WO2018167128A1 - Powder metallurgical method for the production of a dental blank - Google Patents

Abstract

The present invention relates to the production of a dental blank, which optionally contains chromophoric substances, a ceramic or metallic powder as the starting material being mixed with an aqueous, aliphatic polyurethane dispersion to a compressible mass, and to a dental blank produced this way and to the use of the polyurethane dispersions for the production of a dental blank.

Description

 Powder metallurgical process for the preparation of a dental blank. Field of the invention

The present invention relates to the preparation of a dental blank which optionally contains coloring substances, wherein a ceramic or metallic powder is mixed as starting material with an aqueous, aliphatic polyurethane dispersion to a compressible mass, as well as a dental blank thus produced and the use of the polyurethane Dispersions for the preparation of a dental blank. Background of the invention

The use of ceramic or metallic powders for the production of dental restorations, ie dental prostheses or whole teeth, such as implants or inlays, onlays, veneers, crowns or bridges, has long been known. Likewise, composites of ceramics and metal, so-called cermets, are well known. Oxide ceramics are mainly used as scaffolding material for dental restorations, as this material is characterized by excellent biocompatibility and excellent mechanical properties. Ceramics based on partially stabilized or fully stabilized zirconium oxide have lately been used.

In general, a method for producing dental restorations consists of several substeps. In a first step, the starting material is pressed into a green body. Subsequently, usually a pre-and white firing, which results in a stable dental blank for further processing, in particular a further CAD CAM machining. The starting material generally contains a so-called binder to make the powder press-moldable. As binders known polysaccharides such as starch, sugar or cellulose derivatives, polymers such as polyvinyl acetates, polyvinyl alcohols, or polyacrylates, and alginates are used. A binder should preferably have no tendency to stick with the molds, solidify the starting material such that a processing of the green body is possible, and can be burned out completely and residue-free from the blank during the firing processes again. A major disadvantage of the known processes for the production of blanks from green bodies are the long process times of the heat treatment of the green bodies at high reaction temperatures. Conventional procedures require reaction times of several days at temperatures of 800 ° C to 1100 ° C. EP 0 588 171 A1 describes a process for coating a water-sensitive ceramic powder, in particular aluminum nitride, with a hydrophobic coating. This powder can not be processed with water without hydrophilization by additive coating. In the process outlined in EP 0 588 171 A1, a ceramic powder is mixed with a nonionic surfactant and a binder, for example a polyester-modified polyurethane resin, pressed into a binder-containing green body, and sintered at about 1850 ° C. to form a sintered body. This sintered body can no longer be processed using CAD or CAM. Summary of the invention

The invention relates to a method for producing a dental blank which optionally contains coloring substances, the method comprising the following steps:

(A) preparing a mass of a ceramic and / or metallic powder and an aqueous, aliphatic polyurethane dispersion which is free from

Isocyanate groups;

 (b) preparing a granulate from the mass obtained according to step (a);

 (c) producing a green body by compressing the granules obtained in step (b);

(d) heat treatment, in particular tempering or pre-firing, of the green body obtained according to step (c) for the production of the dental blank at temperatures of 60 ° C to 150 ° C; and optional

(e) machining the dental blank obtained according to step (d) to produce a machined dental blank. In one embodiment of the method according to the invention described herein, the dental blank is sintered in a further step.

In one embodiment of the inventive method described herein, the ceramic powder is a zirconia powder.

In one embodiment of the process of the invention described herein, the zirconia powder is a partially stabilized zirconia powder.

In one embodiment of the inventive method described herein, the metallic powder is a chromium-cobalt-molybdenum sintered metal powder. In one embodiment of the process according to the invention described herein, the polyurethane is selected from a polyester-polyurethane, polycarbonate-polyurethane, polyester-polycarbonate-polyurethane, copolymer based on a polycarbonate-polyurethane and a polyacrylate, and / or a polyether-polyurethane.

In one embodiment of the process of the invention described herein, the polyester-polyurethane and / or the polyester-polycarbonate-polyurethane are in colloidal form.

In one embodiment of the process of the invention described herein, the polyester-polyurethane, the polycarbonate-polyurethane, the polyester-polycarbonate-polyurethane and / or the polyether-polyurethane are in anionic form.

In one embodiment of the process of the invention described herein, the polyurethane dispersion contains admixtures or functional groups of polyesters, polycarbonates, polyester-carbonates and / or polyethers. In one embodiment of the inventive method described herein, the polyurethane dispersion has a solids content of about 20-50 wt .-%, preferably from about 29-46 wt .-%, in particular from about 31-46 wt .-%, especially from about 44-46 wt .-%.

In one embodiment of the inventive method described herein, the content of the aliphatic polyurethane based on the solids content of the mixture of from 0.5 wt .-% to 20 wt .-%.

In one embodiment of the inventive method described herein, the content of the aliphatic polyurethane based on the solids content of the mixture of from 0.5 wt .-% to 6 wt .-%. In one embodiment of the inventive method described herein, the polyurethane dispersion additionally contains at least one antioxidant, at least one pressing agent, at least one lubricant and / or at least one plasticizer.

Furthermore, the present invention relates to a dental blank with optionally coloring compounds preparable by a method described herein.

Moreover, the invention relates to the use of an aqueous, aliphatic polyurethane dispersion which is free of isocyanate groups, for the preparation of a composition of a ceramic and / or metallic powder for producing a possibly sintered dental blank, which optionally contains coloring compounds, in particular according to one of the embodiments described herein.

Detailed description of the invention The object of the present invention was therefore to find a binder or a binder mixture which can advantageously be used in the production of a blank, in particular a dental blank.

Another object of the present invention was to provide a method which enables the production of a blank, in particular a dental blank, with short process times and low reaction temperatures.

Surprisingly, it has been found that an aqueous, aliphatic polyurethane dispersion which is free from isocyanate groups can generally be used as binder in the production of a blank, in particular a dental blank.

The invention therefore relates to the use of an aqueous, aliphatic polyurethane dispersion which is free of isocyanate groups, for producing a mass of a ceramic and / or metallic powder for producing an optionally sintered blank, in particular a possibly sintered dental blank, as well a method for producing a blank, in particular a dental blank, and a blank which can be produced by the method according to the invention, as claimed in the independent patent claims. The blank is in particular a dental blank. Further embodiments of the designated article of the invention can be found in the subclaims and in the following description.

The term "dental" used herein means suitability for any application or for use in the dental or orthodontic field, especially in dental restoration.

The particular advantages of the use according to the invention of the polyurethane dispersion described as a binder are, in particular: (a) essentially lacking tendency to stick with the molds, in particular during the press molding; (b) solidification by heat treatment enabling machining; (c) production of an extremely homogeneous green body or compact; (d) producing a green body with significantly higher green strength and thus a significantly higher edge strength than can be achieved with known binder systems, which allows for surface treatment in particular with nanoscale powders; (e) wet working the green body, since the network of binder is no longer redispersible in water; (f) good, residue-free burnout without significant odor development; and / or (g) redispersibility in alcohol, whereby process residues (recycled) can be recovered.

Due to the essentially lack of tendency to stick with molds, the best possible press results can be achieved in biaxial, isostatic and / or postisostatic pressing. Subsequent heat treatment, which results in a network-like curing of the binder, leads to the fact that the green body obtained can be solidified in such a way that a subsequent machining comparable to a polymer processing is possible. The processing properties of the cured composition are therefore significantly better than in conventional binder systems. The ultimately residue-free burning out of the binder makes it possible to maintain the mechanical and optical properties, such as transparency and translucence, of the starting material, which is very advantageous for dental purposes. Processing residues such as fragment-like green bodies or chip wastes of the green bodies are easy to recycle because the binder is redispersible in alcohol and thus can be recovered for the process. The method according to the invention is shown in detail below:

The starting material for the process according to the invention are ceramic or metallic powders. The ceramic and metallic powders are commercially available.

Particularly suitable ceramic powders are zirconium oxide powders, which may be partially stabilized and fully stabilized. The use of partially stabilized zirconia is particularly preferred since the flexural strength is particularly high (eg, higher than 1 .100 MPa). It is also possible to use a mixed phase of tetragonal and cubic zirconium oxide, which can lead to a sufficient flexural strength of greater than 500 MPa. In general, bending strengths of greater than 500 MPa or greater than 800 MPa are desired. Suitable stabilizing substances are oxides such as calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y2O3), erbium oxide (Er ₂ Os) or cerium oxide (CeO 2). The content of the stabilizing oxides may be in the range of 2.5 mol% to 6 mol%, preferably in the range of 2.5 mol% to 5 mol% relative to the total content of zirconium oxide and the stabilizing oxide. Yttrium-fully stabilized or yttrium-partially stabilized zirconium oxide has proven particularly advantageous. The optional presence of alumina (AI2O3) may vary depending on Need to increase strength and decrease light transmission (translucency). The relative content of alumina preferably ranges from 0% by weight (no alumina) to 0.3% by weight, preferably in a range from 0.05% by weight to 0.25% by weight. , The optional presence of silica (S1O2) can further reduce translucency. The content is preferably less than 0.02 wt .-%. Chromium oxide (Cr ₂ O ₃ ), erbium oxide (Er ₂ O), iron oxide (Fe ₂ O), praseodymium oxide (Pr ₆ On), manganese oxide (Mn ₂ O 3) or cobalt oxide (CO ₂ O 3) are examples of optionally available coloring substances. For example, the presence of iron oxide gives a more yellowish hue, the presence of erbia a rather reddish hue and the presence of cobalt oxide a rather gray hue. Based on suitable color mixtures, the color or the color gradient of a natural tooth can thus be largely reproduced. The content of the coloring substances may generally be less than 1% by weight, preferably in a range of 0.03 to 0.6% by weight. Zirconia powders are generally available commercially. Examples of suitable zirconia powders have the following compositions: zirconia (above 90 and 94% by weight, respectively) with hafnium oxide (below 3% by weight), yttria (5 to 9.5% by weight) and alumina (0.05 to 0.3% by weight) at a total content in the mixture of more than 99.9% by weight; with further additions of silicon oxide (<0.02 wt .-%), iron oxide (<0.01 wt .-%) and optionally erbium oxide (0.1 wt .-% to 0.6 wt .-%). As a result, light transmittances of 10% to 35% and brightnesses of 65-90 (L ^* value) can be set. Further examples can be found, for. In US 9,212,065 or EP 1 900 341.

Cobalt-chromium mixtures have proved particularly suitable as metallic powders. The powder mixtures usually contain 50-70% by weight of cobalt and 20-30% by weight of chromium and, if appropriate, further metals such as molybdenum, manganese, silicon, tungsten or iron and, if appropriate, minimum amounts of carbon, in order to increase the strength if necessary increase. An example of a suitable metallic powder is a chromium-cobalt-molybdenum sintered metal powder which is also commercially available. Furthermore, it is advantageous to add an antioxidant or an antioxidant in order to suppress a chemical reaction of the metal powder with water from the binder dispersion. Suitable antioxidants are, for example, oxidation inhibitors for aqueous carbide preparation, such as Metamax 1-15 from Zschimmer & Schwarz GmbH & Co. KG, Lahnstein.

Another starting material for the process are the aqueous, aliphatic polyurethane dispersions according to the invention which are free of isocyanate groups and which serve as a so-called binder. The polyurethane dispersions may contain functional groups or admixtures of polyesters, Polycarbonates, polyester-carbonates or polyethers possess. The polyurethane dispersions may be free of organic solvents but may also contain organic solvents such as alcohol. They can be anionic or neutral depending on the functional group. The properties may be viscous or low viscous and / or colloidal. The aliphatic, isocyanate-free polyurethanes are preferably generally known polyester-polyurethanes, polyether-polyurethanes, polycarbonate-polyurethanes, polyester-polycarbonate-polyurethanes or copolymers of a polycarbonate-polyurethane and a polyacrylate. The solids content in the dispersion moves, for example, at about 20-50% by weight, preferably at about 29-46% by weight, in particular at about 31-46% by weight, especially at about 44% by weight. 46% by weight. The pH is usually 7.0-9.0. The viscosity is 10-1,000 mPa generally ^'s. A preferred polyurethane dispersion is an aqueous, anionic, aliphatic polyether polyurethane dispersion without free isocyanate groups having a solids content of 44-46% by weight, a pH of 7.0-8.0 and a viscosity of 100-1000 mPa ^' s. Other usable polyurethane dispersions can be found in the table below. The aqueous polyurethane dispersions are generally available commercially. However, preparation examples are also found, for example, in EP 1 717 284 or EP 1 773 919.

The content of the aliphatic polyurethane based on the solids content of the mixture is generally from 0.5 wt .-% to 20 wt .-%, preferably from 0.5 wt .-% to 15 wt .-%, in particular of 0.5 % By weight to 10% by weight, especially from 0.5% by weight to 6% by weight. In a particularly preferred embodiment, the content of the aliphatic polyurethane based on the solids content of the mixture of from 0.5 wt .-% to 5 wt .-%. The polyurethane dispersions mentioned can be used as binders in both wet and dry processing. According to the invention in a first step, a mass, also called slip, from a ceramic and / or metallic powder, which advantageously has a primary particle size up to 16 m, wherein the primary particle size can be greater than 16 μιτι, and an aqueous aliphatic polyurethane dispersion , which is free of isocyanate groups. In addition, at least one antioxidant, at least one pressing or lubricating agent, at least one plasticizer, and / or fluidizing agent may be added to the composition. As a result, the pressing and / or Zerspanverhalten of the green body, also called powder compacts, adapted or improved. Suitable pressing or slip aids are based z. B. on fatty acids or hydrocarbons such. As waxes, oleic acids, mineral oils and / or petroleum. Suitable plasticizers are, for example, cellulose derivatives, for. B. Methylcelluloses or ethylcelluloses, glycerol, polyols, polyethylene glycol, e.g. As PEG 400, and / or polyethylene adducts. Suitable condensing agents are for. As fish oils or refined products, acrylates, citrates and / or combinations of carboxylic acids, the z. T. can be neutralized with ammonium or amines. For example, an oxide-ceramic slip for wet processing and downstream spray granulation preferably contains, in addition to the aqueous, aliphatic polyurethane dispersion usable according to the invention, the following additional components:

(a) pressing means for forming sliding layers and reducing the internal friction of the ceramic mass during the pressing process,

 (B) plasticizer for increasing the plasticity and the Einformverhaltens of ceramic materials and

 (c) liquefying agent for influencing the theological behavior of the slurry. The aqueous, aliphatic polyurethane dispersion which can be used according to the invention advantageously performs several tasks simultaneously. For example, the dispersion is highly film-forming and does not show any rheological changes in the slurry viscosities, e.g. through thickening processes. The downstream press molding provides optimally compacted homogeneous green bodies with higher edge strengths. The option of additionally hardening the finished compact in a subsequent heat treatment process allows for exceptionally good and advantageous green processing. In addition, said polyurethane dispersion behaves comparatively odorless.

The preparation of the mass or the slip can be carried out by mixing units with downstream sieve granulation, via a specific re-drying with subsequent granulation or via spray granulation of aqueous or alcoholic slips. As a result, in a further step, a granulate from the resulting mass is produced. Starting from a metal powder, the granule size should advantageously be less than 250 μιτι. Starting from a ceramic powder, the granule size should advantageously be less than 125 μιτι. The pressing moisture of the mass is preferably 4-6%. Subsequently, the granules by means of conventional pressing techniques z. B. biaxially, isostatically and / or postisostatically compressed and possibly compacted. The compression is advantageously biaxial. The densification is advantageously isostatic. The compactness of the green body obtained is advantageously at 5.7-6.4 g / cm ³ when using metallic powders, eg. B. CrCo alloys, and at 3.3-3.4 g / cm ³ when using ceramic powders, eg. B. yttria-stabilized zirconia. The green body obtained according to process step (c) is subjected to a heat treatment (process step (d)) (annealing process) in a further step for the production of the blank. The heat treatment can be carried out at temperatures of 60 ° C and 150 ° C, preferably at temperatures of 80 ° C and 130 ° C, more preferably at temperatures of 100 ° C to 125 ° C. In a particularly preferred embodiment, the heat treatment is carried out at about 120 ° C. At temperatures between 60 ° C and 150 ° C, advantageously at about 120 ° C, the binder can additionally cure. The holding time at these temperatures is advantageously 1-10 hours. In the process according to the invention, the heat treatment of process step (d) is carried out for 1 to 10 hours, for example 1 to 2 hours, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours.

A significant advantage of the method according to the invention is therefore that the heat treatment of process step (d) on the one hand in a much lower temperature range, namely at temperatures of 60 ° C and 150 ° C can be performed, as in conventional methods, which at temperatures of 800 ° C and 1000 ° C are performed. On the other hand, the process time is significantly shorter than in conventional processes for the production of blanks, which is 3 days. Surprisingly, therefore, the energy consumption in carrying out the method according to the invention is significantly lower than in conventional methods, so that the inventive method is environmentally friendly than previously known method.

The blank thus obtained can then be machined wet or dry by conventional processing techniques. These include, for example, turning, milling, grinding, drilling or sawing. Advantageously, the blank can be processed using water. As a result, the milling speed can be up to four times faster than in known methods. The surface quality is also significantly improved. A significant advantage of the blank obtained according to the invention lies in a significantly increased green strength, in particular in a significantly increased edge strength. As a result, much more filigree structures can be worked out. Especially when using zirconium oxide as the starting material, it is possible to produce significantly finer structures and thinner wall thicknesses than, for example, with pre-sintered ceramic blanks. The adjustable milling speeds are also three to four times faster than with conventional manufacturing processes. Advantageous in the inventive method also that the chippings or the green break can then be redispersed in alcohol (eg ethanol) and essentially completely recycled and reused. In this case, recycled masses basically show no negative changes in the properties of the final product. The blank thus obtained is surprisingly more suitable for applications in the dental field or in the orthodontic area than blanks which have been produced by conventional methods.

The processed blank thus obtained may further under normal sintering conditions, for. B. under oxidizing or reducing atmosphere, are sintered. In this case, a so-called debinding of the binder, wherein the binder according to the invention burns out completely and almost odorless. In particular, when using metallic sintered materials found here no negative interaction reactions, eg. B. carbide formation, instead.

The preceding and final shapes may advantageously be computer controlled, e.g. B. by means of a CAD / CAM system or Kopierfräsverfahren performed.

The following examples are intended to illustrate the invention without limiting it.

Examples 1. Useful polyurethane dispersions

According to the present invention, it is possible to use aqueous, solvent-free, or solvent-containing, viscous or low-viscosity, ionic or anionic dispersions based on an aliphatic polyurethane without free isocyanate groups. The polyurethane dispersion may also have admixtures or functional groups of polyesters, polycarbonates, polyester-carbonates or polyethers. The following table gives examples of suitable polyurethane dispersions (PU dispersions):

Polycarbonat-Polyurethans ohne freie

Polycarbonate-polyurethane without free

 isocyanate

 Aqueous, anionic, solvent-free

 colloidal, low-viscous dispersion of a

 3 34-36 8.0-9.0 20-300 aliphatic polyester-polycarbonate-polyurethane without free isocyanate groups

 Aqueous, colloidal, anionic, low viscosity

 4 Dispersion of an aliphatic polyester 31 - 34 8.0 - 9.0 10 - 500 polyurethane without free isocyanate groups

 Aqueous, low viscosity, anionic,

 solvent-free dispersion of a

 5 copolymers based on an aliphatic 34 - 36 7.5 - 8.5 20 - 200 polycarbonate polyurethane and a

 Polyacrylate.

 Aqueous, anionic dispersion of a

 6 aliphatic polyether polyurethane without free 44 - 46 7.0 - 8.0 100 - 1000 isocyanate groups

2. Preparation of a chromium-cobalt sintered metal molding

20 kg of a schmelzverdüsten chromium-cobalt-molybdenum sintered metal powder of the Fa. HC Starck GmbH (AMPERSINT ^® F75) was placed in a mixing tank and subsequently with a liquid binder mixture consisting of 175 g ethanol, 600 g of an anionic aliphatic polyether-polyurethane without free isocyanate groups, with a solids content of 44 to 46 wt .-% (PU dispersion no. 6), and 100 g of the antioxidant Metamax 1-15 of Zschimmer & Schwarz GmbH & Co KG added. The content of the aliphatic polyurethane based on the solids content of the mixture is thus 3 wt .-%. Manufacturer according to the metal powder has a primary particle size of up to 16 μιτι. The use of the antioxidant prevents the chemical interaction of the metal powder with water from the binder. The mixture was then mixed in a 20 liter Lindor Intensive Mixer for 5 to 10 minutes. After the mixing process, a screen granulate was prepared from the resulting moist mixture, which has a granule size smaller than 250 μιτι. The adjusted pressing humidity of the molding compound was 4 to 6%. The sieve granules were then first biaxially pre-pressed and then isostatically densified in a shrink-wrapped plastic foil. Several series of tests showed a preferred biaxial compression pressure of 400 kp / cm ² with a tool diameter of 92 mm. Experiments with isostatic pressing pressures of 1,000 to 3,000 bar gave a preferred densification at 1, 400 bar. This resulted in postisostatic moldings with densities of 5.70 to 6.40 g / cm ³ , usually of 6.05 g / cm ³ , which were subsequently dried and tempered so that solvent residues and moisture were expelled. Another Series of experiments at temperatures between 60 and 150 ° C for 1 to 2 hours gave an additional curing of the binder at preferably 120 ° C. After the heat treatment, the blank or the corresponding blank could be machined wet or dry by turning, milling, drilling, grinding or sawing with conventional processing machines.

 In comparison to the conventional green bodies, the strengths and in particular the present edge strengths of the obtained compacts were much higher with the result that it could be worked out much filigranere structures.

After working, the sintered metal bodies or components were fired in a protective gas sintering furnace under a reducing atmosphere as shown below.

 Sintering program:

 Stage 1: heating from room temperature at 10 ° C / min (600 ° C / h) to 500 ° C; after that the atmospheric debinding is completed;

 Stage 2: heating from 500 ° C at 12 ° C / min (720 ° C / h) to 1 .130 ° C

 Switch on gas flow - small sintering dish (0 80 mm) 1 l / min argon

 - large sintering tray (0 100 mm) 1, 2 l / min argon;

Stage 3: heating from 1 .130 ° C at 5 ° C / min (300 ° C / h) to 1 .280 ° C for the

 Temperature compensation;

 Stage 4: Holding time of one hour on the sintering temperature;

 Step 5: Cooling from 1, 280 ° C at 10 ° C / min (600 ° C / h) to 400 ° C;

 Gas purge is automatically turned off and the oven opens.

 End of program - sintered parts can be removed.

After firing, a chromium-cobalt-sintered metal molding resulted, which had the following optimized and improved material properties:

 Alloy composition:

 Co base

 Cr 27.5% - 29.5%

Mo 5.5% - 6.5%

 Si <0.1%

 Fe <0.1%

 Mn <0.1%

C <0.1% Ni <0.1%

 Technical characteristics:

Density: 7.9 - 8.0 g / cm ³

 Fraction 10%

E-modulus: approx. 190 GPa

 Vickers hardness: about 285 HV 10

Thermal expansion: 14.1 · 10-6Κ ^"1

(25 - 600 ° C)

 Melting range: 1 .390 - 1 .415 ° C

Sintering temperature: 1 .260 ° C

Corrosion resistance: very good

Color: metallic gray

 A comparison with known chromium-cobalt products revealed that the final density achieved was about 5% higher and the flexural strength was 5-10% higher and no carbide formation occurred, resulting in no embrittlement, lower hardness and better post-processing.

3. Preparation of a Zirconia Shaped Body

600 g of binderless partially stabilized zirconia powder containing 3 mol% of grade Zpex (3Y-TZP) from Tosoh Corporation and 400 g of deionized water were placed in a mixing drum and dispersed using 800 g of 1 mm zirconia grinding balls. The processing of the ceramic slurry was carried out on a roller bench for 24 hours at a speed of 60 U / min. After the run time, an anionic aliphatic polyether polyurethane without free isocyanate groups having a solids content of 44 to 46% by weight (PU dispersion No. 6) in a concentration of 6 to 10% by weight was added to the slip in a series of experiments the dry zirconia powder was added. The content of the aliphatic polyurethane based on the solids content of the mixture is thus 2.64 wt .-% to 4.6 wt .-%. 8% by weight, ie 48 g, of solids have proven to be advantageous. After the addition, the slurry was again homogenized for 2 to 3 hours. After conditioning, the slurry was removed from the roll bank and sieved off the milling balls added for processing.

From the ceramic schiicker thus obtained it was subsequently possible to prepare a compressed granulate by means of spray drying or by drying and subsequent sieving. It had been shown that the spray drying a more homogeneous and error-free Press granules delivered as in conventional processes. Spray-drying was carried out on an Anhydro-Technikumsprühturm. The inlet temperature was 250 ° C, the exhaust air temperature 1 10 ° C. The spray tower operated on the countercurrent principle. The spray granules resulting after spray-drying were screen-printed prior to compression down to a size of less than 125 μm.

The spraying of the spray granules was carried out on a biaxial press. The tool diameter was 105 mm and the biaxial pressure was 500 kgf / cm ² . Subsequently, the green body was again vacuum-sealed in foil and postisostatically pressed in a test series with a cold-isosatic pressing pressure of 1 to 4000 bar. As advantageous resulted in a pressing pressure of 3,000 bar. The resulting green body had a compact density of 3.30 to 3.40 g / cm ³ . This was followed by annealing the compact for 1 to 2 hours in a test series in the temperature range from 60 to 150 ° C. As advantageous resulted in a tempering temperature of 120 ° C.

 After heat treatment, the blank or the corresponding blank could be machined wet or dry without any problems by turning, milling, grinding or sawing with conventional processing machines.

Especially with the zirconium oxide, which is fine ceramic, it was possible by mechanical machining to produce significantly finer structures and thinner wall thicknesses than, for example, with presintered ceramic blanks. The adjustable milling speeds were three to four times faster than with the conventionally set dental pre-combustion blanks.

 The sintering of the ceramic was carried out under atmospheric conditions and requires no change in the usual sintering conditions for the material. The additive, which was still in the blank or in the milled parts, burned out completely and almost odorless.

 Chemical composition:

Zirconia (ZrO ₂ / HfO ₂ ): 94%

Yttrium oxide (Y ₂ O ₃ ): 4.5-5.4%

Alumina (Al ₂ O ₃ ): <0.5%

Other <0.5%

 Technical characteristics:

Density: 6.05 / cm ³

Vickers Hardness: 1250 Hvi ₀

 Bending strength: 1 .175 MPa

Modulus of elasticity 210 GPa

CW: 10-10 ^" 6K ^{" 1} Sintering temperature 1 .450 ° C

Holding time: 2 h

 Color white

4. Production of an Alternative Zirconia Shaped Article In order to optimize the compression and compression properties, pressing and sliding aids were additionally added to an offset based on polyurethane dispersion. The process was carried out according to Example 3 based on the partially stabilized zirconium oxide (3Y-TZP) with spray drying and dry treatment. The offset contained an aliphatic polyester polyurethane without free isocyanate groups having a solids content of 29 to 31 wt .-% (PU dispersion no. 1) in a concentration of 1, 5 wt .-% based on the dry zirconia powder. Owing to the further addition of 0.7% by weight of oleic acid and 0.5% by weight of polyethylene glycol 400, an optimized compression and compression behavior was demonstrated. This offset variant also showed complete densification of the sintered body after the final sintering.

Claims

claims Process for the preparation of a dental blank which optionally contains coloring substances, the process comprising the following steps:  (A) preparing a mass of a ceramic and / or metallic powder and an aqueous aliphatic polyurethane dispersion which is free of isocyanate groups;  (b) preparing a granulate from the mass obtained according to step (a);  (c) producing a green body by compressing the granules obtained in step (b);  (d) heat treatment, in particular tempering or pre-firing, of the green body obtained according to step (c) for the production of the dental blank at temperatures of 60 ° C to 150 ° C; and optional  (e) machining the dental blank obtained according to step (d) to produce a machined dental blank. A method according to claim 1, characterized in that in a further step, the dental blank is sintered. A method according to claim 1 or 2, characterized in that the ceramic powder is a zirconium oxide powder. A method according to claim 3, characterized in that the zirconia powder is partially stabilized zirconia. A method according to any one of claims 1 -3, characterized in that the metallic powder is a chromium-cobalt-molybdenum sintered metal powder. Method according to at least one of claims 1-5, characterized in that said polyurethane is selected from a polyester-polyurethane, polycarbonate-polyurethane, polyester-polycarbonate-polyurethane, copolymer based on a polycarbonate-polyurethane and a polyacrylate, and / or a polyether polyurethane. A method according to claim 6, characterized in that the polyester polyurethane and / or the polyester-polycarbonate polyurethane is in colloidal form. 8. The method according to claim 6, characterized in that the polyester polyurethane, the polycarbonate polyurethane, the polyester-polycarbonate polyurethane and / or the polyether polyurethane is present in anionic form. 9. The method according to any one of claims 1-8, characterized in that the polyurethane dispersion contains admixtures or functional groups of polyesters, polycarbonates, polyester-carbonates and / or polyethers. 10. The method according to any one of claims 1 -9, characterized in that the polyurethane dispersion has a solids content of about 20-50 wt .-%, preferably from about 29-46 wt .-%, in particular of about 31 -. 46 wt .-%, especially of about 44-46 wt .-% has. 1 1. Method according to one of claims 1 to 10, characterized in that the content of the aliphatic polyurethane based on the solids content of Mixture of 0.5 wt .-% to 20 wt .-% is. 12. The method according to claim 1 1, characterized in that the content of the aliphatic polyurethane based on the solids content of the mixture of 0.5 wt .-% to 6 wt .-% is. 13. The method according to any one of claims 1 -12, characterized in that the polyurethane dispersion additionally contains at least one antioxidant, at least one pressing agent, at least one slip aid and / or at least one plasticizer. 14. Dental blank with optionally coloring compounds produced by a method according to any one of claims 1 -1 1. 15. Use of an aqueous, aliphatic polyurethane dispersion which is free of isocyanate groups, for the preparation of a mass of a ceramic and / or metallic powder for producing a possibly sintered dental blank, which optionally contains coloring compounds, in particular according to one of the embodiments at least one of claims 3-13.