mean domain size and the volumefractionof therubber phaseareinfavor of thewater-bathmethod.Ahighlysignicantdifferenceinmorphologyindicatedthatitwasimportanttochooseasuitablepowerandcuring(orpolymeriz-ation) time.The results of the temperature measurementandthesolubilityanalysisindicatedthatamicro-waveovenforcuringresinwasmuchfasterthanaconventional water-bath, and the degree of curingalsoincreasedalittle.Thesol partofwater-bathspecimens had the highest

Mwand

Mp; whereas thesol part of the resins after curing at 240 W had thelowest

Mwand

Mp: Itmay be relatedto theweightpercentofsolublepart,39.4fortheformeronesversus 32.0for thelatter. Basedonthelack ofconsistency inthemolecular weight analysis, anadvantage cannotbe claimed for either method ofcuring.Thoughthecuringtimeofresinscouldberemarkably shortened by the use of the microwave,hardness for the microwave cured specimens is veryclosetothat of thewater-bathcuredspecimen.Microwave processing temperatures beyond100.3 8Ccausedvaporizationofthemonomerandproducedporosity. Comparedwiththespecimenscured in the microwave oven, the water-bath curedTable 5 Statistical analysis of displacement.Number ofspecimensDisplacement,mean (mm)Std. dev.(mm)95% Condence interval(mm)Water-bath vs. microwaveLower bound Upper bound P-value P , 0:05Water-bath curing 6 0.83 0.17 0.65 1.01Microwave power Overall 30 2.36 0.35 2.23 2.49 1.576 10217Yes80 W 7 2.33 0.22 2.13 2.54160 W 8 2.34 0.21 2.16 2.51 0.947 No240 W 8 2.35 0.40 2.02 2.69 (Microwave-vs.microwave-)560 W 7 2.43 0.54 1.94 2.93Table 4 Statistical analysis of exural strength.Number ofspecimensMeanstrength (MPa)Std. dev.(MPa)95% Condence interval(MPa)Water-bath vs.micsrowaveLower bound Upper bound P-value P , 0:05Water-bath curing 6 144 17 126 161Microwave power Overall 30 130 10 126 134 0.011 Yes80 W 7 128 9 120 136160 W 8 128 12 118 138 0.861 No240 W 8 132 10 124 140 (Microwave-vs.microwave-)560 W 7 131 11 120 142C.-P. Lai et al. 140resin is considered to have markedly decreasedporosity. Porositybecameinevitablewiththickerspecimens whose voids might be formed by shrink-age in the inner portion. The blocks cured at 240 Wseemedtobesuperiortoothermicrowavecuredblocks becauseof little porosity andtheir suf-ciently cured state.ConclusionsA signicantly large difference in the curingtemperaturewasobserveduponcomparingthesetwo processing methods. This study has shown thatmicrowaveenergycanefcientlypolymerizeandcure denture base polymer fromthe results ofinsolubleweightpercentandthecuringtempera-ture. Highlystatistical differences inmorphologyandexuralpropertieswereevidentinacompari-son of processing methods. The size and the volumefractionof therubber phaseareinfavor of thewater-bath method. However, the amount ofporosityincreasedwithanincreaseinmicrowaveenergy level. Thus, the water-bath cured specimensshowedbetterexuralstrengthandexuralmod-ulus thanthemicrowave-curedspecimens. Therewere no signicant differences in the surfacehardness andthedomainsizedistributionof theeffective rubber phase. It has been stated thatchoice of a suitable power and polymerization timeis important in order to reduce porosity to aminimum level. Microwave processing has a poten-tial for saving time in processing dentures.AcknowledgementsThis work was partially supported by grantVGHNSU88-01fromtheVeterans Affairs Commis-sion, Executive Yuan, ROC, and by grant NSC89-2216-E-110-027 from the National ScienceCouncil, ROC.References1. Craig RG. Restorative dental materials, 7th ed. St Louis: CVMosby Co; 1985. p. 362.2. NishiiM.Studiesonthecuringofdenturebaseresinswithmicrowaveirradiation: withparticular referencetoheat-curing resins. J Osaka Dent Univ 1968;2:2340.3. Kimura H, Teraoka F, Ohnishi H, Saito T, Yato M. Applicationsofmicrowavefordentaltechnique(Part1):dough-formingand curing of acrylic resins. J Osaka Univ Dent Sch 1983;23:439.4. Kimura H, Teraoka F, Saito T. Applications of microwave fordental technique (Part 2): adaptability of cured acrylicresins. J Osaka Univ Dent Sch 1984;23:439.5. Reitz PV, Sanders JL, Levin B. The curing of denture acrylicresinsbymicrowaveenergy:physicalproperties.QuintInt1985;8:54751.6. Peyton FA. Packing and processing denture base resins. J AmDent Assoc 1950;40:5216.7. HaydenWJ.Flexuralstrengthofmicrowave-cureddenturebaseplates. Gen Dent 1986;34:36771.8. Shlosberg SR, Goodacre CJ, Munoz CA, Moore BK, Schnell RJ.Microwave energy polymerization of poly(methyl methacry-late) denture base resin. Int J Prosthodont 1989;2:4538.9. Smith LT, Powers JM, Ladd D. Mechanical properties of newdenture resins polymerized by visible light, heat, andmicrowave energy. Int J Prosthodont 1992;5:31520.10. DeClerckJP. Microwavepolymerizationof acrylicresinsused in dental prostheses. J Prosthet Dent 1987;57:6508.11. Huggett R, Brooks SC, Bates JF. The effect of different curingcycles on levels of residual monomer in acrylic resin denturebase materials. Quint Dent Technol 1984;8:36571.12. OdianG.Principlesofpolymerization, 2nded.NewYork:Wiley; 1981. p. 196.13. ChenM, Siochi EJ, WardTC, McGrathJE. Basicideas ofmicrowave processing of polymers. PolymEngng Sci 1993;33:1092109.14. Al Doori D, Huggett R, Bates JF, Brooks SC. A comparison ofdenture base acrylic resins polymerized by microwaveirradiationandbyconventionalwaterbathcuringsystems.Dent Mater 1988;4:2532.15. ChenM, Siochi EJ, WardTC, McGrathJE. Thedielectricbehaviorofglassyamorphouspolymersat2.45 GHz.PolymEngng Sci 1993;33:111021.Morphology and properties of denture acrylic resins 141