Patent Application
20110160036
The present invention provides a valve metal oxide formulation having organic excipients, wherein the molding pressure necessary for achieving a green density of at least 50% of the theoretic density is 200 MPa or higher, and the force required for the destruction of the blank is 10 MPa or higher in the axial and radial direction. The present invention further relates to a method for the production thereof.
Zirconium oxide is used for production of ceramic bodies as so-called partially or fully stabilized zirconium oxide which are in turn used as components. Examples include medical implants, heat insulation layers, pump rotors or mill linings, but also highly stressed and filigreed construction components. These components are conventionally produced from a ceramic powder by various processes of shaping and subsequent sintering at temperatures above 1000° C. Shaping of the zirconium oxide powder can be carried out by various processes familiar to the person skilled in the art, such as injection molding and belt casting, extrusion, slip casting or electrophoretic deposition, but also more simply and less expensively by axial pressing at or close to room temperature (“cold pressing,” in contrast to hot pressing where axial compaction takes place at high temperatures). A special form of cold pressing is isostatic pressing, in which the powder is filled into a flexible container, which is closed, and compaction is effected by means of a liquid as a pressure transmission medium at pressures of between 300 and 2,200 bar. This shaping is used in particular for large components. In this case, the compact or sintered body is machined in order to bring out the contour of the component.
Since machining of sintered ceramic bodies is expansive due to their high hardness, it is desirable to perform the majority of the mechanical machining in advance by machining the compact by grinding, drilling, milling or machining (“green machining”). A part is thereby produced which is larger in the three spatial dimensions by the expected sintering shrinkage. This is then sintered to the final contour, and subsequently, if at all, subjected to only minor hard machining in order to adhere to the specified dimensions. However, the compact is preferably produced in a way that neither green nor hard machining is necessary (“net shape”). This requires on the one hand a very accurate production of the pressing mold which takes into account the shrinkage of the compact during sintering, and also a sufficiently high internal cohesion (“cohesiveness”) of the compact, which must be ejected from the mold after the pressure is released. A too low cohesiveness leads to fractures on the compact on ejection from the pressing molds and therefore to rejects. A prerequisite for the net shape technology by means of axial pressing already established in powder metallurgy is thus a high cohesiveness, which can, for example, be measured with an apparatus from KZK Powder Tech Corp., USA. From experience in practice, a cohesiveness of at least 0.7, for example, 0.8 or more, for example, 1 or higher, is required for net shape technology. As the cohesiveness increases, the risk of fractures on ejection decreases, and the reject rate is lowered.
The pressed parts are sintered via a suitable temperature program over time, the conventional contents of organic auxiliary substances thereby decomposing and being driven out in the form of gases. The auxiliary substances have diverse tasks, for example, they act as lubricants to make possible the plastic passing of ceramic particles against each other, which are brittle-hard per se, which is necessary during the compaction by pressing. Further organic auxiliary substances act as binders to impart mechanical stability to the compact (“green strength”). Examples of pressing or lubricating auxiliary substances are paraffin, ester or acid amide waxes, and examples of binders are polyethylene glycols, polyvinyl alcohols or polyacrylates. The functional group thereof can also be esterified or alkylated, and copolymers are often used. In the case of very brittle binders, a liquid plasticizer is employed, for example glycol, glycerol or a low molecular weight polyethylene glycol, in order to render the binder auxiliary substance plastically deformable. This list is not exhaustive. Inorganic powders with organic auxiliary substances are formulations.
Zirconium oxide powders which have specific surface areas so high that a sufficient driving force for sintering without pressure exists, serve as the starting material for isostatic or axial pressing. Values of between 3 and 50 m2/g are conventional.
“Stabilizing” in connection with zirconium oxide is understood to mean that other metal oxides which, as pure oxides having a different metal:oxygen ratio to ZrO2, are dissolved in the zirconium oxide lattice. Y2O3, MgO, CaO and further oxides from the group of oxides of the rare earths and combinations of two or more of the abovementioned oxides are conventional. The partial stabilizing has the effect of shifting the phase transformation temperature from tetragonal to monoclinic. Since phase transformations of ZrO2 are associated with changes in volume which would lead to destruction of the ceramic body, industrially manufactured ceramic bodies are chiefly produced from partially or fully stabilized zirconium oxides, conversion of which to the monoclinic lattice type is frozen at room temperature. Other oxides which form crystalline or vitreous foreign phases, such as e.g. Al2O3, or silicates can also be present.
The prevailing teaching has developed from the prior art that for production of ceramic bodies, a partially or fully stabilized zirconium oxide powder should have a monoclinic phase content which is as low as possible, since it is feared that a high monoclinic phase content is an indication of a poor distribution of the stabilizing oxide in the zirconium oxide lattice, which leads to diffusion transportation, and therefore to stresses during sintering, which in turn reduces the strength of the sintered body. Strength is conventionally determined by bending fracture testing on sintered and ground rods in accordance with ISO 843 (4-point flexural strength) or JIS R 1601 (3-point flexural strength).
This teaching has led to production processes for the partially stabilized zirconium oxide powders being aimed at having a monoclinic phase content which is as low as possible. In a zirconium oxide with 3 mol % of Y2O3, values of between 5 and 15 vol. % of monoclinic phase, determined by x-ray diffraction, are conventional. Because of the low monoclinic content, conventional production processes for partially stabilized zirconium oxide powders require high temperatures and long holding times to ensure complete distribution of the Y, which is very energy-consuming. Alternatives to this are processes which require no diffusive transportation of Y, such as hydrolysis of Zr/Y mixed chlorides in spray reactors in the gas phase or coprecipitation of Zr salts and Y salts in an aqueous medium. Gas phase hydrolysis requires handling of hazardous, volatile chlorides, and the coprecipitation process generates waste water and a neutral salt, for example sodium chloride or sulfate. Sodium and sulfate must be washed out of the precipitated product thoroughly and with great effort.
Formulation with organic auxiliary substances takes place after the preparation of the zirconium oxide powders by means of the processes described above. If cold isostatic or axial pressing is envisaged as the intended use and the powder is thus required in the dry state for indirect shaping, the zirconium oxide powder is conventionally dispersed in a liquid, the auxiliary substances required are dissolved or dispersed therein. If appropriate, the suspension is then also subjected to grinding by means of a comminuting apparatus. It is then dried to a formulation by means of spray or fluidized bed drying. Agglomerates having dimensions in the range of from 50 to 1,000 μm are formed thereby. Agglomeration is not absolutely necessary for cold isostatic pressing, and the formulation can also be prepared by a procedure in which the zirconium oxide is wetted with a liquid which contains the required auxiliary substances in a dissolved or dispersed form and the moist powder is dried, for example, in a tumble or paddle dryer. In all cases, the organic auxiliary substances for the most part remain after the drying if unintentional evaporation losses do not occur. Formulated zirconium oxide powders often still contain organic auxiliary substances which facilitate the preparation of the dispersion, such as liquefiers, defoamers, surfactants or reagents for adjustment of the pH.
The liquid used for preparation of the dispersion can be either water, alcohols, hydrocarbons or a ketone or mixtures thereof. Although the use of organic liquids is the industrial standard, it has some disadvantages. These are, for example, easy combustibility, explosiveness of the vapors in a mixture with air and adverse effects on the health of employees exposed to them. Water is also possible as the liquid, but difficulties arise here since the granules obtained are very hard as a result of strong interactions of the powder particles with one another (this phenomenon is used, for example, in the production of pottery, where it imparts stability to the bodies after drying). As a result of the hardness and strength of the granules, fracture rather than plastic deformation thereof occurs during pressing, or undestroyed granules remain. As a consequence, pressing defects are found, which in turn manifest themselves in pores in the sintered body. These in turn lower the strength, which is highly undesirable. Powder formulations for ceramics of zirconium oxides have therefore hitherto been prepared by drying from organic liquids as the dispersing liquid.
The flexural strength of sintered ceramic bodies, which is an important quality criterion, depends on several factors. The most important influencing parameters are structural defects, such as are represented by pores or inclusions. A high sintered density and the absence of foreign phases, inclusions and macropores are therefore prerequisites. A high sintered density is only achieved, however, if the driving force of the sintering is high enough. This is achieved by the specific surface area of the zirconium oxide powder, or by the primary particle size thereof, which can be determined by optical means. A further prerequisite is a high compact density. An isotropic compact density after the pressing is furthermore necessary, since compact defects or regions of low compact density generate macropores, distortion due to sintering or stresses in the ceramic body. A strength of at least 800 MPa, for example, greater than 900 MP in accordance with ISO 843 is targeted so that the components are universally usable.
A further prerequisite for achieving a high sintered density (as close as possible to the theoretical density) is a sufficiently high compact density. In the case of sintering without pressure, a compact density of about 50% of the theoretical density or higher, is generally necessary. If it is below this compact density, residual porosity of the sintered body increasingly occurs. The pressing pressure available is usually limited by the pressing force available, which is a particular disadvantage for flat parts. The pressing force necessary to achieve 50% of the theoretical density is therefore particularly important. Furthermore, wear of pressing tools correspondingly increases with the pressing pressure applied, so that practical considerations set forth a limit.
The zirconium oxide powders known hitherto as formulations with organic auxiliary substances can indeed be produced to give sintered bodies of sufficient strength, but not in net shape technology by means of axial pressing. A large proportion of hard machining, which must be carried out in an involved manner with expensive diamond or cBN tools, is therefore currently necessarily. As an alternative thereto, machining can be carried out in the green state, which requires, however, a corresponding green strength and leads to powder losses which can be recycled only with difficulty. There is therefore a great interest in those zirconium oxides which are formulated with organic auxiliary substances in a way that they can be processed by means of axial pressing in net shape technology with little waste and without excessive hard machining to provide sintered parts without compromising the properties of the sintered bodies. A sufficiently high cohesiveness is therefore necessary. This depends almost entirely on the organic auxiliary substances used, since in contrast to metal powders, ceramic particles have neither ductile nor cold weldability properties during compaction, and the cohesiveness must be established entirely via the organic auxiliary substances.
Various methods exist for determining cohesiveness. Complete characterization of the compaction properties and the properties of the compact is possible by means of the KZK powder tester of KZK Powder Tech Corp. The following are, for example, evaluated:
The bulk and tap density, the ratio of which (Hausner ratio) is a measure of the compactability;
The pressing pressure necessary for a given compact density (for a given compact density, this is a measure of the deformation resistance and should be as low as possible, if possible lower than 200 MPa at 50% of the theoretical density);
The sliding coefficient (a measure of the friction of the powder on the wall of the pressing tool during the compaction operation, the value should be as close to one as possible, since otherwise wear occurs on the pressing molds);
The elastic relaxation after ejection of the compact (a measure of the internal stresses, this should be as low as possible so that the contour of the compact can be controlled);
The cohesiveness (a measure of the internal cohesion of the compact on ejection from the mold, the value). The cohesiveness is calculated from the ratio of the green strength and the ejection force required, and the value should therefore be at least close to one, but at least above 0.8. Damage to the compact on ejection is otherwise expected; and
The force in the axial and radial direction required to destroy the compact (both are a measure of the green strength, and the values should be as high as possible and should be above 10 MPa).
The actual green density measured on the ejected compact may be lower than the predetermined green density as a result of the elastic relaxation.
A further measure of the compactability is the so-called Hausner ratio, which is the ratio between the tap and bulk density. The higher the Hausner ratio above the value of one, the lower the deformation resistance of a powder. A further measure of the compactability is the pressing pressure necessary to achieve a certain compact density. This is relevant for industrial uses since it determines the pressing force necessary for large parts. If the compact density is too low, for example, because no sufficiently strong press is available for axial pressing, an adequate sintered density is not achieved. The strength of the ceramic part is weakened since pores then remain.
An aspect of the present invention is to provide an agglomerated zirconium oxide in a formulation with organic auxiliary substances which at a given pressing pressure has a cohesiveness sufficient for net shape technology of at least 0.7, and which can be pressed to dense compacts under low pressing pressures, and have dense sintered specimens with an adequate strength.
In an embodiment, the present invention provides a formulation of at least one of a partially and a fully stabilized zirconium oxide powder which includes at least one organic auxiliary substance, wherein a pressing pressure so as to obtain a green density of at least 50% of the theoretical density is 200 MPa or less and the cohesiveness is 0.7 or more.
In an embodiment, the present invention provides an agglomerated zirconium oxide formulation with organic auxiliary substances, which can be compacted under pressing pressures of 200 MPa or less to give compacts with at least 50% of the theoretical density and has a force in the axial and radial direction necessary to destroy the compact of 10 MPa or more and an adequate strength in the sintered part.
It has now surprisingly been found that a ceramic body produced via pressing and sintering and with an adequate flexural strength can be produced even from zirconium oxide powder with a monoclinic phase content of significantly above 30% even when using water as the dispersing liquid, if at the same time the organic formulation is modified in a way that one or more carboxylic acids are used in addition to the binder. Although the action of the carboxylic acids is unclear, these evidently facilitate the compaction operation and increase the cohesiveness in a way that net shape technology can be used.
In an embodiment of the present invention, the zirconium oxide in the formulation is stabilized with 2 mol % to 12 mol % of yttrium oxide, for example, with 3 mol % to 8 mol % or with 3 mol % to 6 mol %.
In an embodiment of the present invention, the zirconium oxide has a monoclinic phase content of up to 30%, for example, of up to 40%, or up to 50% and more. However, the monoclinic phase content should at most be 90%.
The pressing pressure under which 50% of the theoretical density is achieved can, for example, be less than 200 MPa, less than 150 MPa, less than 100 MPa, less than 90 MPa, or even less than 80 MPa. This value can even be less than 70 MPa.
The cohesiveness can, for example, be greater than or equal to 0.7, for example, greater than 0.8, or 1 or higher.
The formulation according to the present invention contains organic auxiliary substances.
The formulation can contain at least one carboxylic acid. This carboxylic acid can, for example, be in an amount of from 0.1 to 5 wt. %, for example, in an amount of from 0.25 to 2.4 wt. %, or from 0.5 to 1 wt. %. In general, a use of 0.5 wt. % or more of the carboxylic acid provides good results. The carboxylic acid can, for example, have a melting point of from 35° C. to 100° C.
In an embodiment, the present invention also provides a process for the preparation of a zirconium oxide formulation, wherein at least one carboxylic acid and at least one binder is added to the zirconium oxide in the presence of a solvent.
The zirconium oxide can, for example, be present as a dispersion or as a suspension in the solvent.
This can, for example, be effected with water as the solvent in the preparation of the suspension or dispersion, as an intermediate product into which the carboxylic acid and the binder are introduced.
The carboxylic acid and the binder can, for example, be added jointly, i.e., together as a solution, dispersion or suspension in a solvent, or spatially separately from one another but simultaneously. The can also, for example, be added sequentially to the suspension or to the dispersion of the zirconium oxide. The carboxylic acid can also be added in the solid form or in the form of a melt.
In an embodiment of the present invention, first the carboxylic acid and then the binder is added.
The present invention thus also relates to a process for the preparation of a zirconium oxide formulation with the steps:
provision of a zirconium oxide suspension or dispersion;
provision of a carboxylic acid in the form of a solution, suspension or dispersion;
provision of a binder in the form of a solution, suspension or dispersion;
addition of the carboxylic acid to the zirconium oxide suspension or dispersion to obtain a first suspension or dispersion as an intermediate product;
addition of the binder to the first suspension or dispersion to obtain a second dispersion;
drying of the second dispersion to obtain granules.
In an embodiment, the present invention provides a process for the preparation of a zirconium oxide formulation with the steps:
provision of a zirconium oxide suspension or dispersion;
provision of a short-chained carboxylic acid in the form of a solution, suspension or dispersion;
provision of a long-chained carboxylic acid in the form of a solution, suspension or dispersion;
provision of a binder in the form of a solution, suspension or dispersion;
addition of the short-chained carboxylic acid to the zirconium oxide suspension or dispersion to obtain a first suspension or dispersion as an intermediate product;
addition of the long-chained carboxylic acid to the zirconium oxide suspension or dispersion to obtain a second suspension or dispersion as an intermediate product;
addition of the binder to the second suspension or dispersion to obtain a third dispersion;
drying of the third dispersion to obtain granules.
The addition of the carboxylic acid can, for example, take place at a basic pH and can, for example, be carried out at a pH of from 8 to 12, for example, from 8.4 to 11, or at a pH of from 9 to 10. The addition of the binder is likewise carried out at a basic pH, for example, at a pH of from 8 to 12, or from 8.4 to 11, or from 9 to 10.
The addition both of the binder and of the carboxylic acid can, for example, be carried out at a temperature of less than 60° C., for example, at less than 50° C., or at less than 35° C. The temperature is ideally room temperature, which is, for example, from about 15° C. to about 28° C., or from 18° C. to 23° C.
The drying can in principle be carried out by any known process, such as, for example, through spray drying or related processes. The carboxylic acids employed can, for example, remain in the end product, the granules.
In an embodiment, the present invention provides a process for the preparation of a zirconium oxide formulation with the steps:
provision of a zirconium oxide suspension or dispersion;
provision of a short-chained carboxylic acid in the form of a solution, suspension or dispersion;
provision of a long-chained carboxylic acid in the form of a solution, suspension or dispersion;
provision of a binder in the form of a solution, suspension or dispersion;
addition of the short-chained carboxylic acid to the zirconium oxide suspension or dispersion to obtain a first suspension or dispersion as an intermediate product;
addition of the long-chained carboxylic acid to the zirconium oxide suspension or dispersion to obtain a second suspension or dispersion as an intermediate product;
addition of the binder to the second suspension or dispersion to obtain a third dispersion;
drying of the third dispersion to obtain granules, the addition of the long-chained carboxylic acid, the short-chained carboxylic acid and the binder in each case being carried out at temperatures of less than 50° C. and at a pH of from 8 to 12.
Carboxylic acids are understood as meaning those organic substances which contain at least one or more carboxyl groups or acquire these by reaction in the suspension. According to the present invention, at least one carboxyl group is not esterified but is present in protonated form. The carboxylic acid can also be employed as a salt, such as, for example, as water-soluble salts of the alkali metals or alkaline earth metals, of zirconium or yttrium or ammonium salts. Corresponding acid chlorides can also be used, since they hydrolyze in aqueous media to provide carboxylic acids, and are therefore referred to as carboxylic acids in the context of the present invention.
The formulation according to the present invention can, for example, contain carboxylic acids which are present in their solid form at room temperature since they have a low vapor pressure and it is therefore ensured that they remain in the dried formulation.
A relatively small content of one or more short-chained carboxylic acid(s) which is liquid at room temperature can also be used, for example, acetic acid or salts thereof, if necessary to control pH. In the case of short-chained carboxylic acids, the addition of the abovementioned carboxylic acid salts can, for example, be used to reduce volatility.
In an embodiment of the present invention, a carboxylic acid of wax-like consistency is used with a the melting point or melting range of between 35 and 100° C.
The carboxylic acids can, for example, be unbranched carboxylic acids, however, they can also be branched or straight-chained.
The carboxylic acid can also contain ether and/or hydroxyl groups. At least one carboxyl group can, for example be terminal.
The carboxylic acid can also be short-chained and nevertheless solid at room temperature, but in return have a higher acidity, for example, oxalic acid, tartaric acid or citric acid.
The carboxylic acids used can in general be mono- di- tri- or polycarboxylic acids, which contain 1 to 30 carbon atoms and, for example, have a melting point or melting range of between 35 and 100° C. On the one hand, short-chained carboxylic acids can be used, these being understood according to the present invention to include carboxylic acids having 1 to 8 carbon atoms. These have a melting point or a melting range of, for example, from 35 to 100° C., and are present either as the free carboxylic acid or as an alkali metal or ammonium salt. Mixtures of these carboxylic acids with one another or alkali metal and ammonium salts thereof can also be used.
A long-chained carboxylic acid can likewise be used, these being understood according to the present invention to include carboxylic acids having 10 to 30 carbon atoms, for example, 10 to 23 carbon atoms.
These can, for example, be saturated or unsaturated aliphatic carboxylic acids. The carbon chain can, for example, be linear, branched or cyclic. The carbon chain can also contain ether groups.
The carboxylic acids can, for example, be unsubstituted or substituted Substituents can, for example, include one or more nitro groups, amino groups, F, Cl, Br, I, or hydroxyl groups, or can contain ether or hydroxyl groups, such as hydroxypropionic acid or citric acid.
The carboxylic acids can, for example, be mono- or polyunsaturated.
The long-chained carboxylic acids can, for example, be saturated fatty acids with a melting point range or melting range of between 35 and 100° C., such as, for example, montanic acid, palmitic acid, stearic acid, mixtures thereof with one another or other carboxylic acids or mixtures of alkali metal or ammonium salts thereof with one another or other carboxylic acids of alkali metal or ammonium salts thereof.
Short-chained carboxylic acids according to the present invention include, for example acetic acid, oxalic acid, glycolic acid, propionic acid, methoxyacetic acid, lactic acid, malonic acid, butyric acid, 2-hydroxybutyric acid, 3-hydroxybutyric acid, butanedioic acid, ethoxyacetic acid, 2,2′-oxydiacetic acid, methoxypropionic acid, succinic acid, ascorbic acid, methylmalonic acid, 2-hydroxysuccinic acid, 2,3-dihydroxysuccinic acid (tartaric acid), dihydroxyfumaric acid, valeric acid, trimethylacetic acid, 2-methylbutanoic acid, isovaleric acid, (Z)-2-methyl-2-butenoic acid (angelica acid), glutaric acid, 3,6-dioxaoctanedioic acid, 2-(2-methoxyethoxy)acetic acid, trans-2,3-dimethylacrylic acid (tiglic acid), glutamic acid, caproic acid, 3-methylvaleric acid, cis-propene-1,2,3-tricarboxylic acid (cis-aconitic acid), 2,2-dimethylbutanedioic acid, 2,3-dimethylbutanedioic acid, 2-methylglutaric acid, 3-methylglutaric acid, citric acid, 2,3,4,5-tetrahydroxyhexanoic acid (mucic acid), oenanthic acid, 2-propylpentanoic acid, butylmalonic acid, diethylmalonic acid, tetrahydroxyheptanoic acid (quinic acid), 2-[2-(methoxyethoxy)ethoxy]acetic acid, acelaic acid, (3R,4S,5R)-3,4,5-trihydroxy-1-cyclohexenecarboxylic acid (shikimic acid), caprylic acid, pelargonic acid, nonanedioic acid (azelaic acid), sebacic acid, salts thereof with alkali metal salts or ammonium salts, acid chlorides or mixtures thereof.
Long-chained carboxylic acids, that is to say carboxylic acids having 10 to 30 carbon atoms, include, for example, generally fatty acids, or alkali metal or ammonium salts thereof. The long-chained carboxylic acids can, for example, be solid at room temperature. Saturated and mono- or polyunsaturated fatty acids can also be employed. Suitable and broadly suitable long-chained carboxylic acids according to the present invention include saturated fatty acids, such as lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, monounsaturated fatty acids, such as undecylenic acid, myristoleic acid, palmitoleic acid, petroselic acid, oleic acid, elaidic acid, vaccenic acid, gadoleic acid, icosenic acid, cetoleic acid, erucic acid, nervonic acid, polyunsaturated fatty acids, such as, for example, linoleic acid, alpha-linolenic acid, gamma-linolenic acid, calendic acid, punicic acid, alpha-eleostearic acid, arachidonic acid, timnodonic acid, clupanodonic acid, cervonic acid, vernolic acid, ricinoleic acid and mixtures thereof and mixtures of alkali metal and ammonium salts thereof.
The carboxylic acid can, for example, be added as an aqueous dispersion. In such a case, it can also additionally contain emulsifiers, such as, for example, fatty acid glycerol esters.
The carboxylic acid can also at least partly be used as a solution if it is partly or completely neutralized, with for example, ammonia.
The carboxylic acid can also be a mixture of several different carboxylic acids, in which case reference is made to herein to a carboxylic acid formulation.
A solution or a dispersion of one or more carboxylic acids, both optionally also partly neutralized by ammonia or short-chained amines, is called a carboxylic acid formulation. This can, for example, also be a mixture of a solution and a dispersion.
In an embodiment of the present invention, at least one of the aforementioned short-chained and one long-chained carboxylic acid are used as a carboxylic acid formulation, as a solution, a dispersion or in each case in partly dissolved and dispersed form.
The long-chained and the short-chained carboxylic acid can be added together as a formulation, simultaneously or sequentially. Since a simple reactor can be used, addition can, for example, be sequential, in which case the short-chained carboxylic acid can, for example, be added first.
Table 1 shows suitable combinations of short-chained carboxylic acids with long-chained carboxylic acids and formulations thereof. Individual combinations are designated by the number of the table, followed by the number of the particular combination in Table 1. For example, combination 2.005 means the combination of the carboxylic acids as in Table 1, position no. 5 with the form shown in Table 2 in which the carboxylic acid is present.
Table 2
Table 2 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt. In the case of carboxylic acids with several acid functions, all the acid functions are present in salt form.
Table 3
Table 3 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt. In the case of carboxylic acids with several acid functions, all the acid functions are present in salt form.
Table 4
Table 4 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt. In the case of carboxylic acids with several acid functions, all the acid functions are present in salt form.
Table 5
Table 5 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt. In the case of carboxylic acids with several acid functions, all the acid functions are present in salt form.
Table 6
Table 6 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt. In the case of carboxylic acids with several acid functions, all the acid functions are present in salt form.
These acids and the salts thereof can serve both to adjust the pH and, in the partly or completely neutralized form, for example, with ammonia or water-soluble amines, to buffer the pH. This can improve the controlling of the viscosity and stability of the dispersion.
The carboxylic acid formulation can, for example, be added to the suspension before the drying. If the particle size of the zirconium oxide in the suspension is adjusted by grinding zirconium oxide, the carboxylic acid formulation can also be added at any desired point in time during the grinding step, such as towards the middle or end of the grinding step. It is also possible to add the various contents of the carboxylic acid formulation at various points in time. For example, the addition can occur before the drying, the suspension of the zirconium oxide thereby being stirred or subjected to shearing forces or otherwise mixed in itself and with the carboxylic acid formulation.
The state of the carboxylic acids before and after the drying and the mode of action thereof cannot be defined precisely. It is presumed that any short-chained carboxylic acids present are dissolved in the water or are adsorbed onto the surface of the ceramic particles, the acid carboxyl group undergoing bonding with the alkaline surface of the zirconium oxide particle. If the short-chained carboxylic acid is partly neutralized, ammonium ions would be liberated, which raise the pH of the solution, which can also be observed in practice (such as in Example 2). The long-chained carboxylic acid could form micelles, in the center of which there could be one or more zirconium oxide particles. In this context, the carboxyl group of the long-chained carboxylic acid could be on the surface of the micelles, while the alkyl radical points inwards and undergoes a weak interaction with the enclosed ceramic particle(s), which is possibly covered by a layer of adsorbate. If these micelles are retained during the drying, the ceramic particles are hydrophobized and can no longer form encrustations during drying, which would explain the effect of the good plastic deformability of the granules of Examples 2 and 3. Precise studies on the action mechanism are difficult, however, and require involved methods, since the concentration of carboxyl group is too low for an analysis with known methods.
Polyelectrolytes, such as polyacrylic acid or salts thereof, are not carboxylic acids in the context of the present invention, but can be used as binders in the context of the present invention.
The formulation according to the present invention also contains a binder which ensures the stability of the compact. Examples of a binder are polymers, for example, polymers which have a ceiling temperature of 220° C. or of 200° C. or less, for example, polyethers, such as polyethylene glycols (having, for example, a molecular weight of from 1,000 to 10,000), polyoxymethylene, polytetrahydrofuran, polyvinyl alcohols and esters thereof, such as polyvinyl acetate (with any desired degree of saponification), polyvinylpyrilidone, polyvinylimine, polyacrylic acids and esters thereof, such as polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, poly-tert-butyl methacrylate, polyisobutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, poly-tert-butyl acrylate, polyisobutyl acrylate, blends thereof and copolymers, but also cellulose, starch, derivatives thereof and mixtures or copolymers of the abovementioned polymers. Thus, for example, polyvinyl alcohol-co-polyvinyl acetate, polymethyl methacrylate-co-polymethyl acrylate or polymethyl methacrylate-co-polybutyl acrylate can also be used. Advantageous binders can be burned out in a controlled manner without residue, such as, for example, polyvinyl alcohol, polyacetal and polyvinyl acetate. However, it is also possible to use compositions of the two individual polymers, such as mixtures containing polyvinyl alcohol and polyvinyl acetate, polymethyl methacrylate and polymethyl acrylate or polymethyl methacrylate and polybutyl acrylate. These mixtures are can, for example, be present as suspensions or dispersions of the polymers in water and can be added together, simultaneously or sequentially with respect to the addition of the carboxylic acid(s), for example, sequentially after the addition of the carboxylic acids.
The content of binder can, for example, be in the range of from 0.1 wt. % to 7 wt. %, for example, from 0.1 wt. % to 5 wt. %, or from 0.5 wt. % to 3 wt. %, based on the finished powder.
If the addition of the carboxylic acid formulation sinks the pH so low that the zirconium oxide flocculates out as a result of a too low a zeta potential, the pH should be readjusted, for example, with sodium hydroxide solution, potassium hydroxide solution, blowing in gaseous ammonia or addition of aqueous ammonia, or carboxylic acid formulations which are partly neutralized (advantageously with ammonia), that is to say salts, should be used. The degree of neutralization required can be determined by the person skilled in the art by monitoring the zeta potential of the dispersion.
In the following tables, suitable combinations of carboxylic acids and binders are shown and combinations of short-chained carboxylic acids with long-chained carboxylic acids and formulations thereof with suitable binders are provided. Individual combinations are designated by the number of the table, followed by the number of the particular combination in Table 1. For example, combination 7.005 means the combination of the carboxylic acids as in Table 1, position no. 5 with the form given in Table 7 in which the carboxylic acid is present (here: free carboxylic acid) and the binder shown in Table 7 (here: polyvinyl acetate). In combination 7.005, oxalic acid is thus employed as the long-chained carboxylic acid, stearic acid as the short-chained carboxylic acid and polyvinyl acetate as the binder, the oxalic acid being employed as the free carboxylic acid.
Table 7
Table 7 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polyvinyl acetate is in each case employed as the binder.
Table 8
Table 8 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl acetate is in each case employed as the binder.
Table 9
Table 9 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl acetate is in each case employed as the binder.
Table 10
Table 10 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl acetate is in each case employed as the binder.
Table 11
Table 11 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl acetate is in each case employed as the binder.
Table 12
Table 12 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl acetate is in each case employed as the binder.
Table 13
Table 13 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polyvinyl alcohol is in each case employed as the binder.
Table 14
Table 14 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl alcohol is in each case employed as the binder.
Table 15
Table 15 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl alcohol is in each case employed as the binder.
Table 16
Table 16 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl alcohol is in each case employed as the binder.
Table 17
Table 17 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl alcohol is in each case employed as the binder.
Table 18
Table 18 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinyl alcohol is in each case employed as the binder.
Table 19
Table 19 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polyacrylic acid is in each case employed as the binder.
Table 20
Table 20 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyacrylic acid is in each case employed as the binder.
Table 21
Table 21 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyacrylic acid is in each case employed as the binder.
Table 22
Table 22 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyacrylic acid is in each case employed as the binder.
Table 23
Table 23 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyacrylic acid is in each case employed as the binder.
Table 24
Table 24 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyacrylic acid is in each case employed as the binder.
Table 25
Table 25 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polymethyl methacrylate is in each case employed as the binder.
Table 26
Table 26 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polymethyl methacrylate is in each case employed as the binder.
Table 27
Table 27 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polymethyl methacrylate is in each case employed as the binder.
Table 28
Table 28 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polymethyl methacrylate is in each case employed as the binder.
Table 29
Table 29 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polymethyl methacrylate is in each case employed as the binder.
Table 30
Table 30 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polymethyl methacrylate is in each case employed as the binder.
Table 31
Table 31 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polyethylene glycol (molecular weight 3,000) is in each case employed as the binder.
Table 32
Table 32 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyethylene glycol (molecular weight 3,000) is in each case employed as the binder.
Table 33
Table 33 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyethylene glycol (molecular weight 3,000) is in each case employed as the binder.
Table 34
Table 34 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyethylene glycol (molecular weight 3,000) is in each case employed as the binder.
Table 35
Table 35 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyethylene glycol (molecular weight 3,000) is in each case employed as the binder.
Table 36
Table 36 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyethylene glycol (molecular weight 3,000) is in each case employed as the binder.
Table 37
Table 37 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polyvinylimine is in each case employed as the binder.
Table 38
Table 38 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinylimine is in each case employed as the binder.
Table 39
Table 39 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinylimine is in each case employed as the binder.
Table 40
Table 40 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinylimine is in each case employed as the binder.
Table 41
Table 41 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinylimine is in each case employed as the binder.
Table 41
Table 41 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polyvinylimine is in each case employed as the binder.
Table 42
Table 42 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. A polymethyl methacrylate-polymethyl acrylate blend is in each case employed as the binder.
Table 43
Table 43 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polymethyl methacrylate-polymethyl acrylate blend is in each case employed as the binder.
Table 44
Table 44 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polymethyl methacrylate-polymethyl acrylate blend is in each case employed as the binder.
Table 45
Table 45 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polymethyl methacrylate-polymethyl acrylate blend is in each case employed as the binder.
Table 46
Table 46 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polymethyl methacrylate-polymethyl acrylate blend is in each case employed as the binder.
Table 47
Table 47 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polymethyl methacrylate-polymethyl acrylate blend is in each case employed as the binder.
Table 48
Table 48 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. A polybutyl acrylate-polymethyl methacrylate blend is in each case employed as the binder.
Table 49
Table 49 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polybutyl acrylate-polymethyl methacrylate blend is in each case employed as the binder.
Table 50
Table 50 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polybutyl acrylate-polymethyl methacrylate blend is in each case employed as the binder.
Table 51
Table 51 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polybutyl acrylate-polymethyl methacrylate blend is in each case employed as the binder.
Table 52
Table 52 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polybutyl acrylate-polymethyl methacrylate blend is in each case employed as the binder.
Table 53
Table 53 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. A polybutyl acrylate-polymethyl methacrylate blend is in each case employed as the binder.
Table 54
Table 54 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a free carboxylic acid. Polybutyl methacrylate is in each case employed as the binder.
Table 55
Table 55 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a sodium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polybutyl methacrylate is in each case employed as the binder.
Table 56
Table 56 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a potassium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polybutyl methacrylate is in each case employed as the binder.
Table 57
Table 57 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a zirconium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polybutyl methacrylate is in each case employed as the binder.
Table 58
Table 58 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as an ammonium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polybutyl methacrylate is in each case employed as the binder.
Table 59
Table 59 consists of 700 combinations of the long-chained and short-chained carboxylic acids as described above in Table 1, the short-chained carboxylic acid being present as a yttrium salt and carboxylic acids with several acid functions, all the acid functions being present in salt form. Polybutyl methacrylate is in each case employed as the binder.
After removal of the organic auxiliary substances, the specific surface area, measured by the BET method, of the finished powder from which the binder has been removed is 3 m2/g to 70 m2/g, for example, 7 m2/g to 30 m2/g, or 10 m2/g to 25 m2/g.
The specific surface area, measured herein by the BET method after burning out the organic auxiliary substances, without pressing the powder.
The present invention is hereinafter described in more detail by the following examples. Although all the examples relate to a partially stabilized zirconium oxide with 3 mol % of Y oxide, they can be applied accordingly to any fully or partially stabilized or non-stabilized with zirconium oxide with any content of monoclinic phase, including in a mixture with other oxides. The theoretical density of the present partially stabilized zirconium oxide with 3 mol % of yttrium oxide (Y2O3) is 6.10 g/cm3.
Two commercially available, partially stabilized and formulated zirconium oxide powders with 3 mol % of Y oxide were analyzed in the KZK powder tester. These are the types TZ 3YSB (manufacturer: Tosoh, Japan) and KZ-3YF (SD), type E AC (manufacturer: KCM, Japan). The results are summarized in the following tables:
Green density established: 3.15 g/cm3
Green density established: 3.30 g/cm3
The bulk density and the tap density were also determined with the KZK apparatus, and the ratio obtained. While the TZ 3YSB has a Hausner ratio of 1.11, a value of 1.08 was determined on the KZ 3YF. These values indicate poor compaction properties. The pressing pressures necessary for a compact density of approximately 50% of the theoretical density (=3.05 g/cm3) lie at the limit of that which can be tolerated industrially for TZ 3YSB. The cohesiveness is very significantly below the value of 1, so that industrial processability by means of net shape technology is not expected.
The green machinability was moderate.
Both types of powder are known and conventional raw materials in industry, and are known to achieve strengths above 900 MPa in accordance with ISO 843, but cannot be subjected to green machining in a reliable way and also cannot be processed by means of net shape technology.
The starting substance was a zirconium oxide powder partially stabilized with 3 mol % of Y2O3 and with a specific surface area of 16 m2/g and a monoclinic phase content of 42%, dispersed in demineralized water. The following parameters for the particle size distribution was measured on a diluted sample of the suspension by means of laser diffraction (Coulter counter) using the Mie model: D50 70 nm, D90 170 nm. The value for D50 was confirmed with the aid of a field emission electron microscope.
The solids content of the dispersion corresponded to 50 wt. %, and the pH was 9. An aqueous, partly neutralized formulation of a short-chained carboxylic acid with a pH of 5 was added to this dispersion, with vigorous stirring, so that 0.5 kg of carboxylic acid is present per 100 kg of zirconium oxide. During the addition, the suspension was at room temperature. After the addition, stirring was continued and the pH was determined as 9.8. An aqueous, non-neutralized carboxylic acid formulation was then added, so that 2 kg of carboxylic acid are present per 100 kg of zirconium oxide. Stirring was continued and the pH was determined as 8.5. 2.75 kg of an aqueous dispersion of polymethyl metacrylate, as a binder, per 100 kg of zirconium oxide was then added, stirring was continued, the pH was determined as 9 and the resulting formulation was converted into granules by means of spray drying. The following values were obtained with these granules:
The bulk density and the tap density were furthermore determined, and the ratio was determined. A value of 1.24 was determined. This value indicates very good compaction properties. 50% of the theoretical density is already achieved at 76 MPa. The green strengths are about 50% above the values of the comparison sample from Example 1, and the cohesivenesses of more than 1 are sufficient for net shape technology via axial pressing. The green machinability was very good.
Compacts were furthermore produced for the flexural fracture testing in accordance with ISO 843 by axial pressing and cold isostatic post-compaction under 1,950 bar. The sintered bodies (1475° C./5 h) were then ground to dimensions and to required surface quality.
The following values were obtained: density 6.07 g/cm3, 4-point flexural strength 870 MPa.
An aqueous carboxylic acid formulation as described in Example 2 was added to the dispersion of zirconium oxide in water described in Example 2 so that 3 kg of carboxylic acid are present per 100 kg of zirconium oxide. 2.75 kg of polymethyl methacrylate, in the form of an aqueous dispersion, per 100 kg of zirconium oxide were then added and the resulting formulation was converted into granules by means of spray drying. The following values were obtained with these granules:
The bulk density in accordance with ASTM B329 and the tap density was furthermore determined, and the ratio was obtained. A value of 1.25 was determined. This value indicates good compaction properties. 50% of the theoretical density is already reached at 55 MPa. The cohesiveness is very high, but the green strength is poorer compared with Example 2. The sliding coefficient is comparatively high.
Compacts were furthermore produced for the flexural fracture testing in accordance with ISO 843, as described in Example 2. After thermal removal of the binder and sintering at 1475° C. for 5 h, the following values were obtained: density 6.08 g/cm3, 4-point flexural strength 804 MPa.
The dispersion of zirconium oxide described in Example 2 and 3 was prepared analogously, but without the carboxylic acid formulation, 1 kg of a phenoxyalcohol surfactant as conventional in industry was used instead. The following values were obtained with the granules obtained in this way:
The Hausner ratio was 1.31. However, very high pressing pressures are necessary to achieve 50% of the theoretical density (>178 MPa). The cohesiveness and the green workability were very good.
Compacts were furthermore produced for the flexural fracture testing in accordance with ISO 843 by cold isostatic pressing under 1,950 bar. After removal of the binder by means of heat and sintering at 1475° C. for 5 h, the following values were obtained: density 6.08, 4-point flexural strength 848 MPa.
For the purpose of ascertaining the influence of the compaction properties on the residual porosity and the strength, axially pressed compacts were produced and subjected to cold isostatic post-compaction under only 1,500 bar (instead of 1,950 bar). The lower pressing pressure leads to a lower green density and therefore to a lower sintered density. This indicates how sensitively a powder reacts to the actual pressing pressure, which indeed can vary locally within a relatively large compact and can then lead to pores. Sintering was carried out at 1475° C. for 5 h, the density was then determined, the bodies were ground to dimensions (hard machining), the number of pores on 45×50 mm were determined according to size classes and the strength was determined in accordance with ISO 843. The following table shows the results:
The results of Examples 2 and 5 show the balance of the formulation of Example 2 with respect to processability and strength of the sintered body. The formulation of Examples 3 and 4 show a too low strength, and Example 4 furthermore shows macropores and the lowest sintered density.
The results of Examples 1 to 5 show in direct comparison that the formulation according to the present invention, when the ratio of binder to carboxylic acid is correctly matched, lead to a zirconium oxide of optimum formulation, both the residual porosity, strength and residual pore content in the sintered part and all the parameters which are relevant for the processability of the powder being balanced and optimum. The results can be applied to other zirconium oxide powders if the specific surface area thereof is taken into consideration. In practice, it will therefore always be necessary to determine the optimum content of carboxylic acid by experimentation. Typical contents lie between 0.1 and 5 percent by weight, for example, between 0.5 and 4% by weight.
A long-chained and a short-chained carboxylic acid are employed in the corresponding amounts as carboxylic acid formulations. The long-chained carboxylic acid is employed as the free acid, and the short-chained carboxylic acid is employed as the ammonium salt, potassium salt or free acid. The binders are likewise given, and these are added in the form of an aqueous dispersion or solution. Particular combinations are given in the following table:
The present invention is not limited to embodiments described herein; reference should be had to the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 039 668.0 | Aug 2008 | DE | national |
This application is a U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2009/060194, filed on Aug. 6, 2009 and which claims benefit to German Patent Application No. 10 2008 039 668.0, filed on Aug. 26, 2008. The International Application was published in German on Mar. 11, 2010 as WO 2010/026016 A2 under PCT Article 21(2).
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/60194 | 8/6/2009 | WO | 00 | 3/17/2011 |
