Pre-sintered blank for dental purposes

Abstract

Pre-sintered blanks based on lithium disilicate glass ceramic are described which are suitable in particular for the preparation of dental restorations.

Claims

1. Pre-sintered blank for dental purposes based on lithium disilicate glass ceramic, wherein the blank has a relative density of from 60 to 90%, relative to the true density of the glass ceramic, and wherein the blank has a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 m, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 20 MPa to 200 MPa and (b) the obtained glass powder green compact is heat-treated for 2 to 120 min at a temperature (i) which temperature is at least 500 C., and (ii) which temperature lies in a temperature range which extends over at least 30 C. and in which temperature range the relative density varies by less than 2.5%.

2. Pre-sintered blank according to claim 1, which consists essentially of lithium disilicate glass ceramic.

3. Pre-sintered blank according to claim 1, wherein the glass ceramic includes lithium disilicate as main crystal phase.

4. Pre-sintered blank according to claim 3, wherein the glass ceramic contains more than 10 vol.-% lithium disilicate crystals.

5. Pre-sintered blank according to claim 3, wherein the glass ceramic contains more than 20 vol.-% lithium disilicate crystals.

6. Pre-sintered blank according to claim 1, wherein the lithium disilicate glass ceramic contains at least one of the following components: TABLE-US-00005 Component wt. % SiO.sub.2 50.0 to 80.0 Li.sub.2O 6.0 to 20.0 Me(I).sub.2O 0 to 10.0 Me(II)O 0 to 12.0 Me(III).sub.2O.sub.3 0 to 8.0 Me(IV)O.sub.2 0 to 8.0 Me(V).sub.2O.sub.5 0 to 8.0 Me(VI)O.sub.3 0 to 8.0 nucleating agent 0 to 8.0, wherein Me(I).sub.2O is selected from Na.sub.2O, K.sub.2O, Rb.sub.2O, Cs.sub.2O or mixtures thereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures thereof, Me(III).sub.2O.sub.3 is selected from Al.sub.2O.sub.3, La.sub.2O.sub.3, Bi.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3 and mixtures thereof, Me(IV)O.sub.2 is selected from ZrO.sub.2, TiO.sub.2, SnO.sub.2, GeO.sub.2 and mixtures thereof, Me(V).sub.2O.sub.5 is selected from Ta.sub.2O.sub.5, Nb.sub.2O.sub.5 and mixtures thereof, Me(VI)O.sub.3 is selected from WO.sub.3, MoO.sub.3 and mixtures thereof, and nucleating agent is selected from P.sub.2O.sub.5, metals and mixtures thereof.

7. Pre-sintered blank according to claim 1, which has at least two areas which differ by their coloration or translucence.

8. Pre-sintered blank according to claim 7, which has at least two layers which differ by their coloration or translucence.

9. Pre-sintered blank according to claim 1, which has a holder for securing the blank to a processing device.

10. Pre-sintered blank according to claim 1, which has an interface for connection to a dental implant.

11. Pre-sintered blank according to claim 10, wherein the interface comprises a recess for connection to the dental implant.

12. Pre-sintered blank according to claim 1, wherein the blank has a relative density of from 62 to 88%, relative to the true density of the glass ceramic.

13. Pre-sintered blank according to claim 1, wherein the blank has a relative density of from 65 to 87%, relative to the true density of the glass ceramic.

14. Pre-sintered blank according to claim 1, wherein the lithium disilicate glass ceramic contains at least one of the following components: TABLE-US-00006 Component wt. % SiO.sub.2 50.0 to 80.0 Li.sub.2O 6.0 to 20.0 Me(I).sub.2O 0.1 to 10.0 Me(II)O 0.1 to 12.0 Me(III).sub.2O.sub.3 0.1 to 8.0 Me(IV)O.sub.2 0.1 to 8.0 Me(V).sub.2O.sub.5 0.1 to 8.0 Me(VI)O.sub.3 0.1 to 8.0 nucleating agent 0.1 to 8.0, wherein Me(I).sub.2O is selected from Na.sub.2O, K.sub.2O, Rb.sub.2O, Cs.sub.2O or mixtures thereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures thereof, Me(III).sub.2O.sub.3 is selected from Al.sub.2O.sub.3, La.sub.2O.sub.3, Bi.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3 and mixtures thereof, Me(IV)O.sub.2 is selected from ZrO.sub.2, TiO.sub.2, SnO.sub.2, GeO.sub.2 and mixtures thereof, Me(V).sub.2O.sub.5 is selected from Ta.sub.2O.sub.5, Nb.sub.2O.sub.5 and mixtures thereof, Me(VI)O.sub.3 is selected from WO.sub.3, MoO.sub.3 and mixtures thereof, and nucleating agent is selected from P.sub.2O.sub.5, metals and mixtures thereof.

15. Pre-sintered blank according to claim 1, which has a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 m, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 40 to 120 MPa and (b) the obtained glass powder green compact is heat-treated for 5 to 60 min at a temperature (i) which temperature is at least 540 C., and (ii) which temperature lies in a temperature range which extends over at least 50 C. and in which temperature range the relative density varies by less than 2.0%.

16. Pre-sintered blank according to claim 1, which has a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 m, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 50 to 100 MPa and (b) the obtained glass powder green compact is heat-treated for 10 to 30 min at a temperature (i) which temperature is at least 580 C., and (ii) which temperature lies in a temperature range which extends over at least 70 C. and in which temperature range the relative density varies by less than 1.5%.

17. Process for the preparation of dental restorations, in which (i) the pre-sintered blank based on lithium di silicate glass ceramic according to claim 1 is shaped by machining to form a precursor of the dental restoration, (ii) the precursor is substantially dense-sintered in order to produce the dental restoration, and (iii) optionally the surface of the dental restoration is provided with a finish.

18. Process according to claim 17, in which the machining is carried out with computer-controlled milling and/or grinding devices.

19. Process according to claim 17, in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, (i) wherein the temperature of the heat treatment is at least 500 C., and (ii) wherein the temperature of the heat treatment lies in a temperature range which extends over at least 30 C. and in which temperature range the relative density varies by less than 2.5% and is carried out in order to obtain the pre-sintered blank based on lithium disilicate glass ceramic.

20. Process according to claim 17, in which the dental restorations are selected from crowns, abutments, abutment crowns, inlays, onlays, veneers, shells, bridges and overstructures.

21. A method of using the blank according to claim 1 to prepare dental restorations comprising crowns, abutments, abutment crowns, inlays, onlays, veneers, shells, bridges and overstructures.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages, details and features emerge from the following description of an exemplary embodiment in conjunction with the drawings, in which:

(2) FIG. 1 illustrates the enlargement factor plotted against the temperature applied.

(3) FIG. 2 illustrates the phases usually passed through during heat treatment of a glass powder green compact.

(4) FIG. 3 illustrates a blank which has at least two areas which differ by their coloration or translucence.

(5) FIG. 4 illustrates a blank with a holder attached to it.

(6) The pre-sintered blank according to the invention for dental purposes is characterized in that it

(7) is based on lithium disilicate glass ceramic and

(8) has a relative density of from 60 to 90%, in particular 62 to 88% and preferably 65 to 87%, relative to the true density of the glass ceramic.

(9) The relative density is the ratio of the density of the pre-sintered blank to the true density of the glass ceramic.

(10) The density of the pre-sintered blank is determined by weighing it and ascertaining its volume geometrically. The density is then calculated according to the known formula
density=mass/volume.

(11) The true density of the glass ceramic is determined by grinding the pre-sintered blank to a powder with an average particle size of from 10 to 30 m, in particular of 20 m, relative to the number of particles and ascertaining the density of the powder by means of a pycnometer. The determination of the particle size was carried out by means of laser diffraction in accordance with ISO 13320 (2009) with the CILAS Particle Size Analyzer 1064 from Quantachrome GmbH & Co. KG.

(12) It has surprisingly been found out that not only can the blank according to the invention be machined dry in a simple way, but it can also be prepared at significantly different pre-sintering temperatures, without this resulting in a substantial change in the shrinkage which occurs during a subsequent dense sintering. The enlargement factor taking into consideration the shrinkage that occurs can thus be determined very precisely. These advantageous properties are clearly to be attributed to the particular behaviour of lithium disilicate glass ceramic which was pre-sintered to the relative densities given above.

(13) It is further preferred that the blank consists substantially of lithium disilicate glass ceramic. Particularly preferably, the blank consists of lithium disilicate glass ceramic.

(14) The glass ceramic includes lithium disilicate as main crystal phase in a preferred embodiment. The term main crystal phase denotes the crystal phase which has the highest proportion by volume compared with other crystal phases. In particular the glass ceramic contains more than 10 vol.-%, preferably more than 20 vol.-% and particularly preferably more than 30 vol.-% lithium disilicate crystals, relative to the total glass ceramic.

(15) The lithium disilicate glass ceramic contains SiO.sub.2 and Li.sub.2O, preferably in a molar ratio in the range of from 1.75 to 3.0, in particular 1.8 to 2.6.

(16) In a further preferred embodiment, the lithium disilicate glass ceramic contains at least one of the following components:

(17) TABLE-US-00001 Component wt. % SiO.sub.2 50.0 to 80.0 Li.sub.2O 6.0 to 20.0 Me(I).sub.2O 0 to 10.0, in particular 0.1 to 10.0 Me(II)O 0 to 12.0, in particular 0.1 to 12.0 Me(III).sub.2O.sub.3 0 to 8.0, in particular 0.1 to 8.0 Me(IV)O.sub.2 0 to 8.0, in particular 0.1 to 8.0 Me(V).sub.2O.sub.5 0 to 8.0, in particular 0.1 to 8.0 Me(VI)O.sub.3 0 to 8.0, in particular 0.1 to 8.0 nucleating agent 0 to 8.0, in particular 0.1 to 8.0 wherein Me(I).sub.2O is selected from Na.sub.2O, K.sub.2O, Rb.sub.2O, Cs.sub.2O or mixtures thereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures thereof, Me(III).sub.2O.sub.3 is selected from Al.sub.2O.sub.3, La.sub.2O.sub.3, Bi.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3 and mixtures thereof, Me(IV)O.sub.2 is selected from ZrO.sub.2, TiO.sub.2, SnO.sub.2, GeO.sub.2 and mixtures thereof, Me (V).sub.2O.sub.5 is selected from Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, V.sub.2O.sub.5 and mixtures thereof, Me(VI)O.sub.3 is selected from WO.sub.3, MoO.sub.3 and mixtures thereof, and nucleating agent is selected from P.sub.2O.sub.5, metals and mixtures thereof.

(18) Na.sub.2O and K.sub.2O are preferred as oxides of monovalent elements Me(I).sub.2O.

(19) CaO, MgO, SrO and ZnO are preferred as oxides of divalent elements Me(II)O.

(20) Al.sub.2O.sub.3, La.sub.2O.sub.3 and Y.sub.2O.sub.3 are preferred as oxides of trivalent elements Me(III).sub.2O.sub.3.

(21) ZrO.sub.2, TiO.sub.2 and GeO.sub.2 are preferred as oxides of tetravalent elements Me(IV)O.sub.2.

(22) Ta.sub.2O.sub.5 and Nb.sub.2O.sub.5 are preferred as oxides of pentavalent elements Me(V).sub.2O.sub.5.

(23) WO.sub.3 and MoO.sub.3 are preferred as oxides of hexavalent elements Me(VI)O.sub.3.

(24) P.sub.2O.sub.5 is preferred as nucleating agent.

(25) The lithium disilicate glass ceramic preferably contains colorants and/or fluorescent agents.

(26) Examples of colorants and fluorescent agents are inorganic pigments and/or oxides of d- and f-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr, Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids, e.g. of Ag, Au and Pd, can also be used as colorants and in addition can also act as nucleating agents. These metal colloids can be formed e.g. by reduction of corresponding oxides, chlorides or nitrates during the melting and crystallization processes. For example, doped spinels, zircon silicate, stannates, doped corundum and/or doped ZrO.sub.2 are used as inorganic pigments.

(27) The blank according to the invention preferably has at least two areas, in particular layers, which differ in terms of their coloration or translucence. FIG. 3 shows a blank of at least two areas that differ in terms of coloration or translucence. The blank preferably has at least 3 and up to 10, particularly preferably at least 3 and up to 8, and even more preferably at least 4 and up to 6 areas, in particular layers, differing in coloration or translucence. The imitation of natural tooth material is very successful precisely because of the presence of several differently coloured areas, in particular layers. It is also possible that at least one of the areas or of the layers has a colour gradient to ensure a continuous colour transition.

(28) In a further preferred embodiment, the blank according to the invention has a holder for securing it in a processing device. In another preferred embodiment, the blank according to the invention has an interface for connection to a dental implant. FIG. 4 shows a blank attached to a holder. The blank also includes an interface for connection to a dental implant.

(29) The holder allows the blank to be secured in a processing device, such as in particular a milling or grinding device. The holder is usually in the form of a pin and preferably consists of metal or plastic.

(30) The interface ensures a connection between an implant and the dental restoration fitted thereon, such as in particular an abutment crown, which has been obtained by machining and dense sintering of the blank. This connection is preferably rotationally fixed. The interface is present in particular in the form of a recess, such as a bore. The specific geometry of the interface is usually chosen depending on the implant system used in each case.

(31) The invention also relates to a process for the preparation of the blank according to the invention, in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, wherein the temperature of the heat treatment (i) is at least 500 C., in particular at least 540 C. and preferably at least 580 C., and (ii) lies in a range which extends over at least 30 C., in particular at least 50 C. and preferably at least 70 C. and in which the relative density varies by less than 2.5%, in particular less than 2.0% and preferably less than 1.5%.

(32) In stage (a), lithium silicate glass in powder or granulate form is pressed to form a glass blank.

(33) The lithium silicate glass employed is usually prepared by melting a mixture of suitable starting materials, such as carbonates, oxides, phosphates and fluorides, for 2 to 10 h at temperatures of in particular from 1300 to 1600 C. To achieve a particularly high homogeneity, the obtained glass melt is poured into water in order to form a glass granulate, and the obtained granulate is then melted again.

(34) The granulate is then comminuted to the desired particle size and in particular ground to powder with an average particle size of <100 m, relative to the number of particles.

(35) The granulate or powder is then, optionally together with added pressing auxiliaries or binders, usually placed in a compression mould and pressed to form a glass blank. The pressure applied lies in particular in the range of from 20 to 200 MPa. Uniaxial presses are preferably used for the pressing. The pressing can in particular also be isostatic pressing, preferably cold isostatic pressing.

(36) Through the use of glass powders or glass granulates with different coloration or translucence, glass blanks can be produced which have differently coloured or differently translucent areas and in particular layers. For example, differently coloured powders or granulates can be arranged on top of one another in a compression mould, with the result that a multi-coloured glass blank is produced. The multiple colours make it possible to a great extent to give the finally prepared dental restorations the appearance of natural tooth material.

(37) In stage (b), the obtained uni- or multi-coloured glass blank is subjected to a heat treatment in order to bring about the controlled crystallization of lithium disilicate and thus the formation of a lithium disilicate glass ceramic as well as the pre-sintering. The heat treatment takes place in particular at a temperature of from 500 C. to 900 C., preferably from 540 to 900 C. and particularly preferably from 580 to 900 C. The heat treatment is carried out in particular for a period of from 2 to 120 min, preferably 5 to 60 min and particularly preferably 10 to 30 min.

(38) The temperature range (b)(ii) describes a range in which, despite a change in temperature, the relative density hardly changes. This range is therefore also called plateau in the following. The variation in the relative density possible in this range is calculated in % from the maximum and minimum value of the relative density in the range by
(maximum valueminimum value)/maximum value100

(39) It has surprisingly been shown that during its production and pre-sintering in particular temperature ranges lithium disilicate glass ceramics display essentially no change in the relative density and thus in the linear shrinkage and the enlargement factor during the dense sintering. These ranges are recognizable as plateaus in the graphic representation of relative density, linear shrinkage or enlargement factor against the temperature. Accordingly, properties of the blank that are important for the accuracy of fit of the later dental restoration are essentially not dependent on the temperature in this range. The result of this is the important practical advantage that the blank tends to be unsusceptible e.g. to temperature fluctuations or temperature gradients in the sintering furnace, as long as the temperature is in the plateau range.

(40) According to the invention, therefore, pre-sintered blanks which are obtainable by the process according to the invention are particularly preferred.

(41) Particularly preferred are blanks according to the invention which have a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 m, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 20 to 200 MPa, preferably 40 to 120 MPa and particularly preferably 50 to 100 MPa and (b) the obtained glass powder green compact is heat-treated for 2 to 120 min, preferably 5 to 60 min and particularly preferably 10 to 30 min at a temperature which (i) is at least 500 C., in particular at least 540 C. and preferably at least 580 C., and (ii) lies in a range which extends over at least 30 C., in particular at least 50 C. and preferably at least 70 C. and in which the relative density varies by less than 2.5%, in particular less than 2.0% and preferably less than 1.5%.

(42) FIG. 2 illustrates the phases usually passed through during heat treatment of a glass powder green compact by plotting the enlargement factor against the temperature for a green compact with a composition according to Example 2. In Phase I, up to about 500 C., the heating and the removal of any binder present take place. In Phase II, from about 500 to 600 C., sintering and crystallization take place, and in Phase III, the plateau, from about 600 to about 850 C., there is a pre-sintered blank according to the invention based on lithium disilicate glass ceramic. Then, in Phase IV, from about 850 to about 950 C., the dense sintering of the blank takes place.

(43) The pre-sintered blank according to the invention is preferably present in the form of blocks, disks or cylinders. In these forms, a further processing to form the desired dental restorations is particularly easy.

(44) The pre-sintered blank is further processed in particular to form dental restorations. The invention therefore also relates to a process for the preparation of dental restorations, in which

(45) (i) the pre-sintered blank according to the invention based on lithium disilicate glass ceramic is shaped by machining to form a precursor of the dental restoration,

(46) (ii) the precursor is substantially dense sintered in order to produce the dental restoration, and

(47) (iii) optionally the surface of the dental restoration is provided with a finish.

(48) In stage (i), the machining is usually carried out by material removal processes and in particular by milling and/or grinding. It is preferred that the machining is carried out with computer-controlled milling and/or grinding devices. Particularly preferably, the machining is carried out as part of a CAD/CAM process.

(49) The blank according to the invention can be machined very easily in particular because it is open-pored and has low strength. It is particularly advantageous that it is not necessary to use liquids during the grinding or milling. In contrast to this, so-called wet-grinding processes are often necessary for conventional blanks.

(50) The machining is usually carried out in such a way that the obtained precursor represents an enlarged form of the desired dental restoration. The shrinkage occurring during the subsequent dense sintering is thereby taken into consideration. The blank according to the invention has the particular advantage that the enlargement factor to be applied to it can be determined very precisely. The enlargement factor is the factor by which the precursor has to be ground or milled enlarged out of the pre-sintered blank in order that after the dense sintering the obtained dental restoration has the desired dimensions.

(51) The enlargement factor F.sub.v, the relative density .sub.r and the remaining linear shrinkage S can be converted into each other as follows:
S=1.sub.r.sup.1/3
F.sub.v=1/(1S)

(52) In a preferred embodiment, the blank produced according to the above-described process according to the invention is used as pre-sintered blank.

(53) In stage (ii) the obtained precursor is substantially dense-sintered in order to produce the dental restoration with the desired geometry.

(54) For the dense sintering, the precursor is preferably heat-treated at a temperature of from 700 to 1000 C. The heat treatment usually takes place for a period of from 2 to 30 min.

(55) After the dense sintering, there is a dental restoration based on lithium disilicate glass ceramic in which lithium disilicate preferably forms the main crystal phase. This lithium disilicate glass ceramic has excellent optical and mechanical properties as well as a high chemical stability. Dental restorations which meet high demands can thus be prepared with the process according to the invention.

(56) The dental restorations are preferably selected from crowns, abutments, abutment crowns, inlays, onlays, veneers, shells and bridges as well as overstructures for multi-part restoration frames which can consist e.g. of oxide ceramic, metals or dental alloys.

(57) It can be advantageous for the dense sintering that the precursor of the dental restoration is supported in order to avoid a distortion. It is preferred that the support consists of the same material as the precursor and hence shows the same shrinkage upon sintering. The support can be in form of for example a supporting structure or supporting mould which in terms of their geometry are adapted to the precursor.

(58) In the optional stage (iii), the surface of the dental restoration can also be provided with a finish. It is possible in particular to also carry out a glazing firing at a temperature of from 650 to 900 C. or to polish the restoration.

(59) Because of the described properties of the pre-sintered blank according to the invention, it is suitable in particular for producing dental restorations. The invention therefore also relates to the use of the blank to prepare dental restorations and in particular crowns, abutments, abutment crowns, inlays, onlays, veneers, shells and bridges as well as overstructures.

(60) The average particle sizes given, relative to the number of particles, were determined at room temperature by laser diffraction with the CILAS Particle Size Analyzer 1064 from Quantachrome GmbH & Co. KG in accordance with ISO 13320 (2009).

(61) The invention is explained in more detail below by means of examples.

EXAMPLES

Examples 1 to 4

(62) A total of 4 glass ceramics with lithium disilicate as main crystal phase with the composition given in Table I were prepared by melting corresponding starting glasses and then, by heat treatment, presintering glass powder blanks produced from them, and crystallizing them in a controlled manner.

(63) For this, the starting glasses on a scale of 100 to 200 g were first melted from customary raw materials at 1400 to 1500 C., wherein the melting could be carried out very easily without formation of bubbles or streaks. By pouring the starting glasses into water, glass frits were prepared which were then melted a second time at 1450 to 1550 C. for 1 to 3 h for homogenization.

(64) The obtained glass melts were then cooled to 1400 C. and converted to fine-particle granulates by pouring into water. The granulates were dried and ground to powder with an average particle size of <100 m, relative to the number of particles. These powders were moistened with some water and pressed to form powder green compacts at a pressing pressure of from 20 to 200 MPa.

(65) The powder green compacts were then heat-treated for 2 to 120 min at a temperature which lies in the range given as plateau in Table I for the respective composition. After this heat treatment, blanks according to the invention were present which were pre-sintered and based on lithium disilicate glass ceramic.

(66) TABLE-US-00002 TABLE I Example 1 2 3 4 Component wt. % wt. % wt. % wt. % SiO.sub.2 67.5 75.2 78.4 69.4 Li.sub.2O 14.9 15.6 16.3 19.7 P.sub.2O.sub.5 4.3 3.3 3.4 K.sub.2O 4.2 MgO 0.7 SrO 4.1 ZnO 4.8 Al.sub.2O.sub.3 3.6 3.5 La.sub.2O.sub.3 1.0 Er.sub.2O.sub.3 0.25 CeO.sub.2 2.0 1.0 SnO.sub.2 2.0 Nb.sub.2O.sub.5 V.sub.2O.sub.5 0.1 MoO.sub.3 4.0 Tb.sub.4O.sub.7 0.5 Main crystal phase LS2 LS2 LS2 LS2 Plateau ( C.) 600- 590- 600- 600- 800 860 900 900 LS2 lithium disilicate

Example 5

Examination of Sintering Behaviour of the Composition According to Example 1

(67) A glass with the composition according to Example 1 was melted and ground to a glass powder with an average particle size of less than 50 m, relative to the number of particles. This glass powder was pressed to form cylinders. The sintering behaviour of these cylindrical blanks was examined by heat-treating them at different temperatures in a furnace of the Programat P500 type from Ivoclar Vivadent AG. In each case a heating rate of 20 C./min and a holding time of 2 min at the respective temperature were used. After that the blanks were cooled to room temperature and the relative density of the blanks was then determined in each case in relation to the true density of the glass ceramic. The remaining linear shrinkage as well as the enlargement factor were calculated from the relative density.

(68) The results for temperatures in the range of from 25 to 900 C. are shown in the following Table II. A pre-sintered lithium disilicate glass ceramic blank according to the invention with a relative density of from 69 to 70% was present at between 600 and 800 C.

(69) TABLE-US-00003 TABLE II Temperature [ C.] 25 450 500 550 600 650 700 750 800 850 900 Diameter [mm] 16.10 16.09 16.07 15.47 14.55 14.59 14.61 14.56 14.48 13.39 13.12 Height [mm] 15.16 15.14 15.05 14.91 13.81 13.80 13.83 13.88 13.92 12.65 12.03 Volume [cm3] 3.09 3.08 3.05 2.80 2.30 2.31 2.32 2.31 2.29 1.78 1.63 Mass [g] 4.00 3.98 3.99 4.05 3.98 3.98 3.97 3.99 4.01 4.03 3.98 Density [g/cm3] 1.30 1.29 1.31 1.45 1.74 1.73 1.71 1.73 1.75 2.26 2.45 Relative density [%] 52 52 52 58 69 69 69 69 70 91 98 Linear shrinkage [%] 19.6 19.5 19.2 17.3 11.4 11.5 11.6 11.6 11.5 3.5 0.0 Enlargement factor 1.24 1.24 1.24 1.21 1.13 1.13 1.13 1.13 1.13 1.04 1.00

(70) In FIG. 1, the calculated enlargement factor is plotted against the temperature applied. It can be seen from this that the enlargement factor surprisingly remains substantially constant in the range of from 600 to 800 C. and the curve forms a plateau. Thus, when a heat treatment is applied in this range, a blank according to the invention can be produced for which a very precise specification of the enlargement factor to be chosen is possible.

(71) The same process for determining this range (plateau) was also used for the other compositions given in Table I.

Example 6

Examination of Sintering Behaviour of the Composition According to Example 2

(72) The sintering behaviour of the composition according to Example 2 was examined analogously to Example 5. The values obtained for the relative density, the remaining linear shrinkage and the enlargement factor are listed in Table III.

(73) TABLE-US-00004 TABLE III Temperature [ C.] 30 450 500 550 600 650 700 750 800 850 900 950 Linear shrinkage [%] 14.05 14.10 14.30 12.80 11.45 11.45 11.55 11.45 11.40 11.45 9.95 0.00 Enlargement factor 1.163 1.164 1.167 1.147 1.129 1.129 1.131 1.129 1.129 1.129 1.110 1.000 Relative density [%] 63 63 63 66 69 69 69 69 70 70 73 100

(74) The enlargement factor was plotted against the temperature and the curve obtained is shown in FIG. 2. It can be seen from this that the plateau for the examined lithium disilicate glass ceramic is in a range of about 600 to about 850 C. A pre-sintered lithium disilicate glass ceramic blank according to the invention with a relative density of from 69 to 70% is present in this range.