I.K. Dong, DMD'

Associate Professor

Heat-Pressed Ceramics:Technology and Strength

H. Luthy, PhDAssociate Professor

A. Wohtwend, CDT

P. Schärer, DMD, MSProfessor and Chairman

University of Zurich

The flexural strength of a new heat-pressed ceramic material(IPS-Empress) was tneasured before atid after pressing and/or simulated firing treatmetits (eg, veneering, surfacecoloring, glazing). Heat pressing the material significantlyimproved its flexure strength whereas heat treating thematerial alone did not. Additional firings (heat treatments)after heat pressing further increased material strength. Thefinal strength values ranged between 160 and 180 MPa andwere comparable to some other all-ceramic systems. Noclinical implications were drawn from these data. IntJProsthodont 1992;5:9-16.

R ecently, several new all-ceramic crown systemsthat offer good esthetics and simplify fabrica-

tion have been introduced. With these systems itis possible to fabricate single crowns, inlays, andveneers. On vital teeth the advantages of using atranslucent ceramic material with characteristicssimilar to those of natural enamel are particularlyapparent. Castable glass-ceramic and refractory dietechniques' have been judged to meet theserequirements. The disadvantage of the refractorydie system is that the particles are sinteredtogether, resulting in microporosities and inhom-ogeneities between the ceramic particles. It is wellknown that these microporosities can initiate crackpropagation, leading to early failure of such all-ceramic restorations,^ Many castable glass-ceramicsystems have recently been described (Dicor,'Cera Pearl,"-^ and Olympus Castable Ceramicsi-) in

'Presently at Wonkwang University College of Dentistry. Iri.South Korea.

Reprint requests: H. Luthy, PhD, Associate Professor, Depart-ment of Fixed and Removable Prosthodontics and DentalMaterials, Dental Institute, University of Zuricb, Plattenstrasse11, CH-8028 Zuricb, Switzerland.

which the porosities can be reduced to a minimumby the casting technique. However, the castingprocess is followed by a ceramming procedure(controlled crystallization using an appropriate heattreatment), resulting in additional ceramic shrink-age.' To overcome this disadvantage, a heat-presstechnique (IPS-Empress) was developed in 1983 bythe Department of Fixed and Removable Prostho-dontics and Dental Materials at the University ofZtjrich, Since 1986 the development has pro-ceeded in conjunction with a dental company (Ivo-clar, Schaan, Liechtenstein),

A patented heat-press technique was firstdescribed in 1936 for the construction of ceramiccomplete dentures." However, to the authors'knowledge, a commercially available furnace wasnever developed. In 1969, Dröge^ described aceramic press technique based on the hot-pressresin technique. In his work he was using two-partflasks. Instead of stone, the wax crown wasinvested in a refractory die material. After openingthe two halves of the flask and removing the waxwith boiling water, the cavity was filled withceramic powder and pressed together in the hotstage. Improving Droge's technique, McPhee'" wasable to produce complete-coverage metal-ceramic

Volume 5, Number I, 1992 The International Journal of Prosthodontics

Heal-Pressed Cera : Technology änd Strength Dong et

Fig 1 (Lefíj The IPS-Empress EP ^furnace is especially designed '°' ^^system. Displays show different cacteristics ot the press turnace (s '^ " " *temperature 200X to 8S0'C; heat ngrate S 'C to BO' C; press temperature2OO''C to 1 200°C- vacuum on-on; pres-sure holding times, N 1 to 30 minutes,H 1 to 60 minutes); and the duration ofthe whole press procedure.

Fig 2 (Right)Ttie inlays must tie waxedand placed on a specially designedcylindrical crucible tormer.

restorations that accurately duplicated occlusal sur-faces. The glass-ceramic ingot used in this paper,called IPS-Empress, is precerammed and preco-lored." This system is designed for the fabricationof single units, inlays, and veneers. Natural teethand IPS-Empress crowns appear to be similar, andtherefore good esthetic results can be achievedwith good marginal integrity. For the inlay tech-nique, the IPS-Empress system offers easy handlingand good colormatching because of the similaritybetween the translucency of the base material andthe tooth structure.

The first part of this paper describes the material,its development, and the fabrication technique; thesecond part discusses the material and its strength.Specifically, the purpose was to determine whetherpressing and/or heat treatment (simulated firing)affect the flexural strength of the ceramic.

Materials and Fabrication Procedures

The glass-ceramic material developed for thistechnique is basically a feldspathic porcelain hav-ing the following composition in wt%: 63 SiO^,17.7 AI,O„ n .2 KjO, 4,6 Na^O, 0,6 B^O,, 0,4CeOj, 1,6 CaO, 0,7 BaO, 0.2 TiO^, The crystallinepart of the ceramic consists of leucite crystals,'^

A special furnace (Empress EP 500) was designedfor this system. The pressing furnace contains anenlarged heat dome, a pneumatic pressure system,a reducing valve, and a manometer to control the

pressure; an inductive displacement transducer ismounted on top of the furnace and is connectedto the pneumatic plunger (Eig 1), The crown orinlay was waxed and placed on a specially designedcylindrical crucible former ¡Eig 2) and investedusing a phosphate-bonded investment. The moldwas heated in a burnout furnace to 850°C, Thecylindrical opening into the mold was filled with aceramic ingot and an Al^Oi pushing rod. Theassembly was then placed into the preheated fur-nace (Eigs 3 and 4). The automatic press furnacemakes it possible to change the foiiowing param-eters:1, The rate of temperature increase can be varied

from 5°C to SO^C/min,2, The furnace can be heated to 1,200°C,3, The holding time at the final temperature can

be varied from 0 to 60 minutes.4, If the pneumatic plunger does not continue

to move more than 0,3 mm/min, the pressuremaintenance time wili be activated. A pres-sure maintenance of 1 to 4 minutes is nec-essary depending on the thickness of thecavity that bas to be filled; the time can bevaried from 1 to 20 minutes,

5, The press procedure is performed in a vac-uum, and the beginning and ending poinis fortbe vacuum application can be progran-jnied.

When the start button is pushed, the fumare heatsup automatically to the programmed press tem-perature (1,150°C), After a 20-minute holding time

I of Prosthodontics 10 Volumes, Number 1, 1992

Dong et i • Technology anii Strength

Fig 3 Ceramic ingots are preshaded ancJ preoerammed. Forthe inlay technique, translucent matehal is used.

Fig 4 After filling the cylindrical opening with an already pre-heated ceramic ingot and an M^O^ pushing rod, the cast mustbe placed into the preheated Empress furnace.The aluminiumoxide pusfiing rod is used to transfer the pressure to theceramic mate nal.

Fig 5 After the press procedure, the inlays are devested andprepared tor further treatments.

at this temperature the press procedure was acti-vated and the then-plastic glass-ceramic materialwas pressed (0.3 to 0.4 MPa) into the mold. Themold was filled with the glass-ceramic material andthe furnace stopped automatically. The ceramicrestorations were devested and prepared for fur-ther treatments (Fig 5).

Two methods may be used to obtain the desiredshade.1. Surface coloration; A strongly pigmented sur-

face color is applied until the desired color isestablished. The color pigments are coveredwith a surface glaze that consists of the samecomponents as the hase material. The surface

Volume 5, Number 1, 1992 Journal of Proslli odor tics

Heat-Pressed Ceramics: Technology and Strength Dong el al

Fig 6 The occlusal surface and the inner surface can becovered with a thin layer of suriace stains. The occlusal sur-face will be covered with a glaze.

Figs 7a and 7b Inlays can be made more simply and have good marginal integrity when placed.

Figs 8a and 8b Examples of the surface staining teohnique (Iett) and the veneer technique (right). A complete waxing must bemade by both techniques. When using the veneer technique, the enamel pan is cut back and the dentin structure ''. pressed.The base of the precolored ingot is adjusted to the Ivoclar and Vita shade guide. The final form is established u ing enamelporcelain. Using the surface staining technique with a strongly pigmented surface color, the crown can be charac,Prized untilthe desired coicr is established. These color pigments wili be covered in two additionai firings with a glazing material havingthe same components as the base material.

[jf Prosthodontics • 5, Number 1, 1992

Dong el al L Technology and Strength

Fig 9 Cress section (18X| ol ttie surface colored crownshown in Fig 8b. The surface colors are covered with a glazecompound in a thickness of about 50 fim.

Fig 10 Cross section (18X) of ttie veneered crown shewnin Fig 8b, Many metai oxides that are responsible for ttie colorof the base material are visible.

glaze is applied in a thickness of approxi-mately 50 to 60 ;im (Figs 6 through 10).

2. Veneer technique: A complete-con tour waxpattern is made and then a portion is removedwhere enamel translticency is desired. Theremaining dentin structure is then investedand heat pressed. After devesting and sprueremoval, the dentine structure can be char-acterized using surface colors. Finally, enamelporcelain is applied to complete the desiredform.

Flexural Strength

The 5econd part of the study determined theinfluence of pressing and/or heat treatment (sim-ulated firing) on the flexure strength of the glass-ceramic material. The IPS-Empress material (63-PK/5T II, Ivodar) was used to make (2 X 4 X 14)-mm flexure strength specimens. Eight groups (n =15) of bars were prepared according to the treat-ments listed ¡n Tables 1 and 2. For groups 1 through3, the cylindrical ingots (10,5 mm in diameter and7.5 mm in height) were not heat pressed but weremachined in the as-received state (group 1 ) or afterheat treatments (groups 2 and 3), A slow-speeddiamond wire saw (Well, W Ebner, Switzerland)was used to machine the samples. The bar surfaceswere ground flat with a rectangular cross section,using a metallurgical grinding machine (Knuth-Rotor, Struers, Denmark) with SiC discs (grit size320).

For the remaining groups (4 through 8) the ingotswere first heat pressed into bars of the requiredgeometry. The bars were then heat treated (Tables1 and 2) and ground to the final form. The detailed

procedure was as follows: wax bars were made byinjecting casting wax (S-U-Tauchwachs, SchülerDental, Germany) into a silicone mold. Sprue form-ers (2 mm in diameter] were then attached to aspecially designed crucible former and invested

Table 1 Pressing and Firing Conditions of EachGroup

Group no.

123

456

7

8

Pressing

_--

-1--1--1-

-1-

-1-

Firing conditions

No firing (ingot)Simulated press firingSimulated press and veneerfiring

Practical pressingSimuiated veneer firingSimjiated veneer and giazefiring

Simulated surface coloringand giaze firing

Simulated veneer, surfaceccloration, anO glaze firing

Table 2 Firing Conditions of the Empress System

Firings

Press firing'Veneer firingSurface coloringGiaze firingFinal glaze firing

Temperature(°C)

850-1150400- 920400- 850400- 870iOO- 890

Heatingrate

CC/min)

6060606060

Holdingtime(min)

20

1

1

All firings are under vacuum except tinal glaze firing.•Pressing atter 20-minute t¡oiaing time.

Volume 5, r^umber I. 1992 13 The International lournal of Prostliodonlics

Heaf-Presíed Ceramics: Technology and Strength Dong et al

Table 3 Flexure Strength Results

Grotjp no.Mean strengtti

(MPa)

74 (10)91 (11)82 (11)

126 (15)182 (26)175 (32)159 (28)171 (29)

Standard deviations are in parentheses.

using a phosphate-bonded investment (GilvestEHT/superfine, BN 078664, Giulini Chemie, Ger-many), A total of 200 g of investment powder and48 mL of l iquid were mixed under vacuum,poured, and allowed to set for 15 minutes. Aftersetting, the investment was removed from the cru-cible former and heated in a furnace (MicroTherm,Fink and Hilburg, Germany) from room tempera-ture to 900°C, Concurrently, the press machinewas preheated from room temperature to 85O''Calong with an ingot of the glass-ceramic materialand the pressing rod. The casting ring was thentransferred from the burnout furnace to the heat-press machine and the pressing program wasstarted. The machine stopped automatically afterthe press procedure. After that operation, theinvestment was allowed to air cool and the spec-imen was devested using an air abrasion unit (Sand-master, Wulsag, Switzerland) with a 45 fim-specialglass (Strahlperlen 45P, Benzer Dental, Switzer-land), The sprue was separated from the bars usinga diamond cutting wheel saw (Vari/Cut C-50,Leco), The bar surfaces were then ground flat inthe previously described manner.

The following group-specific heat treatmentswere performed (Table 2):Group 1 No treatment was used.Group 2 The ingots were fired on a flat refractory

tray for 30 minutes (Table 2) at the usualpressing temperature but without press-ing (simulated press firing).

Grotip 3 After a simulated press firing, the ingotswere placed on a flat refractoty tray in aporcelain furnace (Programat P90, Ivo-clar) and subjected to two simulatedveneer firings.

Group 4 There was no heat treatment after press-ing.

Group 5 The heat-pressed bars were subjected totwo simulated veneer firing cycles.

Group 6 The pressed bars were subjected to thesame heat treatment as those in group5, with three simulated glaze firings andone simulated final glaze firing.

Group 7 The pressed specimens were subjectedto three simulated surface coloration andglaze firings and to one simulated finalglaze firing.

Group 8 The pressed bars were stibjected to twosimulated veneer firings, three surfacecoloration firings, three glaze firings, andone final glaze firing.

A three-point bending test was conducted using auniversal testing machine (RM 50, Schenck-Trebel,Germany) at a cross-head speed of 0.5 mm/min.The data were analyzed using ANOVA, Scanningelectron microscope (SEM) analyses (Model 1810,Amray) were done on etched (0,2% hydrofluoricacid/5 min) fracture surfaces to relate the revealedmicrostructure (leucite size, shape, and distribu-tion) to the mechanical behavior of the ceramicmaterial.

Fig 11 Flexure strength comparison of Empress after sim-ulated press and veneer firing with the ingot.

Fig 12 Flexure strength comparison of Empress after press-ing and simuiated veneer firing with the ingot.

The Internationai lournal of FrOBthodontics 14 Volume 5, Number 1, 1992

Dong et . Heat-Pressed Ceramics. Technology and Sirenglli

Results

The experimental strength restilts of the eightgroups—means and standard deviations (SD)expressed in MPa-are listed in Table 3, Eigure 11sbows the strength comparison among groups 1through 3, A statistical evaluation (comparinggroups 3 and 2, groups 3 and 1, and groups 2 and1) indicates that heat treating the material througha simulated firing in the absence of pressing doesnot significantly (P > ,001) affect its strength,

Eigure 12 shows the strength comparison amonggroups 1, 4, and 5, where the influence of pressingis taken into account. The statistical evaluation(comparing groups 4 and 1, groups 4 and 5, andgroups 1 and 5) indicates that heat pressing thematerial (group 4) does significantly (P < ,001)increase its strength. Subseqtient heat treatment(group 5) also significantly increases strength.

There was no significant difference in strengthbetween the four heat-treatment techniques(groups 5 through 8) used after the material hadbeen beat pressed (Eig 13),

Discussion

This study indicates that beat pressing is not onlya method of shape forming, but it also bas a pos-itive influence on tbe strength of the ceramic mate-rial even though tbe applied pressure of 0.4 MPais quite moderate. If pressing is additionally fol-lowed by firing, the strength of the ceramic signif-icantly increased and reached a high mean valueof 182 MPa (group 5). Consequently, the combi-nation of heat pressing and heat treatment (simu-

Fig 13 Flexure strength ot Empress after pressing and sim-ulatea tiring cycles.

lated firing) produced the greatest strength.Evaluation of the microstructure provided addi-

tional information. The SEM photograph (Eig 14) ofan as-received Empress ingot shows the leucitecrystals grouped in cluster forms and located inlarge areas of the glassy phase. In contrast, tbeheat-pressed material exhibited much better leu-cite dispersion (Eig 15), One possible explanationis that the clusters were separated during tbe forc-ing of the softened ingot through the small-diam-eter sprue. The crystals (average sÍ2e, 1 //m) wereconsequently distributed individually and in a morehomogeneous manner in the glassy pbase. It is rea-sonable to suppose that the improved strength oftbe pressed ceramic material resulted from the bet-ter dispersion of the leucite crystals in the glassmatrix.

The additional strength resulting from the sim-ulated firing was reportedly caused by the differentthermal expansion coefficients (a) of glass and crys-

Fig 14 SEM photograph ot the tracture surtace of an Fig 15 SEM photograph ol the fracture surface of a pressedEmpress ingot. 'est bar.

Volumes, Number 1, 1992 15 The International Journal of Prosthodontic

: TechrmlOBV arid Suenglh Dong et :

tals.'^-" Leucite is a high-expansion material witha nonlinear dilation curve.'" If measured between20°C and SOO C its value is 17 X 10V°K; in thetemperature interval of 500°C to 625°C its valuerises to 65 X l O V K . Calculated between 20°Cand 625°C, a has a value of 27 X l O V ' K . Thethermal expansion coefficient of glass is far lower(in order of 10 X !O'V°K). It is known'^'Mhat, oncooling, the presence of a high-expansion coeffi-cient crystal in a matrix of a lower-expansion coef-ficient glass generates tangential compressivestresses in the glass and radial stresses within thecrystal. The compressive stresses have the effectof increasing the specimen's resistance to surfacedamage by opposing the formation of surfacecracks.

All examined firing conditions (Table 2] rangingbetween ßSCC and 920°C have the same bene-ficial effect on flexure strength material when colorwas previously pressed (Fig 13). The values rangebetween 160 and 182 MPa and are close to thestrength results of some other all-ceramic systems(eg, 180 MPa for Hi-Ceram core material from VitaZahnfabrik, Bad Sackingen, Germany) tested underthe same conditions.'^ However, no conclusionwith respect to clinical implication can be drawnfrom these data.

Summary and Conclusions

This paper described a new technique of fabri-cating all-ceramic inlays and crowns using a heat-pressed glass-ceramic material (IPS-Fmpress). Theeffect of heat pressing and subsequent heat treat-ment upon the flexural strength of the material wasstudied and the following conclusions were made:1. Heat pressing of the material as received from

the manufacturer significantly improved itsstrength (from 74 to 126 MPa].

2. Subsequent heat treatments (simulatedveneer firings, surface coloration firings, orglaze firings) further increased materialstrength to values ranging from 160 to 182MPa.

3. Heat treatment in the absence of pressing hadno effect on strength.

AcknowledgmentsThe authors would like to thank Dr Loganathan Vijayan, vis^

iting from National Hospital of Singapore, for hi5 Aliviehelp with the manuscript. We are grateful as well tor me J Jport from itaff members at the Department ol C'own di>uBridge Prosthodonti« and Dental Materials, Dental Instnuie,University of Zurich. Namely, the assistance oí Mr Olivier Loelfel and Mrs Renate Loeffler wilh materials testing and i t Mwork, respectively, was very much appreciated.

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The I mer nation a al oí Prostliodontics 16 . 5, Number I, 1992