Lithium silicate glass ceramic for fabrication of dental appliances

Abstract

The present invention relates to a method of fabricating an improved lithium silicate glass ceramic and to that material for the manufacture of blocks for dental appliances using a CAD/CAM process and hot pressing system. The lithium silicate material has a chemical composition that is different from those reported in the prior art with 1 to 10% of germanium dioxide in final composition. The softening points are close to the crystallization final temperature of 800° C. indicating that the samples will support the temperature process without shape deformation.

Claims

1. A method of fabricating dental restorations of lithium silicate glass, the method comprising the steps of: (a) blending a mix of precursors including precursors of germanium dioxide (GeO.sub.2), silicon dioxide (SiO.sub.2), and lithium oxide (Li.sub.2O) in amounts to provide a silicon dioxide and lithium oxide molar ratio content (SiO.sub.2/Li.sub.2O) of between 1.8 and 1.9; (b) ball milling the mix of precursors to homogenize components of the mix of precursors; (c) melting the resulting mix of step (b); and (d) pouring the melt of step (c) into graphite molds to form shaped blanks and cooling such blanks to room temperature.

2. The method of fabricating dental restorations of lithium silicate glass ceramic as recited in claim 1, the method comprising the additional steps of: (e) heating the blanks and holding the blanks at a temperature to achieve nucleation of lithium silicate crystals; (f) milling the blanks of step (e) into dental restorations; and (g) heating the restoration of step (f) at temperature to achieve full crystal growth of the lithium silicate crystals.

3. The method of fabricating dental restorations of lithium silicate glass ceramic as recited in claim 2, wherein the final crystalline product is lithium silicate.

4. The method of fabricating dental restorations of lithium silicate glass ceramic as recited in claim 1, the method comprising the additional steps of: (e) heating the blanks to a temperature to achieve nucleation and crystallization of lithium silicate crystals in the blanks; and (f) hot pressing the blanks of step (e) into dental restorations.

5. The method of fabricating dental restorations of lithium silicate glass ceramic as recited in claim 4, wherein the final crystalline product is lithium silicate.

6. The method of fabricating dental restorations of lithium silicate glass ceramic as recited in claim 1, wherein the mix of precursors further includes precursors of at least one of aluminum oxide (Al.sub.2O.sub.3), zirconium oxide (ZrO.sub.2), phosphorous pentoxide (P.sub.2O.sub.5), potassium oxide (K.sub.2O).

7. The method of fabricating dental restorations of lithium silicate glass ceramic as recited in claim 1, wherein the mix of precursors further includes single oxide precursors of at least one of cerium, titanium, tin, erbium, vanadium, samarium, niobium, yttrium, europium, tantalum, and magnesium oxides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood herein after as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:

(2) FIG. 1 is an XRD diffraction pattern of a sample of the invention after the intermediate crystallization step (from room temperature to 600° C.) showing the presence of lithium silicate as a main constituent phase in the glass ceramic composition;

(3) FIG. 2 is an XRD diffraction pattern of a sample of the invention after the full crystallization step (from room temperature to 800° C.) showing the presence of lithium silicate as a main constituent phase in the glass ceramic composition. Because the molar ratio of SiO.sub.2/Li.sub.2O is between 1.7 to 1.9, the crystallized phase of the final material shows the presence of only lithium silicate and no lithium disilicate;

(4) FIG. 3 is an XRD diffraction pattern of a sample of this invention after hot pressing in the interval of 800° C. to 840° C. showing the presence of lithium silicate as a main constituent phase in the glass ceramic composition. Because the molar ratio of SiO.sub.2/Li.sub.2O is between 1.7 to 1.9, the crystallized phase of the final material shows the presence of lithium silicate and no lithium disilicate; and

(5) FIG. 4 is a graphical illustration of a dilatometric measurement of a sample of the invention resulting from full crystallization. The softening temperature of the intermediate step is lower than the temperature after full crystallization. This is due to the crystal growth after heating the glass in the Intermediate stage from room temperature to 800° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) The prior art materials are based on the formation of lithium disilicate materials. A principal object of the present invention is to prepare a controlled lithium silicate glass ceramic using in the formulation a specific silicon dioxide and lithium oxide molar ratio with excellent physical properties for manufacturing dental restorations. The glass material subjected to a heat treatment produces an optimal lithium silicate crystal forming a glass ceramic product with outstanding mechanical properties, excellent optical properties, a very good chemical solubility, little contraction and high flexural strength values.

(7) The lithium silicate of the present invention preferably comprises the following components and compositions:

(8) TABLE-US-00001 weight % composition Component minimum maximum SiO.sub.2 53.0 57.0 Al.sub.2O.sub.3 3.0 5.0 K.sub.20 3.0 5.0 CaO 0.0 1.0 B.sub.2O.sub.3 0.0 2.0 CeO.sub.2 0.0 1.0 MgO 0.0 1.0 Fluorine 0.0 1.0 Li.sub.2O 14.0 17.0 Zr0.sub.2 4.0 5.0 TiO.sub.2 0.0 3.0 P.sub.2O.sub.5 2.0 3.0 SnO 0.0 1.0 Er.sub.2O.sub.3 0.0 2.0 V.sub.2O.sub.5 0.0 1.0 GeO.sub.2 0.5 8.0 Ta.sub.2O.sub.5 0.0 3.0 Sm.sub.2O.sub.3 1.0 6.0 Pr.sub.2O.sub.3 0.0 1.0 Eu.sub.2O.sub.3 0.0 2.0 Y.sub.2O.sub.3 0.0 5.0 Nb.sub.2O.sub.5 0.0 1.0

(9) The invention is explained in more detail below with the following examples

(10) The sample preparation and its elemental oxide composition are listed in Table 1.

(11) TABLE-US-00002 TABLE 1 Components % weight. Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 SiO.sub.2 55.03 56.19 56.21 56.21 53.88 Al.sub.2O.sub.3 4.09 4.18 4.18 4.18 3.11 K.sub.2O 4.42 4.52 4.52 4.52 3.44 CaO 0.94 0.96 0.96 0.96 0.00 B.sub.2O.sub.3 1.58 1.61 1.61 1.61 0.00 CeO.sub.2 0.21 0.65 0.34 0.41 0.63 Mgo 0.22 0.23 0.23 0.23 0.00 Fluorine 0.49 0.50 0.50 0.50 0.00 Li.sub.2O 15.81 16.14 16.15 16.15 14.81 ZrO.sub.2 4.70 4.79 4.80 4.80 4.88 TiO.sub.2 2.40 0.80 0.80 0.80 0.63 P.sub.2O.sub.5 2.52 2.58 2.58 2.58 2.94 SnO 0.22 0.07 0.13 0.12 0.00 Er.sub.2O.sub.3 0.37 0.76 0.36 0.21 1.26 V.sub.2O.sub.5 0.39 0.22 0.26 0.11 0.03 GeO.sub.2 0.90 0.92 0.92 0.92 7.75 Ta.sub.2O.sub.5 0.07 0.15 0.22 0.01 0.00 Sm.sub.2O.sub.3 2.03 4.09 4.07 4.09 5.71 Pr.sub.2O.sub.3 0.03 0.33 0.04 0.00 0.88 Eu.sub.2O.sub.3 0.00 0.00 0.00 1.25 0.05 Y.sub.2O.sub.3 3.13 0.11 0.61 0.36 0.00 Nb.sub.2O.sub.5 0.46 0.22 0.53 0.00 0.00 TOTAL 100.00 100.00 100.00 100.00 100.00 Exam- Exam- Exam- Exam- Exam- ple6 ple 7 ple8 ple 9 ple 10 SiO.sub.2 54.08 54.49 56.17 53.49 56.19 Al.sub.2O.sub.3 3.12 3.86 4.18 3.98 4.18 K.sub.2O 3.45 4.20 4.52 4.30 4.52 CaO 0.00 0.00 0.96 0.92 0.96 B.sub.2O.sub.3 0.00 0.00 1.61 1.53 1.61 CeO.sub.2 0.95 0.64 0.00 0.20 0.62 MgO 0.00 0.00 0.23 0.22 0.23 Fluorine 0.00 0.00 0.50 0.48 0.50 Li.sub.2O 14.85 15.25 16.15 15.37 16.14 ZrO.sub.2 4.89 4.88 4.80 4.56 4.79 Ti0.sub.2 0.63 0.64 0.80 0.78 0.80 P.sub.2O.sub.5 2.95 2.97 2.58 2.45 2.58 SnO 0.00 0.00 0.00 0.00 0.07 Er.sub.2O.sub.3 1.52 1.28 0.05 0.16 0.61 V.sub.2O.sub.5 0.06 0.04 0.00 0.48 0.15 GeO.sub.2 7.77 7.70 0.92 0.87 0.92 Ta.sub.2O.sub.5 0.00 0.00 2.33 0.00 0.18 Sm.sub.2O.sub.3 4.82 3.34 1.83 4.90 4.05 Pr.sub.20.sub.3 0.90 0.72 0.00 0.23 0.24 Eu.sub.2O.sub.3 0.00 0.00 0.05 0.00 0.00 Y.sub.2O.sub.3 0.00 0.00 2.33 4.90 0.24 Nb.sub.2O.sub.5 0.00 0.00 0.00 0.18 0.45 TOTAL 100.00 100.00 100.00 100.00 100.00

(12) A particularly preferred lithium silicate material as described in the examples 1 to 10 comprises 53 to 59 wt % of SiO.sub.2, 14 to 19% wt of Li.sub.2O and 1 to 9% of GeO.sub.2, where after nucleation only lithium silicate is formed and then after complete crystal growth only lithium silicate crystals are formed.

(13) The lithium silicate material of this invention is preferably produced by a process which comprises the following steps: (a) A mix of the precursors of the final components of the table 1, are blended together for 10 to 30 min until a mechanical mix is obtained. (b) The mix is ball milled dry or wet using zirconia media for about 1 to 2 hours to homogenize the components and achieve almost the same particle size in all the components (c) The sample is calcined at 800° C. for about 1 to 4 hours in order to decompose the precursors to their primary oxides and eliminate any possibility of formation of gas after the process. (d) Ball-mill the sample of step (c) in order to produce a powder with an average particle size below 30 microns. (e) The powder of step (d) is melted in a platinum crucible at a temperature between 1100 to 1200° C. for 1 to 2 hours. It is then poured into cylindrical or rectangular graphite molds and cooled down to room temperature. (f) The glass ceramic of step (e) is then subjected to an intermediate crystal growth process at a temperature of from room temperature to 600° C. for 10 to 60 min. The growth of the lithium silicate crystals is temporarily stopped for the desired intermediate size by cooling the glass ceramic to room temperature. (g) The glass ceramic of step (f) is subjected to a single step heating cycle from room temperature to 800° C. to achieve full crystallization. (h) For use in a CAD-CAM milling device, the dental restoration is made using a block after intermediate process step (f). After milling, the restoration is heated again from 350° C. to 800° C. or to full crystallization step (g) where the optimal lithium silicate crystal growth in the glass ceramic is achieved in a single step program. (i) For an alternative hot pressing technique, the sample after [step (g)] is pressed into a dental restoration at a temperature of 800-840° C., where the optimal lithium silicate crystal growth in the glass ceramic is achieved.
Coefficient of Thermal Expansion and Softening Point

(14) The percentage linear change vs. temperature was measured using an Orton dilatometer. The coefficient of thermal expansion at 500° C. and the softening point were calculated for all the samples. For this purpose a rectangular rod of approximately 2 inches long was cast and then subjected to the intermediate crystallization cycle at 600° C. for 40 min. After this process the rod is cut into two parts. One part is used for measuring transition temperature, softening point temperature, and coefficient of thermal expansion of that process step. The second part is fully crystallized at 800° C. for about 10 minutes and is used for measuring the same properties. It is expected that after the crystallization step, the softening temperature point increases for the samples due to the formation of larger lithium silicate crystals. Test results are displayed in Table 2.

(15) Flexural Strength.

(16) Biaxial flexural strength tests (MPa) were performed following ISO-6872 procedures. Ten round samples were cut, grinded gradually and polished to a mirror finish in the intermediate stage or step (f). The samples were then fully crystallized in a single stage program from 350° C. to 800° C. for 10 minutes. Then the biaxial flexural strength was measured. For the hot pressing technique the glass ceramic of sample of step (g) is hot pressed into round discs in the interval of 800 to 840° C. Then the discs are grinded gradually and polished to a mirror finish, heated as a simulated glaze cycle, and tested. Test results expressed in MPa are displayed in Table 2.

(17) Chemical Solubility.

(18) A chemical solubility test was performed according to ISO-6872. Ten discs samples subjected to step (g) are placed in a glass flask with an aqueous solution of 4% (V/V) of acetic acid analytical grade (Alfa Aesar). The flask is heated to at temperature of 80+/−3° C. for 16 hours. The change in weight before and after the test is determined and then the chemical solubility expressed as μg/cm.sup.2 is calculated and shown in Table 2.

(19) TABLE-US-00003 TABLE 2 Physical Properties of the Lithium silicate glass ceramic. Exam- Exam- Exam- Exam- Exam- ple #2 ple #3 ple #4 ple #5 ple #8 Softening temperature, ° C., 689 618 690 766 711 Intermediate stage at 600° C. Softening temperature, ° C., 727 744 717 789 724 crystallized sample at 800° C. Coefficient of expansion, ×10.sup.−6/ 11.81 12.58 12.27 11.30 11.61 ° C. Crystallized sample at 800° C. Flexural strength, MPa, 350 +/− 28 402 +/− 56 359 +/− 40 365 +/− 60 370 +/− 50 Crystallized at 800° C. Flexural strength, MPa 393 +/− 48 423 +/− 61 523 +/− 39 345 +/− 20 397 +/− 57 Hot pressed sample Chemical Solubility, μg/cm.sup.2 72 58 65 39 58 Crystallized sample at 800° C.

(20) The preferred range composition (in % wt) of this glass ceramic material is the following:

(21) TABLE-US-00004 TABLE 5 Preferred Range of Composition Components weight % composition Component minimum maximum SiO.sub.2 53.5 56.2 Al.sub.2O.sub.3 3.1 4.2 K.sub.2O 3.4 4.5 CaO 0.0 1.0 B.sub.2O.sub.3 0.0 1.6 CeO.sub.2 0.0 1.0 MgO 0.0 0.2 Fluorine 0.0 0.5 Li.sub.2O 14.8 16.1 ZrO.sub.2 4.6 4.9 TiO.sub.2 0.6 2.4 P.sub.2O.sub.5 2.5 3.0 SnO 0.0 0.2 Er.sub.2O.sub.3 0.1 1.5 V.sub.2O.sub.5 0.0 0.5 GeO.sub.2 0.9 7.8 Ta.sub.2O.sub.5 0.0 2.3 Sm.sub.2O.sub.3 1.8 5.7 Pr.sub.2O.sub.3 0.0 0.9 Eu.sub.2O.sub.3 0.0 1.3 Y.sub.2O.sub.3 0.0 4.9 Nb.sub.2O.sub.5 0.0 0.5

(22) One preferred example of this material has the following specific composition:

(23) TABLE-US-00005 TABLE 6 Preferred Composition Component Weight % Si0.sub.2 55.74 Al.sub.2O.sub.3 4.15 K.sub.2O 4.48 CaO 0.95 B.sub.2O.sub.3 1.60 MgO 0.23 Fluorine 0.50 Li.sub.2O 16.01 ZrO.sub.2 4.76 Ti0.sub.2 0.80 P.sub.2O.sub.5 2.56 GeO.sub.2 0.91 Coloring oxides 7.32

(24) Having thus disclosed a number of embodiments of the formulation of the present invention, including a preferred range of components, a preferred formula thereof and a preferred fabrication process, those having skill in the relevant arts will now perceive various modifications and additions. Therefore, the scope hereof is to be limited only by the appended claims and their equivalents.