Zirconia granules, compacted powder and methods for producing the same

Abstract

The present invention provides: granules suitable for the production of a zirconia sintered body having high light transmittance and high strength and the production of a highly dense and uniform calcined body; a green compact; and methods respectively for producing the granules and the green compact. The present invention relates to granules which comprise zirconia, a stabilizing agent capable of inhibiting the phase transition of zirconia, and a binder, and have a compression ratio R calculated in accordance with formula (1) of 0.46 to 0.53 when the granules are filled in a columnar mold by tapping in accordance with JIS R 1628:1997, are then compressed by uniaxial pressing at a speed of 1 mm / sec, and are then pressurized to 33 MPa.　R = (H-D) / H (1) (wherein H represents a height of the granules filled by tapping in the columnar mold before the pressurization; and D represents a strain amount required until the pressure reaches 33 MPa.)

Description

[Technical Field]

[0001] The present invention relates to granules and powder compacts for obtaining a high-density, uniform calcined body of zirconia (zirconium oxide (IV): ZrO2), and a highly translucent, high-strength zirconia sintered body, as well as methods for producing them. [Background technology]

[0002] Generally, when ceramic powder or granules are fired, they shrink depending on the firing temperature. This shrinkage progresses depending on the primary particle density and internal stress inside the ceramic powder. Therefore, when firing a compact of ceramic powder or granules to produce a sintered body, it is necessary to prepare a high-density, uniform compact.

[0003] For example, a green compact containing zirconia as a main component generally shrinks by approximately 1% during the production of a calcined body and by approximately 20% during the production of a sintered body. Therefore, in order to obtain a zirconia sintered body with high density and high strength, a calcined zirconia body with high density and little density variation, and further a zirconia green compact, are required.

[0004] On the other hand, when using the uniaxial pressing method to manufacture green compacts, uneven density distribution occurs in the green compact, and one method to reduce this is to use cold isostatic pressing (CIP) processing. However, this does not completely eliminate the unevenness inside the green compact, so there is a need for a manufacturing method that can obtain green compacts with high density and minimal density variation.

[0005] Furthermore, to obtain a green compact with high density and minimal density variation, it is necessary not only to uniformly fill the dry powder into the mold, but also to have sufficient fluidity so that the dry powder rearranges in response to the pressure applied during pressing, resulting in a uniform increase in bulk density. To improve the fluidity of the primary particles, a method for obtaining a green compact typically involves adding a binder, which is an organic component. However, before obtaining a calcined or sintered body from the green compact, it is essential to remove the organic component. However, achieving both fluidity and removal is difficult, making it difficult to obtain a green compact with high density and minimal density variation. This has led to the problem of difficulty in obtaining a sintered body with high translucency and high strength.

[0006] For example, polyvinyl alcohol, which is commonly used as a binder, has a wide range of molecular weights, making it easy to adjust the fluidity. However, this cannot be fully removed by firing, and therefore there is a problem that it is prone to carbonization during the process of obtaining a calcined body.

[0007] In relation to the above-mentioned problems, Non-Patent Documents 1 to 3 disclose the compaction and relaxation behavior of granules used in producing a powder compact.

[0008] Furthermore, Patent Document 1 discloses a zirconia sintered body having high sintered density and strength.

[0009] Furthermore, Patent Document 2 discloses ceramic granules used to obtain large or complex-shaped compacts by a pressing method, and discloses granules for obtaining high-density sintered bodies without breakage or deformation, and their compressive fracture strength. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-143178 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-27914 [Non-patent literature]

[0011] [Non-Patent Document 1] Junichiro Tsubaki et al., "Evaluation of Mechanical Properties of Spray-Dried Granules by Compaction and Relaxation Method," Journal of the Ceramic Society of Japan, Vol.107, 11(1999) pp.1093-1098 [Non-patent document 2] Junichiro Tsubaki et al., "Evaluation of Mechanical Properties of Spray-Dried Granules by Compaction and Relaxation Method", Journal of the Ceramic Society of Japan, Vol.107, 12(1999) pp.1183-1187 [Non-patent document 3] Junichiro Tsubaki, "The Role of Chemical Engineering in the Ceramics Industry," Chemical Engineering Vol. 48, No. 1, pp. 34-38 (2003) DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0012] However, Non-Patent Documents 1-3 conducted tests using alumina particles, and the results cannot be said to be universally applicable to zirconia. The inventors' investigation of granules using alumina particles revealed that the compression ratio of the granules using alumina particles was 0.54 or higher when uniaxially pressurized up to 33 MPa, indicating that the granules are less likely to clog at low pressures, i.e., a high-density green compact cannot be obtained. Here, low pressure refers to a pressure of less than 10 MPa, which is undesirable because the granules are not sufficiently crushed, resulting in a green compact with low density and large density variations. Furthermore, Non-Patent Documents 1-3 state that granule deformation and fracture occur after the stress bending point. However, in the case of granules using zirconia particles of the present invention, deformation and fracture of the granules occurred before the bending point, demonstrating this discrepancy.

[0013] Furthermore, Patent Document 1 does not disclose the compaction behavior of the granules, and because the primary particle size of the powder is small, the density of the green compact is low, and furthermore, the density varies greatly, which has been found to pose a problem of large density variations when a sintered body is produced using the powder granules of Patent Document 1. Furthermore, Patent Document 2 uses a binder that is difficult to stretch, which is a combination of polyvinyl alcohol, polyethylene glycol, etc., and has been found to pose a problem of high density being impossible to obtain unless molding is performed at high pressure, and high density is not achieved at low pressure.

[0014] Therefore, an object of the present invention is to provide granules and a green compact suitable for obtaining a zirconia sintered body having little density unevenness, high translucency, and high strength, and a calcined body having little density unevenness, as well as methods for producing the same. [Means for solving the problem]

[0015] As a result of extensive research to solve the above-mentioned problems, the inventors focused on the compaction and relaxation behavior of granules and discovered that granules having a specific compression ratio and stress can produce a green compact with a uniform density even at low pressure, thereby solving the above-mentioned problems. Based on this finding, the inventors conducted further research and completed the present invention.

[0016] That is, the present invention includes the following inventions. [1] Granules containing zirconia, a stabilizer capable of suppressing the phase transition of zirconia, and a binder, The granules are filled into a cylindrical mold by tapping in accordance with JIS R 1628:1997. Granules that, when compressed uniaxially at a rate of 1 mm / sec up to a pressure of 33 MPa, have a compression ratio R calculated by the following formula (1) of 0.46 to 0.53. R=(HD) / H (1) (In the formula, H represents the height of the tapped granules in the cylindrical mold before pressure is applied, and D represents the amount of strain required to reach 33 MPa.) [2] Granules according to [1], in which the stress when the compression ratio R becomes 0.6 during compression of the granules in the cylindrical mold is 7.20 MPa or more. [3] The granules according to [1] or [2], wherein the binder is a (meth)acrylic binder. [4] Granules according to any one of [1] to [3], wherein the average particle size of the primary particles constituting the granules is 45 to 200 nm. [5] The granules according to any one of [1] to [4], wherein the stabilizer is yttria. [6] The granules according to [5], wherein the content of the yttria is 2.5 to 8.5 mol % based on the total moles of zirconia and yttria. [7] A granule according to any one of [1] to [6], having a bulk density of 2.9 to 3.5 g / cm 3 That is, a compacted powder. [8] The powder compact according to [7], wherein the stabilizer contained in the granules is yttria. [9] The powder compact according to [8], wherein the content of the yttria is 2.5 to 8.5 mol % based on the total moles of zirconia and yttria.

[10] A step of mixing zirconia with a stabilizer capable of suppressing a phase transition of zirconia to obtain a zirconia raw material composition; a pulverization step of pulverizing the zirconia raw material composition to obtain particles having an average primary particle size of 45 to 200 nm; a step of adding a binder to the particles and granulating the particles to obtain granules containing zirconia, a stabilizer capable of suppressing the phase transition of zirconia, and a binder; Method for manufacturing granules.

[11] The method for producing granules according to

[10] , wherein the binder is a (meth)acrylic binder.

[12] The method for producing granules according to

[10] or

[11] , wherein the stabilizer is yttria.

[13] The granules according to any one of [1] to [6] are tapped into a cylindrical mold to fill the mold. When compressed uniaxially at a rate of 1 mm / s and pressurized to 33 MPa, the compression ratio R calculated by the following formula (1) is 0.46 to 0.53. A method for manufacturing a powder compact. R=(HD) / H (1) (In the formula, H represents the height of the tapped granules in the cylindrical mold before pressure is applied, and D represents the amount of strain required to reach 33 MPa.)

[14] The method for producing a powder compact according to

[13] , wherein, during compression of the granules in the cylindrical mold, the stress is 7.20 MPa or more when the compression ratio R is 0.6. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide granules, a green compact, and methods for producing them that are suitable for obtaining a zirconia sintered body having little density variation, high translucency, and high strength, and a calcined body having little density variation. Furthermore, according to the present invention, it is possible to provide a green compact having a uniform density even at low pressure. Furthermore, the granules that exhibit the compaction behavior of the present invention can be used to obtain a green compact that is free of cracks and chips, has a high density, and has little density variation. Regardless of where in the green compact is used, it can be suitably used as a dental material or the like that has high translucency and high strength after firing. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a schematic diagram showing a method for measuring a compression ratio R according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The granules of the present invention contain zirconia, a stabilizer capable of suppressing the phase transition of zirconia (hereinafter, sometimes referred to as "stabilizer"), and a binder, and when the granules are filled into a cylindrical mold by tapping in accordance with JIS R 1628:1997 and compressed uniaxially at a rate of 1 mm / sec up to 33 MPa, the compression ratio R calculated by the following formula (1) is 0.46 to 0.53. The uniaxial compression is preferably carried out under atmospheric pressure. R=(HD) / H (1) (In the formula, H represents the height of the tapped granules in the cylindrical mold before pressure is applied, and D represents the amount of strain required to reach 33 MPa.)

[0020] The compression ratio R of the granules of the present invention is 0.46 or more, preferably 0.47 or more, and more preferably 0.48 or more. If the compression ratio R is less than 0.46, the granules tend to be crushed. When a certain pressure is applied during compaction, the granules are crushed locally according to the stress, i.e., density unevenness is likely to occur in the green compact. Furthermore, the compression ratio R of the granules of the present invention is 0.53 or less, preferably 0.52 or less, and more preferably 0.51 or less. If the compression ratio R is greater than 0.53, the granules tend to be difficult to crush. Therefore, a uniform green compact cannot be obtained at a low pressure of less than 10 MPa during compaction, and density unevenness is likely to occur. The green compact is formed by applying an external force to the granules of the present invention, and since it is a product before sintering, it means that there is no necking (sticking). The compression ratio R can be measured by the method described in the Examples below.

[0021] When the compression ratio R of the granules of the present invention is within the above range, a green compact with little density variation can be obtained. Furthermore, using the green compact, a calcined body with little density variation can be obtained, and a zirconia sintered body with little density variation, high translucency, and high strength can be obtained.

[0022] The height of the columnar granules tapped before pressure application, as defined by formula (1), is preferably 8.0 to 13.0 cm, more preferably 8.5 to 12.5 cm, and even more preferably 9.0 to 12.0 cm. Furthermore, the strain required to reach 33 MPa, as defined by formula (1), is preferably greater than 4.60 cm and less than 6.00 cm, more preferably 4.65 cm or more and 5.95 cm or less, and even more preferably 4.70 cm or more and 5.95 cm or less. When the compression ratio R of the granules falls within these ranges, a green compact with minimal density variation can be obtained.

[0023] The granules of the present invention preferably have a stress of 7.20 MPa or more when the compression ratio R is 0.6, and more preferably 7.50 MPa or more. If the stress is less than 7.20 MPa, the granules will be difficult to crush when packed together, which may reduce the uniformity of the density of the green compact and increase density variation. The stress when the compression ratio R is 0.6 can be measured using the method for the compression ratio R described in the Examples below.

[0024] The binder used in the present invention preferably has a tensile elongation in the range of 120 to 800%, more preferably in the range of 135 to 720%, and even more preferably in the range of 150 to 670%. When it is 120% or more, the compression ratio R becomes 0.53 or less, which is preferable, and when it is 800% or less, the green compact does not chip and has excellent shape retention, which is preferable.

[0025] The binder of the present invention is not particularly limited, and known binders can be used, for example, polymers. Examples of polymers constituting the binder include organic binders. Examples of organic binders include commonly used polymers such as (meth)acrylic binders, (meth)acrylic acid binders, paraffin binders, and fatty acid binders. Among these organic binders, those having a carboxyl group in the molecular chain or carboxylic acid derivatives are preferred, (meth)acrylic binders are more preferred, and water-soluble poly(meth)acrylates are even more preferred. The poly(meth)acrylates may be copolymers of acrylic acid or methacrylic acid with maleic acid, or may contain sulfonic acid. Examples of the salt cations include sodium, ammonium, and the like.

[0026] The polymer is preferably one that has a strong interaction with the zirconia surface, as this enhances the shape retention of the green compact. For example, the (meth)acrylic polymer binder is preferred.

[0027] The strength of the interaction between the polymer and the zirconia surface can be estimated, for example, from the wettability of the polymer with the zirconia and stabilizer. The wettability can be calculated, for example, using the Hansen solubility parameter, and the affinity δ can be calculated from the following formula using the solubility parameters, which are the dispersion term (δd), dipole-dipole term (δp), and hydrogen bond term (δh) per molar volume: δ={(δdA-δdB) 2 +(δpA-δpB) 2 +(δhA-δhB) 2}1 / 2 The smaller the affinity δ, the higher the affinity, and the larger the δ, the lower the affinity.

[0028] The molecular weight of the polymer constituting the binder of the present invention, from the viewpoint of its mechanical properties, i.e., moldability of the powder compact, is preferably 10,000 to 1,000,000 in weight-average molecular weight, more preferably 30,000 to 500,000, even more preferably 70,000 to 480,000, even more preferably 80,000 to 470,000, and particularly preferably 100,000 to 450,000. When combined with other components, this range makes it easier to adjust the compression ratio R within a predetermined range, and also makes it easier to adjust the stress at a compression ratio R of 0.6 within a desired range. A higher molecular weight is preferable because it increases the interentanglement molecular weight, improves tensile elongation, and improves moldability during molding into a powder compact. The binder may be composed of two or more polymers with different weight-average molecular weights, and the weight-average molecular weight after mixing is preferably 50,000 to 800,000, more preferably 100,000 to 500,000.

[0029] The weight average molecular weight is a weight average molecular weight calculated in terms of polystyrene, determined by gel permeation chromatography (GPC).

[0030] The polymer constituting the binder of the present invention can have a predetermined glass transition temperature (Tg) or a softening point equivalent thereto. A low Tg is preferable because the compression energy applied during the compact formation reduces the tensile strength of the polymer, improving moldability. The Tg is preferably 50°C or less, more preferably 30°C or less, and even more preferably 10°C or less. The binder may be composed of two or more polymers with different Tg. Preferably, the Tg of the high Tg polymer can be lowered by combining a high Tg polymer with a low Tg polymer. When the Tg or softening point is within the above range, it is easier to adjust the compression ratio R within a predetermined range when combined with other components. The Tg or softening point can be measured, for example, using a Mettler-Toledo DSC (Differential Scanning Calorimetry) measuring device (DSC 822) at a heating rate of 10°C / min in accordance with ISO 3146:2000. The extrapolated onset temperature can be used as the Tg or softening point on the curve obtained. In this specification, Tg is used for crystalline polymers, and softening point is used instead of Tg for non-crystalline polymers (amorphous polymers). The softening point of the polymer constituting the binder is preferably -40°C or higher, more preferably -5°C or higher, and even more preferably above 0°C. The softening point is preferably 50°C or lower, more preferably 30°C or lower, and even more preferably 10°C or lower.

[0031] The method for producing the polymer constituting the binder of the present invention is not particularly limited, and methods based on known techniques can be used. For example, an example of a method for producing the polymer constituting the (meth)acrylic binder is a method in which a (meth)acrylic group (a (meth)acryloyloxy group or a (meth)acrylamide group)-containing monomer is thermally polymerized using a radical polymerization initiator. The (meth)acrylic group-containing monomer and the radical polymerization initiator are not particularly limited as long as they can be set to have the predetermined configuration (e.g., the tensile elongation of the binder, the glass transition point (Tg), softening point, and weight average molecular weight of the polymer constituting the binder), and known methods can be used.

[0032] The binder of the present invention may contain various additives, such as antioxidants, heat stabilizers, lubricants, processing aids, antistatic agents, heat degradation inhibitors, UV absorbers, light stabilizers, polymer processing aids, colorants, and impact resistance aids. In this case, the method for producing the polymer constituting the binder of the present invention is not particularly limited as long as it can uniformly mix the two or more polymers and the additives. Methods similar to known techniques can be used, including dissolving the polymers in water or an organic solvent and mixing them, or melt-kneading them. Furthermore, the glass transition temperature (Tg) and softening point equivalent thereto of the polymer constituting the binder can be adjusted by using various additives.

[0033] The binder content of the present invention is preferably 1.1 parts by mass or more, more preferably 1.3 parts by mass or more, and even more preferably 1.5 parts by mass or more, per 100 parts by mass of the zirconia raw material composition. In this specification, a composition containing zirconia and a stabilizer but no binder is referred to as a "zirconia raw material composition." A composition containing zirconia, a stabilizer, and a binder is referred to as a "zirconia composition." The zirconia may contain hafnia, which is difficult to separate from zirconia. When the binder content is 1.1 parts by mass or more, per 100 parts by mass of the zirconia raw material composition, the powder compact can maintain its shape, and there is no risk of chipping or cracking at its edges. On the other hand, an excessive binder content increases the density of the powder compact and tends to increase the density of the calcined body or sintered body. Therefore, the binder content is preferably 5.0 parts by mass or less, more preferably 4.5 parts by mass or less, and even more preferably 4.0 parts by mass or less, per 100 parts by mass of the zirconia raw material composition. Furthermore, when the binder content is within the above range, it becomes easier to adjust the compression ratio R within a predetermined range when combined with other components. Furthermore, for example, in one embodiment, it is preferable to use a binder whose polymer constituting the binder has a glass transition point (Tg) or softening point within the above range (e.g., from -10°C to 50°C, or from 0°C to 30°C) and whose content is within the above range. In another embodiment, it is preferable to use a binder whose polymer constituting the binder has a weight-average molecular weight within the above range (e.g., from 70,000 to 480,000) and whose content is within the above range.

[0034] Furthermore, in still another embodiment, (i) a binder having a tensile elongation within the above-described range (e.g., 120 to 800%), (ii) a binder having a weight-average molecular weight of a polymer constituting the binder within the above-described range (e.g., 70,000 to 480,000), (iii) a binder having a glass transition point (Tg) or softening point of a polymer constituting the binder within the above-described range (e.g., −10°C or higher and 50°C or lower, or higher than 0°C and 30°C or lower), or a binder combination thereof is used in an amount within the above-described range (by adjusting the compounding ratio (mass ratio) of the zirconia raw material composition and the binder), thereby making it easier to adjust the compression ratio R within a predetermined range, and further making it easier to adjust the stress at a compression ratio R of 0.6 within a desired range.

[0035] The average particle size of the primary particles constituting the granules of the present invention (hereinafter also referred to as "average primary particle size") is preferably 45 to 200 nm, more preferably 50 to 180 nm, and even more preferably 60 to 160 nm. If it is less than 45 nm, the compression ratio will be less than 0.46, which is undesirable, and if it is more than 200 nm, the sintering ability will be low, and it will be difficult for the density to increase after firing, making it difficult to improve the translucency, which is undesirable. The BET specific surface area of ​​the particles constituting the granules is 7.0 m when measured in accordance with JIS Z 8830:2013. 2 / g or more, and 7.5m 2 / g or more is more preferable, and 8m 2 / g or more is more preferable. 2 If the BET specific surface area is less than 30 m / g, sintering is difficult, or even if sintering is possible, the sintered body becomes cloudy. 2 / g or less, and 2 / g or less is more preferable, and 20m 2 / g or less is more preferable. 2If the BET specific surface area exceeds 1 / g, the heating rate (described later) will increase, making the sintered body more susceptible to temperature variations in the sintering furnace. Furthermore, shortening the sintering time will result in a decrease in the translucency of the sintered body. In the present invention, the BET specific surface area is a specific surface area measured without distinguishing between primary particles and secondary particles.

[0036] The compact produced from the granules of the present invention has a bulk density of 2.9 to 3.5 g / cm 3 is preferably 3.0 to 3.45 g / cm 3 More preferably, 3.1 to 3.4 g / cm 3 More preferably, 2.9 g / cm 3 If the density is less than 3.5 g / cm3, it becomes difficult to increase the density of the calcined body or sintered body. 3 If the diameter is larger than this, the gas generated by the thermal decomposition of the binder will expand the compact, which is undesirable because it will reduce the density of the calcined body and cause chipping or cracking.

[0037] The granules or green compact of the present invention contain a stabilizer capable of suppressing the phase transition of zirconia. The stabilizer is preferably one capable of forming partially stabilized zirconia. Examples of the stabilizer include calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (YO) (hereinafter referred to as "yttria"), cerium oxide (CeO), scandium oxide (ScO), niobium oxide (NbO), lanthanum oxide (LaO), erbium oxide (ErO), and praseodymium oxide (PrO). 11Examples of the stabilizer include oxides such as samarium oxide (Sm2O3), europium oxide (Eu2O3), and thulium oxide (Tm2O3), with yttria being preferred. One stabilizer may be used alone, or two or more may be used in combination. The stabilizer content in the zirconia compact of the present invention can be measured, for example, by inductively coupled plasma (ICP) emission spectroscopy, X-ray fluorescence analysis, or the like. In the zirconia compact of the present invention, the stabilizer content is preferably 0.1 to 18 mol%, more preferably 1 to 15 mol%, and even more preferably 3 to 8 mol%, based on the total moles of zirconia and stabilizer.

[0038] From the viewpoint of the strength and translucency of the resulting zirconia sintered body, the zirconia compact preferably contains yttria as a stabilizer. The yttria content is preferably 2.5 mol% or more, more preferably 3 mol% or more, and even more preferably 3.5 mol% or more, based on the total moles of zirconia and yttria. When the yttria content is 3 mol% or more, the translucency of the zirconia sintered body can be further improved. Furthermore, the yttria content is preferably 8.5 mol% or less, more preferably 7.5 mol% or less, and even more preferably 7.0 mol% or less, based on the total moles of zirconia and yttria. When the yttria content is 7.5 mol% or less, the decrease in strength of the resulting zirconia sintered body can be further suppressed.

[0039] A method for producing the granules will be described below. In this specification, the granules containing zirconia, a stabilizer, and a binder may also be referred to as "zirconia granules."

[0040] Examples of the method for producing zirconia granules include a method including: a step (mixing step) of mixing zirconia (preferably zirconia powder) with the stabilizer to obtain a zirconia raw material composition (mixture); a step (pulverizing step) of pulverizing the zirconia raw material composition to obtain particles (pulverized product) having an average primary particle size within a predetermined range; and a step (granulation step) of adding a binder to the pulverized particles (pulverized product) and granulating the particles to obtain zirconia granules.

[0041] First, zirconia (preferably zirconia powder) and a stabilizer are mixed in a predetermined ratio to prepare a mixture (mixing step). For example, when the stabilizer is yttria, the zirconia and yttria can be mixed in a ratio such that the yttria content is as described above. The mixing method is not particularly limited, and a known mixing device can be used. When a solvent is used for mixing, the resulting mixture may be in the form of a slurry or the like. The solvent is not particularly limited, and water or an organic solvent can be used. The mixing may be dry or wet mixing. The mixture can be pulverized (pulverization step) so that the particles constituting the zirconia granules have the above-described average primary particle size (preferably 45 to 200 nm) and, if necessary, so that the particles constituting the zirconia granules have the above-described BET specific surface area. The pulverization method is not particularly limited, and can be performed using a known pulverization device (e.g., a ball mill, a bead mill, etc.). The mixing step and the pulverization step may be performed in the same step.

[0042] The binder is added to the pulverized zirconia raw material composition (the mixture) in the predetermined content described above. In another embodiment, the binder may be added in the mixing process or the pulverization process. When the binder is added in the mixing process, the zirconia composition obtained in the mixing process is pulverized in the pulverization process. The type of binder is as described above. The binder can be added to the mixture so as to achieve the above-described content. For example, when the binder is added in the pulverization process, the mixture and the binder can be dispersed in a solvent such as water (dispersion process), and then pulverized using a pulverizer such as a ball mill or a bead mill until the powder constituting the mixture has a desired average primary particle size. The average primary particle size can be measured, for example, by a laser diffraction / scattering particle size distribution measurement method. For example, using a laser diffraction / scattering particle size distribution measurement device (product name "Partica LA-950" manufactured by Horiba, Ltd.), a slurry diluted with water is irradiated with ultrasound for 30 minutes, and then the volumetric measurement can be performed while applying ultrasound. After the mixing step and / or the pulverization step, the resulting mixture is dried by spray drying to obtain zirconia granules (granulation step). The spray drying method is not particularly limited, and a known spray drying device (such as a spray dryer) can be used.

[0043] It is preferable to prepare the zirconia and the stabilizer (preferably yttria) separately. For example, it is preferable that the zirconia and the stabilizer are not precipitated simultaneously (in the same process), but that the zirconia preparation process (e.g., manufacturing process) and the stabilizer preparation process (e.g., manufacturing process) are separate, independent processes. This makes it possible to prevent the stabilizer (preferably yttria) from dissolving in zirconia during the manufacturing process of the zirconia calcined body described below.

[0044] The following description will be given using the case where the stabilizer is yttria. Zirconia powder with yttria dissolved therein is generally produced by coprecipitation and hydrolysis. In the coprecipitation and hydrolysis methods, a mixture of hydrated zirconia and yttria is produced from zirconium oxychloride and yttrium chloride in the same process, and this mixture is fired at 800°C to 900°C to produce stabilized zirconia powder with yttria (yttrium) dissolved therein. This yttria-dissolved zirconia is mainly of the tetragonal and / or cubic system. The particle size of the resulting zirconia powder is on the order of several tens of nanometers. To use this zirconia powder as a raw material for a zirconia sintered body, the fired product is pulverized to a predetermined particle size and then granulated to produce zirconia granules.

[0045] In the case of zirconia granules produced by such a coprecipitation or hydrolysis method, the bonding temperature in the temperature range for producing a zirconia calcined body is close to the temperature for removing the organic binder, and / or the temperature dependency is high. If the state of the organic binder is uneven, the primary particles constituting the zirconia granules produced by the coprecipitation or hydrolysis method will locally form strong necking depending on their particle diameter. Therefore, when the zirconia calcined body is cut to produce a machined body, the chipping rate of the machined body will increase, which is undesirable.

[0046] Zirconia granules can be molded by applying an external force to form a green compact. The molding method is not limited to a specific method, and a suitable method can be selected depending on the purpose. Examples of molding methods include press molding and injection molding. Multi-stage molding may also be performed. For example, after press-molding the zirconia granules, CIP treatment may be further performed. Examples of methods for producing a green compact include a method in which zirconia granules are tapped into a cylindrical mold and compressed by uniaxial pressing at a speed of 1 mm / sec. When the generated stress reaches 33 MPa, the compression ratio R is 0.46 to 0.53. The uniaxial pressing is preferably performed under atmospheric pressure. To further reduce density unevenness, the green compact obtained by uniaxial pressing may then be subjected to CIP treatment. The CIP pressure is preferably higher than the stress of uniaxial pressing. The CIP pressure is preferably greater than 33 MPa, more preferably 50 MPa or more, even more preferably 100 MPa or more, and particularly preferably 150 MPa or more. By using the zirconia granules of the present invention, a green compact having excellent shape retention and little density variation can be obtained, and this green compact can be used to obtain a calcined body having little density variation. A schematic diagram is shown in Figure 1.

[0047] In Fig. 1, a testing machine having a head, a mold 2, and a table 3 is used, and zirconia granules, which are dry powder, are filled into the mold 2. The mold 2 is then uniaxially pressed in the axial direction 1 of the head at a speed of 1 mm / sec. When the generated stress reaches 33 MPa, the head position is fixed and a green compact can be obtained. As the testing machine, for example, a precision universal testing machine (product name: AG-I 100 kN, manufactured by Shimadzu Corporation) can be used.

[0048] Furthermore, in the method for producing a powder compact, when the granules are compressed in the cylindrical mold, the stress at a compression ratio R of 0.6 is preferably 7.20 MPa to 16 MPa, more preferably 7.50 MPa to 14 MPa, and even more preferably 8.50 MPa to 12 MPa.

[0049] The green compact can have a disk shape, a rectangular parallelepiped shape, or a dental product shape (eg, a dental crown shape).

[0050] A zirconia calcined body is obtained by firing (i.e., calcining) the above-mentioned compact at a temperature below sintering. The zirconia calcined body of the present invention serves as a precursor (intermediate product) of the zirconia sintered body of the present invention, which will be described later. The zirconia calcined body refers to a state in which zirconia particles (powder) are necked (adhered) and not completely sintered. The zirconia calcined body also includes shaped and processed zirconia bodies. The zirconia calcined body of the present invention also includes dental products (e.g., crown-shaped prostheses) obtained by processing calcined zirconia disks using a CAD / CAM (Computer-Aided Design / Computer-Aided Manufacturing) system, for example.

[0051] The contents of zirconia and stabilizer in the zirconia calcined body of the present invention are the same as those in the zirconia granules and green compact before the zirconia calcined body is produced. From the viewpoints of the strength and translucency of the sintered body produced from the zirconia calcined body of the present invention, the stabilizer contained in the zirconia calcined body is preferably yttria.

[0052] In the method for producing a zirconia calcined body of the present invention, the calcination temperature is preferably 830 to 1080°C, more preferably 850 to 1050°C, and even more preferably 895 to 1000°C, from the viewpoint of the strength of the resulting zirconia calcined body.

[0053] It is preferable to set the calcination temperature at the maximum calcination temperature and hold it at the maximum calcination temperature for a certain period of time, as this may result in the hardness of the zirconia calcined body falling within a desirable range and may reduce the chipping rate. The calcination conditions may vary depending on the density of the target zirconia calcined body, the average particle size of the primary particles constituting the zirconia calcined body, and the binder content, but it is preferable to hold the calcination temperature at the maximum calcination temperature for 30 minutes to 6 hours. Furthermore, it is preferable that the rate of temperature increase up to the maximum calcination temperature and the rate of temperature decrease from the maximum calcination temperature be 300°C / min or less.

[0054] The zirconia granules and green compact of the present invention can produce a zirconia calcined body with little density variation.

[0055] The zirconia calcined body of the present invention can be machined to produce a machined body. The cutting method is not limited to a specific method, and a suitable method can be selected depending on the purpose. For example, a machined body can be produced by cutting a zirconia disk, which is also a calcined body, into the shape of a dental product (e.g., a crown-shaped prosthesis) using a CAD / CAM system.

[0056] The surface smoothness of the machined body may be improved using a tool such as an abrasive (for example, trade name "Pearl Surface (registered trademark)" manufactured by Kuraray Noritake Dental Co., Ltd.).

[0057] A zirconia sintered body (hereinafter, sometimes simply referred to as a "zirconia sintered body" or "sintered body") can be produced by subjecting the zirconia calcined body of the present invention or a machined body thereof to a sintering step in which the calcined body is fired at a temperature at which the zirconia particles are sintered (sinterable temperature). The firing temperature is preferably, for example, 1400°C or higher, and more preferably 1450°C or higher. The firing temperature is preferably, for example, 1650°C or lower, and more preferably 1600°C or lower. The heating rate and cooling rate are preferably 300°C / min or lower. A zirconia sintered body refers to a zirconia sintered body in a completely sintered state.

[0058] In the sintering step, the holding time at the sinterable temperature (for example, the maximum firing temperature) is preferably less than 120 minutes, more preferably 90 minutes or less, even more preferably 75 minutes or less, even more preferably 60 minutes or less, particularly preferably 45 minutes or less, and most preferably 30 minutes or less. The holding time is preferably 1 minute or more, more preferably 5 minutes or more, and even more preferably 10 minutes or more.

[0059] In the sintering step, the holding time at the sinterable temperature (for example, the maximum firing temperature) can be, for example, 25 minutes or less, 20 minutes or less, or 15 minutes or less.

[0060] The temperature increase and decrease rates in the sintering step are preferably set so as to shorten the time required for the sintering step. For example, the temperature increase rate can be set so as to reach the maximum firing temperature in the shortest time possible, depending on the performance of the firing furnace. The temperature increase rate up to the maximum firing temperature can be, for example, 10°C / min or more, 50°C / min or more, 100°C / min or more, 120°C / min or more, 150°C / min or more, or 200°C / min or more. The temperature decrease rate from the maximum firing temperature is preferably set so as not to cause defects such as cracks in the sintered body. For example, after heating is completed, the sintered body can be allowed to cool at room temperature.

[0061] The zirconia granules and green compact of the present invention can produce a zirconia sintered body with little density unevenness, high translucency, and high strength.

[0062] The granules of the present invention can be suitably used in ceramics for pressure molding. [Example]

[0063] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.

[0064] <Method for measuring compression ratio R> The universal testing machine used was AGS-X manufactured by Shimadzu Corporation, the control software was TRAPEZIUM LITE X manufactured by Shimadzu Corporation, and the load cell was a 5000N load cell manufactured by Shimadzu Corporation. In accordance with JIS R 1628:1997, the granules obtained in the following examples and comparative examples were tap-filled into a cylindrical mold, sandwiched between fixed punches from above and below, and compression was initiated just before the top surface of the filled granules came into contact with the upper punch of the mold. Just before the start of compression, the distance between the punches was adjusted to the height H of the tap-filled granules in the cylindrical mold before compression. The control software was set to stop punch compression when the pressure reached 33 MPa, and measurements were performed in low-speed mode. The punch compression speed was 1 mm / sec, and the test force was measured every 0.1 sec. The measured test force was divided by the cross-sectional area of ​​the cylindrical mold to define the stress. The punch movement was stopped when the stress of the tap-filled granules reached a predetermined pressure, and the strain at that time was defined as D. The compression ratio R was calculated using the average value of n = 3. The stress at a compression ratio R of 0.6 was also calculated using the above method (average value of n = 3).

[0065] <Measurement of binder elongation rate> Samples of various binders measuring 5 mm long x 30 mm wide x 1 mm thick were obtained by cast film formation. These were hung in the longitudinal direction, and a 200 g weight was attached to the bottom end. The elongation was calculated using the following formula from the length after 180 seconds, with the length before change being 100% (average value of n = 3). (Elongation rate) = (length after 180 seconds) ÷ (initial length) × 100

[0066] <Method for measuring the average particle size of primary particles that make up granules> Using the granules obtained in the following examples or comparative examples, surface images were taken with a scanning electron microscope (product name "VE-9800", manufactured by Keyence Corporation). The average particle diameter was calculated by image analysis of the obtained images. To measure particle diameter, image analysis software (product name "Image-Pro Plus", manufactured by Hakuto Co., Ltd.) was used. The captured SEM image was binarized, and the brightness range was adjusted to clearly show the grain boundaries, allowing particles to be recognized from the field of view (region). For areas where the grain boundaries were unclear, a degeneration filter was applied to the region, and each region was degenerated until it became one or more points. A Voronoi polygon was drawn so that these points became the kernel points of the Voronoi polygon. A line connecting the midpoints of two adjacent kernel points was drawn, and the line was superimposed on the original particle image to separate adjacent particles. For example, in image processing, a single particle may appear gourd-shaped. In such cases, the particle was separated into two by assuming that two circular particles were in contact and appearing as one. In the processing file that recognized the primary particle size, "Diameter" was selected in the "Count / Size Dialog" and the distribution was calculated (n=4). Specifically, for four fields of view of one sample, the particle diameter (primary particle diameter) was measured in each field using image analysis software (product name "Image-Pro Plus") and the average value was calculated.

[0067] <Shape retention of powder compacts> The green compacts obtained when determining the compression ratio R were visually inspected for chipped rectangular corners (n=5). Those without chips were deemed to have high shape retention and rated as "○", while those with even one submillimeter chip were deemed to have no shape retention and rated as "×". The results are shown in Table 2.

[0068] <Method for measuring bulk density of powder compact> The dimensions of the columnar green compacts obtained by the methods of the following Examples or Comparative Examples were accurately measured using a micrometer, and the mass was measured using a precision balance, and the density was calculated by (mass of green compact) / (volume of green compact) (average value of n=3). The results are shown in Table 2. The calculated bulk density was 2.9 g / cm. 3 Less than or equal to 3.5g / cm 3 If it is larger than 2.9 g / cm, it is evaluated as "×" 3If it is more than 3.1g / cm, it is evaluated as "△" 3 If the result was equal to or greater than this, the result was evaluated as "Good".

[0069] <Method for evaluating density uniformity of calcined body> The compacts obtained in the following Examples or Comparative Examples were placed in an electric furnace set at 1,000°C and calcined to obtain calcined bodies. From the obtained calcined bodies, multiple 10 mm square specimens were cut out at random locations while varying the cutting position, and the bulk density of each specimen was calculated from the mass and volume (average of n = 3). The results are shown in Table 2. If the error in the calculated bulk density values ​​was within 3.5%, the density uniformity was evaluated as "Good." If it was greater than 3.5%, the density uniformity was evaluated as "Poor." The error in the bulk density values ​​is preferably within 3.5%, more preferably within 3.4%, and even more preferably within 3.3%. If it is greater than 3.5%, the unevenness in the bulk density of the calcined body may increase the shrinkage rate of the sintered body, making it difficult to obtain a sintered body with the desired shape.

[0070] <Method for evaluating density uniformity of sintered body> From the calcined body obtained in the same manner as above, multiple 10 mm square specimens were cut out at random positions while changing the cutting position. The maximum firing temperature was 1,500°C for Examples 1 to 10 and Comparative Examples 1 to 7, and 1,450°C for Comparative Examples 8 to 10. The specimens were fired at these temperatures for 2 hours to obtain sintered bodies. The density of each sintered body was calculated from its mass and volume (average value of n = 3). The results are shown in Table 2. If the error in the calculated bulk density values ​​of multiple sintered bodies was within 3.1%, the density uniformity was evaluated as "Good", whereas if it was greater than 3.1%, the density uniformity was evaluated as "Poor". The error in the bulk density values ​​is preferably within 3.1%, and more preferably within 3.0%.

[0071] <Method for measuring the light transmittance of sintered bodies> Samples were cut out from the calcined bodies obtained in the same manner as above so as to have a thickness of 1.2 mm after firing, and fired at a maximum firing temperature of 1,500°C for Examples 1 to 10 and Comparative Examples 1 to 7, and 1,450°C for Comparative Examples 8 to 10 for 2 hours, to obtain sintered bodies with a thickness of 1.2 mm. The chromaticity of the obtained sintered bodies was measured using a spectrophotometer (product name "Crystal Eye", manufactured by Olympus Corporation) in a measurement mode of 7 bands with an LED light source against a white background, and the lightness (L W *) and the lightness (L) when chromaticity is measured on the same test piece with the same measuring device, measuring mode, and light source against a black background. B *) and measure the difference between them (ΔL* (W-B) =(L W *)-(L B *)) Translucency (ΔL* (W-B) ) (mean value of n=3). The criteria for judgment were as follows: When the yttria content is 4 mol% or less, if the transmittance is 12.0 or more, it is OK. When the yttria content is more than 4 mol% and less than 5.5 mol%, if the light transmittance is 15.0 or more, it is OK. When the yttria content is over 5.5 mol%, if the transparency is 16.0 or more, it is OK. The results are shown in Table 2.

[0072] <Method for measuring biaxial bending strength of sintered body> Samples were cut from the calcined bodies obtained in the same manner as above so as to have a diameter of 15 mm and a thickness of 1.2 mm after sintering. These samples were sintered for 2 hours at 1,500°C for Examples 1-10 and Comparative Examples 1-7, and at 1,450°C for Comparative Examples 8-10, to obtain sintered bodies with a diameter of 15 mm and a thickness of 1.2 mm. The biaxial bending strength of the resulting sintered bodies was measured at a crosshead speed of 0.5 mm / min using a Shimadzu Corporation universal precision testing machine Autograph (product name "AG-I 100kN") in accordance with JIS T 6526:2012 (average value of n=3). The results are shown in Table 2.

[0073] Examples 1 to 10 and Comparative Examples 1 to 6 > Zirconia raw material (zirconium dioxide, ZrO2) and yttria raw material (yttrium oxide, YO3) were weighed out so that the yttria content relative to the total moles of zirconia and yttria was the amount listed in Table 1, and then added to water. This and zirconia beads were placed in a rotary container, and the raw materials were mixed and pulverized using a ball mill until the desired primary particle size was achieved. The primary particle size was measured on a volume basis using a laser diffraction / scattering particle size distribution analyzer (product name "Partica LA-950") manufactured by Horiba, Ltd., by irradiating the water-diluted slurry with ultrasound for 30 minutes, followed by ultrasonic irradiation. Next, a binder weighed out to the content listed in Table 1 was added to this slurry, and the mixture was stirred with a rotor blade. The stirred slurry was dried and granulated using a spray dryer to obtain granules. The granules were then filled into a cylindrical mold, tapped thoroughly to vibrate them, compressed at a speed of 1 mm / s, and uniaxially pressed to a pressure of 33 MPa, after which a CIP process was performed to a water pressure of 170 MPa to obtain a green compact.

[0074] <Comparative Examples 8 to 10> Zpex (registered trademark), Zpex (registered trademark) 4, and Zpex Smile (registered trademark) manufactured by Tosoh Corporation were used as granules, and the green compact was produced in the same manner as in Example 1 above.

[0075] [Table 1]

[0076] [Table 2]

[0077] As a result, in Comparative Examples 1 to 4 in which the compression ratio R was 0.45 or less, the shape retention of the green compact deteriorated and chipping was observed, while in Comparative Examples 5 to 8 in which the compression ratio R was 0.54 or more, 6It was found that the translucency of the sintered body deteriorated when the compression ratio R was 0.45 or less and the stress at a compression ratio of 0.6 was small in Comparative Examples 8 to 10, in which the bulk density of the green compact was low and only calcined bodies and sintered bodies with low uniformity were obtained. In contrast, it was found that in Examples in which the compression ratio R was in the range of 0.46 to 0.53, the shape retention of the green compact was good, and uniform calcined bodies and sintered bodies were obtained, resulting in a zirconia sintered body with high translucency and high strength. It was confirmed that in the Examples, even when the yttria content was increased, both high translucency and high strength were achieved, providing an excellent balance. [Explanation of symbols]

[0078] 1. Direction of uniaxial pressure on the head 2. Mold 3 units

Claims

1. Granules containing zirconia, a stabilizer capable of suppressing a phase transition of zirconia, and a binder, The granules are filled into a cylindrical mold by tapping in accordance with JIS R 1628:1997. When compressed uniaxially at a rate of 1 mm / sec and pressurized to 33 MPa, the compression ratio R calculated by the following formula (1) is 0.46 to 0.53, the stabilizer contains yttria, and optionally further contains at least one selected from the group consisting of calcium oxide, magnesium oxide, cerium oxide, scandium oxide, niobium oxide, lanthanum oxide, erbium oxide, praseodymium oxide, samarium oxide, europium oxide, and thulium oxide; The content of the stabilizer is 1 to 15 mol% based on the total moles of zirconia and the stabilizer, the binder is a (meth)acrylic binder, the content of the binder is 1.1 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of a zirconia raw material composition that contains zirconia and the stabilizer but does not contain a binder, The weight average molecular weight of the binder is 70,000 to 480,000, Granules, wherein the average particle size of the primary particles constituting the granules is 45 to 200 nm. R=(HD) / H (1) (In the formula, H represents the height of the tapped granules in the columnar mold before pressure is applied, and D represents the amount of strain until 33 MPa is reached.)

2. The granules according to claim 1, wherein the stress when the compression ratio R is 0.6 during compression of the granules in the cylindrical die is 7.20 MPa or more.

3. 3. Granules according to claim 1 or 2, wherein the stabiliser is yttria.

4. The granules according to claim 3, wherein the content of the yttria is 2.5 to 8.5 mol% based on the total moles of zirconia and yttria.

5. A granule according to any one of claims 1 to 4, having a bulk density of 2.9 to 3.5 g / cm 3 That is, a compacted powder.

6. 6. The powder compact according to claim 5, wherein the stabilizer contained in the granules is yttria.

7. 7. The powder compact according to claim 6, wherein the content of the yttria is 2.5 to 8.5 mol % with respect to the total mol of zirconia and yttria.

8. A step of mixing zirconia with a stabilizer capable of suppressing a phase transition of zirconia to obtain a zirconia raw material composition; a pulverization step of pulverizing the zirconia raw material composition to obtain particles having an average primary particle size of 45 to 200 nm; and a step of adding a binder to the particles and granulating the particles to obtain granules containing zirconia, a stabilizer capable of suppressing the phase transition of zirconia, and a binder; the stabilizer contains yttria, and optionally further contains at least one selected from the group consisting of calcium oxide, magnesium oxide, cerium oxide, scandium oxide, niobium oxide, lanthanum oxide, erbium oxide, praseodymium oxide, samarium oxide, europium oxide, and thulium oxide; The content of the stabilizer is 1 to 15 mol% based on the total moles of zirconia and the stabilizer, the binder is a (meth)acrylic binder, the binder content is 1.1 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the zirconia raw material composition, The method for producing granules, wherein the binder has a weight average molecular weight of 70,000 to 480,000.

9. The method for producing granules according to claim 8, wherein the stabilizer is yttria.

10. The granules according to any one of claims 1 to 4 are tapped into a cylindrical mold, When compressed at a rate of 1 mm / s under uniaxial pressure and pressurized to 33 MPa, the compression ratio R calculated by the following formula (1) is 0.46 to 0.

53. A method for manufacturing a powder compact. R=(HD) / H (1) (In the formula, H represents the height of the tapped granules in the columnar mold before pressure is applied, and D represents the amount of strain until 33 MPa is reached.)

11. The method for producing a powder compact according to claim 10, wherein a stress is 7.20 MPa or more when a compression ratio R is 0.6 while the granules are being compressed in the cylindrical die.

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