Low-Melting Glass Ceramic

Abstract

The present invention relates to a glass ceramic for veneering a dental frame structure, wherein said glass ceramic is characterized by a high content of B.sub.2O.sub.3, to a process for the preparation thereof, and to the use thereof in the production of dental restorations.

Claims

1. A glass ceramic for veneering a dental framework structure, characterized in that said glass ceramic contains from 60 to 75% by weight of SiO.sub.2, from 6 to 12% by weight of B.sub.2O.sub.3, and from 6 to 12% by weight of K.sub.2O, respectively based on the total weight of the glass ceramic.

2. The glass ceramic according to claim 1, characterized in that said glass ceramic has a softening point of lower than 790 C., as determined according to a heating microscope. (Currently Amended) The glass ceramic according to claim 1, characterized in that said glass ceramic contains K.sub.2O in an amount of from 7 to 9% by weight, based on the total weight of the glass ceramic.

4. The glass ceramic according to claim 1, characterized in that said glass ceramic further contains Al.sub.2O.sub.3 in an amount of from 3 to 11% by weight, based on the total weight of the glass ceramic.

5. The glass ceramic according to claim 1, characterized in that said glass ceramic further contains Na.sub.2O in an amount of from 4 to 11% by weight, based on the total weight of the glass ceramic.

6. The glass ceramic according to claim 1, characterized in that said glass ceramic further contains Li.sub.2O in an amount of less than 3% by weight, based on the total weight of the glass ceramic.

7. The glass ceramic according to claim 1, characterized in that said glass ceramic has a coefficient of thermal expansion, CTE, of less than 9.5*10.sup.6K.sup.1, as determined by using a dilatometer.

8. The glass ceramic according to claim 1, characterized in that said glass ceramic has a solubility of less than 20 g/cm.sup.2, as determined according to DIN ISO 6872.

9. The glass ceramic according to claim 1, characterized in that said glass ceramic has a translucency above 75%.

10. The glass ceramic according to claim 1, characterized in that said glass ceramic is essentially free of lithium disilicate and/or lithium metasilicate.

11. The glass ceramic according to claim 1, characterized in that said glass ceramic includes leucite.

12. The glass ceramic according to claim 1, characterized in that said glass ceramic includes leucite in an amount of from 0.1 to 10% by weight.

13. A process for preparing a glass ceramic according to claim 1, characterized in that said process comprises the following steps: a) preparing a starting glass by melting the base components and quenching the melt in water; b) milling the glass from step a) to obtain a powder; c) pressing the powder from step b) to obtain a blank; d) thermally treating the blank to obtain a glass ceramic; and e) milling the blank from step d) to obtain a powder.

14. A process claim 1 for veneering a dental framework structure, the process comprising applying the glass ceramic according to claim 1 to the dental framework structure, wherein the dental framework structure comprises a ceramic framework structure based on lithium disilicate or ZrO.sub.2.

15. The process according to claim 14, characterized in that the difference between the coefficient of thermal expansion of the veneer structure, CTE.sub.VS, and the coefficient of thermal expansion of the framework structure, CTE.sub.FS, is not more than 2.5*10.sup.6 K.sup.1, wherein the coefficient of thermal expansion can be respectively determined by dilatometry.

16. A dental restoration including a framework structure and a veneer structure, characterized in that said veneer structure is a glass ceramic according to claim 1.

17. The dental restoration according to claim 16, characterized in that the thickness of the veneer structure is from 0.2 to 3 mm.

18. A paste comprising a liquid medium and a glass ceramic powder according to claim 1 in the form of a powder for veneering a dental framework structure.

19. A process for preparing a dental restoration, wherein the glass ceramic according to claim 1 or a paste according to claim 18 is applied to a framework structure.

20. A process for preparing a dental restoration, wherein the paste of according to claim 18 is applied to a framework structure.

Description

[0080] The present invention is further explained by means of the following Examples, which are by no means to be understood as limiting the idea underlying the invention.

[0081] FIG. 1 shows an SEM micrograph of a glass ceramic according to the invention in accordance with Example Part II. The glass ceramic was treated with 5% HF for 90 seconds. The etched spots indicate the positions where leucite was present.

EXAMPLES

Example Part I

[0082] The glass ceramic according to the invention was applied as a veneering material to different framework structures, and the temperature cycling resistance according to DIN EN ISO 9693-2:2016-07 was tested. Thus, the veneered framework structures were alternately heated in the oven and then quenched in ice water, in which the oven temperature was increased by 15 C. after each quenching. The holding time in the oven was 30 minutes each, and the test specimens were examined for cracking and chipping after each quenching. The results are summarized in the following Tables.

[0083] The glass ceramics according to the invention had a content of B.sub.2O.sub.3 of 8% by weight. For comparison, conventional glass ceramics having a content of B.sub.2O.sub.3 of 1% by weight (Comparison 1) or 5% by weight (Comparison 2) were used.

TABLE-US-00001 TABLE 1 Temperature cycling resistance (DIN EN ISO 9693-2:2016-07) on lithium disilicate as the framework material Glass ceramic according to the Comp. Comp. invention 1 2 fissures at 105 C. none none none fissures at 120 C. none none none fissures at 135 C. none none yes fissures at 150 C. none none yes fissures at 165 C. none yes yes undamaged 7/7 5/7 0/7 specimens

TABLE-US-00002 TABLE 2 Temperature cycling resistance on ZrO.sub.2 frameworks Glass ceramic according to the Comp. Comp. invention 1 2 fissures at 105 C. none none none fissures at 120 C. none none none fissures at 135 C. none yes yes fissures at 150 C. none none yes fissures at 165 C. yes yes undamaged 5/7 1/7 0/7 specimens

[0084] Further, the composite adhesion of the veneering materials with framework materials based on ZrO2 was determined using the detaching/initial cracking test (DIN EN ISO 9693-2:2016-07). The results are summarized in Table 3.

TABLE-US-00003 TABLE 3 Mean Standard value deviation [M Pa] [M Pa] Glass ceramic 45.5 5.9 according to the invention Comp. 1 32.4 6.3 Comp. 2 36.8 4.8

[0085] As can be seen from the Table, the glass ceramic according to the invention has an excellent composite bonding. The softening point of the glass ceramic according to the invention was determined by heating microscopy at 762 C.

Example Part II

[0086] The exemplary compositions MU/034/18, MU/035/18 and MU/036/18 are final frits that are respectively composed of 2 partial frits. One partial frit each is a leucite frit with a high CTE. After the thermal treatment, leucite crystals crystallize in the leucite frit. The other partial frit is respectively a glass frit with a low CTE. The glass frit remains amorphous even after the thermal treatment and does not form any crystals.

[0087] The glass frit and leucite frit are mixed at a ratio in which a certain fraction of the leucite will crystallize after the thermal treatment, by which the desired CTE of the final frit is in turn adjusted.

[0088] The preparation of the final frits is effected by mixing, melting and subsequently quenching in water the components of the glass frit and of the leucite frit, each independently of one another. The respectively quenched powders are ground to a powder. The thus obtained partial frit powders of the leucite frit and the glass frit are subsequently mixed together in the weight percentages as stated in Table 4. Subsequently, the powder mixture is compressed to a blank, and the blank is subjected to thermal treatment to obtain the glass ceramic. During the thermal treatment, leucite crystals are formed. The thermally treated blank is subsequently ground to obtain the final frit powder.

[0089] The following 3 final frits and 4 partial frits are described in the following, and are listed in the Table below:

TABLE-US-00004 TABLE 4 Partial frits Leucite frit (and glass frit its weight (and its weight Final frits percentage) percentage) MU/034/18 SD (22.5%) HAK109 (77.5%) MU/035/18 HAK105 (10%) HAK108 (90%) MU/036/18 HAK105 (22.5%) HAK109 (77.5%)

[0090] Chemical Composition:

[0091] The chemical composition of all Examples was determined. The determination of the contents of the following oxides was performed using X-ray fluorescence: SiO.sub.2, MgO, Na.sub.2O, Fe.sub.2O.sub.3, MnO, TiO.sub.2, P.sub.2O.sub.5, CaO, K.sub.2O, Al.sub.2O.sub.3, BaO, ZnO, ZrO.sub.2, SnO.sub.2, Cr.sub.2O.sub.3, CoO, NiO, Sb.sub.2O.sub.3, La.sub.2O.sub.3, CeO.sub.2. The contents of the following oxides were determined by atomic absorption spectrometry after KOH digestion: B.sub.2O.sub.3, Li.sub.2O.

[0092] The ignition loss was determined according to DIN EN ISO 26845:2008-06 Section 9 (105050 C./1 h). In Table 5 as shown below, only those components that were 0.01% by weight according to the analytical result were considered.

[0093] Since the chemical compositions of all partial frits are available, it is possible to calculate the chemical composition of the final frits from those of the partial frits (Table 6).

[0094] Material Properties:

[0095] For determining the combustion temperature, thin disks (0.7 g) were compressed from ceramic powder and fired in a dental furnace for testing.

[0096] The following material properties were determined according to ISO 6872: Coefficient of thermal expansion (CTE), acid solubility and bending strength. According to ISO 6872, the glass ceramic according to the invention is classified as Type I ceramic and as class 1b. The test instructions and minimum specification for this type or this class apply accordingly. Since Type I ceramics can be influenced by the repeated firing, the CTE is determined after two and four firings of the specimen according to the standard, wherein as low as possible a difference between the two measuring results was to be achieved.

[0097] The following material properties were determined according to ISO 9693: temperature cycling resistance and debonding/crack initiation test. The temperature cycling resistance is possible for all framework materials that match to the ceramic according to the invention in terms of CTE. The debonding/crack initiation test is approved only for zirconia frameworks.

TABLE-US-00005 TABLE 5 Chemical compositions according to analytical result Partial frits Leucite frits Glass frits End frits wt. % SD HAK105 HAK108 HAK109 MU/034/18 MU/036/18 SiO.sub.2 54.41 55.98 70.70 71.62 66.21 67.46 K.sub.2O 14.23 16.04 7.69 6.13 8.16 8.30 Al.sub.2O.sub.3 15.51 14.03 8.85 4.24 7.21 6.32 Na.sub.2O 8.68 6.16 6.45 5.65 6.29 5.50 B.sub.2O.sub.3 4.76 3.12 2.59 8.34 8.31 7.84 CaO 1.28 1.98 1.34 2.13 1.92 2.10 Li.sub.2O <0.01 1.46 1.68 0.90 0.78 1.15 TiO.sub.2 0.37 0.01 0.04 0.02 0.11 0.01 P.sub.2O.sub.5 0.02 <0.01 <0.01 <0.01 0.02 <0.01 Fe.sub.2O.sub.3 <0.01 0.03 0.02 0.02 0.03 0.02 MgO <0.01 0.02 0.02 0.04 0.02 0.03 ZrO.sub.2 <0.01 0.01 0.02 0.03 <0.01 0.02 BaO 0.01 0.08 <0.01 <0.01 0.06 0.02 Ignition loss 0.30 0.27 0.29 0.07 0.11 0.14 Total 99.57 99.19 99.68 99.19 99.23 98.91

TABLE-US-00006 TABLE 6 Chemical compositions according to the calculation from the analytical results of the partial frits: Final frits MU/035/18 wt.% MU/034/18 (Comp.) MU/036/18 SiO.sub.2 66.89 69.23 68.10 K.sub.2O 8.36 8.53 8.36 Al.sub.2O.sub.3 7.34 9.37 6.44 Na.sub.2O 6.48 6.42 5.76 B.sub.2O.sub.3 7.35 2.64 7.17 CaO 1.90 1.40 2.10 Li.sub.2O 0.65 1.66 1.03 TiO.sub.2 0.12 0.04 0.02 P.sub.2O.sub.5 0.01 0.00 0.00 Fe.sub.2O.sub.3 0.01 0.02 0.02 MgO 0.03 0.02 0.04 ZrO.sub.2 0.02 0.02 0.03 BaO 0.00 0.01 0.02 Ignition loss 0.13 0.29 0.12 Total 99.29 99.65 99.21

TABLE-US-00007 TABLE 7 Material properties of the partial and final frits: Firing CTE (25-400 C.) Acid Bending temperature [10.sup.6 K.sup.1] solubility strength Frit Frit type [ C.] 2 fired 4 fired [g/cm.sup.2] Mpa SD Leucite frit 750 18.3 19.5 HAK105 Leucite frit 820 18.4 18.1 HAK108 Glass frit 760 8.5 8.7 5 HAK109 Glass frit 770 7.3 7.2 4 MU/034/18 Final frit 765 8.9 8.9 9 112 29 MU/035/18 Final frit 745 9.2 9.7 1 101 13 (Comp.) MU/036/18 Final frit 760 8.6 8.7 8 93 16

TABLE-US-00008 TABLE 8 Composite strength of the final frits towards lithium disilicate and zirconia frameworks: Temperature cycling resistance on ZrO.sub.2 frameworks MU/035/18 MU/034/18 (Comp.) MU/036/18 fissures at 105 C. none none none fissures at 120 C. none none none fissures at 135 C. none yes none fissures at 150 C. none yes yes fissures at 165 C. yes none yes undamaged 5/7 1/7 4/7 specimens Temperature cycling resistance on lithium disilicate frameworks MU/034/18 MU/036/18 fissures at 105 C. none none fissures at 120 C. none none fissures at 135 C. none none fissures at 150 C. none none fissures at 165 C. none none undamaged 7/7 7/7 specimens Debonding/crack initiation test on commercially available ZrO.sub.2 frameworks Mean Standard Framework value deviation Veneer ceramic material [MPa] [MPa] MU/034/18 VITA YZ.sup.(R) T 44.9 4.2 MU/034/18 VITA YZ.sup.(R) XT 50.8 7.8 MU/036/18 VITA YZ.sup.(R) T 45.5 5.9 MU/036/18 VITA YZ.sup.(R) XT 28.3 3.2

[0098] The framework materials can be purchased from the company Vita Zahnfabrik, Germany.

[0099] Result:

[0100] The glass ceramics MU/034/18 and MU/036/18 according to the invention meet all the desired requirements: sufficiently low firing temperature, stable CTE after repeated firing at 90.510.sup.6 K.sup.1, high bending strength, low acid solubility.

[0101] The Comparative Example MU/035/18 is not CTE-stable after repeated firing. The CTE values after two and four firings deviate from each other by 0.5, which is too much (a maximum of 0.3 would be tolerable for the final frit). Further, the Comparative Example shows a poor temperature cycling resistance. In this case too, the specimen is fired four times to prepare the veneered frameworks.