Method For The Preparation Of Lithium Silicate Glasses And Lithium Silicate Glass Ceramics

Abstract

The invention relates to a method for the preparation of a lithium silicate glass or a lithium silicate glass ceramic which comprise cerium ions and are suitable in particular for the preparation of dental restorations, the fluorescence properties of which largely correspond to those of natural teeth.

The invention also relates to a lithium silicate glass and a lithium silicate glass ceramic which can be obtained using the method according to the invention, the use thereof as dental material and in particular for the preparation of dental restorations, as well as a glass-forming composition which is suitable for use in the method according to the invention.

Claims

1. Method for the preparation of a lithium silicate glass or a lithium silicate glass ceramic, which comprises a step in which a melt of a starting glass which comprises cerium ions is exposed to reducing conditions.

2. Method according to claim 1, in which the melt of the starting glass is reacted with at least one reducing agent.

3. Method according to claim 2, in which the melt of the starting glass is formed from a glass-forming composition which comprises SiO.sub.2, Li.sub.2O, nucleating agent, a cerium compound and at least one reducing agent.

4. Method according to claim 3, in which the at least one reducing agent is a compound which comprises at least one oxidizable carbon atom and is selected from the group consisting of organic salts, carbohydrates and cereal flours.

5. Method according to claim 4, in which the at least one reducing agent is an acetylacetonate.

6. Method according to claim 2, in which the at least one reducing agent is a reducing gas, wherein the gas comprises hydrogen or hydrogen and nitrogen.

7. Method according to claim 6, in which the starting glass comprises up to 5.0 wt.-% alkaline earth metal oxide, wherein the alkaline earth metal oxide comprises CaO, BaO, MgO, SrO or a mixture thereof.

8. Method according to claim 7, in which the starting glass comprises at least one of the following components: TABLE-US-00004 Component wt.-% SiO.sub.2 55.0 to 75.0 Li.sub.2O 9.0 to 21.0 M.sub.2O 1.0 to 12.0 Al.sub.2O.sub.3 0.5 to 5.0 P.sub.2O.sub.5 0.5 to 12.0, wherein M.sub.2O is selected from the group consisting of Na.sub.2O, K.sub.2O, Rb.sub.2O and Cs.sub.2O.

9. Method according to claim 8, in which the starting glass furthermore comprises terbium ions.

10. Method according to claim 1 for the preparation of a lithium silicate glass with nuclei which are suitable for forming lithium metasilicate and/or lithium disilicate crystals.

11. Method according to claim 1 for the preparation of a lithium silicate glass ceramic which comprises lithium metasilicate as main crystal phase and/or comprises more than 10 vol.-% lithium metasilicate crystals.

12. Method according to claim 1 for the preparation of a lithium silicate glass ceramic which comprises lithium disilicate as main crystal phase and/or comprises more than 10 vol.-% lithium disilicate crystals.

13. Method according to claim 1, in which the starting glass is subjected to at least one heat treatment in the range of from 450 to 950 C. in order to form a lithium silicate glass with nuclei which are suitable for forming lithium metasilicate and/or lithium disilicate crystals, or a lithium silicate glass ceramic.

14. Method according to claim 1, in which the lithium silicate glass, the lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or the lithium silicate glass ceramic is present in the form of a powder, a blank or a dental restoration.

15. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic, which are obtainable using the method according to claim 1.

16. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic, which have a fluorescence intensity at 430 nm which is at least 1.5 times the corresponding fluorescence intensity of a reference sample, wherein the reference sample is obtainable by melting a starting glass with the composition: 71.3 wt.-% SiO.sub.2, 15.1 wt.-% Li.sub.2O, 3.2 wt.-% K.sub.2O, 3.5 wt.-% Al.sub.2O.sub.3, 3.3 wt.-% P.sub.2O.sub.5, 1.5 wt.-% CeO.sub.2 and 0.7 wt.-% Tb.sub.4O.sub.7 on a scale of 200 g from suitable raw materials in a platinum-rhodium crucible at 1500 C. for 1 h, pouring 30 g of the glass melt into a pre-heated mould to produce a glass block, and converting the glass block into a glass ceramic by successive temperature treatments at 500 C. for 10 min, 700 C. for 20 min and 850 C. for 10 min, wherein the heating rates between the temperature treatments are 30 K/min in each case.

17. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic, which has a fluorescence intensity at 541 nm which is at least 1.5 times the corresponding fluorescence intensity of a reference sample, wherein the reference sample is obtainable as in claim 15.

18. Method of using the lithium silicate glass, the lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or the lithium silicate glass ceramic according to claim 15 as dental material for the preparation of dental restorations.

19. Method of using the lithium silicate glass, the lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or the lithium silicate glass ceramic according to claim 18, in which the lithium silicate glass, the lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or the lithium silicate glass ceramic is shaped by pressing or machining to form the desired dental restoration comprising an inlay, onlay, veneer, partial crown, crown or facet.

20. Glass-forming composition which comprises SiO.sub.2, Li.sub.2O, nucleating agent, a cerium compound and at least one reducing agent and comprises as cerium compound and reducing agent a cerium compound which comprises at least one oxidizable carbon atom.

21. Method according to claim 5, in which the acetylacetonate, comprises cerium acetylacetonate or cerium(III) acetylacetonate.

22. Method according to claim 8, in which the starting glass comprises the following components: TABLE-US-00005 Component wt.-% SiO.sub.2 59.0 to 73.0 Li.sub.2O 13.0 to 19.0 M.sub.2O 2.0 to 5.0 Al.sub.2O.sub.3 2.5 to 3.5 P.sub.2O.sub.5 2.5 to 7.0.

23. Method according to claim 11 wherein the lithium metasilicate comprises more than 20 vol.-% lithium metasilicate crystals.

24. Method according to claim 11 wherein the lithium metasilicate comprises more than 30 vol.-% lithium metasilicate crystals.

25. Method according to claim 12 wherein the lithium silicate glass ceramic comprises more than 20 vol.-% lithium disilicate crystals.

26. Method according to claim 12 wherein the lithium silicate glass ceramic comprises more than 30 vol.-% lithium disilicate crystals.

27. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic according to claim 16, which have a fluorescence intensity at 430 nm which is at least 2 times the corresponding fluorescence intensity of a reference sample.

28. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic according to claim 16, which have a fluorescence intensity at 430 nm which is at least 4 times the corresponding fluorescence intensity of a reference sample.

29. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic according to claim 16, which have a fluorescence intensity at 430 nm which is at least 6 times the corresponding fluorescence intensity of a reference sample.

30. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic according to claim 17, which has a fluorescence intensity at 541 nm which is at least 2 times the corresponding fluorescence intensity of a reference sample.

31. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic according to claim 17, which has a fluorescence intensity at 541 nm which is at least 3 times the corresponding fluorescence intensity of a reference sample.

32. Lithium silicate glass, lithium silicate glass with nuclei suitable for forming lithium metasilicate and/or lithium disilicate crystals or lithium silicate glass ceramic according to claim 17, which has a fluorescence intensity at 541 nm which is at least 4 times the corresponding fluorescence intensity of a reference sample.

Description

EXAMPLES

[0052] A total of 16 glasses and glass ceramics with the compositions given in Table I were prepared by melting corresponding starting glasses, followed by heat treatment according to Table II for controlled nucleation and crystallization, wherein in Table I the oxidation states of the given oxides refer to the oxidation states of the raw materials used for melting the starting glasses. The following meanings apply in Table II [0053] T.sub.N and t.sub.N temperature and time used for nucleation [0054] T.sub.k1 and t.sub.k1 temperature and time used for first crystallization [0055] T.sub.K2 and t.sub.K2 temperature and time used for second crystallization.

Examples 1 to 10: Use of a Reducing Cerium Compound as Reducing Agent

[0056] To prepare glasses and glass ceramics using a cerium compound as reducing agent, firstly starting glasses corresponding to the compositions given in Table I on a scale of 100 to 200 g were melted from a mixture of usual raw materials at 1500 C. for 2 h in a platinum crucible, wherein cerium(III) acetylacetonate was used as raw material for the given Ce.sub.2O.sub.3 content. By pouring the starting glasses into water, glass frits were prepared which were dried in a drying furnace at 150 C. and then melted a second time at 1500 C. for 2.5 h for homogenization. The obtained glass melts were then poured into pre-heated moulds to produce glass blocks.

[0057] The glass blocks were then converted to glasses and glass ceramics by thermal treatment. The thermal treatments used for controlled nucleation and controlled crystallization are given in Table II.

Examples 11 to 15: Use of Forming Gas as Reducing Agent

[0058] To prepare glasses and glass ceramics using forming gas as reducing agent, firstly starting glasses corresponding to the compositions given in Table I on a scale of 200 g were melted from usual raw materials in a platinum-rhodium crucible at 1450 to 1500 C. for 1 h. Then, 30 g of the glass melts, as reference samples, were poured into pre-heated moulds in order to produce glass blocks. About 3 l/min forming gas (95% N.sub.2, 5% H.sub.2) was passed through the remaining glass melt for 30 to 90 min by means of a quartz glass dip tube. The dip tube was then removed from the melt and the melt surface was flushed with forming gas for about 30 min in order to prevent a reoxidation. The glass melt was then poured into pre-heated moulds to produce glass blocks. The subsequent temperature treatments (nucleation, crystallization and/or pressing) were carried out in a normal furnace atmosphere.

[0059] No effects of the melting under forming gas on the crystallization and/or crystalline structure were observed.

Example 16: Use of an Organic Compound as Reducing Agent

[0060] To prepare glasses and glass ceramics using an organic compound as reducing agent, a starting glass corresponding to the composition given in Table I for Example 11 on a scale of 200 g was melted from a mixture of usual raw materials, accompanied by the addition of 1.5 wt.-% saccharose in a platinum crucible by heating to 1450 C. at a heating rate of 10 K/min. After a holding time of 30 min, the obtained glass melt was fritted in water and then dried. The frit was melted again at 1500 C. for 1 h and poured into a graphite mould in order to produce glass blocks.

[0061] The glass blocks were then converted to glasses and glass ceramics by thermal treatment. For this, the glass blocks were tempered immediately after the casting and demoulding in a muffle furnace at 490 C. for 10 min and then cooled slowly to room temperature.

[0062] A disc about 2 mm thick was sawn off from the glass block and then crystallized in a Programat furnace (Ivoclar Vivadent AG) via a temperature treatment at 840 C. for 7 min. The thus-obtained white lithium disilicate glass ceramic displayed a strong white-bluish fluorescence under excitation by UV light.

[0063] The fluorescence of this sample is strongly increased compared with a conventionally melted glass ceramic and lies in the range of the sample which was prepared by means of passing forming gas through it.

Determination of Biaxial Strengths

[0064] With the aid of a Sirona grinding unit, platelets with thicknesses of about 2 mm were ground out of the blocks obtained after nucleation and first crystallization via the CAD/CAM method. The platelets were then subjected to a further temperature treatment according to Table II in a Programat furnace (Ivoclar Vivadent AG) for the second crystallization. In a further processing step, the platelets were ground to a thickness of about 1.3 mm and the surface was polished with a diamond grinding wheel (15 m). The average biaxial strengths determined using the thus-obtained samples are given in Table II.

Determination of Colour Values

[0065] Discs about 2.5 mm thick were sawn off from the blocks obtained after nucleation and first crystallization and subjected to a further temperature treatment according to Table II for the second crystallization. For the determination of the colour values, the platelets were ground to a thickness of 2 mm with a 1000 grit SiC sandpaper. The measured colour values were measured in the measurement range of 400-700 nm by means of a CM-3700d spectrophotometer (Konica-Minolta). The colour values were determined according to DIN5033 and DIN6174 and the CR value according to British Standard BS56129.

Fluorescence Measurements

[0066] With the aid of a Sirona grinding unit, platelets were ground out of the blocks obtained after nucleation and first crystallization via the CAD/CAM method. The platelets were then subjected to a further temperature treatment according to Table II in a Programat furnace (Ivoclar Vivadent AG) for the second crystallization. In a further processing step, the platelets were ground to the dimensions 17.9 mm15.9 mm2 mm and the surface was polished with an APEX grinding wheel (0.5 m).

[0067] To measure the fluorescence properties, a fluorescence spectrometer of the FL1039 type (Horiba Jobin Yvon GmbH) with an excitation monochromator and an emission monochromator was used. The excitation of the samples was carried out by means of a 450 W xenon lamp. The emission intensity was determined using a photomultiplier detector (PMT) of the PMT 1424M type (Horiba Jobin Yvon GmbH) as pulses per second (counts per second, cps). The calibration of the excitation monochromator was carried out by means of an integrated silicon photodiode. The emission monochromator was calibrated via the position of the water Raman peak. The linearity of the detector in the measurement range was ensured via device-specific correction data sets. The linearity of the excitation intensity was ensured during the determination of the excitation spectra via a mathematical correction of the measured emission intensity via the lamp intensity (division of the measured signal by the reference signal of the integrated silicon photodiode which directly determines the lamp intensity). To protect the detector and in order not to reach the saturation range, a 5% Neutral Density Filter was used in the emission beam path.

[0068] The samples were clamped in a solid sample holder in the right-angle mode. To prevent reflections of the excitation light, the samples were rotated by 30 relative to the excitation beam, with the result that only diffusely scattered emission light was detected. All samples were measured using identical spectrometer settings (gap widths 1 nm (excitation monochromator) and 1.5 nm (emission monochromator), scan range 372 to 700 nm, increment 1 nm, integration time 1 s, excitation wavelength 366 nm).

[0069] FIG. 1 shows, for the glass ceramic sample obtained according to Example 11, the emission spectrum at an excitation wavelength of 366 nm as well as excitation spectra for emission at 430 nm and 541 nm. The emission spectrum displayed a broad maximum at 430 nm, which is to be assigned to the 5d.fwdarw.4f transition of Ce.sup.3+. The corresponding excitation spectrum displayed excitation maxima at 279 nm and 340 nm. Furthermore, the emission spectrum displayed maxima at 483, 541, 585 and 619 nm, which are to be assigned to the transitions .sup.5D.sub.4.fwdarw..sup.7F.sub.6, .sup.7F.sub.5, .sup.7F.sub.4 and .sup.7F.sub.3 of Tb.sup.3+. The associated broad excitation spectrum for emission at 541 nm displayed excitation maxima at 279 nm and 315 nm. The fluorescence emissions shown in the emission spectrum of FIG. 1 are perceived by the human eye overall as white-blue fluorescence.

[0070] FIG. 2 shows emission spectra obtained at an excitation wavelength of 366 nm for the sample according to Example 11, which was prepared under reducing conditions by means of passing forming gas through it, and the corresponding reference sample of the same composition which was melted under normal conditions in oxygen-containing atmosphere. The broad emission maximum of Ce.sup.3+ at about 430 nm and the emission bands of Tb.sup.3+ at 483, 541, 549, 585 and 619 nm can be seen. A comparison of the spectra shows a clear rise in the intensities of the individual emission bands due to the melting under reducing conditions for the sample according to Example 11. A comparison of the total light emission, determined by calculation of the surface integral under the emission curves over the range of from 375 to 700 nm (total measurement range), shows a rise by a factor of 5.4.

[0071] FIG. 3 shows the emission intensities of the samples 1, 14, 8 and 9 compared with the commercial lithium disilicate glass ceramic products IPS e.max CAD HT BL2 and IPS e.max CAD LT A2 (Ivoclar Vivadent AG). The respectively used melting technology (forming gas, reducing raw material) makes it possible to greatly increase the fluorescence intensity compared with the commercial products. Thus, Example 1 showed a very strong fluorescence, which is why the composition is suitable in particular for use as abutment. Examples 9 and 14 are suitable in particular for use as inlay and crown material because of their colour (under normal illumination) and fluorescence (under UV light). By combining different cerium raw materials (CeO.sub.2, Ce(III) acac), such as in the case of Example 9, it is possible to produce an intensive colour effect under D65 normal light and, compared with a commercial product of the same colour (IPS e.max CAD LT A2), a greatly increased fluorescence.

TABLE-US-00002 TABLE I Example 5 10 15 1 2 3 4 wt.- 6 7 8 9 wt.- 11 12 13 14 wt.- Composition wt.-% wt.-% wt.-% wt.-% % wt.-% wt.-% wt.-% wt.-% % wt.-% wt.-% wt.-% wt.-% % SiO.sub.2 69.1 68.0 68.0 59.0 69.9 71.9 70.6 70.7 71.2 71.1 72.7 71.3 67.8 65.0 72.7 GeO.sub.2 2.9 ZrO.sub.2 6.0 6.0 2.0 6.0 9.8 Li.sub.2O 14.4 14.1 14.1 19.0 14.9 15.0 14.6 14.7 14.8 14.8 15.1 14.8 14.1 13.5 15.1 P.sub.2O.sub.5 3.4 3.4 3.4 3.5 4.0 4.0 3.2 3.2 3.4 3.4 3.3 3.2 3.0 2.9 3.3 Al.sub.2O.sub.3 3.4 3.3 3.0 3.0 3.0 3.0 3.4 3.4 3.4 3.4 3.5 3.4 3.2 3.1 3.5 K.sub.2O 3.5 4.0 3.5 3.5 3.8 3.8 3.9 3.9 3.2 3.1 3.7 3.5 3.2 Rb.sub.2O 7.7 Cs.sub.2O 9.2 CaO 2.0 CeO.sub.2 1.5 2.2 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Ce.sub.2O.sub.3* 1.5 2.0 2.0 2.0 1.8 1.8 2.9 1.8 1.5 1.5 Tb.sub.4O.sub.7 0.5 0.5 0.4 0.4 0.7 0.7 0.7 0.7 Tb.sub.2O.sub.3 0.1 0.2 Gd.sub.2O.sub.3 0.4 Er.sub.2O.sub.3 0.1 0.1 0.1 0.1 Eu.sub.2O.sub.3 0.7 V.sub.2O.sub.5 0.1 0.1 0.1 F 0.5 100.0 100 100 100 100.0 100 100 100 100 100 100 100 100 100 100 *used as cerium(III) acetylacetonate

TABLE-US-00003 TABLE II 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 T.sub.N [ C.] 520 560 530 500 500 510 470 490 490 490 500 500 500 500 500 t.sub.N [min] 30 60 20 40 90 80 10 10 10 10 10 10 10 10 10 T.sub.K1 [ C.] 700 700 700 700 700 700 700 700 700 700 700 700 650 650 700 t.sub.K1 [min] 20 20 20 20 20 20 20 20 20 40 20 20 20 20 20 T.sub.K2 [ C.] 850 850 850 850 850 860 840 830 830 830 850 850 840 840 850 t.sub.K2 [min] 7 7 7 7 7 7 7 7 10 7 10 10 7 7 10 .sub.B [MPa] 541.4 L* 93.37 79.04 84.02 81.95 89.08 86.05 a* 0.06 5.61 2.54 3.4 0.14 1.29 b* 4.37 25.03 18.93 19.69 12.22 10.68 CR 93.37 78.38 79.89 83.2 52.59 70.75