JP2000001361A - Method for drying ceramics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a large-sized ceramic molding free from cracks and warpage by immersing a hydrogel-contg. ceramic molding in a solvent having a great affinity for water and having a lower b.p. and a smaller surface tension than water, carrying out solvent displacement and drying the molding. SOLUTION: A slurry contg. a ceramic powder, a gelling component and water is cast in a mold and converted into a hydrogel to form a gelled molding. This molding is immersed in a solvent having a great affinity for water and having a lower b.p. and a smaller surface tension than water, e.g. ethanol having 78.3 deg.C b.p. or acetone having 56.3 deg.C b.p., solvent displacement is carried out optionally under shaking and then the molding is dried to obtain a ceramic molding. The ceramic is, e.g. alumina, zirconia, silicon nitride, silicon carbide or sialon. The gelling component is, e.g. a gel such as methylcellulose, gelatin or starch or a water-soluble polymer of acrylamide and N,N'- methylenebisacrylamide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic molded article containing a hydrogel therein, particularly a ceramic molded article of an aqueous solvent gelled and molded in a mold. The present invention relates to a method for drying ceramics which is free from defects such as cracks and warpage and is suitable for drying.

[0002]

2. Description of the Related Art Moldings formed by wet or semi-wet molding of ceramics, such as cast molding, extrusion molding, and lathe molding, particularly using non-plastic raw materials, are liable to cracks during molding or drying. Then, in order to prevent cracks at the time of molding, it was effective to include hydrogel in the inside of the above-mentioned molded article and to impart flexibility to the wet molded article. However, the molded article containing the hydrogel is more difficult to dry than the molded article not containing the hydrogel, and during drying, the flexibility of the molded article is lost, so that cracks during drying cannot be prevented. Was. In particular, as the molded article became larger and thicker, defects such as cracks and warpage occurred more frequently during drying.

[0003] Here, as one of the ceramic molding methods in which a hydrogel is contained in the inside, gel molding (a slurry obtained by mixing a ceramic powder with a gelling component and water) is hydrogelated in a mold and molded. For example, US Paten
t No. 5,028,326) because near-net-shape molding can be performed and the particle volume ratio (the amount of particles per unit volume of the molded body) at the time of gelation is uniform.
It is expected as a molding method capable of producing a complex-shaped member at low cost and with high quality.

[0004] However, although the particle volume ratio at the time of gelation is uniform, water evaporation occurs only from the surface of the molded body. Crumbles. As a result, stress was generated due to the difference in drying shrinkage in the thickness direction of the molded body and the like, and defects such as cracks and warpage were easily generated. In particular, as the size of the molded product becomes larger and thicker, the stress due to the difference in the drying shrinkage increases, and a good fired product cannot be obtained.

[0005] As a method for drying a gel molded body, U.S. Pat. S.
Patent No. 5,028,326, drying by heating such as microwaves is described in Ceram. Trans.
26101-07 (1992), the inventor of the above-mentioned patent shows high-humidity room temperature drying.

[0006]

SUMMARY OF THE INVENTION However, large
In thick-walled molded products, defects such as cracks and warpage are generated by heating and drying such as microwaves, and it is not possible to obtain a good fired body.Also, even when drying at high humidity at room temperature, In addition to the fact that defects cannot be completely eliminated, an extremely long drying time is required, and neither of them can be applied to the production of large-sized and thick-walled members.

[0007] The present invention provides a ceramic molded article containing a hydrogel therein, particularly a ceramic gelled molded article formed by hydrogelating a gelling component in a mold.
Among them, the present invention provides a drying method for producing ceramics having no defects such as cracks and warpage, particularly in a large-sized and thick molded product.

[0008]

Means for Solving the Problems and Actions / Effects of the Invention The present invention made to solve the problems described above provides a ceramic molded body containing a hydrogel therein, which has a good affinity for water and has a higher affinity than water. It is characterized by being immersed in a solvent having a low boiling point and a low surface tension, replacing the solvent, and then drying.

[0009] Since the molded article is immersed in a solvent having good affinity for water, the solvent can be easily replaced with water. In addition, since the water of the compact is replaced with a solvent having a small surface tension, the capillary suction potential acting between the compact particles is reduced, and damage to the compact when the solvent evaporates can be reduced. Furthermore, the water of the compact is
Since the solvent is replaced with a solvent having a boiling point lower than that of water, drying after the replacement easily proceeds. As a result, defects such as cracks and warpage during drying can be prevented, and the time required for drying can be shortened.

According to the present invention, a drying shrinkage ratio is relatively large, such as gel molding in which a slurry containing ceramic powder, a gelling component and water is cast into a mold and hydrogelated in the mold to form the slurry. It is particularly suitable for drying a molded article in which the hydrogel has relatively excellent water resistance.

According to a further preferred method of the present invention, the solvent used for the solvent replacement is selected from an almost pure organic solvent and a mixed solvent obtained by adding water to the organic solvent. Two or more types of solvents are prepared, and a ceramic molded body containing a hydrogel therein is immersed in a descending order of a mixture ratio of water to a lower one, and the solvent is replaced.

When the water comes into contact with the solvent for solvent replacement and the water moves out by the solvent replacement, the capillary suction potential between the compact particles is much smaller than that in the case of drying in the air where the water is in contact with the air. Works to cause shrinkage of the molded body. Therefore, since the shrinkage when the molded body is immersed in the solvent for solvent replacement also starts from the surface, a stress due to a difference in shrinkage occurs in the thickness direction of the molded body or the like.

Therefore, a mixed solvent to which water is added is prepared, and the molded body is moved from a mixture having a high water mixing ratio to a mixture having a low water mixing ratio. The stress due to the difference can be reduced.

In a further preferred method of the present invention, the solvent is vibrated during part or all of the period during which the ceramic molded body containing the hydrogel is solvent-replaced. By vibrating the solvent, even in a large-sized and thick molded product, the solvent replacement can be quickly performed.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to further clarify the configuration and operation of the present invention described above, preferred embodiments of the present invention will be described below.

The type of ceramic used in the present invention is not particularly limited. For example, alumina, zirconia, silicon nitride, silicon carbide, sialon, and the like can be used.

The ceramic molding method used in the present invention comprises:
The inside of the ceramic molded body is not particularly limited to a molding method as long as it contains a hydrogel for the purpose of binder or plasticization, for example, cast molding, extrusion molding, potter's wheel molding, tape. Molding and the like can be used. Further, the present invention has a relatively large drying shrinkage, such as gel molding in which a slurry containing ceramic powder, a gelling component and water is cast into a mold and hydrogelated in the mold to form the slurry. The hydrogel is particularly suitable for drying molded articles having relatively excellent water resistance.

The type of the hydrogel used in the present invention is not particularly limited. For example, gels of methylcellulose, carboxymethylcellulose, polyvinyl alcohol, gelatin, starch and the like can be used. Further, in the gelling component in the gel forming, for example, U.S.A. S. Patent No. 5,028,326
Or a water-soluble monomer such as acrylamide and N, N'-methylenebisacrylamide, or a water-soluble polymer such as hydraulic urethane.

The ceramic molded body containing the hydrogel therein has a good affinity for water, has a lower boiling point than water, and is immersed in a solvent having a small surface tension to be replaced with a solvent.

[0020] Solvents satisfying this condition include ethanol,
There are organic solvents such as propanol and acetone.
Table 2 shows the surface tension and boiling point of these solvents.

[0021]

[Table 1]

Further, two or more of the above organic solvents may be mixed, or a mixed solvent obtained by adding water to the above organic solvent may be used.

In the solvent replacement, as in the present invention, two or more types of solvents having different mixing ratios of water are prepared, and a ceramic molded body containing a hydrogel therein is removed from one having a higher mixing ratio of water. It is preferable to sequentially immerse the components in ascending order and replace the solvent.

Here, the reason why the present invention is effective in preventing defects such as cracks and warpage during drying will be described in detail with reference to the change in drying shrinkage with time in FIG. When the molded body is replaced with a solvent, the capillary suction potential: ΔP = σ / r (σ: surface tension or interfacial tension, r: meniscus radius) acts between the molded body particles due to the interfacial tension between water and the solvent. Already shrinks during solvent replacement. Since this capillary suction potential is much smaller than the drying in the atmosphere where water is in contact with air, it becomes smaller than the drying shrinkage when left at room temperature or heated and dried.

Next, since the solvent of the compact after solvent replacement is replaced by an organic solvent having a small surface tension, the compact after solvent replacement acts between the compact particles when left at room temperature or heated and dried. The capillary suction potential is small and the drying shrinkage is small.

Therefore, the total shrinkage of drying of the above-mentioned molded article is smaller than the drying shrinkage when it is dried at room temperature or the like. Therefore, by replacing the water in the molded body with an organic solvent having a small surface tension and then drying, the stress due to the difference in drying shrinkage generated in the thickness direction of the molded body can be reduced. Defects such as cracks and warpage can be reduced even with a thick molded product.

Here, before the molded article is immersed in a substantially pure organic solvent and replaced with a solvent, a step of immersing the molded article in a mixed solvent obtained by adding water to the organic solvent and replacing the solvent is added to further increase the capillary suction potential. Can be smaller. Therefore, if the solvent is gradually replaced from a mixture having a high mixing ratio of water to a mixture having a low mixing ratio, drying shrinkage is, for example,
The contraction rate in one substitution can be reduced. Therefore, it is possible to further reduce the stress due to the difference in drying shrinkage generated in the thickness direction of the molded body,
Defects such as cracks and warpage can be prevented.

The stepwise substitution with the mixed solvent of the organic solvent and water according to the present invention may be performed in two steps or three or more steps. Increasing the number of stages is advantageous for preventing defects such as cracks and warpage, but increases the processing time.

Therefore, the time required for solvent replacement can be shortened by performing a part or all of the solvent replacement period while applying vibration to the solvent as in the present invention.

As a method of applying vibration to the solvent, an ultrasonic vibrator or a vibrator may be attached to the solvent tank, or the solvent itself may be stirred or bubbled.
Drying after the completion of the solvent replacement may be performed in the air or under reduced pressure. In addition, room temperature or heating may be performed. The molded body dried by the above method is degreased and fired under conditions suitable for the ceramic powder and hydrogelation component used for this, and a fired body of high quality ceramics can be easily obtained.

[0031]

Embodiment 1 An embodiment of the present invention will be described below. To a mixed aqueous solution of acrylamide and N, N'-methylenebisacrylamide, add an alumina powder having an average particle diameter of 0.6 μm and a deflocculant, and mix in a ball mill to obtain a solid content of about 55 vol.
A 1% slurry was made.

After adding a polymerization initiator and a catalyst to the mixture and defoaming, the mixture was cast into a plastic mold having a casting surface of φ300 × 50 tons and gelled. After immersing the gelled molded body in ethanol for 3 days and replacing the solvent,
It was left at room temperature (about 25 ° C.) for 4 days. Then, it was transferred to a hot air drier at 60 ° C. and dried until the water content became almost zero. The formed body dried by the above method was then fired, and a good fired body free from defects such as cracks and warpage was obtained.

As a comparative example of the present invention, the results of direct drying at room temperature, drying by microwave heating, and humidification drying at room temperature are shown in Table 2 without solvent replacement.

[0034]

[Table 2]

When directly dried at room temperature or dried by microwave heating, cracks are generated during drying.
Drying at room temperature required about twice the drying time as compared to drying at room temperature after solvent replacement. In addition, even when humidified and dried at room temperature, cracks cannot be completely eliminated, and an extremely long drying time is required.

[0036]

Example 2 The same slurry as in Example 1 was used to form a thicker φ300 × 65t than in Example 1. The gelled compact was subjected to ethanol: water = 40: 60 (vol.
%) For 4 days to replace the solvent, further transfer to almost pure ethanol for 4 days, and then leave at room temperature for 6 days, then transfer to a hot air dryer at 60 ° C. until the water content becomes almost zero. Dried. The molded body dried by the above method,
Thereafter, firing was performed, and a good fired body free from defects such as cracks and warpage was obtained.

As a comparative example of the present invention, it was natural that drying was carried out at room temperature and drying by microwave heating without solvent replacement, and cracks occurred during drying, even when humidified and dried at room temperature. Furthermore, cracks also occurred in those which were immersed in almost pure ethanol from the beginning without solvent replacement of the mixed solvent of ethanol and water and solvent replacement was performed.

[0038]

As described above, according to the present invention, a ceramic molded body containing a hydrogel therein can be dried without defects such as cracks and warpages. This makes it easy to manufacture even a large-sized and thick-walled molded product that has been difficult to produce.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change over time in drying shrinkage.

Claims (4)

[Claims] 1. A ceramic molded body containing a hydrogel inside, which has a good affinity for water, has a lower boiling point than water,
A method for drying ceramics, comprising immersing in a solvent having a small surface tension, replacing the solvent, and then drying. 2. A gel molded body formed by casting a slurry containing ceramic powder, a gelling component, and water into a mold, and hydrogel-forming in the mold, has a good affinity for water, and has a boiling point higher than that of water. A method for drying ceramics, comprising immersing in a solvent having a low surface tension and replacing the solvent, followed by drying. 3. The solvent according to claim 1, wherein at least two kinds of solvents having different mixing ratios of water are prepared from a substantially pure organic solvent and a mixed solvent obtained by adding water to the organic solvent. Then, the ceramic molded body containing hydrogel inside,
Immerse in order from high to low water mixing ratio,
A method for drying ceramics, comprising replacing the solvent. 4. The solvent replacement according to claim 1, wherein
A method for drying ceramics, wherein part or all of the solvent replacement period is performed while applying vibration to the solvent.