JP3015402B2 - Freeze-drying method of ceramic molded body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for freeze-drying a ceramic molded body.

[Problems to be Solved by the Related Art and the Invention] When cooled, an exhaust gas purifying apparatus for an internal combustion engine such as a diesel engine filters a carbon soot and the like in the exhaust gas and carries a catalyst for oxidatively decomposing them. There is a filter to carry. As shown in FIGS. 1 and 2, this filter is formed using a porous material having a large number of fine open pores, and has a cylindrical shape and a through-hole (cell) 2 extending in the axial direction having a square shape. 10 per inch
The honeycomb structure 1 includes about 0 to 200 honeycomb structures.

Conventionally, such a honeycomb structure is formed by forming a raw material composition obtained by mixing an organic resin binder and water as a dispersion solvent with ceramic powder such as mullite, cordierite, and silicon carbide into a honeycomb shape, and heating and drying the formed body. Alternatively, water is removed from the molded body by performing drying under reduced pressure, and then the ceramic powder is sintered.

However, when the molded body in the water-containing state is subjected to heating drying or drying under reduced pressure as it is, distortion due to drying shrinkage occurs due to imbalance in the water evaporation rate between the surface and the inside of the molded body, and the surface of the molded body has There is a problem that a coarse crack (macro crack) 3 as shown in FIG. 3 is generated.

On the other hand, a molded body in a water-containing state is charged into a freezer cooled in advance to about −20 ° C., the molded body is temporarily frozen, and then dried under reduced pressure to sublimate the frozen water. There is known a freeze-drying method for preventing the molded article from drying and shrinking and preventing the occurrence of cracks by removing the molded article. However, when the honeycomb formed body was dried by this method, the occurrence of macro cracks as described above was not observed. However, as shown in FIG. 4, the cell walls formed in a lattice shape of the honeycomb structure 1 were formed as shown in FIG. 4 has a new problem that a star-shaped fine crack (microcrack) 5 is generated near the lattice point. For example, even such a fine crack is not cured by the subsequent firing, and causes a significant decrease in the mechanical strength of the sintered body and, consequently, the durability as an exhaust gas purification filter. Had become.

The present inventors considered that the generation of such microcracks was because the compact was not uniformly frozen due to the limit of the cooling capacity of the freezer. In other words, the surface layer of the cell wall 4 that is in direct contact with the cool air is frozen to cause a volume change (expansion), whereas the inside of the cell wall 4 that does not directly contact the cool air is not immediately frozen,
It is speculation that distortion occurs between the surface layer portion and the inside.
Therefore, in order to immediately freeze the entire molded body, an attempt was made to immerse the water-containing honeycomb molded body in liquid nitrogen. However, although no microcracks 5 as shown in FIG. 4 were found on the cell wall 4 after freezing, macrocracks 3 as shown in FIG. The solution was not reached.

The present invention has been made in view of the above circumstances, and its object is to provide a molded article in a state containing a dispersion solvent without causing inconvenience such as generation of cracks, regardless of the shape of the molded article. An object of the present invention is to provide a freeze-drying method for a ceramic molded body that can be dried quickly and reliably.

[Means and Actions for Solving the Problems] In order to solve the above problems, the present inventors have intensively studied and elucidated the mechanism of generation of cracks in the process of cooling the formed body, thereby completing the present invention. That is, in the present invention, the molded body obtained by molding the raw material composition containing the raw ceramic powder and the dispersion solvent is cooled to a primary cooling temperature higher by a predetermined temperature than the solidification point T ° C. of the dispersion solvent over the entire molded body. After cooling uniformly, the molded body was cooled to the freezing point T ° C.
By rapidly cooling to a lower secondary cooling temperature, the dispersion solvent in the molded body is frozen, and then the ceramic dispersion is dried by sublimating the frozen dispersion solvent by vacuum drying and removing it from the molded body. are doing.

According to this method, the preform is cooled to a primary cooling temperature, and the entire preform is cooled to a temperature higher than the freezing point T ° C of the dispersion solvent by a predetermined temperature. For this reason, the compact comes to a state immediately before freezing, and the amount of heat to be removed from the compact when the unfrozen compact is frozen is reduced as much as possible.

Subsequently, the molded body is rapidly cooled to the secondary cooling temperature, thereby further removing heat from the unfrozen molded body. At this time, since the molded body has already been cooled to the vicinity of the freezing point T of the dispersion solvent, the molded body is uniformly frozen over the entire body by a small amount of heat release in a relatively short time. In such a case, no extreme freezing speed difference occurs between the inside and the outer layer of the molded body, and a situation in which cracks occur due to distortion in each part of the molded body is avoided.

Thereafter, the frozen molded body is dried under reduced pressure, whereby the frozen dispersion solvent is sublimated and removed from the molded body. At this time, since the molded body does not cause a volume change such as a sudden shrinkage, the molded body is dried while maintaining the shape at the time of molding without generating a crack due to the sudden change.

The raw material composition according to the present invention is obtained by mixing at least a dispersion solvent with the raw ceramic powder.

Examples of ceramics to which the present invention can be applied include carbon silicon, boron carbide, silicon nitride, boron nitride, aluminum nitride, aluminum oxide, zirconium oxide, mullite, cordierite, aluminum titanate, zirconium boride, sialon, and the like. Each ceramic is used alone or in a mixture of two or more kinds in powder form.

In particular, the quality of a molded body before firing, such as silicon carbide sintered in a gas phase, is greatly affected by the quality of a ceramic to which the present invention is applied, such as the mechanical strength of a sintered body after firing. Is very beneficial.

Examples of the dispersing solvent include water (freezing point: 0 ° C.) and organic solvents such as benzene (freezing point: 5 ° C.). Those having an extremely low freezing point are difficult to freeze, while those having an extremely high boiling point. Is not preferred because drying under reduced pressure becomes difficult. In that regard, it is preferable to use water as the dispersion solvent, and in this case, the mixing ratio of water is 10% or less.
The range of 20 to 50 parts by weight relative to 0 parts by weight is suitable. When the mixing ratio is less than 20 parts by weight, the ceramic powder cannot be uniformly dispersed and the raw material composition cannot be kneaded. On the other hand, when the mixing ratio is more than 50 parts by weight, the viscosity of the raw material composition is unnecessarily reduced and molding is performed. Cause trouble. Further, the space between the ceramic particles is expanded more than necessary due to the volume expansion of the water due to the freezing, and when the frozen water no longer intervenes between the particles due to the drying under reduced pressure, there is a possibility that the molded product may lose its shape.

On the other hand, the raw material composition may optionally contain a molding binder. As the molding binder, for example, phenol resin, lignin sulfonate,
Various organic substances such as polyvinyl alcohol, methylcellulose, carboxymethylcellulose, propylene glycol, constarch, molasses, coal tar pitch, and alginate are used, and these substances are used alone or as a mixture of two or more kinds.

The raw material composition is kneaded and adjusted sufficiently with a Henschel mixer or the like, and formed into a desired shape by extrusion or the like.

The compact obtained in this way is cooled to a primary cooling temperature higher by a predetermined temperature than the freezing point T ° C. of the dispersion solvent. According to the present invention, it is desirable to set the primary cooling temperature in the range of the freezing point of the dispersion solvent T to (T + 2) ° C.

The primary cooling temperature is ideally as close as possible to the freezing point T of the dispersion solvent in order to bring the molded body to a state immediately before freezing as much as possible. However, since it is necessary to avoid local freezing, the freezing point It needs to be slightly higher than T. On the other hand, the compact has a certain weight, and the amount of heat to be removed to freeze the compact is also proportional to the weight of the compact. In view of these points, the upper limit (T + 2) ° C. of the primary cooling temperature range is determined by setting the present invention to a molded article having a general size (in the case of a honeycomb structure, a diameter of 140 mm and a length of 50 to 200 mm).
mm, a molded body having a weight of about 500 to 3000 g), the rapid cooling means assumed in the present invention removes heat from the molded body in a very short time, and quickly removes the molded body without generating cracks or the like. It shows the upper limit of the secondary cooling start temperature that can be frozen.

Further, from the viewpoint of uniformly cooling the entire molded body to the primary cooling temperature, it is preferable to hold the molded body at the primary cooling temperature for a predetermined time. The holding time is suitably about 10 to 30 hours for a molded article having a general size (weight of about 500 to 3000 g).

Subsequently, by rapidly cooling the molded body to a secondary cooling temperature lower than the freezing point T ° C. of the dispersion solvent, the dispersion solvent in the molded body is uniformly and quickly frozen throughout the molded body. When quenching to the secondary cooling temperature, a gaseous or liquid refrigerant cooled to or below the secondary cooling temperature is used.

When the refrigerant is a gaseous refrigerant at the secondary cooling temperature, the molded body is rapidly cooled by flowing the refrigerant around or inside the molded body. Usable gaseous refrigerants include chlorofluorocarbon gas, carbon dioxide gas, argon gas and the like, but there is no problem even if sufficiently cooled nitrogen gas or air is used.

Further, it is preferable to use a liquid refrigerant as a means for rapidly cooling the molded body to the secondary cooling temperature.

Examples of such a liquid refrigerant include halogenated hydrocarbons such as dichloroethane and chlorofluorocarbon, aromatic hydrocarbons such as toluene and xylene, and silicone oil.

Liquid refrigerants have the advantage that they generally have a higher heat absorption capacity than gaseous refrigerants and are less likely to rise in liquid temperature. Also,
By immersing the molded body in the refrigerant, the molded body can be uniformly contacted with the entire molded body, so that heat can be relatively uniformly removed from the molded body in a short time. In the case of the molded article of the general size, the secondary cooling time by the gaseous refrigerant is 1 to 2 hours, whereas if a liquid refrigerant is used, the secondary cooling time is within 30 minutes. Can be.

Further, it is desirable that the liquid refrigerant has poor affinity with the dispersion solvent.

The reason is that even when the molded body is immersed in the dispersion solvent, it is possible to prevent the refrigerant from penetrating the molded body as much as possible.

Furthermore, when water is used as the dispersion solvent, the secondary cooling temperature is preferably set within a temperature range of -20 to -5C.

If the secondary cooling temperature is lower than −20 ° C., the difference from the primary cooling temperature becomes large, and macro cracks are likely to occur particularly in a molded article having a complicated shape such as the honeycomb structure described above. On the other hand, when the secondary cooling temperature exceeds -5 ° C,
The difference from the primary cooling temperature becomes small and it becomes difficult to quickly freeze the molded body, and there is a possibility that micro cracks may occur particularly in a molded body having a complicated shape.

When water is used as the dispersion solvent, the freezing point is −20.
It is preferable to use a refrigerant having a temperature of not more than ° C. The reason is,
In order to quickly freeze the moisture in the compact, it is necessary to increase the heat absorption capacity of the refrigerant, but if the freezing point is high, the temperature of the refrigerant cannot be lowered, so a large amount of refrigerant is required. It is.

After the freezing of the compact is completed, the frozen dispersion solvent is sublimated by drying under reduced pressure and removed from the compact, whereby the ceramic compact is dried.

Drying under reduced pressure is usually performed at room temperature. However, if the temperature is 50 ° C. or lower, there is no possibility that cracks will occur in the molded body. Therefore, drying may be promoted by heating.

Since the frozen dispersion solvent is removed by sublimation, no flow of the dispersion solvent in the molded body occurs.
Therefore, since the ceramic particles constituting the molded body are dried while maintaining the position at the time of molding, even when this is fired, the predetermined pore diameter, reflecting the particle size composition of the ceramic particles, etc. A porous sintered body having a pore distribution can be obtained.

In this way, an ideal compact without cracks is formed.

According to the present invention, even a molded article having a complicated shape can be dried without generating macro and micro cracks. Therefore, the present invention can be applied to a honeycomb structure used as a filter of an exhaust gas purification device. Is preferred. As shown in FIGS. 1 and 2, if the honeycomb formed body 1 has a cylindrical shape and a large number of through holes (cells) 2 formed in the axial direction, the honeycomb that can be effectively dried by the present invention is used. The range of the size of the molded body 1 is 80 to 200 m in diameter.
m, length is 50-200mm, thickness of cell wall 4 is 0.15-0.5mm,
The cell pitch is 1.15 to 2.5 mm, and the number of cells is 100 to 500 per parallel inch.

[Examples 1 and 2 and Comparative Examples 1 to 3] Hereinafter, Examples 1 and 2 in which the present invention is embodied in a honeycomb filter used in an exhaust gas purification device for an internal combustion engine will be compared with Comparative Examples 1 to 3. explain.

(Example 1) <Preparation of molded article> 10 parts by weight of methylcellulose and 25 parts by weight of water were added to 100 parts by weight of silicon carbide fine powder having an average particle diameter of 0.28 μm and 96.2% of β-type crystal. They were blended and uniformly mixed with a Henschel mixer to prepare a raw material composition. And
Using a vacuum extruder, as shown in FIGS. 1 and 2, a honeycomb formed body 1 having a cylindrical shape and formed with a large number of through holes (cells) 2 in an axial direction was formed. This honeycomb formed body 1 is
It has a diameter of 140 mm, a length of 140 mm, a thickness of the cell wall 4 of 0.45 mm, a cell pitch of 1.95 mm, a number of cells of 170 cells / square inch, a weight of about 2200 g, and a water content of about 18 to 20% by weight.
Met.

<Primary Cooling Step> Next, the compact was placed in a refrigerator, and cooling was started at room temperature (15 ° C.) at a cooling rate of 1 ° C./hr under normal pressure to 1 ° C.
, And then kept at this temperature for 30 hours.

<Secondary cooling step> Immediately cool the primary-cooled compact in a refrigerator at -20 ° C.
This was frozen by immersion in about 10 CFCs and kept in the refrigerant for about 30 minutes.

<Decompression Drying Step> Subsequently, the secondary-cooled molded body was taken out of the fluorocarbon, and high-pressure air was blown to remove fluorocarbon remaining in the cell. Then, the formed body was placed in a vacuum dryer, and dried under a reduced pressure of 0.1 to 1 Toll for 20 hours. During this time, the temperature inside the vacuum dryer was controlled at about 40 ° C.

By drying under reduced pressure, a dried molded article having a water content of 0.9% by weight was obtained. No macro cracks were observed on the surface of the molded product, and no micro cracks were observed on the cell walls 4.

<Preparation of Sintered Body> Further, the compact dried under reduced pressure was inserted into a tanman type firing furnace, and fired at 2200 ° C. for 4 hours in an argon gas atmosphere to obtain a honeycomb-shaped porous silicon carbide sintered body. Obtained. Macrocracks, microcracks, etc. were not observed on the surface or inside of the sintered body. The three-point bending strength of this sintered body was 6.0 kgf / mm ² .

(Example 2) A molded article was produced in the same manner as in Example 1, and the molded article was subjected to the same primary cooling. After completion of the primary cooling, the compact was frozen by circulating -20 ° C. gas in the refrigerator. In addition, the circulation of Freon gas is
Time continued. Thereafter, drying under reduced pressure was performed in the same manner as in Example 1 to obtain a dried molded product having a moisture content of 1.1% by weight and showing no cracks.

Further, when the dried molded body was fired in the same manner as in Example 1, there was no crack and the three-point bending strength was 5.3 kN.
A gf / mm ² honeycomb-shaped porous silicon carbide sintered body was obtained.

(Comparative Example 1) After forming a molded body in the same manner as in the above-mentioned Example 1, without passing through the primary cooling step, in the same manner as in the above-mentioned Example 1, the molded body was directly cooled to about −20 ° C. fluorocarbon. It was immersed in 10 and kept in the refrigerant for about 30 minutes.

After 30 minutes, when the molded body was removed from the refrigerant,
Many macro cracks 3 are formed on the surface, and cell walls 4
, Microcracks 5 were observed (see FIGS. 3 and 4).

(Comparative Example 2) After forming a molded body in the same manner as in Example 1, the molded body was placed in a refrigerator, and cooling was started at normal temperature (15 ° C) at a cooling rate of 1 ° C / hr under normal pressure. The temperature was raised to 4 ° C. and maintained at this temperature for 30 hours. Thereafter, secondary cooling and drying under reduced pressure were performed in the same manner as in Example 1. Observation of the obtained dried molded body revealed that many star-shaped microcracks 5 were observed on the cell wall 4 (see FIG. 4).

(Comparative Example 3) A molded article was produced in the same manner as in Example 1, and the molded article was subjected to the same primary cooling. Thereafter, the molded body was immediately immersed in about 10 ° C. of -40 ° C. in a refrigerator to freeze it, and kept in a refrigerant for about 30 minutes.

After 30 minutes, when the molded body was removed from the refrigerant,
Many macro cracks 3 were observed on the surface (see FIG. 3).

(Consideration of Results) From the results of Examples 1 and 2 and Comparative Example 1, it can be seen that a primary cooling step is necessary in order not to cause cracks in the molded body. In Comparative Example 1, a macro-crack 3 was generated due to a large temperature difference between the inside and the outer layer of the molded body when cooled to the freezing temperature, and a micro-crack was generated in the cell wall 4 due to a low freezing rate. 5 appears to have occurred.

From the results of Examples 1 and 2 and Comparative Example 2, it can be seen that cracks occur when the primary cooling temperature is out of the aforementioned range.

From the results of Examples 1 and 2 and Comparative Example 3, it can be seen that cracks occur when the secondary cooling temperature is out of the preferred range when the dispersion solvent is water. In addition, since the crack has a large temperature difference between the molded body and the refrigerant, it is considered that a portion that instantaneously freezes and a portion that does not freeze occur.

[Effects of the Invention] As described above in detail, according to the freeze-drying method of the present invention,
Irrespective of the shape of the molded article, there is an excellent effect that the molded article containing the dispersion solvent can be dried quickly and reliably without inconvenience such as generation of cracks.

[Brief description of the drawings]

FIG. 1 is a front view of a honeycomb structure embodying the present invention,
FIG. 2 is a partial cross-sectional view taken along line AA of FIG. 1, FIG. 3 is a perspective view showing a state in which macrocracks have entered the surface of the honeycomb structure, and FIG. 4 is a microcrack inside the honeycomb structure. FIG. 4 is a partially enlarged view of a cell wall showing a state in which is inserted.

Claims (5)

(57) [Claims] 1. A molded body obtained by molding a raw material composition containing a raw ceramic powder and a dispersion solvent is uniformly spread over the whole of the molded body to a primary cooling temperature higher by a predetermined temperature than a solidification point T ° C. of the dispersion solvent. After cooling, the molded body is rapidly cooled to a secondary cooling temperature lower than the freezing point T ° C., thereby freezing the dispersion solvent in the molded body. A method for freeze-drying a ceramic molded body, comprising removing the ceramic molded body from the body. 2. The method according to claim 1, wherein the primary cooling temperature is set to a freezing point T to (T + 2).
The method according to claim 1, wherein the temperature is set in the range of ° C. 3. A liquid cooling medium is used as means for rapidly cooling the compact to a secondary cooling temperature.
Or the freeze-drying method of the ceramic molded article according to 2. 4. The freeze-drying method for a ceramic molded body according to claim 3, wherein the liquid refrigerant has poor affinity with a dispersion solvent. 5. The method according to claim 1, wherein water is used as a dispersion solvent and the secondary cooling temperature is set within a temperature range of -20 to -5 ° C. A method for freeze-drying ceramic molded bodies.