GB2145710A - Method for producing porous ceramics - Google Patents

Abstract

A method for producing porous ceramics, comprises the steps of: mixing an expandable or swellable resin with raw ceramics material to form a powder mixture; mixing a swelling agent with the mixture to form a gelled body; drying said gelled body; and firing said dried gelled body whereby porous ceramics with numerous pores are formed. As the expandable resin, a powder form resin composed mainly of a highly water-absorbent resin may be used, and as the swelling agent, water or an aqueous solution containing a water-miscible solvent such as an alcohol or ketone solvent may be used.

Description

SPECIFICATION Method for producing porous ceramics The present invention relates to a method for producing porous ceramics.

Porous ceramics are lighter in weight than ceramic products in general use, and they are also highly durable to adverse thermal and chemical conditions.

Consequently, they are in great demand for use as, for example, furnace materials and insulating materials or for use as absorbent materials, catalyst carriers, etc. having pores. For producing such porous ceramics, several methods have been proposed. These methods include: a method of forming a porous body by first mixing an inorganic material, such as a hexagonal boron nitride, which foams and evaporates at firing temperatures, with the raw ceramics materials prior to firing (Japanese Patent Provisional Publication 1983-15062); a method in which a combustible powder, such as sawdust or polymer composite, is added to the powdered raw ceramics materials and then voids are formed by vaporizing the combustible powder during firing (USP 4,325,846); and a method in which raw ceramic material which has been slurried in a dispersing liquid is poured into the holes of a porous resin structure, such as a three-dimensional structure of polyvinyl acetal or soft polyurethane, the structure is dried and then fired whereby the synthetic resin is burned off (Japanese Patent Provisional Publication 1982-47756).

These methods each have merits and demerits, but at the present stage, there are still problems in producing porous ceramic materials economically while maintaining the desired quality of the resulting products. In particular, in the method involving addition of a material that foams on firing it is necessary to match the firing temperature at which the raw ceramic material is melted to the temperature for causing decomposition and gasification of the foaming materials by adjusting those temperatures to be the same level, but it is difficult to exactly predict the temperature suitable for firing when the raw materials used are natural products. Thus, it is in turn difficult to adjust the degree of foaming, and the dimensions and form of the product.Similarly, the method involving the preliminary addition of combustible powder prior to firing and vaporizing the mixture suffers from the following drawbacks. That is, when the additives are naturally produced materials, it is difficult to economically obtain a powder that is uniform in shape, grain size, etc. In addition, since the volume of the grains of the combustible powder fixes the volume of the voids, in order to obtain a porous body with a high void ratio, a combustible powder with a large volume must be mixed with the raw materials for the ceramic. As a result, it is usually difficult to produce porous ceramics having a void ratio above 60%.Moreover, although the method of forming a reticular and stearic porous body, i.e. a sponge with a synthetic polymer material, and then pouring the raw ceramic materials into the pores of the sponge, is logically acceptable as an industrial process, it has the following disadvantages. That is, it requires many process steps to prepare the three-dimensional porous material structure. Furthermore, since it is also necessary that the quantity of polymer material corresponds to the volume of the desired voids, so that a large quantity of polymer material must be used to obtain a ceramic with a high void ratio. Thus, for example, when an aqueous dispersion of raw ceramic material is poured into the above described spongy body, the ceramic structure becomes weak due to the lack of substantial solidness. Thus, this method is not realistic.

We have now discovered a method for producing porous ceramics that can produce ceramics with a desired number and shape of pores, void ratio, etc., regardless of the type of components of the ceramic.

Thus, the invention provides a method for producing porous ceramics, comprising the steps of: mixing an expandable or swellable resin with raw ceramics material to form a powder mixture; mixing a swelling agent with the mixture to form a gelled body; drying said gelled body; and firing said dried gelled body whereby porous ceramics with numerous pores are formed.

The method of the invention enables the production of porous ceramics at low cost and is easily applied to a casting method to produce ceramic structures with a specific shape by using a molding process.

The economical advantage is still further enhanced by using mainly resins which are extremely highly expandable upon the application of water during the production method since the use of such resins allows the utilization of water as the swelling agent to form the voids.

In keeping with the principles of this invention, there is provided the method for the production of porous ceramics wherein an expandable or swellable resin powder is first mixed with powdered raw ceramic material, then this mixture is turned into a sol by adding a swelling agent, the sol is poured into a mold so that it turns into a composite body containing swollen gels. Next, the composite body is heated and fired in order to gasify and purge off the swelling agent so that the spaces filled with the swelling agent form voids in the finished ceramic.

Brief description of the drawings Figure lisa flow chart illustrating one embodiment of a production method in accordance with the present invention.

Detailed description of the invention Referring now in more detail to the present invention, examples of raw ceramic materials to which the production method provided by this invention is applicable include the following minerals: silicious minerals, such as silica rock, silica clay, diatomaceous earth; alumina (aluminous) minerals, such as diaspore, bauxite, fused alumina; silica aluminous minerals, e.g.Gaerome clay and Kibushi clay which are kaolinites as the clay minerals, or bentonite and agalamatolite which are montmorillonite orsillimanite minerals; magnesian minerals, such as magnesite and dolomite; calcinous miner als, such as limestone and wollastonite; chrome minerals, such as chromite and spinel; zirconic minerals, such as zircon and zirconia; and other minerals, such as titanic minerals (titania) and carbonaceous minerals including graphite.

In addition to the natural minerals listed above, artificial minerals such as zirconia, silicon nitride, titania, electro-fused alumina, synthetic magnesia, synthetic dolomite, and synthetic mullite may be used as the raw ceramic materials.

These raw ceramic materials may be used alone or as mixtures and in a form of a pulverized powder, as in a conventional process for manufacturing refractory products. The grain size may also be selected optionally in accordance with the application, and this allows products to be obtained which cannot be prepared by the previously described manufacturing methods using a polymer sponge.

The swellable or expandable resins used in this invention are resins which are convertible from sol to gel by absorbing a swelling agent conveniently comprising water or an aqueous solution of a water-miscible or hydrophilic solvent, such an alcohol or ketone. As the resins described above, the following types listed below may conveniently be used, and all of those listed below are capable of expandingtheirvolume by swelling up to e.g.

several tens of times to 1000 times at maximum, compared with their dry form. Examples of these resins include: cross-linked products of polyvinyl alcohol or its derivatives; saponification products of vinyl ester-unsaturated carboxylic acids (or their esters) copolymers; saponification products of ethylene-vinyl ester-unsaturated carboxylic acids (or their derivatives) copolymers; cross-linked products of alkali salts of a-olefin-maleic anhydride copolymers; cross-linked products of polyethylene oxide; cross-linked products of polyvinyl-pyrrolidone; hydrolysis products of starch-acrylonitrile or methacrylonitrile copolymers; saponification products of cross-linked polyacrylamide; self-bridge polyacrylic acid, saponification products of hydroxyalkyl acrylate-acrylamide copolymers; and cross-linked products of sulfonated polyethylene.

These resins are in powder form, but they have the following property. That is, when they come into contact with water serving as the swelling agent, each individual powder grain of the resins forms a gel by absorbing the water in a certain time, and with an increase in the amount of water absorbed, the volume ofthe gel is enlarged. As will be readily appreciated, the shape of the gel is determined by the shape of the expandable resin that is the solid body. Thus, from a spherical resin particle, a spherical gel body is obtained. Also, the size of the gel body can be determined by the relative adjustment of the size of particles of the expandable resin and the volume of water serving as the swelling agent.In other words, a gel with a large volume may be obtained if the water absorption ratio of the resin is adjusted to be high by increasing the amount of water to be absorbed by the resin, within the water absorption capacity of the resin, it is possible to obtain a porous product with the use of a small amount of expandable resin. Furthermore, with a fixed void ratio, the number of pores can be adjusted by varying the volume of each pore to be large or small. This means that, the volume of each pore determines the number of pores at a fixed void ratio, and this number of pores in turn can be determined by the number of gel bodies. Then, the number of gel bodies can be easily controlled by selecting the combination of the size of the grains of the expandable resin and the water-absorption ratio (the volume expansion ratio through absorption of water) of the expandable resin.The void ratio that is determined by the total volume of the pores formed and the pore size is the important factor affecting the physical as well as thermal properties of ceramics.

For example, with an increase in void ratio, the strength and the filtration resistance decrease, but the gas permeability, the liquid permeability and the thermal conductivity are improved.

A flow chart illustrating the production process for fired products made of porous ceramics using the foregoing raw ceramic materials and the expandable resins is shown in Figure 1.

In order to disperse the gels in the raw ceramic material either one of the following methods may be used. One method is to mixthe expandable resin powder and the powdered raw ceramic material thoroughly in advance, then to pour the swelling agent into the mixture, to effect swelling and gelation. Another method is to form the gel particles from the expandable resin by swelling it to a specified degree, separately, then to mix these gelled particles with the raw ceramic materials. The former method can be applied to a pouring process and a casting process where the desired shape of the ceramic product is obtained before drying and firing.

This is the method providing a process that contributes to the facilitation of operation as well as to economization, and the method provided by this invention makes it possible to employ such a process for the first time because there has previously been a difficulty using a casting process in the various types of conventional methods for producing porous ceramic.

This method is applied and works in the way as described below. That is, when a swelling agent of specified ratio is added into the mixture of the raw material for the ceramic and the expandable resin, gelation of the expandable resin proceeds gradually over a certain time period after the addition of the swelling agent. Therefore, in the course of the foregoing gelation, there exists a time period wherein the expandable resin is partially gelled and is still flowable. By utilizing this state of gelation, the admixture can be processed by three-dimensional or two-dimensional molding by using either a metal mold or a jig. The time required to complete the gelation of the mixture (the gelation of the expandable resin with the swelling agent) from its fluid state varies depending on the shape and size of the powder grains of the highly water-absorbent resin, but this time is also controllable by changing the temperature of the swelling agent to be added. The material to be fired, that is stabilized threedimensionally as a composition of the gels of the expandable resin and the ceramic materials with some amount of free swelling agent captured in this raw ceramic material is heated at a drying temperature below about 110"C for about one hour. By this heating operation, the swelling agent in the gel is mostly evaporated without causing movement of the gelled bodies and the raw ceramic material, and a substantially filled structure frame work is formed.

Thereafter, drying is continued at a temperature below about 1700C until the swelling agent in the gel is nearly completely gone.

Next, in the firing process for melting the raw ceramic material, the porous ceramic is obtained. In order to enhance the three-dimensional stability of the powder form of the ceramic raw material prior to drying and firing if necessary, various types of synthetic polymer resins, tar, etc. which are commonly known as binding agents for inorganic powders may conveniently be added. Thus, the raw ceramic material forms a three-dimensional structure in a stable state together with the gel grains, by using a swelling agent or a bonding agent. As a result, a fired ceramic with stable form can be obtained without causing any deformation by shrinkage that may cause a change in the void ratio during drying and firing.In the case where water is used as the swelling agent, different from the free water, the water contained in the gels after absorbing the water has the property that it takes a relatively long time to move within the volume when heated. Accordingly, such water has a slow so-called evaporation rate. Therefore, the bumping phenomenon is not caused even if the drying temperature is too high. Furthermore, steam does not collect inside the ceramic structure. Thus, for example, even when the drying temperature is increased to 130-1 500C no deformation or crazing occurs in the ceramic. Besides, as the gasified steam moves by passing through the gaps between the raw ceramic material, the passage of the gasified steam forms an air vent.

In this way, the drying process for this material for porous ceramic is completed while the air vent is formed as described above, and thereafter, the porous ceramic material is fired as it is. In this manner, a sintered porous body provided with gas permeability is obtained as a product.

As the raw ceramic material, the previously described natural materials may be used. However, utilizing zirconium, silicon nitride, etc. which have recently become used as artificial raw materials for ceramics, porous ceramics as new ceramics having specific properties can be obtained by processing these artificial raw materials with the same production method as that applied to natural raw materials.

Hence, the production method provided bythis invention is not necessarily limited to natural minerals.

The advantageous features of the production method according to this invention are as described below.

1) The desired combination of dimension, shape, size, etc. which are the characteristic properties of the pore, and the combination of the number of pores in a given volume, (apparent density), and configurational distribution of pores, which are the characteristic properties of porous body, can be effected in a simple and easy manner.

2) in the process for gelation of the admixture of the raw ceramic material and the expandable resin, which are originally in powder form, respectively, during the initial stage of the swelling, the admixture can be kept in the state of sol, i.e. a slurry-like fluid state. Therefore, highly porous materials with a void ratio above 70% up to as 90% and with a bulk density of less than 0.4% can be molded by using a casting process. Accordingly, press forming is not necessarily required.

3) Through the use of highly water-absorbent expandable resins capable of expanding or swelling in volume to several times up to about 1000 times of their dry volume by absorbing e.g. water, with concurrent use of a swelling agent composed primarily of water that is low in cost, gelled bodies are obtained allowing a large number of pores to be formed. Hence, the cost for forming the voids in a structure can be kept low.

4) Assisted by the swelling agent or the binding agent, the powdered raw ceramic material forms a strong three-dimensional structural framework through the initial drying process. The swollen gels are present in the frame, and those gels form the pores while maintaining their volume by the second drying. In this process, as the water content passes through the spaces in the framework, the continuously interconnecting open pores provide gas vents which can be formed easily.

The following non-limiting Examples serve to illustrate the invention.

Example 1 As the raw material for the ceramic, 1 00g of commercial diatomaceous earth containing 92% silica (SiO2) as the primary component and 4% alumina, with 81% of the grains which have a size less than 10 microns in grain size was mixed with 1 0g of commercial bentonite containing 69% silica (SiO2) and 15% alumina, in which 95% of the total contents is smaller than 300 mesh. Into the mixture prepared as described above, as the highly waterabsorbent resin, 2g of commercial Sumicagel S-50 (a saponification product of vinyl acetate-methyl acrylate copolymers, from Sumitomo Chemical Co. Ltd.

Japan) powder was mixed. Furthermore, into the foregoing admixture, 400cc of room temperature water was poured and the mixture thus obtained was stirred. The mixture increased its viscosity while maintaining its flowability. After 3 minutes, the sample was poured in a receptacle for molding and left standing. Five minutes later, the same was hardened into a combination of gels and inorganic material, and it became a molded product that maintains its original form even when it was taken out of the molding receptacle, and thus can be carried easily. Next, this sample was dried in an air bath at 80 degrees centigrade for 12 hours. Then it was dried at 150 degrees centigrade for 8 hours.

After the drying, it was heated in an electric furnace with a heating rate of 10 degrees centigrade per minute. When the temperature reached 1050 degrees centigrade, it was kept at the same tempera ture for 30 minutes before it was allowed to cool.

Then, after 2 hours, the sample was taken out.

The fired product thus obtained was a porous ceramic having numerous pores with diameters from 0.3mm at a minimum to 0.9mm at maximum, which are three-dimensionally and evenly dispersed in the ceramic body. The measurement of the physical properties showed a bulk specific gravity of 0.15 and a void ratio of 84%; Example 2 Commercial light fired magnesia containing magnesium oxide (90% in content ratio), smaller than 170 mesh, was mixed with Kibushi clay and bentonite. In this case, the amount used was 100g of commercial light fired magnesia, 5g of commercial Kibushi clay, and 5g of commercial bentonite. Into the foregoing mixture, 2g of poval (polyvinyl alcohol) smaller than 100 mesh was mixed. Separately, an aggregated body of gel grains was prepared with the following procedure.That is, into 3g of Kl Gel 201 (a cross-linked product of alkali salt of a-olefin- maleic anhydride copolymers, from Ku ra ray Isoprene Co. Ltd. Japan) a highly water-absorbent resin that is smaller than 100 mesh, 300cc of room temperature water and 10cc of methanol were added, and the mixture was stirred. Next, into the mixed powder of light fired magnesia, Kibushi clay, bentonite and poval, the aggregation of gel grains was poured, and they were mixed well to become homogeneous. As a result, a viscous and hard slurry was obtained. When the slurry was charged in a cylindrical mold of 600cc in capacity and manually pressed slightly, a cylindrical dense molded product of 345cc in volume was obtained. This molded sample was heated to 110 degrees centigrade for 8 hours and then at 160 degrees centigrade for 7 hours. Following this heating, the sample was further heated in an electric furnace with a heating rate of 6 degrees centigrade per minute. When the temperature reched 1400 degrees centigrade, the temperature was kept at that level for 30 minutes.

Thereafter, the heating was stopped, and 6 hours later, the sample was taken out of the electric furnace. The resulting product was a porous ceramic having a large number of pores distributed threedimensionally in the ceramic body. The major axis of these pores was about 0.3mm. By the measurement, it was found that the void ratio was 81%, and the bulk specific gravity was 0.37.

Claims (8)

1. A method for producing porous ceramics, comprising the steps of: mixing an expandable or swellable resin with raw ceramics material to form a powder mixture; mixing a swelling agent with the mixture to form a gelled body; drying said gelled body; and firing said dried gelled body whereby porous ceramics with numerous pores are formed.

2. A method for producing porous ceramics according to claim 1, wherein the expandable resin is a resin in powder form containing a highly water-absorbent resin as its primary component.

3. A method for producing porous ceramics according to either of claims 1 and 2, wherein the swelling agent is water.

4. A method for producing porous ceramics according to either of claims 1 and 2, wherein said swelling agent is an aqueous solution containing a solvent.

5. A method for producing porous ceramics wherein an expandable or swellable resin powder is first mixed with powdered raw ceramic material, a swelling agent is then added to the mixture to form a sol, the sol is poured into a mold to form a composite body containing swollen gels, and the composite body is heated and fired to form the desired porous ceramic.

6. A method according to any one of the preceding claims substantially as herein described.

7. A method for producing porous ceramics substantially as herein described in any one of the Examples.

8. Each and every novel composition, process, method and apparatus as herein described.