Process For The Preparation Of A Zirconia Blank

Abstract

The invention relates to a process for the preparation of a zirconia blank by introducing a zirconia suspension into a porous mould and demoulding the blank formed as well as the use of the optionally presintered blank formed for the preparation of a dental restoration using a very short dense-sintering process.

Claims

1. Process for the preparation of a zirconia blank, in which (a) a suspension of zirconia in a liquid medium is introduced into a mould having pores, (b) the liquid medium is at least partially removed via the pores and (c) the blank formed is removed from the mould, wherein the suspension has a zirconia content of from 68 to 88 wt.-%.

2. Process according to claim 1, in which the zirconia in the suspension has a particle size of from 50 to 250 nm, measured as the d.sub.50 value and relative to the volume of the particles.

3. Process according to claim 1, in which the zirconia in the suspension has a primary particle size of from 30 to 100 nm.

4. Process according to claim 1, in which the zirconia is stabilized with Y.sub.2O.sub.3, La.sub.2O.sub.3, CeO.sub.2, MgO and/or CaO.

5. Process according to claim 1, in which the liquid medium comprises water.

6. Process according to claim 1, in which the liquid medium comprises organic components in an amount of not more than 5 wt.-%, relative to the amount of solid in the suspension, or in an amount of 0.05 to 5 wt.-% relative to the amount of solid in the suspension.

7. Process according to claim 1, in which the liquid medium comprises at least one compound selected from amino alcohol, glycol, carboxylic acid and carboxylic acid salt.

8. Process according to claim 1, in which the suspension has a viscosity of from 5 to 500 mPas, measured at a shear rate of 0.1 to 1000 s.sup.1 and a temperature of 25 C.

9. Process according to claim 1, in which the suspension comprises a mixture of zirconia powders with different composition.

10. Process according to claim 1, in which in step (a) suspensions of zirconia powders with different compositions are introduced into the mould one after another.

11. Process according to claim 1, in which the blank is a block, a block with an interface, a disc or a toothlike preform.

12. Process according to claim 1, in which the blank formed in step (c) has a holding device.

13. Process according to claim 1, in which the blank formed in step (c) is dried.

14. Process according to claim 1, in which the blank has a density of from 3.3 to 4.0 g/cm.sup.3 and/or a pore volume of from 0.08 to 0.14 cm.sup.3/g and/or a pore diameter of from 0.02 to 0.12 m, measured as D.sub.50 value relative to the volume of the particles.

15. Process according to claim 1, in which the blank is presintered.

16. Process according to claim 15, in which the presintered blank has a density of from 3.4 to 4.0 g/cm.sup.3 and/or a pore volume of from 0.08 to 0.14 cm.sup.3/g and/or a maximum pore diameter of less than 0.15 m.

17. Zirconia blank which is obtainable by the process according to claim 1.

18. Zirconia blank which is presintered and has a density of from 3.4 to 4.0 g/cm.sup.3 and/or a pore volume of from 0.08 to 0.14 cm.sup.3/g and/or a maximum pore diameter of less than 0.15 m.

19. Zirconia blank according to claim 17 which has a linear shrinkage of less than 18%.

20. Process of using the blank according to claim 17 for the preparation of a dental restoration.

21. Process for the preparation of a dental restoration, in which the blank according to claim 17 is given the shape of the dental restoration and the blank is densely sintered at a sintering temperature of from 1200 to 1600 C.

22. Process according to claim 21, in which the period for heating the blank from room temperature to the sintering temperature, holding at the sintering temperature and cooling to the final temperature is not more than 120 min.

23. Process according to claim 21, in which the blank (a) is subjected to a first heat treatment, (b) is subjected to a second heat treatment and (c) is cooled, wherein the heat treatment in step (a) is effected at a lower pressure than the heat treatment in step (b).

24. Process according to claim 21, in which the dental restoration is a bridge, an inlay, an onlay, a crown, an implant, a veneer, a facet or an abutment.

Description

EXAMPLES

Example 1Suspension with 76 wt.-% Zirconia

[0087] 3.15 g of a dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 1.5 g tetramethylammonium hydroxide were dissolved one after the other in 194.4 g distilled water. The solution had a pH value of 10-10.5.

[0088] This solution was placed in the storage tank of a MicroCer agitator bead mill (from Netzsch), the grinding chamber and rotor of which was made of zirconia. The grinding chamber was filled with 60 ml zirconia grinding beads with a diameter of 0.2-0.3 mm (from Tosoh). At a rotational speed of the rotor of 1500 rpm the solution was pumped continuously through the grinding chamber using a peristaltic pump (tube internal diameter 8 mm). 630 g zirconia powder which was partially stabilized with 3 mol-% Y.sub.2O.sub.3 (TZ-PX-245 from TOSOH Corporation, primary particle size: 40 nm) were then, continuously and with stirring, added to the solution in the storage tank. Once the addition of the zirconia powder was completed, the mixture obtained was pumped through the grinding chamber and back into the storage tank continuously for 45 min at a rate of about 40 l/h. The suspension prepared in this way was transferred into a plastic measuring beaker and stirred very slowly by means of a magnetic stirrer in order to remove trapped air bubbles. In addition, one drop of an alkyl polyalkylene glycol ether was added as defoamer (Contraspum, from Zschimmer & Schwarz).

[0089] The suspension obtained had a zirconia content of 76 wt.-%. The viscosity of the suspension was 7.25 mPas (at a shear rate of 500 s.sup.1 and a temperature of 25 C.)

(A) Preparation of Test Pieces

[0090] Test pieces were produced, on which different properties were determined. For the preparation thereof, the suspension obtained was poured into a disc-shaped mould which consisted of a plate made of porous gypsum with a plastic ring secured thereon. Through the capillary action of the porous gypsum, water was removed from the suspension, and a monolithic green body formed. The green body was left in the mould and dried in air for 24 h. After completion of the drying the test piece had released itself from the mould.

[0091] The respective test pieces had the following properties: [0092] Linear shrinkage: 15.43% (test piece: length=25 mm1 mm, width=5 mm0.5 mm and height=4 mm0.5 mm) [0093] Density: 3.678 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

(B) Preparation of Blocks for a Chairside Application

[0094] A pressure casting mould was used for the preparation of blocks for a chairside application. This pressure casting mould consisted of a top part with a connection for compressed air (positive pressure 0.1 to 100 bar), a middle part which corresponded to the desired shape of the block, and a bottom part which was provided with a membrane filter, paper filter and sintered filter, and on the base had a connection for applying a vacuum. The membrane filter had a pore size of 200 nm.

[0095] The suspension was poured into the pressure casting mould. Then compressed air with a pressure of 6 bar was supplied via the top part and a vacuum of about 0.05 bar was applied via the bottom part. After 4 h, the addition of compressed air and the application of a vacuum was ended. The pressure casting mould was disassembled and the block obtained was dried in air at 20 C. for 24 h.

[0096] The blocks produced in this way were presintered at 950 C. for 2 h. The heating rate was 0.250 K/min. The blocks were then cooled to room temperature within 24 h. The hardness of the blocks was measured by means of the Vickers method according to ISO 14705:2008 and EN ISO 6507-1:2005 at a load of 2.5 kg. The hardness was 992.95 N/mm.sup.2.

[0097] A front tooth crown to which colouring liquid had been applied by means of a brush for staining was dry milled from the presintered blocks by means of a CAD/CAM machine (Zenotec Select from Ivoclar Vivadent AG). After the colouring, the crown was dried and densely sintered in a sintering furnace (Programat CS4, program 1, from Ivoclar Vivadent AG). The sintering took only about 35 min, as the temperature schedule below shows.

TABLE-US-00001 About 25 C. to 900 C. (6.7 min) 900 C. to 1460 C. (11.2 min) 1460 C. to 1460 C. (5.0 min) 1460 C. to 1200 C. (3.7 min) 1200 C. to about 1000-950 C. (45-50 s) (furnace opens) 950 C.-1000 C. to about 50 C. (7 min)

Example 2Suspension with 83 wt.-% Zirconia

[0098] For the preparation of a suspension with 83 wt.-% zirconia and the processing thereof to form test pieces (A) and blocks for chairside application (B), Example 1 was repeated with the variation that the solution contained 164.5 g distilled water, 4.05 g dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 1.5 tetramethylammonium hydroxide, and that 810 g zirconia powder which was partially stabilized with 5 mol-% Y.sub.2O.sub.3 (TZ-PX-430 from TOSOH Corporation, primary particle size: 90 nm) were added and the presintering was carried out at 900 C. for 2 h, wherein the heating rate was likewise 0.250 K/min.

[0099] The viscosity of the suspension was 7.0 mPas (at a shear rate of 1000 s.sup.1 and a temperature of 25 C.)

[0100] The respective test pieces obtained in accordance with (A) had the following properties: [0101] Linear shrinkage: 14.43% (test piece: length=25 mm1 mm, width=5 mm0.5 mm and height=4 mm0.5 mm) [0102] Density: 3.791 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

[0103] The hardness of the presintered blocks obtained in accordance with (B) was measured by means of the Vickers method according to ISO 14705:2008 and EN ISO 6507-1:2005 at a load of 2.5 kg. The hardness was 496.91 N/mm.sup.2.

[0104] A front tooth crown to which colouring liquid had been applied by means of a brush for staining was dry milled from the presintered blocks by means of a CAD/CAM machine (Zenotec Select from Ivoclar Vivadent AG). After the colouring, the crown was dried and densely sintered in a sintering furnace (Programat CS4, program 1, from Ivoclar Vivadent AG). The sintering took only 34 min, as the temperature schedule below shows.

TABLE-US-00002 About 25 C. to 900 C. (6.7 min) 900 C. to 1460 C. (11.2 min) 1460 C. to 1460 C. (5.0 min) 1460 C. to 1200 C. (3.7 min) 1200 C. to about 1000-950 C. (45-50 s) (furnace opens) 950 C.-1000 C. to about 50 C. (7 min)

Example 3Suspension with 80 wt.-% Zirconia

[0105] For the preparation of a suspension with 80 wt.-% zirconia and the processing thereof to form test pieces (A), Example 1 was repeated with the variation that the solution contained 179.5 g distilled water, 3.6 g dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 1.5 g tetramethylammonium hydroxide, and that 720 g zirconia powder which was partially stabilized with 4.25 mol-% Y.sub.2O.sub.3 (TZ-PX-551 from TOSOH Corporation, primary particle size: 90 nm) were added.

[0106] The viscosity of the suspension was 14.6 mPas (at a shear rate of 500 s.sup.1 and a temperature of 25 C.)

[0107] The test pieces obtained according to (A) had the following properties: [0108] Linear shrinkage: 14.59% [0109] Density: 3.780 g/cm.sup.3.

[0110] The test pieces obtained according to (A) were presintered at 1000 C. for 2 h. The heating rate was 0.250 K/min. The test pieces were then cooled to room temperature within 24 h. The presintered test pieces had a linear shrinkage of 13.88%.

Example 4Suspension with 76 wt.-% Zirconia

[0111] For the preparation of a suspension with 76 wt.-% zirconia and the processing thereof to form blocks for chairside application (B), Example 1 was repeated with the variation that 630 g zirconia powder which was partially stabilized with 3 mol-% Y.sub.2O.sub.3 (AuerDent 3Y-5A A2 from Treibacher Industrie AG, primary particle size: 40 nm) were added to the solution, and that the presintering was carried out at 820 C. for 2 h, wherein the heating rate was likewise 0.250 K/min.

[0112] The hardness of the presintered blocks obtained according to (B) was measured by means of the Vickers method according to ISO 14705:2008 and EN ISO 6507-1:2005 at a load of 2.5 kg. The hardness was 440.38 N/mm.sup.2.

[0113] Furthermore, a disc was sawn off the presintered blocks and densely sintered in a sintering furnace (Programat CS4 from Ivoclar Vivadent AG) within only 34 min according to the following temperature schedule:

TABLE-US-00003 About 25 C. to 900 C. (6.7 min) 900 C. to 1460 C. (11.2 min) 1460 C. to 1460 C. (5.0 min) 1460 C. to 1200 C. (3.7 min) 1200 C. to about 1000-950 C. (45-50 s) (furnace opens) 950 C.-1000 C. to about 50 C. (7 min)

[0114] The densely sintered discs had a contrast ratio CR of 89.16. The contrast ratio was determined in accordance with BS 5612 (British Standard) using a spectrophotometer (Minolta CM-3700d). It is a measure of the opacity of a material, wherein a contrast ratio of 100 corresponds to a completely opaque material and a contrast ratio of 0 corresponds to a completely translucent material.

[0115] The measured value of 89.16 shows the high translucence of the samples produced according to the invention and in the case of conventionally produced blanks it can only be achieved with very long sintering times of more than 2 hours.

Example 5Suspension with 83 wt.-% Zirconia

[0116] For the preparation of a suspension with 83 wt.-% zirconia and the processing thereof to form test pieces (A), Example 1 was repeated with the variation that the solution contained 164.5 g distilled water, 3.15 g dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 1.5 g tetramethylammonium hydroxide, and that 810 g zirconia powder which was partially stabilized with 3 mol-% Y.sub.2O.sub.3 (TZ-PX-245 from TOSOH Corporation, primary particle size: 40 nm) were added.

[0117] The test pieces obtained according to (A) were debound at 500 C. and then had the following properties: [0118] Pore volume: 0.11391 cm.sup.3/g [0119] Pore radius: 0.0190 m [0120] Density: 3.5617 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

[0121] The debound test pieces were then presintered at 1050 C. for 2 h. The presintered blanks obtained then had the following properties: [0122] Pore volume: 0.09931 cm.sup.3/g [0123] Pore radius: 0.0242 m [0124] Density: 3.8866 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

[0125] Debound test pieces were likewise presintered at 1000 C. for 2 h. The presintered blanks obtained were densely sintered in a sintering furnace (Cerec Speedfire from Dentsply Sirona) within only 20 min according to the following temperature schedule:

TABLE-US-00004 About 25 C. to 500 C. (88 s) 500 C. to 1580 C. (190 s) 1580 C. to 1580 C. (360 s) 1580 C. to 950 C. (120 s) 950 C. to about 50 C. (420 s)

Example 6Suspension with 83 wt.-% Zirconia+Vacuum

[0126] For the preparation of a suspension with 83 wt.-% zirconia and the processing thereof to form test pieces (A), Example 1 was repeated with the variation that the solution contained 164.5 g distilled water, 4.05 g dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 1.5 g tetramethylammonium hydroxide, and that 810 g zirconia powder which was partially stabilized with 3 mol-% Y.sub.2O.sub.3 (TZ-PX-245 from TOSOH Corporation, primary particle size: 40 nm) were added.

[0127] The test pieces obtained according to (A) were debound and presintered in that they were heated to a temperature of 1000 C. at a rate of 0.25 K/min and held at this temperature for 2 h.

[0128] The presintered blanks obtained were densely sintered in a sintering furnace with MoSi.sub.2 heating elements within a maximum of only 36 min according to the following temperature schedule, wherein the first two steps were carried out under vacuum at a pressure in the range of from about 50 to 100 mbar, and, after feeding air in, the further steps were carried out under ambient atmosphere and with continuous flushing with air.

TABLE-US-00005 About 25 C. to 950 C. (7.1 min) under vacuum 950 C. to 1450 C. (10 min) under vacuum 1450 C. to 1500 C. (1 min) continuous flow of air at ambient atmosphere 1500 C. to 1500 C. (5 min) continuous flow of air at ambient atmosphere 1500 C. to 1100 C. (2.9 min) ambient atmosphere 1100 C. to about 50 C. (5-10 min) ambient atmosphere

[0129] To investigate the optical properties, the densely sintered test pieces were ground plane-parallel (20-m diamond disc) to a thickness of 2 mm0.02 mm and a diameter of 18 mm0.5 mm and then polished (SiC paper, grit 1000).

[0130] The densely sintered discs had a contrast ratio CR of only 78.95%. They therefore showed a very high translucence and the favourable influence of the use of vacuum and of the subsequent atmosphere exchange during the dense sintering.

Example 7Suspension with 83 wt.-% Zirconia

[0131] For the preparation of a suspension with 83 wt.-% zirconia and the processing thereof to form test pieces (A), Example 1 was repeated with the variation that the solution contained 164.5 g distilled water, 3.15 g dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 2.0 g tetramethylammonium hydroxide, and that 810 g zirconia powder which was partially stabilized with 4.25 mol-% Y.sub.2O.sub.3 (TZ-PX-551 from TOSOH Corporation, primary particle size: 90 nm) were added.

[0132] The test pieces obtained according to (A) were debound at 500 C. and then had the following properties: [0133] Pore volume: 0.11087 cm.sup.3/g [0134] Pore radius: 0.0248 m [0135] Density: 3.5474 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

[0136] The debound test pieces were then presintered at 1050 C. for 2 h. The presintered blanks obtained then had the following properties: [0137] Pore volume: 0.10282 cm.sup.3/g [0138] Pore radius: 0.0315 m [0139] Density: 3.6838 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

Example 8Suspension with 83 wt.-% Zirconia

[0140] For the preparation of a suspension with 83 wt.-% zirconia and the processing thereof to form test pieces (A), Example 1 was repeated with the variation that the solution contained 164.5 g distilled water, 3.15 g dispersant containing citric acid or citric acid salt (Dolapix CE64, from Zschimmer & Schwarz) and 2.0 g tetramethylammonium hydroxide, and that 810 g zirconia powder which was partially stabilized with 5.0 mol.-% Y.sub.2O.sub.3 (TZ-PX-430 from TOSOH Corporation, primary particle size: 90 nm) were added.

[0141] The test pieces obtained according to (A) were debound at 500 C. and then had the following properties: [0142] Pore volume: 0.10559 cm.sup.3/g [0143] Pore radius: 0.0253 m [0144] Density: 3.7831 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)

[0145] The debound test pieces were then presintered at 1050 C. for 2 h. The presintered blanks obtained then had the following properties: [0146] Pore volume: 0.10311 cm.sup.3/g [0147] Average pore radius: 0.0318 m [0148] Density: 3.9083 g/cm.sup.3 (test piece: length=5 mm1 mm, width=5 mm1 mm and height=10 mm1 mm)